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The Mechanism and Kinetics of Thiophene Adsorption On THE MECHANISM AND KINETICS OF THIOPHENE ADSORPTION ON NICKEL AT AMBIENT TEMPERATURES AND PRESSURES A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College Khaliq Ahmed Department of Chemical Engineering and Chemical Technology Imperial College of Science and Technology London SW7 United Kingdom January 1987 ABSTRACT An experimental and modelling study of the kinetics of thiophene adsorption on nickel at ambient temperatures and pressures has been made. The objective of the study was to acquire a better understanding of the mechanism involved than is currently available. The reactant (1000 ppm thiophene in hydrogen) was contacted with a nickel on gamma alumina catalyst. Experiments were carried out in a microbalance flow reactor and the uptake of thiophene by the catalyst was recorded as a function of time. Prior to adsorption of thiophene, the catalyst was characterised in terms of its total BET surface area, pore-size distribution, active Ni-area and average crystallite size. All measurements were performed in situ except for that of the crystallite size which was determined by x-ray line broadening. The experimental study revealed that at room temperature thiophene adsorbs on nickel either directly as thiophene molecules, or as a hydrogenated species namely, thiophane ; no gaseous products eluted indicating that thiophene was not undergoing decomposition under these conditions. Overall uptake of thiophene by nickel in the catalyst showed that there were at least two possible modes of adsorption. It is postulated that these are perpendicular and coplanar. The crystallite size of nickel in the catalyst was varied by changing the loading of the catalyst, or by 2 sintering in an atmosphere of hydrogen at elevated temperatures. Adsorption of thiophene on catalysts of varying crystallite size showed that thiophene adsorption on nickel is structure sensitive, with the Ni/C.H.S adsorption b b stoichiometry ranging from a value representing a nearly coplanar mode of adsorption at large values of the crystallite size, to a value approaching a perpendicular mode at some smaller value of the crystallite size. The experimental results were consistent with a model which could be represented by two parallel rate processes, one following a two-site (presumably perpendicular) mechanism and the other following a five-site (presumably coplanar) mechanism. The rate constants for the two processes were approximated from runs on a particular sample and then used to predict the uptake vs. time behaviour of the remaining samples which varied in loading and/or crystallite size. The rate constant x"~ *he adsorption of the five-site (coplanar) species was found to be several orders of magnitude higher than that for the adsorption of the two-site (perpendicular) species, indicating that coplanar adsorption was preferential over perpendicular in the initial stages of adsorption on clean samples. The effective diffusivity parameter was calculated by using a random pore-model. Adsorption on highly sintered samples with particles in the range of .3 to .4 mm was found to be diffusion limited. For these samples a fitted diffusivity had to be used to obtain a satisfactory agreement between 3 experiment and simulation. The value of the fitted diffusivity was consistent with the pore-characteristics and crystallite size of these samples. However, adsorption on powder samples of these catalysts was found to be kinetically controlled. The simulation studies were able to predict all the and observed results within their experimental error,Aexcept for the diffusivity in the highly sintered samples required no fitting of rate constants for individual runs. 4 ACKNOWLEDGEMENTS I wish to express my sincere gratitude to Dr. L.S. Kershenbaum for his constant guidance and supervision throughout the course of this research. It has been a rewarding experience for me. My deepest gratitude to Dr. D. Chadwick for his invaluable suggestions and helpful discussions on certain aspects of the present work. I wish to thank the technical and workshop staff of the Department of Chemical Engineering and Chemical Technology for their backup assistance. I gratefully acknowledge the Scholarship awarded by the Association of Commonwealth Universities, without which this work would not have been possible. I am indebted to Mr. Bryan Spooner of the Old Centralians Association for his generous help in obtaining a grant from the Old Centralians Trust Fund. I am also grateful to Mr. Nigel Wheatley for the award of a Lord Mountbatten Grant in the later stages of this work. I wish to thank Dr. T. Viveros-Garcia for his friendship and for his helpful advice on this work. Thanks are also due to Mr. I. Drummond of this department for his assistance in the analytical experiments and to Mr. R. Sweeney of the Department of Metallurgy and Materials Science for his assistance in the XRD mea surements. 5 My kind est regards and gratitude to my mother and relatives for their constant encouragement and understand ing during this s tudy. Finally, my deepest appreciation for my wife, for her patience. en couragement and understanding, throughout the course of this work. 6 TO MY LATE FATHER TO MY MOTHER Contents 1 INTRODUCTION 1 2 2 LITERATURE SURVEY 1 5 2.1 Catalyst Poisoning 1 6 2.1.1 Adsorption of the Poison Species : Mechanistic Considerations 1 7 2.1.2 Inter- and Intra- pellet Transfer Effects 24 2.1.3 Analysis of the Poisoning Process 30 2.1.3.1 Irreversible Poisoning 30 2.1.3.2 Reversible Poisoning 37 2.1.4 Factors Affecting the Poisoning Process 38 2.2 Thiophene Poisoning of Nickel Catalysts 42 2.3 Thiophene Poisoning of Other Metal Catalysts 55 3 MATHEMATICAL MODELS OF THIOPHENE POISONING 60 3.1 Development of the Mathematical Models 63 3.1.1 Model I 68 3.1.2 Model II 68 3.1.3 Model III 70 3.2 Model Parameters 73 3.3 Numerical Solution of the equations 74 4 EXPERIMENTAL 75 4.1 Apparatus 75 4.2 Materials 80 4.3 Catalyst Preparation 80 8 4.4 Experimental Procedure 81 4.4.1 Reduction 81 4.4.2 BET Area and PSD 83 4.4.3 Ni-Area Measurement 83 4.4.4 Crystallite Size Determination 84 4.4.5 Poisoning Studies 85 4.5 Effect Due to Variation in Room Temperature 87 4.6 Exit-gas Analysis 87 4.7 Investigations with 100ppm Thiophene 90 CHAPTER 5 CATALYST CHARACTERISATION 91 5.1 Literature Survey and Methods 91 5.1.1 Total Surface Area and PSD 91 5.1.2 Metal Area 95 5.1.3 Crystallite Size by X-Ray Methods 97 5.2 Results and Discussion 100 5.2.1 Total Surface Area 102 5.2.2 Pore-Size Distribution 108 5.2.3 Nickel Area 114 5.2.4 Mean Crystallite Size 124 5.2.5 Characterisation of the Support 129 CHAPTER 6 THIOPHENE ADSORPTION STUDIES 132 6.1 Experimental Results 132 6.1.1 Runs on Catalyst Particles 132 6.1.2 Runs on Powdered Samples 143 6.1.3 Saturation Uptake of Thiophene and the Influence of Crystallite Size 158 6.1.4 Effect of Flow Rate 160 9 6.1.5 Interparticle Diffusion 160 6.2 Simulation 164 6.3 Model Parametrs 209 6.4 Discussion 209 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 220 7.1 Conclusions 220 7.2 Suggestions for Further Work 223 REFERENCES 225 APPENDICES 233 A Discretization of the Models 233 B Instantaneous Adsorption Stoichiometry in Model III 237 C Estimation of Effective Diffusivity 239 0-1 Calculation of Crystallite Size from Chemisorption Data 242 D-II Evaluation of ZD^ (Dispersion) from CO Chemisorption 245 D-III Determination of the Extent of Reduction 245 E Calculation of the Pore-size Distribution from Type IV Isotherms 247 F Results of Different Forms of Variation of o, the Instantaneous Stoichiometry of Adsorption 252 G Sample Calculations 254 G-I Total BET Surface Area 254 G-II Pore-Size Distribution 254 G-III Metal Area of the Catalyst 255 1 0 G-IV Extent of Reduction 255 G-V Metallic Dispersion 256 G-VI Crystallite Size Determination a By X-Ray Diffraction 256 b By CO Chemisorption 256 G-VII Effective Diffusivity 257 G-VI11 Fractions of Coplanar and Perpendicular sites in Model II 257 G-IX Thiophene uptake of Ideal Crystallites 258 CHAPTER 1 Introduction The phenomenon of catalyst poisoning is one of the most severe problems associated with the industrial application of catalysts. The overall behaviour of catalyst poisoning has been studied extensively in the last 25 years and the results of these studies have been used to predict the life and behaviour of industrial catalysts. However, because of a lack of sufficient careful studies of the poisoning process, which is due, largely, to the complexity of the process, a quantitative understanding of the intrinsic rates and mechanism of catalyst poisoning is still not available. According to Bartholomew et al (19B2), the poisoning effect of sulphur on metal catalysts is probably the most severe form of poisoning encountered in catalytic systems. Sulphur induced poisoning is so severe that the activity of the catalyst is reduced markedly at extremely low gas-phase concentrations of sulphur-containing compounds. In industrial practice the catalyst life may be reduced to only a few months or even weeks in the presence of only ppm concentrations of sulphur contaminants in the feedstream. The practical importance of sulphur poisoning has led to investigations of sulphur induced poisoning of metal catalysts. Recent investigations have resulted in the accumulation of fundamental information regarding the interaction of sulphur with metal surfaces. Most of these studies have been conducted with HgS as the poison and the results obtained thence constitute the majority of the literature on sulphur poisoning.
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