Effect of microwave radiation on Fe/ZSM-5 for catalytic

Effect of microwave radiation on Fe/ZSM-5 for catalytic conversion

of methanol to hydrocarbons (MTH)

by

TAU SILVESTER NTELANE

submitted in accordance with the requirements for

the degree of

MAGISTER TECHNOLOGIAE

in the subject

ENGINEERING: CHEMICAL

at the

UNIVERSITY OF SOUTH AFRICA

SUPERVISOR: PROF M S SCURRELL

CO-SUPERVISOR: PROF C M MASUKU

MARCH 2018

ii

DECLARATION

I Tau Silvester Ntelane declare that this is my own work and that all the sources that I have used

or quoted have been indicated and acknowledged by means of complete references.

I further declare that I have submitted the thesis chapters to originality checking software.

I further declare that I have not previously submitted this work, or part of it, for examination at

University of South Africa for another qualification or at any other higher education institution.

……………………...

TS NTELANE (58521623)

…………………day of…………………….2018

iii

ACKNOWLEDGEMENTS

First and foremost, I will like to thank God, the Almighty for giving me the courage

and strength to complete this work. Without God’s grace, this journey would have been

impossible.

I would like to express my genuine gratitude to my supervisors Prof. MS Scurrell and

Prof. CM Masuku for their continuous guidance, encouragement, and support during

the course of this work. Their supervision gave me hope in life and forever I will remain

thankful.

Also, I would like to thank Dr. Themba Tshabalala and Mr. Charles Noakes (from

Poretech) for helping me to understand the concepts of using AutoChem II 2920

equipment.

Equally important, many thanks to my colleagues in CREATE (Catalytic REaction

Technology And Transformation of Energy) Research Flagship – UNISA. You guys

were amazing especially; Tumelo Seadira, Sibusiso Dubazana and Thabelo Nelushi.

A very special thanks to my guardian Mrs. ‘Nyane Mzamo, my elder sister Tsèpiso

Ntelane, little sister Lemohang Ntelane and my niece Karabo Ntelane. To my late

parents, thank you for everything.

“…I consider that our present sufferings are not worth comparing with the glory that will be

revealed in us…” Romans 8:18

iv

PRESENTATIONS AND PUBLICATIONS

Conference Presentations:

10th World Congress of Chemical Engineering (WCCE 10) 2017 conference,

Barcelona, Spain, 1st-5th October 2017. Poster presentation tilted: Temperature-

Programmed Surface Reaction (TPSR) study: Effect of Microwave radiation on

Fe/ZSM-5 for catalytic conversion of Methanol to Hydrocarbons (MTH).

Catalysis Society of South Africa (CATSA) 2017 conference, North-West, South

Africa, 19-22 November 2017. Poster presentation tilted: Temperature-Programmed

Surface Reaction (TPSR) study: Effect of Microwave radiation on Fe/ZSM-5 for

catalytic conversion of Methanol to Hydrocarbons (MTH).

Publications:

Manuscript titled, “Temperature-Programmed Surface Reaction (TPSR) study:

Effect of Microwave radiation on Fe/ZSM-5 for catalytic conversion of Methanol

to Hydrocarbons (MTH)” submitted to Microporous and Mesoporous Materials

journal.

v

ABSTRACT

The effect of microwave radiation on the prepared 0.5Fe/ZSM-5 catalysts as a post-synthesis

modification step was studied in the methanol-to-hydrocarbons process using the temperature-

programmed surface reaction (TPSR) technique. This was achieved by preparing a series of

0.5Fe/ZSM-5 based catalysts under varying microwave power levels (0–700 W) and over a 10

s period, after iron impregnating the HZSM-5 zeolite (Si/Al = 30 and 80). Physicochemical

properties were determined by XRD, SEM, BET, FT-IR, C3H9N-TPSR, and TGA techniques.

It was found that microwave radiation induced few changes in the bulk properties of the

0.5Fe/ZSM-5 catalysts, but their surface and catalytic behavior were distinctly changed.

Microwave radiation enhanced crystallinity and mesoporous growth, decreased coke and

methane formation, decreased the concentration of Brønsted acidic sites, and decreased surface

area and micropore volume as the microwave power level was increased from 0 to 700 W.

From the TPSR profiles, it was observed that microwave radiation affects the peak intensities

of the produced hydrocarbons. Application of microwave radiation shifted the desorption

temperatures of the MTH process products over the HZSM-5(30) and HZSM-5(80) based

catalysts to lower and higher values respectively. The MeOH-TPSR profiles showed that

methanol was converted to DME and subsequently converted to aliphatic and aromatic

hydrocarbons. It is reasonable to suggest that microwave radiation would be an essential post-

synthesis modification step to mitigate coke formation and methane formation and increase

catalyst activity and selectivity.

Keywords: Methanol-To-Hydrocarbons (MTH); Microwave radiation; Post-synthesis

modification step; Temperature Programmed Surface Reaction; 0.5Fe/ZSM-5 catalysts; Coke

deposition; Methane formation; Multi-mode microwave oven; Catalyst deactivation; Bed

shape; ZSM-5 zeolite.

vi

TABLE OF CONTENTS

DECLARATION ....................................................................................................................... ii

ACKNOWLEDGEMENTS ..................................................................................................... iii

PRESENTATIONS AND PUBLICATIONS ........................................................................... iv

ABSTRACT ............................................................................................................................... v

TABLE OF CONTENTS .......................................................................................................... vi

LIST OF FIGURES .................................................................................................................. ix

LIST OF TABLES .................................................................................................................... xi

LIST OF SCHEMES................................................................................................................ xii

LIST OF ABBREVIATIONS ................................................................................................ xiii

CHAPTER 1 .............................................................................................................................. 1

1.1 Introduction ..................................................................................................................... 1

Figure 1.1: Methanol to hydrocarbons (MTH) process (Baliban et al., 2012). ........................ 2

1.2 Problem Statement .......................................................................................................... 3

1.3 Aims and Objectives ....................................................................................................... 4

1.3.1 Aim .......................................................................................................................... 4

1.3.2 Objectives ................................................................................................................ 4

1.3.3 Thesis Statement ...................................................................................................... 5

1.3 Structure of Thesis .......................................................................................................... 5

References .................................................................................................................................. 6

CHAPTER 2 .............................................................................................................................. 9

2.1 Methanol Economy ......................................................................................................... 9

2.2 MTH Process ................................................................................................................ 14

2.3 Commercialized MTH Processes .................................................................................. 15

2.3.1 Commercialized MTG Processes .......................................................................... 16

2.3.2 Commercialized MTO Processes .......................................................................... 17

2.3.3 Commercialized MTP Processes ........................................................................... 18

2.4 ZSM-5 Zeolite ............................................................................................................... 20

2.5 Reaction Mechanisms ................................................................................................... 22

2.5.1 Olefin Methylation ................................................................................................ 24

2.5.2 Olefin Cracking ..................................................................................................... 26

2.5.3 Hydrogen Transfer ................................................................................................. 27

2.5.4 Cyclization ............................................................................................................. 27

2.5.5 Aromatic Methylation............................................................................................ 28

vii

2.5.6 Aromatic Dealkylation .......................................................................................... 30

2.6 Factors Influencing Selectivity towards MTH Products ............................................... 31

2.6.1 Influence of Si/Al ratio .......................................................................................... 31

2.6.2 Influence of reaction temperature .......................................................................... 33

2.6.3 Influence of Partial Pressure and Co-feeding water .............................................. 33

2.6.4 Influence of contact time ....................................................................................... 34

2.6.5 Influence of modifying ZSM-5 with elements ...................................................... 34

2.7 Catalyst Deactivation due to Coke Formation .............................................................. 36

2.8 Studies to Mitigate Coke Formation ............................................................................. 37

2.9 Microwave Radiation .................................................................................................... 40

2.9.1 Application of Microwave Radiation in Catalysis ................................................ 41

2.9.2 Mechanisms and types of Microwave heating ...................................................... 42

2.9.2.1 Direct heating and Selective heating .............................................................. 43

2.9.3 Heterogeneous catalysts and Microwave absorption ............................................. 44

2.9.3.1 Microwave heating of ZSM-5 zeolite ............................................................ 45

2.9.4 Microwave oven and Bed shape ............................................................................ 46

References ................................................................................................................................ 49

CHAPTER 3 ............................................................................................................................ 67

3.1 Introduction ................................................................................................................... 67

3.2 Reagents used................................................................................................................ 67

3.3 Experimental Procedures and Instruments used ........................................................... 68

3.3.1 Catalysts preparation and post-synthesis modification ......................................... 68

3.3.1.1 Microwave post-synthesis modification of catalysts ..................................... 69

3.3.2 Catalysts Characterization ..................................................................................... 70

3.3.2.1 Nitrogen Adsorption-desorption Analysis ..................................................... 70

3.3.2.2 Powder X-Ray Diffraction (p-XRD) analysis ................................................ 70

3.3.2.3 Thermogravimetric analysis (TGA) ............................................................... 70

3.3.2.4 Scanning electron microscopy (SEM) analysis .............................................. 71

3.3.2.5 Fourier Transform Infrared (FT-IR) Spectroscopy ........................................ 71

3.3.2.6 Propylamine-Temperature programmed surface reaction (C3H9N-TPSR) .... 71

3.3.3 Catalysts testing (MeOH-TPSR experiments) ....................................................... 72

References ................................................................................................................................ 73

CHAPTER 4 ............................................................................................................................ 74

4.1 Introduction ................................................................................................................... 74

4.2 Catalysts characterization ............................................................................................. 74

viii

4.2.1 X-ray diffraction analysis ...................................................................................... 74

4.2.2 Nitrogen adsorption-desorption analysis ............................................................... 76

4.2.3 Thermogravimetric analysis .................................................................................. 80

4.2.4 Scanning electron microscopy (SEM) analysis ..................................................... 81

4.2.5 Fourier Transform Infrared (FT-IR) Spectroscopy ............................................... 84

4.2.6 C3H9N-TPSR for acidic sites determination .......................................................... 87

4.3 Catalysts Testing (MeOH-TPSR experiments) ............................................................. 90

4.3.1 MeOH-TPSR ......................................................................................................... 90

References .............................................................................................................................. 101

CHAPTER 5 .......................................................................................................................... 105

5.1 Conclusions ................................................................................................................. 105

5.2 Recommendations ....................................................................................................... 107

APPENDIX A ........................................................................................................................ 108

ix

LIST OF FIGURES

Figure 1.1: Methanol to hydrocarbons (MTH) process. ............................................................ 2

Figure 2.1: Methanol production processes operating worldwide. .......................................... 10

Figure 2.2: Global Methanol demand by end use (a) 2011 demand = 55.4 Million Metric tons

(b) 2016 demand forecast = 92.3 Million Metric Tons. ........................................................... 12

Figure 2.3: 2021 Global Methanol Demand by end use, 2021 Demand = 95.2 Million Metric

Tons: The Importance of MTO – 1 in 5 tons of methanol. ...................................................... 13

Figure 2.4: ExxonMobil photo of Mobil’s commercial MTG plant in New Zealand in the 1990s.

.................................................................................................................................................. 17

Figure 2.5: (a) Skeletal diagram of the (100) face of ZSM-5 zeolite (10-membered ring

openings), and (b) 3D channel structure of ZSM-5 zeolite. .................................................... 22

Figure 2.6: Published areas in which microwave radiation has been applied. ........................ 41

Figure 2.7: Electromagnetic spectrum. .................................................................................... 41

Figure 2.8: Types of microwave heating. ................................................................................ 43

Figure 2.9: Multi-mode microwave oven schematic diagram. ................................................ 48

Figure 3.1: Experimental set-up for post-synthesis modification step. ................................... 70

Figure 4.1: XRD patterns for the parent zeolites (HZSM-5) and microwave-untreated and

treated catalysts: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700 ......................................................... 75

Figure 4.2: N2 adsorption-desorption isotherms of microwave-treated and untreated catalysts

at -196 °C: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700 catalysts. .................................................. 77

Figure 4.3: SEM micrographs of microwave-treated and untreated catalysts. ........................ 83

Figure 4.4: FT-IR spectra for the parent zeolites (HZSM-5) and microwave-untreated and

treated catalysts: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700 catalysts. ......................................... 84

Figure 4.5: C3H9N-TPSR data profiles for n-propylamine decompostion to propylene and

ammonia. .................................................................................................................................. 87

Figure 4.6: Propylene profiles obtained from the C3H9N-TPSR experiments over varying Si/Al

ratio; 30 (a) and 80 (b) for determining the concentrations of Brønsted acidic sites (cB). ...... 88

Figure 4.7: MeOH-TPSR data profiles for methanol conversion to C1–C5 aliphatic and C6–C8

aromatic hydrocarbons. ............................................................................................................ 91

Figure 4.8: C1 Alkane (Methane) profiles obtained from the MeOH-TPSR experiments over

varying Si/Al ratio; 30 (a) and 80 (b). ...................................................................................... 93

x

Figure 4.9: C2 Aliphatic hydrocarbons (Ethane and Ethylene) profiles obtained from the

MeOH-TPSR experiments over varying Si/Al ratio; 30 (a) and 80 (b). .................................. 95

Figure 4.10: C3 Alkane (Propane) profiles obtained from the MeOH-TPSR experiments over

varying Si/Al ratio; 30 (a) and 80 (b). ...................................................................................... 96

Figure 4.11: C3-C4 Alkenes (Propylene and Butene) profiles obtained from the MeOH-TPSR

experiments over varying Si/Al ratio; 30 (a) and 80 (b). ......................................................... 97

Figure 4.12: C6 Aromatic hydrocarbon (benzene) profiles obtained from the MeOH-TPSR


Figure 4.13: C7-C8 Aromatics (Xylene and Toluene) profiles obtained from the MeOH-TPSR


Figure 4.14: Increasing power level against maximum peak temperature for MTH process over

0.5FeZ10/0-700 catalysts (a) and 0.5FeX10/0-700 catalysts (b)..................................................... 100

Figure A1: SEM micrographs of microwave-treated catalysts. ............................................. 108

Figure A2: C5 Alkene (Pentene) profiles obtained from the MeOH-TPSR experiments over

varying Si/Al ratio; 30 (a) and 80 (b). .................................................................................... 109

Figure A3: C4-C5 Alkanes (butane and Pentane) profiles obtained from the MeOH-TPSR

experiments over varying Si/Al ratio; 30 (a) and 80 (b). ....................................................... 110

xi

LIST OF TABLES

Table 3.1: List of reagents used together with their suppliers. ................................................ 68

Table 4.1: Physicochemical properties of microwave-untreated and treated catalysts

(0.5FeX10/0-700 and 0.5FeZ10/0-700). ............................................................................................ 79

Table 4.2: Coke quantity for Methanol-To-hydrocarbons conversion over microwave-treated

and untreated catalysts (0.5FeX10/0-700 and 0.5FeZ10/0-700). ...................................................... 81

Table 4.3: Effect of Fe loading, and microwave post-modification on the ratio of the band

intensities of the FT-IR spectra and relative crystallinity estimated from FT-IR spectra. ...... 86

Table 4.4: Determination of Brønsted acidic sites from C3H9N-TPSR. .................................. 90

Table A1: Maximum desorption peak temperatures (Tmax values) from MeOH-TPSR data for

adsorbed methanol over microwave untreated (0.5FeX10/0 & 0.5FeZ10/0) and treat (0.5FeX10/119-

700 & 0.5FeZ10/119-700) catalysts. .............................................................................................. 109

xii

LIST OF SCHEMES

Scheme 2.1: Dual-cycle mechanism for Methanol to Hydrocarbons over H-ZSM-5. ............ 24

Scheme 2.2: Two proposed olefin methylation mechanisms; Co-adsorbed mechanism and

Surface methoxide mechanism. ............................................................................................... 26

Scheme 2.3: Two proposed aromatic methylation mechanisms; top (direct mechanism) and

bottom (stepwise mechanism).................................................................................................. 29

xiii

LIST OF ABBREVIATIONS

BET Brunauer‐Emmett‐Teller

BTX Benzene, Toluene, and Xylene

CTAB Cetyltrimethylammonium Bromide

DICP Dalian Institute of Chemical Physics

DMTO Dimethyl ether or Methanol-To-Olefin

DMTP Dimethyl ether or Methanol-To-Propylene

FCC Fluid Catalytic Cracking)

FMTP Fluidized-bed Methanol-To-Propylene

FT-IR Fourier Transform Infrared Spectroscopy

GHG Greenhouse gas

GHz Gigahertz

IUPAC International Union of Pure and Applied Chemistry

KTA Kilo Tons per Annum

LHSV Liquid Hourly Space Velocity

MGC Mitsubishi Gas Chemical

MTA Mega Tons per Annum

MTBE Methyl-tert-butyl ether

MTG Methanol-To-Gasoline

MTH Methanol-To-Hydrocarbons

MTO Methanol-To-Olefins

MTP Methanol-To-Propylene

OGD Olefin-to-Gasoline/Distillate

PDH Propane Dehydrogenation

SAPO-34 Silicoaluminophosphate-34

SEM Scanning Electron Microscopy

SMTO SRIPT Methanol-To-Olefins

SMTP SRIPT Methanol-To-Propylene

SRIPT Shanghai Research Institute of Petrochemical Technology

TGA Thermogravimetric Analysis

TIGAS Topsøe’s improved gasoline synthesis

TPSR Temperature Programmed Surface Reaction

xiv

wt.% Weight percent

XRD X-Ray Diffraction

ZSM-5 Zeolite Socony Mobil-5

Chapter 1: Introduction

1

CHAPTER 1

Introduction and Problem Statement

1.1 Introduction

In recent years, research and development of alternative processes for clean fuel sources to

replace petroleum energy sources has attracted tremendous attention. This is due to the

challenges facing the transport sector which heavily relies on petroleum as the primary energy

source in countries like South Africa (Singh, 2006), and the ever-growing propylene gap

amongst others (Plotkin, 2014). These challenges include; greenhouse gas (GHG) emissions

which are the results of fossil fuels consumption to meet the energy demands like in South

Africa (Escobar et al., 2009; Pone et al., 2007). These emissions cause many harmful effects

such as climate change, rise in sea level, receding of glaciers, loss of biodiversity, etc. (Gullison

et al., 2007). Moreover, uncertainty over the future price of crude oil, caused by political

instability in the crude oil exporting countries (Nkomo, 2017), and the rising exhaustion of

fossil fuel resources (Escobar et al., 2009), are also driving forces to search for alternative fuel

sources and petrochemical derivatives. There is a need to develop alternative, sustainable,

renewable, efficient and cost-effective energy sources which result in lower or zero emissions.

Various alternative energy resources that have been explored so far to produce fuels include

biomass, biogas (Murphy and McCarthy, 2005), alcohols, vegetable oils (Agarwal, 2007),

natural gas and coal (Han and Chang, 2000). Due to its physical and chemical properties,

methanol has been considered and verified as an eye-catching cleaner fuel to produce liquid

fuel (high octane gasoline) and petrochemicals from methanol derivatives. Methanol-to-

hydrocarbons (MTH) and Fischer–Tropsch (F-T) processes have been used as alternative

routes to petroleum refineries (Baliban et al., 2012).

Methanol is converted to hydrocarbons (gasoline and olefins) using acidic zeolites or zeotypes

as catalyst. The commercially used zeolites are ZSM-5 and SAPO-34 (Amghizar et al., 2017;

Keil, 1999). In the MTH process, methanol is converted to the mixture of hydrocarbons in the

range, C2 to C10 which includes; paraffins, olefins and aromatics. Since diesel or jet fuel

fractions are not produced in this process, Mobil developed another route which is abbreviated


2

as MOGD. MOGD is the combination of methanol-to-olefins (MTO) and Mobil olefin-to-

gasoline/distillate (OGD) processes. The distillate that is produced, is also hydrotreated to form

diesel or kerosene (Figure 1.1) (Baliban et al., 2012; Keil, 1999).

Figure 1.1: Methanol to hydrocarbons (MTH) process (Baliban et al., 2012).

The ZSM-5 zeolite is mainly used as a catalyst to produce a high yield of hydrocarbons due to

its catalytic properties (Keil, 1999). However, in the MTH process, catalyst deactivation by

coke deposition is the main challenge. Using other conventional commercial catalysts, coke

deposition is even increased. Coke deposition is attributed to diffusion limitation of larger

molecules within the zeolite pore cavity which then hinders the contact between the reactant

and product species with zeolite active sites. Coke deposition decreases the catalyst activity

and selectivity (Benito et al., 1996; Liu et al., 2016; Tian et al., 2015; Ye et al., 2015).

Much interest has been paid in the preparation of nanoporous, mesoporous or hierarchical

ZSM-5 from microporous ZSM-5 zeolite to overcome diffusional limitations. Hierarchical or

mesopores in zeolites can be created during synthesis directly using templating techniques or

in a post-synthesis modification step. Post-synthesis modification involves the dealumination


3

or desilication of part of the zeolite using a chemical leaching approach. As it can be observed

from the literature, several attempts have been conducted to increase the accessibility of the

catalyst active sites in order to overcome catalyst deactivation due to coke deposition in the

MTH process (Ahmadpour and Taghizadeh, 2015; Chen et al., 2016; Wang et al., 2014; Wei

et al., 2015).

However, all the studied techniques to overcome coke deposition have some drawbacks and

are covered in detail in the literature review in this present work. For example, the chemical

leaching approach involves multiple steps to create the hierarchical or mesoporous zeolites,

and an undesired Si/Al ratio is sometimes attained. The templating approach involves the use

of expensive templates, and it is complicated to use templates as there is phase separation

between micro and mesoporous phases when using surfactants as templates. Moreover,

creating nano-sized ZSM-5 zeolite also has difficulties, as it is difficult to separate the seeding

particles and the nano-sized ZSM-5 zeolite aggregates (Bjørgen et al., 2008; Majano et al.,

2009; Meng et al., 2016; Meng et al., 2009; Shen et al., 2014). In this present study, we seek

to use microwave radiation as post-synthesis modification step to overcome coke deposition

and methane production in the MTH process by either creating the hierarchical or mesoporous

catalysts.

1.2 Problem Statement

Despite the superior results of ZSM-5 in the MTH process, which include high selectivity

towards desired hydrocarbons, resistance to deactivation due to coke deposition as compared

to other zeolites, etc. (Chen et al., 2017; Mokrani and Scurrell, 2009; Wu et al., 2013; Zhang

et al., 2017), the microporous characteristic of ZSM-5 zeolite significantly influences mass

transfer of the reactants and products to/from the active sites, and as a result, ZSM-5 still suffers

from diffusion limitations of larger molecules resulting in restrictions in the catalyst activity

and lifetime (Bjørgen et al., 2008; Hu et al., 2014). Due to ZSM-5’s unique pore structure, only

gasoline range hydrocarbons are allowed to diffuse out, and larger molecules than those found

in this range are responsible for ZSM-5 zeolite deactivation. Catalyst deactivation due to coke

deposition is still a significant challenge which needs to be solved.


4

In this study, we investigate the possibility of using microwave radiation to modify metal

impregnated ZSM-5 by creating either mesopores or hierarchical structures in ZSM-5 zeolite

in order to overcome catalyst deactivation which is associated with high methane production

in the MTH process. Microwave ovens are cheap and readily available for industrial, domestic

and laboratory use. Already exploited modification steps like chemical leaching, templating,

etc. have some drawbacks which limits their wider application.

1.3 Aims and Objectives

1.3.1 Aim

The aim of this work is to create mesoporous or hierarchal iron-impregnated ZSM-5 (Fe/ZSM-

5) catalysts using microwave radiation as s post-synthesis modification step to overcome

catalyst deactivation in MTH process.

1.3.2 Objectives

The objectives set to achieve the aim of this work are:

(i) To prepare HZSM-5 from NaZSM-5 zeolite by varying Si/Al ratio; a low value of

30 and a high value of 80 was used.

(ii) To prepare 0.5Fe/ZSM-5 catalysts using the incipient wetness impregnation (IWI)

method from HZSM-5 parent zeolite and iron(III) nitrate nonahydrate.

(iii) To modify the prepared 0.5Fe/ZSM-5 catalysts using microwave radiation under

varying microwave power levels (0-700 Watts) and over a 10 s duration.

(iv) To characterize the microwave treated and untreated catalysts prepared above

using: nitrogen adsorption-desorption analysis, powder x-ray diffraction (p-XRD)

analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM)

analysis, Fourier transform infrared spectroscopy (FT-IR), and propylamine-

temperature programmed surface reaction (C3H9N-TPSR).


5

(v) To study the effect of microwave radiation on the product distribution in the MTH

process using methanol-temperature programmed surface reaction (MeOH-TPSR)

technique.

1.3.3 Thesis Statement

Microwave radiation is an effective post-synthesis modification step to overcome catalyst

deactivation due to coke deposition in MTH process.

1.3 Structure of Thesis

This thesis is divided into five chapters:

Chapter 1: this chapter presents the introduction, problem statement, research aim and

objectives, and the thesis statement.

Chapter 2: this chapter presents a literature review of this research giving the details on

methanol economy, methanol to hydrocarbons, ZSM-5 zeolite, catalyst deactivation,

studies to mitigate coke deposition and their limitations, and microwave radiation.

Chapter 3: this chapter presents the general experimental procedures followed, list of

reagents used, and list of characterization techniques used.

Chapter 4: this chapter presents the Results and Discussion obtained.

Chapter 5: this chapter presents the conclusions and recommendations necessary for

future work.


6

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Chapter 2: Literature Review

9

CHAPTER 2

Literature Review

2.1 Methanol Economy

Thanks to the Nobel Laureate George Olah and coworkers, and some researchers before him

for advocating for the use of methanol and its derivatives as alternative fuel sources. The aim

was to replace the petroleum-based chemicals and fuels in order to use methanol and its

derivatives as a route to sustainable development in the future (Olah, 2005; Yang and Jackson,

2012). This advocacy is a result of the recognized rapid population growth and expanding

economies which causes increasing global energy demand. To meet this energy demand,

petroleum-based fuels have been and are still being used as primary energy sources. The

bottlenecks for using these petroleum-based fuels are fluctuating petroleum prices, greenhouse

gas emissions which constantly possesses threats to the environment, and the depletion of

world fossil resources (Riaz et al., 2013). This has augmented the search and use of

environmentally friendly alternative fuels.

A few promising alternative energies being studied so far include hydrogen energy, nuclear

energy, bio energy, solar energy, hydro energy and wind energy. However, due to the extreme

importance of petroleum-based fuels and chemicals, the latter which serve as raw materials in

various chemical industries, the use of petroleum-based fuels will not completely leave the

history stage. For that matter, methanol is a promising alternative fuel as it is also a vital raw

material in chemical industries, and because of its use in the transportation sector (Su et al.,

2013), hence the so called term, “methanol economy”.

Besides the methanol economy, the hydrogen economy as an alternative, is an option also being

studied worldwide (Seadira et al., 2018). However, as pointed out by Olah (2005), hydrogen

generation and its utilization as a clean fuel have some drawbacks. Its use as a fuel is not

convenient, and its transportation possesses some safety concerns. Moreover, the needed

infrastructure is not available for a hydrogen economy although it may eventually be

developed, and the hydrogen volumetric power density is also a drawback. A methanol

economy can serve as a feasible and realistic means of storing hydrogen energy by producing

methanol from syngas. Methanol is easy to handle and to transport essentially using already


10

existing infrastructure, and it can be used conveniently as a fuel and a raw material for

manmade hydrocarbons and their products (Olah, 2005). Moreover, processes used for

methanol production have cleaner production with some challenges related to catalyst

deactivation (Riaz et al., 2013).

Methanol is produced starting from multiple sources either traditionally or renewably from

coal, natural gas, biomass, landfill gas and recycling captured carbon dioxide (CO2) from

power plant/industrial emissions and atmospheric CO2 (Bozzano and Manenti, 2016; Pérez-

Fortes et al., 2016). The latter has recently been receiving great interest as a green-methanol

synthesis pathway as this will alleviate the main manmade cause of global warming (Olah,

2005; Pérez-Fortes et al., 2016). Despite that, mature technologies are available for natural gas

conversion to syngas in order to produce methanol and are widely used in chemical process

industries. Producing methanol from natural gas still dominates the industry (Amigun et al.,

2010; Riaz et al., 2013). Figure 2.1 depicts methanol production processes currently operating

worldwide under the following operating conditions; pressures of 50-100 atm and temperatures

of 200-300 °C (Bozzano and Manenti, 2016).

Figure 2.1: Methanol production processes operating worldwide (Bozzano and Manenti,

2016).


11

Methanol plays a vital role as a major chemical intermediate and its transformation into very

important products and commodities drive and span our daily life (Riaz et al., 2013). Methanol

and its derivatives are utilized for the following: fuels, medicines, pesticides (Yang and

Jackson, 2012), plastics, resins, adhesives, paints, antifreeze, silicones, polymers, single-cell

proteins (Bozzano and Manenti, 2016), as a hydrogen carrier for fuel cells, wastewater de-

nitrification, electricity generation, transesterification of vegetables oils for biodiesel

production (Riaz et al., 2013), and so on. Among the methanol derivatives, the most important

include: formaldehyde, methyl-tert-butyl ether (MTBE), acetic acid and dimethyl ether

(Bozzano and Manenti, 2016). Figure 2.2–2.3 shows a summary of methanol demand by end

use for 2011 and projected demand forecast for 2016 and 2021.


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Figure 2.2: Global Methanol demand by end use (a) 2011 demand = 55.4 Million Metric tons

(b) 2016 demand forecast = 92.3 Million Metric Tons (Riaz et al., 2013).


13

Figure 2.3: 2021 Global Methanol Demand by end use, 2021 Demand = 95.2 Million Metric

Tons: The Importance of MTO – 1 in 5 tons of methanol (Alvarado, 2005).

As shown in Figure 2.2-2.3, in 2011, formaldehyde production was the largest methanol

consumer accounting for almost 32.0% of methanol demand worldwide, and with the 2016 and

2021 methanol demand forecast, formaldehyde production become the second largest after

falling to 25.0% and 26.9% and the gasoline/fuel applications becoming the largest methanol

demanding sector rising to 31.0% and 28.5% respectively as anticipated. Compared to 2011

end use demand (6.0%), the methanol-to-olefins/propylene (MTO/MTP) demand is anticipated

to become a high growing sector, rising to 22.0% and 19.3% by 2016 and 2021. With some

methanol derivatives declining, such as formaldehyde, while others are strongly increasing

such as MTO/MTP, and biodiesel for example, the methanol market is in a changing state with

overall world methanol demand anticipated to grow to 95.2 million metric tons by 2021

(Alvarado, 2005). This shows that there is an increasing interest in methanol and its derivatives

(synthetic gasoline, diesel, olefins, and some wide-ranging chemical products). Ensuring novel

ways to design the catalysts to overcome catalyst deactivation by coke, green-methanol

synthesis pathways, further technology development and increasing practical application, a

methanol economy is a realistic and feasible alternative to replace petroleum-based fuels and

mitigate greenhouse emissions and atmospheric CO2.


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2.2 MTH Process

Out of two teams of Mobil scientists working on different projects, one team was attempting

to convert methanol to ethylene oxide over ZSM-5 zeolite at Mobil Chemical Edison, New

Jersey, while the other team was attempting to methylate isobutene with methanol using ZSM-

5 at Mobil Oil’s Central Research Laboratory in Princeton; this led to the discovery of

methanol-to-hydrocarbons (MTH) by coincidence as neither of the aforementioned reactions

proceeded according to their expectations (Keil, 1999; Mokrani and Scurrell, 2009). This gave

rise to a large number of detailed investigations on the reaction mechanisms involved and

catalyst optimization.

The discovery of methanol into value-added hydrocarbons (gasoline with high octane number)

over solid acidic zeolite in the 1970s, and the first and second oil catastrophes in 1973 and 1978

respectively, triggered immense interest for industrial development of the reaction and the

search for alternatives to petroleum (Chang, 1983; Keil, 1999). Since methanol can be

produced from various carbon sources by proven technology, the discovery by Mobil offered

a new alternative source for petroleum and classical synthetic fuel processes like Fischer-

Tropsch (Chang, 1983).

This led to the demonstration and development of the first Mobil pilot plant (4 barrels per day)

using fixed-bed reactor to check the feasibility of the MTG process in response to the New

Zealand government request (Keil, 1999; Ye et al., 2015). In addition to the fixed-bed MTG

process, a 4 barrels per day MTG pilot plant using a fluidized-bed technology which surpassed

fixed-bed reactor technology in terms of excellent heat transfer properties, high octane

numbers, and continuous catalyst regeneration (Keil, 1999); this was developed in Paulsboro,

New Jersey and later on, scaled-up to a demonstration plant (100 barrels per day) in Wesseling,

Germany during 1981-1984 (Ye et al., 2015). This plant successfully demonstrated the

performance of the fluidized-bed reactor system for MTG and methanol-to-olefins (MTO)

technology (Keil, 1999). In 1985, the first Mobil MTG commercial plant was implemented in

New Zealand with a gasoline capacity of 14,500 barrels per day based on natural gas converted

into methanol through syngas (Ye et al., 2015).

After Mobil’s discovery about the synthetic shape-selective ZSM-5 zeolite in MTG process,

Union Carbide reported the successful synthesis of silicoaluminophosphate (SAPO) catalyst


15

and developed the MTO process in 1986 using SAPO with the olefins yield exceeding 90%.

They reported that by modifying the process, about 60% selectivity towards ethylene and

propylene can be attained (Keil, 1999). In the Mobil’s MTG process, the hydrocarbons

produced spin from C1 to C11 with roughly 80% selectivity towards C5+ (benzene fraction)

hydrocarbons (Ye et al., 2015). This boils down to a conclusion that the zeolite topology and

the operating conditions used determine classes of hydrocarbons produced from methanol over

acidic zeolites or zeotypes (Ilias and Bhan, 2012).

As the oil prices dropped again in the 1980s, further developments of commercial processes

were forced to stop. However, this never thwarted the investigations on the bench scale and

submissions of the patent applications (Keil, 1999). Recently, much interest has been paid into

the MTH processes especially in catalyst synthesis, reaction mechanism, reaction kinetics,

process development and reactor scale-up. This is driven by roaring high energy demand,

global warming, search for petrochemicals from non-petroleum based sources, and depleting

fossil fuels which resulted in a search for alternatives for fossil fuels. Many researchers, for

example Stöcker have reviewed the catalyst details and reaction mechanisms for MTH

processes especially MTO/MTP, MTG and Mobil’s olefin-to-gasoline and distillate (MOGD)

(Stöcker, 1999). Therefore, understanding the reaction mechanism and shape selectivity of the

targeted products over relevant zeolite catalyst is very important.

2.3 Commercialized MTH Processes

Owing to enormous demands of petrochemicals which drive and span our daily life which can

be derived alternatively from methanol and its derivatives, especially propylene, ethylene, and

high octane gasoline. Numerous companies and institutions have put more effort in the research

and development of the MTH process since it was first discovered by Mobil Corporation. Major

progress has been accomplished regarding catalyst synthesis, the reaction mechanisms

involved, and process research and development (Stöcker, 1999). Several MTH processes have

either been commercialized or are ready to be commercialized.


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2.3.1 Commercialized MTG Processes

In October 1985, the world’s first commercial MTG plant using a fixed-bed system was set

into operation in New Zealand (Allum and Williams, 1988). The plant was designed to produce

14,450 barrels per day of high octane gasoline, and for 1986 as a whole, 584,780 tons of

gasoline were produced which was equivalent to nearly 35% of New Zealand’s premium

gasoline consumption (Maiden, 1988). Several parallel reactors were used in the commercial

MTG unit in New Zealand, and alternating catalyst regeneration was done to maintain the

continuous operation, as the ZSM-5 was slowly deactivated by coke deposition even though it

was shown as a feasible catalyst of choice for the MTG process (Ye et al., 2015).

Mobil corporation currently held by ExxonMobil, continued further improving the MTG

process and started to license the improved MTG process. In 2010 the first MTG unit was

started up in Shanxi, China with a production capacity of 100 kilo tons of gasoline per annum

using the improved ExxonMobil MTG technology (Ye et al., 2015). Figure 2.4 shows the

ExxonMobil photo of Mobil’s commercial MTG plant in New Zealand in the 1990s (Hindman,

2017). Besides the Mobil Oil methanol-to-gasoline (MTG) process, the Haldor-Topsøe TIGAS

(Topsøe’s improved gasoline synthesis) process was also commercialized (Losch et al., 2015).


17

Figure 2.4: ExxonMobil photo of Mobil’s commercial MTG plant in New Zealand in the

1990s (Hindman, 2017).

Some improved ExxonMobil MTG fixed-bed licenses issued are listed below (Hindman,

2017):

(i) 2006, Shanxi, China, 100 KTA

(ii) 2007. Wyoming, U.S., 600 KTA

(iii)2011, Shanxi, China, 1 MTA

(iv) 2012, Louisiana, U.S., 1.6 KTA

(v) 2012, Louisiana, U.S., 500 KTA

(vi) 2014, U.S., 640 KTA

2.3.2 Commercialized MTO Processes

For more than 30 years, the DICP (Dalian Institute of Chemical Physics) has been keen on the

research and development of the MTO process, with the aim of developing a commercially

available MTO process. In the 1990s, DICP scientists successfully upgraded the MTO

technology to DMTO (dimethyl ether or methanol-to-olefin) technology after completing an

MTO pilot test in 1991. In late 2010, China witnessed a couple of milestone developments of

its own MTO technology based on DMTO technology. The world’s first commercial MTO


18

process was started up successfully in its first trial run with production capacity of 600,000

tons of light olefins (ethylene and propylene) per annum in Baotou, China (Ling, 2011).

Later on in October 2010, the first licencing agreement on DMTO-II was signed with the

production capacity of 670,000 tons of light olefins per annum (Ling, 2011). So far, twelve

commercial units applying DICP’s DMTO technology have been realized in China with annual

production capacity of 6.5 million tons of light olefins (Liu, 2017). Several MTO processes

have either been commercialized or are ready for commercialization; these include in addition

to the DMTO process, the MTO process by Norsk Hydro/UOP, and the SINOPEC’s MTO

(SMTO) process by SINOPEC (Bleken et al., 2012; Ye et al., 2015).

2.3.3 Commercialized MTP Processes

Propylene is the second largest olefin feedstock, and it is produced traditionally as a by-product

of the steam cracking process for ethylene production, and from FCC (fluid catalytic cracking)

units (Wei et al., 2011). Since it is produced as a by-product, its production volume is

determined by ethylene and gasoline production, respectively. The above mentioned traditional

two propylene sources are now facing a down-turn as stream crackers in North America and

the Middle East are now shifting to ethane feedstocks resulting in a decrease in propylene

production volume, and the propylene demand continuously growing and outpacing the growth

of new steam crackers and new FCC units. With the 2015 forecast, about 80% of propylene

demand was supplied by the traditional refineries and steam crackers, and 20% was projected

to be met by alternative processes (Ding. and Hua, 2013). However, crude oil cracking to

produce light olefins is expected to be in a short supply in the foreseeable future (Zhang et al.,

2014). Therefore, new processes with high propylene yield are required to avoid a continuously

growing propylene gap.

High demands for propylene derivatives and propylene scarcity are the driving forces for

propylene prices overshooting the projected 2015 forecast. Therefore, the research and

development of alternatives for traditional propylene sources and propylene derivatives has

been extensively pursued. These alternatives include; propane dehydrogenation (PDH), olefin

metathesis, propylene derivatives from non-propylene feedstocks, and methanol-

olefin/propylene especially in the countries where naphtha and natural gas are very scarce like

China (Ding and Hua, 2013).


19

However, there are a few limitations that can be pointed out for the each of the following

alternative process; even though PDH has received much attention, few feedstock sources are

available to sustain the process in the near future as fossil fuels used to produce propane are

depleting unlike the MTH process where methanol has diversified carbon sources. Thus, MTH

process can be regarded as the most promising option because of methanol on its own and

various petrochemicals obtained from methanol derivatives (Bozzano and Manenti, 2016; Riaz

et al., 2013; Su et al., 2013; Yang and Jackson, 2012). Even though olefin metathesis is a well-

developed technology, the lucrativeness of the metathesis process is determined by the price

difference between ethylene, butane, and propylene. Olefin metathesis units also need access

to large C4 streams that are free of isobutylene and butadiene (Ding and Hua, 2013).

The non-propylene feedstocks that have been proven to produce propylene derivatives include,

butanol, acrylic acid, propylene glycol, epichlorohydrin, and 1,3-propanediol processes. These

processes are at or close to the commercialization stage. These no-propylene feedstocks are

produced using bio and fossil-based routes but there are also some limitations towards vast

implementation of each one of them. For example, the cost of the fermentation process, the

price of purchased propylene and ethylene, the price of biomass, the cost of producing ethylene

oxide from ethylene, the cost and number of catalysts used to produce 1,3-propanediol, the cost

of chlorine as a raw material for producing glycerin, the cost of alcohols to produce biodiesel

and by-product (glycerin) (Ding and Hua, 2013), and the price of vegetable oils used to produce

biodiesel which surpasses the price of crude oil (Kennedy, 2017). These aforementioned

limitations are very crucial in determining whether the process will be competitive in the

market or not compared to other commercially available propylene producing processes. This

is not to say that they are not feasible alternatives for producing propylene and its derivatives

apart from the MTH process.

Propylene with high yield can be obtained from the MTP process. The MTP process was first

put into the commercialization stage in 2010 in China by the Lurgi company using a fixed-bed

reactor and a high-silica nanosized HZSM-5 zeolite as a catalyst (Ahmadpour and Taghizadeh,

2015; Zhang et al., 2017). In 2011, two MTP processes were successfully started up in Ningxia

Autonomous Region, and in Duoteng, Inner Mongolia Autonomous Region, China by Shenhua

Ningxia Coal Chemical, and Datang International Power Generation Company Ltd.

respectively (Ding and Hua, 2013). The JGC/Mitsubishi DTP (dimethyl ether-to-propylene)


20

process and Lurgi MTP process are major processes based on MTP technology. Several MTP

technologies were also developed, including: the SMTP technology, FMTP (fluidized-bed

methanol-to-propylene) technology, and the DMTP technology developed by Shanghai

Research Institute of Petrochemical Technology, Tsinghua University, and the DICP based on

their DMTO process, respectively (Ding and Hua, 2013).

2.4 ZSM-5 Zeolite

Numerous types of catalysts with different topologies, compositions, morphologies, and under

different reaction conditions have been examined extensively in the MTH process before

(Chang, 1983), and even after the discovery of the MTH process over acidic ZSM-5 zeolite in

the 1970s by Mobil Corporation (Rostamizadeh and Yaripour, 2016; Yaripour et al., 2015;

Yaripour et al., 2015; Zhang et al., 2017). Molten ZnCl2, P2O5, Al2O3, silica gel, silica-alumina,

activated Al2O3 or Type-C Al2O3, amorphous silica-alumina, and metal molybdate catalyst

modified with nickel chromite were used in the MTH process prior to ZSM-5 zeolite. Some of

the products obtained were light alkanes, mostly methane, hexamethylbenzenes, isobutene,

dimethyl ether, carbon monoxide and carbon dioxide (Chang, 1983).

For the last four decades, a comprehensive screening of a large variety of catalysts

characterized by different compositions (distribution and number of acidic sites), topologies

(cavity/channels networks and dimensions), and morphologies (micro and mesoporosity,

crystal dimensions) has been made (Olsbye et al., 2012). The aims were to increase selectivity

towards light olefins, gasoline range hydrocarbons, and to find a catalyst which is resistant to

deactivation by coke deposition (Mokrani and Scurrell, 2009). The MTH process has been

assessed over a range of catalysts starting from small-pore zeolites (SAPO-34, ZSM-58,

Sigma-1, HFU-1, etc.), medium-pore zeolites (EU-2, ZSM-5, etc.), and large-pore zeolites

(CON, modified Y zeolites, Ba/dealuminated mordenite, Mn/ZSM-12, etc) (Mikkelsen and

Kolboe, 1999; Yaripour et al., 2015; Yaripour et al., 2015; Zhang et al., 2017).

Among all the catalysts studied, Zeolite Socony Mobil-5 (ZSM-5) was recognized to be a

remarkably effective catalyst for the MTH process in an industrial application owing to its


21

inherent advantages: high octane number gasoline produced (Mokrani and Scurrell, 2009), high

selectivity towards propylene (Zhang et al., 2017), strong resistance to catalyst deactivation

caused by coke deposition which permits reasonable cycle lengths to be attained without

extreme catalyst requirements (Wu et al., 2013; Yurchak, 1988), a 10-ring interconnected

channel system with high/adjustable Si/Al ratio, high thermal/hydrothermal stability, and high

catalytic activity (Chen et al., 2017; Yaripour et al., 2015). The appropriate choice of the

catalyst is very critical as it plays a major role in the success of the process.

Beside medium-pore ZSM-5 zeolite, SAPO-34 zeolite with small-pores is used as a catalyst in

industrial MTH processes for high selective production of light olefins (Howe et al., 2016).

Despite the fact of high selectivity towards ethylene, SAPO-34 is deactivated by coke

deposition quicker than ZSM-5 zeolite. Thus, fluidized-bed technology is the best choice for

any MTH processes using SAPO-34 zeolite as a catalyst (Ye et al., 2015). The high selectivity

towards light olefins is still a challenge and there is still ample opportunity for research to

design novel catalysts for the selective production of light olefins. ZSM-5 zeolite as a catalyst

is still a subject of interest owing to its properties towards deactivation by coke deposition.

ZSM-5 zeolite is a porous aluminosilicate framework composed of AlO4 and SiO4 tetrahedra

(Fathi et al., 2014). ZSM-5 zeolite has 10-membered ring openings, and three-dimensional

(3D) pore structure consisting of straight channels intersecting with sinusoidal channels. Figure

2.5(a) demonstrates the skeletal diagram of the face of ZSM-5, where the 10-membered ring

openings are the entrances to the sinusoidal channels. Figure 2.5(b) demonstrates two channel

structures in ZSM-5 zeolite; straight channels (0.53 nm × 0.56 nm), and sinusoidal channels

(0.51 nm × 0.55 nm) which create the 3D network responsible for ZSM-5 zeolite’s high

selectivity and stability (Omojola et al., 2018).


22

Figure 2.5: (a) Skeletal diagram of the (100) face of ZSM-5 zeolite (10-membered ring

openings), and (b) 3D channel structure of ZSM-5 zeolite (Weitkamp, 2000).

2.5 Reaction Mechanisms

There has been lively debate surrounding the MTH process since it was discovered regarding

the following two aspects: the origin of the first C−C bond from the C1 units, that is methanol


23

or DME, and the mechanism by which MTH proceeds. Theoretical and experimental work

indicated that direct coupling of two methanol molecules does not occur (Lesthaeghe et al.,

2006; Lesthaeghe et al., 2007; Marcus et al., 2006). Early work postulated that the MTH

reaction is autocatalytic in the sense that the small amounts of hydrocarbons greatly enhance

the rate of C1 units conversion until a steady-state is reached, and with an observable catalytic

induction period (Ono and Mori, 1981). It has been shown that by co-feeding methanol with

higher hydrocarbons (C2+), the catalyst induction period is significantly reduced, and this

shows that both olefins and C2+ species play a critical role in the MTH process (Langner, 1982;

Ono and Mori, 1981).

Dahl and Kolboe, (1996) proposed the hydrocarbon pool mechanism in which the pool of

(CH2)n species within the zeolite pores is formed by methanol to produce light olefins, alkanes,

and aromatics. They showed that high hydrocarbons are not formed by successive methylations

of ethylene. In recent years, it is widely accepted that the MTH process proceeds through a

dominating indirect route called “hydrocarbon pool mechanism” (Ilias and Bhan, 2012). Mole

et al., (1983) indicated that 12C-atoms of ethylene matched exactly the 12C-atoms of toluene

when co-feeding 13C-methanol with 12C-toluene over ZSM-5, and this shows that ethylene was

formed by dealkylation of methylbenzenes, not by direct C-C coupling of C1 units.

Bjørgena et al., (2007) assessed the reactivity of the organics residing in the zeolite pores

during the MTH reaction over ZSM-5 using transient 12C/13C methanol-switching experiments.

It was observed that 13C content and evolution of ethylene resembled exactly the ones for p/m-

xylene and trimethylbenzenes, indicating that ethylene was formed predominantly from the

lower methylbenzenes. In contrast to ethylene, the 13C content incorporation of C3+ olefins

matched each other, indicating that propylene and higher alkenes are formed to a considerable

extent from alkene methylations and interconversions (e.g., cracking reactions) not from

ethylene methylation.

It has been shown that two catalytic cycles run simultaneously during the MTH reaction over

H-ZSM-5 namely: ethylene formation from the lower methylbenzenes followed by

remethylation-aromatics carbon pool cycle, and a methylation/cracking cycle involving only

the C3+ alkenes–olefin carbon cycle (Scheme 2.1) (Bjørgen et al., 2007; Svelle et al., 2006).


24

These two catalytic cycles co-exist in the MTH reaction over ZSM-5; as a result, the

aromatics/ethylene carbon pool cycle cannot run without the olefin-carbon pool cycle.

Scheme 2.1: Dual-cycle mechanism for Methanol to Hydrocarbons over H-ZSM-5 (Ilias and

Bhan, 2012).

Ilias and Bhan, (2012) proposed six major chemistries that occur within the dual cycle

mechanism for the MTH process over ZMS-5 zeolite namely, (i) olefin methylation, (ii) olefin

cracking, (iii) hydrogen transfer, (iv) cyclization, (v) aromatic methylation, and (vi) aromatic

dealkylation.

2.5.1 Olefin Methylation

Scheme 2.1 indicates that olefin homologation (methylation of the olefin carbon double bond)

exists and is one route by which methyls are incorporated into hydrocarbon products (Ilias and

Bhan, 2012). Tau and Davis, (1993) indicated that olefin methylation reactions exist over ZSM-

5 zeolite using isotopic tracer studies. Several researchers have also shown that the olefin

methylation route overwhelm in some one-dimensional 10-membered ring zeolites (ZSM-22,

and ZSM-23) as these zeolites hinder both the hydrocarbon pool mechanism and secondary


25

reactions (Cui et al., 2008; Teketel et al., 2011). These catalysts (ZSM-22, ZSM-23, and ZSM-

48) gave high selectivity towards hydrocarbon products rich in C5+ aliphatic hydrocarbons

(Teketel et al., 2011).

Ilias and Bhan, (2012) indicated that for all feed compositions that were tested using 13C-

labelled co-feeds (propylene and toluene), the C5-C7 olefins all had very similar fractions of

isotopomers containing three 13C atoms as expected based on methylation of the (n - 1) olefins,

showing that these olefins were formed primarily from olefin homologation reactions. In

contrast, the expected 13C atoms of propylene based on ethylene homologation did not match

the observed 13C distribution of propylene for any of the feed compositions tested, showing

that ethylene homologation is not a significant route for propylene formation, and ethylene

does not play any significant role in any olefin-based catalytic cycle. The contributions of olefin

methylation are hard to be accessed due to some difficulties to separate the olefin methylation

reactions from the reactions caused by the hydrocarbon pool mechanism and secondary

reactions overwhelm the primary reactions over ZSM-5 or SAPO-34 as catalysts (Song et al.,

2000).

Two mechanisms for olefin methylation by methanol and dimethyl ether have been proposed

namely (Scheme 2.2): (1) coadsorbed mechanism in which methanol/DME and an olefin are

adsorbed on a single acid site and react in a single, associative/concerted step without formation

of a surface methoxide intermediate; and (2) a surface methoxide mechanism in which

protonation of methanol or DME by the zeolite acidic site protonate to form a surface

methoxide that desorbs upon reacting with an olefin with lower carbon content (ethylene) to

form higher carbon content olefin (propylene) (Ilias and Bhan, 2012; Maihom et al., 2009).


26

Scheme 2.2: Two proposed olefin methylation mechanisms; coadsorbed mechanism and

surface methoxide mechanism (Ilias and Bhan, 2012).

2.5.2 Olefin Cracking

Olefin cracking is the important route used commercially in Lurgi’s methanol-to-propylene

(MTP) process to increase the production of propylene using highly siliceous ZSM-5 catalyst

(Olsbye et al., 2012). Ilias and Bhan, (2012) showed that there was a high content of 12C atoms

from 12C-DME in propylene when a 4 kPa partial pressure of 13C-propylene was co-reacted

with a 70 kPa partial pressure of 12C-DME, showing that propylene was formed by cracking of

larger olefins.

The olefin cracking mechanism requires an olefin to be protonated to form an alkoxide

intermediate, followed by β-scission of the alkoxide to form a smaller alkoxide and smaller

olefin. Subsequently, the smaller alkoxide desorbs to form another olefin and leaves behind a

proton to regenerate the acid site of the zeolite. Speciation of olefin isomers is critical in

determining the product selectivity for the MTH process. For example, if olefin cracking is

considerably faster than olefin methylation, then the product distribution should be rich in light

olefins. In contrast, if olefin cracking is slower than olefin methylation, then the product


27

distribution may be rich in larger olefins, which may cyclize to eventually form aromatics (Ilias

and Bhan, 2012).

2.5.3 Hydrogen Transfer

Hydrogen transfer is a bimolecular reaction which involves transfer of a hydrogen atom

between an adsorbed surface alkoxide and a cyclic or acyclic alkane or alkene. The identity of

species that can undergo hydrogen transfer is changing with conversion, and as a result, the

rate of hydrogen transfer varies with conversion (Ilias and Bhan, 2012).

Hydrogen transfer involves the abstraction of a hydrogen atom and is mediated by

carbocationic transition states. As compared to linear alkanes, branched alkanes are facile

hydrogen donors because the resulting carbocationic transition states are more stable as

inferred by density functional theory (Boronat et al., 2000). Alkenes with tertiary allylic C−H

bonds are even more reactive hydrogen donors than branched alkanes because they delocalize

positive charge more effectively, which leads, in turn, to more stable carbocations (Ilias and

Bhan, 2012).

2.5.4 Cyclization

As shown in Scheme 2.1, the olefin- and aromatic-based cycles are not independent of one

another and “communicate” through cyclization and aromatic dealkylation steps. Cyclization

is related to aromatization in that cycloalkanes and cycloolefins are not stable products of MTH

and are quickly dehydrogenated to form aromatics. There are two possible generalized routes

to olefin cyclization and aromatization. One involves dehydrogenation of olefins to form dienes

and trienes that undergo cyclization to aromatics. In the second route, olefins first form

cycloalkanes and are subsequently dehydrogenated to form aromatics. In both of these routes,

dehydrogenation occurs through hydrogen transfer reactions in which olefins or cycloalkanes

donate hydrogen to other hydrocarbons that act as hydrogen acceptors (Ilias and Bhan, 2012).

Dass and Odell, (1988) studied the conversion of C1-C7 alkanes, methanol, and co-feeding

benzene over ZSM-5 zeolite as catalyst. They found that the gasoline produced from n-heptane

at reaction temperature > 400 °C, is comparable to that one produced from methanol, at 370

°C and 410 °C reaction temperatures; the main products of n-heptane conversion were mainly


28

alkanes presumably from n-heptane cracking, and gasoline range products at elevated

temperatures, respectively. By co-feeding benzene, they found that the yields to ethylbenzene

are higher for n-heptane than for methanol. Moreover, they found that toluene is also a

significant product of n-heptane and its yield is not attributed to adding benzene. They

suggested that toluene is probably produced by ring closure of the heptane ring. The formation

of these large aromatics shows dehydrogenation through hydrogen transfer, cyclization and

side reactions of cyclization, such as olefin oligomerization and alkylation of aromatics. The

question of whether cyclization occurs prior to olefin dehydrogenation or vice versa still needs

to be addressed.

2.5.5 Aromatic Methylation

Aromatics, specifically polymethylbenzenes, play a critical part in MTH catalysis in that these

species, along with olefins, act as scaffolds for methylation reactions. The isotopic labeling

studies of co-feeding methanol with aromatic benzene or toluene show that 13C atoms from

methanol were situated in the aromatic ring as well as in the methyls groups – an excess of 13C

was most pronounced in tri- and tetramethylbenzenes over ZSM-5 catalyst. This shows that

aromatic methylation involves sequential methylation and it is the main reaction taking place

over ZSM-5 catalysts (Mikkelsen et al., 2000).

However, Ilias and Bhan, (2012) showed that aromatic methylation is not the only route for

producing other methylbenzenes. From their 12C/13C isotopomer distributions, o-xylene and p-

xylene containing seven 13C atoms were formed from 13C-toluene methylation, but when the

feed composition was varied using 13C-toluene and 12C-DME and/or 12C-propylene, the

isotopic distribution of o-xylene closely matched the expected isotopic distribution, indicating

that o-xylene comes from toluene methylation entirely, whereas the observed 13C content for

p-xylene and lager polymethylbenzenes differ from the expected 13C content by methylation of

the (n - 1) methylbenzene, showing that these aromatics are not exclusively formed from

methylation reactions instead cyclization reactions occur predominantly for C8+ aliphatics to

form p-xylene and larger aromatics.

There are two proposed distinct mechanisms for aromatic methylation like in olefin

methylation: A stepwise, (consecutive, or dissociative) mechanism and a direct (concerted, or

associative) mechanism (Martinez-Espin et al., 2017; Svelle et al., 2012), and are shown in


29

Scheme 2.3. The primary difference between the two, is that the stepwise mechanism involves

the formation of the covalently bonded surface methoxy group as a reaction intermediate, while

in the direct mechanism, there is no such reaction intermediate formed.

In the stepwise mechanism, methanol or DME is physisorbed unimolecularly on the Brønsted

acid site through hydrogen bonding and subsequent Brønsted proton transfer to form water and

methoxy intermediate. The formed methoxy group is then attached to the species to be

methylated resulting in a cationic species, which is assumed to be quickly re-oriented and

deprotonated to regenerate the original Brønsted acid site of the zeolite and the neutral

methylated product. In the direct mechanism, methanol or DME reacts directly with the species

to be methylated in a concerted fashion resulting in a protonated methylated product. Then,

quick re-orientation and back protonation of the zeolite occur which result in the neutral

methylated product and water or methanol, respectively (Svelle et al., 2011).

Scheme 2.3: Two proposed aromatic methylation mechanisms; top (direct mechanism) and

bottom (stepwise mechanism) (Martinez-Espin et al., 2017).


30

2.5.6 Aromatic Dealkylation

At the commercial scale, SAPO-34 zeolite is used as a catalyst to produce light olefins almost

exclusively through aromatic dealkylation (Olsbye et al., 2012); this shows the vital role played

by this chemistry route to olefin production. Experimental evidence has clearly shown that

methylbenzenes and their protonated counterparts (or other related cyclic species) are central

reaction intermediates for alkene formation in the MTH reaction (Olsbye et al., 2012).

Two distinct reaction mechanisms have been proposed on how these alkenes are formed from

these species (mechanisms for aromatic dealkylation) namely: the paring, and the side-chain

methylation mechanism. In the paring mechanism, the gem-methylation of a methylbenzene

results in ring contraction. Subsequently, an alkyl substituent is formed which then cracks to

produce light olefins. The side-chain methylation mechanism is also commenced by gem-

methylation of a methylbenzene species which results in the removal of a methyl hydrogen,

thus forming an exocyclic double bond, which can undergo side chain methylation. This side

chain can then crack to form ethylene or propylene. The starting point for both aforementioned

mechanisms is the gem-methylation step in which the aromaticity is broken down and the

charged species is formed due to aromatic ring being doubly methylated. The key intermediate

of the side-chain methylation mechanism that distinguishes it from the paring mechanism is

the formation of methylbenzenes that also have ethyl, propyl, or other alkyl groups. The paring

mechanism provides the mechanism that results in the ring carbon of methylbenzene being

incorporated into the olefin formed, and also methyl group carbons on the aromatic to become

incorporated into the benzene ring (Ilias and Bhan, 2012).

Ilias and Bhan, (2014) using isotopic labeling studies indicated that at 250–450 °C and low

conversions (< 10 C%) with varying isotopic feed compositions of 13C/12C over ZSM-5 zeolite,

the 13C-content of ethylene and propylene from tetramethylbenzene most closely match the

experimentally observed 13C-contents of ethylene and propylene. This shows that aromatic

dealkylation to form ethylene and propylene occurs through the paring mechanism and that

tetramethylbenzene is the predominant aromatic precursor for light olefin formation for MTO

conversion over ZSM-5 zeolite. Understanding the communication between olefin- and

aromatic-based cycles is a critical step in controlling selectivity of the MTH process over ZSM-

5, since both cycles are not isolated from one another. Current interest in the MTH process has

shifted to the production of olefins.


31

2.6 Factors Influencing Selectivity towards MTH Products

From the dual cycle mechanism over ZSM-5 zeolite, it has been shown that these two cycles

namely – namely, the olefin-carbon cycle and aromatic-carbon cycle mechanism -

communicate and are not independent of one another. Thus, to enhance the selectivity towards

either light olefins or gasoline range hydrocarbons, one cycle must be suppressed and the other

enhanced. Therefore, understanding this dual cycle is crucial. As an example, in order to

enhance selectivity towards light olefins, it is essential to inhibit their conversion to aromatics.

Numerous approaches that influence high selectivity towards either light olefins or gasoline

over ZSM-5 have been investigated comprehensively. These approaches include: modifying

the reaction condition (co-feeding water and/or varying methanol partial pressures, varying

reaction temperature, varying contact time, etc.), varying Si/Al ratio of ZSM-5, varying ZSM-

5 crystal size, modifying ZSM-5 with elements, etc. (Chang et al., 1984; Chang and Silvestri,

1977; Khanmohammadi et al., 2016; Chang, 1984; Luk'yanov, 1992; Mokrani and Scurrell,

2009; Stöcker, 1999; Spivey et al., 1992).

2.6.1 Influence of Si/Al ratio

As it has been shown here, the MTH process has been assessed over a range of catalysts starting

from small-pore zeolites, medium-pore zeolites, and large-pore zeolites under different

reaction conditions (Mikkelsen and Kolboe, 1999; Yaripour et al., 2015; Yaripour et al., 2015;

Zhang et al., 2017). Most of the open literature deals with either SAPO-34 or ZSM-5 catalyst

in the MTH process. SAPO-34 shows high selectivity towards ethylene production in the MTO

process due to its small pore openings and moderate acid strength (Behbahani et al., 2014);

however, high rate of catalyst deactivation due to coke deposition is observed on SAPO-34

(Álvaro-Muñoz et al., 2014; Chen et al., 1999). For that matter, ZSM-5 zeolite is a very

promising candidate for MTO as a catalyst owing to its inherent advantages outlined in the

previous sections, and it has been used at large MTO scale (e.g. Lurgi’s methanol-to-propylene

(MTP) process) (Olsbye et al., 2012). However, the selectivity towards light olefins in the

MTO over ZSM-5 catalyst is relatively low (Behbahani and Mehr, 2014). ZSM-5 zeolite has

also been used in the MTG process to produce high octane gasoline.

ZSM-5 zeolite is a porous aluminosilicate framework composed of AlO4 and SiO4 tetrahedra

(Fathi et al., 2014). The framework is negatively charged, and this is attributed to the partial


32

substitution of Si in the framework by Al. The charge of the unit cells is usually localized by

the extra-framework cations (usually Na+), and these cations are exchangeable. Therefore,

protonated zeolite can be derived by exchanging the cations with ammonium ones first,

followed by thermally decomposing exchanged ammonium cations providing strong acid sites

which give ZSM-5 zeolite high catalytic activity. As inferred above, the strength and density

of acidic sites, more precisely the number of accessible Brønsted acid sites per unit cell are

vital in determining the activity of the ZSM-5 zeolite and product distribution in MTH process.

Zeolite acid sites may be related to tetrahedrally coordinated Al content (Benito et al., 1996;

Primo and Garcia, 2014); thus, Si/Al ratio plays a vital role in determining the life span and

activity of the ZSM-5 catalyst. ZSM-5 zeolite with low Si/Al ratio (i.e., high Al content)

generally has a high density of acidic sites, and therefore high activity which lead to fast

catalyst deactivation by rapid coke formation (Wan et al., 2016). Increasing Si/Al ratio of the

zeolite can enhance the selectivity towards light olefin in the MTH process (Chang et al., 1984;

Prinz and Riekert, 1988).

Wan et al., (2016) studied the effect of varying Si/Al ratio (23-411) of ZSM-5 zeolite in the

MTG process. It was observed that decreasing the Si/Al ratio facilitated higher methanol

conversion, and also decreased the catalytic activity which can be increased by increasing the

Si/Al ratio; C2-C4 alkenes decreased, C1-C4 alkanes increased, a low gasoline yield was

produced, and the rate of the aromatization increased reaction, which led to formation of coke

precursors. As a result, rapid catalyst deactivation occured. The ZSM-5 zeolite with Si/Al ratio

of 217 indicated 100% methanol conversion, low coke formation and high gasoline yield under

studied reaction conditions.

Benito et al., (1996) showed that the greater proportion of higher molecular olefins was

observed as Si/Al ratio increases. Moreover, the ratio of Brønsted/Lewis acid site increases,

and total acidity decreases as the Si/Al ratio increases. This was explained using the mechanism

of propagation of oxonium ions. From small scale to larger scale in MTP process, high-silica

ZSM-5 zeolite normally with Si/Al ratio greater than 170 is being used as a suitable catalyst

due to its inherent advantages to give high propylene selectivity and catalytic stability

(Ahmadpour and Taghizadeh, 2015; Hu et al., 2012; Khanmohammadi et al., 2016;

Rostamizadeh and Taeb, 2015).For the MTG process, ZSM-5 zeolite with Si/Al ratio in the

range of 30-70 is used as a suitable catalyst with better catalytic activity and life span (Chang


33

et al., 1984; Chang and Grover, 1977; Bjørgen et al., 2008; Wan et al., 2014; Zaidi and Pant,

2004). The choice of suitable Si/Al ratio is very crucial to ensure high catalytic activity and

long life span.

2.6.2 Influence of reaction temperature

The reaction temperature for the MTH (MTG and MTO) process has an effect on the product

distribution. It was shown that at high reaction temperature (>500 °C), lower selectivity

towards light olefins was observed accompanied by the formation of increasing number of

carbon oxide molecules over ZSM-5 catalyst (Chang and Silvestri, 1977; Zhang et al., 2014);

however, as the reaction temperature was decreased (≤ 500 °C), the selectivity towards the light

olefins was enhanced (Chang et al., 1984; Chang and Silvestri, 1977; Zhang et al., 2014).

Generally, the optimum reaction temperature used to harvest high yield of olefins especially

propylene, ranges from 450°C to 500 °C (Chang et al., 1984; Wu et al., 2013; Zhang et al.,

2014). It was also shown that reaction temperature has an effect on the aromatic selectivity,

and as the reaction temperature was increased, the selectivity towards aromatics also increased

(Di et al., 2013; Inoue et al., 1995). For example, Di et al., (2013) showed that when the MTG

reaction temperature was increased from 340 to 420 °C, the aromatics increased from 70.8

wt.% to 84.3 wt.%, respectively.

2.6.3 Influence of Partial Pressure and Co-feeding water

Olefin selectivity is greatly enhanced by conversion at subatomospheric methanol partial

pressures. Chang et al., (1984) indicated that when the methanol partial pressure was increased

from 1 atm to 6 atm using pure methanol as feed, C2–C5 olefins decreased from 58 wt.% to 41

wt.%, and aromatics increased from 13 wt.% to 20 wt.%. In another set of experiments, when

methanol partial pressure was decreased from 1 atm to 0.4 atm by co-feeding water and keeping

the total pressure constant at 3 atm, C2–C5 olefins increased from 65 wt.% to 77 wt.%, and

aromatics decreased from 12 wt.% to 7 wt.%. This indicated that subatomospheric and/or

atmospheric methanol partial pressures (Wu et al., 2013), and partial pressures greater than

atmospheric (Di et al., 2013; Wan et al., 2014) enhance olefin, and aromatic selectivity,

respectively. Moreover, this indicates the effect of water in the feed stream on the hydrocarbons

distribution from methanol conversion over ZSM-5 catalyst. The presence of water in the feed


34

stream slows down the catalyst deactivation (Ghavipour et al., 2013), and increase the ethylene,

and propylene selectivity (Wu et al., 2013).

2.6.4 Influence of contact time

Several authors have shown the influence of varying contact time on the MTH product

distribution over zeolites (ZSM-5) (Chang, 1984; Chang et al., 1984; Chang and Silvestri,

1977; Inoue et al., 1995; Luk'yanov, 1992; Stöcker, 1999), and the trade-offs between the

contact time and the catalyst Brønsted acidity (Chang, 1984; Chang et al., 1984). Chang and

Silvestri, (1977) performed experiments on methanol conversion at 371 °C and liquid hourly

space velocity (LHSV) equal to 1080, 108, and 1. At lowest contact time (LHSV = 1080), the

dehydration of methanol to dimethyl ether and C2-C4 alkenes formation dominated, and as the

contact time was increased to LHSV of 1, aromatics and paraffins dominated. This indicated

that as the contact time was increased, the yield of light olefins increased, and reached the

maximum at the expense of decreasing oxygenates concentration. As the contact time further

increased, the yield of light olefins decreased while the yield of aromatics and paraffins

increased indicating that those alkenes were intermediates to aromatics.

It is clear that the selectivity of olefins can be enhanced by decreasing contact time, and

alternatively to reducing contact time, decreasing the Brønsted acidity of the zeolite also plays

a significant role. Decreasing the Brønsted acidity can be achieved by either increasing the

Si/Al ratio during the crystallization process for zeolites like ZSM-5 or by partially neutralizing

the acidic sites. These two approaches have been found to give corresponding selectivity for

olefin formation from methanol conversion over zeolites when compared on the same basis of

proton concentration. From the plots of selectivity vs Si/Al ratio, and selectivity vs contact

space time, similar trajectory was observed (Chang, 1984). When the contact time was adjusted

to achieve complete conversion in the high Si/Al ratio region, significant aromatization

occurred indicating the trade-off between contact time and catalyst Brønsted acidity (Chang,

1984; Chang et al., 1984).

2.6.5 Influence of modifying ZSM-5 with elements

The effect of modifying the zeolites with certain elements (e.g. metals, nonmetals, etc.) to

increase product selectivity in the MTH process has been reported and dealt with extensively


35

(Froment et al., 1992; Khanmohammadi et al., 2016; Mokrani and Scurrell, 2009). The

zeolite’s inner pore system is the catalytically active surface, and its structure depends on the

zeolite topology, composition and the extra-framework cations in the interior space channels

(Khanmohammadi et al., 2016). As mentioned above, these cations are sources of positive

charge for compensating the negative charge of the framework, and they can be exchanged

using other cations. These cations play a very critical role in determining the catalytic activity

of the zeolite via their interaction with the Brønsted acid sites. Even the nonmetallic elements

such as phosphorus also interact with the Brønsted acid sites, thereby affecting the catalytic

activity of the zeolite. To introduce the elements into the zeolites, several modification

procedures have been applied, and these includes: ion exchange, incorporation, and

impregnation. Many attempts have been made using various elements across the periodic table

ranging from alkali metal, alkaline earth metal, transition metal, nonmetal elements, etc. to

improve the selectivity towards the hydrocarbons (gasoline and olefins) from methanol

conversion using zeolites.

Elements such as phosphorus, Cs, Ca, Fe, Co, Pt, La, Zr, Mn, etc. have been used to modify

ZSM-5 zeolite and studied in the conversion of methanol to hydrocarbons process. The results

indicated that in all cases, modification increased the shape-selectivity to light olefins

(Hajimirzaee et al., 2015; Inui et al., 1986; Rostamizadeh and Taeb, 2015; Zarei et al., 2017;

Zhang et al., 2014; Zhao et al., 2006). Liu et al., (2009) reported the modification of ZMS-5

zeolite (Si/Al = 220) using a large variety of promoter elements such as P, Ce, W, Fe, Mn, Gr,

Mo, Ga, V, and Ni. P was found to have improved the propylene selectivity, 55.6% in

comparison with 46.4% in H‐ZSM‐5 zeolite. The rank order of propylene selectivity over

different promoters was P > Ce >W > Mn > Fe > Gr > Mo > Ga > V > Ni.

Zaidi and Pant, (2004) used Zn and Cu to modify ZSM-5 zeolites (Si/Al = 45) in the MTH

process, and the results indicated that the selectivity towards gasoline range hydrocarbons was

increased for modified ZSM-5 more than for parent ZSM-5 zeolite alone. Ga was also used to

modify the ZSM-5 zeolite, and the addition of Ga to the ZSM-5 zeolite noticeably increased

the selectivity towards the aromatic hydrocarbons at the expense of C2–C4 alkenes, without

affecting the overall conversion (Freeman et al., 2002). Moreover, Dai et al., (2018) modified

ZSM-5 zeolite (Si/Al = 25) with varying Ga metal loadings (1-5 wt.%), they reported that with

increasing Ga contents, the selectivity towards gasoline range hydrocarbons increased from


36

28% to 66.7% for the parent ZSM-5 zeolite and 5%Ga/ZSM-5 catalyst, respectively. However,

the catalyst lifetime gradually decreases with increasing Ga content, and dropped down to 15

h compared to ~ 30 hours for the parent zeolite. The reason for this drop can be better explained

using the dual cycle mechanism in the MTH process. The Ga species promoted the olefin cycle

to aromatics cycle, accompanied by rapid accumulation of coke deposits. This shows that

introducing some elements into the ZSM-5 zeolite is an efficient way to modify the acidity of

the zeolite, thereby enhancing the selectivity towards the desired hydrocarbons from methanol

conversion.

2.7 Catalyst Deactivation due to Coke Formation

Zeolites, especially ZSM-5, are widely employed in the MTH process. Despite the superior

results of ZSM-5 in the MTH process, which include high selectivity towards desired

hydrocarbons, resistance to deactivation due to coke deposition as compared to other zeolites,

etc. (Chen et al., 2017; Wu et al., 2013; Zhang et al., 2017), the microporous characteristic of

ZSM-5 zeolite significantly influences mass transfer of the reactants and products to/from the

active sites, and as a result, ZSM-5 still suffers from diffusion limitations of larger molecules

resulting in restrictions in the catalyst activity and lifetime (Bjørgen et al., 2008; Hu et al.,

2014). Due to ZSM-5’s unique pore structure, only gasoline range hydrocarbons are allowed

to diffuse out, and large molecules than those found in this range, are responsible for ZSM-5

zeolite deactivation.

In principle, there are two main causes of zeolite deactivation, namely dealumination and

coking. Dealumination involves the removal of the tetrahedrally coordinated aluminium

framework which is compulsory for generating Brønsted acid sites, by so doing breaking down

the acidic structure of the zeolite catalyst (De Lucas et al., 1997; Gayubo et al., 2003). Coking

involves the blockage of the active sites, pore mouths, and intersections of the zeolite by

polycondensed aromatics formed from adsorbed carbocation intermediates on the strong acidic

sites of the zeolite (Guisnet and Magnoux, 2001; Guisnet et al., 2009; Ilias and Bhan, 2012;

Schulz, 2010). Catalyst deactivation due to dealumination is irreversible unlike deactivation

caused by coking which is reversible by burning off the residual carbon deposits or washing in


37

industrial applications. However, the initial catalyst activity and lifetime cannot be achieved

again (Gayubo et al., 2001; Gayubo et al., 2003).

The location, composition of coke, and the rate of coke formation are determined by the zeolite

properties (channels, pores, cavities, active sites, Si/Al ratio, crystal size, etc.), reaction

conditions (temperature, pressure, etc.), and the features of the reaction system (characteristics

of the reactor, shape and size of reactant and product molecules, etc.) (Guisnet, 2002; Guisnet

et al., 2009; Guisnet and Magnoux, 2001; Schulz, 2010; Ye et al., 2015). Catalyst deactivation

by coke deposition still remains the challenging topic for economic use of ZSM-5 zeolite in

the MTH process. Thus, knowledge about the catalyst deactivation is very important for

catalyst and process development.

2.8 Studies to Mitigate Coke Formation

Catalyst deactivation remains a challenging topic in the MTH process, and numerous methods

(morphological modification, varying amount of acid sites, etc.) have been tried to mitigate the

rapid deactivation of ZSM-5 catalyst caused by coke deposition in the MTH process in order

to improve diffusivity of the reactants and products to and from the active sites, and

subsequently increase catalyst lifetime. Morphological modification involves the preparation

of either nano-sized, mesopore or hierarchial ZSM-5 zeolites from microporous ZSM-5

zeolites (Liu et al., 2014; Meng et al., 2017; Meng et al., 2017; Pan et al., 2014; Shen et al.,

2014; Wang et al., 2014).

Crystal size also influences greatly the ZSM-5 catalyst performance, and recently more efforts

have been put into reducing the crystal size to the nanoscale in order to overcome diffusion

limitations encountered by the ZSM-5 zeolites in the MTH process which subsequently leads

to catalyst deactivation (Pan et al., 2014; Shen et al., 2014). The crystal size reduction has been

successfully achieved via hydrothermal crystallization to synthesize nanocrystalline ZSM-5

(Mintova et al., 2013; Mohamed et al., 2005). The advantage of synthesizing nanocrystalline

ZSM-5 includes increased external surface area which provides more accessibility of active

sites to the reactants, and more importantly, crystal size reduction shortens the diffusion path


38

length resulting in significantly enhanced mass transfer of both reactant and product species in

or out. Nanocrystalline ZSM-5 show better performance than their micro-sized ZSM-5

counterparts (Choi et al., 2009; Rownaghi and Hedlund, 2011; Shen et al., 2014). For example,

Shen et al., (2014) showed that Zn/ZSM-5-AP60 exhibits aromatics selectivity above 62% over

42 h time on stream, the coke formation rate was about 4.41 mg gcat−1 h−1 compared with its

counterpart conventional Zn/ZSM-5 catalyst with aromatics selectivity below 54% over 3.6 h

time on stream, and the coke formation rate was 26.33 mg gcat−1 h−1. Moreover, Zn/ZSM-5-

AP60 exhibits lower selectivity towards overall total olefin compared to Zn/ZSM-5 catalyst,

possibly due to enhanced hydrogen transfer ability to harvest more aromatics. The greater

catalytic performance of Zn/ZSM-5-AP60 catalyst may be attributed to the size of

nanocrystalline ZSM-5 which favors diffusion of both reactant and product species, and thus

inhibiting coke deposition.

However, nano-sized zeolites might be thermodynamically unstable because of high surface

energy and enormous amounts of surface defects. Nanocrystals are difficult to handle and have

low yields during synthesis, in which the majority of the building units are left unused in the

mother liquid. This limits the application of the nanocrystalline zeolites in the industrial sector

(Zhu et al., 2011). An alternative approach (seed-induced method) to synthesize

nanocrystalline ZSM-5 have been developed with the following advantages; fast crystallization

of ZSM-5, and low consumption of templates (Majano et al., 2009; Xue et al., 2012). However,

crystal intergrowth and secondary growth typically occurs leading to compact aggregates with

larger crystal sizes formed with respect to the seed particles, and separation is a problem

(Majano et al., 2009).

Many methods to synthesize nanocrystals ZSM-5 have been extensively reported in the

literature, for example; seed crystallization (Majano et al., 2009; Ren et al., 2010), using

organosilanes (Gao et al., 2016), surfactants (Chen et al., 2017; Frunz et al., 2006; Goncalves

et al., 2008), growth in confined space (Frisch et al., 2009; Yoo et al., 2008), using large

amounts of templates under autogenous pressure (Kalita and Talukdar, 2009; Reding et al.,

2003) and, etc. However, separation of the nanocrystals from the reaction mixture remains a

big challenge, and the use of expensive surfactants and organosilanes, and large amounts of

templates restricts the application of these methods in the industrial sector.


39

Another particular promising approach in morphological zeolite modification to overcome

ZSM-5 deactivation due to coke deposition, is to synthesize mesoporous or hierarchical zeolite

catalysts (Ahmadpour and Taghizadeh, 2015; Bleken et al., 2013; Wan et al., 2014; Zhang et

al., 2014). The term, “hierarchical” refers to the presence of at least two porosity types in the

ZSM-5 zeolite, namely; micropores and mesopores size range (Wan et al., 2014). As a result,

hierarchical ZSM-5 takes both the advantage of microporous and mesoporous materials, thus

showing superior catalytic performance. Numerous methods to prepare mesoporous or

hierarchical ZSM-5 zeolite have been developed, these methods include; chemical leaching,

synthesis using templates, surfactants, and etc. (Čejka and Mintova, 2007; Egeblad et al., 2007;

Mintova and Čejka, 2007; Van Donk et al., 2003; Verboekend and Pérez-Ramírez, 2011).

Much concentration has been paid in preparing mesoporous or hierarchical ZSM-5 using

chemical leaching (dealumination and desilication) approaches (Behbahani and Mehr, 2014;

Triantafillidis et al., 2001; Wei et al., 2015), as well as steaming (Jia et al., 2018; Wei et al.,

2017; Xing et al., 2017). However, for the past few years, the use of soft and/or hard template

(Ahmadpour and Taghizadeh, 2015; Kim et al., 2010; Meng et al., 2009; Tao et al., 2006;

Wang et al., 2014; Xue et al., 2012; Zhu et al., 2011) to create mesoporous or hierarchical

ZSM-5 with high efficiency has been drawing growing attention because there is better control

of the pore size, and the mesoporous or hierarchical structure can be achieved without

crystallinity being lost. Nevertheless, the use of templates has some drawbacks, as these

templates are expensive, complicated and energy-intensive to be used during synthesis and/or

to be prepared, and produce large amounts of aqueous pollutants, thus limiting their industrial

applications (Ahmadpour and Taghizadeh, 2015; Wang et al., 2017; Zhang et al., 2014).

Moreover, the use of conventional cationic surfactants (e.g. cetyltrimethylammonium bromide

(CTAB)) remains a significant challenge due to phase separation between mesoporous and

microporous-phase when preparing mesoporous or hierarchical ZSM-5 zeolite (Zhang et al.,

2014; Zhu et al., 2011). Chemical leaching approaches to create hierarchical structures may be

seen as relatively cheap and easy to carry out, but treating zeolites with chemicals frequently

results in major losses of zeolites because of chemical dissolution, and frequently undesired

changes to the zeolite Si/Al ratio.


40

2.9 Microwave Radiation

It was not until 1946, at the end stage period of World War II when Dr. Percy Spencer with the

Raytheon Corporation noticed something unusual – the candy bar melted in the pocket while

standing in front of an active radar set. Dr. Spencer was the first to examine this incident, and

he later on carried out an experiment on popcorns and subsequently, on an egg (Pradhan et al.,

2012; Rana and Rana, 2014). This gave birth to the development and commercialization of

microwave ovens in 1947-1978, and in 1971, Liu and Wightman reported the decomposition

of alcohols, ethers and ketones using microwave radiation (Liu and Wightman, 1971).

Following that, in 1986 two important papers were published regarding successful application

of microwave radiation in organic synthesis (Gedye et al., 1986; Giguere et al., 1986). Figure

2.6 summarizes published areas in which microwave radiation has been applied. It is obvious

that the use of microwave radiation has shifted from food processing to processing advanced

materials (Mishra and Sharma, 2016).


41

Figure 2.6: Published areas in which microwave radiation has been applied (Mishra and

Sharma, 2016).

Microwaves are regarded as electromagnetic waves lying in between the infra-red and radio

frequencies, with wavelengths between 0.01-1m (corresponding to frequencies of 30 - 0.3

GHz) in the electromagnetic spectrum as shown in Figure 2.7 (Rana and Rana, 2014). Most

domestic microwave ovens are authorized to operate in the 2.45 GHz. The 0.915 GHz and

2.172×10-5 GHz radio frequencies are frequently used for industrial electromagnetic heating to

avoid interfering with telecommunication and radar frequencies (Zlotorzynski, 1995).

Figure 2.7: Electromagnetic spectrum (Rana and Rana, 2014).

2.9.1 Application of Microwave Radiation in Catalysis

Among the areas (environmental, nuclear chemistry, engineering, and etc.) in which

microwave technology has been used (Zlotorzynski, 1995), in catalysis microwave technology

offers many advantages in chemical reactions (microwave-assisted synthesis) as compared to

traditionally used methods, such as conventional heating. These advantages include: increased

rates of reaction, save time, offers fast and relatively low-cost access to very high temperatures


42

and pressures, clean and higher yields, etc. (Caddick and Fitzmaurice, 2009; Gedye et al., 1986;

Menéndez et al., 2010; Rana and Rana, 2014; Zhang and Hayward, 2006).

In recent years, the application of microwave radiation in heterogeneous catalysis has received

great interest due to the nature of microwave radiation’s ability to quickly, widely, uniformly

and internally heat the material (Ohgushi et al., 2009). Microwave radiation has been applied

widely in the synthesis (Anuwattana et al., 2008; Somani et al., 2003; Arafat et al., 1993; Li

and Yang, 2008), ion exchange (Kuroda et al., 2002; Romero et al., 2007), modification (Hasan

et al., 2015), and reactivation of zeolites (Ohgushi and Nagae, 2003; Polaert et al., 2010).

Anuwattana et al., (2008) synthesized ZSM-5 zeolite using both conventional and microwave

heating from cupola slag waste. As compared to conventional heating, microwave heating

increased the rate of ZSM-5 formation by 4 times at 150 °C, and produced smaller ZSM-5

particles with 0.3 µm in size, and 3 µm obtained by using conventional heating. Moreover,

Katsuki et al., (2005) also reported that the rate of ZSM-5 formation increased by 3-4 times at

150 °C, and produced smaller ZSM-5 particles with 0.3-5 µm when using microwave heating.

It is clear that, synthesizing zeolites from microwave heating provides many advantages such

as, narrow particle size distribution, and fast crystallization.

Modern domestic microwave ovens are readily available for academic and industrial

applications due to their relatively low-cost. Alkylation, aromatic and nucleophilic substitution,

condensation, oxidation, reduction reaction, and etc. have been successfully carried out via

microwave-assisted synthesis. Several useful reactions can be carried out using domestic

microwave ovens using different microwave-assisted procedures, including non-solid state

reaction and solid state reaction (Pradhan et al., 2012).

2.9.2 Mechanisms and types of Microwave heating

Microwave heating occurs through two distinct mechanisms, which are dipolar polarization

and conduction. The heating primarily results from the interaction of charged particles in the

material and/or magnetic dipoles with the electric field. In dipolar polarization, the electric

field of the microwave induces motion on the dipoles until they align themselves with the


43

alternating field and this is significant in water and other polar fluids. In conduction, the electric

field of the microwave gives rise to a current traveling in phase with the field and causing

resistive heating. This mechanism is common in solid materials with mobile charge carriers

(Kitchen et al., 2013).

Figure 2.8 shows the types that characterize and distinguish microwave heating from

conventional heating both in theory and in practice. These are the types that offer microwave

heating outstanding advantages over the conventional heating method. As shown, these types

of microwave heating can be classified into: (a) direct heating, (b) selective heating, and (c)

hybrid heating. In direct heating, the material is directly exposed to the microwave radiation,

for example, when heating food. In selective heating, the material is still directly exposed to

the microwave radiation. However, the specific region is heated while the other is not as a

result a desired part of the material is heated without disturbing the rest of the volume

properties. Hybrid heating combines the direct and selective heating properties as shown below

(Mishra and Sharma, 2016).

Figure 2.8: Types of microwave heating (Mishra and Sharma, 2016).

2.9.2.1 Direct heating and Selective heating

Selective heating offers the most obvious advantage of heating the catalyst in heterogeneously

catalyzed reaction whereas in homogeneously catalyzed reaction, both the catalyst and the

medium are heated to the temperature needed to start the reaction (Stiegman, 2015). Selective

heating allows the generation of high temperatures on specific regions of the material while

others remain at lower temperatures and this is generally used in heating specific active sites

in supported metal catalysts (Zhang et al., 2003). Similarly, direct heating allows selective

heating of specific reactants that interact with the microwave radiation to be heated more

strongly than others (Kitchen et al., 2013). Some zeolites when are directly subjected to


44

microwave radiation without solution or solvent; they are easily and effectively heated to a

glowing (melting) temperature (Whittington and Milestone, 1992; Ohgushi and Nagae, 2003;

Ohgushi et al., 2001). This melting of certain zeolites is related to sufficient absorption of

microwave radiation caused by prolonged microwave treatment and further lead to possibly

the migration of cations into the large cavities of the zeolite (Whittington and Milestone, 1992).

2.9.3 Heterogeneous catalysts and Microwave absorption

The absorption of microwave power in a material is influenced by the electromagnetic

properties of the material and most significantly, the penetration depth. As a result, microwave

radiation penetrates inside different materials in different ways. Thus, under the same

conditions of microwave heating, some materials are capable of absorbing more microwave

radiation than others. For nonmetallic materials, microwave penetration is defined in terms of

Penetration Depth (Dp), and it is expressed mathematically as (Chandrasekaran et al., 2012;

Rattanadecho, 2006):

Dp =1

α=

1

2πf(

2

μ′μ0ε0κ′)

1 2⁄

[((1 + tan2δ)1 2⁄ − 1)]−1 2⁄

(2.1)

Where;

α = attenuation factor

ε0= permittivity of free space

f = frequency

μ′= magnetic permeability

μ0 = permeability of free space

κ′ = relative dielectric constant

For metallic materials, microwave penetration is defined in terms of Skin Depth (Ds), and it is

expressed mathematically as (Mishra and Sharma, 2016):

Ds =1

√(πfμσ)= 0.029(ρλ0)0.5 (2.2)


45

Where;

σ = electrical conductivity

ρ = electrical resistivity

λ0 = incident wavelength

μ = magnetic permeability

f = frequency

Different materials possess different microwave absorption characteristics due to their

differences in magnetic and electric field strengths during microwave irradiation. This leads to

their classification into different groups, as: microwave transparent, absorber, opaque and

mixed absorbers (Mishra and Sharma, 2016). Heterogeneous catalyst materials are classified

into different groups and each exhibits different microwave absorption characteristics, namely

(Stiegman, 2015):

(i) Solid oxide catalysts: in this category we have any bulk oxide employed as

catalysts. For example, simple binary oxides like Al2O3, ZrO2, SiO2 and TiO2,

ternary oxides like perovskites and spinels, and porous aluminosilicates and

silicates like templated mesoporous sieves and zeolites.

(ii) Metals: in this category, we have metals used as catalysts in chemical reactions like

Ag, Ni, Cu and etc.

(iii) Support catalysts: in this category, we have an active site like metal particle or

transition metal or complex deposited on an oxide support surface performing part

or all of the catalytic function.

All these catalysts respond to microwave radiation in different ways and as a result affect the

chemical reactions differently.

2.9.3.1 Microwave heating of ZSM-5 zeolite

The precise main mechanism of direct absorption of microwave energy, and consequently the

transformation to heat depends on the temperature, the amount of water adsorbed and the

structure (type, location and cluster of mobile cations between different sites in the zeolite

framework). The reason being, the cations are accountable for neutralizing the charge of the

primary building block of the zeolites (AlO4- tetrahedrons) (Kraus et al., 2015). The number of

cations per unit cell of the zeolite decreases with increasing Si/Al ratio and vice versa (Mentzen


46

et al., 2006; Simon and Flesch, 1999; Ohgushi and Kawanabe, 1994). For instance, Mentzen

et al., (2006) showed that for Cs/ZSM-5 with Si/Al = 70, and for Cs/ZSM-5 with Si/Al = 10,

there are 0.7 and 7.7 cesium cations per unit cell, respectively. As the increasing Si/Al ratio

causes the decrease in the number of exchangeable cations per unit cell that provide an active

site for water absorption, this increases the hydrophobicity of the zeolites as the weakly bonded

cations are expelled by increasing Si/Al ratio (Ohgushi and Kawanabe, 1994). As a result, this

reduces the ability of the zeolites to absorb energy under microwave irradiation (Whittington

and Milestone, 1992), so in these materials, microwave irradiation is favorably based on a

cation hopping mechanism (Gracia et al., 2013; Ohgushi et al., 2009; Whittington and

Milestone, 1992).

Whittington and Milestone, (1992) reported that either hydrated or dehydrated Na/ZSM-5 or

Na/USY do not undergo any large temperature increase under microwave irradiation and stated

that this may be due to the low sodium content, but may be correlated to the cavity size of

ZSM-5, and the fact that these zeolites are hydrophobic. Ohgushi and Kataoka, (1992) found

that there exist two types of Na+ ions (NaI and NaII) in Na/ZSM-5 with Si/Al ratio = 13. Also

for Na/ZSM-5, there exist a single type of Na+ ion (NaI) with Si/Al ratio = 685 (Ohgushi and

Kawanabe, 1994). The differences between the two are due to the difference in binding strength

of the Na+ ion to the zeolite framework. The weakly bonded NaII are favourably expelled by

increasing Si/Al ratio. NaI have higher activation energy for movement, less affinity for water

molecules adsorbed and it is more strongly bonded to the zeolite framework than NaII (Ohgushi

and Kawanabe, 1994). A stronger cation-zeolite framework interaction results in higher

activation energy and a lower resonance frequency. With regards to microwave radiation, that

material can be labelled as microwave-transparent.

2.9.4 Microwave oven and Bed shape

Microwave ovens are often classified into two categories, namely single-mode and multi-mode

ovens. In a single-mode microwave oven, the targeted material being heated is positioned at

the antinodal position of the standing electromagnetic field to ensure faster processing. The

cavity size in single-mode microwave oven is equivalent to one wavelength, and only one


47

vessel can be irradiated at a time. Multi-mode microwave ovens have larger cavities due to

larger dimensions than for single-mode microwave ovens, and they provide a more

homogeneous electromagnetic field due to the rotation inside the cavity and stirrer (Figure 2.9).

Multi-mode microwave ovens can accommodate a number of samples to be heated at the same

time. Household microwave ovens fall under this category (Chandrasekaran et al., 2012;

Mishra and Sharma, 2016; Rana and Rana, 2014).

Single-mode microwave ovens have been used in material processing with phase change,

mineral and environmental processing, ceramic and metal sintering, and in continuous flow

microwave processing. Multi-mode microwave ovens have been used in food processing,

metallurgical, polymer and ceramic applications (Chandrasekaran et al., 2012). Recently,

multi-mode microwave ovens have been used in heterogeneous catalysis because they are

cheap, readily available, easy to operate, save time and provide better catalytic activity and

selectivity in chemical reactions (Dlamini et al., 2016; Dlamini et al., 2015; Janjic and Scurrell,

2002; Mohiuddin et al., 2017; Linganiso and Scurrell, 2016).

However, heating several kilograms or more of the pure dry hydrophobic zeolites is difficult

at the industrial scale (Kraus et al., 2015). By taking advantage of the findings on the effect of

bed shape during microwave irradiation using multi-mode microwave oven (Linganiso and

Scurrell, 2017), we further use the findings (uniformly and thinly spreading of material to be

irradiated promotes better microwave absorption than having cone-shaped bed) to see the if

microwave radiation can be used to post-modify metal impregnated ZSM-5 zeolite in the

methanol-to-hydrocarbon process.


48

Figure 2.9: Multi-mode microwave oven schematic diagram (Rana and Rana, 2014).

Summary of Chapter 2

The MTH process is a promising alternative process to replace the use of petroleum sources as

primary energy resources. However, use of microporous metal-based ZSM-5 catalysts leads to

diffusion limitations as larger molecules block the pore channels and cover the catalyst active

sites. This diffusion limitation results in coke deposition which causes catalyst deactivation and

high methane production. This is a significant challenge facing the MTH process. In this

present study, we seek to investigate the use of microwave radiation as a post-synthesis

modification step to create mesoporous or hierarchical metal-based ZSM-5 catalysts to

overcome coke deposition. In order to achieve the set aim, the multi-mode microwave oven

was used to carry out the post-synthesis modification step.


49

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65

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Chapter 3: Experimental Procedures and Reagents

67

CHAPTER 3

Experimental Procedures and Reagents used

3.1 Introduction

In this chapter, the reagents and the instruments used are presented. The experimental

procedures used for synthesizing and characterizing the microwave treated and untreated iron-

containing HZSM-5 catalysts are described in detail. The following techniques were used to

characterize the microwave treated and untreated catalysts to study the effect of microwave

radiation as post-synthesis modification step: powder x-ray diffraction (p-XRD),

thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform

infrared (FT-IR) spectroscopy, nitrogen adsorption-desorption analysis, and n-propylamine-

temperature programmed surface reaction (C3H9N-TPSR). The effect of microwave radiation

on the product distribution in the methanol conversion to hydrocarbons (MTH) process was

examined using methanol-temperature programmed surface reaction (MeOH-TPSR)

technique.

3.2 Reagents used

The reagents used in this work were used as received without any purification, and are

presented in Table 3.1.


68

Table 3.1: List of reagents used together with their suppliers.

REAGENTS SUPPLIERS

Aluminium sulfate octadecahydrate, ≥97% Sigma Aldrich

Ammonium nitrate, , ≥99.0% Sigma Aldrich

Iron(III) nitrate nonahydrate, ≥98% Sigma Aldrich

Methanol, ≥99.9% Sigma Aldrich

n-Propylamine, 98% Sigma Aldrich

Silica, 99.8% Sigma Aldrich

Sodium hydroxide, 99.99% Sigma Aldrich

Tetrapropylammonium bromide, 98% Sigma Aldrich

ZSM-5 (Si/Al = 30) Zeolyst International

3.3 Experimental Procedures and Instruments used

This section presents the experimental procedures and instruments used.

3.3.1 Catalysts preparation and post-synthesis modification

0.5wt.%Fe/ZSM-5 with Si/Al ratio = 30 was prepared as follows. As received, 30.0 g ZSM-

5 (Si/Al = 30 from Zeolyst International) in the ammonium form was calcined for 5 h at 550

°C to obtain the protonated form of the zeolite. The 0.5Fe/ZSM-5 catalyst was prepared using

the incipient wetness impregnation method, whereby 0.95 g iron(III) nitrate nonahydrate

(Fe(NO3)3 9H2O) was dissolved in enough distilled water. 26.0 g HZSM-5 (on dry basis) was

taken in its powdered form and to it, the iron nitrate solution was slowly added with constant

stirring. The mixture was then kept at room temperature and allowed to equilibrate overnight.

The mixture was then evaporated to dryness at 100 °C followed by calcination for 5 h at 550

°C.

0.5wt.%Fe/ZSM-5 with Si/Al ratio = 80 was prepared as follows using incipient wetness

impregnation. The Na/ZSM-5 sample with Si/Al = 80 was synthesized under hydrothermal


69

conditions at 150 °C for 72 h following a procedure previously reported by Yu et al. (2013).

The molar composition of the synthesis gel was: 9.0 Na2O : 1.0 Al2O3 : 80 SiO2 : 0.53 (TPA)2O

.1300 H2O. The product was then dried in the oven at ~80 °C overnight, and subsequently

calcined in a furnace at 500 °C for 5 h. 11.0 g of NaZSM-5 was refluxed in 110 mL 0.8M

NH4NO3 solution at 80 °C for 5 h, the mixture was then filtered and washed with distilled

water. This aforementioned step was repeated four times to obtain NH4ZSM-5. The NH4/ZSM-

5 was dried overnight in an oven at 110 °C and then calcined at 550 °C for 5 h in a furnace to

produce HZSM-5. To get 0.5Fe/ZSM-5 catalyst, 0.27 g iron(III) nitrate nonahydrate (Fe(NO3)3

9H2O) was dissolved in enough distilled water. 7.40 g HZSM-5 (on dry basis) was taken in its

powdered form and to it, the iron nitrate solution was slowly added with constant stirring. The

mixture was kept at room temperature and allowed to equilibrate overnight. The mixture was

then evaporated to dryness at 100 °C followed by calcination for 5 h at 550 °C.

3.3.1.1 Microwave post-synthesis modification of catalysts

The 0.5Fe/ZSM-5 catalysts were modified using microwave irradiation as follows; ~1.00 g of

0.5Fe/ZSM-5 catalyst (on a dry basis for both Si/Al = 30 and 80) was placed and uniformly

dispersed on a flat-bottomed domestic microwave oven plate (glass tray). The sample was then

subjected to microwave radiation for 10 s on varying power level (0-700 W) using domestic

microwave oven (DEFY, DM0368 MWM 2030M) (Figure 3.1). Microwave-treated samples

were then denoted as 0.5FeZtime/power level for Si/Al = 30 and 0.5FeXtime/power level for Si/Al = 80.


70

Figure 3.1: Experimental set-up for post-synthesis modification step.

3.3.2 Catalysts Characterization

3.3.2.1 Nitrogen Adsorption-desorption Analysis

The specific surface area (SBET), micropore volume and total pore volume were analyzed by

N2 adsorption-desorption measured at -196 °C using Micromeritics TriStar II 3020 instrument.

Prior to the analysis, about 0.20 g of the sample was degassed for 3 h at 400 °C.

3.3.2.2 Powder X-Ray Diffraction (p-XRD) analysis

The XRD analysis was performed on a Rigaku D/MAX – 1400 instrument with a Cu-Kα

radiation, with a scan speed of 15°/min and a scan range of 10-90° at 40 kV and 40 mA.

3.3.2.3 Thermogravimetric analysis (TGA)

The TGA was applied to estimate the coke content deposited on microwave-treated and

untreated catalysts after the methanol conversion to hydrocarbons tests. The analysis was

conducted using Thermogravimetric Analyzers (Discovery TGA 5500) in the presence of air

and the temperature range of 25–1000 °C with the heating ramp rate of 10 °C/min.

Glass tray

Sample

Microwave oven


71

3.3.2.4 Scanning electron microscopy (SEM) analysis

JEOL JSM-6010PLUS/LA analytical scanning electron microscope (SEM) was used to

determine material morphology. The samples were coated with a thin gold film prior to

analysis.

3.3.2.5 Fourier Transform Infrared (FT-IR) Spectroscopy

The structural properties of the catalysts were analyzed using VERTEX 70 FT-IR spectrometer

coupled with RAM II FT-Raman module with spectral resolution of 4 cm-1 in the range from

400-4000 cm-1 at room temperature.

3.3.2.6 Propylamine-Temperature programmed surface reaction (C3H9N-TPSR)

The effect of microwave radiation on the Brønsted acidic sites was determined using

Propylamine-Temperature programmed surface reaction (C3H9N-TPSR) on the Micromeritics

AutoChem II 2920 coupled with MKS CirrusTM2 Benchtop Atmospheric Pressure Gas

Analysis System. The C3H9N-TPSR experiment was carried out as follows; the samples

(0.5FeXtime/power level and 0.5FeZtime/power level) about ~20 mg per trial each was pretreated in situ

at 500 °C for about an hour maximum under flowing helium gas (He) and then cooled to

ambient temperature. n-Propylamine was chemisorbed at 120 °C by passing He as a carrier gas

through a saturator containing n-propylamine. The sample was then purged for 120 minutes

afterwards in flowing helium to remove any physically adsorbed n-propylamine to ensure 1 :

1 ratio of adsorbed n-propylamine to Brønsted acidic sites, and then the temperature was

increased at a temperature ramp of 10 °C/min up to 500 °C.

For C3H9N-TPSR technique in the present study, the temperature of the sample (the

microwave-treated and untreated catalysts) was increased linearly with time. The desorbing

species were continuously monitored by their characteristic mass fragments (m/e): 17

(ammonia), 30 (n-propylamine), and 41 (propene). The signal of propene from the TPSR was

used to quantify the corresponding number of Brønsted acid sites (Kresnawahjuesa et al., 2002;

Milina et al., 2013; Milina et al., 2014; Tittensor et al., 1992).


72

3.3.3 Catalysts testing (MeOH-TPSR experiments)

The effect of microwave radiation on hydrocarbon yields from methanol conversion was tested

on the Methanol-Temperature Programmed Surface Reaction (MeOH-TPSR) using

Micromeritics AutoChem II 2920 coupled with MKS CirrusTM2 Benchtop Atmospheric

Pressure Gas Analysis System. The MeOH-TPSR experiment was carried out as follows.

About 0.20 g of sample was pretreated in situ at 500 °C for about an hour maximum under

flowing helium gas (He) and then cooled to ambient temperature. Methanol was chemisorbed

at ambient temperature by passing He as a carrier gas through a saturator containing methanol.

The sample was then purged for about an hour afterwards in flowing helium to remove any

physically adsorbed methanol, and then the temperature was increased at a temperature ramp

of 10 °C/min up to 550 °C.

For MeOH-TPSR technique in the present study, the temperature of the sample (the

microwave-treated and untreated catalysts) was increased linearly with time, and the

production of hydrocarbons and consumption of methanol was also monitored. The desorbing

species were continuously monitored by their characteristic mass fragments (m/e): 16

(methane), 28 (ethane and ethene), 29 (propane), 41 (propene and butene), 42 (pentene), 43

(butane and pentane), 78 (benzene), and 91 (toluene and xylene). The intensities were then

corrected to eliminate interferences from other compounds to account for contributions to our

desired hydrocarbon yield from methanol conversion.


73

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Chapter 4: Results and Discussion

74

CHAPTER 4

Results and Discussion

4.1 Introduction

The results that were found from catalysts characterization and MeOH-TPSR experiments in

the MTH process are stated and discussed in this chapter to observe the effect of microwave

radiation.

4.2 Catalysts characterization

In this section, the results obtained from characterizing prepared catalysts to see the effect of

microwave radiation are presented and discussed in detail.

4.2.1 X-ray diffraction analysis

The XRD analysis was conducted to investigate the influence of loading Fe cations and the

effect of microwave irradiation on the zeolite structure. Figure 4.1(a-b) shows the powder XRD

patterns for the HZSM-5 with varying Si/Al ratio (30 and 80) and for microwave-treated and

untreated catalysts after iron impregnation on the HZSM-5 zeolites. The samples were analyzed

to check the influence of iron loading and the effect of microwave treatment. The

diffractograms of the microwave-treated and untreated (0.5FeX10/0-700 and 0.5FeZ10/0-700)

catalysts and for the parent HZSM-5 zeolites were similar, and identical to that of a typical

MFI structure (Moreno-Recio et al., 2016), depicting that the MFI framework structure of

ZSM-5 zeolite was well maintained even after both iron impregnation and microwave post-

treatment of the catalysts. The diffraction signals associated to iron oxide phases were not

detected on the XRD profiles of iron-containing samples (0.5FeX10/0-700 and 0.5FeZ10/0-700),

indicating that the iron cations were uniformly dispersed onto the zeolite framework (Moreno-

Recio et al., 2016; Aziz et al., 2016), and low iron loading on the parent HZSM-5 zeolite was


75

the cause of undetected diffraction signals associated to any iron oxide phases (Rasouli et al.,

2017).

Figure 4.1: XRD patterns for the parent zeolites (HZSM-5) and microwave-untreated and

treated catalysts: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700.

Slight intensity decreases in the diffraction peaks at 2θ angle between 23–25o on the

diffractogram of the parent HZSM-5(30) zeolite were also observed after iron impregnation,

and these were due to the higher adsorption coefficient of iron oxide for x-ray radiation (Figure

4.1(a)). The intensity decreases of iron-containing catalysts confirmed the existence of iron

species. As for microwave-treated catalysts in Figure 4.1(a), there were some changes in the

diffractogram intensities compared to that of the parent HZSM-5 (30) zeolite. When the

microwave power level was increased from 280 to 595 W, there was an increase in the

diffractogram intensities of 0.5FeZ10/280 to 0.5FeZ10/595 catalysts and then decreases for

0.5FeZ10/700 catalyst. The diffractogram intensities of the 0.5FeZ10/0 and 0.5FeZ10/119 catalysts

were the same but lower than for the above-mentioned catalysts. The increasing diffractogram

intensities of the microwave-treated catalysts, with increasing microwave power level maybe

associated with crystal growth within the catalysts and pronounced response to microwave

irradiation due to the higher number of iron cations per unit cell for the HZSM-5 (30) than for

HZSM-5 (80).


76

The crystal growth may proceed under microwave irradiation provided that there are many

nuclei in the sample being irradiated which then undergo crystallite growth (Anuwattana et al.,

2008; Somani et al., 2003). Inui et al., (1986) showed that small amount of iron ions plays a

role in providing nuclei in the crystal growth in high-silica zeolites. The decrease in

diffractogram intensity for 0.5FeZ10/700 catalyst may suggest the partial disintegration of the

zeolite framework. The structure directing agent (tetrapropylammonium bromide), used when

preparing the used HZSM-5 zeolites in the present study, is well known to suffer Hoffmann

degradation during synthesis time (Arafat et al., 1993), and also fast degradation under

microwave irradiation (Li and Yang, 2008; Arafat et al., 1993). Figure 4.1(b) shows slight

changes at 2θ angle between 23–25o on the diffractogram intensities of the parent HZSM-5

(80) zeolite and its based microwave-treated catalysts (0.5FeX10/119-700). This indicates that

microwave-treated catalysts (0.5FeX10/119-700) did show some crystallinity changes upon

microwave irradiation.

4.2.2 Nitrogen adsorption-desorption analysis

The nitrogen adsorption-desorption analysis was conducted to investigate the influence of

loading Fe cations and the effect of microwave irradiation on the physicochemical properties

of the prepared catalysts. The physicochemical properties of microwave-treated and untreated

catalysts are summarized in Table 4.1. The BET surface areas for 0.5FeX10/0-700 and 0.5FeZ10/0-

700 catalysts were found to be 343–366 m2/g and 346–376 m2/g, respectively. This difference

is attributed to different parent HZSM-5 zeolites used with varying Si/Al ratios (30 for

0.5FeZ10/0-700 based catalysts and 80 for 0.5FeX10/0-700 based catalysts). Table 4.1 shows that

there was a linear BET surface area decrease (376 m2/g to 346 m2/g) and a linear micropore

volume (0.119 cm3/g to 0.108 cm3/g) decrease, with increasing microwave power level from 0

to 595 W for the 0.5FeZ10/0-595 catalysts except for the 0.5FeZ10/700 catalyst, where the BET

surface area and micropore volume was 356 m2/g and 0.111 cm3/g, respectively. This effect

may be related to the microwave absorption capability of the iron cations within the zeolite

framework with lower Si/Al ratio. This also shows that the iron cations may have agglomerated

in the HZSM-5 zeolites micropores and decreased the BET surface area and the total pore

volume.


77

Figure 4.2: N2 adsorption-desorption isotherms of microwave-treated and untreated catalysts

at -196 °C: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700 catalysts.

Figure 4.2(a) shows the N2 adsorption-desorption isotherms of the microwave-treated and

untreated (0.5FeZ10/0-700) catalysts with type IV(a) isotherms with H3-shaped hysteresis loops

according to IUPAC classification (Thommes et al., 2015), Type IV(a) isotherms are given by

mesoporous adsorbents (Thommes et al., 2015). This shows that these catalysts possessed

mainly mesopores. The hysteresis loop of 0.5FeZ10/700 catalyst in Figure 4.2(a) was smaller

than that of the corresponding catalysts (0.5FeZ10/0-595), and this could be associated with the

agglomeration of iron oxide particles leading to a blocking of the channels and/or the

degradation of the HZSM-5 zeolite framework under microwave irradiation.

As for the 0.5FeX10/0-700 based catalysts, there was no such linear agreement between the

microwave power level and their corresponding BET surface areas (Table 4.1). However, there

was still an effect on the BET surface areas and an increase in mesopore volume (0.0919 cm3/g

to 0.0995 cm3/g) of the catalysts as the microwave power level was increased. There was a


78

linear increase in mesopore volume as the microwave power level was increased (119 W to

700 W). The microwave-treated catalysts (0.5FeX10/119-700) have lower BET surface areas (343

m2/g to 357 m2/g) than that of the microwave-untreated catalyst (0.5FeX10/0) with BET surface

area of 366 m2/g. This may be ascribed to the agglomeration of the iron particles on the

micropores and resulting in decreasing the BET surface area upon microwave irradiation of the

catalysts. The absence of linear agreement between the BET surface area and microwave power

level maybe ascribed to the deficiency of microwave absorbing cations within the parent

HZSM-5 zeolite, as the weakly bonded iron cations are likely to be expelled by the increasing

Si/Al ratio (Inui et al., 1986).

The N2 adsorption-desorption isotherms of the microwave-treated and untreated (0.5FeX10/0-

700) catalysts at different power levels (0–700 W) are illustrated in Figure 4.2(b). All the

catalysts exhibited the Type IV isotherms with the H4-shaped hysteresis loops at p/po > 0.4

according to IUPAC classification (Thommes et al., 2015), due to the formation of textural

mesoporosity originating from the crystal intergrowth. The pronounced hysteresis loops for

0.5FeX10/336-700 catalysts as the microwave power level was increased from 336 W to 700 W,

may be associated with the microwave treatment’s ability to enhance the generation of

mesopores between very small iron oxide crystallites dispersed on the surface of the HZSM-5

zeolite.


79

Table 4.1: Physicochemical properties of microwave-untreated and treated catalysts

(0.5FeX10/0-700 and 0.5FeZ10/0-700).

Catalyst BET Surface

Area /(m2/g)a

Total Pore

Volume

/(cm3/g)b

Micropore

Volume

/(cm3/g)c

Mesopore

Volume

/(cm3/g)d

0.5FeX10/0 366 0.194 0.0950 0.0990

0.5FeX10/119 343 0.182 0.0901 0.0919

0.5FeX10/280 348 0.183 0.0904 0.0926

0.5FeX10/336 348 0.187 0.0943 0.0927

0.5FeX10/462 357 0.192 0.0958 0.0962

0.5FeX10/595 352 0.192 0.0929 0.0991

0.5FeX10/700 355 0.193 0.0935 0.0995

0.5FeZ10/0 376 0.233 0.119 0.114

0.5FeZ10/119 374 0.233 0.118 0.115

0.5FeZ10/280 367 0.229 0.117 0.112

0.5FeZ10/336 364 0.229 0.113 0.116

0.5FeZ10/462 357 0.225 0.111 0.114

0.5FeZ10/595 346 0.216 0.108 0.108

0.5FeZ10/700 356 0.222 0.111 0.111

a Brunauer‐Emmett‐Teller (BET) surface area;

b Total pore volume at p/po = 0.99;

c micropore volume determined by t‐plot;

d mesopore volume calculated as total pore volume – micropore volume.


80

4.2.3 Thermogravimetric analysis

The thermogravimetric analysis was conducted to determine the amount of deposited coke on

the prepared catalysts. The amounts of coke deposited on the used microwave-treated and

untreated catalysts after the MeOH-TPSR tests are given in Table 4.2. The deposited coke

amounts were determined by the weight loss upon the thermogravimetric analysis (TGA).

Microwave-untreated catalysts, 0.5FeX10/0 and 0.5FeZ10/0 possessed higher coke amounts, 31.7

mgcokegcat.-1 and 46.4 mgcokegcat.

-1 respectively, than their corresponding microwave-treated

catalysts (0.5FeX10/119-700 and 0.5FeZ10/119-700). 0.5FeX10/119-700 and 0.5FeZ10/119-700 catalysts

possessed the coke amounts in the range of 2.84-31.0 mgcokegcat.-1 and 16.2-44.7 mgcokegcat.

-1,

respectively. As the microwave power level under which the catalysts were irradiated at was

increased from 0–700 W, the coke deposition decreased. The results show that the microwave-

untreated catalysts have a comparatively higher carbon capacity, which was later reduced

somewhat after microwave irradiation.

Moreover, some researchers used changes in methane formation as the indication for measuring

catalyst deactivation by coking (Wen et al., 2016; Schulz, 2010; Villacampa et al., 2003).

Microwave-treated catalysts produced less methane gas than the microwave-untreated catalysts

(Figure 4.8(a-b)). From methane profiles in Figure 4.8(a-b), microwave-treated catalysts

(0.5FeX10/119-700 and 0.5FeZ10/119-700) produced less methane. As the microwave power level

was increased, the peak intensities of methane profiles for 0.5FeZ10/119-700 catalysts were

effectively decreased relative to those of 0.5FeX10/119-700 catalysts (Figure 4.8(a-b)), and this

seems to be related to the high ability of 0.5FeZ10/119-700 based catalysts to respond to

microwave radiation due to the lower Si/Al of 30. 0.5FeZ10/0-700 based catalysts have higher

coke amounts and methane formation than 0.5FeX10/0-700 based catalysts (Table 4.2 and Figure

4.8(a-b)). There is a proportional relationship between coke deposition, methane formation and

Si/Al ratio. Similar observations were previously reported by Gao et al. (2016); As the Si/Al

ratio was increased from 30 to 80, Gao et al, 2016 showed that the coke amount was decreased


81

from 126.0 mgcokegcat.-1 to 78.8 mgcokegcat.

-1 and methane formation decreased from 8.1% to

5.1%, respectively.

Table 4.2: Coke quantity for Methanol-To-hydrocarbons conversion over microwave-treated

and untreated catalysts (0.5FeX10/0-700 and 0.5FeZ10/0-700).

Used Catalyst Coke deposited /(mgcokegcat.-1)

0.5FeX10/0 31.7

0.5FeX10/119 21.7

0.5FeX10/280 31.0

0.5FeX10/336 27.4

0.5FeX10/462 23.8

0.5FeX10/595 14.0

0.5FeX10/700 2.84

0.5FeZ10/0 46.4

0.5FeZ10/119 24.3

0.5FeZ10/280 44.7

0.5FeZ10/336 38.1

0.5FeZ10/462 33.8

0.5FeZ10/595 25.0

0.5FeZ10/700 16.2

4.2.4 Scanning electron microscopy (SEM) analysis

Figure 4.3 and Figure A1 show SEM images of microwave-untreated (0.5FeX10/0 and

0.5FeZ10/0) and microwave-treated catalysts (0.5FeX10/119-700 and 0.5FeZ10/119-700) with non-


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uniform size. SEM images of 0.5FeZ10/0-700 catalysts depicted that the small crystallites

agglomerate together and formed larger spherical stacks. This may be attributed to their high

surface Gibbs energy. No differences in surface morphologies were clearly observed between

the SEM images of 0.5FeZ10/119-700 catalysts and microwave-untreated (0.5FeZ10/0) catalyst.

Unlike 0.5FeZ10/119-700 catalysts SEM images, microwave-treated (0.5FeX10/119-700) catalysts

SEM images clearly showed that upon increased microwave power level, the nanosized

aggregates clustered together into micro- and mesosized stacks with visually seen spaces

between the stacks. SEM images of 0.5FeX10/0-700 catalysts depicted that the crystallites exhibit

coffin-like shapes.


83

Figure 4.3: SEM micrographs of microwave-treated and untreated catalysts.


84

4.2.5 Fourier Transform Infrared (FT-IR) Spectroscopy

The FT-IR spectra of the parent zeolites and the microwave-untreated and treated catalysts

(0.5FeZ10/0-700 and 0.5FeX10/0-700) in the range 400–1500 cm-1 are shown in Figure 4.4(a-b). The

ratios of the band intensities of the FT-IR spectra and the relative crystallinity estimated from

FT-IR spectra are given in Table 4.3. As it can be observed in Figure 4.4(a-b), in all the samples

basically similar bands appeared, even though upon Fe-loading and subsequent microwave

treatment, their intensities were found to be drastically changed. The band intensities of HZSM-

5(80) based samples were found to be increasing with increasing microwave power level from

0-700 W and even after Fe-loading as shown in Figure 4.4(b). However, the band intensities

for HZSM-5(30) based samples were found to fluctuate with increasing microwave power level

and even after Fe-loading as shown in Figure 4.4(a).

Figure 4.4: FT-IR spectra for the parent zeolites (HZSM-5) and microwave-untreated and

treated catalysts: (a) 0.5FeZ10/0-700 and (b) 0.5FeX10/0-700 catalysts.

All the FT-IR spectra depicted all the bands which typically characterize the MFI zeolite

structure, with absorption bands at 1221-1225, 1072-1105, 795-800, 544-550, and 432-451 cm-

1 as shown in Figure 4.4(a) and at 1223-1225, 1072-1095, 785-797, 544-546, and 434-446 cm-


85

1 as shown in Figure 4.4(b). The band around 1225 cm-1, 1080 cm-1, 796 cm-1, 546 cm-1 and

441 cm-1 were associated with external asymmetric stretching of Si-O which clearly

characterized the zeolites containing one or four chains of five-membered rings (Jacobs et al.,

1981; Jansen et al., 1984), internal asymmetric stretching of Si-O-T (Ismail et al., 2006),

symmetrical stretching of Si-O-Si (Jiang et al., 2015), double five-membered rings of the ZSM-

5 zeolites (Rasouli et al., 2017), and T-O bending vibration of the SiO4 and AlO4 internal

tetrahedral (Coudurier et al., 1982), respectively.

The ratio of the band intensities near A and B, as shown in Table 4.3, can be used to estimate

the zeolite crystallinity even though quantitative FT-IR data are not very accurate (Ismail et

al., 2006; Coudurier et al., 1982). There are slight differences between crystallinity values

estimated using FT-IR and XRD data (Coudurier et al., 1982). As it can be observed from

Table 4.3 after 0.5 wt.% Fe was loaded on HZSM-5(30) and HZSM-5 (80), the relative

crystallinity value decreased from 46.6-46.3% and increased to 43.0-55.8%, respectively. Upon

increased microwave power level to 119–700 W, the relative crystallinity values fluctuated in

the range 48.5–58.2% for 0.5FeZ10/119-700 catalysts and the relative crystallinity values for

0.5FeX10/119-700 catalysts increased to ≥ 61.3%. This showed that microwave radiation enhanced

crystallinity. The results were almost similar to the XRD results reported in Figure 4.1(a-b). As

shown in Table 4.3, the 0.5FeX10/462-700 catalysts were said to be highly crystalline than other

microwave treated catalysts since their A/B ratios were greater than 0.7 and any material having

A/B ratio less than 0.7 can be regarded as amorphous (Coudurier et al., 1982).


86

Table 4.3: Effect of Fe loading, and microwave post-modification on the ratio of the band

intensities of the FT-IR spectra and relative crystallinity estimated from FT-IR spectra.

Sample A/B ratio* Relativity crystallinity /(%)

HZSM-5(80) 0.430 43.0

0.5FeX10/0 0.558 55.8

0.5FeX10/119 0.677 67.7

0.5FeX10/280 0.613 61.3

0.5FeX10/336 0.651 65.1

0.5FeX10/462 0.724 72.4

0.5FeX10/595 0.714 71.4

0.5FeX10/700 0.712 71.2

HZSM-5(30) 0.466 46.6

0.5FeZ10/0 0.463 46.3

0.5FeZ10/119 0.485 48.5

0.5FeZ10/280 0.582 58.2

0.5FeZ10/336 0.473 47.3

0.5FeZ10/462 0.511 51.1

0.5FeZ10/595 0.574 57.4

0.5FeZ10/700 0.486 48.6

*A/B ratios for HZSM-5(30) based samples were calculated using the band intensities in these

ranges; A (544–550 cm-1) and B (432–451 cm-1) bands and A/B ratios for HZSM-5(80) based

samples were calculated using the band intensities in these ranges; A (544–546 cm-1) and (434–

446 B cm-1) bands.


87

4.2.6 C3H9N-TPSR for acidic sites determination

The effect of microwave radiation on the concentration of Brønsted acidic sites (cB) was studied

using n-propylamine-temperature programmed surface reaction (C3H9N-TPSR), and the results

are presented in Figure 4.5(a-b), Table 4.4, and Figure 4.6(a-b). As an alternative to ammonia,

Gorte and co-workers have reported successive works regarding the use of TPSR of reactive

amines to characterize solid acids (Farneth and Gorte, 1995; Gorte, 1999; Kresnawahjuesa et

al., 2000; Parrillo et al., 1995). Reactive amine which are primarily used includes but not

limited to primary amines which can be stoichiometrically adsorbed on the Brønsted acidic

sites, and then undergo Hofmann elimination upon heating to alkenes and ammonia

(Kresnawahjuesa et al., 2000; Kresnawahjuesa et al., 2002; Milina et al., 2013; Milina et al.,

2014). Figure 4.5(a-b) depict the C3H9N-TPSR profiles for the desorption of n-propylamine,

propylene and ammonia between 120 °C and 500 °C. All 0.5FeX10/0-700 and 0.5FeZ10/0-700

catalysts follow a similar trajectory to one presented in Figure 4.5(b), and Figure 4.5(a),

respectively. However, the only differences are brought about the differences in peak

intensities and desorption temperatures due to increasing microwave power level (0-700

Watts). The first peak around 205 °C and 198 °C in Figure 4.5(a) and Figure 4.5(b),

respectively may be attributed to the desorption of physisorbed n-propylamine which may be

associated with hydroxyl defects, Lewis acid sites or hydrogen bonded to other protonated

amines at Brønsted acidic sites (Gorte, 1999).

Figure 4.5: C3H9N-TPSR data profiles for n-propylamine decompostion to propylene and

ammonia.


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At elevated temperatures above 270 °C, propylene and ammonia were evolved at different

temperatures for every sample studied. As shown in Figure 4.6(a), 0.5FeZ10/0-700 catalysts have

two well-resolved peaks – the first ones centered at 337-345 °C temperature range (Table 4.4)

and the second small ones around 400 °C which seem to be fading away upon increasing

microwave power level (0-700 W). 0.5FeX10/0-700 catalysts have only one and first well-

resolved peak centered at 339-347 °C temperature range as shown in Table 4.4 and Figure

4.6(b). Those first peaks centered at aforementioned temperature ranges are indicative

characteristics of surface reactions of adsorbed n-propylamine on the Brønsted acidic sites.

Consequently, the amount of desorbed propylene can be used alternatively as way to quantify

the concentration of Brønsted acidic sites (cB), and the data is presented in Table 4.4. Upon

increasing microwave power level (0-700), the concentration of Brønsted acidic sites decreased

from 88.8 µmolC3H6 g-1 to the lowest value of 69.6 µmolC3H6 g

-1 for 0.5FeZ10/0-700 catalysts, and


-1 for 0.5FeX10/0-700 catalysts. It

can be observed that microwave untreated catalysts, 0.5FeX10/0 and 0.5FeZ10/0 exhibited

highest concentration of Brønsted acidic sites of 57.4 µmolC3H6 g-1 and 88.8 µmolC3H6 g-1,

respectively than microwave treated catalysts (Table 4.4). Similar remarks can also be made in

Figure 4.6(a-b), that microwave untreated catalysts (0.5FeX10/0 and 0.5FeZ10/0) have higher

peak intensities than microwave treated catalysts (0.5FeX10/119-700 and 0.5FeZ10/119-700). This

indicated that microwave radiation reduced the concentration of Brønsted acidic sites.

250 300 350 400 450

B

C

D

E

F

G

H

0.5FeZ10/0

0.5FeZ10/119

0.5FeZ10/280

0.5FeZ10/336

0.5FeZ10/462

0.5FeZ10/595

0.5FeZ10/700

(a)

Temperature/(oC)

C3H

6 s

ign

al/

(a.u

)

250 300 350 400 450

B

C

D

E

F

G

H

0.5FeX10/0

0.5FeX10/119

0.5FeX10/280

0.5FeX10/336

0.5FeX10/462

0.5FeX10/595

0.5FeX10/700

(b)

Temperature/(o

C)

C3H

6 s

ign

al/

(a.u

)

Figure 4.6: Propylene profiles obtained from the C3H9N-TPSR experiments over varying Si/Al

ratio; 30 (a) and 80 (b) for determining the concentrations of Brønsted acidic sites (cB).


89

The concentration and strength of acidic sites play a vital role in the MTH process. Thus,

altering the acidic sites, especially the Brønsted acidic sites, affects the product distribution and

also the catalyst lifetime. Modifying ZSM-5 zeolite with the metal does not only increases the

selectivity towards desired hydrocarbons, but also increase the catalyst lifetime as metal

loading effectively decreases the concentration of Brønsted acidic sites (Chen et al., 2015;

Dyballa et al., 2013; Li et al., 2016; Li et al., 2016). Several approaches have been developed

to overcome coke by preparing mesoporous or hierarchical ZSM-5 from microporous ZSM-5

zeolite. Chemical leaching as one of the developed approaches for example, decreases the

concentration of Brønsted acidic sites, and by so doing increases the catalyst lifetime, decreases

coke formation and increases selectivity towards desired hydrocarbons (Wei et al., 2015; Zhou

et al., 2017).

As it can be observed in Table 4.4, there is no such linear relationship between increasing

microwave power level (0-700) and decreasing concentration of Brønsted acidic sites. This

may be attributed to low iron loading (0.5wt.% Fe) and the fact that ZSM-5 zeolite, on its own,

is microwave transparent. Whittington and Milestone, (1992) reported that either hydrated or

dehydrated Na/ZSM-5 or Na/USY do not undergo any large temperature increase under

microwave irradiation and stated that this may be due to the low sodium content, but may be

also correlated to cavity size of ZSM-5, and the fact that these zeolites are hydrophobic.

Microwave absorption in metal-based ZSM-5 catalysts can be further increased by taking

advantage of increasing the metal loading which already has some benefits. Therefore, further

decreasing the concentration of Brønsted acidic sites, and increasing the selectivity and catalyst

lifetime without using expensive templates and performing many steps to create mesoporous

or hierarchical catalysts can be easily and cheaply achieved within a short period of time using

microwave radiation.


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Table 4.4: Determination of Brønsted acidic sites from C3H9N-TPSR.

Catalyst cB* /(µmolC3H6 g-1) Temperature+ /(°C)

0.5FeX10/0 57.4 342

0.5FeX10/119 41.6 347

0.5FeX10/280 41.2 347

0.5FeX10/336 42.8 342

0.5FeX10/462 39.8 341

0.5FeX10/595 37.5 339

0.5FeX10/700 39.8 339

0.5FeZ10/0 88.8 339

0.5FeZ10/119 83.1 341

0.5FeZ10/280 83.8 337

0.5FeZ10/336 84.6 341

0.5FeZ10/462 69.6 345

0.5FeZ10/595 73.9 343

0.5FeZ10/700 73.7 339

* Concentrations of Brønsted acidic sites (cB).

+ Temperature at which first maximum peak appears.

4.3 Catalysts Testing (MeOH-TPSR experiments)

In this section, the results obtained from MeOH-TPSR experiments in the MTH process to see

the effect of microwave radiation are presented and discussed in detail.

4.3.1 MeOH-TPSR

MeOH-TPSR experiments on the iron (III)-exchanged MFI zeolite (0.5wt.%Fe/ZSM-5, with

Si/Al ratio = 30 and 80) samples were performed to study the effect of microwave radiation on

the catalyst characteristics regarding the MTH process product distribution, maximum

desorption peak temperatures (Tmax values) and peak intensities. MeOH-TPSR profiles for the


91

as-prepared catalysts (0.5FeX10/0-700 and 0.5FeZ10/0-700) were divided into three temperature

regions for clarity: 172-277 °C and 202-254 °C (region I), 252-290 °C and 246-265 °C (region

II), and 324-376 °C and 327-363 °C (region III) for 0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts,

respectively. In every temperature region, the detailed information (Tmax values and peak

intensities) of mainly the desired reactant (methanol) and products (H2O, DME, aliphatic and

aromatic hydrocarbons) are listed and shown in Table A1 and Figure 4.7 for clarity.

Figure 4.7: MeOH-TPSR data profiles for methanol conversion to C1–C5 aliphatic and C6–C8

aromatic hydrocarbons.

It is well-known that methanol is converted to hydrocarbons via three main reaction steps

namely: methanol is firstly dehydrated to produce dimethyl ether (DME), and the equilibrium

mixture is then formed, comprised of methanol, DME, and water. Produced DME undergoes

dehydration also to produce light olefins. Subsequently, the conversion of light olefins to

paraffins, higher olefins, aromatics, and naphthenes occurs (Lee et al., 2014). As it can be

observed in Figure 4.7, the desorption peaks of methanol, DME and the first peak of water

appeared in the region I. It could be understood that methanol dissociatively adsorbs on the

surfaces of the as-prepared catalysts (0.5FeX10/0-700 and 0.5FeZ10/0-700) to generate DME and


92

water. The dissociative adsorption may be proposed to take place via two ways: (i) by

adsorption on surface hydroxyl groups with the formation of methoxy groups and water, or (ii)

methoxy groups react on the surface of the catalyst to produce DME via condensation of two

neighboring methoxy groups (Bianchi et al., 1995).

As for region II, the second peak of water which is more intense than the first broad asymmetric

one which appeared in region I and C1-C5 aliphatic hydrocarbons peaks were observed. The

second peak of water is attributed to DME dehydration to produce the C1-C5 aliphatic

hydrocarbons. Lastly, as for region III in Figure 4.7, C6-C8 aromatics (benzene, toluene and

xylene) peaks were observed, showing the occurrence of an aromatization reaction as the

reaction temperature was increased to 550 oC. Previously, it has been accepted that propylene

and ethylene are primary intermediates for producing BTX aromatics by a series of processes

such as dehydrogenation, cyclization and aromatization at elevated temperature.

Table A1 shows the typical TPSR data obtained for methanol on the microwave-untreated

catalysts (0.5FeX10/0 and 0.5FeZ10/0) and for other microwave treated catalysts. Methanol

profiles have two peaks – a broad asymmetric profile with a Tmax at 183 °C and a more intense

narrow profile with a Tmax at 237 °C for 0.5FeX10/0 catalyst, and one broad asymmetric profile

with Tmax at 246 °C for 0.5FeZ10/0 catalyst (Table A1). Upon increasing microwave radiation

power level from 0-700 W, the Tmax values for desorbed methanol and evolved products (H2O,

DME, aliphatic and aromatic hydrocarbons) from methanol conversion shifted to either lower

or higher Tmax values than for those of 0.5FeX10/0 and 0.5FeZ10/0 catalysts as shown in Table

A1. Moreover, the profile shapes for methanol desorption and for products formation were

found to be almost similar upon increasing microwave level (0-700 W) for each Si/Al ratio.

However, only the peak intensities were found to be significantly changing. Very different peak


93

intensities of the profiles and the Tmax values observed for different catalysts obtained using

different Si/Al ratios (30 and 80) and varying microwave power levels (0-700 W) indicated the

presence of different active surface sites having very different chemical activity and selectivity

properties.

Figure 4.8: C1 Alkane (Methane) profiles obtained from the MeOH-TPSR experiments over

varying Si/Al ratio; 30 (a) and 80 (b).

During the MeOH-TPSR experiments over 0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts, water and

DME formation profiles were included, but for simplicity, more focus was paid on the aliphatic

and aromatic hydrocarbon profiles to see the effect of microwave radiation. As for methane

profiles, upon increasing microwave power level from 0-700 W (Figure 4.8(a)), the peak

intensities decreased. Microwave-untreated catalyst (0.5FeZ10/0) showed pronounced peak

intensity on methane profile than microwave-treated catalysts (0.5FeZ10/119-700). This shows

that less methane was formed upon increasing microwave power level. Under increased

microwave power level (0-700 W), the Tmax values for methane profiles were found to be

fluctuating in the range of 246–260 oC (Table A1 and Figure 4.14(a)).


94

Microwave-untreated catalyst (0.5FeX10/0) showed pronounced peak intensities for methane

formation compared to microwave-treated catalyst (0.5FeX10/119-700) as shown in Figure 4.8(b).

Upon increasing microwave power level, there was no such tremendous decrease in methane

peak intensities when using 0.5FeX10/119, 0.5FeX10/336 and 0.5FeX10/700 catalysts unlike when

using 0.5FeX10/280, 0.5FeX10/462 and 0.5FeX10/595 catalysts. The fluctuating peak intensity

response for methane profiles over microwave-treated catalysts (0.5FeX10/119-700) may be

attributed to increased Si/Al ratio (80) which expels the weakly bonded cations to the zeolite

framework. Therefore, the number of active iron cations per unit cell were decreased, resulting

in uneffective response of these catalysts (0.5FeX10/119-700) to microwave radiation compared

to other microwave-treated 0.5FeZ10/119-700 catalysts prepared using ZSM-5 zeolite with Si/Al

= 30.

Under 119–700 W microwave irradiation (Table A1 and Figure 4.14(b)), the Tmax values for

methane profiles shifted from 252 oC to Tmax values ≥ 255 oC. It can be deduced that microwave

irradiation of the catalysts enhanced the rate of reorientation of the intermediates (C7H9+-

C12H19+) in the MTH process and facilitated the proton back-donation process to the zeolite

framework. This process facilitated the recovery of the acidic site of the catalysts and the

release of the corresponding arene to overcome coke formation, as the rate of methane

formation was slow. Wen et al, (2016) showed that the rate of reorientation of the intermediates

and the rate of methane formation via demethylation are two competing reactions.


95

Figure 4.9: C2 Aliphatic hydrocarbons (Ethane and Ethylene) profiles obtained from the

MeOH-TPSR experiments over varying Si/Al ratio; 30 (a) and 80 (b).

Figure 4.9(a) shows similar profiles in shape for C2 aliphatic hydrocarbons (Ethylene and

Ethane) like the ones for methane formation when using 0.5FeZ10/0-700 catalysts. Upon

increasing microwave power level, there was a decrease on the C2 aliphatic hydrocarbons peak

intensities except for microwave-untreated catalyst (0.5FeZ10/0). 0.5FeZ10/0 catalyst shows

pronounced C2 aliphatic hydrocarbons peak intensities than microwave-treated catalysts

(0.5FeZ10/119-700). Figure 4.9(b) shows that microwave-treated (0.5FeX10/119 and 0.5FeX10/700)

catalysts exhibit pronounced peak intensities on the C2 aliphatic hydrocarbon profiles than

other microwave-treated catalysts (0.5FeX10/280-595) and microwave-untreated catalyst

(0.5FeX10/0). The shapes of the C2 aliphatic hydrocarbon profiles remained similar, and only

the peak intensities were affected during increased microwave power level (0–700 W) as shown

in Figure 4.9(b).


96

Figure 4.10: C3 Alkane (Propane) profiles obtained from the MeOH-TPSR experiments over


Figure 4.10(a-b) show that upon increased microwave power level (0–700 W), the peak

intensities on the propane profiles were suppressed. As a result, microwave-untreated catalysts

(0.5FeX10/0 and 0.5FeZ10/0) showed the highest peak intensities on the propane profiles.

Microwave irradiation resulted in decreased propane formation. The microwave-untreated

catalyst (0.5FeZ10/0) shows pronounced peak intensities on the C3-C5 aliphatic hydrocarbon

profiles namely, propylene, butene, pentene, butane and pentane compared to the microwave-

treated catalysts (0.5FeZ10/119-700) as shown in Figure 4.11(a) and Figure A2-A3(a). Figure

4.11(b) and Figure A2-A3(b) show that microwave-treated catalyst (0.5FeX10/462) showed the

highest peak intensity on C3-C5 aliphatic hydrocarbon profiles than microwave-untreated

catalyst (0.5FeX10/0) and corresponding microwave-treated catalysts.


97

Figure 4.11: C3-C4 Alkenes (Propylene and Butene) profiles obtained from the MeOH-TPSR

experiments over varying Si/Al ratio; 30 (a) and 80 (b).

As for Figure 4.11(a) and Figure A2-A3(a), and Figure 4.11(b) and Figure A2-A3(b), similar

C3-C5 aliphatic hydrocarbon profiles in shape, distribution and adscription but with different

peak intensities were observed over 0.5FeZ10/0-700 and 0.5FeX10/0-700 catalysts, respectively.

Moreover, C3-C5 aliphatic hydrocarbons appeared to have similar Tmax values like for C1-C2

aliphatic hydrocarbons (Table A1 and Figure 4.14(a-b)). From the Tmax values for C1-C5

aliphatic hydrocarbons, upon increased Si/Al ratio to 80, and increased microwave power level,

the Tmax values shifted to higher temperatures from 252 C. This shows that increased Si/Al

ratio and microwave power level resulted in increased desorption temperatures for the C1-C5

aliphatic hydrocarbons (Table A1). Upon decreased Si/Al ratio to 30, and increased microwave

power level (Table A1), Tmax values were below 260 C for C1-C5 aliphatic hydrocarbons. This

shows that the C1-C5 aliphatic hydrocarbons were easily formed at lower temperatures than for

Si/Al ratio of 80.


98

Figure 4.12: C6 Aromatic hydrocarbon (benzene) profiles obtained from the MeOH-TPSR


As for 0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts in region III, in each figure (Figure 4.12(a-b)

and Figure 4.13(a-b)), the shape, distribution and adscription of the peaks for the profiles were

found to be similar, but the peak intensities and the Tmax values of the products (benzene (Figure

4.12(a-b)), toluene and xylene (Figure 4.13(a-b))), were tremendously different. The

microwave-treated catalyst (0.5FeZ10/700) showed the highest peak intensity in benzene profiles

than microwave-untreated catalyst (0.5FeZ10/0) and other corresponding microwave-treated

catalysts (0.5FeZ10/119-595) as shown in Figure 4.12(a). This implies that more benzene was

generated on the 0.5FeZ10/700 catalysts. However, upon increased Si/Al ratio to 80 from 30 and

under the same microwave power levels for irradiation and Fe loading, microwave-untreated

catalyst (0.5FeX10/0) showed the highest peak intensity on benzene profiles than microwave-

treated catalysts (0.5FeX10/119-700) as shown in Figure 4.12(b).


99

Figure 4.13: C7-C8 Aromatics (Xylene and Toluene) profiles obtained from the MeOH-TPSR


The microwave-untreated catalyst (0.5FeZ10/0) showed the highest peak intensity in toluene

and xylene profiles followed by 0.5FeZ10/700 catalyst as shown in Figure 4.13(a). When the

Si/Al ratio was increased from 30 to 80, 0.5FeX10/700 catalyst showed the highest peak intensity

in toluene and xylene profiles followed by 0.5FeX10/0 and 0.5FeX10/595 catalysts as shown in

Figure 4.13(b). It is plausible that under optimized reaction conditions, feed composition and

suitable metal loading on the zeolite, upon increased microwave power level for catalyst post-

treatment, the rate of aromatization in the MTH process intermediates can be enhanced to form

BTX. Compared with the peak intensities for C2-C5 aliphatic hydrocarbons, only trace amounts

of BTX can be observed over 0.5FeZ10/0-700 and 0.5FeX10/0-700 in region III (as shown in Figure

4.12(a-b), and Figure 4.13(a-b)) as the peak intensities of the BTX profiles were lower. This

shows that most of the primary intermediates (ethylene and propylene) obtained over

0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts were not converted to BTX products. The enhanced

peak intensities in region II (as shown in Figure 4.9-4.11(a-b) and Figure A2-A3(a-b)) for C2-

C5 aliphatic hydrocarbons compared with the peak intensities for BTX in region III (as shown

in Figure 4.12(a-b) and Figure 4.13(a-b)), indicated that incorporation of iron promotes the

formation of the intermediates in MTH process over the aromatization steps to form BTX, even

under microwave post-treatment of the catalysts.


100

As observed in Table A1 and Figure 4.14(a-b), the Tmax values for benzene over the 0.5FeX10/0-

700 and 0.5FeZ10/0-700 catalysts fluctuates at lower temperature ranges, 324–361 C and 327–348

C, respectively as compared to Tmax values for xylene and toluene. This suggests that the

benzene was more easily generated at lower temperatures, followed by a benzene methylation

process occurring at higher temperatures to produce xylene and toluene. Based on the shift on

Tmax values for toluene and xylene to higher temperatures, ≥ 331 C and ≥ 339 C over

0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts, respectively (Table A1), it is thought that increased

microwave power level under which the catalysts were post-treated can lead to an increased

toluene and xylene formation temperatures. As shown in Figure 4.14(a-b), the microwave

power level has an effect on the Tmax values in the MTH process.

Figure 4.14: Increasing power level against maximum peak temperature for MTH process over

0.5FeZ10/0-700 catalysts (a) and 0.5FeX10/0-700 catalysts (b).


101

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Chapter 5: Conclusions and Recommendations

105

CHAPTER 5

Conclusions and Recommendations

5.1 Conclusions

The iron-containing precursors containing 0.5 wt.% iron (0.5Fe/ZSM-5), were prepared using

the incipient wetness impregnation method and subsequently post-modified under microwave

irradiation. The resulting 0.5FeX10/0-700 and 0.5FeZ10/0-700 catalysts were characterized and

tested in the MeOH-TPSR experiment to understand the effect of microwave radiation on these

catalysts and their activity for methanol conversion to hydrocarbons. Upon increased

microwave power level from 0 to 700 W, it was found that:

(i) The MFI zeolite structure for all the catalysts was maintained even after iron loading

as confirmed by XRD and FT-IR analysis. However, their peak intensities and

crystallinities were found to be drastically changing. The relative crystallinity

values increased from 46.3% to values ≥ 47.3% for 0.5FeZ10/0-700 catalysts and the

relative crystallinity values for 0.5FeX10/0-700 catalysts increased from 55.8% to

values ≥ 61.3%. The 0.5FeX10/0-700 catalysts were more crystalline than 0.5FeZ10/0-

700 catalysts.

(ii) BET surface area and micropore volume decreased from 376 m2/g to 346 m2/g and

0.119 cm3/g to 0.108 cm3/g, respectively for 0.5FeZ10/0-595 catalysts except for the

0.5FeZ10/700 catalyst. The BET surface area decreased from 366 m2/g to values ≤

357 cm2/g and the mesopore volume increased to 0.0995 cm3/g for 0.5FeX10/0-700

catalysts.

(iii) Coke amounts decreased from 46.4 mgcokegcat.-1 to 16.2 mgcokegcat.

-1 and from 31.7

to 2.84 mgcokegcat.-1 for 0.5FeZ10/0-700 and 0.5FeX10/0-700 catalysts, respectively. As a


106

result, the 0.5FeX10/0-700 catalysts shown more resistance to coke deposition than

the 0.5FeZ10/0-700 catalysts.

(iv) SEM images for 0.5FeX10/0-700 catalysts clearly showed that nanosized aggregates

clustered together into micro- and mesosized stacks with visually seen spaces

between the stacks as compared to SEM images for 0.5FeZ10/0-700 catalysts.

(v) From the C3H9N-TPSR results, the concentration of Brønsted acidic sites decreased


-1 for 0.5FeZ10/0-700

catalysts, and from 57.4 µmolC3H6 g-1 to the lowest value of 37.5 µmolC3H6 g

-1 for

0.5FeX10/0-700 catalysts, respectively.

(vi) From the MeOH-TPSR results, the Tmax values for desorbed methanol and evolved

products (H2O, DME, aliphatic and aromatic hydrocarbons) shifted in either lower

or higher Tmax values than for those of 0.5FeX10/0 and 0.5FeZ10/0 catalysts. The peak

intensities for methane profiles decreased over 0.5FeX10/119-700 and 0.5FeZ10/119-700

catalysts. This shows that microwave radiation inhibits methane production.

Microwave radiation enhanced the rate of reorientation of the intermediates (C7H9+-

C12H19+) in the MTH process as methane production was decreased. Ethane and

ethene peak intensities decreased and fluctuated, propane peak intensities

decreased, propene, butene, pentene, butane, pentane peak intensities decreased and

fluctuated, over the 0.5FeZ10/0-700 and 0.5FeX10/0-700 catalysts, respectively. The

0.5FeZ10/700 catalyst showed pronounced peak intensity on benzene profiles while

peak intensities for benzene profiles decreased over 0.5FeX10/0-700 catalysts. This

shows that 0.5FeZ10/700 give rise to more benzene formation than all the catalysts

studied. The peak intensities for xylene and toluene decreased over 0.5FeZ10/0-700

catalysts while 0.5FeX10/700 catalysts showed pronounced peak intensity on xylene

and toluene profiles.


107

It was observed that, generally the microwave radiation has an effect on the catalytic properties

of 0.5Fe/ZSM-5 catalysts prepared which then results in either positive or negative

enhancement of the peak intensities in the MeOH-TPSR experiments. This work successfully

met the set aim which was to use microwave radiation to create mesoporous or hierarchical

catalysts to overcome coke formation which is associated with high methane production.

5.2 Recommendations

This work may be considered successful; however, there are some recommendations that could

be further carried over for future work, and some recommendations are stated below.

Owing to successful application of microwave radiation as a post-synthesis

modification step, apart from the already reported methods in the literature like

chemical leaching and templating approaches to create mesoporous or hierarchical

catalysts from microporous catalysts, to reduce mass transfer limitations encountered

in microporous catalysts in the process like MTH, microwave radiation should be

employed especially in heterogeneous catalysis.

To increase the ability of metal-based ZSM-5 catalysts to absorb microwave radiation

effectively, the effect of metal loading should be carried out.

Examining of the zeolite samples using micro-XRD to see if there are any structural

changes induced by microwave radiation should be carried out.

The effect of microwave radiation should be further tested over a number of elements

across the periodic table using ZSM-5 zeolite as a support in the MTH process.

Further work using microwave radiation should be carried out to be able to quantify

how much is being produced of the desired hydrocarbons (olefins/aromatics) from

methanol conversion.

Appendix

108

APPENDIX A

Supplementary information for Chapter 4

Figure A1: SEM micrographs of microwave-treated catalysts.

Appendix

109

Table A1: Maximum desorption peak temperatures (Tmax values) from MeOH-TPSR data for

adsorbed methanol over microwave untreated (0.5FeX10/0 & 0.5FeZ10/0) and treat (0.5FeX10/119-

700 & 0.5FeZ10/119-700) catalysts.

Figure A2: C5 Alkene (Pentene) profiles obtained from the MeOH-TPSR experiments over


Methane

Ethane

&Ethene Propane

Propene

& Butene Pentene

Butane &

Pentane

Dimethyl

Ether Benzene

Toluene

& Xylene

0.5FeZ10/0 254 207 257 253 253 253 253 253 248 339 347

0.5FeZ10/119 256 203 258 256 256 256 256 256 247 327 339

0.5FeZ10/280 260 207 265 260 260 260 260 260 254 347 363

0.5FeZ10/336 249 205 252 249 249 248 248 248 243 334 356

0.5FeZ10/462 246 202 252 246 246 246 246 246 243 333 357

0.5FeZ10/595 249 205 255 249 249 249 249 249 243 345 357

0.5FeZ10/700 250 205 256 250 250 250 250 250 248 337 359

0.5FeX10/0 252 195 254 252 252 183 237 252 252 252 241 324 331

0.5FeX10/119 255 196 256 255 255 172 230 255 255 255 237 325 340

0.5FeX10/280 289 218 290 289 289 197 276 289 289 289 277 361 376

0.5FeX10/336 275 204 276 275 275 189 254 275 275 275 253 353 364

0.5FeX10/462 282 212 283 282 282 195 258 282 282 282 266 356 365

0.5FeX10/595 278 209 279 278 278 188 257 278 278 278 264 354 356

0.5FeX10/700 273 210 275 273 273 187 256 273 273 274 262 354 362

241

237

246

243

247

245

Catalyst

Water Methanol

Maximum desorption peak temperature/°C

246

Appendix

110

Figure A3: C4-C5 Alkanes (butane and Pentane) profiles obtained from the MeOH-TPSR


Appendix

~ THE END ~

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Effect of microwave radiation on Fe/ZSM-5 for catalytic