14.139.121.10614.139.121.106 › pdf › 2013 › jan13 › 4.pdf · Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda,

SYNTHESIS, CHARACTERIZATION AND APPLICATION OF

MESOPOROUS MATERIALS

Submitted To

The Maharaja Sayajirao University of Baroda

For

The Degree of

DDOOCCTTOORR OOFF PPHHIILLOOSSOOPPHHYY

in

Applied Chemistry

by

Rajeshkumar M. Patel

Applied Chemistry Department Faculty of Technology and Engineering

The Maharaja Sayajirao University of Baroda Vadodara - 390001, Gujarat, India

August 2012

Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda, Post Box No. 51, Kalabhavan, Vadodara - 390001, Gujarat, India.

Prof. Uma V. Chudasama Research Guide

Tel: +265-2434188 Extn. 415, 212

Date: 27/08/2012

CCEERRTTIIFFIICCAATTEE

The thesis entitled “SYNTHESIS, CHARACTERIZATION AND APPLICATION

OF MESOPOROUS MATERIALS” incorporates the original research work

carried out by MR. RAJESHKUMAR M. PATEL under my supervision.

Head,

Applied Chemistry Department

Dean,

Faculty of Technology and Engineering

Dedicated

To

My beloved

Parents

With profound sense of indebtedness and

reverence, I wish to express my sincere feelings of

gratitude to my research guide Dr. Uma V.

Chudasama, Professor, Applied Chemistry

Department, Faculty of Technology and

Engineering, The M. S. University of Baroda,

Vadodara, for her inspiring and enthusiastic

guidance throughout the research work. Special

thanks and recognition go to her, whose drive and

commitment to research has inspired, motivated

and enabled me to complete this endeavour. She

has played a fundamental role in defining my

future.

(Rajeshkumar Patel)

*** ACKNOWLEDGEMENTS ***

Primarily, I would like to express my profound gratitude to

Dr. (Mrs.) Uma V. Chudasama, Professor, Applied Chemistry Department,

Faculty of Technology and Engineering, The M. S. University of Baroda,

who constantly encouraged, inspired and guided me right from the

beginning of this work. Her receptive attitude, untiring enthusiasm and

positive approach will always remain a source of inspiration. Her deep

passion for perfection of work has not only taken care of my present study,

but shall always be a guide to me in future.

I wish to express my gratitude to the Dean, Faculty of Technology

and Engineering and to the Head, Applied Chemistry Department for

providing me the required facilities to carry out my research work.

My sincere thanks to the Applied Chemistry Department and

Department of Metallurgical and Material Engineering for providing

Thermal analysis, FTIR and SEM, EDX facilities respectively.

I wish to express my gratitude to the management of Sud-Chemie

India Pvt. Limited for providing me instrumentation and other Laboratory

facilities for conducting my research work. My special thanks to Mr. T. A.

Siddiquie (Asstt. Vice president-Operation), Mr. V. Sreedharan (Asstt. Vice

president-Works), Mr. R. M. Cursetji (Head- R & D(Tech. Development)),

Dr Arun Basarur (GM- R & D), Dr. D. P. Sabde (Manager- R & D), Mr. S.

A. Shaikh (Manager- Per.& HRD), and my research staff at Sud-Chemie

for helping me in many ways.My special thanks to Dr. Rikesh Joshi (Asstt.

Manager- R & D) for his help, support and useful suggestions at various

stages of my research work.

I extend my sincere thanks to all the teaching staff members of the

Applied Chemistry Department who have helped me in many ways during

my research work.

I had the pleasure of working with an excellent group of lab mates,

Dr. Rikesh Joshi, Dr. Rakesh Thakkar, Dr. Parimal Patel, Mr. Tarun

Parangi, Mr. Brijesh Shah and Mr. Shrinivas Ghodke for making my work

an enjoyable one. I would like to thank my lab mates for all the help.

I cannot forget the cooperation extended to me by the Research

students working in the Department and non-teaching staff members.

I thank my family members for their love and encouragement, who

above all have taught me that patience, determination, persistence and

faith are the keys to success.

There were so many people who did so many things to help me out

at critical times. In this short space, it is impossible to acknowledge all of

them individually. I thank all of you, my friends, colleagues and well

wishers, who have always been with me, in times of need.

(Rajeshkumar Patel)

CONTENTS

CHAPTER 1 INTRODUCTION 1-37 1.1 Introduction 1 1.2 Towards a workable definition 3 1.3 Catalyst development and commercialization 5 1.4 Catalysis concepts and terminologies 7 1.5 Catalyst characterization 13 1.6 Present trends in catalysis 16 1.7 Solid acid catalysts-An alternate approach to

liquid acid catalysts 18

1.8 Pores and porosity 23 1.9 From micropores to mesopores 25

1.10 Ordered mesoporous materials 26 1.11 Summarising porous materials 28 1.12 Aim and scope of the present work 30

References 33 CHAPTER 2 Synthesis and Characterization of MCM-41

Based Catalysts and their Applications as Solid Acid Catalysts in Esterification and Friedel-Crafts Acylation and Alkylation

38-130

2.1 Introduction 38 2.2 Sol-gel process 38 2.3 Sol-gel process as applied to silica 44 2.4 Sol-gel synthesis as applied to Zeolites 46 2.5 Sol-gel process using templates 49 2.6 The formation of mesoporous structures 53 2.7 Generalized LCT mechanism 56 2.8 Removal of template 58 2.9 Synthesis strategies and characterization

methodologies- A literature survey 60

2.10 Experimental 71 2.11 Materials characterization 75 2.12 Application of Al-MCM-41 and 12TPA-MCM-41

as solid acid catalysts 83

2.13 Esterification 84 2.14 Literature survey in the current area of study 87 2.15 Experimental 91 2.16 Results and discussion 93

2.17 Friedel-Crafts acylation and alkylation 101 2.18 Literature survey in the current area of study 106 2.19 Experimental 112 2.20 Results and Discussion 113 2.21 Conclusions 118

References 119 CHAPTER 3 Synthesis and Characterization of Zr-

MCM-41 and Ti-MCM-41 and their Application as Oxidation Catalysts

131-160

3.1 Introduction 131 3.2 Literature survey in the current area of study 131 3.3 Synthesis of Zr-MCM-41 and Ti-MCM-41 134 3.4 Material characterization 137 3.5 Application 143 3.6 Experimental 148 3.7 Results and discussion 149 3.8 Conclusions 153

References 155 CHAPTER 4 Synthesis and Characterization of a

Palladium Loaded Perovskite MCM-41 material and its Application as an Automotive Catalyst

161-181

4.1 Introduction 161 4.2 Automotive exhaust purification catalysis –

Terms and concepts 162

4.3 Materials used as automotive exhaust purification catalysts

168

4.4 Synthesis strategies for automotive catalysts- A literature survey

171

4.5 Aim and scope of the present work 172 4.6 Experimental 173 4.7 Results and discussion 175 4.8 Conclusions 179

References 180 SUMMARY PUBLICATIONS

CCHHAAPPTTEERR 11 Introduction ________________________________________________________

Chapter 1 - Introduction

1

1.1 INTRODUCTION Catalysts have been successfully used in the chemical industry for

more than 100 years, examples being the synthesis of sulfuric acid, the

conversion of ammonia to nitric acid, and catalytic hydrogenation. Later

developments include new highly selective multi component oxide and

metallic catalysts, zeolites, and the introduction of homogeneous transition

metal complexes in the chemical industry. This was supplemented by new

high-performance techniques for probing catalysts and elucidating the

mechanisms of heterogeneous and homogenous catalysis [1]. A brief

historical survey given in Table 1.1 shows how closely the development of

catalysis is linked to the history of industrial chemistry.

Catalysis provides the chemist and the technologist with a valuable tool

for developing existing industries on a more economic and sound footing,

besides discovering new industrial processes. It is estimated that over 80% of

the existing chemical processes are catalytic, and of the newly developed

processes, 90% are catalytic based. Technological advances in chemical,

petrochemical, oil processing, food and many other industries, involve the

application of catalysts. Catalysts not only reduce the cost of production but

are directed primarily at improving the quality of products. The answer to

some problems of atmospheric pollution is also sought through catalysis.

Catalysts play an important role in purifying waste gases and reducing

pollution. Obviously, the importance of catalysis, both fundamental and

applied, in the economic and industrial growth of a country cannot be

overestimated [2].

From a commercial point of view, a catalyst is supposed to lower the

raw material consumption or the energy requirement of a chemical reaction.

The former can be achieved, by increasing the yield or the selectivity towards

a particular product, whereas for the latter, a catalyst must bring down the

activation energy and thereby reaction temperature. Thus, a major target of

the catalyst is to obtain chemical products in high purity and high yields with

excellent selectivity at a considerably lower temperature. Further, the catalyst

also plays a significant role in pharmaceutical and fine chemicals by reducing

the number of steps involved in producing a desired chemical. It thereby


2

reduces potential environmental hazards by reducing the large amount of

waste in each individual step [3]. Table 1.1 History of Industrial catalytic processes [1]

Catalytic reaction Catalyst Discoverer or company/ year

Sulphuric acid (lead- chamber process) NOx Désormes, Clement, 1806 Chlorine production by HCl oxidation CuSO4 Deacon, 1867

Sulfuric acid (contact process) Pt, V2O5 Winkler, 1875; Knietsch, 1888 (BASF)

Nitric acid by NH3 oxidation Pt/Rh nets Ostwald, 1906 Fat hardening Ni Normann, 1907

Ammonia synthesis from N2, H2 Fe

Mittasch, Haber, Bosch, 1908;Production,1913 (BASF)

Hydrogenation of coal to hydrocarbons Fe, Mo, Sn Bergius, 1913; Pier, 1927 Oxidation of benzene, naphthalene

to MSA or PSA V2O5

Weiss, Downs, 1920

Methanol synthesis from CO/H2 ZnO/Cr2O3 Mittasch, 1923

Hydrocarbons from CO/H2 (motor fuels) Fe, Co, Ni Fischer, Tropsch, 1925 Oxidation of ethylene to ethylene oxide Ag Lefort, 1930

Alkylation of olefins with isobutane to gasoline

AlCl3

Ipatieff, Pines, 1932

Cracking of hydrocarbons Al2O3/SiO2 Houdry, 1937 Hydroformylation of ethylene to

Propanal Co Roelen, 1938 (Ruhrchemie)

Cracking in a fluidized bed aluminosilicates Lewis, Gilliland, 1939

(Standard Oil) Ethylene polymerization,

low-pressure Ti compounds

Ziegler, Natta, 1954

Oxidation of ethylene to acetaldehyde Pd/Cu chlorides Hafner, Smidt (Wacker)

Ammoxidation of propene to Acrylonitrile

Bi/Mo

Idol, 1959 (SOHIO process)

Olefin metathesis Re, W, Mo Banks, Bailey, 1964 Hydrogenation, isomerization,

Hydroformylation Rh-, Ru

complexes

Wilkinson, 1964

Asymmetric hydrogenation Rh/chiral phosphine

Knowles, 1974; l-Dopa (Monsanto)

Three-way catalyst Pt, Rh/monolith General Motors, Ford, 1974 Methanol conversion to

Hydrocarbons Zeolites

Mobil Chemical Co., 1975

α- olefines from ethylene Ni/chelate

Phosphine Shell (SHOP process) 1977

Sharpless oxidation, epoxidation Ti/ROOH/tartrate May & Baker, Upjohn, ARCO,

1981 Selective oxidations with H2O2 titanium zeolite

(TS-1) Enichem, 1983

Hydroformylation Rh/phosphine/ aqueous

Rhône-Poulenc/Ruhrchemie, 1984

Polymerization of olefines zirconocene/MAO Sinn, Kaminsky, 1985 Selective catalytic reduction

SCR (power plants) V, W, Ti oxides/

Monolith ~1986

Acetic acid Ir / I– / Ru “Cativa”-process, BP Chemicals, 1996


3

Studying catalysis is an academically valuable exerise and clearly

demonstrates the essential unity of science and technology. An understanding

of the phenomenon of catalysis, requires some familiarity with all three

classical branches of chemistry as well as the fundamental concepts of

chemical engineering. The catalysts used are mostly inorganic or

organometallic compounds while the reactions carried out using the

developed catalyst are organic transformations. Hence a catalyst chemist

must be familiar with inorganic chemistry, coordination chemistry, and organic

chemistry. The overall performance of a catalyst depends on various factors

such as turn over number, turn over frequency, conversion, selectivity and

rate of reaction linked to the kinetics of the reactions. Therefore a sound

knowledge of physical chemistry is required to understand the kinetics of

reaction and hence reaction mechanism, reactor design and its effect on the

performance of a catalyst. In most industrial applications, it is a requirement to

predict the overall rate of a chemical process with respect to entire reactor,

which depends on several factors such as interface mass transfer, mixing of

the reactants, temperature profile and intrinsic kinetics of the reaction.

Therefore, the large scale application of a developed process requires the

skills and knowledge of a chemical engineer [4].

1.2 TOWARDS A WORKABLE DEFINITION Several substances have the ability to exercise a force on other

substances and are able to bring about a transformation in the reactants

giving rise to new products without themselves undergoing a chemical

change. This force was termed as catalytic force and the transformation

brought about by this force was termed “Catalysis” which was introduced by

Berzelius as early as 1836.

Ostwald in 1895 applied the principles of thermodynamics to show that

a catalyst just modifies the rate at which the chemical transformation takes

place (provided it is thermodynamically feasible). A catalyst does not alter the

equilibrium concentration of the various species present in a chemical

transformation. Catalysis therefore emerges as a kinetic phenomenon, and

one can define a catalyst as a substance that alters the rate of a reaction


4

without itself being consumed in it and also without modifying its equilibrium

constant.

While it was formerly assumed that the catalyst remained unchanged

during the course of the reaction, it is now known that the catalyst is involved

in chemical bonding with the reactants during the catalytic process, termed as

intermediate which in most cases are highly reactive and difficult to detect. In

theory, an ideal catalyst would not be consumed, but this is not the case in

practice. Owing to competing reactions, the catalyst undergoes chemical

changes, and its activity becomes lower (catalyst deactivation). Thus catalysts

must be regenerated or eventually replaced.

The basic concept is that a catalyzed reaction involves the transitory

adsorption of one or more of the reactants on the surface of the catalyst,

rearrangement of bonding and desorption of the products. The catalyst does

not remain as it is during the reaction. It participates in the reaction at some

stage, but this occurs in a cycle and the catalyst is regenerated at the end of

every cycle. Often the catalyst undergoes a structural change, or change in

the stoichiometry in course of the reaction. But the total quantity of the

catalyst remains more or less unchanged.

Catalysis is a kinetic phenomenon which occurs due to a catalyst in

action. Catalyst is a substance that aids in the attainment of chemical

equilibrium by reducing the potential energy barriers in the reaction path. The

catalyst can neither force a reaction to occur, nor can it alter the equilibrium

concentration of various species present in the reaction mixture. It can be said

to alter the rate of a reaction, without itself being consumed in the process.

Catalyzed reactions usually involve a reaction intermediate formed by the

reaction of a catalyst with one or more of the reactants. This transitory

intermediate then leads to product formation. Thus, it is clear that it

participates in the reaction at some stage and is regenerated at the end of the

reaction, may be in a different physical form but the mass essentially remains

the same. In some cases, an additional substance is added to the active

catalyst, termed as “Promoter” or a “Co-catalyst”. It not only enhances the

activity of the catalyst but in some cases also helps in combating catalyst

sintering and poisoning [4].


5

Apart from accelerating reactions, catalysts have another important

property; they can influence the selectivity of chemical reactions. This means

that completely different products can be obtained from a given starting

material by using different catalyst systems. Industrially, this targeted reaction

control is often even more important than the catalytic activity. A good catalyst

is one which not only produces, selective products but also does not undergo

deactivation quickly. There are various reasons for deactivation, for example,

the catalyst may restructure as a consequence of selective adsorption of

impurities from the reactant stream, carbon may be deposited and thus

suitable additives must be incorporated to resist these changes on the

catalyst surface.

1.3 CATALYST DEVELOPMENT AND COMMERCIALIZATION For the successful development of a practical catalyst, the following

infrastructural facilities are essential:

Basic Research provides a thorough understanding of the catalyst science

through morden scientific tools and techniques. Basic Research generates

data leading to the understanding of the catalyst phenomena, kinetics and

mechanism of catalytic reactions and other fundamental aspects. The

literature available in the relevant fields provide the starting point, as also the

guidelines for planning the working program. The data generated in the

laboratory are used in identifying the catalyst for a particular reaction

mechanism, studying the phenomena associated with its deactivation, which

help to predict the performance, life and selectivity of the catalyst in actual

use.

Applied Research leads to the development of the “real” catalyst and

optimization of the production through pilot scale trial production and

evaluation facilities. Applied research in catalysis is based on two distinct

types of activities: the first ,related to developmental research aimed at the

formulation of practical catalysts, and the other related to operational research

involving catalyst application techniques in the industry.

Developmental Research aims at the formulation of a practical catalyst on a

bench scale level, and its commercial rationalisation through pilot scale

investigation and standardisation. The developmental research is carried out


6

with close interaction and cooperation of various disciplines of science,

technology and engineering. The first step in this direction is catalyst

preparation after ascertaining the probable combination of active components,

carriers, promoters etc. Catalyst preparation involves ways and means of

compounding the formulation and its activation. There may be several

possible routes to arrive at the desired chemical composition of the catalyst,

but its physical and physico-chemical properties and hence the catalytic

activities may differ widely. Judicious selection of the method of formulation of

a catalyst through bench scale experimentation constitutes the major activity

in the first step of developmental research.

Efficient functioning of mordern chemical plants depends, to a great

extent, on performance, life and stability of the catalyst used. High activity,

selectivity, good mechanical strength, good thermal stability and resistance to

poisons are the prime requisites of an industrial catalyst. In addition, it should

withstand plant instabilities, and abnormal operating conditions. It is also

essential to avoid a sudden failure of the catalyst in a plant which can cause a

heavy loss of production. So, a commercial catalyst formulation should have

the following essential features:

Optimum activity: to achieve a close approach to equilibrium with minimum

catalyst volume and under economic operating conditions.

Sufficient mechanical strength and Good thermal stability : to withstand

handling and charging operations as well as the cycle of stress and strain

involved in plant operation including shut-downs and start-ups.

Stable activity: to sustain satisfactory performance for a reasonable period of

time.

Sufficient resistance: to normal level of poisons present in the feed.

The above qualities of a catalyst depend on certain essential factors

like surface chemical composition, concentration and distribution of active

sites, and structure and textural stability of the catalyst support or matrix. A

catalyst cannot work in isolation in an industrial reactor as its efficiency

depends,to a great extent, on the environment in which it is operating. This, in

turn, is greatly influenced by reaction conditions, reactor design and mode of


7

operation. Optimization of all these factors is essential for the efficient

performance of a catalyst [5]. Commercial Production of a catalyst recipe is taken up on the basis of the

procedures standardized during pilot trial production. During commercial

production of a catalyst, the active involvement of the following disciplines is

essential.

• Process design and engineering: to establish efficient facilities for catalyst

production and catalyst improvements.

• Plant operation and maintenance: to ensure the regular production of

catalyst.

• Quality control of catalyst at different stages of production.

• Commercial strategies to establish linkages with customers.

1.4 CATALYSIS CONCEPTS AND TERMINOLOGIES The suitability of a catalyst for an industrial process depends mainly on

the following properties such as Activity, Selectivity, Stability (deactivation

behavior) and Environment compability [1].

Activity It can be defined as the rate at which the catalyst causes the reaction

to proceed to equilibrium. Active sites are the specific sites of importance,

present on the catalyst surface which induces a catalytic action. The rate of a

catalytic reaction is site dependant and hence can be increased by increasing

the surface area of a catalyst. It is generally believed that higher the surface

area of a catalyst, higher will be the activity. It is notable, that only a fraction of

the whole catalyst surface (active site) is active during the reaction. However,

the nature of activity of a catalyst differs under different reaction conditions.

Hence it is important to distinguish the active behavior of a catalyst under

different conditions. Activity is a measure of how fast one or more reactions

proceed in the presence of the catalyst. Activity can be defined in terms of

kinetics or from a more practically oriented viewpoint. In a formal kinetic

treatment, it is appropriate to measure reaction rates in the temperature and

concentration ranges that will be present in the reactor. There are three ways


8

of expressing catalyst activity: Reaction rate (r), Rate constant (k) and

Activation energy (Ea).

Reaction rate (r)

The reaction rate (r is calculated as the rate of change of the amount

of reactant molecules nA of reactant A with time relative to the reaction volume

or the mass of catalyst:

h h (1.1)

Rate constant (k)

Kinetic activities are derived from the fundamental rate laws. For a

simple irreversible reaction A P:

(1.2)

Where, k = rate constant, f cA = concentration term that can exhibit a first or

higher order dependence on adsorption equilibria.

Activation energy (Ea)

The effect of a catalyst is always to reduce the activation energy

(enthalpy) of a reaction. If the temperature dependence of rate constant (k) is

given by the Arrhenius equation (1.3), then the implication is that is always

reduced and A remains more or less unchanged.

(1.3)

where k = rate constant, Ea = activation energy, k0 = pre-exponential factor

and R = gas constant.

Empirical rate equations are obtained by measuring reaction rates at

various concentrations and temperatures. If, however, different catalysts are

to be compared for a given reaction, the use of constant concentration and

temperature conditions is often difficult because each catalyst requires its own

optimal conditions. In this case it is appropriate to use the initial reaction rates

ro obtained by extrapolation to the start of the reaction. For comparative measurements following activity measures are used:

A given constant conversion is expressed by space velocity.

(1.4)

Where V0 = volume flow rate and mcat = relative to the catalyst mass. If we

replace the catalyst mass in Equation (1.4) with the catalyst volume, then we


9

see that the space velocity is proportional to the reciprocal of the residence

time. For conversion under constant reaction conditions, since catalysts are

often investigated in continuously operated test reactors, in which the

conversions attained at constant space velocity are compared, the conversion

XA is the ratio of the amount of reactant A n1A0 that has reacted to the

amount that was introduced into the reactor. For a batch reactor:

, / , / % (1.5)

Often the performance of a reactor is given relative to the catalyst

mass or volume, so that reactors of different size or construction can be

compared with one another. This quantity is known as the space time yield

(STY):

(1.6)

Temperature required for a given conversion is another method of

comparing catalysts. The best catalyst is the one that gives the desired

conversion at a lower temperature. This method cannot however, be

recommended since the kinetics are often different at higher temperature,

making misinterpretations likely. This method is better suited for carrying out

deactivation measurements on catalysts in pilot plants.

Catalysis is a kinetic phenomenon. The speed of a catalyzed reaction

is often designated by the parameter called “turn over number” which denotes

the number of reactant molecules that are converted on an active site or on a

unit catalyst surface area per second at a given temperature, pressure and

concentration of reactants and products. Since the catalytic reaction occurs at

specific sites on solid surfaces, the rate of catalytic reaction can be increased

by using catalysts with very high surface area. This could be achieved by

dispersing the active species and therefore a parameter termed ‘catalyst dispersion’ defined as the number of surface atoms per total number of

atoms is important which can assume any value from 1 when all of the

surface area active sites to approximately 0.01. The catalyst particle sizes

may vary between 10-500Å. This dispersed system must maintain structural

and chemical stability for thousands of hours under the conditions of high

temperature (400-900K) and high pressure (1-102 atm). The design of new


10

and stable, catalysts which resist chemical attrition and sintering of the small

particles is a constant concern of the catalyst scientist.

The common terminologies used to describe the efficiency of a catalyst

in terms of rate of conversion are described as follows:

• Turn Over Rate (TOR): The speed of a catalyzed reaction is often

described in terms of TOR, defined as the conversion of the number of

reactant molecules to products, per unit surface area of the catalyst at a

given temperature, pressure and concentration.

• Specific Rate: It indicates the number of reactant molecules reacting or

product molecules produced per unit catalyst area per second at a given

temperature, pressure and concentration. Its value can be used to judge a

suitable catalyst by comparing the activity of different catalysts for the same

reaction under similar conditions.

• Turn Over Number (TON): The effectiveness of a catalyst can be

expressed in terms of its “turn over number” – which is the number of

molecules of the substrate, reacting per minute due to the intervention of

one molecule of the catalyst. This depends on temperature, concentration

of substrate and the number of active sites on the catalyst. TON can also

be defined as the number of moles of substrate converted to product by a

mole of catalyst (metal or active compound in case of supported catalysts).

TON can also be defined as, mass/volume of substrate converted to the

product per unit mass/volume of the catalyst.

• Reaction Probability (RP): It is defined as the number of product

molecules formed per number of reactant molecules, incident on the

catalytic surface. It is readily obtained by dividing the specific rate of

product formation by the rate of incident reactants (on catalytic surface) in a

flow reactor. Rp reveals the overall efficiency of the catalyst and it is often

quoted in place of turnover rate.

Selectivity It is the efficiency with which the catalyst causes the reaction to

proceed in a direction to give the desired product. A chemical reaction leads

to the formation of several different thermodynamically feasible products. A

selective catalyst will facilitate the formation of one product molecule, while


11

inhibiting the formation of other molecules, even though formation of other

products is thermodynamically feasible.

The selectivity Sp of a reaction is the fraction of the starting material that is

converted to the desired product P. It is expressed by the ratio of the amount

of desired product to the reacted quantity of a reaction partner A and therefore

gives information about the course of the reaction. In addition to the desired

reaction, parallel and sequential reactions can also occur (Fig. 1.2).

Fig. 1.2. Parallel and sequential reactions

Since this quantity compares starting materials and products, the

stoichiometric coefficients i of the reactants must be taken into account,

which gives rise to the following equation:

υ⁄

, | |⁄|υ |

, ⁄ % (1.7)

In comparative selectivity studies, the reaction conditions of

temperature and conversion or space velocity must, of course, be kept

constant. If the reaction is independent of the stoichiometry, then the

selectivity SP = 1. The selectivity is of great importance in industrial catalysis.

Stability The chemical, thermal, and mechanical stability of a catalyst

determines its lifetime in industrial reactors. Catalyst stability is influenced by

numerous factors, including decomposition, coking, and poisoning. Catalyst

deactivation can be followed by measuring activity or selectivity as a function

of time. Catalysts that lose activity during a process can often be regenerated

before they ultimately have to be replaced. The total catalyst lifetime is of

crucial importance for the economics of a process. Today the efficient use of

raw materials and energy is of major importance, and it is preferable to


12

optimize existing processes than to develop new ones. For various reasons,

the target quantities should be given the following order of priority:

Selectivity >Stability > Activity

The following are the terminologies recognized for catalyst deactivation [4]. Poisoning – It is a chemical effect and occurs on the catalyst surface due to

the chemisorption of impurities, reactants, products or byproducts. Catalyst

activity is affected due to blocking of the active centers. This blocking occurs

either due to the permanent chemisorption of species at the active centre or

due to the blockage of the pathway of adsorbed reactive species towards the

active centers. Fouling – It occurs when a carbonaceous material is deposited on the

catalyst. Carbonaceous materials, either coke or carbon is the major cause of

deactivation in most cases. Coke or carbon affects catalyst activity by

adsorbing strongly on the active centers or by plugging the micro and

mesopores of the catalyst. Thermal degradation – It is common in case of supported metal catalyst

systems or with oxides of high surface area. It comes into effect either due to

loss of metal surface area due to crystallite growth (Metal Sintering) or due to

loss of support surface area due to pore collapse (Support Sintering). Attrition – It is due to the inherent low mechanical strength of the catalyst.

The catalyst when subjected to shear and stress, collapses, thereby losing its

activity.

Environmental compatibility Catalytic processes should produce zero or minimum emission with

potential environmental hazards Some popular terminologies used in the

current scenario, to define process environmental compatibility [3] include:

E-factor: Indicates the amount of waste produced per kilogram of the product.

E-factor for various segments of chemical industry is given as. Industry Product tonnage kg byproduct/kg product (E-factor)

Petroleum 106–108 <0.1

Bulk chemicals 104–106 <1–5

Fine chemicals 102–104 5–>50

Pharmaceuticals 10–103 25–>100


13

Environmental Quotient (EQ): Indicates the environmental impact of the

waste produced, during a particular process.

_ , (1.8)

(where, Q = 1 for NaCl and 100 -1000 for heavy metal salts)

Atom efficiency: Refers to the effectiveness with which a desired product is

obtained in a particular process.

∑ ⁄ (1.9)

Zero Emission Technology is the emission of waste products that do not pollute the environment or disrupt the climate. It is focused towards achieving

high selectivity towards desired product and recycle/reuse of reactants. A lot

of attention is focused on development of zero emission technologies for

major industrial processes. To meet these challenges, the industry requires

innovative catalytic technologies that offer high space-time yield, improved

selectivity, higher atom efficiency, as well as low solvent requirement.

1.5 CATALYST CHARACTERIZATION Catalyst characterization is an important aspect in catalysis as it gives

an idea about the physico-chemical properties associated with a material.

Generally used characterization techniques are:

Elemental analysis Elemental analysis gives us an idea about the composition of the

catalyst. It is important to know the composition of a catalyst before use,

during use and after being used for a number of cycles. Conventional

methods involve gravimetric/volumetric analysis. Instrumental methods used

for elemental analysis are Flame photometry, Atomic absorption spectroscopy

(AAS) and Inductively coupled plasma-Atomic emission spectroscopy (ICP-

AES) which are both popular as well as accurate.

Chemical resistivity The chemical resistivity of the catalyst in various media (e.g. acids,

bases and organic solvents etc) or in the media/environment where catalyst

would operate, gives us an idea about the stability/resistivity of the material in

these environments.


14

Thermal analysis (TGA) It gives an idea about the thermal stability of the catalyst and the

possible phase changes that occur during the thermal treatment of the

catalyst. An understanding of the thermal behavior is of basic importance for

utilizing the catalyst in various temperature ranges where it is thermally stable.

FTIR spectroscopy It is routinely used to derive information regarding the various chemical

bonds, functional groups and the interactions among them. In case of solid

acid catalyst, the catalyst material is adsorbed with ammonia or pyridine. The

IR spectrum provides a direct evidence for the existence of Bronsted and

Lewis acid sites on the surface of catalysts, a technique very useful for solid

acid catalysts.

X-ray Diffraction It indicates the amorphous or crystalline nature of the material. In case

of crystalline materials, distinct peaks at characteristic 2θ values are obtained,

whereas absence of peaks indicates amorphous nature of material. Besides,

if any impurity is present, can be detected by the characteristic 2θ value of the

peak in the X-ray diffractogram.

Microscopy Scanning electron microscopy (SEM) and Transmission electron

microscopy (TEM) are used to study the morphology of the material. Besides

it also gives an idea about the changes in shape, size and surface that occur

in a used catalyst.

Energy-dispersive X-ray spectroscopy (EDX) This analytical technique is used for both identification of an element as

well as to have a rough estimate of the composition of the materials. EDX is

used in coordination with and as supportive analysis with ICP-AES which is

more accurate compared to EDX.

Surface area (BET method) BET surface area can be obtained by N2 adsorption under liquid

nitrogen atmosphere. The activity of any catalyst is linked to its surface area

and hence the number of active sites present. Further, pore size can also be


15

determined from the adsorption desorption curve obtained, during the

measurement process.

Temperature programmed desorption It involves Temperature programmed desorption of ammonia

(NH3TPD), Temperature programmed reduction (TPR) and Temperature

programmed oxidation (TPO). NH3TPD gives an idea about the nature of acid

sites present in the material through an adsorption desorption programme.

TPR and TPO give an idea about the active metal surface area of the

material.

Electron spin resonance (ESR) spectroscopy ESR provides information about the symmetry of the catalyst sites, the

oxidation state and coordination environment of the metal as well as

interaction of the adsorbed species with the active sites.

X-ray photoelectron spectroscopy (XPS) XPS is a quantitative spectroscopic technique that measures the

elemental composition, empirical formula, chemical state and electronic state

of the elements that exist within a material. XPS is also known as ESCA, an

abbreviation for Electron Spectroscopy for Chemical Analysis. XPS is a

surface chemical analysis technique that can be used to analyze the surface

chemistry of a catalyst in its as synthesized state, or after catalytic run.

Diffuse Reflectance UV-visible spectroscopy This technique measures the scattered light reflected from the surface

of samples in the UV-visible range (200-800 nm). For most of the

isomorphously substituted molecular sieves, transitions in the UV region (200-

400) nm are of prime interest. This spectroscopic technique is used to

determine the coordination state of transition metal ions substituted in the

matrix of the molecular sieves, involving ligand-to- metal charge transfer

transitions at ~ 200- 220 nm.

Mechanical properties Mechanical properties of a catalyst are important, as it gives an idea

about the utility of the catalyst in a reactor. The properties to be monitored are

abrasion and attrition resistance, crushing strength etc.


16

The field of catalyst characterization is so widespread and important that

ASTM has developed few standard test procedures of catalyst

characterization which is practiced universally. A list of such procedures is

summarized in table 1.2. Table 1.2 ASTM procedures for catalyst characterization [6]:

Properties ASTM NO. Surface area of catalyst D3663 Pore volume distribution by mercury intrusion porosimetry D 4284 Pore distribution of catalysts from N2 desorption isotherms D 4641 Hydrogen chemisorption for platinum on alumina catalyst D 3908 Catalyst acidity by ammonia chemisorption D 4824 Particle size determination by laser light scattering D 4464 Attrition and abrasion of catalysts and catalyst carriers D 4058

1.6 PRESENT TRENDS IN CATALYSIS Over the past few years, there has been an increasing concern for

pollution prevention and the approach to solve this problem has been towards

the development of processes and technologies that produce minimum or

zero waste. This new approach is popularly known as Green Chemistry and

involves the synthesis, processing and use of chemicals so as to reduce the

potential risks for human health and the environment. This new approach is

also popular by the names like environmentally benign chemistry, clean

chemistry, Sustainable chemistry, Atom economy and Benign by design

chemistry. Today, green chemistry is a frontier area of research and is

receiving considerable attention.

Green chemistry Green chemistry focuses on the design, manufacture, and use of

chemicals and chemical processes that have little or no pollution potential or

environmental risk, and processes that are economically and technologically

feasible, thus providing the best opportunity for chemists, manufacturers, and

processors to use chemicals safely and to carry out their work under safe

conditions. The basic idea of green chemistry is to increase production

efficiency through atom economy, and at the same time eliminate or at least

minimize wastes and emissions at their source, rather than treat them at the

end of the process, after they have been generated.

Twelve principles [7] outlined below, provide a significant guideline in

dealing with the concept of green chemistry.


17

• It is better to prevent waste than to treat or clean up waste after it is

generated.

• Synthetic methods should be designed to maximize the utility of all

materials used in the process, in the conversion of final product.

• Whenever practicable, synthetic methodologies should be designed to use

and generate substances that possess little or no toxicity to human health

and the environment.

• Chemical products should be designed to preserve efficacy of function

while reducing toxicity.

• The use of auxiliary substances (solvents, separation agents etc.) should

be made unnecessary whenever possible and when used, innocuous.

• Energy requirements should be recognized for their environmental and

economic impacts and should be minimized. Synthetic methods should be

conducted at ambient temperature and pressure.

• A raw material or feedstock should be renewable, rather than depleting

whenever technically and economically practical.

• Unnecessary derivatization (blocking groups, protection/deprotection, and

temporary modification of physical/chemical processes) should be avoided

whenever possible.

• Catalysts (as selective as possible) are superior to stoichiometric reagents.

• Chemical products should be designed, so that at the end of their function

they do not persist in the environment and instead break down into

innocuous degradation products.

• Analytical methodologies need to be further developed, to allow for real

time in-process monitoring and control prior to the formation of hazardous

substances.

• Substances and the form of a substance, used in a chemical process

should be chosen such, so as to minimize the potential for chemical

accidents, including releases, explosions and fires.

The principles of green chemistry can be applied to all areas of

chemistry including synthesis, reaction conditions, separations, analysis,

monitoring and catalysis.


18

Green chemistry and catalysis Catalysis provides an important opportunity to achieve the goals of

green chemistry. Both, greener catalytic processes and catalytic processes for

greener products, must play key roles in green chemistry. Asymmetric

catalysis, biocatalysis, heterogeneous catalysis, environmental catalysis,

shape selective catalysis, phase transfer catalysis and solid acid catalysis are

just a few examples that have a direct and significant impact on

accomplishing the goals of green chemistry. Amongst the various catalytic

systems used, solid acid catalysts are making a huge impact.

1.7 SOLID ACID CATALYSTS - AN ALTERNATE APPROACH TO LIQUID ACID CATALYSTS

Liquid acids such as H2SO4, HF, H3PO4 have been extensively used as

catalysts in a variety of organic transformations for long. Though they are very

effective, liquid acid catalysts are cited as potential environmentally

hazardous chemicals and are becoming a major area of concern mainly due

to operational difficulties such as toxicity, corrosiveness, effluent disposal,

product separation, storage and handling. Owing to increasing environmental

awareness and a quest for zero emission technologies, much attention is

focused on developing alternatives to these existing acids. Solid acids are

safe alternatives for conventional liquid acid catalysts, used in synthetic

organic chemistry in petroleum refineries, fine chemical synthesis,

pharmaceuticals etc [8].

In general terms, a solid acid can be described as a solid on which the

color of a basic indicator changes, or as a solid on which a base is chemically

adsorbed. More strictly, following both the Bronsted and Lewis definitions, a

solid acid shows a tendency to donate a proton or to accept an electron pair.

Though they differ in structure from liquid acids, solid acid catalysts work on

the same principles.

The ability to lend protons makes solid acids valuable as catalysts.

Protons are often released from ionisable hydroxyl groups in which the bond

between hydrogen and oxygen is severed to give H+ and O-. Protons may

also be released in the form of hydrated ions such as H3O+. When a reactant

receives and incorporates a proton from an acid, it forms a reactive


19

intermediate. This positively charged intermediate may change shape and

configuration. It may then undergo either isomerization or rearrangement by

shedding the proton or may undergo some organic transformation, leading to

the formation of a new molecule. In any case the proton is returned to the

catalyst [9].

Solid acid catalysts are appealing, since the nature of the acid sites are

known and it is possible to modify the acidic properties of these materials by

adopting various synthetic and post synthetic treatments. The main

characteristic of solid acids, as compared to liquid acids, is that solid acids

encompasses different population of sites, differing in their nature and

strength (weak acid and strong acid sites) and hence depending on the

reaction conditions, the same catalyst can be active for one reaction and

inactive for another [10]. The effectiveness of a particular solid acid catalyst

for a given reaction, depends on various factors including surface area,

porosity, acidity, crystallinity and nature of acid sites. Several review articles

have been published, dealing with the use of solid acid catalysts for the

preparation of speciality and fine chemicals [11-13].

Advantages of solid acid catalysts • Though environmental benefits have been the major reasons for the

introduction of solid acids in many chemical processes, these catalysts

have proved to be more economical and often produce better quality

products.

• They are very effective and some of them are known to exceed the acidity

of concentrated H2SO4. Besides, they hold their acidity internally and thus

easy to handle and also reaction vessels or reactors are not corroded.

• They can allay concerns about safety and environmentally hazardous

emissions as they are nontoxic and nonvolatile.

• They possess high catalytic activity and selectivity.

• Being heterogeneous in nature, separation from reaction mixture is easy

and the catalyst can be regenerated and reused.

• Problems associated with the disposal of used solid acids is less compared

to the disposal of liquid acids that require much money and efforts, for post

use treatment and effluent neutralization.


20

Some important solid acid catalysts A drive for clean technology associated with the problems encountered

while using the liquid acids has led to the development of a variety of solid

acids. Several inorganic materials tested as solid acid catalysts include silica-

alumina gels, zeolites, oxides and hydrous oxides, heteropolyacids, clays,

solid superacids and TMA salts.

• Silica Alumina Gels: Silica-alumina gels have no well defined structures

and contain many pores, which range in diameter from few to few hundred

angstroms. Further, their amorphous nature make them less than ideal

catalysts. These compounds tend to lose their activity due to clogging of

pores. In addition, because of their irregular structure, protons are unevenly

distributed and are therefore difficult to control catalysis precisely. Silica

gels were replaced by zeolites in mid 1960's.

• Zeolites: Zeolites are crystalline solids with three dimensional frame work,

containing micropores of uniform size throughout the structure. They are

formed by the corner sharing of (SiO4)4- and (AlO4)5- tetrahedra. Due to the

excess negative charge on the tetrahedron, a counter ion is required to

neutralize the charge. When this counter ion used is a proton, the material

behaves as a solid acid (Bronsted acidity). If the zeolite is heated, water

may then be eliminated from the Bronsted sites leaving aluminum atoms

coordinated to only three oxygen atoms. These will act as Lewis acids [14].

Zeolites have found widespread application as solid acid catalysts in

petroleum industries [15]. The acid strength of the protons in some zeolites

can be very high, quite often being 100 % stronger than H2SO4 and hence

the H-zeolite makes excellent solid acid catalysts [16]. However, zeolites

are unstable in acid media, have low aberration resistance and undergo

rapid deactivation due to plugging of the pores.

• Oxides and Hydrous Oxides: In oxide based solid acids, protons balance

net negative charges produced by the replacement of a high valent cation

with one of lower valence or by the attachment of an anion to the surface of

a neutral oxide support. These protons are responsible for acidity. They

include oxide and hydrous oxides of Zr4+, Ti4+, Fe3+, Al3+, Nb5+, Cr3+, Th4+,

etc. Of the hydrous oxides, zirconia has received much attention as a solid


21

acid catalyst [17]. A mechanism for the generation of acid sites by mixing

two oxides has been proposed by Tanabe [18]. They suggest that the

acidity generation is caused by excess of a negative or positive charge in

model structure of a binary oxide related to the coordination number of a

positive and negative element. Zr(OH)4 and Ti(OH)4 is synthesized by

traditional sol gel method. Obtaining high acid strength comparable to

sulfuric acid, halides, or oxy halides in these catalysts remains a challenge

due to the smaller electronegativity difference in metal-oxygen bond

compared to the metal halide bonds [19].

• Solid superacids: Solid acids with low surface acidity could be converted

to solid acids with high surface acidity. Such catalysts are known as solid

superacids. Solid super acids of oxides and hydrous oxides are prepared

by introducing sulphate ion (sulphation). Sulphation is carried out by

washing hydrous ZrO2 at room temperature thoroughly with

H2SO4/(NH4)2SO4 or passing H2SO4 (SO42-) through a packed column of

ZrO2 or immersing ZrO2 into H2SO4, stirring and filtering. Though different

methods are used, enough H2SO4 should be added to form a monolayer.

These processes lead to adsorption of sulphate ions on ZrO2. Finally, the

resultant material is calcined at > 500 °C to remove excess H2SO4. Some

get firmly grafted on surface- introducing acidity. The solids consist mainly

of the metal dioxides with sulphate ions coordinated to the metal ions on

the surface. They have the general formula S042-/MxOy (M = Zr, Ti)

exhibiting acidity higher than 100 % H2SO4. They can be easily prepared,

are stable at elevated temperatures and can be easily regenerated. They

have been extensively used for a range of important organic

transformations such as isomerization, alkylation, acylation, esterification,

etherification, oligomerization, oxidation [20-26] etc. The main limitation of

these catalysts is, they get easily deactivated by loosing the sulfate ions.

• Heteropolyacids: Heteropoly acids offer appealing characteristics as solid

acid catalysts in many acid-catalyzed reactions [27]. Various types of

heteropoly compounds [28] are known, but the most popular is based on

Keggin structure corresponding to the formula [XM12O40]n-, M being a

transition element (usually Mo or W) and X a hetero atom (P, Si, As, Ge


22

etc.). It displays a tetrahedral symmetry based on a central XO4

tetrahedron, surrounded by twelve MO6 octahedra arranged in four groups

of M3O13 of three edge-bridged octahedra. The central atom is the

important factor in determining the acid strength and the acidity is related to

the total charge on the anion.

• Clays: Clays are some of the most abundant, porous and benign materials

on the earth. Clays are complex layered oxides essentially comprising of

parallel tetrahedral silicate and octahedral aluminate sheets [29]. For

charge compensation, various cations (especially Na+ and Ca2+), may

occupy the interlayer gallery. When these cations are replaced by hydrated

protons in the form of H3O+, acidity is introduced. They have been used in a

variety of reactions including Friedel-Crafts alkylation, acylation and

production of methyl tertiary butyl ether (MTBE) [30].

• Tetravalent Metal Acid salts: Abbreviated as TMA salts possess the

general formula M(IV)(HXO4)2.nH2O [31]. TMA salts possessing structural

hydroxyl groups (the H+ of the –OH being the exchangeable sites) indicates

good potential for application as solid acid catalysts due to presence of

surface protons/acidity. TMA salts can be obtained in both amorphous and

crystalline forms. It is observed that both surface area and surface acidity

decreases with increasing crystallinity of the material [32, 33]. Hence, their

acidity can be tailored for a specific application by controlling the

crystallinity of the material. The preparation procedure thus affects the

structural hydroxyl groups, which is reflected in the performance of TMA

salts as solid acid catalysts. Besides they possess excellent thermal

stability and chemical resistivity. TMA salts have been used as catalysts in

various organic transformations by various groups - Dr. A. Clearfield (USA),

Dr. G. Alberti (Italy), Dr. W. Holderich (Germany), Dr. D. Whittaker (UK)

and Dr. U. V. Chudasama (The M. S. University of Baroda, India).

TMA salts have been investigated for cyclohexanol dehydration to

cyclohexene [32,34,35], oxidative dehydrogenation of cyclohexene to

benzene[36,37], conversion of ethylbenzene to styrene[38,] amination

reactions [39-41] hydrogenation of alkenes [42-44], reverse Prins reaction

[45], dehydration of cyclohexanol and methylcyclohexanols [46], terpene


23

rearrangements [33], ionic and radical rearrangements of α and β-pinene [47]

and Friedel-Crafts alkylation of anisole with alcohols [48]. The catalytic

aspects of TMA salts have been extensively studied from our laboratory that

include esterification [49-57], dehydration of alcohols [58,59], hydration of

nitriles to amides [60], ketalization of ketones [61] and Pechmann

condensation reactions [62].

1.8 PORES AND POROSITY Porous materials have attracted the interest of scientists and industry

due to various applications for instance in molecular separation,

heterogeneous catalysis, adsorption technology as well as new challenges in

the fundamental materials research, owing to their high surface area, tunable

pore size, adjustable framework and surface properties [63]. The behavior

and performance of such materials can be determined by many

characteristics such as surface area, porosity and pore size distribution.

Physical properties such as density, thermal conductivity and strength are

dependent on the pore structure of the solid .Porosity influences the chemical

reactivity of a solid and the physical interaction of solids with gases and

liquids.

In general, the surface area of a porous material is higher than the

surface of an analogous non-porous material. Thereby the internal surface

area is usually much higher than the one contributed by the external surface.

Due to the fact that heterogeneous catalyzed chemical reactions basically

occur on surfaces or at phase boundaries, a higher surface area would,

theoretically, directly yield to an improved reactivity. Apart from the surface

area, other important characteristics of porous solids are the crystallinity or

regularity if present, the distribution of pore sizes and the chemistry of the

walls. In an ideal porous material these attributes should be tailored exactly to

the needs of the application.

The science and technology of porous materials has progressed

steadily and is expanding in many new directions with respect to processing

methods and applications. Many synthetic pathways have been reported for

the synthesis of porous materials, either with a disordered pore system or

ordered with various structures, which can meet the demands of the target


24

application. To be commercially interesting, such a material should be

inexpensive and highly stable for regeneration. Materials based on silicate

show up such kind of flexibility and have hence found wide fields of industrial

applications.

A solid material that contains cavities, channels or interstices can be

regarded as porous.The voids between linked atoms in any material arranged

in an ordered manner are called pores.

Based on the accessibility of an external fluid, pores can be classified

into closed pores and open pores. Closed pores are totally isolated from their

neighbours and surface of the particle. They influence macroscopic properties

such as bulk density, mechanical strength and thermal conductivity, but are

inactive in processes such as fluid flow and adsorption of gases. Pores which

have a continuous channel of communications with external surface of the

body are called open pores. In Fig. 1.3 region ‘a’ represents the closed pores,

and regions such as b, c, d, e and f represent open pores, ‘b’ and ‘f’ are also

described as blind or saccate pores.

Fig.1.3. Classification of pores

The international Union of Pure and Applied Chemistry (IUPAC)

classifies three categories for pore sizes in solids. Pore size distributions

larger than 500 Å are macroporous, materials having pores between 20 Å to

500 Å represent mesoporous materials, and materials with pore size

distribution less than 20 Å represent microporous materials. Mesoporous

materials belong to a new family of material with sizes intermediate to those

usually studied by chemists and material scientist, and therefore mesoporous

materials pose new challenge in their synthesis and characterization.


25

1.9 FROM MICROPORES TO MESOPORES It is possible to say that zeolites are the most widely used catalysts in

industry. They are crystalline microporous materials which have become

extremely successful as catalysts for oil refining, petrochemistry, and organic

synthesis in the production of fine and speciality chemicals, particularly when

dealing with molecules having kinetic diameters below 10 Å. The reason for

their success in catalysis is related to the following specific features of these

materials:[64] (i) They have very high surface area and adsorption capacity.

(ii) The adsorption properties of the zeolites can be controlled, and they can

be varied from hydrophobic to hydrophilic type materials. (iii) Active sites,

such as acid sites for instance, can be generated in the framework and their

strength and concentration can be tailored for a particular application. (iv) The

sizes of their channels and cavities are in the range typical for many

molecules of interest (5-12 Å), and the strong electric fields [65] existing in

these micropores together with an electronic confinement of the guest

molecules [66] are responsible for a preactivation of the reactants. (v) Their

intricate channel structure allows the zeolites to present different types of

shape selectivity, i.e., product, reactant, and transition state, which can be

used to direct a given catalytic reaction towards the desired product avoiding

undesired side reactions. (vi) All of these properties of zeolites, which are of

paramount importance in catalysis and make them attractive choices for the

types of processes listed above, are ultimately dependent on the thermal and

hydrothermal stability of these materials. In the case of zeolites, they can be

activated to produce very stable materials not just resistant to heat and steam

but also to chemical attacks.

Despite these catalytically desirable properties of zeolites they become

inadequate when reactants with sizes above the dimensions of the pores

have to be processed. In this case the rational approach to overcome such a

limitation would be to maintain the porous structure, which is responsible for

the benefits described above, but to increase their diameter to bring them into

the mesoporous region. The strategy used by the scientist to do this was

based on the fact that most of the organic templates used to synthesize

zeolites affect the gel chemistry and act as void fillers in the growing porous


26

solids. Consequently, attempts were made that employed larger organic

templates that would result in larger voids in the synthesized material. This

approach did not give positive results in the case of zeolites, but in contrast

was quite successful when using Al and P or Ga and P as framework

elements [67-77]. A 14-member ring (MR) unidirectional zeolite (UTD-1) could

be synthesized using a Co organometallics complex as the template [78,79].

The template can be removed, and the thermal stability of the framework of

the organometallic-free material is high, resisting calcination temperatures up

to 1000 °C. The presence of framework tetrahedral Al generates Brønsted

acidity which is strong enough to carry out the cracking of paraffins. In a

general way, the different strategies directed towards the synthesis of

ultralarge pore zeolites has been summarized [80].

However, when the zeolite and zeotypes with the largest known

diameters were considered for their possible use as catalysts, it was observed

that cacoxenite,[81] which is a naturally occurring mineral with a 15 Å pore

system, is thermally unstable and thus cannot be used as a catalyst. Though

Cloverite, has potentially large pores, the diffusion of large molecules is

restricted, owing to the unusual shape of the pore openings which are altered

due to protruding hydroxyl groups. Likewise in VPI-5, stacking disorder or

deformation of some of the 18-membered rings during dehydration results in a

decrease in the pore size from 12 Å to about 8 Å. In the case of the new

zeolite UTD-1, the fact that it has to be synthesized with an organometallic

Cobalt complex, which has then to be destroyed, and the Cobalt left has to be

acid leached raises strong questions concerning its practical application,

which remains in doubt unless a more suitable template and activation

procedure can be found.

In conclusion, it can be said that despite the outstanding progress

made in producing large pore molecular sieves, the materials synthesized

were not suitable to be used in catalytic processes.

1.10. ORDERED MESOPOROUS MATERIALS It is true to say that one of the most exciting discoveries in the field of

materials synthesis over the last years is the formation of mesoporous silicate

and aluminosilicate molecular sieves using liquid crystal templates. In 1992,


27

researchers at the Mobil Oil Company reported a novel family of materials

called M41S [82-84]. The breakthrough assured a bright future due to their

properties with a well-defined pore size between 15-100Å. With the discovery

of this new type of material, the pore size constraint of microporous materials,

with pore diameter smaller than 15Å, was overcome. The family of

mesoporous M41S material consists of three types, as summarized below in

the Fig 1.4.

Fig.1.4 The mesoporous M41S family [82]

About MCM-41 • MCM-41 is one of the most studied and promising member of the M41S

family.

• MCM-41 is the abbreviation for Mobil Crystalline Material or Mobil

composition of matter.

• The mesoporous material presents regular arrays of uniform channels,

which has a honeycomb structure as a result of hexagonal packing of uni-

dimensional cylindrical pores.

• By choosing adequate reactants and reaction conditions, it is possible to

tailor the channel dimension in the range of 15-100Å or even larger.

• The BET surface area is typically over 1000 m2/g [83,84]. The pore is

usually between 0.7 and 1.2 cm3/g with long-range order.

• With increasing pore size, the regularity of the structure is affected.

• The adsorption capacity is exceptionally high (more than 50 wt% for

cyclohexane at 40 Torr, 67 wt% for benzene at 50 Torr).

• MCM-41 possesses excellent thermal and hydrothermal stability (up to

800°C).

• It is relatively stable in acidic medium (pH 2) [85]. However, it is destroyed

in a basic medium (pH12).


28

• MCM-41 is composed of silica framework, which is almost catalytically

inactive.

• The isomorphous substitution of silicon by a variety of metals (Al, Ga, Fe)

gives rise to acidic properties [86].

• The possibility of using the pore channels of MCM-41 as a support for

existing catalysts has also been considered [80, 87].

There is no doubt that the synthesis of these materials opens definitive

new possibilities for preparing catalysts with uniform pores in the mesoporous

region. Obviously when a new type of material such as these is discovered,

an explosion of scientific and commercial development swiftly follows, and

new investigations on every conceivable aspect of their nature, the synthesis

procedures and synthesis mechanisms, heteroatom insertion,

characterization, adsorption, and catalytic properties, rapidly occurs.

1.11. SUMMARISING POROUS MATERIALS Well known microporous materials are zeolites [88] and

aluminophosphate molecular sieves [89] which are inorganic composites

having a crystalline three-dimensional framework woven with tetrahedral

atoms (T-atoms) like aluminium, silicon, phosphorous etc. bridged by oxygen

atoms. These materials possess uniform channels or cavities circumscribed

by rings of a definite number of T-atoms. The exploitation of the architectural

features of zeolites resulting in different acid sites and acid strengths,

exchangeable ions, shape and size selective channels and pores etc. has

been well established by now. Modification of the framework and extra-

framework composition makes these materials useful for catalyzing organic

reactions.

Even though, zeolites, having pore dimensions of 5 to 7 Å, have served

the purpose for most of the industrial reactions by providing high surface area,

the pore dimensions are not sufficient enough to accommodate broad

spectrum of larger molecules. The performance of the zeolitic systems is

limited by diffusional constraint associated with smaller pores. To a certain

extent, it is possible to overcome this problem with aluminophosphates, with

pore dimensions up to 13 Å. However, these materials suffer from limited

thermal stability as well as negligible catalytic activity due to framework


29

neutrality. Moreover, there is a need for present day catalytic studies dealing

with processing of hydrocarbons with high molecular weights. These factors

led to the discovery of mesoporous solids and there has been an ever-

growing interest in expanding the pore size of the zeotype materials from the

micropore region to mesopore region.

Materials with super large pores possessing catalytic properties are

attractive candidates as catalysts for non-shape selective conversion of large

molecules, especially for the cracking of heavier petroleum feed stock. The

large void volumes of such materials make them potential adsorbents with

high adsorption capacity. The large pore can also act as host for various

guest species so that the alteration of redox properties can also be achieved.

With the first successful report on the mesoporous materials (M41S) by

Mobil researchers, with well defined pore sizes of 20 – 500 Å, the pore size

constraint (15 Å) of microporous zeolites observed a breakthrough. The high

surface area (> 1000 m2/g) and the precise tuning of the pores are among the

desirable properties of these materials. Mainly, these materials ushered in a

new synthetic approach where, instead of a single molecule as a templating

agent as in the case of zeolites, self-assembly of molecular aggregates or

supra-molecular assemblies are employed as templating agents. The basic

difference in the synthesis of microporous and mesoporous molecular sieves

can be shown pictorially.

Fig. 1.5 Microporous materials using single molecule as template

Fig. 1.6 Mesoporous materials using molecular aggregates or supra-molecular assemblies as

template [90]


30

1.12. AIM AND SCOPE OF THE PRESENT WORK Though it is possible to overcome the existing pore size constraints of

microporous solids as mentioned earlier, the MCM-41 based materials have

negligible catalytic activity due to framework neutrality, however with

advantageous properties like mesoporous nature of the material, good

thermal stability, high surface area and retention of surface area at high

temperatures. Thus, the main aim of the present study was to encash the

advantageous properties of MCM-41 and enhance its practicability in the area

of catalysis using Green Chemistry principles. There are a number of ways by

which catalytic activity can be generated into the MCM-41 neutral framework.

(i)Substitution of an M3+ cation e.g. Al3+ in the Si4+ framework, leading to

negatively charged framework, followed by balancing these charges by H+

ions to create Bronsted acid sites (via NH4+ ion exchange and subsequent

thermal decomposition to give H+ and NH3) to result in a material with inherent

acidity.

(ii)Immobilization/anchoring/impregnation of homogenous acid catalyst e.g.

heteropoly acids (HPAs) onto MCM-41 to result in a material with induced

acidity.

(iii)Isomorphous replacement of Zr4+and Ti4+ in the siliceous MCM-41

framework to induce redox properties.

Chapter II of the thesis includes the synthesis of mesoporous (i)

Siliceous MCM-41, (ii) Al-MCM-41 and (iii) 12TPA-MCM-41, (where 12-TPA =

12-Tungstophosphoric acid a HPA). Materials (i) and (ii) have been

synthesized by sol-gel method, using templates varying several parameters

such as silica source, templating agent/types, reaction conditions such as pH,

time of reaction, aging, temperature etc. and these parameters optimized,

using surface area as an indicative tool. In case of Al-MCM-41 SiO2:Al2O3

ratios have been varied in order to obtain material with maximum surface

acidity and hence in case of Al-MCM-41 surface acidity has been used as an

indicative tool. The salient feature is the synthesis of MCM-41 and Al-MCM-41

at room temperature. 12-TPA supported MCM-41 was prepared by a process

of anchoring and calcination, with varying 12-TPA loading (10-40 wt.%) in


31

order to obtain material with maximum surface acidity and hence, here also

surface acidity has been used as an indicative tool.

All synthesized materials were characterized for Elemental analysis by

ICP-AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),

Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy

(EDX), Surface area (BET method), Pore volume and pore distribution(BJH

method), surface acidity by temperature programmed desorption (TPD) of

ammonia, Diffuse reflectance spectroscopy (UV-DRS), Fourier transform

infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA).

The potential use of Al-MCM-41 and 12-TPA-MCM-41 as solid acid

catalysts was explored studying (i) Esterification and (ii) Friedel-Crafts

alkylation and acylation as model reactions. In case of esterification

monoesters such as ethyl acetate (EA), propyl acetate(PA), butyl acetate (BA)

and benzyl acetate (BzA) and diesters such as diethyl malonate (DEM),

dioctyl phthalate (DOP) and dibutyl phthalate (DBP) have been synthesized.

Friedel-Crafts acylation of anisole and veratrole with acetic anhydride

and alkylation of toluene with benzyl chloride have been performed to obtain

4-methoxy acetophenone (4MA), 3,4-dimethoxy acetophenone (3,4DMA) and

parabenzyltoluene (PBT) under solvent free condition.

In the above reactions, parameters such as catalyst amount, reaction

time and reaction temperature, mole ratio of reagents etc. have been

optimized including catalyst regeneration capacity. The catalytic activity of

both catalysts have been compared and the results have been correlated with

surface properties of the materials.

Chapter III aims at synthesizing oxidation catalysts Zr-MCM-41 and Ti-

MCM-41 by a sol-gel method using templates towards achieving

mesoporosity, with high surface area, good thermal stability and maximum

M4+ incorporation. For incorporation of maximum Zr4+ and Ti4+, SiO2:ZrO2 and

SiO2:TiO2 ratios have been varied. The salient feature is that the material is

synthesized at room temperature.

All synthesized materials were characterized for Elemental analysis by ICP-

AES, X-ray diffraction (XRD), Transmission electron microscopy (TEM),



32



ammonia, Diffuse reflectance spectroscopy (UV DRS), Fourier transform


Further, the catalytic potential of the materials Zr-MCM-41 and Ti-

MCM-41 has been explored by studying Epoxidation as a model reaction

using H2O2 as an oxidant in the conversion of allyl chloride to selectively

produce Epichlorohydrin in a single step at room temperature.

In the above reaction, parameters such as catalyst amount, reaction



both catalysts have been compared.

Chapter IV involves the use of mesoporous MCM-41 as a support in

the synthesis of automobile catalyst. Automobile emissions form an important

source of atmospheric pollution. Automobile catalysts are now widely

recognized for the conversion of mainly CO → CO2, oxides of nitrogen → N2

and unburnt hydrocarbons → CO2 and H2O. At present, noble metals

particularly Rh and Pt catalysts are used for these purposes which are

expensive. There is thus a search for relatively cheaper materials that must

operate efficiently under a wide variety of conditions. Perovskite type oxides

have attracted much attention recently in environment pollution control.

In the present endeavour, LaCoO3 (LC) a Perovskite has been

synthesized on the surface of mesoporous MCM-41 (LCM) by citrate solution

combustion route. Pd has been incorporated in the Perovskite lattice as well

as on the MCM-41 surface and the materials characterized for XRD, surface

area (BET method) and temperature programmed reduction (TPR). In order to

check the thermal stability/durability, the materials have been aged at 1000oC

for 3h and again characterized. Catalytic activity of fresh and aged materials

have been explored for oxidation of CO, hydrocarbons and reduction of NOx

in a tubular down flow test reactor under simulated exhaust conditions and

known air/fuel ratios and the results correlated with surface and redox

properties of the materials.


33

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CCHHAAPPTTEERR 22

Synthesis and Characterization of MCM-41 based Catalysts and their Applications as Solid Acid Catalyst in Esterification and Friedel- Crafts Acylation and Alkylation. _____________________________________________

Chapter 2. Synthesis and Characterization of MCM-41 based materials

38

2.1 INTRODUCTION

Advanced materials are the enablers of new technologies. The present

demands are better materials, new materials and cheaper materials prepared

by ecofriendly routes. An important goal of materials research is the ability to

design and synthesize in high yields, materials whose structures and

properties can be predicted, varied and controlled. This is the challenge for

the synthetic chemist by the demands of material technology. Traditional

ceramic processes use high temperatures. The present demands are making

use of soft chemistry routes (low temperatures) which is popularly known as

“Chemie Douce” by the French. Sol-gel method of synthesis is a soft

chemistry route. Advantages of materials prepared by sol-gel synthesis is high

homogeneity, high purity, low temperature processing, structural control of

materials formed, materials with improved or desired properties and

preparation of porous materials by use of templates.A great deal of interest

has been shown in the application of sol-gel chemistry in various fields of

technology. A majority of the materials prepared using the sol-gel method are

ceramics, refractories and glasses. Attempts to apply the sol-gel technique for

the preparation of catalysts is a relatively new venture, made since the last

decade.

2.2 SOL-GEL PROCESS

Concepts and Terminologies

The sol-gel process is a wet-chemical technique for the fabrication of

materials, employing low temperature, starting either from a chemical solution

or colloidal particles (sol for solution or nanoscale particle) to produce an

integrated network (gel). In general, sol-gel process can be regarded as the

preparation of the sol, gelation of the sol and removal of the solvent. The

overall sol-gel process can be represented by the following sequence of

transformations [1]:

Precursor Sol Gel Product

Precursors are starting materials, in which the essential basic entities for

further network formation are present in the correct stoichiometry. Typical

precursors are metal alkoxides and metal chlorides.


39

Sol is a colloidal suspension of particles in a liquid, the particles typically

ranging from 1-100 nm in diameter.The solid particles in the colloidal phase

are stable due to short-range forces such as Van der Waals attraction and

surface charges.

Gel is a semi–rigid solid, in which solvent is contained in a network/framework

of the material, which is either colloidal (essentially a concentrated sol) or

polymeric.Gel is defined as a substance that contains a continuous solid

skeleton enclosing a continuous liquid / fluid phases of colloidal dimensions.

In sol-gel processing, a sol of a given precursor is prepared, which

involves the dissolution of the required metal ions either as alkoxides or other

metallo-organic salts in a suitable solvent (alcohol) or as inorganic salts in

water, which undergo hydrolysis,followed by condensation and polymerisation

reactions to produce highly condensed and branched network polymers, the

gel. The networking depends on the functionality of the metal. Silicon with

coordination number four, forms highly branched networks [2] (Fig. 2.1a). In

the gelation step,the fluid sol is transformed to a semirigid solid gel. Two types

of gels are usually formed, colloidal and polymeric. Colloidal gels are formed

from metal salt solutions, oxides and hydroxide sols, while polymeric gels are

formed from metal alkoxide based sols. The name “sol-gel” is thus given to

the process, because of the distinctive viscosity increase that occurs at a

particular point in the sequence of steps. A sudden increase in viscosity is the

common feature in sol-gel processing, indicating the onset of gel formation.

Sol-gel process can be distinguished from precipitation by its specific property

to stabilize a finely dispersed (mostly colloidal) phase in solution.

Typically, formation of a metal oxide via sol-gel route involves

connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges,

generating metal-oxo or metal-hydroxo polymers in solution.The

transformation of sol to gel takes place via hydrolysis and condensation

reactions of the precursors. The hydrolysis reaction is represented taking

silicon as an example:

Si (OR) 4 + n H2O (HO) n-Si (OR) 4-n + ROH (2.1)

In case of metal alkoxide precursors, R represents an alkyl group. The metal

is totally hydrolyzed when n = 4. For any other value of n, partial hydrolysis


40

takes place. In the condensation reaction, the two partially hydrolyzed

molecules link together and liberate a small molecule such as H2O or ROH.

The general reaction is represented as:

(OR)4-nSi(OH)n+(HO)nSi (OR)4-n(OR)4-n(OH)n-1 Si-O-Si (HO)n-1 (OR) 4-n+ H2O(2.2)

The condensation takes place in such a way so as to maximize the

number of M-O-M bonds and minimize terminal hydroxyl groups through

internal condensation. Initially monomers add to form rings, creating 3-D

structures. These compact structures are formed by leaving the hydroxyl

groups outside, (Fig.2.1b) that serve as nuclei for further particle growth[3],

that proceeds by Ostwald ripening mechanism, a process by which small

particles precipitate on relatively larger insoluble particles, indicated by arrow

heads in (Fig.2.1c). As particles grow in size the number of particles

decrease. The polymerization behaviour of aqueous silica via sol-gel process

at different pH is presented in (Fig.2.1d).

(a) (b)

(c) (d)

Fig. 2.1(a) Highly branched networks of silicon, (b)Condensation reactions leading to closed

ring 3D structure (c) SEM showing Ostwald ripening mechanism[3].(d)Polymerization

behavior of aqueous silica. A = in presence of salts / acidic medium, B = alkaline medium.

Sequential steps involved in sol-gel synthesis


41

Hydrolysis:It involves reaction of inorganic or organometallic precursor with

water or a solvent, at ambient or slightly elevated temperature. Acid or base

catalysts are added to speed up the reaction.

Polymerization:This step involves condensation of adjacent molecules

wherein water/alcohol are eliminated and metal oxide linkages are formed.

Polymeric networks grow to colloidal dimensions in the liquid (sol) state.

Gelation: It leads to the formation of a three dimensional network throughout

the liquid, by the linking up of polymeric networks.

Ageing:A continuous change in structure and properties of a completely

immersed gel in liquid is called ageing and represents the time between

formation of the gel and removal of solvent.Aggregation of smaller polymeric

units to the main network, progressively continues on ageing the gel. Solvent

molecules however, remain inside the pores of the gel. Extensive ageing

however, causes shrinkage of gel. Factors that affect ageing processes

include temperature, time and pH of the pore liquid.

Drying:Here, solvent is removed at moderate temperatures (<200 °C) leaving

the residue behind. During drying, the gel initially shrinks due to loss of pore

fluid maintaining the liquid-vapour interface at the exterior surface of the gel.

At the final stage of drying, liquid-vapour menisci recede into the gel

interior[4]. The magnitude of the capillary pressure, Pc, exerted on the

network, depends on the surface tension of the liquid, γ, the constant angle θ,

and the pore size, r: given by Pc = 2 γ Cosθ/r.If the pore size is very small, the

capillary pressure will be large. The original gel network collapses due to this

pressure[5]. Aging may be used to reduce the extent of collapse of the gel

structure during drying.The resulting materials are identified based on drying

conditions. Conventional evaporative drying such as heating a gel in an oven

induces capillary pressure associated with the liquid-vapour interface within a

pore, resulting in the collapse of the porous network. The sample thus

obtained is called a xerogel, which has a relatively low surface area and pore

volume.In supercritical drying, on the other hand, these deleterious effects are

minimized due to differential capillary pressure and the resultant materials are

known as aerogels. Consequently, they have high pore volumes, surface

areas and low bulk densities.A third method of drying involves the freeze


42

Hydrolysis Condensation

Polymerization

drying of the solvents at low temperature under reduced pressure. This

method is similar to the lyophilization technique adopted in pharmaceutical

industries and the product is called cryogel.Another method of drying is

subjecting the gel to ultrasonic vibration at room temperature to remove the

solvent. The gel thus obtained is called a sonogel.

Dehydration:This step is carried out between 400 °C and 800 °C to drive off

the organic residues and chemically bound water. A thermal treatment

firing/calcination may be performed in order to favour further

polycondensation and enhanced mechanical property, when following

changes, such as loss of solvents, pyrolysis of the organics, structural

rearrangement and densification or crystallization are observed.

Densification:Heating the porous gel at high temperatures, leads to

formation of a dense oxide product. The densification temperature depends

considerably on the dimensions of the pore network, the connectivity of the

pores, and surface area. Sequential steps involved in sol-gel process is

presented in Fig.2.2

Fig.2.2. Sequential steps involved in sol-gel process

Conditions for sol-gel synthesis

Precursor Solvent Catalyst

Inorganic Salt

Metal alkoxides

Water

Organic

Optional

Sol

Gel

Ageing

Washing

Drying

Xerogel Aerogel

Sonogel

Cryogel

Calcination

Structural rearrangement

Pyrolysis of the organic

Crystallization/ Densification

Improved mechanical property

Final Material


43

It is possible to tailor specific properties in a material by tuning the

various conditions of the sol-gel process, outlined as follows:

pH of the hydrolysis : The pH during hydrolysis mainly decides the nature of

the pores, surface area and density of the materials. In general, acid

catalyzed hydrolysis gives a microporous network, while base catalyzed

hydrolysis leads to the formation of a mesoporous network. The preparation of

microor mesoporous materials in a neutral medium has been reported [6].

Rate of addition of water: The rate of addition of water during the

preparation of the sol affects the rate of hydrolysis and condensation which in

turn influence the texture and morphology of the gel.

Temperature of gelation: The rate of gelation depends on the temperature at

which the sol is aged or heated for the removal of the solvent or for the

facilitation of hydrolysis. If the temperature is lower, the rate of hydrolysis is

slower and the particle size is relatively smaller. This results in reduced pore

collapse and yields a well defined porous network.

Aging of Gels:Aging is the process of keeping the gels in various solutions

for a period of time in order to increase the strength of the gel network, so that

cracking of gels during drying can be prevented. The chemical reactions that

cause gelation, continue long after gel point strengthening, stiffening and

shrinkage of the network [4,7]. The composition, structure and properties of

gel change during aging. The changes that occur during aging are

categorized as,

Polymerization: Increase in connectivity of the gel network by condensation

reactions.

Coarsening: Process of dissolution and reprecipitation driven by differences

in solubility between surfaces with different radii of curvature.

Syneresis: Shrinkage of the gel and the resulting expulsion of liquid from

the pores.

Drying control chemical agents (DCCAs):The presence of DCCAs has a

significant influence on the particle texture and morphology. Various DCCAs

useful in controlling the porosity and bulk density are formamide, glycerol and

oxalic acid.


44

Calcination temperature: the calcination temperature is also important in

controlling the pore size and density of the materials.

Conclusions, Advantages and Disadvantages of Sol-Gel process

The sol-gel method, thus offers the possibility to prepare solids with

pre-determined structure, by varying the experimental conditions such as the

choice of reagents, concentration, mode and rate of mixing, temperature, pH,

ageing and drying conditions. Variation in any of these parameters yields

materials with different characteristics. The preparation procedure thus affects

the composition and structure, which is further reflected in the

properties/performance such as porosity, surface polarity and crystallinity. The

various steps involved in the sol-gel technique described above may or may

not be followed. In practice, however, a modified sol-gel route is followed.

Advantages of sol-gel process include increased homogeneity, high

purity, low processing temperature and high surface area of the gels or

powders obtained. The inherent usefulness of this approach is largely due to

the ease with which sol-gel derived materials can be prepared, modified, and

processed. The mild reaction conditions afford an opportunity to incorporate

various organic moieties into inorganic compounds. Furthermore, the average

pore size, pore size distribution, surface area, refractive index and polarity of

the resultant matrix can also be controlled and tailored by manipulations in the

sol-gel processing conditions.Disadvantages of the Sol-Gel Process include,

large shrinkage during processing, creation of fine pores, presences of

hydroxyl groups when hydroxides are used, residual carbon in final material

originating from templates, health hazards of organic solvents, and finally long

processing times.

2.3SOL-GEL PROCESS AS APPLIED TO SILICA

Silica gels are most often synthesized by hydrolysing monomeric,

tetrafunctional alkoxide precursors employing a mineral acid or base as a

catalyst. Most common tetraalkoxysilanes used in the sol-gel process are

tetraethoxysilane [TEOS, (SiOC2H5)4] and tetramethoxysilane [TMOS,

Si(OCH3)4]. The sol-gel process involving silica can be described as follows,


45

The hydrolysis reaction replaces alkoxide groups with hydroxyl groups.

Subsequent condensation reactions involving the silanol groups produce

siloxane bonds and the by-products, alcohol or water. Under most conditions,

condensation commences before hydrolysis is complete. Since water and

alkoxysilanes are immiscible, a mutual solvent such as alcohol is normally

used as a homogenising agent. In order to reduce the reactivity of the

alkoxide precursor, organic functional groups are introduced into the siloxane

network. This type of alkoxide precursors are known as organoalkoxysilanes.

The surface and structural characteristics of the silica gels are affected

by various parameters that control the rate of hydrolysis and condensation

and these can be summarized into three categories[8]:

Composition: Type of starting materials, quantity of water, catalysts and

solvents.

Reactions (up to gel formation): Rate of mixing, reaction temperature and

gelation schemes.

Process variables (after gel formation): Type of dehydration, drying

temperature and heating.

Factors affecting silica gel synthesis are mole ratio of H2O to alkoxides

and pH values that play a significant role in determining reaction rates,

morphology and composition. Changes in pH values during the sol

preparation stage significantly affects the appearance of sample, porosity,

density, viscosity, gelation time, activation energy, surface area, pore volume,

and pore size distribution. Increase in the gelation temperature could make

the gelation process shorter,while increase in drying temperature would cause

the pore diameter to become bigger. Intermediate conditions produce

structures intermediate to these extremes.

Si OR H2OHydrolysis

Esterification

Si OH ROH

Si OR SiHO

+

+

Alcohol Condensation

Alcoholysis

Si O Si ROH +

+

Si OH + SiHO Si O Si +

Hydrolysis

WaterCondensation

H2O


46

2.4SOL-GEL SYNTHESIS AS APPLIED TO ZEOLITES

The factors that influence the synthesis of crystalline zeolite are summarized

as follows

Composition of the reaction mixture.

Nature of reactants

Initial and final pH of the system.

Temperature of the process and its variation with time (if any).

Ambient- 25 to 60 °C

Low –90 to 120 °C

Moderate- 120 to 200 °C

High- 250 °C or higher

Time allowed for the reaction to take place, including the calcination time.

Mixture, whether homogeneous or heterogeneous.

Seeding.

Template molecules (if any).

Other factors

Aging

Stirring (fast/slow) (time)

Nature of mixing

Order of mixing

Composition of the Reaction Mixture:

The composition of the reaction mixture is one of the most important factor,

governing the product properties,which includes :

Silica to alumina ratio.

Hydroxyl, ion concentration.

Inorganic cations.

SiO2 / Al2O3in the gel phase decides the framework composition of the

zeolite. The hydrophobic / hydrophilic nature of zeolite is also affected by this

ratio as aluminium is hydrophilic and silicon is hydrophobic. Also, high

aluminium contents give higher acidic sites, which are useful for many

applications. Zeolites with higher silica / alumina ratio are used for catalytic

applications in cracking and isomerization. As the aluminium content is


47

increased the acid resistance & thermal stability of the zeolite reduces. These

effects can be summarized as follows:

Increasing the silica / alumina ratio affects following physical properties of the

zeolite:

Increases acid resistance.

Increases thermal stability.

Increases hydrophobicity.

Decreases affinity for polar adsorbents.

Decreases cation content.

Decreasing the silica / alumina ratio affects following physical properties of the

zeolite:

Increases hydrophilicity.

Increases cation exchange properties.

Decreases the pore size for same numbered ring, as aluminium has lower

atomic radius than silicon.

Hydroxide ion concentration:It functions as structure director through

control of the degree of polymerization of silicates in solution. OH¯ ion

modifies the nucleation time by influencing transport of silicates from the solid

phase to solution. It enhances the crystal growth and controls the phase

purity. In a study[9] it was found that the OH¯/Si ratio influences the pore size.

i.e. higher the ratio, wider were the pores.

Role of Inorganic Cations: Inorganic cations are added to the reaction

mixture, to induce crystallization of specific zeolite structures, that could not

have been formed in their absence. These cations are used in the zeolite

synthesis for following reasons:

They act as structure directing agents.

They balance the framework charge.

They govern the morphology of the zeolite.

They affect the crystal purity

They also affect the product yield.

Due to their charge and their orientation in the reaction mixture,

inorganic cations alter the pore sizes of the zeolites. Most commonly used

cations are alkyl ammonium ions such as tetramethyl ammonium (TMA),


48

tetraethyl ammonium (TEA), etc. Apart from silica to alumina ratio and pH, the

amine concentration is also a very important variable [10].

Zeolite synthesis is described in two steps: nucleation and

crystallization. Nucleation is a process where small aggregates of precursors

give rise to germ nuclei (embryos), which become larger with time. The rate of

nucleation of a new phase from a melt increase by decreasing the

temperature of the system.Crystallization starts by involving nuclei and

ingredients from the solution mixture. The deposition on a seed or stable

nucleus increases with the extent of stirring and temperature. The yield of the

crystals increases at a rate proportional to total free external surfaces of the

crystal.

Crystallization can be split into four stages:

Formation of water icebergs. These icebergs are ring like structures of

water molecules, interconnected by hydrogen bonding.

Depolymerizationof the condensed large molecules of the precursors then

takes place.

These depolymerized molecules start orienting around the water icebergs,

thus forming thenuclei.

These nuclei form terminated tetrahedral. resulting into crystalline

structures.

pH of the Reaction Mixture

The zeolite synthesis via sol-gel process is carried out in alkaline pH (>

10). Crystal formation is accompanied by increase in pH indicating that SiOH

in colloidal state is incorporated in the framework in the form of SiO2. Since

SiOH is acidic in nature, the crystallization is accompanied by increase in pH.

Further, the pH of the system is also very important for stabilizing the sol and

control of the particle size.

Role of additives

Addition of colloidal particles e.g. Polystyrenecauses organization of the

pores. Polystyrene acts as a template as well as nucleating agent.


49

2.5SOL-GEL PROCESS USING TEMPLATES

Introduction

One of the important modified routes in sol-gel process involves use of

templates. Templates are structure directing agents, that find applications in

synthesis of porous materials with tailor made dimensions. They are organic

molecules (singular or assembly) around which the main structure is built up –

a process very similar to a casting process. Template synthesis began from

using quaternary alkyl ammonium ions - with alkyl chain length containing 10-

20 carbons.

Templates, when used at optimum concentration, referred to as Critical

Micelle Concentration (CMC), orient themselves to form an assembly with the

polar head groups pointing outside, around which the anions orient to form a

network. The layers of inorganic materials seem to distort and crosslink

around the polar head groups to form a new mesoporous structure. The

driving force for this layer folding, is most likely the ion pairing between the

positively charged head groups and the negatively charged inorganic

components. The template can subsequently be removed from the system,

either by solvent extraction method or by calcination, to obtain finished

product with predetermined pore size and structure. Excellent up-to-date

reviews on the use of various organic templates and the mechanism of

structure directing agents are available in the literature [11-13].

Concepts of templating

The main concept of obtaining well defined mesostructures is to use a

surfactant templated polymerization instead of an uncontrolled reaction. In

general, the Lyotropic (i.e. amphiphilic) molecules of the surfactant form a

liquid crystal by aggregation in aqueous solution[14,15]. Formation of the

liquid crystal matrix is strongly dependent on the conditions in the solution and

the structure of the liquid crystal is the so called mesostructure. Important

parameters for the mesophase formation are, the temperature, concentration

and pH- value of the solution. Depending on these conditions, the structure of

the mesophase can be for example ordered with spherical, cylindrical,

lamellar or cubic phases or disordered.


50

In the broadest sense, a template may be defined as a central structure

about which a network forms in such a way, that removal of the template

creates a cavity with morphological and/or stereochemical features related to

those of the template[16]. A general template approach is illustrated in Fig.

2.3, where primary structural units are organized around a molecular template

and solidified to form a matrix.

Fig.2.3Schematic of the organic template approach showing the incorporation and removal of

the template.

The fidelity of the imprint created by template removal depends on several

factors:

The nature of the interaction between the template and the embedding

matrix.

The ability of the matrix to conform to the template.

The relative sizes of the template and the primary units used to construct

the matrix. Examples of single molecules used as templates are

ethylamine(EA), isopropylamine(IPA), ethylmethylamine (EMA),

diethylamine (DEA), n-propylamine(nPA), ethylenediamine (en)

tetramethyl ammonium(TMA), and tetraethyl

ammonium(TEA).Surfactants/assemblies used as templates are cetyl

trimethyl ammonium bromide (CTABr), cetyl trimethyl ammonium chloride

(CTACl), cetyl pyridinium bromide (CPBr),and cetyl pyridinium

chloride(CPCl).

Surfactants are amphiphilic molecules which consist of a hydrophilic, polar

head group and a hydrophobic, non-polar tail. Due to their amphiphilic nature,

surfactant molecules have a high affinity towards surfaces and interfaces,

thereby the term “surfactant” emerges which is an abbreviation for “surface

active agent”. Surfactants may be classified into four different groups,

depending on the nature of the polar head group: anionic surfactants, cationic

surfactants, zwitterionic surfactants and non-ionic surfactants[17].In aqueous

solutions, the surfactants associate into aggregates called micelles, if


51

theconcentration is above the critical micelle concentration (CMC). If the

concentration is increased even further, or a polar additive is added, the

surfactants self-assemble into liquid crystalline mesophases.

The geometry of the surfactant aggregates formed in solution is

dependent upon the shape of surfactant and its concentration, which is

expressed by the term surfactant packing parameter:[18,19]v / a lwhere v is

the volume of the surfactant tail, a is the effective head group area and l is the

length of the extended surfactant tail, Fig. 2.4a

Due to variations in size of different types of surfactant tails and head

groups, this ratio willvary for different types of surfactants. The relative sizes

of the tail and head group thereforegovern the optimal way of packing the

surfactants together into aggregates of differentgeometry,[17,19] as shown in

Fig. 2.4b. If the packing parameter is below 1/3, only spherical micelles exist

in the solution. An increase in concentration causes these spherical micelles

toorganize themselves in the solution, which creates a three-dimensional,

cubic ordering. A v/alratio above 1/3 creates aggregates with a rod-like shape.

If the concentration of the surfactantsis sufficiently high, these rod-shaped

micelles assemble into a hexagonal array, therebycreating a hexagonal liquid

crystal.

(a)

(b) (C)


52

Fig. 2.4(a) Visualization of the geometrical considerations for the surfactant packing Parameter [19].(b) Different liquid-crystalline phases as a function of the surfactant packing Parameter [17]. (c). Curvatures of different types of surfactant surfaces.

For v/al = 1 there is a balance between sizes of the head group and

tail, which causes the surfactants to form planar aggregates with a sheet-like,

bilayer structure. With sufficient concentration, three-dimensional liquid

crystals with lamellar or bicontinuous cubic structures are created. If v/al is

increased above 1, the surfactants form reversed “water-in-oil” systems.The

value of the packing parameter may be influenced by the addition of co-

solutes in the surfactant solution[17,19]. Hydrophobic moleculesdissolve in

theinterior of the micelles, causing an increase of ‘v’. Short-chained alcohols

reside in the palisade layer in the micelles, thereby decreasing

‘a’andincreasing ‘v’. Electrolytes in the solution adsorb on the micelle surface

(of ionic surfactants), whichdecrease a due to screening of the repulsion

between the head groups.For non-ionic surfactants, the polarity and thereby

the solubility of the head group isdependent upon temperature. An increase in

temperature decreases this polarity, whichdecreases ‘a’.Introduction of other

types of surfactants into the solution also affects the packingparameter. The

surfactants interact with each other and form mixed micelle systems,

whichhas an average packing parameter.

The surface area of surfactant mesophases is highly dependent upon

the curvature of the surface, as visualized in Fig.2.4c. A convex surface has a

higher surface area than a concave surface, with the planar surface in

between.For ionic surfactants, the surface of the aggregates is charged. The

charge density of a charged surface is defined [20] as: σ = Q / A where σ is

the charge density, Q is the surface charge and A is the surface area. Fig.2.4c

shows that concave surfaces have higher charge densities than convex

surfaces, due to the lower surface area. The different liquid-crystalline

structures shown in Fig.2.4b have different surface curvatures and thereby

different surface charge densities. Itcan be seen that the surface charge

density of the surfactant mesophases increases withincreasing packing

parameter, due to the closer packing of the charged head groups.


53

2.6THE FORMATION OF MESOPOROUS STRUCTURES

Introduction

A number of models have been proposed to explain the formation of

mesoporous materials and to provide a rational basis for the various synthetic

routes followed. All these models are proposed on the basis of structure

directing ability of surfactants or templates in solution. Surfactants with

hydrophilic head groups and hydrophobic tail within the same molecule get

self-organized so as to minimize the contact with incompatible ends. The

principal difference amongst various synthetic routes is the way in which the

surfactants interact with inorganic species. Earlier it was thought that the

formation of these materials is a result of electrostatic complementary

between charged surfactant and inorganic species. But later the

mesostructured materials have been prepared by exploring other possible

interactions other than electrostatic pathways. The formation of these

materials can be viewed alternatively as the interface chemistry between the

surfactant and inorganic species. The mesoporous materials can be prepared

by exploiting ionic, hydrogen bonding as well as covalent bonding

interactions.Different synthesis mechanisms have been proposed to explain

theformation of mesoporous materials. A few of them are described below:

Liquid Crystal Templating (LCT) Mechanism

There are threemain liquid crystalline phases with hexagonal, cubic

and lamellar structures. Because of the similarity between the different M41S

phases MCM-41 hexagonal, MCM-48-cubic, MCM-50-lamellar and known

liquid crystalphases, the first mechanism proposed for the synthesis of these

materials was the liquidcrystal templating mechanism [14,21-23]. In aqueous

solution, surfactant molecules exist asrandomly dispersed monomolecules at

low concentrations. With increasing concentration,the surfactant molecules

aggregate with their hydrophobic tails together exposing their polarheads to

the aqueous solution to reach a minimum energy configuration and thus

formspherical micelles decreasing the system entropy. The lowest

concentration at which monomolecules aggregate to form spherical isotropic

micelles is called critical micellie concentration (CMC1). There exists a


54

second critical concentration(CMC2) corresponding tothe further aggregration

of spherical into cylindrical or rod-like micelles. The hexagonalphase is the

result of hexagonal packing of cylindrical micelles, the lamellar

phasecorresponds to the formation of surfactant bilayers and the cubic phase

may be regarded as abicontinuous structure. The structure of the mesophase

depends on the composition of themixture, the pH and the temperature [24].

Two possible pathways have been proposed [14] for the LCT

mechanism which are schematically shown in Fig. 2.5. Pathway-1 is a

surfactant controlled pathway, in which the surfactant arrays, prior to the

condensation of framework materials. In this pathway, it is considered that

first there is aformation of the surfactant hexagonal liquid-crystal phase

around which the growth of theinorganic materials is directed. Thesurfactant

micelles aggregate to formhexagonal arrays of rods. Silicate anions present in

the reaction mixture interact with thesurfactant cationic head groups.

Condensation of the silicate species leads to the formation ofan inorganic

polymer.

Fig. 2.5 Possible mechanistic pathways for the formation of MCM-41: (1) liquid crystal phase

initiated and (2) silicate anion initiated [14].

Pathway-2 is a silicate controlled mechanism,in which the silicate

species condense continuously around micelles, as they form rods and pack

into a hexagonal structure.This is thought of as a cooperative self assembly

pathway.In this pathway, it has been proposed that the randomly ordered

rodlike micelles interact with silicate species by coulombic interactions in the

reaction mixture to produce approximately two or three monolayers of silicate

around the external surfaces of the micelles. These randomly ordered


55

composite species spontaneously pack into a highly ordered mesoporous

phase with an energetically favourable hexagonal arrangement, accompanied

by silicate condensation. With the increase in heating time, the inorganic wall

continues to condense. The absence of hexagonal liquid crystalline

mesophases, either in the synthesis gel or in the surfactant solution (used as

template) it was concluded that formation of MCM-41 phase is possibly via

pathway 2 rather than pathway 1.

Transformation mechanism from lamellar to hexagonal phase

It has been proposed that[24-26] in a surfactant/silicate aqueous

mixture with relatively low pH, low degree of polymerization of silica species,

and low temperatures, small silica oligomers (three to eight silicon atoms)

interact with surfactant cations by coulombic interactions at the interfaces

forming multidentate binding between them. These subsequently polymerize

to form larger ligands, enhancing the binding between the surfactant and

silicate species. These surfactant silicate multidentate ligands lead to a

lamellar biphase governed by the optimal surfactant average head group area

(A). As the polymerization of silicate species proceeds, the average

headgroup area of surfactant assembly increases due to the decrease in the

charge density of larger silicate layers and ultimately results in the hexagonal

mesophase precipitation(Fig.2.6).

Fig. 2.6 Transformation mechanism from lamellar to hexagonal phase [25].

Folded Sheet Mechanism

Yanagisawa et al. [27] and Inagaki et al[28,29] synthesized crystalline

mesoporous silicate andaluminosilicate materials designated as FSM-16

(Folded Sheet Mesoporous Materials). Theyproposed a folded sheet

mechanism (Fig. 2.7) for the formation of mesostructures derivedfrom

kanemite (layered silicate). The surfactant cations intercalate into the bilayers

ofkanemite by ion-exchange process. The transformation to the hexagonal


56

phase occurs duringhydrothermal treatment by condensation of silanol

groups. MCM-41 and FSM-16 are similarbut show slightly different properties

in adsorption[22]and surface chemistry[30].

Fig. 2.7 Folded sheet mechanism [29]

2.7GENERALIZED LCT MECHANISM

Based on the specific type of electrostatic interaction between a given

inorganic precursor “I” and surfactant head group “S” various synthesis routes

have been evolved as presented in scheme 2.1.

Scheme 2.1 Possible synthetic routes for mesoporous materials

Direct pathways:

In the case of direct pathway, surfactants are directly bonded to

inorganic precursors through electrostatic interactions. So far, most of the

materials have been prepared with cationic surfactants like

CTABr/CTACl.Based on the original LCT mechanism, which involves the


57

anionic silicate species and cationic quaternary ammonium surfactant, it could

be categorized as the S+I– pathway. However, a variety of surfactants can

serve the purpose. In all these cases, control of pH is critical. Various

materials have been prepared with S+I- and S- I+ pathways [14,31-35].

Mediated Pathways:

Here charge interaction pathways are S-I+,S+X-I+ (where x is a counter

anion) and S- M+ I- (where M is metal cation). By operating well below the

isoelectric point of silica under acidic conditions (pH ~2), the silicate species

are cationic. In this case the halide ions (X-) act as mediators.In charge

reversed situation, where the surfactants and inorganic species are negatively

charged the metal ion M+ (where M = Na+ or K+) act as mediators. By

maintaining higher pH conditions, it is possible to prepare mesoporous

materials by S- M+ I- pathway through metal ion mediation. Materials prepared

by S+X- I+ and S- M+ I- path ways are reported [33-35].

Neutral Pathways:

Tanev and Pinnavaia [36] proposed a neutral templating mechanism

based on hydrogen bonding interactions (S°-I°) betweenneutral primary amine

(S°) which acts as template and neutral inorganic precursors (I°). Hydrolysis

of tetraethylorthosilicate in an aqueous solution of primary amine yields the

neutral Si(OC2H5)4-X (OH)X species which then binds through H-bonding to the

surfactant head group. This leads to the formation of rod like micelles.

Further, hydrolysis followed by condensation, leads to Hexagonal

Mesoporous Silica (HMS). The neutral templating route provides several

advantages over materials prepared by electrostatic pathways, mainly the

synthesis can be carried out at room temperature and the surfactant can be

removed by extraction with ethanol.

Even though this process, S°I° offers the practical advantage of facile

template recovery by non- corrosive solvent extraction or evaporation

methods, these surfactants have some limitations. Neutral amines are costly

and toxic and not ideally suitable for the industrial scale preparation of

materials. So there exists a need to think of a process with low cost and

environmentally compatible neutral templating route. Polymeric polyethylene

oxide (PEO) surfactants have been used to prepare these materials which are


58

relatively inexpensive and biodegradable. The main advantage of using

polymeric surfactants is the requirement of the lower concentration of the

surfactant. These surfactants form spherical to flexible rod or worm like

micelles at critical concentrations approximately one hundredth of those

required for ionic surfactants [37]. It is also observed that polymeric

polyethylene oxide tri-block copolymer is a promising surfactant. In the

presence of suitable solvents or combination of solvents these

PEO/PPO/PEO tri-block copolymeric surfactants arrange into different

lyotropic phases.

Ligand Assisted interactions

By a different synthetic approach [38], it is possible to prepare

mesoporous materials through covalent interactions. Instead of relying on

charge interaction, the surfactants were pretreated with the metal alkoxides in

the absence of water to form metal-ligand covalent bonded complexes. High

quality materials are formed by the use of amine surfactants, due to the strong

affinity for nitrogen-metal bond formation between surfactant head group and

the inorganic precursor. In this ligand assisted templating approach, the

control of the mesostructure was found possible by adjusting the

metal/surfactant ratio, and it has been established that the M41S family of

mesoporous materials can be prepared through this approach.Mesoporous

materials prepared by various methods are presented in Table2.1.

2.8 REMOVAL OF TEMPLATE

Once the framework condenses around the micellar rods, a nonporous solid is

formed. To produce the porous species, the template must be removed from

the framework. The method of template removal depends on the desired

morphology and the thermal stability of the synthesized compound. MCM-41

shows reasonable thermal stability. As such, the surfactant template is

removed by calcination. In this context, calcination refers to simply heating the

sample sufficiently to burn out the organic phase and leave behind the porous

framework [38,39]. The calcination has to be done in the flow of inert gases at

the initial stages followed by the flow of air. This is to maintain the crystallanity

of the material. The template can also be removed by washing the as

synthesized material with extracting solvents.


59

Table 2.1Summary of Mesoporous materials prepared by various methods [14,31-38]

Class of Materials

Type Preparation Method

Observed phases

M41S MCM-41 S+ I

– Hexagonal

MCM-48 “ Cubic

MCM-50 “ Lamellar

T-M41S (T=Transition metals)

“

---------

SBA SBA-1 S+ X

– I

+ Cubic

SBA-2 “ 3D-Hexagonal

SBA-3/APM “ MCM-41 Like Hexagonal

SBA-11 “ Cubic

SBA-12 “ 3D-Hexagonal Material

SBA-14 “ Lamellar

SBA-15 “ 2D-Hexagonal

SBA-16 “ 3D-Cubic cage structure

T-SBA (T=Ti,V,Mn,Mo,Cr,Zr)

“ -----------

MSU MSU-1 Silica N◦ I

◦ Worm-like

disordered

MSU-2 Silica “ Worm-like disordered

MSU-3 Silica “ Worm-like disordered

MSU-V “ Lamellar

Ti-MSU- Silica “ Worm-like disordered

Zr-MSU- Silica “ Worm-like disordered

Nb-TMS1

Ta-TMS1 Hexagonal

TMS Nb-TMS2 Hexagonal

Nb-TMS3 Hexagonal

Nb-TMS4 Cubic Layered

PHTS PHTS S+ X

– I

+ Analogue to SBA-15

MCF MCF Swelling agent added to

Synthesis of SBA-15

Sponge-like foam with 3D-structure with large uniform

spherical cell

HMS T-HMS (T=Si,V,Al,Ga,Fe,Cr,Mo)

S◦ I

◦ Hexagonal

MSU- Michigan State University HMS- Hexagonal Mesoporous Silica PHTS- Plugged Hexagonal Templated Silica, MCF - Meso Cellular Form SBA - Santa Barbara Amorphos or Santa Barbara Acid Material, TMS - Transition Metal Oxide Mesoporous Molecular Sieves, M41S- Mobil Composition of Material or Mobil Composition of Matter


60

Some of the more effective solvents are ethanol and supercritical CO2.

When using supercritical extraction (SCE), polar modifiers are added to

enhance the extraction capability of the supercritical fluid. Commonly used

modifiers are dichloromethane (DCM) and methanol [40]. In the case of

materials prepared through hydrogen bonding interactions, the removal of the

template can be achieved either by repeated extraction with ethanol or by

calcination.

Each method of template removal has advantages and disadvantages.

Standard calcination programs are effective at removing all of the template,

but the high temperatures generally cause some loss of long range order and

shrinkage of pores within the MCM structure. Extractions with ethanol are

easy to perform at room temperature but are not as effective at completely

removing the template. Supercritical extractions are extremely efficient at

removing surfactants. It is a more environmentally benign extraction method

as the surfactant can be recovered for use in subsequent synthesis as

opposed to being burned to oxidation products in typical calcinations. Due to

the interesting properties of supercritical fluids, SCE does not induce

structural degradation whereas pore shrinkage and some loss of long-range

order arise in thermal calcinations [40]. Unfortunately, supercritical extraction

is expensive and difficult to perform.

2.9SYNTHESIS STRATEGIES AND CHARACTERIZATION

METHODOLOGIES - A LITERATURE SURVEY

Synthesis strategies of Siliceous MCM-41- A Literature Survey

The discovery of MCM-41, has attracted considerable attention due to

large and uniform pore size distribution, high surface area (>800 m2/g) and

distinct adsorption properties [21].Siliceous MCM-41 when synthesized is

chemically inert in which catalytic activity has to be imparted. It is essential

that the MCM-41 thus synthesized should be hydrothermally stable for

functionalizing, after which it can be held at relatively higher temperatures and

even under harsher conditions, which is generally prevalent when used in

industries. In addition, if one needs to scale up for producing larger quantities,

the synthesis has to be simple, lesser energy demanding, cost effective and


61

environment friendly, like handling and usage of appropriate concentration of

chemicals without much of secondary wastes.

Several synthesis methods have been proposed and successfully used

to synthesize mesoporous MCM-41 molecular sieves [24,41-44].These

materials are normally synthesized by hydrothermal procedures and their

structures are obtained from amorphous inorganic silica walls around

surfactant molecules. It is now well documented in literature that the formation

of mesostructures is influenced by surfactant concentration, pH, presence of

co-surfactant, and its concentration and temperature [45-52].

Generally siliceous MCM-41 materials are synthesized from gels with

surfactant/silica molar ratio of more than 0.12 [45-48, 53-59] and involves

hydrothermal treatment of precursor gel in the temperature range between

60–150°C for a long time (1–6 days) in presence of quaternary ammonium

surfactants, CnH2n+1 (CH3)3N+, with different alkyl chain lengths (n = 8–18),

using sodium silicate or tetraalkylorthosilicate as sources of silica

[15,48,50,53-56]. Siliceous MCM-41has also been synthesized under

refluxing [53,57] and microwave irradiation [58]. Attempts have been made to

synthesize siliceous MCM-41 materials under ambient conditions [60-68].

Voegtlin et al. [64] have prepared highly ordered MCM-41 at room

temperature in 1 h; however, stability above 873 K has not been reported.

Different strategies have been employed to improve thermal and hydrothermal

stability of these materials, such as synthesis of materials with thicker pore

wall under hydrothermal condition by using low surfactant to silica molar ratio

in the range of 0.06 to 0.1 [56], by addition of salts in the synthesis gel before

or during hydrothermal crystallization [69-72] and/or intermittent pH

adjustment with acid during hydrothermal treatment [71,73] and using highly

condensed silica source such as fumed silica [74,75] and calcined MCM-41

silica [76]. Cheng et al [77] have synthesized MCM-41 using fumed silica at

438 K in 48 h with pore wall thickness of 2.68 nm and showed improved

thermal stability. Kumar et al [78] have synthesized MCM-41 analogue at

room temperature using hexadecylamine as templating agent that exhibited

improved stability. However, most of the approaches suffer process difficulties


62

in large scale preparation, as the synthesis involves longer crystallization time

at higher temperatures (100°C-150°C).

A highly ordered mesoporous siliceous MCM-41 has been synthesized

in, using a simple synthesis methodology at room temperature with surfactant

to silica ratio 0.1, using sodium silicate as silica source and CTAB as structure

directing agent, adding silicate solution to the surfactant solution at a

controlled rate, resulting in synthesis of a well crystalline MCM-41, with

improved thermal and hydrothermal stability at room temperature in less than

3 h [79].

Modified MCM-41 materials

As already mentioned in chapter I, MCM-41 based materials have

negligible catalytic activity due to framework neutrality. MCM-41 can be used

as solid acid catalyst by generation of acid properties in MCM-41 that

enhances its practicability and exhibit remarkable catalytic performance.

A purely siliceous framework is electronically neutral.When lattice Si4+

cations are replaced by Al3+cations,puresiliceous MCM-41 loses neutrality.

The negatively charged framework is balanced by Na+ ions present in the

system. In order to form acidic mesoporous materials, ion exchange with

ammonium nitrate is carried out, followed by thermal decomposition of the

NH4+ cations into protons and ammonia. The Brønsted acid sites are protons,

loosely attached to lattice oxygen atoms in the vicinity of aluminium. With

increased Al3+ incorporation in the MCM-41lattice the acidity increases. Thus,

by incorporation of Al3+ in the MCM-41 framework, material with inherent

acidity is obtained, generally entitled as Al-MCM-41.

Heteropoly acids (HPAs) have proved to be the alternative to traditional

mineral acid catalysts due to both strong acidity and appropriate redox

properties. However, limitations for HPAs to be used as solid acid catalysts

are low thermal stability, low surface area (1-10 m2/g) and difficulty in

separation from reaction mixture due to their high solubility in polar solvents.

For HPAs to be effective as catalysts, they should be supported on a carrier

with a large surface area. Owing to a very large surface area and a uniform

large pore size, the MCM-41 materials can act as excellent supports that

provide an opportunity for HPAs to be dispersed over a large surface area


63

and hence increased catalytic activity. Further, such mesoporous materials,

which have relatively small diffusion hindrance, can aid the easy diffusion of

bulky organic molecules in and out of their mesopores.Thus, heteropoly acids

(HPAs) supported onto MCM-41 by process of anchoring and calcination

yields material with induced acidity [80,81].

Characterization Methodologies - A Literature Survey

Characterization of MCM-41

As the most investigated member of the M41S family, MCM-41

provides an excellent example in characterizing mesoporous materials. MCM-

41 has a honeycomb structure that is the result of hexagonal packing of

unidimensional cylindrical pores. Reliable characterization of the porous

hexagonal structure requires the use of three independent techniques [82]: X-

ray diffraction (XRD), transmission electron microscopy (TEM) and adsorption

analysis.

The XRD pattern of MCM-41 shows typically three to five reflections

between 2=2° and 5° (Fig. 2.8), although samples with more reflections have

also been reported [83,84]. The reflections are due to the ordered hexagonal

array of parallel silica tubes and can be indexed, assuming a hexagonal unit

cell as (100), (110), (200), (210) and (300). Out of the XRD peaks exhibited in

the low angle region for mesoporous phase, the most intense peak is the

(100) reflection. The powder pattern is the finger print of the molecular sieve

structure and can be ascertained by comparing with the standard pattern for

the molecular sieves under investigation. Since the materials are not

crystalline at the atomiclevel, no reflections at higher angles are observed.

Moreover, these reflections would only bevery weak in any case, owing to the

strong decrease of the structure factor at high angles. By means of XRD it is

not possible to quantify the purity of the material. Samples with only one

distinct reflection have also been found to contain substantial amounts of

MCM-41. It is reported that even when the hexagonal pore structure contains

a large number of defects, a hexagonally indexable three-reflection pattern

can be calculated [85].

Transmission Electron Microscopy (TEM)is used to elucidate the

pore structure of mesoporous molecular sieves [14,22,35] It provides


64

topographic information of materials at near atomic resolution. However, the

exact analysis of pore sizes and thickness of the pore walls is very difficult

and not possible without additional simulations because of the ‘focus’

problem.Chen et al [86] have reported that the thickness of MCM-41 depends

strongly on the focus conditions, and careful modelling is necessary for

precise analysis.More than one model with a hexagonal array of large

cylindrical pores with thin walls gives a similar XRD pattern, but TEM gives a

direct, precise and simultaneous measurement of the pore diameter and pore

thickness. HRTEM can be successfully used to examine the microstructural

feature of mesoporous molecular sieves [87,88]. In addition to structural

characterization, it can also be used to detect the location of metal clusters

and heavy cations in the framework [88].Fig. 2.9 shows a TEM image of the

hexagonal arrangement of uniform, 4 nm sized pores in a sample of MCM-41.

Most MCM-41 samples not only show ordered regions but also disordered

regions, lamellar and fingerprint-like structures [87]. The existence of a

lamellar phase after calcination is unlikely, because silicate layers are too

distant from one another to preserve the spacing in the silicate organic phase

and collapse without additional post-treatments.

Scanning electron microscopy (SEM) is also used to study the

morphology of the material. It also gives an idea about the changes in shape,

size and surface that occur in a used material.

Adsorption of probe molecules has been widely used to determine the

surface area and to characterizethepore size and pore-size distribution of

MCM-41 type materials.

The Braunauer-Emmett-Teller (BET) volumetric gas adsorption

technique using nitrogen, argon, etc. is a standard method for the

determination of the surface areas and pore size/pore size distribution of

finely divided porous samples [89]. The relation between the amount

adsorbed and the equilibrium pressure of the gas at constant temperature is

defined by the adsorption isotherm.The physisorptionof gases such as N2, O2

and Ar has been studiedto characterize the porosity [90-93]. The nitrogen

adsorption isotherm for MCM-41 with pores of around 4.0 nm, which is type IV

in the IUPAC classification [94], shows two distinct features: a sharp capillary


65

condensation step at a relative pressure of 0.4 and no hysteresis between the

adsorption and desorption branches (Fig.2.10).The adsorption at very low

relative pressure, p/p0, is due to monolayer adsorption of N2 on the wallsof the

mesopores and does not represent the presence of any micropores

[95,96].The steep increase in N2 adsorption (within the p/p0 range between

0.2 to 0.4) corresponds to capillary condensation within uniform pores. The

sharpness and the height of this step reflects the uniformity of the pore size

and the pore volume respectively. However, in thecase of materials with pores

larger than 4.0 nm [97] or using O2 or Ar as adsorbate [91], the isotherm is still

type IV but also exhibits welldefined hysteresis loops. The presence and size

of the hysteresis loops depend on the adsorbate [91], pore size [97] and

temperature [98].

The traditional method for analyzing pore-size distributions in the

mesopore range is the Barrett–Joyner–Halenda (BJH) method [99,100] which

is based on the Kelvin equation and, thus, has a thermodynamic origin.

However, compared with new methods that rely on more localized

descriptions such as density functional theory (DFT) [96,101] and Monte Carlo

(MC) simulation [102], the thermodynamically based methods over estimate

the relative pressure at desorption and therefore underestimate the calculated

pore diameters by 1.0 nm. Moreover, the theoretical basis for the BJH

analysis becomes fairly weak if the step at 77 K lies below p/p0=0.42, because

this is considered to be the stability limit of the meniscus. Pore sizes

calculated in such cases are still probably in the right range, but a sound

theoretical foundation for such values is missing.

Fourier transform infrared spectroscopy (FTIR) yields information

concerning the structural details of a siliceous inorganic material [103,104]. In

addition, it can be used to confirm surface characteristics (such as acidity)

and isomorphous substitution by other elements in the material. The

technique allows to relate different materials by their common structural

features, such as a classification of zeolite structures.

Broad bands in the region ~3400 cm-1are assigned to –OH stretching

vibration of MCM-41 which could be associated to Si-OH and water vibrations,

confirming the presence of the silanol groups [80,105] or bridged hydroxyl


66

groups. Bands ~1650 cm-1are attributed to H-O-H bending vibration. A broad

band ~1300-1000 cm-1is assigned Si-O-Si asymmetric stretching mode.

Bands at 800cm-1and 458 cm-1are attributed to symmetric stretching vibration

and bending vibration (rocking mode) of Si-O-Si. The band at 960 cm-1is

assigned to the presence of Si-OH stretching vibration.

Fig.2.8X-ray diffraction pattern of high-quality calcined MCM-41 made by Huo and Margolese [46].

Fig. 2.9 TEM of MCM-41 featuring 4.0 nm sized pores, hexagonally arranged [85].

Fig. 2.10 Adsorption isotherm of nitrogen on MCM-41 with 4.0 nm pores at 77 K [91].

Thermal Analysisgives an idea about the thermal stability of the

material and the possible phase changes that occur during the thermal

treatment of the material. An understanding of the thermal behavior is of basic

importance for utilizing the material in various temperature ranges where it is

thermally stable.

Determining the quantity and strength of the acid sites is crucial to

understanding and predicting the performance of a catalyst.Temperature-


67

Programmed Desorption (TPD) is one of the most widely used and flexible

techniques for characterizing the acid sites on surfaces. Ammonia is a very

basic molecule which is capable of titrating weak acid sites, which may not

contribute to the activity of catalysts. The strongly polar adsorbed ammonia is

also capable of adsorbing additional ammonia from the gas phase.

The acidity of different materialscan be determined using the TPD of

ammonia. This method involves three steps. The sample is first degassed and

then saturated with a mixture of 5% NH3+He gas at 120 oC, when ammonia

gets chemisorbed on the acidic sites of the catalyst. After removal of any

physisorbed ammonia from the surface by purging He at 120oC for 30 min, the

temperature programmed desorption is carried out at a heating rate of 10 oC/

min. The desorbed gas concentration is continuously monitored and recorded

with temperature by a thermal conductivity detector (TCD). This

concentration-temperature plot is referred to as the TPD profile. The area

under the profile is proportional to the amount of gas desorbed. Acid sites with

varying acid strength differ in their heat of adsorption, which is reflected in the

TPD profile by way of a number of distinct peaks representing the acid sites of

the catalyst. The acidity is reported as ml/g.The area under the curve

indicates the amount of NH3 desorbed and hence the number of surface acid

sites. In general, amorphous materials exhibit broad desorption peaks

compared to crystalline ones. Though the crystalline materials show sharper

peaks indicating less number of acid sites, the desorption temperatures of

NH3 are high indicating strong acid sites. Since siliceous MCM-41 have a

neutral framework, sharp desorption bands are absent in the NH3-TPD

profiles indicative of negligible surface acidity.

Elemental analysis gives us an idea about the composition of the

catalyst.It is important to know the composition of a catalyst before

use.Instrumental methods used for elemental analysis are Flame photometry,

Atomic absorption spectroscopy (AAS) and Inductively coupled plasma-

atomic emission spectroscopy (ICP-AES) which are both popular as well as

accurate.ICP-AES is a widely used analytical technique for the determination

of elements present in a wide variety of samples. The technique is based on

atomic emission spectroscopy, and as the name suggests, plasma is used as


68

the source of excitation/ionization of atoms.The intensity obtained for each

sample is matched against a calibration plot prepared for the particular

element for quantitative estimation. The concentration of different elements is

actually measured at ppm level, which can be later converted into the %

weight of the element, by incorporating the dilution factor .These values are

then converted into moles of each element.

Energy dispersive X-ray analysis (EDX) is used for both identification

of an element as well as to have a rough estimate of the composition of the

materials. EDX is used in coordination with and as supportive analysis with

ICP-AES which is more accurate compared to EDX.

Diffuse Reflectance UV-visible spectroscopyis a technique that

measures the scattered light reflected from the surface of samples in the UV-

visible range (200-800 nm). For most of the isomorphously substituted

molecular sieves, transitions in the UV region (200-400) nm are of prime

interest. This spectroscopic technique is used to determine the coordination

state of transition metal ions substituted in the matrix of the molecular sieves,

involving ligand-to-metal charge transfer transitions at ~ 200- 220 nm.

Characterization of Al-MCM-41

Isomorphous substitution of a heteroatom in the framework of the

molecular sieves results in changes in the unit cell parameters and unit cell

volume. This is one of the ways to confirm isomorphous substitution. The

XRD diffraction patterns for Al-MCM-41 are shown in Fig.2.11. The patterns

illustrate the characteristics of a typical mesoporous MCM-41 structure. The

d100reflections of Al-MCM-41 have been shifted to higher values compared to

its as-synthesized analogue.This is in agreement with Borade and Clearfield

[106], suggesting the framework substitution of Al3+ in MCM-41 structure.

The FTIR spectrum of MCM-41 has already been discussed earlier in

the text. It is observed that there is no significant change in the FTIR bands of

Al-MCM-41 compared to silicon MCM-41. The band at 960 cm-1is assigned to

the presence of Si-OH stretching vibration as well as metal ion substituted

MCM-41. A slight shift in band positions is observed due to ismorphous

substitution of metal ion for Si4+.


69

For the Al3+ incorporated MCM-41, sharp desorption bands are

observed in the NH3-TPD profiles indicating presence of surface acidity,

combined with increase in surface acidity, with increase in Al3+ content.

. Fig.2.11 XRD pattern of the materials: (a) Al-MCM-41(25), (b) Al-MCM-41(50), (c) Al-MCM-41(75) and (d) Al-MCM-41(100) [80]

Characterization of Heteropoly acid (HPA) supported MCM-41

Heteropoly acid supported onto MCM-41have been prepared and

characterized exhaustively [80,105]. The three main reliable techniques used

for characterization of these materials are XRD, FTIR and surface properties-

pore size, pore size distribution, surface area and surface acidity which

ensure the anchoring of HPA’s onto MCM-41, wherein comparisons are made

between pure HPA’s, pure MCM-41 and a combination of these.

The XRD patterns of a typical mesoporous MCM-41 structure have

been discussed earlier in the text, where importance of the d100 reflection has

been indicated. Fig.2.12 illustrates the effect of the HPA loading on the XRD

of MCM-41 samples. HPA has a striking effect on the width and intensity of

the main reflection at high d100 spacing and this line becomes broader and

weaker as the loading increases. This suggests that the long-range order of

Si-MCM-41is decreased noticeably by the presence of HPA. MCM-41

presentsthe highest surface area and pore volume, with all pores being in the

mesopore range. The pore size distribution of MCM-41 shows a unique peak

centered at about 25Å diameters as given in the literature [107]. With

increasing HPA loading, a reduction in surface area, pore volume and a

notable compression of the pore size distribution and increase in surface


70

acidity are observed compared to MCM-41 (Table 2.2). Table 2.2 presents

changes in textural properties with increase in HPA wt. %.

Table 2.2Textural properties of various wt.%. HPA loaded materials[105]

.Catalysts d100

(Å)

Unit cell

a0 (nm)

Surface

area

(m2/g)

Pore size

BJHAds

(nm)

Pore

volume

BJHAds

(cc/g)

Si-MCM-41 44.21 5.10 938 2.60 0.60

10wt.% 12-TPA-MCM-41 42.15 4.86 526 2.55 0.32

15wt.% 12-TPA-MCM-41 38.45 4.44 265 2.40 0.12

20wt.% 12-TPA-MCM-41 35.37 4.08 235 1.90 0.11

Fig.2.12 XRD pattern of the materials (a) Si-MCM-41, (b) 10wt.%12-TPA-MCM-41, (c) 15wt.%12-TPA-MCM-41, (d) 20wt.%12-TPA-MCM-41 [80].

Fig.2.13FTIR spectra of mesoporous materials: (a) Al-MCM-41(25), (b) Al-MCM-41(50), (c) Al-MCM-41(75), (d) Al-MCM-41(100), (e) 10wt.% 12-TPA-MCM-41, (f) 15wt.% 12-TPA -MCM-41, (g) 20wt.% 12-TPA-MCM-41 and (h) 12-TPA[80].

The FTIR spectra of HPA supported onto MCM-41 are given in Fig

2.13. A broad band due to –OH stretch of water in ~3400cm-1region and the

corresponding –OH2 bending mode around ~1637 cm-1 very well correlate

with the water adsorption property (hydrophilic property) of the catalysts. Pure

HPA spectra with a Keggin structure with four strong bands, at 1082 cm-1 (P-

O), 988 cm-1 (W=O) and 800 cm-1 (W-O-W), and a weak band at 525 cm-1 (W-

O-P) [108] are observed. The framework bands of Si-MCM-41 at 1236, 1090,

965, 800, 564 and 465 cm-1[109] easily overlap with those of HPA i.e. 12-

tungsto phosphoric acid (12-TPA). For 10 wt.%12-TPA-MCM-41, none of the

12-TPA bands are observed, except for a slight increase in the intensity of the


71

800 cm-1 band. For 15 wt.%12-TPA-MCM-41, the bands at 988 and 891 cm-1

become visible. With 20 wt.% of the 12-TPA loading, the 1082 cm-1 band

become sharper and the intensity of the two bands at 988 and 891 cm-1

increases. Furthermore, it is observed that the intensity of 800 cm-1 band is

almost proportional to the increase in the amount of 12-TPA.The final and

most important evidence is the increase in surface acidity with increased 12-

TPA loading.

2.10 EXPERIMENTAL

Materials: Commercial grade sodium silicate (Na2SiO3) with composition

28%SiO2 and 7.5% Na2O was procured from Sapna Chemicals, Vadodara.

CTABr, CPBr, sodium hydroxide flakes, aluminium sulfate, ammonium nitrate

and 12-tungstophosphoric acid (12-TPA) were purchased from Loba

Chemicals, Mumbai. Tetra ethylortho silicate (TEOS) and analytical grade

sulphuric acid were obtained from E.Merck, Mumbai. All other chemicals and

reagents used were of analytical grade. Double distilled water(DDW) was

used for all studies

Synthesis of MCM-41:In the present endeavor, the objective is to synthesize

mesoporous MCM-41 at room temperature with good thermal stability and

high surface area as well as retention of surface area at high temperature. A

sol–gel method has been used to synthesize MCM-41 to achieve this

objective. Several sets of materials were prepared varying conditions in each

case, using surface area as an indicative tool in all cases. Table 2.3 describes

the parameters that have been optimized for synthesis of MCM-41.

Synthesis of MCM-41 at optimized condition

As indicated in table-2.3 entry No.-8 indicates optimum condition. We

hereby describe synthesis of MCM-41 under optimized conditions. The molar

gel composition for MCM-41 is 1 SiO2:0.5 CTABr:0.25 Na2O:80 H2O. In a

typical 500g batch size experiment, 63.02 g of Na2SiO3was mixed with 183

gDDW under continuous stirring at room temperature for~15 min. in a

polypropylene container (A). An aqueous solution of CTABr was prepared by

dissolving 54.14 gm CTABr in 200 gm DDW under continuous stirring at room

temperature (B). Template solution B was added to a precursor solution A

dropwise with continuous stirring within ~15 min. and the solution further


72

stirred for 15 min. The pH of the resultant solution was adjusted to 10.5 using

1:1 H2SO4(diluted 1:1V/V). A gel was obtained at this stage which was stirred

further for 30 min. The polypropylene container was now closed and allowed

to age at room temperature without stirring for 24 h. The resultant gel was

filtered, washed with DDW to remove adhering ions and dried at 120oC,

followed by calcination at 550oC for 6 h at a heating rate of 2oC/min. The final

material obtained was used for all further studies.

Note:In the present synthetic endeavor Na2SiO3 as a silica source is

preferred to TEOS, reasons being higher cost of TEOS. Further, the thermal

stability of final material obtained is better when Na2SiO3 is used as silica

source compared to TEOS. While preparation it is preferable to add template

to silica source. The pH of template being almost neutral and pH of silica

being >12, if template is added to silica source, the pH variation window is

narrowed down due to which pH of gel formation is easily adjusted. The pH in

the synthesis was adjusted to 10.5 because gel viscosity is maximum at this

pH, which can also be stirred with ease for homogenization. Finally at the

optimized condition, surface area retention between 550 - 900 oC is fairly

good.

Synthesis of Al-MCM-41

In the present synthetic endeavor the objective is to synthesize

mesoporous Al-MCM-41 at room temperature with good thermal stability and

high surface acidity. A sol-gel method has been used to achieve this

objective. Several sets of materials were prepared varying silica to alumina

ratios, apart from other conditions, where surface acidity has been used as an

indicative tool in all cases. Table 2.4 describes parameters that have been

optimized for synthesis of Al-MCM-41.

Synthesis of Al-MCM-41 at optimized condition

As indicated in optimization table 2.4 entry No-4 indicates optimum

conditions. We hereby describe synthesis of Al-MCM-41 under optimized

conditions. The molar gel composition of Al-MCM-41 is

1SiO2:0.033Al2O3:0.4CTABr:0.25Na2O:90H2O. In a typical 500g batch

experiment, the first step was preparation of the precursor solution. 57.48 g of

Na2SiO3was mixed with 197.5 g DDW under continuous stirring at room


73

temperature for ~15 min, in a polypropylene container, to which was added an

aqueous solution of aluminium sulfate (prepared by dissolving 5.643 g

aluminium sulfate in 40 g DDW) dropwise and with constant stirring within ~15

min. This is the precursor solution (A). An aqueous solution of CTABr was

prepared by dissolving 39.5 g CTABr in 160 g DDW under continuous stirring

at room temperature (B). Template solution (B) was added to precursor

solution (A), dropwise and under constant stirring within ~15 min. The pH of

the resultant solution was adjusted to ~10.5 using 1:1 H2SO4 (diluted 1:1V/V).

A gel was formed which was further stirred for 30 min. The polypropylene

container was now closed and allowed to age at room temperature without

stirring for 24 hrs. The resultant gel was filtered, washed with DDW to remove

adhering ions and dried at 120°C followed by calcination at 550°C for 6h, at a

heating rate of 2°C/min. After thermal treatment, the samples with various

Silica : Alumina ratios Al-MCM-41-20,Al-MCM-41-30,Al-MCM-41-50, and Al-

MCM-41-100 were subjected to ion exchange by treating them with aqueous

1.0%NH4NO3 solution under continuous stirring for 3h, followed by

calcination at 550◦C for 3h at a heating rate of 2°C/min. in air flow.As indicated

in Table 2.5 the sample Al-MCM-41-30 exhibits maximum surface acidity and

was used for all further studies.

Note: In the present synthetic endeavor Na2SiO3 as a silica source is

preferred to TEOS, reasons being higher cost of TEOS, thermal stability of

final material obtained is better, and excess negative charge developed in the

framework due to substitution of Si4+ by Al3+, requires Na+ for charge balance.

While preparation, it is preferable to add template to precursor source. The pH

of template being almost neutral and pH of precursor being >12, if template is

added to precursor source, the pH variation window is narrowed down due to

which pH of gel formation is easily adjusted. The pH in the synthesis was

adjusted to 10.5 because gel viscosity is maximum at this pH, which can also

be stirred with ease for homogenization.

Further, lower the SiO2:Al2O3 ratio, higher will be Al3+ content and

hence better is the acidity generated in the resulting materials. At the

optimized condition surface acidity is high as well as surface area retention

between 550 - 900 oC is fairly good.


74

Table 2.3MCM-41 Synthesis Strategies - Parameters optimized

Parameters No SiO2

source Template

source Template

mole H2O mole

Temp. (°C)

Aging Time (h)

pH

BET Surface area at different Temperature

( m2/g)

550°C 700°C 900°C

Aging time

1 Na2SiO3 CTABr 0.25 80 RT 1 10.5 640 490 370

2 Na2SiO3 CTABr 0.25 80 RT 3 10.5 770 510 342

3 Na2SiO3 CTABr 0.25 80 RT 6 10.5 820 550 446

4 Na2SiO3 CTABr 0.25 80 RT 18 10.5 900 570 428

5 Na2SiO3 CTABr 0.25 80 RT 24 10.5 917 612 433

Temperature 6 Na2SiO3 CTABr 0.25 80 70 24 10.5 864 735 460

7 Na2SiO3 CTABr 0.25 80 100 24 10.5 974 820 300

Template mole 8 Na2SiO3 CTABr 0.5 80 RT 24 10.5 1136 1121 805

9 TEOS CTABr 0.5 40 RT 24 9.5 1126 1116 706

Template source 10 Na2SiO3 CPBr 0.5 80 RT 24 10.5 736 593 483 SiO2 mole = 1 mole; RT= Room Temperature (30±3 °C); % SiO2 = 99.89 (ICP-AES).

Table 2.4Al-MCM-41 Synthesis Strategies - Parameters optimized

Parameters No SiO2 mole

Al2O3 mole

SiO2 /Al2O3 Input mole ratio

Template mole

Element Analysis (ICP-AES)

SiO2 /Al2O3 Output mole ratio

*Total Acidity (ml/g)

BET Surface area at different Temperature

(m2/g)

%SiO2 %Al2O3 550°C 700°C 900°C

Template mole

1 1 0.01 100 0.5 98.59 1.35 124.15 0.86 1191 1028 554

2 1 0.01 100 0.4 98.22 1.55 107.70 1.14 1220 1044 577

SiO2/Al2O3 Mole ratio

3 1 0.02 50 0.4 93.31 3.05 52.01 1.74 884 730 600

4 1 0.033 30 0.4 94.90 4.90 32.92 3.03 580 510 470

5 1 0.05 20 0.4 89.42 6.64 22.89 2.60 500 438 280 SiO2 Source = Na2SiO3 ; Al2O3 source = Al2(SO4)3 ; Template Source = CTABr; H2O mole = 90; RT = (30±3 °C); pH = 10.5; Aging Time = 24 h

*Details in Table 2.5.


75

Synthesis of 12-TPA supported MCM-41

The aim was to load different wt. % of 12-TPA onto MCM-41 and

induce acidity into the material, using surface acidity as an indicative tool in all

cases. Four samples of 12-TPA supported MCM-41 catalysts were prepared

with varying 12-TPA loading (10-40 wt.%). In a typical setup (10% 12-TPA

loading) 1g of 12-TPA was dissolved in 100 ml DDW, to which was added 9 g

of MCM-41 synthesized as described earlier in the text, and the resultant

slurry stirred continuously for 24h at room temperature. The excess solution

was removed under vacuum, dried and subsequently calcined at 350 ◦C for 2h

at a heating rate of 2◦C/min. Table 2.6 shows that 20%12-TPA loaded sample

exhibits maximum surface acidity and used for all studies.

2.11 MATERIAL CHARACTERIZATION

The synthesized materials in the present study MCM-41, Al-MCM-41

and 12-TPA supported onto MCM-41 were subjected toinstrumental methods

of analysis/characterization. Al-MCM-41 with SiO2:Al2O3 = 30, abbreviated as

Al-MCM-41-30 and 12-TPA supported onto MCM-41 with 12-TPA loading =

20 wt.%, abbreviated as 12TPA-MCM-41-20 have been used for

characterization, as they exhibit highest surface acidity and used for all

catalytic studies.

Instrumental Methods of Analysis

Elemental analysis was performed on ICP-AES spectrometer (Thermo

Scientific iCAP 6000 series).X-ray diffractogram(2= 1° - 40°)was obtained on

X-ray diffractometer (Bruker D8 Focus) with Cu-Kα radiation with nickel

filter.FTIR spectra was recorded using KBr pellet on Shimadzu (Model

8400S). Thermal analysis (TGA) was performed on a Shimadzu (Model TGA

50) thermal analyzer at a heating rate of 10 ºC·min-1. SEM and EDX of the

sample were scanned on Jeol JSM-5610-SLV scanning electron microscope.

TEM was performed using Philips CM30 ST electron microscope operated at

300kv. Surface area measurements weredetermined using Micromeritics

Gemini at -196oC using nitrogen adsorption isotherms. Surface acidity was

determined by NH3-TPD method using Micromeritics Chemisorb 2720. UV-

VIS.-diffuse reflectance spectra was obtained using Shimadzu(Model UV-

DRS 2450).


76

Characterization of MCM-41

The XRD of MCM-41 is presented in (Fig. 2.14). A peak for 2θ between

2º and 3º is observed which is characteristic of the Bragg plane reflection

(100). This is evidence for MCM-41 structure [80,83,84,106].

A TEM image (Fig.2.15) shows hexagonal arrangement of uniform, ~3

nm sized pores indicative of MCM-41 structure.SEM (Fig.2.16) exhibits

irregular morphology indicating amorphous nature of material. Elemental

analysis performed by ICP-AES shows %SiO2 to be 99.89 (Table 2.3), which

is also well supported by EDX (Fig.2.17) which shows atomic % of Si = 33.33

and atomic % of O = 66.67.

Surface areas (ABET) determined by N2adsorption BET method, exhibits

isotherms of type IV, in accordance with the IUPAC classification for

mesoporous materials [94] (Fig.2.18 and Fig.2.19). Pore diameters (~3 nm)

confirm the mesoporous nature of the synthesized materials with pore size

distribution between 2-6 nm which are in the range usually observed for

MCM-41 samples [110]. Surface area of MCM-41 calculated by BET method

is 1136 m2/gat 550 ◦C,1121m2/g at 700◦C and805 m2/gat900°C.

The FTIR spectrum presented in Fig. 2.20 exhibits presence of broad

bands in the region ~3400 cm-1 assigned to –OH stretching vibration of MCM-

41 which could be associated to Si-OH and water vibrations. Bands ~1630

cm-1are attributed to H-O-H bending vibration. A broad band ~1260-1000 cm-

1is assigned Si-O-Si asymmetric stretching mode. Bands at 800 cm-1and 450

cm-1are attributed to symmetric stretching vibration and bending vibration

(rocking mode) of Si-O-Si. The band at 960 cm-1is assigned to the presence of

Si-O-H stretching vibration.

TGA thermogram (Fig. 2.21) exhibits an initial weight loss of ~10%in

the temperature range 30-100 ◦C due to loss of moisture and hydrated water.

Thereafter in the region 200-600◦C there is a marginal/negligible weight loss

which indicates fairly stable nature of the materials.

Absence of sharp desorption bands as well as negligible acidity in NH3-

TPD profiles (Fig.2.22) is indicative of a neutral MCM-41 framework.


77

Fig. 2.14 XRD of MCM-41

Fig. 2.15TEM of MCM-41

Fig. 2.16SEM of MCM-41

Fig. 2.17EDX of MCM-41

Fig. 2.18Nitrogen adsorption isotherm of MCM-

41

Fig. 2.19Pore distribution of MCM-41

Element Wt.% At. % Comp. %

Si 46.74 33.33 100.00

O 53.26 66.67

Totals 100

Formula SiO2


78

Fig. 2.20FTIR of MCM-41

Fig. 2.21 TGA of MCM-41

Fig. 2.22Ammonia TPD of MCM-41

Characterization of Al-MCM-41-30

XRD of Al-MCM-41 is presented in Fig. 2.23. It is observed that there is

shift in position of the peak characteristic of the Bragg plane reflection (100)

for 2θ between 2º and 3º positionindicating incorporation of Al3+ in the

framework of siliceous MCM-41. Further, with increase in SiO2:Al2O3 ratio

there is further shift in band position towards higher 2θ value.

TEM image of Al-MCM-41-30 (Fig.2.24) exhibits a well-defined

hexagonal arrangement with a fairly uniform pore structure. SEM (Fig.2.25) of

Al-MCM-41-30 exhibits irregular morphology. Elemental analysis performed

by ICP-AES shows %SiO2and %Al2O3to be 94.90 and 4.90 respectively(Table


79

2.4), which is also well supported by EDX (Fig.2.26) which shows atomic % of

Al = 1.73, atomic % of Si = 31.89 and atomic % of O = 66.38.

Surface areas (ABET) determined by N2adsorption BET method exhibits


mesoporous materials [94] (Fig.2.27 and 2.28). Pore diameters (~3.5 nm)

confirm the mesoporous nature of the synthesized material with pore size

distribution between 2.2-5 nm which are in the range usually observed for Al-

MCM-41 [110]. Surface area of Al-MCM-41-30 calculated by BET method is

580 m2/gat 550◦C,510 m2/g at 700◦C and 470 m2/gat900◦C (Table 2.3).

The FTIR spectrum presented in Fig. 2.29 exhibits of broad bands in

the region ~3400 cm-1 assigned to –OH stretching vibration of MCM-41 which

could be attributed to Si-OH and water vibrations. Bands ~1650 cm-1are

attributed to H-O-H bending vibration. A broad band ~1260-1000 cm-1is

assigned Si-O-Si asymmetric stretching mode. Bands at 795 cm-1and 450 cm-

1are attributed to symmetric stretching vibration and bending vibration (rocking

mode) of Si-O-Si. The band at 960 cm-1is assigned to the presence of Si-O-Al

stretching vibration.

TGA thermogram (Fig. 2.30) exhibits an initial weight loss of ~20%in

the temperature range 30-100 ◦C due to loss of moisture and hydrated water.

Thereafter in the region 200-600◦C there is a marginal/negligible weight loss

which indicates fairly stable nature of the material.

NH3-TPD patterns of Al-MCM-41 with varying SiO2:Al2O3ratios are

presented in Fig.2.31. Al-MCM-41-30 exhibits highest surface acidity (table-

2.5). Surface acidity of samples is presented in Table 2.5.

Fig. 2.32 presents UV-VIS-DRS of Al-MCM-41 with varying SiO2:Al2O3

ratio. Rise in intensity of band ~ 210 nm region is observed with increasing

Al3+ incorporation. The incorporation of Al3+ in the silica framework is further

evident from surface acidity values and elemental analysis results (Table 2.5).


80

Fig.2.23 XRD of Al-MCM-41-30

Fig. 2.24TEM of Al-MCM-41-30

Fig.2.25 SEM of Al-MCM-41-30

Fig. 2.26EDX of Al-MCM-41-30

Fig.2.27Nitrogen adsorption isotherm of Al-

MCM-41-30

Fig.2.28 Pore distribution of Al-MCM-41-30


Al 2.33 1.73 4.41

Si 44.69 31.89 95.59

O 52.98 66.38

Totals 100

Formula SiO2 and Al2O3


81

Fig.2.29FTIR of Al-MCM-41-30

Fig.2.30 TGA of Al-MCM-41-30

Fig 2.31 Ammonia TPD of Al-MCM-41

Fig 2.32UV-DRS of Al-MCM-41

Characterization of 12-TPA-MCM-41

FTIR spectra of HPA supported onto MCM-41 have been detailed

earlier in the text. The FTIR spectra (Fig. 2.33)with different 12-TPA loading

coincide well with those observed in literature [105,106,108].The surface

acidity increases gradually with increasing 12-TPA loading from 10 to 20

wt.%, after which a decrease is observed (Table 2.6 ).

SEM image of 12TPA-MCM-41-20 (Fig.2.34) exhibits irregular

morphology. EDX of 12TPA-MCM-41-20, presented in Fig.2.35 shows the

presence of W as atomic % = 2.20, Si atomic % = 30.44 and O atomic % =

67.40. Absence of P in EDX is probably due very low % of P in the original

compound, which is probably not detected in EDX due to instrument

limitations.


82

Fig. 2.33 FT-IR of 12-TPA-MCM-41

Fig. 2.34SEM of12-TPA-MCM-41-20

Fig. 2.35EDX of12-TPA-MCM-41-20

Fig. 2.36Ammonia TPD of 12-TPA-MCM-41

Fig. 2.37XRD of 12-TPA-MCM-41

A=MCM 41, b=10% TPA-MCM-41 c=20%, d=30%, e=40% , f=12-TPA


W 17.30 2.20 21.81

Si 36.55 30.44 78.19

O 46.15 67.40

Totals 100

Formula SiO2 and WO3


83

Comparative TPD pattern of MCM 41 samples with different 12-TPA

loading (Fig.2.36) exhibits two distinct peaks at 200oC and 600oC indicating

presence of medium and strong acid sites respectively in all the

samples.Surface acidity of samples is presented in Table 2.6.

XRD of different wt% loading of 12-TPA-MCM-41 are presented in Fig.

2.37. With reference to MCM-41 (a) and 12-TPA (f) b, c, d, e shows that 12-

TPA is loaded onto MCM-41. e with the highest loading shows that the 12-

TPA remains on the surface and the X-ray pattern is close towards (f). The

observed results coincide well with those observed in literature [105,106].

Table2.5.Surface acidity of Al-MCM-41 with various SiO2:Al2O3 ratios

Materials Pore

volume (cc/g)

Surface acidity (ml/g) Total

Acidity (ml/g)

Weak acid Strong acid

Temp. (0C)

Volume (ml/g)

Temp. (0C)

Volume (ml/g)

Al-MCM-41-20 0.44 203 1.59 287 1.02 2.60

Al-MCM-41-30 0.27 207 2.91 287 0.12 3.03

Al-MCM-41-50 0.60 204 1.65 304 0.09 1.74

Al-MCM-41-100 0.91 201 1.09 311 0.05 1.14

Table 2.6.Surface acidity of different wt.% 12TPA loaded onto MCM-41

Materials

Surface acidity (ml/g)

Total Acidity (ml/g)

Weak Acid Strong Acid

Temp. (°C)

Volume (ml/g)

Temp. (°C)

Volume (ml/g)

Siliceous MCM-41 171 0.15 470 0.06 0.21

10% 12-TPA-MCM-41 209 1.67 651 3.74 5.41

20% 12-TPA-MCM-41 201 1.60 662 5.78 7.39

30% 12-TPA-MCM-41 200 2.40 609 2.90 5.30

40% 12-TPA-MCM-41 188 2.50 620 2.50 5.00

2.12 APPLICATION OF Al-MCM-41 AND 12-TPA-MCM-41 AS

SOLID ACID CATALYSTS

In chapter I, the importance of Green Chemistry, 12 principles of Green

Chemistry, and how Green Chemistry goals can be achieved through

catalysis has been discussed in details. Further, solid acid catalyst as an


84

alternative approach to liquid acid catalyst and its advantages over liquid acid

catalyst and important materials used as solid acid catalysts for various

organic transformations has been discussed. Siliceous MCM-41 has

poor/negligible catalytic activity due to framework neutrality, however with

high thermal stability and surface area. Thus, the aim of the present work was

to induce catalytic properties into MCM-41 and encash the advantageous

properties such as thermal stability and surface area, and put the resulting

material to practical use. The answers to these questions are emergence of

inherent acidity in Al-MCM-41 and induced acidity in 12TPA-MCM-41. Theory,

synthesis and characterization of these materials are well described in the

preceding pages in the text of this chapter (Section 2.10 and 2.11). In the

present study the utility of Al-MCM-41 and 12TPA-MCM-41 as solid acid

catalysts using Esterification and Friedel-Crafts alkylation and acylation as

model reactions has been explored.

2.13 ESTERIFICATION

Esterification is a widely employed reaction in the organic process

industry. Esters fall under a very wide category, ranging from aliphatic to

aromatic with various substitutions and multifunctional groups,organic esters

being valuable intermediates in the chemical industry. Esters are mostly used

as plasticizers, solvents, perfumery and flavour chemicals, and also as

precursors to many pharmaceuticals, agrochemicals and fine chemicals.

Esters are carboxylic acid derivatives with the general formula R-

COOR’ (R,R’=H, alkyl or aryl). When a carboxylic acid is treated with a large

excess of an alcohol, in presence of an acid catalyst, an ester is formed. The

reaction is called acid catalyzed esterification or sometimes Fischer

esterification after the great German chemist Emil Fischer.The esterification

process introduced by E.Fischer and A. Speier(1895) consists in refluxing

acid and excess methanol or ethanol in presence of about 3% hydrogen

chloride. The reaction is represented as follows.

RCOOH + R'OHH+

RCOOR'+ H2O (2.3)

The conventional esterification is an equilibrium reaction. For the

stoichiometric mixture of acid and alcohol, equilibrium generally reaches


85

~68% [111] of conversion for the straight chain saturated alcohol. In order to

obtain maximum yields Le Chatlier’s principle, is followed and the reaction is

driven to the right hand side/forward direction, as follows:

Addition of one of the reactants in excess:The reaction usually reaches a

point of equilibrium at ~60% conversion, but in a small scale experiment a

conversion of 60-80% can be achieved by use of a large excess of either acid

or alcohol.

Removal of one of the products:Either ester or water formed is removed as

soon as it is formed. Generally, suitable organic solvents are employed to

remove the water formed during the reaction as a binary azeotrope or by

employment of dehydrating agent such as anhydrous magnesium sulfate or

molecular sieves [112].

Synthetic routes to monoesters and diesters

Monoesters are typically synthesized by [113,114] (1) Solvolytic

reactions (2) Condensation reactions (3) Free radical processes (4)

Miscellaneous processes. The synthesis of mono esters can be presented as

shown in equation 2.1.

Diesters are prepared in two stages (Scheme 2.2) [115] e.g. reaction

between phthalic anhydride with alcohol. The first stage is very rapid and can

be carried out in the absence of a catalyst. However, esterification of the

second carboxylic group is very slow and needs to be facilitated by an acid

catalyst, resulting in the formation of water as a byproduct. The reaction is an

equilibrium one and hence to facilitate it in the forward direction, the water

molecule must be removed by azeotrope formation. The current commercial

process is a batch method which is very efficient with respect to its feed

stocks. Conversion (based on phthalic anhydride) and selectivity can reach

99.2 and 99.8 %, respectively. To reach this high conversion, a 20 % excess

of alcohol is used. The excess is recovered after reaction by a steam stripping

process. Hardly any purification is required after synthesis because of the

high selectivity, usually only a decolorization is carried out. A typical byproduct

is the dialkyl ether formed by the condensation of two molecules of alcohol.


86

First step

+ ROH

O

O

OHeat

C

C

O

O

OR

OH

Second step

C

C

O

O

OR

OH

+ ROH Heat

Solvent, Solid acid

C

C

O

O

OR

OR

+ H2O

Phthalic anhydrideMonoester

Monoester Diester

R = 2-ethyl hexanol

Scheme 2.2Schematic presentation of diester formation

Problems / limitations in esterification

For the preparation of perfumery and flavour grade esters, only a few

of the above mentioned routes can be considered, due to the stringent

specifications of the final product. Normally, liquid phase catalysts such as

sulphuric acid, p-toluene sulfonic acid, methanesulfonic acid, hydrochloric

acid, phosphoric acid etc. have been used, that are cited as potential

environmentally hazardous chemicals that pose problems such as difficulty in

handling, causing acidic waste water, difficulty of catalyst recovery etc.

[116,117]. These catalysts are known to colour the product and cannot be

reused. As already mentioned, these liquid acids have several disadvantages.

Due to these problems, accompanied by the increasing environmental

awareness, there is a global effort to replace the conventional liquid acids by

suitable solid acids. The most widely employed and supposedly cleaner

production technique for such esters, involves the reaction of the appropriate

carboxylic acid with an alcohol using a heterogeneous catalyst such as solid

acid catalyst under reflux conditions, followed by separation of the ester by

distillation.


87

2.14 LITERATURE SURVEY IN THE CURRENT AREA OF

STUDY

Monoesters using various solid acid catalysts

Cation exchange resins Dowex 50W and Amberlite IR-120 [118,119]

have been used as solid acid catalysts in the esterification of acetic acid with

isobutanol. Esterification of acrylic and lactic acids with butanol using

Amberlyst-15 [120, 121] and lactic and salicylic acids with methanol using

Dowex 50W resin as solid acid catalysts [122,123] has been reported. Salmi

et al [124] have studied methyl acetate formation on new polyolefin supported

sulfonic acid catalysts. Meunier has reported esterification reactions using

Nafion as solid acid catalyst [125]. Kaolinite [126] as well as montmorillonite

[127] clay has been used as catalyst in the esterification of carboxylic acids.

Giovanni Sartori has written an excellent review on clay catalyst for

monoesterification reaction [128].Manohar et al [129] have reported

esterification of acetic acid and benzoic acid using ZrO2 and Mo-ZrO2 as solid

acid catalysts and found that Mo-ZrO2 exhibits better catalytic activity than

ZrO2.Vishwanathan et al.[130] have reported esterification by solid acid

catalysts including clays, zeolites, sulphated metal oxides and

heteropolyacids.Toor et al.[131] have reported kinetic study of esterification of

acetic acid with n-butanol and isobutanol catalyzed by ion exchange

resin.Silicotungstic acid supported zirconia is reported as an effective catalyst

for esterification reactions using formic, acetic, propionic, n-butyricacid and n-

butyl alcohol, isobutyl alcohol and sec-butyl alcohol [132]. Chu et al [133]

have reported the vapour phase synthesis of ethyl and butyl acetate by

immobilized dodecatungstosilicic acid on activated carbon. The rate of

esterification was found to be dependent on the partial pressure of the

reactants. Dupont et al [134] have reported heteropolyacids supported on

activated carbon as catalysts for the esterification of acrylic acid by butanol.

Deactivation of the catalyst was observed under flow conditions (from 43 to

32% conversion) and was due to the dissolution of the supported HPA in the

reaction medium (25 %). Timofeeva et al [135] have reported esterification of

acetic acid and n-butyl alcohol using Keggin and Dawson type HPAs and

found that the reaction rate depends on the acidity, as well as on the structure


88

and composition of HPAs. The Dawson type heteropoly acids exhibited higher

activity compared to the Keggin type HPAs. In the esterification reaction of

acetic acid and n-butyl alcohol, the catalytic activity of HPAs has a good

correlation with the dissociation constant of HPAs. 12-TPA supported on

hydrous zirconia was used as solid acid catalyst in esterification of primary

and secondary alcohols [136]. Sharath et al studied benzyl acetate formation

in the presence of zeolites and their ion exchanged forms. They reported

reasonably good yield with 100 % selectivity [137]. Ma et al have reported the

synthesis of ethyl, butyl and benzyl acetates with high yields using

zeolitecatalyst [138]. From our laboratory, TMA salts have been widely

investigated as solid acid catalysts for synthesis of monoesters such as ethyl

acetate (EA), propyl acetate (PA), butyl acetate (BA) and benzyl acetate

(BzA) [139-146].

Diestersusing various solid acid catalysts

Suter has reported a noncatalytic process for the manufacture of DOP,

at very high temperatures, at which autocatalysis occurs [147]. Bekkum and

Schwegler investigated the use of HPAs (homogeneous and carbon

supported) for DOP synthesis [148]. They obtained a superior activity at low

temperatures in both homogeneous and supported form. Yadav et al [149]

have reported the use of solid super acids (sulfated and HPA supported onto

oxides) for the synthesis of DOP. They have reported a selectivity > 99 % and

demonstrated that selection of optimum calcination temperature is a must for

the optimum yield. Yadav et al[150] also have reported esterification of maleic

acid with ethanol over cation-exchange resin catalysts.G Lu [151] also

investigated DOP synthesis over solid superacids SO42-/Ti-M-O (M = Al, Fe,

Sn). They obtained superior activity in case of SO42-/Ti-Al-Sn-O system and

found that acid strength, surface area and catalytic activity of the system is

affected by the preparation conditions. Ma et al [152] studied the synthesis of

DOP using ZSM-5 and HY zeolites. Z H Zhao [153] has also reported the use

of aluminophosphate and silicoaluminophosphate molecular sieves as solid

acid catalyst for the synthesis of DOP. Amini et al [115] have reported the use

of heteropoly acids for the production of DOP and DBP. DEM synthesis has

been reported by Reddy et al [154] using montmorillonite clay, but the yield is


89

low (41 %) and relatively high amount of catalyst (0.5 g) was used. In another

report, DEM was synthesized by Jiang et al [155] using the reaction of CO

with ClCH2COOC2H5. In this case high yield was observed but the reaction

was carried out at high pressure.Kolah et al[156] have reported esterification

of succinic acid with ethanol and also have reported esterification of tri-ethyl

citrate via mono and di-ethyl citrate catalyzed by macroporous Amberlyst-15

ion exchange resin. From our laboratory, TMA salts have been widely

investigated as solid acid catalysts for synthesis of diesters such as dioctyl

phthalate (DOP), dibutyl phthalate (DBP) anddiethyl malonate(DEM) [144-

146,157].

Monoesters and diesters using MCM-41 type materials

Jiang et al [158] have studied catalytic activity of mesoporous TiO2

solid super acid for esterification of iso-amyl alcohol and salicylic acid. Salmi

et al [159] have studied methyl acetate formation on polyolefin supported

sulfonic acid catalysts. Sugi et al [160] have reported 12-TPA supported onto

MCM-48 as an efficient catalyst for the esterification of long chain fatty acid

and alcohols in supercritical CO2.Yarmo et al[161] have reported 12-TPA

supported on MCM-41 for esterification of fatty acid under solvent free

condition. The workers have also reported synthesis, characterization and

catalytic performance of porous nafion resin/silica nanocomposites for

esterification of lauric acid and methanol [162].Nascimento et al.[163] have

reported catalytic esterification of oleic acid over SO42-/MCM-41

nanostructured materials.Helen et al[164] have reported use of mesoporous

silica supported diarylammonium catalysts for esterification of free fatty acid in

greases.Zhu et al.[165] have reported synthesis, characterization and

application of sulfated zirconia/hexagonal mesoporous silica (HMS) catalyst in

the esterification of gossypol.Pandurangan et al. [166] have reported vapour

phase esterification of butyric acid with 1-pentanol and tert-butylbenzene with

iso-propyl acetate over Al-MCM-41 mesoporous molecular sieves.Srinivas et

al. [167] have reported the kinetics of esterification of fatty acids over solid

acid catalysts including large pore zeolite-β (Hβ), micro-mesoporous Fe/Zn

double-metal cyanide (DMC) and mesoporous Al-MCM-41.Said et al[168]

have reported perspective catalytic performance of Brønsted acid sites during


90

esterification of acetic acid with ethyl alcohol over 12-TPA supported on silica.

Rhijn et al[169] have reported sulfonic acid functionalized ordered

mesoporous materials as catalysts for condensation and esterification

reactions.Zhang Yijun et al. [170] have reported synthesis, characterization

and catalytic application of HPA/MCM-48 in the esterification of methacrylic

acid with n-butyl alcohol.Lingaiah et al. [171] have reported 12-TPA with

varying contents on SnO2 as efficient solid acid catalysts for esterification of

free acids with methanol for the production of biodiesel.Valdeilson et al [172]

have studied esterification of acetic acid with alcohols using supported

niobium pentoxide on silica-alumina catalysts.These catalysts were found to

be highly stable and active in esterification reaction.Patel et al [173] have

reported synthesis and characterization of 12-TPA anchored to MCM-41 as

well as its use as environmentally benign catalyst for synthesis of succinate

and malonate Diesters.

Conclusions

As mentioned in the above quoted literature, esterification reactions have

been widely investigated using several solid acids such as sulfated metal

oxides, zeolites, ion exchange resins, HPA, metal oxides, pillared clays etc.

Though solid acid catalysts are emerging as alternatives to liquid acid

catalysts a literature survey reveals that there are several limitations e.g.

though sulfated metal oxide is a very good esterification catalyst, it gets easily

deactivated by losing the sulfate ions, thereby recycling of the catalyst is

restricted. In case of HPA, the separation is difficult and when supported on

carbon the activity decreases.Sulfonic acid based resin (Nafion-H) has also

been found to be unsatisfactory due to its low operating temperature. Hence,

new materials are continuously being synthesized and explored as solid acid

catalysts to overcome the above mentioned limitations.Thus, the endeavour –

“Global effort to replace conventional liquid acid catalysts by solid acid

catalysts” is on.

As already indicated earlier in the text siliceous MCM-41 has a

poor/negligible catalytic activity due to framework neutrality. Catalytic

properties have been induced into siliceous MCM-41 via incorporation of Al3+

into the MCM-41 lattice to result in a catalyst with inherent acidity. HPAs have


91

proved to be the alternative for traditional acid catalysts due to both strong

acidity and appropriate redox properties. The major disadvantage of HPAs, as

catalyst lies in their low thermal stability, low surface area (1-10m2/g) and

separation problems from reaction mixture due to its high solubility in polar

solvent. HPAs can be made eco-friendly, insoluble solid acids, with high

thermal stability and high surface area by supporting them onto suitable

supports. The support provides an opportunity for HPAs to be dispersed over

a large surface area which increases catalytic activity. Thus, catalytic

properties have been induced into MCM-41 by supporting a HPA onto MCM-

41 by process of anchoring and calcination to result in a catalyst with induced

acidity.

In the present endeavour 12TPA-MCM-41(Induced Acidity) and Al-

MCM-41(Inherent Acidity) have been used as solid acid catalysts using

esterification as a model reaction wherein monoesters such as Ethyl Acetate

(EA), Propyl acetate (PA), Butyl acetate (BA) and Benzyl acetate (BzA) and

diesters such as Dioctyl phthalate (DOP), Dibutyl phthalate (DBP) andDiethyl

malonate(DEM) have been synthesized.The catalytic activity of 12TPA-MCM-

41 and Al-MCM-41have been compared and correlated with surface

properties of the materials.

2.15EXPERIMENTAL

Materials: Acetic acid, ethanol, 1-propanol, 1-butanol, benzyl alcohol,

cyclohexane, toluene, xylene, phthalic anhydride, 2-ethyl 1-hexanol, malonic

acid were procured from Merck.

Catalyst Synthesis:The synthesis and characterization of 12-TPA-MCM-41

(with various wt.% of 12-TPA loading) and Al-MCM-41(with varying SiO2:Al2O3

ratios) have been discussed earlier in the text (Section 2.10 and 2.11). It is

observed that 12-TPA-MCM-41 with 20wt.% 12-TPA loading abbreviated as

12TPA-MCM-41-20 and Al-MCM-41 with SiO2:Al2O3 ratio 30 abbreviated as

Al-MCM-41-30 exhibit highest surface acidity and thus used for all catalytic

studies.

Synthesis of monoesters (EA, PA, BA and BzA):In a typical reaction, a 100

mL round bottomed flask equipped with a Dean and Stark apparatus,

attached to a reflux condenser was used and charged with acetic acid (0.05 -


92

0.10 M), alcohol (0.05 - 0.10 M), catalyst (0.10 - 0.20 g) and a suitable solvent

(15 mL). Cyclohexane was used as a solvent for the synthesis of ethyl

acetate and toluene for propyl acetate, butyl acetate and benzyl acetate. The

reactions were carried out varying several parameters such as amount of

catalyst, mole ratio of reactants, reaction time and these parameters

optimized. After completion of reaction, catalyst was separated by decantation

and reaction mixture was distilled to obtain the product.

Synthesis of diesters (DEM, DOP and DBP): The diesters were synthesized

in two steps. In the first step, equimolar proportion (0.025 mole) of acid and

alcohol (malonic acid and ethanol for DEM, phthalic anhydride and 2-ethyl 1-

hexanol for DOP, phthalic anhydride and 1-butanol for DBP) were taken in a

round bottomed flask and the reaction mixture stirred at ~80 °C for DEM,

~145 °C for DOP, and ~115 °C for DBP for about 10-15 min in absence of any

catalyst and solvent. The dicarboxylic acid and anhydride are completely

converted to the monoester, so that the acid concentration at this stage is

taken as the initial concentration. The obtained product (monoester) was then

subjected to esterification reaction by addition of a second mole (0.025 mole)

of respective alcohol, catalyst (0.10 - 0.20 g) and 15 mL solvent (toluene for

DEM and DBP and xylene for DOP). The reactions were carried out varying

several parameters such as amount of catalyst, mole ratio of reactants,

reaction time and these parameters optimized. In all cases the round

bottomed flask was fitted with Dean and Stark apparatus, with a condenser to

remove water formed during the reaction. After completion of reaction,

catalyst was separated by decantation and reaction mixture was distilled to

obtain the product.

Regeneration and recyclability of catalyst: During the course of the

reaction, many a time the catalyst colour changes. This is probably due to the

adsorption of reacting molecules coming onto the surface of the catalyst. After

separation of catalyst in reaction mixtureby decantation, it is first refluxed in

ethanol for 30 minutes, followed by drying at 120°C. This material was used

as recycled catalyst. This regeneration procedure was followed in subsequent

recycle reaction.


93

Calculation of % yield of esters: For both mono and diesters, % yields were

determined by titrating the reaction mixture with 0.1 M alcoholic KOH solution.

The yields of the esters were calculated using the formula, % yield = [(A - B) /

A] M 100, where A and B are acid values of the sample withdrawn before

and after reaction and M is mole ratio of acid: alcohol. The yield of ester

formed was also determined using GC.

2.16RESULTS AND DISCUSSION

Monoesterification

Equilibrium constants of the esterification reactions are low. As in any

equilibrium reaction, the reaction may be driven to the product side by

controlling the concentration of one of the reactants (Le Chatlier’s Principle).

When concentration of one of the reactant relative to the other is increased,

the reaction is driven to the product side. In order to obtain higher yield of

esters,Le Chatlier’s Principle has been followed. Solvents cyclohexane and

toluene have been employed to remove the water formed during the reaction

as a binary azeotrope. Monoesters EA, PA, BA and BzA were synthesized as

described in experimental section.

Firstly, reaction conditions were optimized using 12TPA-MCM-41-20 as

solid acid catalyst for BzA synthesis by varying such parameters as catalyst

amount, initial mole ratio of the reactant (alcohol to acid) and reaction time,

and the results obtained presented in Table 2.7 and a graphical presentation

(Fig. 2.38 and 2.39).

During optimization of reaction time, it is observed that the conversion

rate was very high initially, indicating that the reaction obeys first order

kinetics. As reaction time increases, percentage yield increases. However,

there is not much gain in product after 8 h. Hence, the optimum reaction time

was selected as 8 h.With increasing amount of the catalyst, the % yield

increased which is due to proportional increase in the number of active sites.

The influence of reactant ratio (alcohol: acid) was studied by increasing from

1:1.5 to 1.5:1. The yield can be increased by increasing the concentration of

either alcohol or acid. As observed from Table 2.7, the % yield of ester

increases with increase in mole ratio of acid while decreases with increasing

mole ratio of alcohol. This may be attributed to preferential adsorption of


94

alcohol on the catalyst which results in blocking of active sites. For economic

reasons also, the reactant that is usually less expensive of the two is taken in

excess. In the present study, acids were used in excess. The temperature

parameter has not been varied, due to the fact that the reaction temperature

is sensitive to boiling points of reactants as well as solvents used as

azeotrope.

At the optimum condition (mole ratio of reactants, alcohol:acid = 1:1.5,

amount of catalyst = 0.15 g, time = 8 h) mono esters EA, PA and BA have

been synthesized and the yields of esters are presented in Table 2.8. Further,

for comparative study, Al-MCM-41-30 is used as solid acid catalyst for

synthesis of mono esters BzA, EA, PA and BA at the above mentioned

optimum condition.The results are presented in Table 2.8.

It is observed that, the order of % yield of ester formedfor both

catalysts is BzA > EA > PA > BA. Though the yields in case of mono esters

using both catalysts are comparable, higher yields are observed in case of

12TPA-MCM-41-20 which could be attributed to higher surface acidity. Turn

over number (TON) reflects the effectiveness of a catalyst and this also

follows the order of ester formation.

Esterification of monoesters EA, PA and BA has been reported [153] in

absence of catalyst and exhibit poor yields. Therefore catalyst is a must for

these reactions. In case of BzA however, it is observed that with an excess of

acetic acid and in the absence of any catalyst the yield is as high as 90.6 %

which is attributed to auto catalysis. In another report [137] high yields of BzA

were obtained with small amount of the catalyst but the reaction time was

relatively high. In the present study, yields of BzA are ~95% for both catalysts

compared to above literature reports. Higher yields in case of benzyl acetate

could be attributed to an enhanced nucleophilicity due to presence of aromatic

ring in benzyl alcohol. The order of % yield of ester formed is EA > PA >

BAcould be explained due to increase in carbon chain length in the respective

alcohols used for ester formation. During condensation of these alcohols with

acetic acid probably steric effects are responsible for explaining decreasing

yields from EA through PA to BA.


95

% yields obtained in recycled catalyst and % decrease in yields in

subsequent cycles is presented in Table 2.8 and 2.9 respectively, and a

graphical presentation (Fig. 2.40). It is observed that in subsequent cycles

decrease in % yields is less for Al-MCM-41-30 compared to 12TPA-MCM-41-

20. This could probably be due to the leaching of 12-TPA from surface of

MCM-41. It is observed that the colour of the catalyst changes after each

catalytic run. This gives an indication that during the course of the reaction the

reacting molecules come onto the surface of the catalyst. Some of them enter

into reaction to give the product while a few of them get adsorbed on the

surface, which is marked by the change in the colour of the catalyst. The fact

that the reactant molecules are weakly adsorbed is evident from the catalyst

regaining its original colour, when treated with ethanol. The possibility of

molecules entering interstices cannot be ruled out. This is observed from the

fact that the yields go downafter every regeneration, leading to deactivation of

the catalyst.

Fig.2.38Reaction time variation for BzA synthesis

Fig.2.39Catalyst amount variation for BzA

synthesis

Fig.2.40Comparative catalyst performance in the formation of monoesters

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10

% Y

ield

Reaction Time (h)

40

50

60

70

80

90

100

0.05 0.1 0.15 0.2 0.25

% Y

ieild

Catalyst amount (g)

0 10 20 30 40 50 60 70 80 90

100

Fresh 1st Cycle

2nd Cycle

Fresh 1st Cycle

2nd Cycle

Fresh 1st Cycle

2nd Cycle

Fresh 1st Cycle

2nd Cycle

% Y

ield

BzA EA PA BA

12-TPA-MCM-41-20 Al-MCM-41-30


96

Table 2.7Optimization of reaction conditions for monoesters using 12-TPA-MCM-41-20.

Sr.

No.

Reactants with

their mole ratio Product

Catalyst

amount

(g)

Time

(h)

Temp.

(0C)

%Yield

12-TPA-MCM-

41-20

(A) Time variation

1 Bz + AA (1:1) BzA 0.1 1 115 31.54

3 Bz + AA (1:1) BzA 0.1 2 115 46.43

4 Bz + AA (1:1) BzA 0.1 3 115 55.35

5 Bz + AA (1:1) BzA 0.1 4 115 67.26

6 Bz + AA (1:1) BzA 0.1 5 115 69.04

7 Bz + AA (1:1) BzA 0.1 6 115 70.23

8 Bz + AA (1:1) BzA 0.1 7 115 71.43

9 Bz + AA (1:1) BzA 0.1 8 115 71.65

(B) Catalyst amount optimization

10 Bz + AA (1:1) BzA 0.05 8 115 63.49

11 Bz + AA (1:1) BzA 0.15 8 115 90.63

12 Bz + AA (1:1) BzA 0.2 8 115 91.41

(C) Mole ratio optimization

13 Bz + AA (1:1.5) BzA 0.15 8 115 95.23

14 Bz + AA (1.5:1) BzA 0.15 8 115 62.50

Bz= Benzyl alcohol; AA= Acetic acid; Entry No.13 is optimum condition.


97

Table 2.8% yields of monoesters using 12-TPA-MCM-41-20 and Al-MCM-41-30 at

optimized condition

Sr.

No Reactants Product

12TPA-MCM-41-20 Al-MCM-41-30

%Yield *TON %Yield *TON

1 Bz + AA BzA 95.23 34.12 93.55 34.50

2 E + AA EA 92.31 33.10 90.35 33.10

3 P + AA PA 90.23 33.00 88.25 33.56

B + AA BA 74.13 18.50 73.16 17.59

A Reusability of catalyst

4 Bz +AA1st cycle BzA 87.15 31.35 89.73 31.70

5 Bz + AA2nd cycle BzA 77.45 20.82 83.80 27.80

6 E + AA 1st cycle EA 87.10 31.89 85.20 29.20

7 E + AA 2nd cycle EA 78.65 21.40 79.15 22.00

8 P + AA 1st cycle PA 84.42 28.51 85.41 29.15

9 P + AA 2nd cycle PA 78.35 21.14 80.25 23.00

10 B + AA 1st cycle BA 69.00 17.50 69.45 15.45

11 Bz +AA1st cycle BA 61.48 12.00 64.32 14.55

Bz= Benzyl alcohol; AA= Acetic acid; E=Ethanol; P= 1-Propanol; B=1-Butanol; Mole ration of the reactants = 1.5:1 (Acid:Alcohol);Reaction Time = 8 h. Catalyst amount = 0.15 g; Reaction temperature 80˚C for EA and 115˚C for PA, BA and BzA; *TON = Turn over number, gram of ester formed per gram of catalyst.

Table.2.9% Decrease in yields of monoesters using regenerated catalysts in

subsequent cycles

Monoesters

% Decrease in yields

12TPA-MCM-41-20 Al-MCM-41-30

1st Cycle 2nd Cycle 1st Cycle 2nd Cycle

BzA 8 10 4 6

EA 5 9 5 6

PA 6 8 4 5

BA 4 8 3 4


98

Diesterification

Diesters DEM, DOP and DBP were synthesized as described in

experimental section

Firstly, reaction condition was optimized using 12TPA-MCM-41-20as

solid acid catalyst for DEM synthesis by varying such parameters as catalyst

amount, reaction time, temperature and mole ratio of the acid and alcohol.The

results obtained are presented in Table 2.10 and a graphical presentation

2.42 and 2.43.

Diesters are prepared in two stages (Scheme 2.2). The first stage is

very rapid and can be carried out in the absence of a catalyst. In second

stage with increasing amount of the catalyst, the % yield increases which is

due to proportional increase in the number of active sites. In diester formation

the acid/anhydride taken as substrate possesses two attacking sites

responsible for ester formation. Thus, only concentration of alcohol was varied

following Le Chatlier’s Principle in the present study. The influence of reactant

ratio (alcohol: acid) was studied increasing from 2:1 to 2.4:1. It is observed

that the % yield ofdiester was maximum with 2:1 mole ratio.As reaction time

increases, percentage yield increases. However, there is not much gain in

product after 8 h. Hence, the optimum reaction time was selected as 8 h.

At optimum condition (mole ratio of reactants, alcohol: diacid/anhydride

= 2: 1, amount of catalyst = 0.15 g, time = 8 h) diesters DOP and DBP have

been synthesized. Results are presented in Table 2.11. Further, for

comparative study, at optimized condition Al-MCM-41-30 is used as solid acid

catalyst for synthesis of diesters DEM, DOP and DBP. The results are

presented in Table 2.11.

From Table 2.11, it is observed that, there is no marginal difference in

the yields of DOP and DBP and order of the % yields of diesters formation is

DEM > DOP ≈ DBP, which is probably due to less steric hindrance felt by

incoming ethanol from monoethyl malonate formed in the first step in case of

DEM.

The mechanism of diester formation over solid acid catalyst is similar to

that of conventional mechanism involving the formation of protonated

dicarboxylic acid, using proton donated by the catalyst, followed by


99

nucleophilic attack of alcoholic group to yield the respective monoester. The

second carboxylic group present in monoester gets further esterified by the

same mechanism in a repeat reaction, which ultimately results in the diester

formation [154].

Fig.2.41Reaction time variation for DEM

synthesis

Fig.2.42Catalyst amount variation for DEM

synthesis

Fig. 2.43Comparative catalyst performance in the formation of diesters




decrease in % yields is less for Al-MCM-41-30 compared to 12TPA-MCM-41-

20. This is could probably be due to leaching of 12-TPA from surface of

catalyst. The change in colour of catalyst is also observed after each catalytic

run.The same explanation can be forwarded as discussed in case of

monoester formation.

0

10

20

30

40

50

60

0 2 4 6 8 10

% Y

ield

Reaction Time (h)

30

35

40

45

50

55

60

0 0.05 0.1 0.15 0.2

% Y

ield

Catalyst amount (g)

0

10

20

30

40

50

60

Fresh 1st Cycle

2nd Cycle

Fresh 1st Cycle

2nd Cycle

Fresh 1st Cycle

2nd Cycle

% Y

ield

DEM DOP DBP

12TPA-MCM-41-20 Al-MCM-41-30


100

Table 2.10Optimization of reaction conditions for DEM using 12TPA-MCM-41-20.

Sr.

No Reactants with

their mole ratio Product

Catalyst

amount

(g)

Time

(h)

Temp.

(0C)

%Yield

12-TPA-

MCM-41-20

A Time variation

1 E + MA (2:1) DEM 0.1 2 115 16.66

2 E + MA (2:1) DEM 0.1 4 115 41.66

3 E + MA (2:1) DEM 0.1 6 115 50.01

4 E + MA (2:1) DEM 0.1 8 115 53.33

B Catalyst amount variation

5 E + MA (2:1) DEM 0.05 8 115 44.21

6 E + MA (2:1) DEM 0.15 8 115 53.79

C Mole ratio variation

7 E + MA (2.2:1) DEM 0.15 8 115 44.44

8 E + MA (2.4:1) DEM 0.15 8 115 46.45

E= Ethanol, MA = Malonic Acid; Entry No. 6 is optimum condition.

Table 2.11% yield of diesters using 12TPA-MCM-41-20 and Al-MCM-41-30 at

optimized condition

Sr.

No Reactants Product

12TPA-MCM-41-20 Al-MCM-41-30

%Yield *TON %Yield *TON

1 E + MA DEM 53.79 20.18 48.40 23.79

2 PA + O DOP 53.36 19.89 42.32 16.25

3 PA + B DBP 52.92 19.00 43.05 18.00

A Reusability of catalyst

4 E + MA1st cycle DEM 48.41 18.80 45.10 18.08

5 E + MA2nd cycle DEM 42.01 14.41 35.54 13.75

6 PA + O1st cycle DOP 50.12 19.25 40.82 14.15

7 PA + O2nd cycle DOP 44.70 16.00 36.2 14.20

8 PA + B 1st cycle DBP 49.36 19.00 40.38 14.00

9 PA + B 2nd cycle DBP 44.00 15.86 34.58 12.60

PA = Phthalic anhydride; O = 2-ethyl-1-hexanol; B = 1-Butanol; E = Ethanol; MA = Malonic Acid; Mole ration of the reactants = 1:2 (Acid/Anhydride:Alcohol); Reaction Time = 8 h. Catalyst amount = 0.15 g; Reaction temperature 115˚C for DOP and 145˚C DEM and DOP; *TON = Turn over number, gram of ester formed per gram of catalyst.


101

Table 2.12% Decrease in yields of diesters using regenerated catalysts in

subsequent cycles

Monoesters


12TPA-MCM-41-20 Al-MCM-41-30


DEM 5 6 3 3

DOP 3 6 2 4

DBP 8 5 3 6

Conclusions

The work outlined herein reveals the promising use of 12TPA-MCM-41-

20 and Al-MCM-41-30 as solid acid catalysts in the synthesis of monoesters

and diesters, the advantages being operational simplicity, mild reaction

conditions and eco-friendly nature of catalyst. The monoesters and diesters

formed can be simply distilled over, there is no catalyst contamination in

products formed, no acid waste formation and products are colourless a

limitation in the conventional process. The catalysts can be regenerated and

reused. Since the reactions are driven by surface acidity of the catalyst, there

is scope of obtaining better/higher yield by synthesizing material with high

surface acidity by modifying synthesis procedure. Though yields of

monoesters are high, the diester yields are low however, with the only

advantage of the product having no colour contamination.

2.17 FRIEDEL-CRAFTS ACYLATION AND ALKYLATION

Charles Friedel and James Crafts in 1877 developed a set of reactions

popularly known today as Friedel-Crafts reactions, involving electrophilic

aromatic substitution of two types, acylation and alkylation.

Friedel-Crafts acylation

Friedel-Crafts acylation (Scheme 2.3) involves the reaction of an acyl

chloride or acid anhydride with aromatic compounds in presence of a strong

Lewis acid catalyst. Due to the electron-withdrawing effect of the carbonyl

group, the ketone product is always less reactive than the original molecule,

therefore multiple acylations do not occur, which is an advantage over the

alkylation reaction (described later in the text).Also, there are no

http://en.wikipedia.org/wiki/1877

http://en.wikipedia.org/wiki/Organic_reaction

http://en.wikipedia.org/wiki/Acylation

http://en.wikipedia.org/wiki/Alkylation

http://en.wikipedia.org/wiki/Acyl_chloride

http://en.wikipedia.org/wiki/Acyl_chloride

http://en.wikipedia.org/wiki/Lewis_acid

http://en.wikipedia.org/wiki/Carbonyl

http://en.wikipedia.org/wiki/Ketone


102

carbocationrearrangements, as the carbonium ion is stabilized by a

resonance structure in which the positive charge is on the oxygen, inhibiting

intra molecular reactions.

+ CH3COClAlCl3

+

COCH3

HCl

Scheme 2.3 Reaction scheme for Friedel Crafts acylation

Mechanism for Friedel Crafts Acylation:

As seen from Scheme 2.4, the first step consists of dissociation of a

chlorine atom to form an acyl cation. This is followed by nucleophilic attack of

the arene towards the acyl group. Finally, a chlorine atom reacts to form HCl,

and the AlCl3 catalyst is regenerated:

R

C

Cl

O

AlCl3

H COCH3

+COCH3

H COCH3

Al

Cl

Cl

Cl ClAlCl3

COCH3

+ + HCl

Scheme 2.4 Reaction mechanism for Friedel Crafts acylation

The viability of the Friedel-Crafts acylation depends on the stability of

the acyl chloride reagent. For example, in synthesis of benzaldehyde via the

Friedel-Crafts pathway using formyl chloride as an acylating agent, since

formyl chloride is too unstable to be isolated, formyl chloride has to be

http://en.wikipedia.org/wiki/Carbocation

http://en.wikipedia.org/wiki/Acyl

http://en.wikipedia.org/wiki/Benzaldehyde


103

synthesized in situ. This is accomplished via the Gattermann-Koch reaction,

accomplished by reacting benzene with carbon monoxide and hydrogen

chloride under high pressure, catalyzed by a mixture of aluminium chloride

and cuprous chloride.

Friedel–Crafts acylation of aromatic compounds and aromatic

heterocyclic compounds is a ubiquitous reaction in the production of aromatic

ketones, largely used as intermediates in the synthesis of pharmaceuticals,

naproxen, dextromethorphan, ibuprofen and dyes, fragrances, and

agrochemicals [174-179]. In particular, the synthesis of substituted

acetophenones employing acylation is an important step for the production of

a variety of precursors which find application in the production of

pharmaceuticals, paint additives, photo initiators, fragrances, plasticizers,

dyes and other commercial products [180-184].

Friedel-Crafts alkylation

Friedel-Crafts alkylation (Scheme 2.5) involves the alkylation of an

aromatic ring and an alkyl halide using a strong Lewis acid catalyst. With

anhydrous aluminium chloride as a catalyst, the alkyl group substitutes the

chloride ion.

+ R-Cl

R

AlCl3

+ HCl

Scheme 2.5 Reaction scheme for Friedel Crafts alkylation

Mechanism for Friedel Crafts Alkylation

Al

Cl

Cl

Cl Cl

R-Cl + AlCl3 R+ +AlCl4

-

R+HR

R

Scheme 2.6 Reaction mechanism for Friedel Crafts alkylation

http://en.wikipedia.org/wiki/Gattermann-Koch_reaction

http://en.wikipedia.org/wiki/Benzene

http://en.wikipedia.org/wiki/Carbon_monoxide

http://en.wikipedia.org/wiki/Hydrogen_chloride

http://en.wikipedia.org/wiki/Hydrogen_chloride

http://en.wikipedia.org/wiki/Aluminium_chloride

http://en.wikipedia.org/wiki/Cuprous_chloride

http://en.wikipedia.org/wiki/Aromatic_ring

http://en.wikipedia.org/wiki/Alkyl_halide


http://en.wikipedia.org/wiki/Ferric_chloride

http://en.wikipedia.org/wiki/Catalyst


104

As seen from Scheme 2.6, the first step consists of dissociation of a

chlorine atom to form an alkyl cation. This is followed by nucleophilic attack of

the arene towards the alkyl group. Finally, a chlorine atom reacts to form HCl,

and the AlCl3 catalyst is regenerated.

In this reaction, the product is more nucleophilic than the reactant due

to the electron donating effect of alkyl-chain, therefore, another hydrogen is

substituted with an alkyl-chain, which leads to overalkylation of the molecule.

Further, if the chlorine is not on a tertiary carbon, carbocationrearrangement

reaction occurs, attributed to the relative stability of the tertiary carbocation

over the secondary and primary carbocations.Steric hindrance can be

exploited to limit the number of alkylations, as in the tertiary butylation of 1,4-

dimethoxybenzene (Scheme 2.7)

Scheme 2.7 Reaction scheme for t-butylation of 1,4-dimethoxybenzene.

Scheme 2.8 Reaction scheme for Friedel Crafts alkylation using bromonium ion as

electrophile

Alkylations are not limited to alkyl halides. Friedel-Crafts alkylation is

possible with any carbocationic intermediate such as those derived from

alkenes and a protic acid or lewis acid, enones and epoxides. In one study,

the electrophile is a bromonium ion derived from an alkene and N-

http://en.wikipedia.org/wiki/Acyl

http://en.wikipedia.org/wiki/Nucleophile

http://en.wikipedia.org/wiki/Tertiary_carbon




http://en.wikipedia.org/wiki/Steric_hindrance


http://en.wikipedia.org/wiki/Alkene

http://en.wikipedia.org/wiki/Protic_acid


http://en.wikipedia.org/wiki/Enone

http://en.wikipedia.org/wiki/Epoxide

http://en.wikipedia.org/wiki/Bromonium_ion


105

bromosuccinimide(NBS). In this reaction samarium(III) triflate is believed to

activate the NBS halogen donor in halonium ion formation(Scheme 2.8).

The liquid phase benzylation of benzene and other aromatic

compounds by benzyl chloride is important for the production of

diphenylmethane and substituted diphenylmethanes which are industrially

important compounds used as pharmaceutical intermediates and fine

chemicals [185-189].

The use of acyl halides or anhydrides as acetylating agents and

soluble Lewis acids as catalysts is polluting, expensive and difficult to work

with. In normal practice, strong mineral acids, such as H2SO4, HF, or

supported Lewis-acid catalysts like anhydrous AlCl3/SiO2 and BF3/SiO2 are

used for such reactions. However, these Lewis acids are consumed in more

than stoichiometric amounts due to the formation of 1:1 molar adduct with

aromatic ketones and further, the subsequent separation of the product by

hydrolysis is cumbersome and generates a large amount of environmentally

hazardous and corrosive waste.

Friedel-Crafts alkylation reactions catalyzed by homogeneous Lewis

acid catalysts generally give complex reaction mixtures. The formation of

reactant (and product) catalyst complexes, the increased tendency of

alkylated products towards further alkylation and isomerization, coupled with

the long contact of the reactant with the catalyst, result in decreased product

selectivity.

As indicated earlier in the text owing to stringent and growing

environmental regulations worldwide, there is a global effort to replace the

conventional liquid acid catalysts by solid acids, which are less toxic, easily

regenerable from the product, easy to handle and reuseable. In this context,

the focus has been towards design of processes to replace homogeneous

Lewis acid catalysts with environmentally benign heterogeneous catalysts.

The acid sites in solid acids being milder than the conventional Lewis acids,

would also inhibit side reactions such as polyalkylation, isomerization,

transalkylation, dealkylation and polymerization that occur in traditional

procedure.There is, therefore, substantial interest to carry out alkylation

reactions with solid acid catalysts which decrease these side reactions.

http://en.wikipedia.org/wiki/N-Bromosuccinimide

http://en.wikipedia.org/wiki/Samarium(III)_triflate


106

2.18LITERATURE SURVEY IN THE CURRENT AREA OF

STUDY

Acylation/alkylation of aromatic compounds have been reported using

several solid acid catalysts in recent years. Kantam et al [189]have reported

Friedel–Crafts acylation of aromatics and heteroaromatics using micro

crystalline zeolites with different acid anhydrides.The micronized -zeolite

shows manifold activity over normal zeolite in acylation reactions of aromatics.

Deutsch et al [190] have reported acylation and benzoylation of various

aromatics on sulfated zirconiaand observed that the rate of acylation reactions

is dependent on the nature of the respective aromatic compound. The

application of sulfated zirconia as a catalyst for the acetylation of aromatics

was only successful in case of anisole amongst various aromatic compounds

used. Kaur et al[191] have reported Friedel–Crafts acylation of anisole and

toluene with acetic anhydride using HPA supported on silica as catalyst as

well as H- Zeolite. In contrast to anisole, the acylation of toluene with HPA is

far less efficient than that with H-The inhibited activity of HPA for toluene

could be attributed to preferential adsorption of acetic anhydride on the

catalyst. Beers et al [192] have reported use of dealuminated zeolites as

solid acid catalyst for acylation of anisole with octanoic acid and proposed a

structure–activity relation for the same. After dealumination, increased activity

and selectivity were found in the acylation of anisolewith octanoic acid.The

enhanced activity is suggested to resultfrom higher accessibility of the active

sites associated with framework-connected aluminum atoms.Bachiller-Baeza

et al[193] have studied and compared the behaviours of HPA catalysts

supported on a commercial silica and on a silica–zirconia mixed oxide for the

acylation of anisole with acetic anhydride.The yields of p-

methoxyacetophenone were highest for HPA/SiO2. Castro et al [194] have

reported a mechanistic overview on the acylation reactions of anisole

usingunsaturated organic acids as acylating agents and solid acids as

catalysts.The mechanism of acylation of anisole with unsaturated acids,

i.e. acrylic, crotonic and methylcrotonic acid,have been investigated using 12-

PTA, supported on SiO2 and in the form of cesium saltsas catalysts. Since


107

unsaturated acid can either alkylate and/or acylate the aromatic

compound, the influence of the catalyston the selectivity for these two

competing reactions was studied. Analysis of products obtained on the

acylation of aromaticcompounds with unsaturated acids shows that all the

catalysts are more active for acylation than alkylation. Secondaryproducts

coming from intermolecular reactions of the acylated product with anisole as

well as tertiary products coming fromits further decomposition and

recombination with another anisole molecule were observed. Heteropolyacids

supported onsilica were found to be more active and selective towards

acylation reactions than zeolites HY and H.Melero et al [195] have reported

Friedel Crafts acylation of aromatic compounds overarenesulfonic acid

containing mesostructured SBA-15 materials.Arenesulfonic acidcenters

anchored on the pore surface of a mesostructured SBA-15 material show

greater activity (normalised to the concentration of sulfonic groups) as

compared to other homogeneous and heterogeneous sulfonatedcatalysts and

even in solventless conditions. This high activity is accompanied with a

remarkable thermal stability of the acidcenters, without leaching of sulfur

species during the reaction.

Cardoso et al [196] have reported silica supported HPA catalyst for

acylation of anisole using acetic anhydride as acylating agent. High

conversions and very high p-selectivity were attained in the temperature

range of 61–110 ◦C. However, deactivation was observed due to strong

adsorption of the products. Ma et al [197] have reported Friedel-Crafts

acylation of anisole over Y-zeolite catalystwith alkanoic acids, anhydrides and

substituted benzoic acids.When carboxylic acids were used as acylating

agents, the activity ofthe Y zeolite increased with its Lewis acidity, showing

that the Lewis acid sites were more active than the Bronsted acid

sites.Further, the reaction mechanism was found to be similar to the

homogeneous catalysis, that is, the electrophilicintermediate formed from the

acylating agent over zeolite acid sites attacked the aromatic ring of anisole.

Gaare et al [198] have reported effect of lanthanum ion exchange and Si/Al

ratio of Y-zeolite on the Friedel-Crafts acylation of anisole by acetyl chloride

and aceticanhydride.For the rare-earth modified zeolites, the activity was


108

found to be dependent on the lanthanum content, and the yieldincreased with

the level of lanthanum, even up to 93% exchange. Dealunminated Y-zeolites

were also found to be very active,and an almost linear increase in the yield

with decreasing aluminium fraction was found attributed to theincreased

hydrophobicity of dealuminated zeolites.Heidekum et al [199] have reported

use of Nafion/Silica composite materials as solid acid catalysts for acylation

reactions and claimed that entrapping nanosized Nafion particles in a silica-

matrix, effectivelyenhances the accessibility of the acid sites in comparisonto

the original material, Nafion resin. Chaudhari et al [200] have reported AlClx-

grafted Si-MCM-41 prepared by reacting anhydrous AlCl3 with terminal Si–OH

groups as an active and a reusable (if not exposed to atmosphere)

mesoporous solid catalyst for the Friedel–Crafts benzylation and acylation

reactions. However, like anhydrous AlCl3, it is highly moisture sensitive and

loses its activity on exposure to moist atmosphere. The active species on the

catalyst are (–Si-O)nAlCl3-n (n = 1–3). Cseri et al [201] have reported alkylation

of benzene and toluene with benzyl chloride and benzyl alcohol over a series

of clays obtained by exchanging the original cations of K10 by Ti4+, Fe3+, Zr4+,

Cu2+, Zn2+, Ce3+, Cr3+ and Sn2+ cations. The acidity of these solids has been

determined by infra red spectrometry using pyridine as molecular probe. The

acidity of K1O clays can be changed to a great extent by cation exchange and

by the thermal treatments applied to the solids. The rate of alkylation is

roughlyrelated to acidity when the substrate is benzyl alcohol, but not when

benzyl chloride is used. In that case, the catalysts containingreducible cations

( Fe3+, Sn4+, Cu2+) exhibit high activities in spite of their low number of acid

sites. Bachari et al [202-204] have investigated benzylation of benzene and

substitutedbenzenes, employing benzyl chloride as the alkylating agent over

mesoporous silica with different Sn, Cu and Ga contents. The mechanism

involves aredox step at the reaction initiation. The large pores of the

mesoporous catalyst donot limit the size of the molecules that could be

reacted. Chaudhary et al [205] have investigated benzylation of benzene by

benzyl chloride to diphenyl methane over InCl3, GaCl3, FeCl3 and ZnCl2

supportedon commercial clays (viz. Montmorillonite-K10, Montmorillonite-KSF

and Kaolin) or on high silica mesoporous MCM-41. The redox function


109

created due to the impregnation of the clays or Si-MCM-41 by InCl3, GaCl3,

FeCl3 or ZnCl2seems to play a very important role in the benzylation process.

Kinetics of the benzene benzylation (using excess of benzene)over the

supported metal chloride catalysts has also been investigated and a plausible

reaction mechanism for thebenzylation over the supported metal chloride

catalysts is proposed. Silva et al [206] have evaluated catalytic activity of gel

and macroreticular ion-exchange resins (Lewatit and Amberlyst-15) for the

reaction of benzene with benzyl alcohol and benzyl chloride at 80°C in the

liquid phase.With benzylchloride, the monobenzylation product,

diphenylmethane, was obtained in low yield, both with the gel and

themacroreticular resins. Better results were obtained with benzyl alcohol as

benzylation agent and the most active resin was Amberlyst-15, the conversion

of benzyl alcohol being proportional to the concentration of acid sites on the

resin. Mantri et al [207] have investigated Friedel–Crafts alkylation of

aromatics with benzyl alcohol as alkylating agent over rare earth metal

triflates, Sc(OTf)3,Hf(OTf)4, La(OTf)3, and Yb(OTf)3 supported on MCM-

41.The catalytic activity of triflates, was enhanced after being loaded onto

MCM-41 due to increased dispersion, and gave the benzylated product in

high yield. The rate of the benzylation of benzene was accelerated by electron

donating groups and retarded by electronwithdrawing groups. Narender et al

[208] have studied benzylation of benzene and toluene with benzyl alcohol

over a series of zeolites and metal modified zeolites. A reaction mechanism

has been proposed for formation ofdiphenylmethane and benzyl ether. Benzyl

ether formationfrom benzyl alcohol is explained on the basis of

theintermolecular reaction pathway, involving Bronsted acidsites of the

zeolite. Bachari et al [209] have reported benzylation of benzene by benzyl

chloride to diphenylmethane over FeCl3, InCl3, GaCl3, ZnCl2, CuCl2 and NiCl2

supported on mesoporous SBA-15.Further it is claimed that the redox

property due to the impregnation of the SBA-15 by transition metal chloride,

seems to play a very important role in the benzene benzylation process. Zhou

et al [210] have reported silica-supportedpolytrifluoromethanesulfosiloxane

(SiO2–Si–SCF3) catalyzed Friedel–Craftsbenzylation of benzene and

substituted benzenes.It was found that SiO2–Si–SCF3could catalyze Friedel–


110

Crafts benzylation of benzene and substituted benzenes with benzyl alcohol

under relatively mild experimental conditions. Reactions are very clean and

water is the only by-product of the reaction. The yields amounted to 97–100%.

Vinu et al [211] have reported benzylation of benzene and other aromatics by

benzyl chlorideover mesoporous Al-SBA-15 catalysts. Okuhara et al [212]

have explained various HPAs, like H3[PW12O40] and H4[SiW12O40], were used

as solid acid catalysts for the alkylation of 1,3,5-trimethylbenzene with -

butyrolactone to form 4-(2,4,6-trimethylphenyl) butyric acid. The catalysts

could be reused. Sugi et al [213] have reported Friedel–Crafts benzylation of

aromatics with benzyl alcohols catalyzed by heteropoly acids such as 12-TPA

(H3PW12O40·xH2O)(HPW), molybdophosphoric acid (H3PMo12O40·xH2O)

(HPMo) and tungstosilicic acid (H4SiW12O40·xH2O) (HSiW) supported on

mesoporous silica such as MCM-41, FSM-16 and SBA-15 by the

impregnation method to enhance the catalytic activity of these solid acids by

their dispersion on the support with high surface area. They also have used

rare earth metal triflates supported on MCM-41 mesoporous silica. Donghao

et al[214] have successfully prepared mesoporous silica materials, SBA-15,

functionalized with strong (-SO3H), moderate (-PO3H2) and weak (-COOH)

acid groups and these mesoporous acid catalysts have been applied to the

alkylation of phenol with tert-butanol. Subramaniam et al [215] have reported

synthesis and characterization of HPA catalysts and their cesium salts,

catalysts have been evaluated for the alkylation of isobutane with 1-butene.

Angelis et al [216] have reported solid acid catalysts based on HPAs

supported on different oxides catalyze the alkylation of isobutane with n-

butenes to yield high-octane gasoline components. Ramos-Galvan et al [217]

have reported alkylation of benzene with propylene over 12-TPA supported on

MCM-41 and -48 type mesoporous materials. Chaudhariet al [218] have

reported highly active Si-MCM-41 supported Ga2O3 and In2O3catalysts for

Friedel-Crafts-type benzylation and acylation reactions in presence or

absence of moisture.Mohammed et al [219] have reported

benzylation of benzene over sulfated zirconia supported as MCM-41 using a

single source precursor.Kalabasi et al. have reported vapor phase alkylation

of toluene using various alcohols over H3PO4/MCM-41 catalyst: influence of

http://www.chemspider.com/236


111

reaction parameters on selectivity and conversion.Selvaraj at al [220] have

reported synthesis of 2-acetyl-6-methoxynaphthalene using mesoporous

SO42- /Al-MCM-41 molecular sieves. Murugesan et al [221] have reported

synthesis, characterization and catalytic activity of Al-MCM-41, Fe,Al-MCM-41

and Zn,Al-MCM-41 in the vapor phasealkylation and acylation of

ethylbenzene with ethyl acetate in the temperature range between 250 and

400 °C. Endud et al [222] have reported cubic aluminated mesoporous

materials, Al-MCM-48 as highly effective catalysts for Friedel-Crafts acylation

of 2-methoxynaphthalene and 2-acetyl-6-methoxynaphthalene. Iwamoto et al

[223] have reported Friedel-Crafts acylation of anisole with carboxylic

anhydrides of large molecular sizes on mesoporous silica MCM-41 catalyst.

Halligudi et al [224] have reported 12-TPA supported over zirconia in

mesoporous channels of MCM-41 as catalyst in veratrole acetylation.Liquid-

phase Friedel–Crafts alkylation and acylation reactions have been reported

using aluminosilicate MCM-41 [225-229].

Conclusions

As mentioned in the above quoted literature, Friedel-Crafts acylation

and alkylation reactions have been widely investigated using various solid

acids, however with several limitations such as,Friedel-Crafts acylation and

alkylation was successful only with certain substrates [191], leaching in case

of supported catalysts, generation of secondary products [194], catalyst

deactivation when exposed to moisture [200] etc. Many of the reactions using

solid acid catalysts were also not economically viable. Such catalysts must be

designed that are both economically viable and able to withstand industrial

conditions. Thus, our endeavour as mentioned earlier in the text– “Global

effort to replace conventional liquid acid catalysts by solid acid

catalysts”is on. In continuation we herein report the catalytic activity of

12TPA-MCM-41-20(Induced Acidity) and Al-MCM-41-30 (Inherent Acidity) as

solid acid catalysts using Friedel-Crafts acylation and alkylation as model

reactions wherein 4-methoxy acetophenone (4MA), 3,4-dimethoxy

acetophenone (3,4DMA) and p-benzyl toluene (PBT) have been synthesized

under solvent free conditions. The activity of both catalysts have been

compared and correlated with surface properties of the materials.


112

2.19 EXPERIMENTAL

Materials: Anisole, acetic anhydride, veratrole, benzyl chloride and toluene

were procured from Merck India.

Catalyst Synthesis:The synthesis and characterization of 12-TPA-MCM-41

(with various wt.% of 12-TPA loading) and Al-MCM-41(with varying SiO2:Al2O3

ratios) have been discussed earlier in the text (Section 2.10 and 2.11). It is

observed that 12-TPA-MCM-41 with 20wt.% 12-TPA loading abbreviated as

12TPA-MCM-41-20 and Al-MCM-41 with SiO2:Al2O3 ratio 30, abbreviated as

Al-MCM-41-30 exhibit highest surface acidity and thus used for all catalytic

studies.

Experimental setup for Friedel-Crafts acylation and alkylation:The

reactions were carried out in a two necked 50 ml round bottomed flask

equipped with a magnetic stirrer under heating in an oil bath. In a typical set

up, a mixture of anisole or veratrole (10 mmol) and acetic anhydride (15

mmol) for acylation and toluene (10 mmol) and benzyl chloride (15 mmol) for

alkylation, along with the catalyst (0.2 g) were taken in a round bottomed flask

and stirred at 110oC for three hours.In all the reactions the substrates were

used as solvents and hence the reaction temperature was kept according to

solvent used (reflux temperature) for all the studies. The reactions were

monitored by GC. After completion of reaction, the catalyst was separated by

decantation, and reaction mixture was distilled to obtain the products 4 MA,

3,4DMA and PBT, the boiling points being ~273oC,286oC and 300oC

respectively.

Regeneration and recyclability of catalyst: During the course of the

reaction, many a time the catalyst colour changes. This is probably due to the

adsorption of reacting molecules coming onto the surface of the catalyst. After

separation of catalyst in reaction mixtureby decantation, it is first refluxed in

ethanol for 30 minutes, followed by drying at 120°C. This material was used

as recycled catalyst. This regeneration procedure was followed in subsequent

recycle reaction.


113

2.20 RESULTS AND DISCUSSION

Firstly, reaction condition was optimized using 12TPA-MCM-41-20as

solid acid catalyst for 3,4DMA synthesis by varying such parameters as

catalyst amount, initial mole ratio of the reactants, reaction time and

temperature. The results obtained have been presented in Table 2.13 and a

graphical presentation (Fig. 2.44 to 2.47).

Friedel crafts acylation of veratrole with acetic anhydride, gave

selectively 3,4 DMA. It is observed that yield increases with reaction time until

equilibrium is reached within 4 h. For the same reaction time, yield increases

with increasing catalyst amount, since the number of active sites per gm of

substrate increases. The influence of reactant ratio (veratrole:acetic

anhydride) was studied increasing from 1:0.75 to 1:2. It is observed that the %

yield of3,4DMAwas maximum with 1:1.5 mole ratio. (Table 2.13)

It has been reported earlier that there is no significant effect of solvents

in the acylation of anisole and veratrole and best results were obtained when

aromatic ethers were used as self solvents [189]. In the present study

therefore anisole and veratrole (aromatic ethers) have been used both as

substrates and solvent and for this reason while optimizing reaction condition,

concentration of only the acylating agent was varied. Thus, the Green

Chemistry principle 5 which states that the “use of solvents should be

made unnecessary whenever possible and when used, innocuous” is

implemented.Further, the boiling point of solvent was taken as reaction

temperature. However, when reaction temperature is varied (100 ˚C and 120

˚C), there is no significant change in % yield. Therefore 70˚C is optimized as

reaction temperature for 3,4DMA synthesis.

At optimum condition (mole ratio of reactants, veratrole:acetic

anhydride = 1: 1.5, amount of catalyst = 0.2 g, time = 4 h) 4MA and PBT have

been synthesized. Acylation of anisole with acetic anhydride gave selectively

4-methoxy acetophenone (4 MA) and alkylation of toluene with benzyl

chloride gave selectively p-benzyl toluene (PBT). Results are presented in

Table 2.14. Further, for comparative study, at this optimized condition Al-

MCM-41-30 is used as solid acid catalyst for synthesis of 3,4DMA, 4MA and

PBT. The results are presented in Table 2.14.


114

When comparison is made between anisole and veratrole the product

yield and turn over number (TON) are higher for veratrole. The rate-

determining step of the Friedel-Crafts acylation is the formation of the

electrophilic intermediate. The presence of an additional electron donating

methoxy group in veratrole makes it a more active compound for electrophilic

substitution of acyl group in the para position than anisole due to an increased

electron density at para position and resultant increased susceptibility for

attack by the electrophile.

It is reported that the mechanism for Friedel-Crafts acylation and

alkylation over solid acid catalysts is the same as homogeneous Lewis acid

catalysts, [197,202,203]. The proposed mechanism for the acylation and

alkylation reaction on solid acid catalyst implies the formation of an adsorbed

species by interaction of the acylating/alkylating agent with a Brønsted acid

site [202,203] (acyl/alkyl cation). The Brønsted acid site generates an acyl

carbonium ion, which in turn affects the electrophilic substitution. A higher

density of acid sites increases number of acyl cations enhancing activity of the

reaction. Catalytic activity is a function of number as well as type of acid sites

present on the catalyst surface. The acylation and alkylation reactions are

thus driven by the surface acidity of the catalyst.Probably this is the reason

why 12TPA-MCM-41-20 gives higher yields compared to Al-MCM-41-30 due

to higher surface acidity observed in case of the former catalyst.In an earlier

study using TMA salts as solid acid catalyst in Friedel-Crafts acylation and

alkylation a probable mechanism [230,231] has been proposed by us, as

given in Scheme 2.9 and 2.10. The same mechanism is thought to be

operating in the present study.




decrease in % yields is less for Al-MCM-41 compared to 12TPA-MCM-41-20.

This could be due to the leaching of 12TPA from surface of MCM-41. It is

observed that the colour of the catalyst changes after each catalytic run.


115

+C+

O

CH3

OCH3H

H3CO

COCH3

H3CO

COCH3

CH3COO-H+

+ CH3COOH +H+

C

O

0H3C

C

O

CH3

CH3COO-H+

Catalyst

H+

+C+

O

CH3

+

-

Anisole

Acetic Anhydride

2-Methoxy Acetophenone (2-MA)

COCH3

OCH3

4-Methoxy Acetophenone (4-MA)

+

major product minor product

Catalyst

Catalyst

Catalyst

Scheme 2.9 Reaction mechanism of Friedel-Crafts acylation of anisole using solid acid

catalyst.

+

+Catalyst

H+

CH3

H3C

Cl-H+

Cl-H+

+ HCl + H+

CH2ClCH2

+

CH2+

H2C

H

H3C H2C

Catalyst

Catalyst

Catalyst

Scheme 2.10 Reaction mechanism of Friedel-Crafts alkylation of toluene using solid acid

catalyst.


116

This gives an indication that during the course of the reaction the

reacting molecules come onto the surface of the catalyst. Some of them enter

into reaction to give the product while a few of them get adsorbed on the

surface, which is marked by the change in the colour of the catalyst. The fact

that the reactant molecules are weakly adsorbed is evident from the catalyst

regaining its original colour, when treated with ethanol. The possibility of

molecules entering interstices cannot be ruled out. This is observed from the

fact that the yields go down by 3-6% after every regeneration, leading to

deactivation of the catalyst.The deactivation of the catalyst might be due to

the strongly adsorbed acetic acid and the product on the acid sites. It is

known that acetic acid is generally strongly absorbed on the acidic sites. The

above two reasons are responsible for decrease in % yield.

Fig.2.44 Reaction time variation

Fig. 2.45Catalyst amount variation

Fig. 2.46Reaction temperature variation

Fig. 2.47Catalyst amount variation

20

30

40

50

60

70

80

0 2 4 6 8

% Y

ield

Reaction Time (h)

72

72.5

73

73.5

74

74.5

75

60 80 100 120 140

% Y

ield

Temperature (˚C)

1:0.75 1:1 1:1.5 1:20

10

20

30

40

50

60

70

80

% Y

ield

Mole ratio if the reactants


117

Fig. 2.48Comparative catalyst performance in the Friedel-Crafts alkylation and acylation

Table 2.13Optimization of reaction conditions for Friedel Crafts acylation and

alkylation using 12TPA-MCM-41-20

No Reactants with mole ratio

Catalyst

Amount

(g)

Time

(h)

Temp.

(°C)

% Yield of

3,4DMA

A Time variation

1 V: AA (1:1.5) 0.10 1 70 47.46

2 V: AA (1:1.5) 0.10 2 70 53.69

3 V: AA (1:1.5) 0.10 3 70 55.69

4 V: AA (1:1.5) 0.10 4 70 61.55

5 V: AA (1:1.5) 0.10 5 70 60.85

6 V: AA (1:1.5) 0.10 6 70 61.35

B Catalyst amount variation

7 V: AA (1:1.5) 0.15 4 70 69.42

8 V: AA (1:1.5) 0.20 4 70 74.33

9 V: AA (1:1.5) 0.25 4 70 74.47

C Mole ratio variation

10 V: AA (1:0.75) 0.20 4 70 47.80

11 V: AA (1:1) 0.20 4 70 53.54

12 V: AA (1:2) 0.20 4 70 74.31

D Temperature variation

13 V: AA (1:1.5) 0.20 4 100 73.45

14 V: AA (1:1.5) 0.20 4 120 72.49

A = Anisole; AA = Acetic Anhydride; V = Veratrole; T = Toluene; BzCl= Benzyl Chloride

0

10

20

30

40

50

60

70

80

Fresh 1st Cycle 2nd Cycle



% Y

ield

3,4DMA 4MA PBT

12-TPA-MCM-41-20 Al-MCM-41-30


118

Table 2.14Friedel Crafts acylation and alkylation using12TPA-MCM-41-20and Al-

MCM-41-30 at optimized condition

No Reactants Product Temp. (C°)

12TPA-MCM-41-20 Al-MCM-41-30

% Yields *TON % Yields *TON

1 V: AA 3,4DMA 70 74.33 10.81 62.14 9.02

2 A: AA 4MA 70 59.39 9.00 59.21 9.00

3 T: BzCl PBT 110 72.02 9.82 58.95 8.92

A Catalyst reusability

1 V: AA 1st Cycle 3,4DMA 70 72.89 10.62 59.43 9.00

2 V: AA 2nd Cycle 3,4DMA 70 68.37 10.05 56.17 8.08

3 A: AA 1st Cycle 4MA 70 56.40 8.20 56.00 8.00

4 A: AA 2nd Cycle 4MA 70 51.63 7.10 52.71 7.52

5 T: BzCl1st Cycle PBT 110 66.15 10.00 54.35 7.40

6 T: BzCl 2nd Cycle PBT 110 61.74 9.45 47.89 6.50

A = Anisole; AA = Acetic Anhydride; V = Veratrole; T = Toluene; BzCl= Benzyl Chloride;

Catalyst amount = 0.20g; reaction time = 4 h. mole ratio of the reactants = 1:1.5

(Veratrole/Anisole/Toluene:acylating/alkylating agent); *TON = Turn over number, gram of

product formed per gram of catalyst.

Table 2.15% Decrease in yields of 3,4DMA, 4MA and PBT using recycled catalysts

in subsequent cycles

Product


12TPA-MCM-41-20 Al-MCM-41-30


3,4DMA 2 4 3 3

4MA 3 5 3 4

PBT 5 7 4 5

2.21 CONCLUSIONS

In the present study, Green Chemistry goals have been achieved by

using solid acid catalysts (replacing liquid acid catalysts used in conventional

reactions) and under solvent free conditions with high selectivity of the

products. The products formed can be simply distilled over, there is no

catalyst contamination in product and no acid waste formed. The catalyst can

be regenerated and reused. Further, since the studied reactions are driven by

surface acidity of the catalyst, there is scope to obtain higher/better yields by

synthesizing materials with higher surface acidity. Finally, the MCM-41 neutral

framework has been successfully modified and put to practical use.


119

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CCHHAAPPTTEERR 3 Synthesis and Characterization of Zr-MCM-41 and Ti-MCM-41 and their Application as Oxidation Catalysts. _________________________________________________________________________

Chapter 3. Zr-MCM-41 and Ti-MCM-41 as oxidation catalysts

131

3.1 INTRODUCTION As already discussed in Chapter II, since Si-MCM-41 possesses

poor/negligible catalytic activity, the properties were modified by inducing

acidity into the material via Al3+ substitution and anchoring of 12-TPA onto

MCM-41.The catalysts Al-MCM-41-30 and 12-TPA-MCM-41-20 with

maximum surface acidity were explored as solid acid catalysts using

esterification and Friedel-Crafts alkylation and acylation as model reactions.

Literature survey prompted us to incorporate Zr4+ and Ti4+ in the silica

framework and examine the redox properties created in the material by

carrying out catalytic test reactions.

3.2 LITERATURE SURVEY IN THE CURRENT AREA OF STUDY Recently, many efforts have been made worldwide by researchers for

the modification of siliceous silica MCM-41 mesoporous molecular sieve in

order to enhance its practicability, of which the incorporation of transition

metal ions into the siliceous MCM-41 framework for promoting redox

properties, thermal and hydrothermal stabilities has been found to be an

effective strategy. Consequently, the study of metal based mesoporous

composites has been a hot subject. Various transition metal ions such as Fe

[1], Nd, Cu, Mo [2] Co [3-5], Ni [6, 7], Al [8], Mn [9], V [1, 10], Ti [11-14], Zr

[15-17] and Ce [18] have been introduced into the framework of MCM-41

mesoporous molecular sieve to give it some redox sites, and these prepared

mesoporous materials exhibits remarkable catalytic performance.

Much attention has been focused towards Zirconium containing

mesoporous materials as potential candidates in catalytic applications, due to

their both moderate acidity and oxidizing capacity. The limitation of low

surface area (lower than 50m2/g) can be overcome by supporting Zirconia on

high surface oxides mainly silica. The resulting catalysts display strong acidity

and show satisfactory activity in a diversity of organic reactions (alcohol

dehydration, alkene isomerization, and cumene dealkylation) [19, 20].

To extend the Zirconia catalytic capabilities, to larger organic substrates,

the feasibility of incorporating Zr atoms into mesoporous silica has been

explored. Zr atoms have been introduced into the framework of ordered

(MCM-41 [15, 21-23] and MCM-48 [24]) and disordered (HMS) [16, 25, 26]


132

mesoporous silicas. These solids combine high surface area (usually higher

than 800m2/g) and size selectivity, but the synthetic procedures used severely

affect the catalytic performances of the resulting materials.

Since the reported synthesis of microporous TS-1 [27], titanium

substituted zeolites have been attracting great attention because of their

remarkable catalytic performance for selective oxidations of various organic

substrates [28-34]. However, they cannot effectively catalyze conversion of

bulky molecules, which have no access to the active sites located inside the

micropores (0.7 nm). Thus, attention has increasingly been directed towards

the study of metal-containing mesoporous M41S type molecular sieves with

large pores (20–100 Å diameter) suitable for the transformation of bulky

organic compounds [35-40]. Titanium-containing MCM-41 were reported for

the first time in 1994 [41, 42]. Corma and co-workers reported Ti-MCM-41 [42]

causing great interest on its structure and catalytic features. The discovery of

ordered mesoporous titanosilicates with wider pore sizes (2–10 nm) [41-49]

and mesoporous titanium-containing zeolite [50] offers an opportunity to use

titanosilicates as versatile catalysts in the oxidation of bulky reactant

molecules. The most important features of the ordered mesoporous

titanosilicates are the high surface area, which potentially allows an efficient

dispersion of active sites, and the large and uniform pore diameters in the

mesopore range, which favour the diffusion of bulky molecules. Ti-MCM-41

materials have been studied as catalysts for various reactions, such as the

epoxidation of olefins [42], unsaturated alcohols [51], plant oils [52] and the

oxidation of organic sulphides [53]. In the oxidation of small reactant

molecules, however, they show much lower catalytic activity than TS-1 and Ti-

beta, probably due to the lower hydrophobicity that promotes water

adsorption, which poisons the catalytically active centers [43]. Ti-MCM-41

also suffers from leaching of the titanium active species in the presence of

H2O, which causes the gradual deactivation of the catalyst. Therefore, an

enhancement of the hydrophobicity is considered important to improve the

activity of Ti-MCM-41 and retard the Ti leaching in liquid phase oxidation [54].

Literature reports that most of the mesoporous molecular sieves have

been synthesized by traditional hydrothermal methods. Usually, transition


133

metal doped MCM-41 materials are focused on the hydrothermal synthesis

under basic conditions, in which the desired metal precursors are added into

the mixture of silicate / surfactant / base / water [55]. The highly basic solution

(pH≥10) required for the synthesis of MCM-41 seems to limit the incorporation

of metal cation dopants. Under such conditions, the metal dopants tend to

form a second phase, because of incompatible condensation and precipitation

rates [56]. To avoid the problem, another doping method under acid

conditions was developed [57, 58]. In this route, large amount of mineral

acids, such as HCl, is often needed, which is not environment friendly. Wong

et al. attempted to replace HCl by salt, such as KCl, in the synthesis of Zr

doped mesoporous material, but only nonmeso structured precipitate was

obtained [56].

An important condition for having very active metal ion supported

catalysts, lies in site isolation. The metal ion must be isolated and well-

dispersed throughout the silica network, thus avoiding the formation of metal

oxide clusters [59]. In this sense, a major problem in the preparation of mixed

oxides from aqueous media by sol-gel related procedures is the unequal

hydrolysis and condensation rates of the metal-containing reagents.

Zirconium species usually hydrolyze faster than silicon precursors. In most

cases, this results in partial segregation of ZrO2 together with a decrease of

the Zr/Si molar ratio in the resulting mixed oxide with respect to the initial (gel)

composition. In such cases, the catalysts’ performance substantially depends

on their purity and chemical homogeneity, thus phase segregation must be

avoided.

Metal incorporated MCM-41, could be considered as mesoporous mixed

oxide materials, wherein metal oxides are highly dispersed and more active

sites can be available. Moreover, the mixed oxides generate some particular

properties, which are not possessed by the silica or transition metal oxide

alone. For example, it has been reported that zirconium incorporated

mesoporous silica exhibited strong acidity, although the acidities of both

zirconia and silica are weak [60-62].

In the present chapter, Zr4+ and Ti4+ have been incorporated into

mesoporous siliceous MCM-41 framework, varying SiO2:ZrO2 and SiO2:TiO2


134

ratio via sol-gel process. The materials have been characterized and the

catalytic property created in the material has been examined using

epoxidation as a model reaction wherein, allyl chloride is converted to

epichlorhydrin, an industrially important compound/reaction.

3.3 SYNTHESIS OF Zr-MCM-41 AND Ti-MCM-41 In the present endeavour, the main objective is to synthesize

mesoporous Zr-MCM-41 and Ti-MCM-41 by incorporating (isomorphous

substitution) maximum amount of Zr4+and Ti4+ at room temperature with good

thermal stability, high surface area as well as retention of surface area at high

temperature, along with creation of redox properties. A sol-gel method has

been used to achieve this objective. Several sets of materials were prepared

varying conditions in each case, using surface area as an indicative tool in all

cases. Table 3.1 and 3.2 describes the parameters that have been optimized

for synthesis of Zr-MCM-41 and Ti-MCM-41 respectively.

Materials Tetra ethyl-ortho silicate(TEOS) and analytical grade liquor ammonia

were obtained from Merck. Tetra butyl ortho titanate(TBOT) was procured

from Sigma Aldrich. Cetyl trimethyl ammonium bromide (CTABr), 20% tetra

propylammonium hydroxide (TPAOH), and zirconium oxychloride

(ZrOCl2.8H2O) were purchased from Loba chemicals. Double distilled water

(DDW) was used for all studies.

Synthesis of Zr-MCM-41 at optimized condition As indicated in optimization table-3.1 entry No-3 indicates optimum

conditions. We hereby describe synthesis of Zr-MCM-41 under optimized

conditions. The molar composition of Zr-MCM-41 is 1SiO2:0.2 ZrO2:0.6CTABr:

40H2O. In a typical 500g batch experiment the first step was preparation of

the precursor solution. TEOS 87.42 g was mixed with 100 g DDW under

continuous stirring at room temperature for ~15 min, in a polypropylene

container, to which was added an aqueous solution of ZrOCl2 (prepared by

dissolving 28.72 g in 60 g DDW) dropwise and with constant stirring within

~15 min. This is the precursor solution (A). An aqueous solution of CTABr was

prepared by dissolving 90.11 g CTABr in 133.74 g DDW under continuous

stirring at room temperature (B).Template solution B was added to precursor


135

solution A, dropwise and under constant stirring within ~15 min. The pH of the

resultant solution was adjusted to ~9.5 using 20% TPAOH. A gel was formed

which was further stirred for 30 min. The polypropylene container was now

closed and allowed to age at room temperature without stirring for 24 hrs. The

resultant gel was filtered, washed with DDW to remove adhering ions and

dried at 120°C followed by calcination at 550°C for 6h, at a heating rate of

2°C/min. This material was used for all further studies.

Note: In the present endeavour, TEOS as a silica source is preferred to

Na2SiO3, because Na2SiO3 causes hydrolysis and precipitation of Zirconium

component/source. Further, residual Na2O in the product may pose problems

for use of this material as selective oxidation catalyst. Entry no-3 does not

give maximum surface area, however it is important that maximum Zr4+ be

incorporated in the silicate matrix to give improved redox property. The pH in

the synthesis was adjusted to 9.5 because gel viscosity is maximum at this

pH, which can also be stirred with ease for homogenization.

Synthesis of Ti-MCM-41 at optimized condition As indicated in optimization table 3.2, entry no. 2 indicates optimum

conditions. We hereby describe synthesis of Ti-MCM-41 under optimized

conditions. The molar composition of Ti-MCM-41 is 1SiO2:0.033

TiO2:0.25CTABr: 40H2O. In a typical 500g batch experiment the first step was

preparation of the precursor solution. TEOS 103.25 g was mixed with 5.46 g

TBOT under continuous stirring at room temperature for ~15 min, in a

polypropylene container. This is the precursor solution (A). An aqueous

solution of CTABr was prepared by dissolving 44.36 g CTABr in 346.93 g

DDW under continuous stirring at room temperature (B). Template solution B

was added to precursor solution A, dropwise and under constant stirring

within ~15 min. The pH of the resultant solution was adjusted to~9.5 using

20% TPAOH. A gel was formed which was further stirred for 30 min. The

polypropylene container was now closed and allowed to age at room

temperature without stirring for 24 hrs. The resultant gel was filtered, washed

with DDW to remove adhering ions and dried at 120°C followed by calcination


136

Table 3.1 Zr-MCM-41Synthesis strategies-Parameters optimized:

Parameters No SiO2 Mole

ZrO2 Mole

SiO2/ZrO2 Input Mole ratio

Template Mole

Elemental Analysis (ICP-AES)

SiO2/ZrO2 output Mole

ratio

BET Surface area At different Temperature (m2/g)

%SiO2 %ZrO2 550°C 700°C 900°C

Template mole 1 1 0.2 5 0.25 68.10 39.98 4.50 550 536 464 2 1 0.2 5 0.4 69.00 30.70 4.60 550 526 417 3 1 0.2 5 0.6 70.01 29.51 4.86 832 700 372

SiO2/ZrO2 Mole ratio

4 1 0.05 20 0.6 89.07 10.80 16.90 841 743 731 5 1 0.025 40 0.6 92.80 5.45 34.90 905 849 748 6 1 0.016 60 0.6 96.31 3.75 52.64 948 918 857

SiO2 Source = TEOS ; ZrO2 source = ZrOCl2.8H2O ; Template Source = CTABr; H2O mole = 40; Temperature = (30±3 °C); pH = 9.5; Aging Time = 24 h Table 3.2 Ti-MCM-41Synthesis strategies-Parameters optimized:

Parameters No SiO2 mole

TiO2 mole

Template Mole

SiO2/TiO2 Input mole ratio

Element Analysis (ICP-AES)

SiO2/TiO2 Output mole

ratio

BET Surface area at different Temperature (m2/g)

%SiO2 %TiO2 550°C 700°C 900°C

SiO2/TiO2 mole ratio

8 1 0.05 0.25 20 86.51 5.95 24.86 1098 970 735 9 1 0.033 0.25 30 88.85 4.61 32.96 1617 1134 948

Template mole 10 1 0.033 0.4 30 89.13 4.55 33.50 1564 1095 912 SiO2 Source = TEOS ; TiO2 source = TBOT ; Template Source = CTABr; H2O mole = 40; Temperature = (30±3 °C); pH = 9.5; Aging Time = 24 h


137

at 550°C for 6h, at a heating rate of 2°C/min. This material was used for all

further studies.

Note: In the present synthetic endeavour TEOS as a silica source is preferred

to Na2SiO3, because Na2SiO3 causes hydrolysis and precipitation of Titanium

component/source. Further residual Na2O in the product may pose problems

for use of this material as selective oxidation catalyst. The pH in the synthesis

was adjusted to 9.5 because gel viscosity is maximum at this pH, which can

also be stirred with ease for homogenization. In the synthesis of mesoporous

Ti-MCM-41, the SiO2:TiO2 ratio was adjusted such that the resulting material

has high surface area with a maximum of Ti4+ incorporation to induce redox

properties and enhanced hydrophobicity in the material so as to avoid the

leaching of Ti4+ during use in an aqueous environment [54].

3.4 MATERIAL CHARACTERISATION: Instrumental Methods of Analysis:

Elemental analysis was performed on ICP-AES spectrometer (Thermo

Scientific iCAP 6000 series). X-ray diffractogram was obtained on X-ray

diffractometer (Bruker D8 Focus) with Cu-Kα radiation with nickel filter. FTIR

spectra was recorded using KBr pellet on Shimadzu (Model 8400S). Thermal

analysis (TGA) was carried out on a Shimadzu (Model TGA 50) thermal

analyzer at a heating rate of 10 ºC·min-1. SEM and EDX of the sample were

scanned on Jeol JSM-5610-SLV scanning electron microscope. TEM was

performed using Philips CM30 ST electron microscope operated at 300kv.

Surface area measurement was carried out on Micromeritics Gemini at -

196oC using nitrogen adsorption isotherms. UV-Visible-diffuse reflectance

spectra was obtained using UV-DRS, 2450 Shimadzu. Conversion of allyl

chloride to epichlorohydrin was determined by GC using Chemito ceres 800

plus equipped with flame-ionization detector (FID).

Characterisation of Zr-MCM-41: XRD of Zr-MCM-41 is presented in Fig. 3.1. A peak for 2θ between 2o

and 3o characteristic of the Bragg plane reflection (100) is observed, which is

sufficient evidence to indicate the incorporation of Zr4+ in the framework of

MCM-41. With increasing incorporation of Zr4+ the diffraction peak (100) of the


138

Zr-MCM-41 becomes broad and weak, accompanied by decrease in intensity

and long range ordering of Zr-MCM-41, which is in agreement with results

obtained from the literature [63, 64].

XRD high-angle pattern of Zr-MCM-41 is presented in Fig. 3.2. Bulk

ZrO2 phase exhibits characteristic peaks in the high-angle XRD pattern. In Zr-

MCM-41 synthesized in the present study, this peak is absent, which is further

evidence to incorporation of Zr4+ in the MCM-41 framework with good

dispersion and absence of ZrO2 cluster [65].

TEM image of Zr-MCM-41-5 (Fig. 3.3) shows hexagonal arrangement

of uniform pores in the sample. SEM image of Zr-MCM-41-5 (Fig. 3.4) exhibits

irregular morphology. Elemental analysis for Zr-MCM-41-5 performed by ICP-

AES shows % ZrO2 and % SiO2 to be 29.51 and 70.01 respectively (Table

3.1), which is also supported by EDX, which shows atomic % of Si = 27.56,

atomic % of Zr = 5.66 and atomic % of O= 66.78 (Fig. 3.5).

Surface area (ABET) determined by N2 adsorption BET method exhibits


mesoporous materials [66]. Pore diameter (~3.8 nm) confirms the

mesoporous nature of the synthesized material with pore size distribution

between ~2.5 – 8.0 nm, which is in range usually observed for Zr-MCM-41

(Fig. 3.6 and 3.7) [67]. Surface area of Zr-MCM-41 with various SiO2:ZrO2

ratio calculated by BET method are presented in Table 3.1.

The FTIR spectrum presented in Fig. 3.8 exhibits broad band in the

region ~3400 cm-1 assigned to –OH stretching vibration of mesoporous MCM-

41 structure. Band around ~ 1640 cm-1 is attributed to H-O-H bending

vibration. A broad band between 1250 cm-1 to 1050 cm-1 is attributed to

asymmetrical stretching of Si-O-Si. The band at ~960 cm-1 is attributed to Si-

O-Zr stretching. Bands around ~795 cm-1 and 450 cm-1 are attributed to

symmetric Si-O-Si stretching and bending respectively [68].

TGA thermogram of Zr-MCM-41-5 (Fig. 3.9) exhibits an initial weight

loss of ~ 14 % in the temperature range of 30-1500C due to loss of moisture

and hydrated water. Thereafter in the region 150-8000C, there is a

marginal/negligible weight loss in the material indicating good thermal stability

of the material.


139

UV-DRS spectrum of Zr-MCM-41 (with various SiO2:ZrO2 ratio) (Fig.

3.10) can offer additional information on the dispersion and environment of Zr

atoms in the MCM-41 framework. The intense absorption band ~ 210 nm is

attributed to a charge transfer transition from an oxygen atom to an isolated Zr

cation in a tetrahedral environment. Further, the absorbance of band at 210

nm increases with increasing ZrO2 content, indicating more isomorphous

substitution of Zr4+ [16,66].

Characterisation of Ti-MCM-41: XRD of Ti-MCM-41 is presented in Fig. 3.11. A peak for 2θ between 2o

and 3o characteristic of the Bragg plane reflection (100) is observed, which is

sufficient evidence to indicate the incorporation of Ti4+ in the framework of

MCM-41. With increasing incorporation of Ti4+ the diffraction peak (100) of the

Ti-MCM-41 becomes broad and weak, accompanied by decrease in intensity

and long range ordering of Ti-MCM-41, which is in agreement with results

obtained from the literature [54].

TEM image of Ti-MCM-41-30 (Fig. 3.12) shows hexagonal

arrangement of uniform pores in the sample. SEM image of Ti-MCM-41-30

(Fig. 3.13) exhibits irregular morphology. Elemental analysis for Ti-MCM-41-

30 performed by ICP-AES shows % TiO2 and % SiO2 to be 4.61 and 88.85

respectively (Table 3.2), which is also supported by EDX, which shows atomic

% of Si= 32.79, atomic % of Ti= 0.54 and atomic % of O= 66.67(Fig. 3.14).

Surface area (ABET) determined by N2 adsorption BET method exhibits


mesoporous materials [66]. Pore diameter (~3.2 nm) confirms the

mesoporous nature of the synthesized material with pore size distribution

between ~2.0 – 8.0 nm, which is in range usually observed for Ti-MCM-41-30

(Fig. 3.15 and 3.16). Surface area of Ti-MCM-41 with various SiO2:TiO2 ratios

calculated by BET method are presented in Table 3.2.

The FTIR spectrum presented in Fig. 3.17 exhibits broad band in the

region ~3426 cm-1 assigned to –OH stretching vibration of mesoporous MCM-

41 structure. Band around ~ 1640 cm-1 is attributed to H-O-H bending

vibration. A broad band between 1225 cm-1 to 1030 cm-1 is attributed to

asymmetrical stretching of Si-O-Si. The band at ~960 cm-1 is attributed to Si-

O

sy

lo

a

m

o

3

a

a

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Fig. 3

Chapter 3

hing. Band

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thermogram

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rial.

DRS spectr

ffer addition

e MCM-41

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tetrahedral

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of Ti4+ [70].

3.1 XRD of Z

3.3 TEM of Zr

3. Zr-MCM-

s around

tching and

m of Ti-MC

temperature

Thereafte

ight loss in

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ransfer tran

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.

Zr-MCM-41

r-MCM-41-5

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~795 cm-1

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CM-41-30 (F

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MCM-41-5

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Ti

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XRD)


141

Fig 3.5. EDX of Zr-MCM-41-5

Fig. 3.6 N2 Adsorption Isotherm of Zr-MCM-41-5

Fig.3.7 Pore size distribution of Zr-MCM-41-5

Fig.3.8 FTIR of Zr-MCM-41-5

Fig.3.9 TGA of Zr-MCM-41-5

Fig.3.10 UV-DRS of Zr-MCM-41

Element Wt.% At. % Comp. % Si 32.71 27.56 69.98 Zr 21.80 5.66 29.45 O 45.49 66.78 Totals 100 Formula SiO2 and ZrO2


142

Fig.3.11 XRD of Ti-MCM-41

Fig.3.12 TEM of Ti-MCM-41-30

Fig 3.13 SEM of Ti-MCM-41-30

Fig 3.14 EDX of Ti-MCM-41-30

Fig-3.15 N2 Asorption Isotherm of Ti-MCM-41-

30

Fig-3.16 Pore size distribution of Ti-MCM-41-30


Si 45.74 32.79 97.85 Ti 1.29 0.54 2.15 O 52.97 66.67 Totals 100 Formula SiO2 and TiO2

3In

c

u

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re

E

e

in

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a

Fig.3.

3.5 APPLIntroductio

Epoxid

ompounds.

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ynthesis. T

eactions an

Epoxides ar

poxy resins

Epichlo

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Chapter 3

17 FTIR of Ti

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This reactiv

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re widely us

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an extrem

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18 TGA of Ti-

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14

-MCM-41-30

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144

consumption of ECH is used to make epoxy resins. It is also used in the

production of epichlorohydrin elastomers, polyamide-epichlorohydrin resins,

water treatment chemicals, synthetic glycerol, polyols and glycidyl derivatives

[72].

The traditional epoxidation processes are, (i) non-catalytic process using

chlorine, (ii) co-epoxidation processes and (iii) catalytic processes based on

organic peroxides and peracids [73]. These processes are very capital-

intensive. The use of chlorine has environmental disadvantages due to the

large output of chlorine-laden sewage. The employment of peracids is not a

clean method as an equivalent amount of acid waste is produced.

Furthermore, the homogeneously catalytic processes usually suffer from the

difficulty of product separation and catalyst recovery. As to the co-epoxidation

processes, the coupling product should be an equivalently commercial

desired one [74]. Therefore, enantioselectivity of the epoxidation reaction

plays an important role, since absolute configuration is essential in the

synthesis of many industrially important products as mentioned above.

Hydrogen peroxide (H2O2) is an attractive option of oxidants that can

epoxidize olefinic compounds in the presence of various transition metal-

containing catalysts such as Ti, V, Cr and Mo etc [74]. As the oxidant, it does

not cause environmental contamination, and the reaction with its contribution

produces water, besides the major product under mild reaction conditions. An

additional advantage of the application of H2O2 in various chemical processes

is associated with its relatively low price. Due to good oxidative properties,

H2O2 has found applications in many processes, displacing other chemical

oxidants which are associated with the formation of wastes [75].

Literature survey in the current area of study: The first truly heterogeneous epoxidation catalyst was presented by

Shell workers in 1970 [76]. Impregnating a silica surface with a TiCl4 precursor

lead to the so called TiO2-on-SiO2 catalysts. The Ti-precursors were attached

by free silanol groups on the surface of the silica carrier. In this way, Ti (IV)

site isolation was achieved and Lewis acidity created by electron withdrawal

through the Si-O-ligands [77]. When using alkyl hydroperoxides as oxidants,

this supported oxide showed high activity. Today, this catalyst is still applied in


145

half of the worldwide propene oxide production performed by continuous liquid

phase epoxidation. However, when using H2O2, the hydrophilic catalyst is

rapidly deactivated by leaching of Ti.

An important milestone in the history of heterogeneous epoxidation

catalysts is the discovery of the crystalline microporous TS-1 by Taramasso et

al. [27]. This titanium-substituted molecular sieve was found to be an active

and selective oxidation catalyst using H2O2 as oxidant and due to its

hydrophobic surface, leaching of Ti could be prevented. Epoxidation takes

place at low temperatures and is carried out in a polar solvent.

Currently, new methods for the preparation of epoxides are being

developed while previous ones are being improved. Considerable effort is

directed towards the use of catalysts and reactants that are environment

friendly. Another desirable thing is to use heterogeneous catalysts because of

their easy separation, regeneration and operational simplicity. Further, the

epoxidation reaction using heterogeneous solid catalysts with H2O2 as an

oxidant are environment friendly routes to produce extensively useful

epoxides which are traditionally obtained from capital-intensive or

environmentally polluted processes. Therefore, development of

heterogeneous catalytic processes for epoxidation using H2O2 as an oxidant

is very demanding [74].

Zhou et. al. [74] have reported various types of solid catalysts for the

epoxidation of olefins with H2O2 as an oxidant. In their review, they have

reported: (i) microporus and mesoporous molecular sieves such as

framework-substituted zeolites, aluminium phosphate molecular sieves and

transition metal substituted mesoporous materials, (ii) layered type materials

such as hydrotalcites, tungstates and tungstic acid, (iii) inorganic oxides and

supported catalysts such as mixed oxides (Al2O3-ZrO2, Al2O3-TiO2, SiO2-TiO2

and ZrO2-TiO2 etc.), Re2O3 supported catalysts, Al2O3 supported catalysts,

(iv) porous materials encapsulated metal complexes, (v) polyoxometalates

and (vi) supported porphyrins.

All above mentioned catalysts have shown potential in olefin

epoxidation, sometimes depending on the reaction conditions. Among these,

the catalyst systems with W, Ti and Mo have much better prospect of

industrial application from the economical view point, although these catalysts


146

and reaction conditions should be further optimized [74]. However, the

catalytic performance of most catalysts still cannot satisfy all requirements of

commercialization. Further improvements should still be sought to meet the

need of industrial processes with H2O2 as an oxidant [74].

In recent years, catalytic systems which selectively activate hydrogen

peroxide are used which includes titanium silicalites such as: TS-1, TS-2, and

Ti-Beta that have replaced more dangerous acid catalysts (HF, HCl, H2SO4).

Their additional advantage is associated with frequent elimination of the

intermediate stages, which contributes to the reduction of the waste amounts.

As a result of heterogeneity, they can be easily separated from the reaction

environment and subjected to regeneration. One of the advantages is the

possibility of multiple regeneration and reduced corrosion of apparatus. The

application of titanium silicalite catalyst in alkene epoxidation eliminates the

formation of by-products, which are usually formed in the conventional

epoxidation processes. The titanium silicalite zeolites are used not only during

the epoxidation of alkenes [78-83], but also for the oxidation of alcohols to

aldehydes and ketones [84-86], for monoepoxidation of dienes [87],

stereoselective epoxidation of cis- and trans-isomers [88], and for the

oxidation of vinylbenzenes to corresponding aldehydes [89].

Since the application of titanium silicalite catalysts and hydrogen

peroxide in the synthesis processes causes an improvement in the selectivity

of transformation and the yields, extensive research has been generated in

this area [90-92]. Further, the processes can be carried out under mild

conditions, which results in the reduction of operating costs of the installations

[93].

Adam et al. [94, 95] have proposed the mechanism of allyl alcohol

epoxidation using H2O2 over titanium silicalite catalysts that assume the

formation of the active adduct A which is a five-membered hydroperoxide

adduct, in which both the H2O2 molecule and alcohol molecule (protic solvent)

are associated with the active centre (the Ti4+ ion). In this adduct there is also

a hydrogen bond between oxygen of the alcohol molecule and hydrogen of

the hydroperoxide group. ROH is an additional solvent molecule or the water

molecule, which is coordinatively bonded to the active centre [96]. In the

structure B the allyl alcohol molecule is associated with the active adduct A


147

(3.1)

(3.2)

through the hydrogen bond formed between the —OH group of allyl alcohol

and the oxygen atom located close to the Ti atom in the active adduct A. This

hydrogen bond stabilises the entire system. The presented mechanism

reveals that the nature of the solvent plays a very important role in the course

of epoxidation [96].

Cheng-Hua et al. [97] have reported that, the framework Ti is the active center

with tetrahedral coordination. In this process, H2O2 is first adsorbed on the

surface of the molecular sieves and interacts with the framework Ti to form

titanium peroxocomplex Ti-OOH species, an active intermediate, which can

carry out the surface reaction with the substrates to form the products.

According to them [97] the epoxidation reaction can be expressed as follows:

+ +(SiO3)Ti-OH H2O2 (SiO3)Ti-OOH H2O(fast)

CH

H2C Cl+ H2C

O

CHH2C Cl

Allyl chloride Epichlorohydrin

H2C(SiO3)Ti-OOH + (SiO3)Ti-OH(slow)

Scheme 3.1 Mechanism of allyl alcohol epoxidation over the titanium silicate catalyst [96].


148

Literature survey reveals that much of the researches are focused

towards Ti substituted molecular sieves. However, not much has been

reported on catalytic / redox behavoiur of Zr incorporated MCM-41.

Importance of epichlorohydrin prompted us to carry out epoxidation reaction

using Zr-MCM-41 and Ti-MCM-41 as heterogeneous solid catalysts in the

catalytic conversion of allyl chloride to epichlorohydrin.

3.6 EXPERIMENTAL Materials: Methanol, allyl chloride, tertiary butyl hydrogen peroxide (TBHP,

17.8% active oxygen), NaOCl (21.6% active oxygen) and hydrogen peroxide

50% (47.0% active oxygen) were obtained from Merck India. Sodium

thiosulfate (Na2S2O3) was obtained from Loba Chemicals.

Catalyst Synthesis: Synthesis and characterization of mesoporous Zr-MCM-

41 and Ti-MCM-41 has been discussed in detail in section 3.3. Experimental set up for conversion of allyl chloride to epichlorohydrin: In a typical reaction, 2.5 g catalyst (Zr-MCM-41-5), 10 mmol methanol (MT)

(as a solvent), 10 mmol allyl chloride (AC) and 10 mmol H2O2 (50%) were

taken in a 50 ml three necked round bottomed flask, equipped with a water

condenser and placed on a magnetic stirrer. The reaction mixture was stirred

at room temperature for 5h after which sample was withdrawn and analyzed

by GC. H2O2 concentration before and after reaction was determined by

iodometric titration (Eq.3.3). All reactions were carried out varying several

parameters such as reaction time, amount of catalyst used, catalysts used

with varying SiO2:ZrO2 and SiO2:TiO2 ratio in Zr-MCM-41 and Ti-MCM-41

samples respectively, and amount of oxidant used and these parameters

were optimized using Zr-MCM-41-5 (Table 3.3). Using these optimized

conditions, the activity of the other synthesized catalysts with different Zr and

Ti content has been studied. Calculation of % conversion based on H2O2 concentration

3.3

Where, A= Initial concentration of H2O2, B= Final concentration of H2O2 after

completion of reaction (unreacted H2O2), C= Amount of sample withdrawn, N=

Normality of Sodium thiosulfate (Na2S2O3).


149

GC chromatogram showing conversion of allyl chloride to ECH using

Zr-MCM-41-5 and Ti-MCM-41-30 have been presented in fig. 3.20 and 3.21

respectively.

Fig. 3.20 Gas chromatograph of allyl chloride conversion to ECH using Zr-MCM-41-5.

Fig. 3.21 Gas chromatograph of allyl chloride conversion to ECH using Ti-MCM-41-30.

Three peaks are observed in GC chromatogram corresponding to

solvent, allyl chloride and ECH. Since no other peaks are observed in the GC

chromatogram it is concluded that % selectivity for ECH formed is ~100 %.

Further, the % H2O2 consumed can be directly correlated to ECH formed.

Regeneration and recyclability of catalyst During the course of the reaction, many a time the catalyst colour

changes. This is probably due to the adsorption of reacting molecules coming

onto the surface of the catalyst. After separation of catalyst in reaction mixture

by decantation, it is first refluxed in ethanol for 30 minutes, followed by drying

at 120oC. This material was used as recycled catalyst. This regeneration

procedure was followed in subsequent recycle reaction.

3.7 RESULTS AND DISCUSSION The epoxidation reaction of allyl chloride using H2O2 as oxidant to give

ECH is presented as follows:

CH

CH3OH H2O2Catalyst R.T.

H2C Cl + + H2C

O

CHH2C Cl

Allyl chloride Epichlorohydrin

H2C

Scheme 3.2 Epoxidation of allyl chloride using H2O2.

Firstly, reaction conditions for conversion of allyl chloride to ECH were

optimized using Zr-MCM-41-5 catalyst and results presented in table 3.3. All


150

the reactions were carried out at room temperature, to avoid thermal

decomposition of hydrogen peroxide using methanol as a solvent. It is

observed that yield increases with reaction time until equilibrium is reached

within 5h (Fig. 3.20). For the same reaction time yield increases with

increasing catalyst amount, since the number of active sites per gm of

substrate increases(Fig. 3.21). In all cases selectively only ECH was formed

with selectivity ~99%, indicating no by-product formed.

Better catalytic activity of H2O2 as oxidant, compared to NaOCl and

TBHP can be explained due to, formation of active adduct with H2O2

compared to NaOCl and TBHP (Table 3.3, Entry No. 13 and 14) (Fig. 3.22).

When concentration of H2O2 was decreased (Table. 3.3, Entry No. 12) the %

yield decreases. The presence of methanol in the reaction medium causes

that methanol participates in the formation of active adduct [96]. This role is

performed by water in the presence of an aprotic solvent. However, the

electrophilic properties of the active adduct with the participation of water are

weaker than those with methanol as a solvent [96]. For this reason the

epoxidation in the aprotic solvent proceeds slowly. Therefore, protic solvent

methanol was used in the present study.

When mole ratio of SiO2/ZrO2 was increased from 5 to 20, 40 and 60

catalytic activity decreased which could be attributed to decrease in Zr4+

content in the silica framework. Zr-MCM-41-5 thus exhibits highest catalytic

activity (Table 3.3, Entry No. 10) (Fig. 3.23). Amongst Ti-MCM-41-20 and Ti-

MCM-41-30, the later is catalytically more active (Fig. 3.24).

Based on ESR studies, Chaudhary et al. [54] have reported that Ti4+

undergoes easy reduction compared to Zr4+ and thus higher catalytic activity

for Ti containing catalyst is expected compared to Zr containing catalyst. In

the present study, there is not much difference in % yield using Ti-MCM-41-30

compared to Zr-MCM-41-5 where, Zr4+ content is greater than Ti4+ content.

However, the average % yield of ECH calculated from entry no. 15 and 16

(Table 3.3), with SiO2: ZrO2 ratio 20 and 40 respectively, is ~37.69 %

compared to % yield of ECH from entry no. 19 (Table 3.3) with SiO2:TiO2 ratio

30 which is 39.45 % which is in line with observation made in literature [54],

indicating that the Ti4+ incorporated catalyst is catalytically more active

compared to Zr4+ incorporated catalyst. The mechanism involved in the


151

formation of ECH could be proposed as suggested in literature reports [96]

(Scheme 3.1) shown earlier in the text where, methanol is used as solvent

using TS-1 and TS-2 as catalysts. Table 3.3 Optimization of reaction conditions for conversion of allyl chloride to ECH*. Sr. No.

Reactants with their mole ratio

Catalyst used Amount of

Catalyst (g)

ReactionTime (h)

Oxidant Used

(mole)

% Conversion

of H2O2

%Selectivity

Optimization of Reaction Time 1 MT:AC (1:1)

Zr-MCM-41-5 0.5 2 H2O2 (1) 18.84 -

2 MT:AC (1:1) Zr-MCM-41-5 0.5 3 H2O2 (1) 25.50 - 3 MT:AC (1:1) Zr-MCM-41-5 0.5 4 H2O2 (1) 28.40 - 4 MT:AC (1:1) Zr-MCM-41-5 0.5 5 H2O2 (1) 30.86 - 5 MT:AC (1:1) Zr-MCM-41-5 0.5 6 H2O2 (1) 31.01 - 6 MT:AC (1:1) Zr-MCM-41-5 0.5 8 H2O2 (1) 31.30 -

Optimization of Catalyst amount 7 MT:AC (1:1) Zr-MCM-41-5 1.0 5 H2O2 (1) 39.08 99.45 8 MT:AC (1:1) Zr-MCM-41-5 1.5 5 H2O2 (1) 40.58 99.50 9 MT:AC (1:1) Zr-MCM-41-5 2.0 5 H2O2 (1) 41.01 99.50 10 MT:AC (1:1) Zr-MCM-41-5 2.5 5 H2O2 (1) 42.85 99.5611 MT:AC (1:1) Zr-MCM-41-5 3.0 5 H2O2 (1) 43.20 98.80

Optimization of Oxidant amount 12 MT:AC (1:1)

Zr-MCM-41-5 2.5 5 H2O2 (0.5) 31.14 98.80

Optimization of various oxidant used 13 MT:AC (1:1) Zr-MCM-41-5 2.5 5 TBHP (1) 37.23 99.20 14 MT:AC (1:1) Zr-MCM-41-5 2.5 5 NaOCl (1) 31.50 99.10

Optimization of SiO2:ZrO2 ratio of catalyst used 15 MT:AC (1:1) Zr-MCM-41-20 2.5 5 H2O2 (1) 38.46 99.60 16 MT:AC (1:1) Zr-MCM-41-40 2.5 5 H2O2 (1) 36.92 99.40 17 MT:AC (1:1) Zr-MCM-41-60 2.5 5 H2O2 (1) 30.76 99.25

Optimization of SiO2:TiO2 ratio of catalyst used 18 MT:AC (1:1) Ti-MCM-41-20 2.5 5 H2O2 (1) 36.81 99.17 19 MT:AC (1:1) Ti-MCM-41-30 2.5 5 H2O2 (1) 39.45 99.78

Recyclability of Zr-MCM-41-520 MT:AC (1:1) Zr-MCM-41-5

(1st Cycle) 2.5 5 H2O2 (1) 39.59 99.35

21 MT:AC (1:1) Zr-MCM-41-5 (2nd Cycle)

2.5 5 H2O2 (1) 36.12 99.28

Recyclability of Ti-MCM-41-30 22 MT:AC (1:1) Ti-MCM-41-30

(1st Cycle)2.5 5 H2O2 (1) 37.57 99.57

23 MT:AC (1:1) Ti-MCM-41-30 (2nd Cycle)

2.5 5 H2O2 (1) 35.84 99.42

*All reactions were carried out at room temperature. MT: Methanol, AC: Allyl Chloride


152

Fig.3.22 Optimization of reaction time

Fig.3.23 Optimization of catalyst amount

Fig. 3.24 Optimization of oxidants used

Fig 3.25 Effect of SiO2:ZrO2 ratio on ECH conversion

Fig 3.26 Effect of SiO2:TiO2 ratio on ECH

conversion

Fig. 3.27 Comparative catalytic performance in the conversion of ECH

Zr-MCM-41-5 Ti-MCM-41-30

Zr-MCM-41-5Zr-MCM-41-5

0

5

10

15

20

25

30

35

0 2 4 6 8 10

% C

onve

rsio

n of

EC

H

Reaction time (h)

38.539

39.540

40.541

41.542

42.543

43.5

0 1 2 3 4

% C

onve

rsio

n of

EC

H

Catalyst amount (g)

0

5

10

15

20

25

30

35

40

45

H2O2 TBHP NaOCl

% C

onve

rsio

n of

EC

H

05

1015202530354045

% C

onve

rsio

n of

EC

H

32

34

36

38

40

42

44

1st Cycle

2nd Cycle

3rd Cycle

1st Cycle

2nd Cycle

3rd Cycle

% C

onve

rsio

n of

EC

H

3535.5

3636.5

3737.5

3838.5

3939.5

40

Ti-MCM-41-20 Ti-MCM-41-30

% C

onve

rsio

n of

EC

H


153

Cheng-Hua et al. have reported the synthesis of ECH using allyl chloride

with H2O2 using Ti-ZSM-5 as catalyst [97]. The maximum conversion reported

by them based on allyl chloride, is ~67%, however using 8 g/L of catalyst (Ti-

ZSM-5) with reaction time ~4.5 h and at 450C temperature. In the present

study, ECH is obtained at room temperature. % yield of ECH (based on %

conversion of H2O2) is ~39% using only 2.5 g/L of catalyst (Ti-MCM-41-30)

with reaction time ~5h.

Recyclability study of the catalysts: Entry no. 20 to 23 (Table 3.3) indicates that there is not much decrease

in % yield, which indicates that catalysts synthesized are stable and there is

no indication of metal ion leaching (Fig. 3.25). It was observed that the colour

of the catalyst changes after each catalytic run. This gives an indication that

during the course of the reaction the reacting molecules come onto the

surface of the catalyst. Some of them enter into reaction to give the product

while a few of them get adsorbed on the surface, which is marked by the

change in the colour of the catalyst. The fact that the reactant molecules are

weakly adsorbed is evident from the catalyst regaining its original colour,

when treated with ethanol. The possibility of molecules entering interstices

cannot be ruled out. This is observed from the fact that the yields go down

after every regeneration, leading to deactivation of the catalyst. The

deactivation of the catalyst might be due to the adsorption of allyl chloride and

ECH formed on the active sites of the catalyst.

3.8 CONCLUSIONS The study indicates the promising use of Zr-MCM-41 and Ti-MCM-41

as environment friendly heterogeneous solid acid catalysts for epoxidation of

allyl chloride to ECH with ~99% selectivity at room temperature. The

epoxidation reaction involves operational simplicity. The product formed can

be simply distilled over and the catalyst can be regenerated and reused. The

conversion of allyl chloride to ECH by using Zr-MCM-41-5 and Ti-MCM-41-30

as heterogeneous solid acid catalysts, along with H2O2 as an oxidant

eliminates the use of corrosive chlorine and peracids (used in conventional

process) as well as generation of chlorine-laden sewage and acid waste.

Further, the reaction with H2O2 has the advantage of mild reaction condition,


154

production of only water as by-product with added contribution to the

epoxidation reaction. Finally, the conventional method for production of ECH

is a non-catalytic process using chlorine. By using eco-friendly solid acid

catalysts such as Zr-MCM-41 and Ti-MCM-41 the green chemistry principle

no. 9, “Catalysts (as selective as possible) are superior to stoichiometric reagents” is implemented.


155

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CCHHAAPPTTEERR 44 Synthesis and Characterization of a Palladium Loaded Perovskite MCM-41 material and its Application as an Automotive Catalyst ________________________________________________________

Chapter 4. Automotive Catalysis

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4.1 INTRODUCTION: Automotive catalysts designed to detoxify the exhaust were

implemented in production in US on vehicles of the model year 1975 and, as

we are reaching a full quarter century of their use, there is ample information

available to allow us to declare that these devices, which are the principal

emission control tools, have proved to be an unqualified success. Following

the positive experience in US, in short order Japan and thereafter Europe, in

1986, adopted the use of automotive catalysts. Less affluent developing

societies have come to the realization that emission control in heavily

populated areas is not a costly frill but a tangible benefit for the quality of life

and the use of automotive catalysts is rapidly spreading around the globe.

Even a subjective, casual visitor to urban centers where these devices have

not yet been widely implemented will quickly notice the difference in air

quality. The ubiquity of automobiles, and by extension of catalysts, has made

catalysts and their function much more familiar to the population at large.[1]

The significant environmental implications of vehicles cannot be

denied. The need to reduce vehicular pollution has led to emission control

through regulations in conjunction with increasingly environment-friendly

technologies. In India, it was only in 1991 that the first stage emission norms

came into force for petrol vehicles and in 1992 for diesel vehicles. The

following chart indicates, step wise enforcement of norms for emission of

pollutants from two wheelers.

Fig. 4.1 Progressive Reduction of Indian Emission Norms 2 Wheelers (Both 2 and 4 stroke):


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Table 4.1 Stepwise enforcement of norms for emission of pollutants from two wheelers

Year 1991 1996 2000 2005 CO g/km 12 4.5 2.0 1.5

HC+NOx g/km 8 3.6 2.0 1.5

From April 1995 mandatory fitment of catalytic converters in new petrol

passenger cars sold in the four metros of Delhi, Calcutta, Mumbai and

Chennai along with supply of Unleaded Petrol (ULP) was effected. Availability

of ULP was further extended to 42 major cities and now it is available

throughout the country. The emission reduction achieved from pre-89 levels is

over 85% for petrol driven and 61% for diesel vehicles to 1991 levels. Since

the year 2000, passenger cars and commercial vehicles have been meeting

Euro I equivalent India 2000 norms, while two wheelers have been meeting

one of the tightest emission norms in the world. Euro II equivalent Bharat

Stage II norms are in force from 2001 in 4 metros of Delhi, Mumbai, Chennai

and Kolkata. Since India embarked on a formal emission control regime only

in 1991, there is a gap in comparison with technologies available in the USA

or Europe. Currently, we are behind Euro norms by few years, however, a

beginning has been made, and emission norms are being alligned with Euro

standards and vehicular technology is being accordingly upgraded. Vehicle

manufactures are also working towards bridging the gap between Euro

standards and Indian emission norms.

4.2. AUTOMOTIVE EXHAUST PURIFICATION CATALYSIS-

TERMS AND CONCEPTS: Catalytic converter is an exhaust emission control device which

converts toxic chemicals in the exhaust of an internal combustion engine into

less toxic substances. Inside a catalytic converter, a catalyst stimulates a

chemical reaction in which toxic byproducts of combustion are converted to

less toxic substances by way of catalysed chemical reactions. The specific

reactions vary with the type of catalyst installed. Most present-day vehicles

that run on gasoline are fitted with a "three way" converter, so named because

it converts the three main pollutants in automobile exhaust: an oxidizing

reaction converts carbon monoxide (CO) and unburnt hydrocarbons (HC), and


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a reduction reaction converts oxides of nitrogen (NOx) to produce carbon

dioxide (CO2), nitrogen (N2), and water (H2O).

The catalytic converter consists of several components:

The catalyst core. For automotive catalytic converters, the core is usually a

ceramic monolith with a honeycomb structure. Metallic foil monoliths made of

FeCrAl are used in some applications. This is partially a cost issue. Ceramic

cores are inexpensive when manufactured in large quantities. Metallic cores

are less expensive to build in small production runs. Either material is

designed to provide a high surface area to support the catalyst washcoat, and

therefore is often called a "catalyst support". The cordierite ceramic substrate

used in most catalytic converters was invented by Rodney Bagley, Irwin

Lachman and Ronald Lewis at Corning Glass, for which they were inducted

into the National Inventors Hall of Fame in 2002.

Monolith The combined requirements of compactness, high volumetric flow rates

and low back pressure led to the adoption of a monolithic embodiment for

automotive catalysts, quite different from the packed-bed forms that prevailed

in almost all industrial and petroleum catalysis. The monoliths were multi-

channeled ceramic catalyst bodies (square, triangular or honeycomb channel

configurations), with the exhaust gas flowing through the channels on whose

walls there is a coated high surface area porous layer with finely dispersed

noble metal catalytic particles. (At the dawn of the implementation period,

some manufacturers stuck to the more familiar granular catalysts. In use,

these catalyst granules or beads suffered from attrition and ultimately were

abandoned in favour of the monoliths and also due to poorer warm-up

characteristics.) Presently, monolithic catalysts (mostly ceramic but also

metallic in some special instances) are universally used in automotive

catalysis. Moreover, the “monolith” catalytic technology is migrating into the

realm of industrial catalysis, most often in processes for treatment of industrial

effluents.

The washcoat. A washcoat is a carrier for the catalytic materials and is used

to disperse the materials over a high surface area. Aluminum oxide, Titanium

dioxide, Silicon dioxide, or a mixture of silica and alumina can be used. The

catalytic materials are suspended in the washcoat prior to applying to the


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core. Washcoat materials are selected to form a rough, irregular surface,

which greatly increases the surface area compared to the smooth surface of

the bare substrate. This in turn maximizes the catalytically active surface

available to react with the engine exhaust.

Precious metal. The catalyst itself is most often a precious metal. Platinum is

the most active catalyst and is widely used, but is not suitable for all

applications because of unwanted additional reactions and high cost.

Palladium and rhodium are two other precious metals used. Rhodium is used

as a reduction catalyst, palladium is used as an oxidation catalyst, and

platinum is used both for reduction and oxidation. Cerium, iron, manganese

and nickel are also used, although each has its own limitations.

Types of catalytic converters: Two-way A two-way (or "oxidation") catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide:

2CO + O2 → 2CO2 (4.1)

2. Oxidation of hydrocarbons (unburnt and partially burnt fuel) to carbon

dioxide and water:

CxH2x+2 + [(3x+1)/2] O2 → xCO2 + (x+1) H2O (a combustion reaction) (4.2)

This type of catalytic converter is widely used on diesel engines to

reduce hydrocarbon and carbon monoxide emissions. They were also used

on gasoline engines in American- and Canadian-market automobiles until

1981. Because of their inability to control oxides of nitrogen, they were

superseded by three-way converters.

Three-way Since 1981, "three-way" (oxidation-reduction) catalytic converters have

been used in vehicle emission control systems in the United States and

Canada; many other countries have also adopted stringent vehicle emission

regulations that in effect require three-way converters on gasoline-powered

vehicles. The reduction and oxidation catalysts are typically contained in a

common housing, however in some instances they may be housed

separately. A three-way catalytic converter has three simultaneous tasks:


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I. Reduction of nitrogen oxides to nitrogen and oxygen:

2NOx → xO2 + N2 (4.3)

II. Oxidation of carbon monoxide to carbon dioxide:

2CO + O2 → 2CO2 (4.4)

III. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water:

CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O (4.5)

These three reactions occur most efficiently when the catalytic

converter receives exhaust from an engine running slightly above the

stoichiometric point. This point is between 14.6 and 14.8 parts air to 1 part

fuel, by weight, for gasoline. The ratio for Autogas (or liquefied petroleum gas

(LPG)), natural gas and ethanol fuels is each slightly different, requiring

modified fuel system settings when using those fuels. In general, engines

fitted with 3-way catalytic converters are equipped with a computerized

closed-loop feedback fuel injection system using one or more oxygen

sensors, though early in the deployment of three-way converters, carburetors

equipped for feedback mixture control were used.

Three-way catalysts are effective when the engine is operated within a

narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas

oscillates between rich (excess fuel) and lean (excess oxygen) conditions.

However, conversion efficiency falls very rapidly when the engine is operated

outside of that band of air-fuel ratios. Under lean engine operation, there is

excess oxygen and the reduction of NOx is not favored. Under rich conditions,

the excess fuel consumes all of the available oxygen prior to the catalyst, thus

only stored oxygen is available for the oxidation function. Closed-loop control

systems are necessary because of the conflicting requirements for effective

NOx reduction and HC oxidation. The control system must prevent the NOx

reduction catalyst from becoming fully oxidized, yet replenish the oxygen

storage material to maintain its function as an oxidation catalyst.

Light off Temperature: Car catalyst do not work at low temperature due to

kinetic considerations and if the temperature of the catalyst is increased,

increase in conversion from zero to high levels occurs over a short

temperature range, the inflexion point of this curve is often called the light off

temperature. It is important that this temperature is as low as possible for


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efficient conversion quickly after the cold start of the engine. This is especially

so since the testing cycle require the catalyst to achieve total averaged

conversion from a cold start and include cycles of low temperature operation.

Precious metals are highly active materials which encourage low temperature

light-off , but below this temperature the metals tend to be covered by carbon

monoxide and light off only occurs when this begins to desorb from the

surface and liberate surface sites for reaction. Of late, the light off temperature

has gained significance in defining the efficiency of automotive catalysts, and

the definition has been conveniently labeled for individual pollutants (CO,

Hydrocarbon and NOx) as the temperature at which 50 % conversion is

achieved for the respective pollutant gas.

Air fuel ratio (Lambda,λ): Air–fuel ratio (AFR) is the mass ratio of air to fuel present in an

internal combustion engine. If exactly enough air is provided to completely

burn all of the fuel, the ratio is known as the stoichiometric mixture. AFR is an

important measure for anti-pollution and performance-tuning reasons.

For gasoline fuel, the stoichiometric air–fuel mixture is approximately 14.7; i.e.

for every one molecule of fuel, 14.7 molecules of O2 are required (the fuel

oxidation reaction is: 25/2 O2 + C8H18 -> 8 CO2 + 9 H2O). Any mixture less

than 14.7 to 1 is considered to be a rich mixture; any more than 14.7 to 1 is a

lean mixture – given perfect (ideal) "test" fuel.

Lambda (λ) is the ratio of actual AFR to stoichiometry for a given

mixture. Lambda is 1.0 at stoichiometry, less than 1.0 for rich mixtures, and

greater than 1.0 for lean mixtures. There is a direct relationship between

lambda and AFR. To calculate AFR from a given lambda, multiply the

measured lambda by the stoichiometric AFR for that fuel. Alternatively, to

recover lambda from an AFR, divide AFR by the stoichiometric AFR for that

fuel. This last equation is often used as the definition of lambda.

(4.6)

Because the composition of common fuels varies seasonally, and because

many modern vehicles can handle different fuels, when tuning, it makes more

sense to talk about lambda values rather than AFR.


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Oxygen storage Three-way catalytic converters can store oxygen from the exhaust gas

stream, usually when the air-fuel ratio goes lean. When insufficient oxygen is

available from the exhaust stream, the stored oxygen is released and

consumed. A lack of sufficient oxygen occurs either when oxygen derived

from NOx reduction is unavailable or when certain maneuvers such as hard

acceleration enrich the mixture beyond the ability of the converter to supply

oxygen.

Thermal stability The catalyst can cycle from cold start to temperatures as high as

900oC, and this is a very demanding situation not usually experienced by

catalysts in chemical processing. Thus both mechanical and chemical stability

are required. Mechanical support and thermal shock resistance are provided

by monolith support, as stated earlier. Clearly, the chemically active

components of the catalyst must also be thermally stable and this eliminates

many possible elemental compositions, including low melting metals and

oxides which may be reducible to more volatile species during the engine

cycle. Platinum group metals (Pt, Pd and Rh) are relatively high melting point

metals, compared to other base metals like Cu and Ag.

Lifetime The catalyst must have considerable longevity and maintain high

conversion over at least 50000 miles operation in Europe or 100000 miles in

the USA. Thus all the materials used are also designed for great time stability

and many of the features described above would aid in catalyst engineering.

Catalyst Architecture The automotive catalyst assembly is required to operate with very high

efficiency at high gas flows. As a result much care must be taken in design

and construction of the catalyst to ensure that these requirements are met. Of

particular interest are the pore size distribution in the catalyst, the metal

particle size and the distribution of active metal within the pores. Two main

factors influence the design: the requirement for effective mass transport so

that reactant gases can reach the catalyst and product can be efficiently

removed, and chemical considerations to ensure that catalysis is effectively


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carried out with minimum need for expensive resources, particularly precious

metals. The mass transport usually needs a network of both

macropores(d>500 Ao) which carry most of the gas load, and meso pores (20

Ao<d<500 Ao) where most of the precious metal component is located and

catalysis occurs.

4.3 MATERIALS USED AS AUTOMOTIVE EXHAUST PURIFICATION CATALYSTS:

Gasoline-powered passenger cars, which comprise a large majority of the US market, emit CO, unburnt HC and oxides of nitrogen (NOx). Starting in 1981, the automobile industry was mandated to sharply reduce the emissions of all three (previously only CO and HC had to be removed). This required "three-way-conversion" (TWC): the simultaneous oxidation of CO/HCs and reduction of NOx, a feat without precedent in the chemical industry. It can only be accomplished by keeping the exhaust gas composition extremely close to the stoichiometric point. Cerium oxide was soon recognized as indispensable to the success of the TWC catalyst, due to its ability to rapidly change oxidation states at the surface. This enables it to "store" and "release" oxygen in response to changes in the gas phase composition. The use of ceria was greatly expanded by the addition of zirconia; the ceria-zirconia was far more thermally stable than ceria itself and allowed the TWC catalyst to survive much higher temperatures

Diesel exhaust differs from gasoline exhaust in important respects. It

is always "lean" (i.e., net oxidizing) and three-way conversion is thus ruled

out. The chief concern is particulate matter, which includes dry soot and a

"soluble organic fraction" (SOF), comprised of mainly C20-C28. Prior to 2000,

attention was mainly focused on SOF conversion, as dry soot emissions

could be controlled within the standards by optimizing fuel delivery, air intake

systems, and the combustion process. To meet the new emission standards

proposed for 2007-2009 in the U.S, Europe and Japan, the diesel particulate

filter (DPF) was created, in which the channel wall filters out the soot

particles. The latter are burned off, at suitable intervals, by raising the

temperature. DPFs were originally limited to trucks and buses, but their

proven effectiveness has led to their planned use on passenger cars as well.


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A final topic in diesel after treatment is NOx removal. A successful approach,

already in place in Europe, is the use of urea, carried on-board as an

aqueous solution. Urea hydrolyzes to release NH3, which is a highly effective

agent for converting NOx to N2.

Zeolites: With the continued improvements in three-way catalyst (TWC)

technology, most of the hydrocarbon (HC) emissions from gasoline engines

occur during engine cold start before the TWC would reach its effective

operating temperature. Since the TWC is deemed to be at or near its lowest

feasible light-off temperature, alternate approaches to achieve the reduction

are necessary. One approach is to use a zeolite to adsorb HC at low

temperatures followed by desorption at temperatures where the TWC is

active. Several Japanese car manufacturers have reported the use of zeolites

as HC traps for gasoline powered passenger cars to meet these stringent

emission standards [2, 3]. The concept of using zeolites as hydrocarbon traps

can be traced back to the early 1970s [4,5], and various zeolites have since

been considered as adsorbents for trapping exhaust HCs [6–15]. For

example, silicalite (MFI) was studied because of its hydrophobic properties

that lead to the ability to preferentially adsorb HCs over water present in the

exhaust [7]. Other zeolites such as H-ZSM-5 [7, 12], Ag-ZSM-5 [8], Beta

zeolite [9, 12], mordenite [10], EUO [10], SSZ-33 [11] and USY [12] have also

been investigated as HC trapping materials.

In diesel automotive emission control systems, zeolites have been

incorporated into the diesel oxidation catalysts (DOC) to help reduce HC

emissions [13]. Zeolites Beta and ZSM-5 have been considered to be

effective for DOC [14, 15].

Perovskites: Perovskite-type oxides have general formula as ABO3. Fig. 4.2 shows

structure of perovskite where red spheres are oxygen atoms, the blue

spheres are B atoms (a smaller metal cation, such as Ti4+), and Green

spheres are the A-atoms (a larger metal cation, Ca2+). When A is rare earth

or alkaline earth metal and B is transition metal, the perovskite are typically

P-type semiconductors.


170

Fig 4.2. Schematic Perovskite structure

Pictured is the undistorted cubic structure, however the symmetry is lowered

to orthorhombic, tetragonal or trigonal in many perovskites. Their

composition can be varied in a wide range by partial substitution of lower

valent cation in A or B site yielding additional mobile anion vacancies. Their

mixed conductivity by both ion and electron migration and their high

nonstoichiometric composition have resulted in the applications of this group

of materials in the areas such as electrochemistry, catalysis, solid oxide fuel

cells, oxygen separation membranes, chemical sensors for the detection of

humidity, alcohol and gases such as oxygen, hydrocarbon and nitric oxide

[16]. Earlier studies reported on perovskite oxide LaCoxFe1-xO3 mainly

involved methane oxidation catalysis. Recently, noble metals combined with perovskites have developed into

an emerging field. Addition of small amounts of noble metals to perovskites

could improve their catalytic activity. Incorporation of small amounts of

precious metals into a perovskite structure can prevent their sintering, reduce

losses due to volatalization at high operating temperatures and avoid

reactions with the support that slow down the catalyst. Further, attention has

been concentrated on the use of palladium based catalyst for TWC (three way

catalyst) formulation. Pd is well known to have a good resistance to thermal

sintering, is much cheaper than Pt and Rh and also has a good activity for

oxidation of CO and hydrocarbons [16].


171

Oxygen Storage and release in Perovskites: Y. Zhang-Steenwinkel et al [17] have studied the oxygen storage/

releasing property by reduction of La0.8Ce0.2MnO3 type Perovskite, using

labeled 1 vol.% C18O and balance He(99%), as reducing gas mixture over the

catalyst and monitoring the production of C16O, C18O, C18O16O and C16O18O,

and have provided following reaction mechanism of CO oxidation on

Perovskite surface.

Fig.4.3 The reaction mechanism of the reduction of La1−xCexMnO3 by C18O at 473 K [17].

4.4 SYNTHESIS STRATEGIES FOR AUTOMOTIVE

CATALYSTS-A LITERATURE SURVEY LaMnO3 perovskites supported noble metal (Pt, Pd, Rh) catalysts

prepared by the citrate method are used for the total oxidation of methane and

it is observed that the oxidation activity is enhanced [18]. Tanaka [19]

prepared LaFe0.95Pd0.05O3 perovskite catalyst by the alkoxide method. ‘‘The

intelligent catalyst’’ has a self-regenerative function. Pd in this catalyst moves

back and forth between the B-site in the perovskite structure and the metal

lattice.

Furthermore, La(Fe, Co)PdO3 [20,21] (La0.6Sr0.4)(Co0.94 Pt0.03Ru0.03)O3

[22,23] and LaMn0.976Rh0.224O3.15 [22,24] perovskite composition were

explored for TWC catalysis for exhaust treatment from single cylinder engine

and compared with synthetic CO + NO + C3H6 mixture .

Co-precipitation [25], sol-gel methods [26, 27] and combustion

methods [28, 29, 30–33] are common synthetic approaches for synthesis of


172

perovskites. Amongst these synthetic methods, the solution combustion is an

attractive route. It is very facile and energy efficient. It can produce high purity,

homogeneous crystalline product with high specific surface area [28, 29,30–

33]. However, not much attention has been focussed towards the preparation

of perovskite catalysts incorporating noble metal, using a solution combustion

method. There is very limited information available on developing new active

catalysts, especially the perovskite oxides substituted by Pd via solution

combustion method. Perovskite-type oxides have been used for simultaneous

catalytic removal of NOx and diesel soot [34]. LaCoO3 has excellent catalytic

activity for oxidation. Pd could modify its structure and the physical properties

and increase its catalytic activity [22].

4.5 AIM AND SCOPE OF THE PRESENT WORK Although perovskite materials have not yet found application as

commercial catalysts, their importance in efforts to correlate solid-state

chemistry with catalytic properties, dependence of their properties on the

preparation methods, and the fact that they can be tailored for specific

catalytic needs make these oxides, prototype models for heterogeneous

catalysts.

Perovskite produced via conventional synthesis methods are found to

exhibit relatively low specific surface areas and low catalytic activity in the

reactions, thus its commercial applications are limited. Siliceous MCM-41 a

material with a neutral framework exhibits negligible catalytic activity.

However, one can encash its advantageous properties such as high surface

area, chemical inertness, high thermal and chemical stability. Thus, one of the

approaches for overcoming the limitations of Perovskites is to support them

on to materials with high surface area inert materials like MCM 41. Supporting

Perovskites on a high surface area inert material can ensure uniform and high

dispersion throughout the surface of carrier along with thermal stability of

resultant material and hence durability of the catalyst and generating

structural defects due to interaction with support and thereby improvement of

redox functionality.

In the present endeavor LaCoO3 Perovskites(LC) have been

synthesized on the surface of siliceous MCM 41 materials by citric acid


173

solution combustion method abbreviated as LCM. Different wt.% Pd was

loaded by equilibrium adsorption using excess solution of palladium nitrate..

The materials have been characterized for XRD, BET surface area and

temperature programmed reduction(TPR). Catalytic activity of these materials

have been evaluated in a down flow tubular micro reactor using a simulated

exhaust gas mixture, where in concentration of CO,HC and NOX in the gas

mixture passing through the catalyst bed has been monitored as a function of

catalyst bed temperature.

4.6 EXPERIMENTAL Materials:

Lanthanum nitrate, Cobaltous nitrate, Palladium nitrate and citric acid

were procured from Merck India Ltd. as AR grade reagents.

Synthesis of Perovskite based automotive catalysts The synthesis consists of three parts.

(i) Synthesis of MCM 41 The synthesis has been performed as described in Chapter 2 section 2.10.

(ii) Synthesis of LC onto siliceous MCM 41-(LCM)

In a typical set up for preparation of LCM, lanthanum nitrate and cobaltous

nitrate were dissolved in water in a 1:1 mole ratio. To this solution was added

an aqueous solution of 20% citric acid. (Amount of Citric acid solution was

taken such that weight of citric acid was equivalent to oxide weight of resultant

LaCoO3). To this resultant solution, MCM 41 powder was added and kept

under agitation for 0.5 h. The slurry was transferred to a ceramic crucible,

dried at 120oC for 6 h and then calcined at 550 oC for 3 hrs at the rate of 2

◦C/min. This sample was designated as LCM. For comparison, LaCoO3 was

prepared following the same procedure as described above except that MCM

41 was not added. This sample was designated as LC.

(iii) Incorporation of precious metal (Pd) onto LCM to give Pd-LCM:

Palladium impregnation on LCM sample was carried out using equilibrium

adsorption using excess solution of Pd(NO3)2. In a typical set up, 1 g LCM

powder was kept with 1 % solution of palladium nitrate for 12 hours. After 12

hours, powder was separated by filtration, dried at 120oC, followed by


174

calcination at 550oC for 3 hrs at a heating rate of 2 ◦C/min. This sample was

designated as Pd-LCM.

In the present study following materials were prepared LC, LCM-15 wt.%-

LC loading, LCM-40wt%-LC loading, 0.2wt%Pd-LCM-15wt%-LC loading, 0.2

wt%Pd-LCM 40 wt%- LC loading. 1wt%Pd-LCM and 1wt%-Pd-MCM-41.

Instrumental methods of characterization X-ray diffractograms (2θ=5-90o) were obtained on X-ray diffractometer

(Bruker D8)with Cu-Kα radiation and Nickel filter. Surface area measurement

(BET method) was carried out on Micromeritics Gemini 2120 at -196oC using

nitrogen adsorption isotherms. TPR was carried out on Micromeritics 2720,

using 10 % H2+N2 gas up to 800oC at a heating rate of 10oC/ min.

Evaluation of Catalytic Activity Catalytic activity of the above synthesized materials was evaluated

using a simulated exhaust gas mixture in a down flow tubular micro reactor,

fitted with a programmable furnace and data recording device which logs the

data of gas concentration and temperature at every 10 seconds to the

computer attached. Catalyst pellets were placed in the middle of the reactor

tube on a sintered disc and a thermocouple placed near the catalyst bed

detects the temperature and sends temperature signals to computer every 10

seconds. In a typical test set up, 1 g of material was pelletized using a

hydraulic die press and placed inside the reactor on the disc. Gas mixture

containing 0.4 % CO, 0.13 % H2, 0.04 % Propylene, 12.5 % CO2, 0.06 %

NOx, 0.4 % O2 and balance N2 was passed through the reactor at the rate of

125 L/h to give gas hourly space velocity of 125 KL/h. Temperature ramp up

was given at the rate of 10oC/min. from room temperature to 450oC and

temperature and composition of gases flowing through the reactor were

recorded using a Horiba gas analyzer. Air to fuel ratio (λ) of the exhaust gas

composition was calculated as,

(4.7)

Catalytic activity of the materials tested has been reported as (i) Light

off temperature for CO,HC and NOx and (ii) conversion of CO,HC and NOx at

400 oC, the temperature at which activation energy is enough for conversion


175

to take place on the active sites, and there are no thermodynamic limitations

for CO and HC conversion.

4.7 RESULTS AND DISCUSSION XRD of LC is presented in Fig. 4.4 and matches with LaCoO3 structure

with cubic morphology (JCPDS card no.-075-0279). XRD of LCM-40 wt%-LC

loading is presented in Fig. 4.5 and matches with LaCoO3 structure with cubic

morphology (JCPDS card no.-075-0279). The intensity of peaks is very less

for 40 wt% LCM sample compared to 100 % LC sample, indicating that

LaCoO3 is well dispersed on the surface of MCM-41.

Fig.4.4 XRD of LC

Fig.4.5 XRD of 40wt% LCM

Fig.4.6 TPR patterns of LCM and Pd LCM Fig.4.7 Conversion curves for 1 % Pd LCM


176

Fig.4.8 Conversion curves for 0.2 % Pd LCM

Since the materials are proposed to be used as automotive exhaust

catalysts, thermal stability is an important aspect of the same, as stated

earlier in the introduction part. In order to check the thermal stability, the

materials were subjected to calcination at 1000 oC in a muffle furnace for 3 h.

and checked for BET surface area. BET surface area of MCM-41, MCM-

41(LCM) fresh and aged samples are presented in (Table-4.2). BET surface

area of MCM-41 decreases after impregnation with perovskite. This may be

attributed to pore blockage by loading of perovskite on MCM-41. Further drop

in surface area is observed after thermal ageing at 1000 oC. This may be

attributed to sintering of perovskite structure as well as collapse of MCM-41

pores after severe thermal treatment. Table 4.2: BET surface area of fresh and thermally aged samples

Sample BET surface area (m2/g) Fresh Aged

MCM-41 1126 700 LC 16 10

LCM-15 wt.%-LC loading 535 25 LCM-40wt%-LC loading 158 3

0.2wt%Pd-LCM-15wt%-LC loading 487 17

0.2 wt%Pd-LCM 40 wt%- LC loading 119 14

1% Pd-LCM 89 2

TPR patterns of LCM and Pd LCM (Fig. 4.6) shows peaks due to

hydrogen consumption for reduction of perovskite materials. As observed,


177

reduction temperature of perovskite decreases post impregnation with Pd

which may be attributed to Pd catalyzing reduction of perovskite resulting in a

lower reduction temperature. Area under the curve in TPR pattern indicates

amount of metal being reduced. It is observed that area of TPR curve

increases after Pd metal loading and as a function of amount of Pd metal

loading. The decrease in reduction temperatures can be explained by the

activated hydrogen atoms on the Pd particles [35]. These H atoms could

reduce the Con+ species at lower temperature [18]. Similar results have been

reported for Pd doped LaFeCoO3 catalysts [36] and LaMnPdO3 catalysts [18].

Lowering in the reduction temperature may be attributed to Pd

catalyzing complete reduction of perovskite material. Reduction temperature

in TPR pattern can be correlated with flexibility of oxygen up take and release

(redox functionality) under transient conditions (rich and lean) in automotive

exhaust. Lower the reduction temperature, lower the temperature of oxygen

release in rich condition in exhaust. Hence, from the TPR pattern it can be

concluded that LCM indicates potential as oxygen storage material for

automotive exhaust catalyst. Pd impregnation of LCM enhances reducibility of

perovskite and oxygen storage capacity of the material.

To check the activity of Perovskite for conversion of CO and HC, 15 %

LCM without Pd was evaluated first. As seen from (Table 4.3) entry no 2, no

light off for either CO or HC was observed for this sample and conversion of

CO was 8% at 400 oC, while no HC conversion was observed. Thus 40 wt%

LCM entry no 3 was subjected to light off with no response to CO and HC

though active sites were increased, however conversion of CO and HC at 400

◦C improved marginally (Table-4.4) entry no 3.

Further it is observed that when Pd is introduced in samples to provide

active sites for CO and HC conversion (Table-4.3 entry no 4 & 5) while light

off was not observed, for 15 % LCM, CO and HC conversion were

dramatically improved and for 40 % LCM, CO and HC light off were observed

at ~ 400 oC. These observations indicate that, CO and HC conversion occur

on Pd metal surface, and Pd is the active site for CO and HC conversion. ~ 10

% conversion of CO and HC for samples without Pd may be attributed on

account of free radical oxygen being released from Perovskite and reacting

with CO and HC.


178

Pd concentration was further increased to 1 wt. % on 40 % LCM

sample and as seen from table 4.3( entry no 6), CO and HC light off were

significantly improved to 220 oC. There was no unreacted CO and HC at 400 oC for 1 % Pd on 40 % LCM sample (table 4.4 entry no 6).

For all the samples tested, NOx light off is not observed, which is

expected since there is no Rhodium present, which is necessary for NOx

reduction to occur.

For comparison purpose a catalyst with 1 % Pd on MCM 41 (Table-4.3

entry no 7) was evaluated for catalytic activity for which no light off for CO and

HC was observed upto 400 oC. This indicates that, CO and HC are oxidized

only when Pd is present as an active site on catalyst. Since Perovskites

reduce the activation energy of oxidation of CO and HC, it may even help to

reduce precious metal content in commercial catalysts. Table 4.3: Light off temperatures for synthesized catalysts

No Sample CO Light off T, oC

HC light off T ,oC

NOX Light off T, oC

1 LC No light off No light off No light off 2 LCM-15 wt%-LC loading No light off No light off No light off 3 LCM – 40 wt% -LC

loading No light off No light off No light off

4 0.2wt% Pd-LCM-15 Wt% LC Loading

No light off No light off No light off

5 0.2wt% Pd-LCM -40 Wt% LC Loading

410 394.5 No light off

6 1% Pd-LCM 220 223.5 No light off 7 1% Pd-MCM41 No light off No light off No light off

Table 4.4: Conversion of CO,HC and NOx at 400 oC

No Sample CO Conversion,%

HC Conversion,%

NOX Conversion,%

1 LC 15.5 0 3 2 LCM-15 wt%-LC loading 8 0 3 3 LCM – 40 wt% -LC

loading 10 5 3

4 0.2wt% Pd-LCM-15 Wt% LC Loading

25 20 3

5 0.2wt% Pd-LCM -40 Wt% LC Loading

37.5 50 5

6 1% Pd-LCM 100 100 10 7 1% Pd-MCM41 10 25 3


179

4.8 CONCLUSIONS In the present endeavour a humble attempt has been made towards

synthesis of materials that could be probably used for automotive catalysts.

Since it is evident that storage oxygen is used for oxidation, a material that

can store oxygen is highly essential for automotive catalyst, which is met with

by use of perovskite (LC) in the present study.

Oxidation of CO and HC by free radical oxygen, released from surface

of Perovskite(LCM) has been demonstrated. The conventional precious metal

used is Pt. Using Pd makes the catalyst cost effective. In view of the relatively

larger drop of surface area of the samples post thermal ageing, and due to

low conversion for NOx, in the synthesized materials, it could be useful for

diesel oxidation catalyst, wherein exhaust temperatures are low, and only

oxidation function of the catalyst is required, since NOx reduction is treated

separately in diesel exhaust treatment system. It is also proposed that further

work on LCM system with incorporation of Rh can be studied, for three way

catalysis wherein NOX reduction will also occur on the same catalyst.


180

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Summary ________________________________________________________

182

One of the most exciting discoveries in the field of materials synthesis

over the last years is the formation of mesoporous silicate by researchers at

the Mobil Oil Company who reported a novel family of materials called M41S

[1-3]. The family of mesoporous M41S materials consists of three types, as

summarized below in the Fig 1.

Fig.1 The mesoporous M41S family [1]

About MCM-41 • MCM-41 is one of the most studied and promising member of the M41S

family.

• MCM-41 is the abbreviation for Mobil Crystalline Material or Mobil

composition of matter.

• The mesoporous material presents regular arrays of uniform channels,

which has a honeycomb structure as a result of hexagonal packing of uni-

dimensional cylindrical pores.

• By choosing adequate reactants and reaction conditions, it is possible to

tailor the channel dimension in the range of 15-100Å or even larger.

• The BET surface area is typically over 1000 m2/g. The pore is usually

between 0.7 and 1.2 cm3/g with long-range order.

• With increasing pore size, the regularity of the structure is affected.

• The adsorption capacity is exceptionally high (more than 50 wt% for

cyclohexane at 40 Torr, 67 wt% for benzene at 50 Torr).

• MCM-41 possesses excellent thermal and hydrothermal stability (up to

800°C).

• It is relatively stable in acidic medium (pH 2). However, it is destroyed in a

basic medium (pH12).

• MCM-41 is composed of silica framework, which is almost catalytically

inactive.

• The isomorphous substitution of silicon by a variety of metals (Al, Ga, Fe)

gives rise to acidic properties [4].

183

• The possibility of using the pore channels of MCM-41 as a support for

existing catalysts has also been considered [5,6].

There is no doubt that the synthesis of these materials opens definitive

new possibilities for preparing catalysts with uniform pores in the mesoporous

region. Obviously when a new type of material such as these is discovered,

an explosion of scientific and commercial development swiftly follows, and

new investigations on every conceivable aspect of their nature, the synthesis

procedures and synthesis mechanisms, heteroatom insertion,

characterization, adsorption, and catalytic properties, rapidly occurs.

With the first successful report on the mesoporous materials (M41S) by

Mobil researchers, with well defined pore sizes of 20 – 500 Å, the pore size

constraint (15 Å) of microporous zeolites observed a breakthrough. The high

surface area (> 1000 m2/g) and the precise tuning of the pores are among the

desirable properties of these materials. Mainly, these materials ushered in a

new synthetic approach where, instead of a single molecule as a templating

agent as in the case of zeolites, self-assembly of molecular aggregates or

supra-molecular assemblies are employed as templating agents. The basic

difference in the synthesis of microporous and mesoporous molecular sieves

can be shown pictorially.

Fig. 2 Microporous materials using single molecule as template

Fig. 3 Mesoporous materials using molecular aggregates or supra-molecular assemblies as

template.

Though it is possible to overcome the existing pore size constraints of

microporous solids as mentioned earlier, the MCM-41 based materials have

negligible catalytic activity due to framework neutrality, however with

advantageous properties like mesoporous nature of the material, good

184

thermal stability, high surface area and retention of surface area at high

temperatures. Thus, the main aim of the present study was to encash the

advantageous properties of MCM-41 and enhance its practicability in the area

of catalysis using Green Chemistry principles. There are a number of ways by

which catalytic activity can be generated into the MCM-41 neutral framework.

(i)Substitution of an M3+ cation e.g. Al3+ in the Si4+ framework, leading to

negatively charged framework, followed by balancing these charges by H+

ions to create Bronsted acid sites (via NH4+ ion exchange and subsequent

thermal decomposition to give H+ and NH3) to result in a material with inherent

acidity. (ii)Immobilization/anchoring/impregnation of homogenous acid catalyst

e.g. heteropoly acids (HPAs) onto MCM-41 to result in a material with induced

acidity. (iii)Isomorphous replacement of Zr4+and Ti4+ in the siliceous MCM-41

framework to induce redox properties.

Chapter II of the thesis includes the synthesis of mesoporous (i)

Siliceous MCM-41, (ii) Al-MCM-41 and (iii) 12TPA-MCM-41, (where 12-TPA =

12-Tungstophosphoric acid a HPA). Materials (i) and (ii) have been

synthesized by sol-gel method, using templates varying several parameters

such as silica source, templating agent/types, reaction conditions such as pH,

time of reaction, aging, temperature etc. and these parameters optimized,

using surface area as an indicative tool. In case of Al-MCM-41 SiO2:Al2O3

ratios have been varied in order to obtain material with maximum surface

acidity and hence in case of Al-MCM-41 surface acidity has been used as an

indicative tool. The salient feature is the synthesis of MCM-41 and Al-MCM-41

at room temperature. 12-TPA supported MCM-41 was prepared by a process

of anchoring and calcination, with varying 12-TPA loading (10-40 wt.%) in

order to obtain material with maximum surface acidity and hence, here also

surface acidity has been used as an indicative tool.






ammonia, Diffuse reflectance spectroscopy (UV-DRS), Fourier transform


185

The importance of green chemistry, 12 principals of green chemistry

and how green chemistry goals can be achieved through catalysis is a well

established concept [7,8]. Further solid acid catalysts as an alternative

approach to liquid acid catalyst and its advantages over liquid acid catalyst

and important materials used as solid acid catalysts for various organic

transformations is well documented in literature. New materials are

continuously synthesized and explored as solid acid catalysts. The endeavour

“Global effort to replace conventional liquid acid catalysts by solid acid catalysts” is on. This prompted us to use Al-MCM-41 and 12-TPA-MCM-41

as solid acid catalysts by studying (i) Esterification (ii) Friedel- Crafts

alkylation and acylation as model reactions. Since 12-TPA-MCM-41-20 and

Al-MCM-41-30 exhibit highest surface acidity amongst the various samples

prepared, they have been used for all catalytic test reactions.

In case of esterification monoesters such as ethyl acetate (EA), propyl

acetate(PA), butyl acetate (BA) and benzyl acetate (BzA) and diesters such

as diethyl malonate (DEM), dioctyl phthalate (DOP) and dibutyl phthalate

(DBP) have been synthesized.



optimized including catalyst regeneration capacity. The catalytic activities of



It is observed that the order of % yield of ester formed for both

catalysts is BzA> EA> PA> BA. Though the yield in case of mono esters using

both catalysts are comparable, higher yields are observed in case of 12TPA-

MCM-41-20 which could be attributed to higher surface acidity. Turn over

number, reflects the effectiveness of a catalyst and this also follows the order

of ester formation.

In case of diesters there is no marginal difference in the yields of DOP

and DBP and order of the % yields of diesters formation is DEM> DOP ~

DBP., which is probably due to less steric hindrance felt by incoming ethanol

from monoethyl malonate formed in the fist step in case of DEM.

The above esterification reveals the promising use of 12TPA-MCM-41-

20 and Al-MCM-41-30 as solid acid catalysts in the synthesis of monoesters

186

and diesters, the advantages being operational simplicity, mild reaction

conditions and eco-friendly nature of catalyst. The monoesters and diesters

formed can be simply distilled over, there is no catalyst contamination in

products formed, no acid waste formation and products are colorless a

limitation in the conventional process. The catalysts can be regenerated and

reused. Since the reactions are driven by surface acidity of the catalyst, there

is scope of obtaining better/higher yield by synthesizing material with high

surface acidity by modifying synthesis procedure. Though yields of

monoesters are high, the diester yields are low however, with the only

advantage of the product having no colour contamination. Friedel-Crafts acylation of anisole and veratrole with acetic anhydride

and alkylation of toluene with benzyl chloride have been performed to obtain

4-methoxy acetophenone (4MA), 3,4-dimethoxy acetophenone (3,4DMA) and

parabenzyltoluene (PBT) under solvent free condition.






Friedel-Crafts acylation of veratrole with acetic anhydride, gave

selectively 3,4-DMA. Acylation of anisole with acetic anhydride gave

selectively 4MA and aklyation of toluene with benzyl chloride gave selectively

PBT.

The acylation and alkylation reactions are driven by the surface acidity

of the catalyst. Probably this is the reason why 12-TPA-MCM-41-20 gives

higher yields compared to Al-MCM-41-30 due higher surface acidity observed

in case of former catalyst.

In the above synthesis, Green Chemistry goals have been achieved by

using solid acid catalysts (replacing liquid acid catalysts used in conventional

reactions) and under solvent free conditions with high selectivity of the

products. The products formed can be simply distilled over, there is no

catalyst contamination in product and no acid waste formed. The catalyst can

be regenerated and reused. Further, since the studied reactions are driven by

surface acidity of the catalyst, there is scope to obtain higher/better yields by

187

synthesizing materials with higher surface acidity. Finally, the MCM-41 neutral

framework has been successfully modified and put to practical use.

Further, Chapter III of the thesis aims at synthesizing oxidation

catalysts Zr-MCM-41 and Ti-MCM-41 by a sol-gel method using templates

towards achieving mesoporosity, with high surface area, good thermal stability

and maximum M4+ incorporation. For incorporation of maximum Zr4+ and Ti4+,

SiO2:ZrO2 and SiO2:TiO2 ratios have been varied. The salient feature is that

the material is synthesized at room temperature.






ammonia, Diffuse reflectance spectroscopy (UV DRS), Fourier transform


Further, the catalytic potential of the materials Zr-MCM-41 and Ti-

MCM-41 has been explored by studying Epoxidation as a model reaction

using H2O2 as an oxidant in the conversion of allyl chloride to selectively

produce Epichlorohydrin in a single step at room temperature.

In the above reaction, parameters such as catalyst amount, reaction



both catalysts has been compared.

In the present study ECH is obtained at room temperature with very

small amount of catalyst, compared to previous reports that use a large

amount of catalyst and reaction performed at higher temperature. Ti-MCM-41

exhibits higher catalytic activity compared to Zr-MCM-41.

The study indicates the promising use of Zr-MCM-41 and Ti-MCM-41

as environment friendly heterogeneous solid acid catalysts for epoxidation of

allyl chloride to ECH with ~99% selectivity at room temperature. The

epoxidation reaction involves operational simplicity. The product formed can

be simply distilled over and the catalyst can be regenerated and reused. The

conversion of allyl chloride to ECH by using Zr-MCM-41-5 and Ti-MCM-41-30

as heterogeneous solid acid catalysts, along with H2O2 as an oxidant

188

eliminates the use of corrosive chlorine and peracids (used in conventional

process) as well as generation of chlorine-laden sewage and acid waste.

Further, the reaction with H2O2 has the advantage of mild reaction condition,

production of only water as by-product with added contribution to the

epoxidation reaction. Finally, the conventional method for production of ECH

is a non-catalytic process using chlorine. By using eco-friendly solid acid

catalysts such as Zr-MCM-41 and Ti-MCM-41 the green chemistry principle

no. 9, “Catalysts (as selective as possible) are superior to stoichiometric reagents” is implemented.

Chapter IV involves the use of mesoporous MCM-41 as a support in

the synthesis of automobile catalyst. Automobile emissions form an important

source of atmospheric pollution. Automobile catalysts are now widely

recognized for the conversion of mainly CO → CO2, oxides of nitrogen → N2

and unburnt hydrocarbons → CO2 and H2O. At present, noble metals

particularly Rh and Pt catalysts are used for these purposes which are

expensive. There is thus a search for relatively cheaper materials that must

operate efficiently under a wide variety of conditions. Perovskite type oxides

have attracted much attention recently in environment pollution control [9,10].

In the present endeavour, LaCoO3 (LC) a Perovskite has been

synthesized on the surface of mesoporous MCM-41 (LCM) by citrate solution

combustion route. Pd has been incorporated in the Perovskite lattice as well

as on the MCM-41 surface with various wt.% of Palladium and the materials

characterized for XRD, surface area (BET method) and temperature

programmed reduction (TPR). In order to check the thermal stability/durability,

the materials have been aged at 1000oC for 3h and again characterized.

Catalytic activity of fresh and aged materials have been explored for oxidation

of CO, hydrocarbons and reduction of NOx in a tubular down flow test reactor

under simulated exhaust conditions and known air/fuel ratios and the results

correlated with surface and redox properties of the materials.

Conversion occurs only in the presence of Pd, therefore Pd is the

active site for CO and HC conversion.1 wt% Pd LCM(40 wt% LC) exhibited

light off at 220 ◦C ,leaving no unreacted CO and HC.

In conclusion, since it is evident that storage oxygen is used for

oxidation, a material that can store oxygen is highly essential for automotive

189

catalysts, which is met with by use of Perovskite LC in the present study.

Further, the conventional precious metal is Pt. Using Pd makes the catalyst

cost effective. Since no conversion of NOx is observed the Pd-LCM could be

useful for diesel oxidation catalyst, wherein exhaust temperature are low and

only oxidation function of the catalyst is required, since NOx reduction is

treated separately in diesel exhaust treatment system. It is proposed that

further work on LCM system with incorporation of Rh can be studied, for three

way catalysis wherein NOx reduction will also take place on the same

catalyst.

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Publications ________________________________________________________

PUBLICATIONS IN SCIENTIFIC JOURNALS [1] An Investigation into the Thermal and Surface Properties of MCM-41 type

Materials for Catalytic Application Proceedings of DAE-BRNS 17th National

Symposium on Thermal Analysis, P. C. Kalsi, Rajesh V. Pai, Mrinal R. Pai,S

Bharadwaj, V.Venugopal (Eds), THERMANS (2010) 254-256.

[2] Synthesis and characterization of mesoporous materials possessing induced

and inherent acidity and their application as solid acid catalysts in some esterification reactions.(communicated)

[3] Friedel- Crafts acylation and alkylation of aromatic compounds using

mesoporous solid acid catalysts possessing induced and inherent acidity.(communicated)

[4] Mesoporous Zr-MCM-41 and Ti-MCM-41 as solid oxidation catalysts in the synthesis of epichlorohydrin.(communicated)

[5] Synthesis and characterization of a Palladium loaded Pervoskite MCM-41

material and its application as an automotive catalyst.(communicated)

PAPERS PRESENTED AT NATIONAL/INTERNATIONAL CONFERENCES

[1] “Synthesis and Characterization of Mesoporous Zirconium Silicate

using Templates for Catalytic Applications”, 2nd International Symposium on

Materials Chemistry (ISMC-2008) held at Bhabha Atomic Research Centre, Mumbai

organized by Society for Materials Chemistry, 2nd to 6th December, 2008.

[2] “An Investigation into the Thermal and Surface Properties of MCM-41 type

Materials for Catalytic Application”, 17th National Symposium on Thermal Analysis

(THERMANS 2010) held at Kurukshetra University Kurukshetra, Haryana,India,

organized by Indian Thermal Analysis Society, 9th -11th March 2010.

[3] “Friedel- Crafts Alkylation and Acylation using Acid Induced mesoporous Si-

MCM-41 Molecular Sieves”, 3Rd International Symposium on Materials Chemistry

(ISMC-2010) held at Bhabha Atomic Research Centre, Mumbai organized by Society

for Materials Chemistry, 7th to 11th December, 2010.

[4] “Synthesis and Characterization of Perovskites on MCM 41 for Automotive Exhaust Gas Purification”, 15th National Workshop on The role of Materials in

Catalysis, organized by Catalysis Society of India (CSI), held at National Centre for

Catalysis Research (NCCR), IIT-Madras, December 11th – 13th, 2011.

[5] “Synthesis and Characterization of Ti-MCM-41 for Catalytic Applications”,

National Seminar on Catalysis for Sustainable Development, The M. S. University of

Baroda, Vadodara, organized by Catalysis Society of India(CSI), Baroda Chapter,

January 27th -28th , 2012.

Documents

14.139.121.10614.139.121.106 › pdf › 2013 › jan13 › 4.pdf · Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda,