DECLARATION. (Signature of candidate) .. day of

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1 DECLARATION I declare that this thesis is my own, unaided work. It is being submitted for the Degree of Doctor of Philosophy in the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination in any other University... (Signature of candidate).. day of i

2 Abstract The study of the aromatisation of methane was conducted at 750 o C over metalimpregnated H-ZSM-5 catalysts with a feed flow rate of 13 ml/min and the composition of the feed was 90% methane balance argon. Typical products that were detected from the outlet stream were ethene, ethane, benzene and toluene. The amount of coke produced was determined by using 10% argon as an internal standard. The effects of different parameters such as the type of the support material, the molybdenum content, the %XRD crystallinity and SiO 2 /Al 2 O 3 ratio of H-ZSM-5, the reaction temperature, the feed flow rate, the type of the molybdenum precursor, the catalysts preparation method, the addition of dopants, silanation and the regenerability of the catalysts were investigated. The results obtained showed that H-ZSM-5 was a better support for the preparation of catalysts used for the aromatisation of methane. Mo/H-ZSM-5 catalysts were more active when the molybdenum loading was between 2 and 4 wt% and loadings higher than 4% led to lower activities. The lower activities observed at higher molybdenum loadings was related to the poor dispersion and decrease in the pore volumes and surface areas observed due to the formation of MoO 3 crystallites. Furthermore, the zeolite structure collapsed under the reaction conditions when the molybdenum loading was more than 4 wt%. The study showed that the conversion of methane increased linearly with increasing reaction temperature and the apparent activation energy of the reaction was found to be 64.5 kj/mol. The results of the effect of the %XRD crystallinity of H-ZSM-5 on the performance of H-ZSM-5 catalysts showed that 2%Mo/H-ZSM-5 catalysts were more active when the crystallinity of the zeolite was between 50 and 70%. The conversion of methane decreased with an increase in the SiO 2 /Al 2 O 3 ratio of H-ZSM-5. Higher aromatisation activities were observed when the SiO 2 /Al 2 O 3 ratio of H-ZSM-5 was ii

3 60. The type of the molybdenum precursor used in the preparation of 2%Mo/H- ZSM-5 catalysts did not have a significant influence on the conversion of the catalysts, but higher selectivities for aromatics were observed when ammonium heptamolybdate was used as a source of molybdenum. The catalysts prepared by physical mixing of MoO 3 and H-ZSM-5 catalysts were more active than those prepared by impregnation with solutions of ammonium heptamolybdate. The presence of dopants such as boron, silver and alkali metal ions (Li +, Na + and K + ) in 2%Mo/H-ZSM-5 catalysts was also investigated. Boron ( wt%) did not affect the conversion level of the catalysts but changed their selectivity properties. The selectivity for C 2 hydrocarbons increased with boron content, while the selectivity for aromatics decreased. The addition of silver ions (0.5 wt%) significantly improved the conversion of the catalysts. This was attributed to the enhancement of the acvidity of the catalysts upon addition of silver ions which was observed by temperature programmed desorption of ammonia and pyridine adsorption studies of the infrared spectra of the catalysts. The addition of alkali metal ions in the Mo:Metal ratio of 0.5 led to decreased catalytic activities, due to the lowered acidities of the catalysts. The silanation of H-ZSM-5 improved the conversion of methane but lowered the selectivity for aromatics. A comparative study of the W-based and Mo-based catalyst at equivalent molar contents showed that molybdenum-based catalysts were more active than tungsten based catalysts. The study also showed that the catalytic performance of 2%Mo/H-ZSM-5 catalysts could be regenerated to appreciable levels by treatment of the catalysts in air at 600 o C. The possibility of using Mo/H-ZSM-5 catalysts for the aromatisation of propane was also evaluated at 530 o C, with consideration of three variables, namely, the molybdenum loading, the reaction temperature and %XRD crystallinity. The results indicated that impregnation H-ZSM-5 catalysts with molybdenum led to lower iii

4 propane aromatisation activities. This lower activity was attributed to the lower Brønsted acid sites in the Mo/H-ZSM-5. The activities of the catalysts could be improved by operation at higher temperatures, but the rate of deactivation was also improved at higher temperatures. In line with the observations from the conversion of methane, higher activities were observed when the %XRD crystallinity of the catalyst was 61%. iv

5 This thesis is dedicated to the memory of my grandfather Ngwako William Marutha ( ) v

6 Acknowledgements I would like to express my gratitude to the following people and organisations whose contributions during the project will always be remembered: My supervisor, Prof. M.S. Scurrell, for believing in me and guiding me throughout this study. You have been a very patient teacher, and always encouraged me to think and work independently. My mentor, Dr. C.P. Nicolaides, for giving helpful ideas throughout the study. You have helped achieve my goals; your contributions have been invaluable. The late Prof. J.J. Prinsloo who was supervisor during my studies for a M.Sc. degree for encouraging me to go on with my studies until nature becomes the only limit. Dr. B.S. Nkosi for organising me a bursary during my M.Sc. studies, I probably would not have reached this far if it was not for that help. Basil Chassoulas for the great technical assistance he provided throughout the duration of the project, making sure that the equipment used in this study is working properly. The glass blowers, Barry Fairbrother and Steve Ganon, for making sure that the quartz reactors are available whenever needed. Members of Cat-Om-Mat group, who always provided a friendly working environment. Thank you guys for all the suggestions and ideas you were always willing to offer. vi

7 The students who got involved in the project during their honours studies, namely, Tefo Selikane, Warren Thompson and Themba Tshabalala. Your contributions in this project have been invaluable and did not go unnoticed. My family for giving me all the support I needed to make it through this project. You have all taught me something about perseverance. All my friends, the list is so long I cannot mention names, for the support you gave me and always encouraging me and always giving me the reasons to go on. University of the Witwatersrand for providing the facilities and financial assistance. Sasol Ltd., DAAD and Mellon for making funds needed for carrying out this project available. vii

8 Presentations arising from this work Poster presentations Nonoxidative methane conversion over supported Mo catalysts, Catalysis Society of South Africa (CATSA) annual conference, Potchefstroom, South Africa, Methane conversion to chemicals, carbon and hydrogen (MCCH) over modified molybdenum-nas catalysts, The 8 th Natural Gas Conversion Symposium, Natal, Brazil, Oral presentations The nonoxidative conversion of methane over Mo/H-ZSM-5: The effect of dopants, Catalysis Society of South Africa (CATSA) annual conference, Midrand, South Africa, Methane conversion to chemicals, carbon and hydrogen (MCCH)) over modified molybdenum-nas catalysts, Catalysis Society of South Africa (CATSA) annual conference, Mosselbay, South Africa, Paper published and in preparation Methane conversion to chemicals, carbon and hydrogen (MCCH) over modified molybdenum-nas catalysts, M.W. Ngobeni, M.S. Scurrell and C.P. Nicolaides, in viii

9 F.B. Noronha, M. Schmal and E.F. Sousa-Aguiar, eds., Proceedings of the 8 th Natural Gas Conversion Symposium, Stud. Surf. Sci. Catal. 167 (2007) ix

10 Table of contents Declaration Abstract Dedication Acknowledgements Presentations and publications arising from this work Table of contents List of Figures List of Tables List of Abbreviations and Symbol (i) (ii) (v) (vi) (viii) (x) (xvi) (xxvii) (xxx) Chapter 1: Introduction The aromatisation of light alkanes Considerations of methane conversion Aims and objectives Outline of the thesis References 7 x

11 Chapter 2: Preparation and characterisation of supportedmetal catalysts Introduction Catalyst preparation in heterogeneous catalysis Methodologies of catalysts preparation Drying Calcination and activation Synthesis and structure of zeolites Characterisation of catalysts X-Ray diffraction Surface area determination by nitrogen physisorption Temperature-programmed analysis Electron microscopy Vibrational spectroscopy References 43 Chapter 3: The nonoxidative dehydroaromatisation of methane Catalysts used in the nonoxidative dehydroaromatisation of methane The preparation of catalysts for the nonoxidative dehydroaromatisation of methane 49 xi

12 3.3 Factors that influence the catalytic performance of methane dehydroaromatisation catalyst The active sites and the proposed mechanisms The role of Brønsted acidity in methane aromatisation The nature of carbon deposited on the catalysts Methods used for the suppression of poisonous carbonaceous species production Methane dehydroaromatisation over W/H-ZSM-5 catalysts An overview of the nonoxidative conversion of methane References 79 Chapter 4: The aromatisation of propane Propane aromatisation over Ga-based ZSM-5 type zeolites The effect of hydrogen pretreatment The effect of co-feeding propane with other gases The effect of the calcination, hydrothermal treatment, and factors affecting zeolitic acidity The effect of the reaction temperature The effect of space velocity Mechanism of propane aromatisation over Ga-loaded zeolites Deactivation and regeneration of the catalysts Propane aromatisation over Mo-based catalysts 99 xii

13 4.5 Overview of the aromatisation of propane References 101 Chapter 5: Experimental Reagents The preparation of catalysts Characterisation of the catalysts X-ray powder diffraction (XRD) Surface area and porosity analysis Temperature programmed reduction (TPR) Temperature programmed desorption of ammonia (NH3-TPD) Temperature programmed oxidation (TPO) Diffuse reflectance spectroscopy (DRS) Transmission electron microscopy (TEM) FT-IR spectroscopy Carbon content analysis Catalytic conversion of methane The aromatisation of propane over Mo/H-ZSM-5 catalysts References 119 Chapter 6: Results from the nonoxidative conversion of methane over Mo/H-ZSM-5 catalysts 120 xiii

14 6.1 The effect of the support The effect of molybdenum loading The effect of flow rate The effect of the reaction temperature The effect of the percentage XRD crystallinity The effect of the SiO2/Al2O3 ratio The effect of the molybdenum precursor The effect of the catalyst preparation method The effect of dopants Boron Silver Alkali metal ions The effect of silanation Regenerability Mo/H-ZSM-5 catalysts On the use of tungsten as replacement for molybdenum in Mo/H- ZSM-5 catalysts Conclusions References 194 Chapter 7: The aromatisation of propane over Mo/H-ZSM-5 catalysts The effect of the molybdenum loading 197 xiv

15 7.2 The effect of the reaction temperature The effect of %XRD crystallinity of H-ZSM References 207 Chapter 8: Conclusions 209 xv

16 List of Figures Figure 1.1 Flow chart of processing natural gas into motor fuel. 2 Figure 1.2 Distribution of proven reserves in 1985, 1995 and Figure 2.1 The IUPAC classification of adsorption isotherms for gasadsorption equilibria. 28 Figure 2.2 t-plots of (a) nonporous, (b) mesoporous, (c) microporous and (d) micro- and mesoporous solids. 31 Figure 4.1 Reaction path for the formation of different aromatics in the aromatisation of propane (B = benzene, T = toluene, X = xylene, TMB = trimethyl benzene). 93 Figure 4.2 Pathway of propane aromatisation over H-ZSM-5 and Ga/H-ZM-5 catalysts. 94 Figure 4.3 Mechanism for the bifunctional activation of propane on Ga-containing H-ZSM-5 catalysts. 96 Figure 4.4 Ga-acid bifunctional mechanism of propane dehydrogenation over Ga/H-ZSM-5 catalysts. 96 Figure 4.5 The overall mechanism for the formation of propane over Ga-containing H-ZSM-5 zeolite catalysts. 97 xvi

17 Figure 5.1 Schematic representation of the catalytic rig setup. 113 Figure 5.2 Figure 5.3 Schematic representation of the quartz reactor used in this study. 114 A typical GC profile from the catalytic conversion of methane over a Mo/H-ZSM-5 catalyst at 750 o C. 115 Figure 5.4 A typical GC profile for the conversion of methane over Mo/H-ZSM-5 at 530 o C. 118 Figure 6.1 The effect of the support on the catalytic conversion and stability of supported molybdenum catalyst at 750 o C. 123 Figure 6.2 The effect of the Mo loading on the catalytic conversion and stability of Mo/H-ZSM-5 catalysts with a crystallinity of 66% for the nonoxidative conversion of methane at 750 o C. 125 Figure 6.3 The effect of Mo loading on the conversion of methane at 750 o C over Mo/HZSM-5 catalysts with a crystallinity of 66%, after a time on stream of 5 hrs. 125 Figure 6.4 The effect of Mo loading on the products selectivity from methane aromatisation over Mo/H-ZSM-5 catalysts for a time on stream of 5 hrs at 750 o C, for a conversion of 10%. 127 Figure 6.5 The X-ray diffractograms of Mo/H-ZSM-5 catalysts of different Mo contents. 129 xvii

18 Figure 6.6 The results of the phase identification of the 15%Mo/H- ZSM-5 catalyst using database of X-ray structures. 130 Figure 6.7 H 2 -TPR profiles of (a) MoO 3, together with the profile for (b) Mo/H-ZSM-5 catalysts of different Mo contents. 131 Figure 6.8 The UV-Vis diffuse reflectance spectra of (a) MoO 3, sodium molybdate (Na 2 MoO 4 ) and ammonium heptamolybdate (AHM), and (b) Mo/H-ZSM-5 catalysts with different molybdenum contents. 134 Figure 6.9 The variation of the absorption UV-Vis absorption edge energies of Mo/H-ZSM-5 catalysts with the molybdenum content. 135 Figure 6.10 The NH 3 -TPD profiles of Mo/H-ZSM-5 catalysts with different molybdenum contents. 136 Figure 6.11 The TPO profiles of Mo/H-ZSM-5 catalysts with different molybdenum contents that were used for the conversion of methane at 750 o C. 138 Figure 6.12 The X-ray diffractograms of the used Mo/H-ZSM-5 catalysts of different Mo contents. 140 Figure 6.13 The results of the phase identification of the used 15%Mo/H-ZSM-5 catalyst using database of X-ray structures. 141 xviii

19 Figure 6.14 The variation %conversion of methane with the flow rate of the feed during the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 54% at 750 o C. 143 Figure 6.15 The variation of %selectivity for ethene, ethane and toluene with %conversion from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 54% at 750 o C. 143 Figure 6.16 The variation of the %selectivity for benzene and coke with %conversion from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 54% at 750 o C. 144 Figure 6.17 The variation of methane %conversion with the flow rate of the feed during the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts at 750 o C. 145 Figure 6.18 The variation of %selectivity for ethene, ethane and toluene with %conversion from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts at 750 o C. 146 Figure 6.19 The variation of the %selectivity for benzene and coke with %conversion from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts at 750 o C. 146 Figure 6.20 The effect of the reaction temperature on the catalytic conversion and stability of 2%Mo/H-ZSM-5 catalysts with xix

20 a crystallinity of 61% for the nonoxidative conversion of methane at 750 o C. 147 Figure 6.21 The effect of the reaction temperature on the %conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 61%. 148 Figure 6.22 The Arrhenius plot used for the determination of the apparent activation energy for the conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 61%. 149 Figure 6.23 The effect of the reaction temperature of the selectivity for C 2 hydrocarbons from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 61%, after a time on stream of 1 h. 150 Figure 6.24 The effect of the reaction temperature of the %selectivity for aromatics and coke from the nonoxidative conversion of methane over 2%Mo/H-ZSM-5 catalysts with a crystallinity of 61%, after a time on stream of 1 h. 150 Figure 6.25 The TPO profiles of 2%Mo/H-ZSM-5 catalysts used for the aromatisation of methane at different reaction temperatures. 151 Figure 6.26 The X-ray diffractograms of 2%Mo/H-ZSM-5 catalysts used for the aromatisation of methane at different reaction temperatures. 153 xx

21 Figure 6.27 The effect of %XRD crystallinity on the stability of 2%Mo/H-ZSM-5 catalysts during the conversion of methane at 750 o C. 154 Figure 6.28 The effect of the %XRD crystallinity on the percentage conversion of methane over Mo/H-ZSM-5 catalysts at 750 o C, after a time on stream of 5 h. 155 Figure 6.29 The effect of the %XRD crystallinity on the product selectivity from the conversion of methane at 750 o C, for a conversion of about 10%. 157 Figure 6.30 The effect of the %XRD crystallinity on the ethene/ethane ratio from the conversion of methane at 750 o C, for a conversion of about 10%. 157 Figure 6.31 A TEM micrograph of the carbon nanotubes formed from the aromatisation of methane over Mo/H-ZSM-5 catalyst (with a crystallinity of 60%) at 750 o C. 158 Figure 6.32 TPD profiles of 2%Mo/H-ZSM-5 catalysts with different SiO 2 /Al 2 O 3 ratios. 159 Figure 6.33 The TPR profile of 2%Mo/H-ZSM-5 catalysts with different molybdenum. 161 Figure 6.34 The effect of the SiO 2 /Al 2 O 3 on the catalytic conversion and stability of 2%Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C. 162 xxi

22 Figure 6.35 The effect of the SiO 2 /Al 2 O 3 ratio on the catalytic conversion of methane over 2%Mo/H-ZSM-5 catalysts at 750 o C, after a time on stream of 5 h. 163 Figure 6.36 The effect of the SiO 2 /Al 2 O 3 ratio on the selectivity for C 2 hydrocarbons from the conversion of methane over 2%Mo/H-ZSM-5 catalysts, for a conversion of 10% at 750 o C. 164 Figure 6.37 The effect of the molybdenum precursor on the conversion and stability of 2%Mo/ H-ZSM-5 catalysts for the conversion of methane at 750 o C. 165 Figure 6.38 The effect of the precursor of molybdenum on the product selectivity of 2%Mo/H-ZSM-5 for the conversion of propane at 750 o C, for a conversion of 10%. 166 Figure 6.39 The effect of the method used for the preparation of 2%Mo/ H-ZSM-5 catalysts on the stability and conversion methane at 750 o C. 167 Figure 6.40 The effect of the catalysts preparation method on the product selectivity of 2%Mo/H-ZSM-5 for the conversion of propane at 750 o C, for a conversion of 10%. 168 Figure 6.41 The effect of boron on the conversion and stability of Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C. 169 xxii

23 Figure 6.42 The effect of boron loading on the selectivity for C 2 hydrocarbons and the ethene/ethane ratio during the conversion of methane at 750 o C for a conversion of 10%. 170 Figure 6.43 The effect of boron loading on the selectivity for aromatics and coke during the conversion of methane at 750 o C for a conversion of 10%. 170 Figure 6.44 The TPR profiles for 2%Mo/H-ZSM-5 catalysts with different boron loadings. 171 Figure 6.45 NH 3 -TPR profiles of 2%Mo/H-ZSM-5 catalyst with and without boron. 172 Figure 6.46 The UV-Vis diffuse reflectance spectra of 2%Mo/H-ZSM-5 catalysts with different boron loadings. 173 Figure 6.47 The variation of the absorption UV-Vis absorption edge energies of Mo/H-ZSM-5 catalysts with the boron content. 174 Figure 6.48 The effect of silver on the conversion and stability of Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C. 175 Figure 6.49 The effect of the silver ions on the product selectivity from the conversion of methane over Mo/H-ZSM-5 catalysts at 750 o C, for a conversion of 10%. 176 Figure 6.50 NH 3 -TPD profiles of 2%Mo/H-ZSM-5 catalyst with and without silver ions. 177 xxiii

24 Figure 6.51 The IR spectra of adsorbed pyridine on H-ZSM-5 catalysts with and without metal additives. 178 Figure 6.52 The effect of alkali metal ions on the conversion and stability of Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C. 179 Figure 6.53 UV-Vis diffuse reflectance spectra of 2%Mo/H-ZSM-5 catalyst with and without alkali metal ion dopants. 180 Figure 6.54 TPR profiles of 2%Mo/H-ZSM-5 catalysts with and without alkali metal ion dopants. 182 Figure 6.55 The TPO profiles of Mo/H-ZSM-5 catalysts with and without alkali metal ions used in the conversion of methane at 750 o C. 183 Figure 6.56 The NH 3 -TPD profiles of 2%Mo/H-ZSM-5 catalysts with and without alkali metal ions. 184 Figure 6.57 The effect of silanation on the catalytic performance of Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C. 185 Figure 6.58 The effect of silanation of the product selectivity of 2%Mo/H-ZSM-5 catalysts from the conversion of methane at 750 o C. 186 xxiv

25 Figure 6.59 The effect of regeneration on the conversion of methane over the 2%Mo/H-ZSM-5 catalyst at 750 o C. 187 Figure 6.60 The effect of regeneration on the selectivity for C 2 hydrocarbons from the conversion of methane over the 2%Mo/H-ZSM-5 catalyst at 750 o C. 188 Figure 6.61 The effect of regeneration on the selectivity for aromatics from the conversion of methane over the 2%Mo/H-ZSM-5 catalyst at 750 o C. 188 Figure 6.62 The effect of regeneration on the selectivity for coke from the conversion of methane over the 2%Mo/H-ZSM-5 catalyst at 750 o C. 189 Figure 6.63 The effect of %XRD crystallinity on the stability of W/H- ZSM-5 catalysts during the conversion of methane at 750 o C. 190 Figure 6.64 The effect of %XRD crystallinity on the performance of W/H-ZSM-5 catalysts during the conversion of methane at 750 o C, compared with that for 2%Mo/H-ZSM-5 catalysts. 191 Figure 7.1 The catalytic conversion of propane as a function of time on stream over Mo/H-ZSM-5 catalysts of different molybdenum loading at 530 C. 198 Figure 7.2 The effect of molybdenum content on the catalytic conversion of propane at 530 o C at a TOS of 5 h and propane flow rate of 20 ml/min. 199 xxv

26 Figure 7.3 The catalytic conversion of propane as the function of time on stream over 2%Mo/HZSM-5 catalysts at different temperatures. 201 Figure 7.4 The catalytic conversion as the function of temperature over the 2%Mo/HZSM-5 catalysts at time of stream of 1, 5 and 10 h. 202 Figure 7.5 The catalytic conversion of propane as a function of timeon-stream over the 2%Mo/H-ZSM-5 catalysts of various %XRD crystallinities. 204 Figure 7.6 The effect of the percentage XRD crystallinity of H-ZSM-5 on the catalytic conversion of propane at 530 o C over 2%Mo/H-ZSM-5 catalysts at TOS of 5 h. 205 Figure 7.7 The effect of the crystallinity on the yield of aromatic products from the aromatisation of propane over 2%Mo/H- ZSM-5 catalyst at 530 o C and a time on stream of 5 hours. 206 Figure 7.8 The effect of %XRD crystallinity on the product selectivity during the aromatisation of propane over 2%Mo/H-ZSM-5 at 530 o C at a conversion of about 10%. 207 xxvi

27 List of Tables Table 2.1 The isoelectric points (IEPS) of various oxides 15 Table 5.1 Reagents used, supplier and their purity or composition 106 Table 6.1 The effect of the support on the nonoxidative dehydroaromatisation of methane at 750 o C over 2%Mo/Support catalysts for a time on stream of 1 h 121 Table 6.2 The effect of the molybdenum loading on the surface and pore volumes of Mo/H-ZSM-5 catalysts with a crystallinity of 66% 124 Table 6.3 The effect of the molybdenum loading on the catalytic performance of Mo/H-ZSM-5 catalysts for the nonoxidative aromatisation of methane at 750 o C, after 5 h on-stream 126 Table 6.4 A summary of the effect of the Mo loading on the reducibility of the Mo/H-ZSM-5 catalysts 132 Table 6.5 The amounts of carbon that remained on the catalysts after use in the dehydroaromatisation of methane at 750 o C 137 Table 6.6 The effect of the flow rate of the feed on the catalytic performance of 2%Mo/H-ZSM-5 catalysts with a xxvii

28 crystallinity of 54% for the conversion of methane at 750 o C 142 Table 6.7 The effect of the reaction temperature on the catalytic performance of 2%Mo/H-ZSM-5 catalysts with a crystallinity of 61% at 750 o C and a time on stream of 1 h 148 Table 6.8 The effect of the %XRD crystallinity of H-ZSM-5 on the catalytic performance of Mo/H-ZSM-5 catalysts for the nonoxidative conversion of methane at 750 o C for a time on stream of 5 h 156 Table 6.9 A summary of the physicochemical properties of 2%Mo/H-ZSM-5 catalysts with different SiO 2 /Al 2 O 3 ratios 160 Table 6.10 The effect of the SiO 2 /Al 2 O 3 on the catalytic performance of 2%Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C, after a time on stream of 5 h 162 Table 6.11 The effect of alkali metal ions on the catalytic performance of 2%Mo/H-ZSM-5 catalysts for the conversion of methane at 750 o C after a time on stream of 5 h 179 Table 6.12 A comparison of the product selectivity of W/H-ZSM- 5 and 2%Mo/H-ZSM-5 catalysts during the conversion of methane at 750 o C 191 xxviii

29 Table 7.1 The effect of molybdenum loading on the catalytic aromatisation of propane over Mo/H-ZSM-5 catalysts at 530 o C after a time on stream of 5 h and with a flow rate of propane = 20ml/min 199 Table 7.2 The effect of molybdenum loading on the selectivity and yield of aromatics and cracking products at a conversion of about 2% and a reaction temperature of 530 o C 200 Table 7.3 The effect of the reaction temperature on the catalytic performance of 2%Mo/H-ZSM-5 catalysts for the aromatisation of propane at 530 o C and propane flow rate of 20 ml/min 203 Table 7.4 The effect of %XRD crystallinity of H-ZSM-5 on the catalytic performance of 2%Mo/H-ZSM-5 catalysts for the aromatisation of propane at 530 o C and propane flow rate of 20 ml/min 205 xxix

30 List of Abbreviations and Symbols Ag AHM AM B BE BET BJH BTX CNG CO CO 2 CO x DDMP DRIFTS DRS DSC DTA ERS FCC FID FT-IR h IEP LPG min Mo Silver Ammonium heptamolybdate Ammonium molybdate Boron Binding energy Brunauer-Emmet-Teller Barret, Joyner, Halender Benzene, toluene and xylene Compressed natural gas Carbon monoxide Carbon dioxide Carbon oxides Dodecylmolybdophosphoric acid Diffuse reflectance spectroscopy Diffuse reflectance spectroscopy Differential scanning calorimetry Differential thermal analysis Electron spin resonance Fluid catalytic cracking Flame ionisation detector Fourier Transform infrared spectroscopy hour Isoelectric points Liquefied petroleum gas minute Molybdenum xxx

31 nm nanometre N A NMR OCM Avogadro s number Nuclear magnetic resonance Oxidative coupling of methane OPEC Organization of the Petroleum Exporting Countries S g SEM STEM TCD TEM TG TOS TPD TPO TPR TPSR UV-Vis V m W XAS XPS XRD Specific surface area Scanning electron microscopy Scanning transmission electron microscopy Thermal conductivity detector Transmission electron microscopy Thermogravimetry Time on stream Temperature programmed desorption Temperature programmed oxidation Temperature programmed reduction Temperature programmed surface reaction Ultraviolet-visible Monolayer volume Tungsten X-ray absorption spectroscopy X-ray photoelectron spectroscopy X-ray powder diffraction ZSM-5 Zeolite Saucony Mobil number 5 Wavelength Bragg angle Full line-width at half-maximum intensity Area covered by one molecule of adsorbate Surface tension xxxi