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CHAPTER 1
INTRODUCTION
Catalysis is one of the most powerful branches of chemistry
successfully applied in chemical industry and has a major impact on the
quality of human life as well as economic development. Several “pushers”
and “pullers” for innovative catalysis have been identified with regard to the
challenges for catalysis research in the 21st century. Among the pushers
appear new approaches to discover potentially new catalytic material,
including innovative preparation methods. Catalysts are considered as
chemical components capable of directing and achieving thermodynamic
equilibrium. Catalyst is a substance that increases the rate at which a chemical
reaction reaches equilibrium without being consumed during the course of
reaction. Catalysis is the word used to describe the action of the catalyst. The
word catalysis was first coined by Berzelius in 1835 to describe a number of
experimental observations such as ammonia decomposition by metals and
decomposition by potassium chlorate by manganese dioxide. Berzelius
suggested that reactions could occur at the surfaces of the solids provided the
latter possessed a catalytic force. Most important advances have been
achieved in the last few decades in the area of catalysis.
The catalysts aid the attainment of chemical equilibrium by
reducing the potential energy barriers in the reaction path. Catalyst in some
form or other is involved in more than 90% of the processes in the petroleum,
petrochemical and fertilizers industries. Catalytic processes are widely
employed in the manufacture of bulk as well as fine chemicals. The
2
developments during the last 20 years have been mandated mainly by
considerations related to the abatement and prevention of pollution,
conservation of raw materials and energy, use of alternate methodologies and
productions of more efficient drugs. Catalytic technologies play a key role in
the economic development and growth of the chemical industry due to the
increasing demand for new and green process for the production of many
industrially important chemicals. Catalysis has a wide range of applications in
chemical industry and has major impact on the quality of human life as well
as economic development. The catalytic reactions are generally classified as
homogeneous or heterogeneous, depending on the physical state of the
catalysts. Catalysts are essential, for the present technology for the production
and consumption of fuels, manufacture and processing of chemical feed
stocks and plastics. Today world wide catalysts sales exceed $5.9 billion per
year (Figure 1.1).
Figure 1.1 Chemical catalyst business
Polymerisation 39%
21%
18%
9%
8% 5% 1%
Dehydrogenation Aromatization Organic Synthesis
Oxidation
Hydrogenation
3
1.1 HOMOGENEOUS CATALYSIS
Homogeneous catalysis occur when the catalyst and the reactants
are in the same phase. Wacker process is a typical example for homogeneous
catalysis. The industrially important reactions are primarily hydroformylation,
carbonylation and addition of hydrogen cyanide. The manufacture of
sulphuric acid by lead chamber process from SO2 was catalysed by the
metastable intermediate nitrosylsulphuric acid, HNOSO4 in which NO2 acts as
catalyst. Though homogeneous liquid-phase catalytic process and
heterogeneous vapour phase process produce the same product, the factors to
be considered are the relative degree of selectivity in the two processes and
the ease of control to avoid run away reactions or explosions. A review of the
basics of the homogeneous catalysis from the industrial point of view was
given by Parshall (1980). The disadvantages of homogeneous catalysts are,
they may be poisoned and hence its separation from the product in the
reusable form is difficult. Since permanent deactivation is much more
susceptible in homogeneous catalysts, it would be better choice for chemical
industries to opt for heterogeneous catalysts, as they are much less prone to
poisoning.
1.2 HETEROGENEOUS CATALYSIS
Heterogeneous catalysis occurs when the catalyst and the reactants
are in different phase. Clays, zeolites, crystalline microporous
aluminosilicates, mesoporous materials and zeotype catalysts such as
aluminophosphates have been playing a vital role as catalysts in this area. It is
based on the adsorption concept, in which the reactants are activated by
certain active sites on the surface of solid catalyst particles followed by the
interaction of adsorbed species to produce a product and vacating the site for
continuing the catalytic cycle. Heterogeneous catalysis is crucial to chemical
4
technology. It has been the basis for most of the commercial processes in
petroleum and the petrochemical industry. In addition to the innovations in
reducing the environment impact associated with production, heterogeneous
catalysts also play a vital role in the development of new technologies in
cleaning the polluting emissions (Armor 1992).
The application of heterogeneous catalysts, especially in fine
chemical production, is expected to grow in future, as they offer several
intrinsic advantages over homogenous catalysis. There are many factors
affecting heterogeneous catalysis and the effectiveness of a particular catalyst
for a given reaction is likely to be the result of combination of factors
including surface area, porosity, acidity-basicity as well as the crystalline or
amorphous nature of the material. Some of the factors include reduced
dimensionality, increased local concentration on the surface, increased surface
area, safe handling, eco friendliness and availability.
1.3 CHOICE OF THE SUPPORT
(i) Naturally occurring materials, diatomaceous silica, activated
carbon, pumice and a variety of clays e.g naturally occuring
kaolin, sepiolite which are often mentioned in the literature
as supports for dispersing metals.
(ii) Standard inorganic supports, include silica, alumina,
magnesia, thoria, zirconia and molecular sieves.
Novel forms of materials of growing interest, are mesoporous
materials, carbon molecular sieves, fullerens, polymers. Recently, anionic
clays (hydrotalcite or layered double hydroxide) are used for dispersing
metals. In the present study, the use of anionic clays, as catalyst support for
5
dispersing metals and their use in hydrogenation reaction is described in
detail.
1.4 CLAYS
Clays are among the most common minerals on the earth’s surface
and they are indispensable to our existence. Clays are versatile materials and
hundreds of millions of tons currently find applications in many different
areas. In addition to ceramics, including building materials, clays also are
used as paper coatings, fillings, drilling muds, pharmaceuticals, cat litters, etc.
Furthermore, clays can be used as adsorbents, catalysts or catalyst supports,
ion exchangers, decolorizing agents, etc., depending on their specific
properties. For example, the high surface area and surface polarity of some
clays determine high adsorption and water-retention, capacities. Clays also
play an important role in agriculture, considering that many soils contain large
amounts of clay materials, which determine key soil properties. (structure,
texture, water retention, fertility, etc.) (Schoonheydt et al 1991, Fowden et al
1984). Clays are usually layered compounds and based on the structure and
charges of layers and interlayers they can be classified broadly into two
categories namely,
(i) Smectic type cationic clays have negatively charged
alumino-silicate layers, with small cations in the interlayer
space to balance the charge.
(ii) Hydrotalcite (HT) type anionic clays also known as layered
double hydroxides have positively charged brucite-type
metal hydroxide layers with balancing anions and water
molecules located interstitially.
6
1.4.1 Cationic clays
Cationic clays exist in vast quantities in nature and are mainly
prepared starting from the minerals (Grim 1968, Newman 1987) and are
commonly used for many chemical transformations. One example for cationic
clay is commercially modified montmorillonites, K10 or KSF, widely used as
acid catalyst or adsorbent in hydrocarbon cracking. Analogously, “Clayfen”
and “Claycop” are the acronyms for iron (III) or copper (II) nitrate supported
on K10 clay. Cationic clays are mainly obtained from natural materials, that
invariably contain impurities such as quartz, calcite, feldspars, etc., although,
they may also be synthesized, such as for example laponite1 (analog of
hectorite, by Laporte Industries) (Vaccari 1998).
1.4.2 Anionic clays
Anionic clays also known as hydrotalcites are natural or synthetic
lamellar mixed hydroxides with interlayer spaces containing exchangeable
anions (Miyata 1977, Cavani 1991) and different names are used depending
on the composition and polytype form (Drits et al 1987). Anionic clays may
be defined by their chemical composition, basal spacing and stacking
sequence (Vaccari 1999). Anionic clays are usually synthesized by
precipitation methods and are widely used as industrial adsorbents, ion-
exchangers and scavengers in industry and laboratory producing regio and
stereo selective process. Anionic clays based on hydrotalcite-like compounds
have produced sensational success in many organic syntheses in recent years
and represent a puzzle catalyst in many more reactions.
7
1.5 FACTORS INFLUENCING THE SYNTEHSIS
Structural variables
Cation size
Value of x [M3+/(M3+ + Mz+)
ratio]
Cation stereochemistry
Cation mixture (nature and
ratio)
Nature of balancing anions
Amount of interlayer water
Crystal morphology and size
Preparation variables
pH
precipitation method
precipitation temperature
Reagent concentration
Aging
Washing and drying
Presence of impurities
1.6 HYDROTALCITE LIKE ANIONIC CLAYS
1.6.1 Historical background
Half way through the nineteenth century, hydrotalcite (HT), a
mineral that can be easily crushed into a white powder similar to talc was
discovered in Snarum, Sweden during 1842. It is a hydroxyl carbonate of
magnesium and aluminium, and occurs in nature as foliated and contorted
plates of masses of the earth’s surface. The exact formula,
Mg6Al2(OH)16CO3·4H2O, was presented by Manasse (1915) Professor of
Minerology at the University of Florence (Italy). The first patent appeared in
1970 referring to a hydrotalcite like structure as an optimal precursor for the
preparation of hydrogenation catalysts (Brocker and Marosi 1970), which led
to the catalytic research into hydrotalcite like compounds in later years. For a
long time, the structure was assumed to exist of consecutive layers of brucite,
Mg(OH)2, and Al(OH)3, as proposed by Feitknecht (1942), to which he gave
the name “doppelschichtstrukturen” (Double sheet hydroxides). Feitknetcht’s
idea, was that the compounds synthesised were constituted by a layer of
8
hydroxide of one cation, intercalated with a layer of the second one. This
hypothesis was refuted by Allmann (1970) by means of X-ray analysis of
monocrystals. In fact they concluded that the two cations are localized in the
same layer and only the carbonate ions and water are located in an interlayer
(Cavani et al 1991). The HT structure closely resembles that of brucite, in
which the magnesium cations are octahedrally coordinated by hydroxyl ions,
giving rise to edge-shared layers of octahedra (Figure 1.1a). In HT, part of the
Mg2+ ions is replaced by Al3+ ions resulting in positively charged cation
layers. The compensating negative charge is provided by anions, situated in
the interlayer, which is the space between the brucite-like layers.
1.6.2 Structural features of anionic clays
Hydrotalcite (HT) like compounds can be represented by the
general formula: [Mz+1-x M3+
x(OH)2]b+[An-b/n].mH2O where M=metal;
A=interlayer anion, b=x or 2x-1 for z=2 or 1 and x is given by the ratio M3+/
(M3++ Mz+) respectively. HT like compounds have structures similar to that of
brucite Mg(OH2). The most detailed investigation of hydrotalcites was carried
out by Allmann (1970). Brucite crystallizes in a layer-type lattice as a
consequence of the presence of relatively small positively charged divalent
cations in a close proximity to the non-spherico symmetrical and highly
polarizable OH-anions. Each Mg2+ ion is octahedrally surrounded by six OH-
ions and the successive octahedral share edges to form infinite sheets. These
sheets are stacked on top of each other and are held together by weak
interactions through hydrogen (Trifiro et al 1996, Oswald et al 1977). By
isomorphous replacement of some of octahedrally coordinated Mg2+ cation in
the brucite layer with a cation having higher charge more or less similar in
radius such as Fe3+, Cr3+ and Al3+ (Shannon 1976), and the excess positive
charge produced is compensated by carbonate anions located in the disordered
interlayer domains containing water molecules. The OH sheets may exhibit
9
two stacking sequences (Allmann 1970, Frondel 1941), rhombohedral (3R), a
three layer poly type sequence, which is the normal form obtained at high
temperatures. HT structure can accommodate a wide range of variables giving
rise to the possibility of producing tailor made materials synthetically. The
main feature of hydrotalcite like structure are determined by the nature of the
brucite like sheet (Figure 1.2b) by the position of anions and water in the
interlayer spacing and by the type of stacking of the brucite like sheet. The
sheet containing cations are built as in brucite, where the cations randomly
occupy the octahedral holes in the close packed configuration of the OH ions.
The anion and water molecules are randomly located in the interlayer region
via hydrogen bonding and being free to move by breaking the bonds and
forming new ones. The oxygen atoms of the water molecules and of the
carbonate groups are distributed approximately closely around the symmetry
axes that pass through the hydroxyl groups (0.56ºA apart) of the adjacent
brucite like sheet (Cavani et al 1991). These hydroxyls are tied to the
carbonate groups directly or via intermediate H2O through hydrogen bridges
(Allmann 1970). The CO32- groups are situated in the interlayer while H2 is
loosely bound and they can be eliminated without destroying the structure. In
fact layered double hydroxide made with similar divalent and trivalent metal
ions in the hydroxyl carbonate forms represent the family of HTs and HT like
compounds.
10
Figure 1.2a Structure of Hydrotalcite (Anionic clays)
Figure 1.2b Schematic representation of brucite layer
11
A broad spectrum of divalent and trivalent cations in different
atomic compositions constitute layered double hydroxide with HT like
network. Interlayer anions can be varied which allows the tailoring of desired
catalytic properties in these materials.
1.6.3 Interlayer anions
A variety of interlayer anions can be introduced between the brucite
like sheets having different M2+ and M3+ cations. Generally carbonate is the
preferred interlayer anion and it is held more tenaciously than any other anion
between the adjacent metal hydroxide sheets. The stability of the anions
approximately are in the order of: CO32->>SO4
2-->>OH->F>Cl->Br->NO3->I-
(Miyata 1983). Chloride and nitrate ions are significantly easier to replace
than carbonate ion. The ability of hydrotalcites to scavenge the anionic
species is now being exploited in environmental chemistry and in the new
clean technologies.
1.6.4 Properties of calcined hydrotalcites
The anionic clays based on hydrotalcite like compounds have been
used as such or mainly after calcination. The catalysts obtained by calcination
of Mg-Al hydrotalcites are potential substitutes for the most common bases
such as hydroxides or carbonates of alkaline or alkaline earth metals, salts of
ammonium or amines, among others used in industry (Bastiani et al 2004).
The most interesting properties of the oxides obtained may be summarized as
follows: (Cavani et al 1991, Vaccari 1998).
(i) High Surface area: Hydrotalcite upon calcination upto 723 K
form high surface area (120-300 m2g-1) mixed oxides with
12
numerous fine pores and considerable pore volume
(0.1-0.6 cm3g-1).
(ii) Homogenous inter-dispersion : The mixed oxides formed
show homogenous interdispersion of the elements which are
also thermally stable under reducing conditions, with the
formation of very small and stable metal crystallites (with
high metal dispersion).
(iii) Basic Properties: The synergistic effects between the
elements due to the intimate interdispersion, which mainly
favours the development of unusual basic or hydrogenating
properties. The basic properties depend significantly on
composition and calcination temperature. On account of
high basic nature, HTs (mixed oxides from HT) are found to
catalyze various base catalyzed reactions like hydrogenation,
condensation, isomerization etc.
(iv) Memory effect: It is the capacity of the sample obtained by
thermal decomposition of hydrotalcite like precursors,
containing a volatile anion such as CO32- to reconstitute the
original layered structure upon the adsorption of various
anions or on exposure to air.
1.6.5 Generation of basic sites
In general hydrotalcite and HT like compounds are basic
compounds. However, calcination of M-M type mixed oxides produces very
high basicity. The basic sites thus created act as catalytically active centers for
several reactions. The activity of layered double hydroxides (LDHs) or parent
HT like compounds could be attributed to the surface water adherence,
inclusion of other gases and substrate access to the basic centers being
13
inhibited. The final basicity of these catalysts and the electronic state of the
metallic phase depend on parameters such as M/M ratio, the nature of
compensating anions and thermal treatments.
1.6.6 Memory effect of hydrotalcite
The term memory effect has been called for the capacity of the
samples obtained by thermal decomposition of HT type precursors. These
samples contain a volatile anion such as carbonate for the reconstruction of
the original hydrotalcite structure upon the adsorption of various anions or
simply upon exposure to the atmospheric air or moisture. The effect is
strongly dependent on calcination temperature (Figure 1.3). Generally, the
first stage of heating, results in the loss of the physisorbed water and heating
from 550 to 723 K leads to the simultaneous loss of hydroxyl and carbonate
groups as water and carbon dioxide respectively.
Figure 1.3 Schematic representation of Memory Effect
Therefore a lamellar microstructure was retained after thermal
decomposition and reconstitution of the HT precursors was thus permitted.
Meixnerite, a natural compound is easily obtained by reconstruction of
Mg(Al)O by memory effect (Tichit and Coq 2003).
14
1.6.7 Exchange properties of hydrotalcites
Anionic clays have good anion exchange capacities, although the
true exchange capacity (1.0 ± 1.5 meq/g) (Reichle 1986) is usually much less
than the theoretical one (3.3 meq/g for hydrotalcite). They show a resistance
towards temperature higher than that of anion exchange resins and thus are
used in some high temperature applications, such as treatment of the cooling
water of nuclear reactors. The selectivity in the exchange capacity increases
with increasing anion charge density (Miyata 1983) i.e., anionic clays
strongly prefer multiple charged anions and compounds containing nitrates or
chlorides are the best precursors for exchange reactions. However, very
important is the pH of the solution, which may favor or prevent the exchange
(Bish 1980), and has to be compatible with both the range of stability of the
starting anionic clay and the anion. There is practically no limitation to the
nature of the anions, which can compensate for the positive charge of the
brucite like sheet in the HT like network. One method of anion exchange has
been described by (Bish 1980) using the reaction of dilute mineral acids with
the carbonate form of LDH, the expulsion of carbon dioxide results in anion
exchange according to the equation,
LDH.CO3 + 2HCl LDHCl2 + CO2 + H2O (1.1)
The number of exchangeable anions, which in the structure depends
upon the charge density are carried on the host layer. The readily incorporated
CO32- anion in the preparative stages is held tenaciously within the layer and
is difficult to exchange. The presence of atmospheric CO2 during the
synthesis is therefore highly undesirable for preparing non-carbonate LDHs
(Jones and Chibwe 1986). Water molecules are localized in the interlayer, in
those sites which are not occupied by anions. It is possible to calculate the
maximum amount of water present in the interlayer on the basis of number of
15
sites present in the interlayer by assuming a close-packed configuration of
oxygen atoms and substracting the sites occupied by the anions (Miyata
1975).
A wide variety of anions can be incorporated into the network,
which indicates its promising position as exchangers and polyoxometalate
pillared LDHs which are more desirable since they are suitable for developing
materials with unique two-dimensional galleries and zeolite porosites. Such
porous solids can be used as catalysts and photo catalysts.
1.6.8 Structure and surface properties of hydrotalcites
The structure and surface properties of Mg-Al hydrotalcites and of
the resulting mixed oxides depend strongly on the chemical composition and
synthesis procedures (Cavani et al 1991). In catalysis, surface area, intimate
contact between two or more oxide components, and the size of the resulting
metal oxide clusters strongly influence the rate and selectivity of chemical
reactions (Di Cosimo et al 1998). The activity of calcined MgAl hydrotalcites
with low Al content (Mg:Al atomic ratio between 5 and 15) in the double
bond isomerization of 1-pentene shows that the activity depends strongly on
chemical composition and calcination temperature (Shaper et al 1989). The
alkylation of phenol with methanol on Mg-Al calcined hydrotalcites with ratio
3:10 has been studied and concluded that the participation of both acidic and
basic sites is required for alkylation reactions and that surface acid-base
properties are determined by the Al content (Velu and Swamy 1994). The
influence of chemical composition of Mg-Al calcined hydrotalcites on the
dehydrogenation of isopropanol was widely investigated by Corma et al
(1994) and concluded that on calcined MgAl hydrotalcites, the catalytic
activity and selectivity depend on chemical composition.
16
1.6.9 Swelling properties of hydrotalcites
Swelling of layered double hydroxides is possible and has been
observed a dependence of basal spacing for hydrotalcite on levels of humidity
and concluded that 7.9A spacing is characteristic of fully dehydrated phase.
Observations by Bish (1980) indicated that the LDH-sulphate and LDH-
chloride could be solvated with glycols, glycerol etc. Swelling of LDHs
depends on several factors similar to those of cationic clays:
a) Dependence on the nature of the exchangeable anion
(i.e., charge, mass, structure, etc.)
b) Nature of the solvent (polarity, molecular dimensions etc.)
c) The charge of the layer
The available data on the swelling properties of layered double
hydroxides is limited when compared to cationic clays.
1.6.10 Applications of hydrotalcites (HTs)
Figure 1.4 shows the commercial and industrial applications of
HTs. HTs have been used for various diverse applications from medicine to
catalysis. Anionic clays are, most importantly, the precursors for the synthesis
of multicomponent catalysts. Because of high thermal stability and
remarkable ion-exchange properties of HTs they have been widely recognized
as ion-exchangers. Hydrotalcites are used as catalysts and catalyst supports:
17
(i) HT as base catalysts
Thermally activated hydrotalcites have been successfully applied as
heterogeneous base catalysts for various organic reactions such as,
depolymerisation of paraldehyde (Walvekar and Halgeri 1973), methylation
of phenol and methanol (Velu and Swamy 1994), selective methylation of
aniline (Santhanalakshmi and Thirumalaiswamy 1996). Hydrotalcites are used
in aldol condensation reactions such as, self-aldol condensation of acetone
(Reichle 1984,1985), cross aldol condensation of formaldehyde and acetone
to give methyl vinyl ketone (Suzuki and Ono1988), condensation of
benzaldehyde with ethyl acetoacetate (Corma et al 1992), Claisen-Schmidt
condensation of substituted acetophenones and benzaldehyde for the
production of chalcones and flavanones (Climent et al 1995). Hydrotalcites
are used as base catalysts in Meerwin-Pondorf verley reduction of carbonyl
compounds in liquid phase (Kumbhar 1998a), cyanoethylation of alcohols
(Kumbhar 1998b), liquid phase hydroxylation of phenol with H2O2 over
Zr-containing hydrotalcite (Velu et al 1997), decomposition of nitrous oxide
(Kannan and Swamy 1994), steam reforming of hydrocarbons, methanation of
CO, methanol synthesis, higher alcohol synthesis and Fischer-Tropsch
reaction (Cavani 1991).
(ii) HT as catalyst supports/precursors
Hydrotalcites are found to be suitable supports or precursors for
dispersing noble metals and have been studied for the following reactions,
Aromatization of n-hexane to benzene over Pt/Mg(Al)O (Davis and Derouane
1991), methanol decomposition to synthesis gas over supported Pd catalysts
on Mg-Cr hydrotalcites (Shiozaki et al 1999), partial oxidation of methane
over highly dispersed Rh or Ni supported catalysts from Mg-Al HT as the
precursor (Basile et al 1996), Hydrogenation of acetonitrile over Ni/Mg-Al
18
HT (Cabello et al 1997), Synthesis of methyl isobutyl ketone from
hydrogenation of acetone over Mg-AlHT supported Pd or Ni catalysts (Chen
et al 1998), supports of Zeigler – Natta catalysts for ethylene polymerization
or DeSOx additives to FCC catalysts.
Figure 1.4 Applications of anionic clays
1.7 SUPPORTED METAL CATALYSTS
The supported metal catalysts differ from massive metal catalysts,
in that they consist of metal particles, which are to some extent separated
from one another. There are a number of advantages in depositing
catalytically active metals on a support such as alumina, charcoal or silica.
The metal can be highly dispersed throughout the pore system of the support.
As a result a large active metal area is produced relative to the weight of the
metal used, which is especially advantageous in case of precious metals. The
Anionic Clays Anionic Clays
Catalyst - hydrogenation - polymerization - steam reforming
Catalyst Support - Zeiglar –Natta - CeO
- For Pt and Pd
Encapsultated fullerens conductors
Protonic
Adsorbents
- halogen scavenger - PVC stabiliser - waste water treatment
Medicinal - antacid - antipepsin - stabiliser
Industrial - molecular sieves - flame retardent - ion-exchanger - filler
19
support can also improve dissipation of reaction heat and retard sintering of
metal crystallites, which results in the loss of active surface and increase
poison resistance, thus promoting a longer catalyst life. These advantages
make supported metal catalysts preferable over bulk metal catalysts for
chemical processing. The support may also have a catalytic role to perform.
For example, in petroleum reforming on Pt/Al2O3, Pt serves as
dehydrogenation site and Al2O3 as isomerisation site. The support may have
strong influence on metal dispersion and active metal surface area available
for the reaction. Hydrogenation activity and selectivity of supported metal
catalyst can be influenced by the choice of support, metal and also the
conditions under which the reaction is carried out. From the economic sense,
it is often more important to achieve highly dispersed catalyst with small
metal content and cheaper metal precursor especially when noble metals such
as Pt, Pd, Ru and Rh are used. Studies on new supported metal catalysts, such
as Pd/hydrotalcite are significant from the point of view of understanding
catalysis and for the development of new polyfunctional catalytic systems and
catalytic process. The influence of catalyst preparation conditions on the
activity, selectivity and the life of a catalyst has led to the development of
new support for metals. The properties of these catalysts depend strongly on
the state and dispersion of the metal component, the nature of the supported
metal and the nature of the support.
1.8 CHOICE OF THE METAL
Metals are classified based on their capacity to chemisorb different
gases. The characteristic feature of transition metals is that they have one or
more unpaired d electrons in the outermost electron shell. The weakly
chemisorbing non transition metals have only s or p valence electrons. It has
been suggested that unpaired electrons are necessary to bring the absorbing
molecule to the surface strongly. This is an empirical way of correlating the
20
percentage d-character of the metallic bond with the adsorption or absorption
data (Anderson 1975). There are many reports available in the literature on
the use of metals for various hydrogenation reactions. Among the transition
and noble metals, Platinum and palladium are used extensively for the
hydrogenation reactions. Palladium is most frequently used metal in the
organic synthesis in laboratory scale as well as industrial scale. Many organic
reductions in the laboratory are carried out with carbon supported palladium
catalysts. The reduction of phenol to cyclohexanone is performed with
alumina supported palladium and platinum catalysts (Neri et al 1994) and
hydrotalcite supported palladium catalysts (Narayanan and Krishna 1998).
1.8.1 Palladium
Palladium, first of platinum group metals other than platinum found
in the strange ore platina discovered by Wallson in 1803, has got many unique
and useful properties. These include its selective transformation of hydrogen
and its ability to form workable alloys with more elements than any other
metal. Its catalytic properties are outstanding in many types of reactions
including hydrogenation, isomerization, disproportionation, dehydrogenation
and oxidation reactions (Wise 1968). Palladium is one of the most important
hydrogenation catalysts, and widely employed in industrial processes (Dalla
Betta 1993, Morikawa et al 1991, Moore and Kell 1992) and in basic research
(Stacchiola et al 2001, Shen et al 2001, Surnev et al 2000). Palladium
(Figure 1.5) is very active under ambient conditions and in some reactions
palladium is highly selective in giving a product virtually free of by-products.
Also Pd is unaffected by many chemical reagents. All these properties have
made palladium very effective in many catalytic applications.
21
Figure 1.5 Palladium crystal structure: interactively and
non-interactively
Pd has proved to be an excellent catalytic hydrogenation reagent for
most of the functional groups (acids, anhydrides, amides, esters, anilines,
aldehydes, acetylenes, nitriles and aromatic systems etc.,) which can be
reduced under mild conditions. It can be supported on many substances such
as clays, carbon, alumina silica etc. Catalytic hydrogenolysis has got much
importance in synthetic and degradative chemistry. In such type of reductions,
use of palladium as catalyst is frequently met with outstanding success. The
structure of PdCl2 is shown below: (Figure 1.6).
Figure 1.6 Structure of Palladium chloride
22
1.8.2 Hydrogen effects in metals with reference to palladium
The interaction between hydrogen and palladium metal has been
attributed due to subsurface bonding of hydrogen (Lagos 1982). According to
his calculations on energy requirements, hydrogen would prefer to bind to
subsurface sites which can facilitate hydrogen diffusion into the bulk of the
metal. According to Van Hove and Hermann (1988) sites between the top of
the first layer and second layer of Pd are the surface sites and the interaction
between these sites with hydrogen is chemisorption in nature. Sites below the
second layer of Pd interstitial site, is a real subsurface site and the interaction
between these interstitial and hydrogen is absorption in nature. The sites much
below these subsurface are responsible for the hydrogen bulk diffusion. The
value of desorption energy for bulk diffusion is ~4.4 kcal/mol, which is
always smaller than the energy of desorption from a surface chemisorbed
state (20 K cal/mol). Hydrogen adsorption on the Pd surface or absorption in
its subsurface is responsible for the formation of PdHx species. The
decomposition of PdHx species during temperature rise occurs ~100C.
The migration of chemisorbed hydrogen atoms from a metal to its
support is called spill over. This is reviewed by Sermon and Bond (1973). The
mobility/diffusion of active hydrogen as atoms or ions on the surface of the
heterogeneous catalysts is an important phenomenon for the activity and
selectivity of these catalysts and for the kinetics and mechanism of the
reactions on them. Studies of tritium radioisotope retention by alumina
supported metal catalysts revealed the ratio of hydrogen atoms to surface
metal atoms in case of Pd is 5.7 (Taylor et al 1968). The ratio of hydrogen
atoms to the total amount of metal atoms is 0.7 and they concluded that the
retention must be attributed to the spillover from the above observation.
Noble metals are generally affected by hydrogen treatment, even at room
temperature which in turn changes the morphology of the metal like
23
crystallinity. It is reported that the hydrogen can be retained in a metallic
catalyst system after reduction and cooling to a reaction temperature and this
hydrogen may affect the catalyst in following ways (Paal and Menon 1983).
(i) Too strongly chemisorbed hydrogen may simply block the
active sites, which are needed for the reactant molecule. For
example in hydrogenolysis reactions, hydrogen is one of the
reactant, its strong adsorption can lead to self-inhibition of
the reaction.
(ii) Hydrogen present in the surface layers of a metal can change
the electronic properties and hence the catalytic performance
of the metal.
(iii) When a metal supported on a carrier like oxide/carbon etc,
the spillover of hydrogen to the support impart unusual
properties to the support.
In following are some of the general considerations on hydrogen
effects in metals and are listed below: (Paal and Menon 1983)
(i) There is certainly more than one type of hydrogen
interacting with surface layers of metals.
(ii) It is possible to conclude the position and form of various
types of hydrogen.
(iii) The energy necessary to remove the hydrogen in most cases
is measured.
When one tries to understand the exact role of hydrogen under
experimental conditions, most of the time, there exists lot of conflicts.
24
Sometimes hydrogen may migrate to the support or may interact with support
or it may reduce the support partially.
1.9 METAL-SUPPORT INTERACTION
The interaction of metal with the support influences the properties
and behaviour of supported metal catalyst in a significant way. The extent of
interaction is a function of the various physico-chemical properties if the
support as well as the dispersed phase in addition to their method of
preparation, activation and the metal content. Factors enhancing the catalyst
support interaction are given below.
(i) High temperature fabrication or use
(ii) Small particle size of all components
(iii) Intimate association of finely divided components
(iv) Compression of components so as to decrease the distance
between intimately associated finely divided particles.
(v) Low melting point of one or more ingredients
(vi) Prolonged exposure to coalescing conditions
(vii) Presence of flux or mineralizers in catalytic components or
in environment where catalyst is used. Minerlaizers are
alkali, alkali sulfates, phosphates, borates or hydrothermal
conditions.
At severe operating conditions, the interaction between the metal
and support becomes pronounced and it can affect the performance of the
catalyst significantly. This is due to the solid state reaction between the metal
and support resulting in the formation of some non-reducible compounds.
25
This was attributed to a strong metal-support interaction (SMSI) (Tauster et al
1978).
1.10 HYDROTALCITE SUPPORTED METAL CATALYSTS
Hydrotalcite supported palladium catalysts are generally employed
for several organic reactions such as hydrogenation, condensation etc. The
structural features of hydrotalcites on metal dispersion, CO adsorption and
phenol hydrogenation activity were studied (Narayanan and Krishna 1998).
Studies on hydrogenation of acetone using metal containing layered double
hydroxide were made in detail by Unnikrishnan and Narayanan (1999).
Calcined hydrotalcite supported palladium catalyst is the most promising
catalyst for selective hydrogenation of phenol to cyclohexanone in a one step
process (Chen et al 1999). Structural influence of hydrotalcite on palladium
dispersion and hydrogenation of phenol was discussed (Narayanan and
Krishna 1997). A comprehensive study on hydrotalcite-based catalysts for the
single-stage liquid-phase synthesis of MIBK from acetone and H2 under mild
conditions was made (Winter et al 2006). Hydrogenation of phenol was
studied over uncalcined and calcined hydrotalcite supported palladium
catalysts and their catalytic activity is compared with that of the conventional
supports such as MgO and -alumina (Narayanan and Krishna 1996).
Sonogoshira coupling reaction was studied using palladium and copper
supported on mixed oxides derived from hydrotalcites (Corma et al 2006).
Palladium and Platinum hydrotalcite-derived Mg–Al mixed oxide catalysts
were used for condensation of acetone and its selective hydrogenation to
MIBK (Nikolopoulos et al 2005). Vapour phase hydrogenation of naphthalene
was investigated in detail (Albertazzi et al 2004) using Pd/Pt catalysts
supported on basic Mg-Al mixed oxide obtained by calcination of hydrotalcite
precursor. Calcined Mg/Al hydrotalcite-supported palladium or nickel
26
catalysts was tested for hydrogenation of acetone to methyl isobutyl ketone
(Chen et al 1998).
Several reports are available on the hydrogenation reactions using
hydrotalcite supported palladium catalyst (Chen et al 1999, Winter et al 2006,
Narayanan and Krishna 1997). Hydrogenation activity and selectivity of
supported metal catalysts can be influenced by the choice of support, metal
and other conditions under which the reaction is carried out. From the
economic sense, it is often more important to achieve highly dispersed
catalyst with small metal content and cheaper metal precursor especially when
using noble metal such as Pt, Pd, Ru and Rh. Studies on hydrotalcite
supported palladium catalyst are significant and are gaining importance from
the point of view of understanding catalysis and for the development of new
polyfunctional catalytic systems and catalytic processes.
1.11 HYDROGENATION REACTION
The French chemist Paul Sabatier is the father of the hydrogenation
process. In 1897 he discovered that the introduction of a trace of nickel as a
catalyst facilitated the addition of hydrogen to molecules of gaseous carbon
compounds. Wilhelm Normann was awarded a patent in Germany in 1902
and in Britain in 1903 for the hydrogenation of liquid oils using hydrogen gas.
Hydrogenation is one of the common organic reactions used in the synthesis
of chemicals. Catalytic hydrogenation is used in the hardening of fats and
production of alcohols from aldehydes, and ketones. Hydrogenation of
nitrobenzene to aniline is an industrially important reaction. Aniline is an
important raw material used for the production of methylene diphenyl
diisocyante (MDI). It is also used as an additive for rubber process,
intermediate dyes and pigments, pesticides and herbicides. About 85% of
global aniline is produced by catalytic hydrogenation of nitrobenzene. Carbon
27
nanotube supported platinum catalyst has been used for the hydrogenation of
nitrobenzene (Li et al 2005). Platinum nanoparticle core-polyaryl ether
trisacetic acid ammonium chloride dendimer shell nanocomposites were
employed for hydrogenation of nitrobenzene to aniline with molecular
hydrogen under mild conditions (Yang et al 2006). Active carbons were used
as supports for palladium in the liquid phase hydrogenation of nitrobenzene to
aniline (Bouchenafa-Saib et al 2005). Hydrogenation of nitrobenzene was
studied over Pt/C catalysts in supercritical carbondioxide and ethanol (Zhao
et al 2004). Polymer anchored metal complex catalyst has been used for the
hydrogenation of nitrobenzene (Patel and Ram 1998). Liquid phase
hydrogenation of nitrobenzene was studied over Pd-B/SiO2 amorphous
catalyst (Yu et al 2000). Three activated carbon supports for palladium are
employed to study the hydrogenation of nitrobenzene in liquid phase
condition (Gelder et al 2002). In the present work vapour phase
hydrogenation of nitrobenzene is studied over hydrotalcite supported
palladium catalysts and a comparative study is also done with commercial
supports such as MgO and -Al2O3.
1.12 SELECTIVE HYDROGENATION REACTION
Selective hydrogenation of , unsaturated ketones, aldehydes and
esters in general is used in perfumery, hardening of fats, drugs and in
synthesis of organic chemical intermediates. The direct catalytic
hydrogenation of unsaturated aldehydes and ketones is however a challenging
proposition. The reduction of C=C bond is thermodynamically more favoured
than that of the C=O bond and hence the reaction becomes difficult in
compounds having both the bonds present in it. Selective hydrogenation of
compounds such as furfural and acetophenone, is difficult since these
molecules has got both C=C and C=O bond present in it. Therefore it is
desirable to find catalysts which will control the intramolecular selectivity by
28
hydrogenation preferentially the C=O group while keeping the olefenic bond
intact (Claus 1998). Chemoselective hydrogenation catalysts are mostly based
on supported metals involving Pt, Ru, Rh and Pd. Hydrogenation of furfural
to furfuryl alcohol is an important reaction in the chemical industries. Furfuryl
alcohol is an important fine chemical in the polymer chemistry. It is widely
used in the production of dark thermostatic resins which have strong resistant
ability against acids, bases and other solvents. It is also used for the
manufacture of liquid resins for galvanic bath tube. Ce promoted Ni-B
amorphous alloy catalyst (Ni-Ce-B) has been employed for the liquid phase
hydrogenation of furfural (Li et al 2004).
Liquid phase selective hydrogenation of furfural to furfuryl alcohol
was studied on Raney nickel catalyst modified by impregnation of salts of
heteropolyacids (Baijun et al 1998). Cu-Ca/SiO2 catalysts were prepared by
impregnation and sol-gel method and their catalytic performance was tested
for the vapor phase hydrogenation of furfural (Wu et al 2005). Furfural
hydrogenation was studied over copper dispersed on three forms of carbon,
such as activated carbon, diamond and graphitized fibres (Rao et al 1999).
Vapor phase hydrogenation of furfural to furfuryl alcohol at atmospheric
pressure over Cu-MgO catalyst prepared by co-precipitation method was
studied (Nagaraja et al 2003). The role of both SMSI and SOOI effects for the
selective hydrogenation of furfural to furfuryl alcohol was demonstrated using
platinum deposited on monolayer supports (Kijenski et al 2002). Polymer
stabilized NiB catalysts were used to test the catalytic activity of furfural
hydrogenation (Liaw et al 2005). Platinum catalysts deposited on monolayer
support containing titania have been tested for the hydrogenation of ,
unsaturated aldehyde (Kijenski and Winiarek 2000). In the present work
vapour phase hydrogenation of furfural is tried over palladium catalysts on
different supports such as hydrotalcite, MgCr, MgAl mixed oxide and MgO.
29
The catalytic activity is compared and well correlated with their characteristic
features.
Selective hydrogenation of , unsaturated ketone is a challenging
task. The catalytic hydrogenation of organic compounds containing a
carbonyl group is important in the synthesis of fine chemicals,
pharmaceuticals, dyes and agrochemicals. An important class of this reaction
is the hydrogenation of acetophenone to 1-phenyl ethanol using supported
metal catalysts. 1-phenyl ethanol finds its application in the manufacture of
perfumery products and pharmaceuticals. The control of hydrogenation
selectivity of a complex molecule containing both C=C and C=O bonds have
been studied rather intensively by several workers. However, the competitive
hydrogenation between phenyl and carbonyl group in one molecule has not
been significantly done. Chen et al (2003) have recently studied the selective
hydrogenation of acetophenone on Pt/SiO2 catalyst under gas-phase
conditions. The reduced Pt/SiO2 containing 4.7 wt.% of Pt prepared by
impregnation gave around 4% conversion and 40-80% selectivity towards
1-phenyl ethanol and 11-39% for cyclohexyl ethanol at room temperature.
Oxidized Pt catalyst provides a lower catalytic activity than the reduced Pt,
but selectivity shifts to 1-phenyl ethanol and ethylbenzene. The hydrogenation
of aromatic ring is almost suppressed by oxygen. The oxidized Pt induces
activation of C=O and hence favouring C=O hydrogenation than C=C
hydrogenation. The adsorption geometry of acetophenone on Pt surface is
assumed to be an important factor in controlling molecular decomposition and
hydrogenation selectivity. Kinetic and reaction mechanism for acetophenone
hydrogenation was proposed by Rajashekharam et al (1999) using Ni
supported on zeolite Y catalyst in the temperature range 353-392 K.
Hydrogenation of acetophenone and its derivatives using monometallic
Ni-supported and bimetallic Ni-Pt supported on zeolite Y catalysts were
studied and the structure activity and structure stability behaviour of various
30
catalysts have been studied (Malyala et al 2000). Liquid phase acetophenone
hydrogenation on Ru/Cr/B catalysts supported on silica was studied
(Casagrande et al 2002). The influence of the nature of the solvent was
studied for the selective hydrogenation of acetophenone on chromium
promoted Raney nickel catalysts and concluded that using 2-proponol-water
and sodium hydroxide as solvent, 1-phenyl ethanol can be obtained with
higher selectivity (Masson et al 1997). The use of oxidized Pd/SiO2 as the
catalyst to rapidly convert acetophenone to ethylbenzene through dehydration
process. The adsorbed oxygen on Pd was expected to have a significant
influence on the reaction selectivity and reaction pathway (Chen and Chen
et al 2004).
1.13 DEHYDROGENATION REACTION
Catalytic dehydrogenation of cyclohexanol to cyclohexanone is an
industrially important reaction for the manufacture of nylon because the two
major raw materials in producing polyamide fiber are caprolactum and adipic
acid, both of which can be obtained from cyclohexanone. Cyclohexanone
plays an important role in chemical and petrochemical industries. It may be
also used as a solvent and as a building block in the synthesis of many organic
compounds used as pharmaceuticals, insecticides etc., The catalytic
dehydrogenation of cyclohexanol has gained much importance in recent
years. Sivaraj et al (1988) reported highly selective dehydrogenation of
cyclohexanol to cyclohexanone over Cu-ZnO-Al2O3 catalysts prepared by a
deposition – precipitation method. The dehydrogenation of cyclohexanol was
studied to test the catalytic activity of these catalysts at 523 K. The activity
and stability of copper oxide and zinc oxide catalysts for the oxidative
dehydrogenation of cyclohexanol to cyclohexanone was studied (Lin et al
1988). From the industrial point of view the production of cyclohexanone is
quite limited, since, the reaction is highly endothermic and the conversion is
31
limited by thermodynamic equilibrium, the selectivity and stability are
drastically affected by increasing the reaction temperature. Therefore different
works are carried out related to catalyst preparations and modifications of
conventional catalysts. Lin et al (1988) developed a Cu-ZnO catalyst
promoted by Na, K, Rb, Cs, Ba and Zr, to enhance cyclohexanone formation
and found that the promoters increases resistance to the presence of water in
the feed and decrease the reaction temperature. According to Jeon and Chung
(1994) copper well dispersed on alkali-doped, silica-support catalysts with
specific preparation conditions are resistant to sintering. Cu/SiO2 catalysts
prepared by electrodless plating procedure indicates that the preparation
method influences significantly the activity and selectivity. A number of
kinetic models are considered for dehydrogenation of cyclohexanol on Cu+
(Fridman et al 2004). They are listed as follows,
(i) Dissociative cyclohexanol adsorption as a first step of a
reaction where cyclohexanol interacts with Cu+ and
hydrogen adsorption occurs on a X adsorption site other than
Cu+.
(ii) Dissociative cyclohexanol adsorption as a first step and
hydrogen recombination in the gas phase.
(iii) Molecular adsorption of cyclohexanol with subsequent
abstraction of the hydroxylic hydrogen as a second step,
with hydrogen adsorption on an X site other than Cu+.
In contrast to the monovalent copper, a number of kinetic models
for dehydrogenation of cyclohexanol on zero valent copper are given. The
most important model considered is as follows:
32
(i) Dissociative adsorption of cyclohexanol on metallic copper
followed by the formation of cyclohexanol alcoholate and
dissociatively adsorbed hydrogen.
(ii) Molecular adsorption of cyclohexanol with subsequent
abstraction of the hydroxylic hydrogen followed by the
formation of cyclohexanol alcoholate and dissociatively
adsorbed hydrogen.
There are several reports available for the dehydrogenation of
cyclohexanol using copper catalysts in both vapor phase and liquid phase
conditions. In literature it is very rare to find the use of palladium based
catalysts for the dehydrogenation of cyclohexanol. Hence, in the present work
vapor phase dehydrogenation of cyclohexanol to cyclohexanone is being tried
using hydrotalcite supported palladium catalyst for the first time.
1.14 COMBINATION REACTION
Catalytic hydrogenation and dehydrogenation reactions are
generally employed in the chemical industries, and such reactions involving
oxygenated compounds are particularly important in the preparation of
pharmaceuticals and fine chemicals. Researchers have challenged today to
develop processes that have minimal detrimental effects on the environment,
and applications of environmental catalysis, are increasing in such diverse
areas as alternative fuels and global pollution clean up and control (Ertl et al
1999). Some of the examples of simultaneous reactions are given below:
(i) 1,4-butanediol dehydrogenation and maleic anhydride
hydrogenation.
(ii) 1,4-butanediol dehydrogenation and furfural hydrogenation.
(iii) Cyclohexanol dehydrogenation and furfural hydrogenation.
33
Coupled catalytic reaction through hydrogen transfer between two
reactants, for example 1,4-butanediol (BDO) and maleic anhydride to produce
one valuable product -butyrolactone is an economic way by energy
utilization and the environmental friendly process. The reaction scheme is
given below in Figure 1.7.
Figure 1.7 Combination reaction of furfural and 1,4-butanediol
Catalytic hydrogenation transfer reactions have widely been
investigated in the past years mostly for the reduction of organic compounds
by using hydrogen donor, which often leads to undesirable products
(Zassinovich et al 1992). In the coupled hydrogen transfer reaction, maleic
anhydride (hydrogen acceptor) is reduced by the hydrogen from 1,4-
butanediol (hydrogen donor) to form the same desired product
-butyrolactone (Figure 1.8). -butyrolactone is currently being manufactured
by the vapour phase hydrogenation of maleic anhydride using reduced copper
chromite catalyst containing some physical and chemical promoters
(Castinglioni et al 1995). The new catalytic process combines reactions of
Maleic anhydride and 1,4-butanediol into one catalytic system to significantly
improve the yield of the catalytic hydrogenation of maleic anhydride, apart
from the better thermal balance and the effective usage of hydrogen through
hydrogen transfer between the two reactants. In addition this new catalytic
process produces one desired product (-butyrolactone) with a substantially
34
increased hydrogenation yield, and the no-hydrogen, operation also simplifies
the technical problem (Zhu et al 2005).
Figure 1.8 Combination reaction of maleic anhydride and
1,4-butanediol
The reaction represents that producing 1 mol of gamma
butyrolactone requires 1 mol of maleic anhydride and 3 mol of hydrogen,
releasing 211 KJ of heat (Messori and Vaccari 1994). Due to the strong
exothermic reaction, the temperature control over this process is very difficult
and this leads to apparent hotspots, in a tubular fixed bed reactor, which very
often causes thermal runaway and lowers the selectivity to desired product,
gamma butyrolactone. Cu based catalytsts are generally employed for the
catalytic dehydrogenation of 1,4-butanediol and maleic anhydride (Tatsumi
et al 1993, 1994). Thus the gamma butyrolactone production by coupling
process eliminates the separate hydrogen preparation procedures leading to
environmental-friendly clean process. From the reaction enthalpies of
simultaneous furfural hydrogenation and 1,4-butanediol dehydrogenation, it is
clear that the coupling process that combine furfural and 1,4-butanediol into
one catalytic process, achieve a better heat balance. The released heat of
reaction can be used for furfural hydrogenation, which drastically decrease the
exothermic heat of the process, leading to a much easier temperature control.
35
In addition to this, the hydrogen released by the dehydrogenation of
1,4-butanediol can be utilized by the hydrogenation of furfural, leading to
more perfect hydrogen mass balance. The detailed study of the coupling
process is given by Yang et al (2004). Recently US, patent by Rao et al
(2006) describes the advantage of combining the hydrogenation and
dehydrogenation reaction over Cu-MgO catalysts to overcome the equilibrium
limitations in the dehydrogenation of cyclohexanol and utilization of the
hydrogen produced in this process to carry out hydrogenation of furfural there
by avoiding the external pumping of hydrogen (Figure 1.9). The exothermic
furfural hydrogenation and endothermic equilibrium constrained cyclohexanol
dehydrogenation, are industrially important reactions. Thus furfural alcohol is
produced from furfural hydrogenation and cyclohexanone is obtained from
cyclohexanol dehydrogenation. The combination reaction in single step over
bifunctional catalysts is an important technology, as an energy efficient
process giving greater yield and hence making it cost effective.
Figure 1.9 Combination reaction of furfural and cyclohexanol
The main aim of the present work is to conduct both the
hydrogenation and dehydrogenation reactions on a single catalyst
simultaneously. In vapour phase reaction conditions, cyclohexanol
dehydrogenation and nitrobenzene hydrogenation takes place simultaneously
to yield cyclohexanone and aniline by using hydrotalcite supported palladium
catalyst. The advantage of this process is that no hydrogen is required for the
hydrogenation reaction because the hydrogen generated in the
36
dehydrogenation reaction is sufficient to promote the hydrogenation reaction
and there is a possibility to shift equilibrium towards the product side.
1.15 ECONOMY OF HYDROGEN
The hydrogen economy describes a system in which our energy
needs are predominantly met by hydrogen, rather than fossil fuels. The
economy would rely on renewable resources in the form of hydrogen gas
water, drastically changing pollution, electrical sources, infrastructure,
engines and international trade, without compromising our quality of life. In
zero emission vehicles like cars and airplanes hydrogen fuel cells are used for
power, rather than petroleum distillates. By considering hydrogen economy,
we are referencing our increasing demand for clean burning fuels that do not
cause air and water pollution nor make us dependent on dwindling energy
sources. It is important to see the ideal of the hydrogen economy as
simultaneously addressing several problems with the current state of
petroleum reliance. It is motivated by a combination of economic and
environmental-friendly process.
1.16 SCOPE OF THE WORK
Hydrotalcite upon calcination leads to the formation of mixed
oxides and it has been the subject of numerous studies especially with respect
to their characterisation. Most part of the heterogeneous catalysis on
hydrotalcites are dealt with catalyst preparation and characterisation. However
catalysis, over these materials are still at its infancy. Most of the studies are
devoted to the electronic effect of the HT support on the metal. The
hydrotalcite supported metals have not been generally well characterised and
the hydrogenation reactions over them have not been correlated with
adsorption properties such as CO or Hydrogen uptake, metal surface area,
37
dispersion and crystallite size. The support plays an important role in the
metal support interaction making the metal available for hydrogenation. With
respect to hydrotalcite supported metal catalysts, such studies are rather
limited. Therefore the preparation of hydrotalcite materials and their use as
support for palladium as well as the characterisation of such catalysts and
their catalytic evaluation for industrially important reaction such as
hydrogenation and dehydrogenation of aromatic compounds are studied.
The present study is motivated by the need to gather information on
the properties of metal supported on HTs, and correlating the structural
properties of HT with respect to metal dispersion and catalytic activity. The
aim of the present investigation is to exploit hydrotalcite materials as supports
for Pd metal. Literature survey has indicated that the hydrotalcite-like
compounds have number of advantages over the conventional base catalysts.
These catalysts are eco-friendly and possess highly desired property such as
memory effect, renumerable surfaces longevity, high selectivity and high
turnover for reactions. The vapour phase reaction can be easily carried out
over these type of catalysts in the temperature range 573-623 K. The present
work aims at the use of these Envirocats as catalysts for specific organic
reactions.
1.17 AIMS AND OBJECTIVES OF THE PRESENT STUDY
The objectives of the present study include
(i) Preparation of support MII-MIII HTs at high and low
supersaturation
(ii) Preparation of support MgCr and MgAl mixed oxide
38
(iii) Preparation of catalysts with different loadings of palladium
(0.5, 1, 2 and 5 wt.%) on hydrotalcite support by wet
impregnation method.
(iv) Preparation of hydrotalcite supported palladium catalyst by
polyol reduction method using ethylene glycol as solvent.
(v) Preparation of palladium catalysts on MgCr and MgAl
mixed oxide.
(vi) Preparation of palladium catalysts on commercial support
such as MgO and -Al2O3.
(vii) Characterisation of the supports by XRD, FT-IR,
TGA/DTA, BET surface area and SEM.
(viii) Characterisation of the palladium catalysts by XRD, FT-IR,
TGA/DTA, BET surface area, UV-Vis DRS, TPR, SEM,
TEM, and XPS.
(ix) CO chemisorption studies to find the dispersion, metal area
and particle size of palladium metal.
(x) Vapour phase hydrogenation of nitrobenzene over high and
low supersaturation hydrotalcite supported palladium
catalysts in the temperature range of 498-573 K.
(xi) Vapour phase hydrogenation of nitrobenzene over different
loadings of palladium on calcined hydrotalcites.
(xii) A comparative study over other conventional supports such
as MgO and -Al2O3 for the hydrogenation of nitrobenzene.
(xiii) Vapour phase dehydrogenation of cyclohexanol to
cyclohexanone in a fixed bed reactor using hydrotalcite
supported palladium catalyst.
39
(xiv) Simultaneous dehydrogenation and hydrogenation of
cyclohexanol and nitrobenzene over hydrotalcite supported
palladium catalysts and its correlation of catalytic activity
with the characterization results.
(xv) Selective hydrogenation of furfural out over palladium
catalysts-A comparative study.
(xvi) Selective hydrogenation of acetophenone to 1-phenyl
ethanol over calcined hydrotalcite supported palladium
catalysts.