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Chapter Four Role of External Sites on Catalytic Polymer Degradation Plastic Catalytic Degradation Study of the role of external catalytic surface, Catalytic Reusability and Temperature Effects By Toju S. Kpere-Daibo April 2009 A Thesis submitted for the degree of Doctor of Philosophy of the University of London Department of Chemical Engineering University College London, London, WC1E 7JE 1 Abstract Technological advancements over the last century have lead large and continuous growth in the output of plastic materials. This exponential growth has created public concern over the environmental impact caused by the polymeric waste produced. These have acted as driving forces for a lot of current research aimed at the development of plastic recycle processes. As a result, the conversion of plastic waste to useful products is gaining increasing attention. The aim of this work was to study aspects of polymer catalytic degradation using zeolite based catalysts. More specifically the study focused on identifying the role of the external catalytic surface on overall polymer decomposition reactions, the reusability of the catalysts as well as temperature and acidity effects. The first stage of this investigation aimed to explore the premise behind the assumption that polymer catalytic degradation takes place initially on the external catalytic surface by selectively poisoning the external sites of a zeolite catalyst (ZSM- 5). Degradation results in a semi-batch reactor as well as thermogravimetric analysis demonstrated that the activity of poisoned catalyst samples was indeed lower than that of fresh catalyst. The next stage of the study involved an investigation of the extent of catalytic reusability of four zeolite catalysts - HZSM-5, USY and two commercial cracking catalysts containing 20 % and 40 % USY respectively. While the performance of US- Y showed deterioration with each cycle, ZSM-5 and both commercial cracking catalysts retained consistent levels of activity that enabled full polymer conversion in each cycle. Finally, the temperature effect on catalytic reactions was studied as well as the effect of catalyst acidity. While temperature effects were not conclusive regarding selectivity towards gas or liquid products prompting the suggestion of further work using a continuous flow reactor system, the formation of liquid products showed a maximum with the acidity content. 2 Acknowledgements I wish to thank my supervisor, Dr George Manos, all his patience and guidance throughout the entire course of this work. His contribution to my academic progress is immensely significant. The many sacrifices made by all members of my family, particularly my Mum and Dad cannot be overemphasised enough. I am so grateful for all the love and support you have given me throughout this period. I am especially grateful for the unfaltering belief and confidence you held in me. And the constant prayers and encouragement you continue to give me. I would also like to acknowledge the support from my friends and would like to thank you all for your devotion and loyalty towards me. I would like to say that even when you haven’t known it you have all been there for me and given me that extra support when I have needed it at difficult moments. Finally I am thankful to God for all his blessings and for everything bestowed upon me. 3 Dedication Dedicated to the loving memory of my wonderful sister Mojirin 4 Table of Contents Abstract......................................................................................................................2 List of Figures..........................................................................................................10 List of Tables ...........................................................................................................14 Chapter One ...............................................................................................................15 Introduction.............................................................................................................15 1.1 Background .........................................................................................................15 1.2 Motivations and Objectives of this Work ...........................................................16 1.3 Outline of this Thesis..........................................................................................17 Chapter Two...............................................................................................................19 Literature Survey....................................................................................................19 2.1 Plastics ................................................................................................................19 2.2 Basics of Polymers..............................................................................................21 2.2.1 Polyethylene (PE) ............................................................................................21 2.2.2 Polypropylene (PP) ..........................................................................................22 2.2.3 The plastic-linear low-density polyethylene (LLDPE)....................................22 2.3 Classification of Plastic Recycling .....................................................................22 2.3.1 Primary recycling.............................................................................................23 2.3.2 Secondary Recycling .......................................................................................24 2.3.3 Tertiary Recycling ...........................................................................................24 2.3.4 Quaternary Recycling ......................................................................................25 2.4 Thermal and Catalytic Degradation ....................................................................25 5 2.5 Mechanism of the Degradation Process..............................................................28 2.5.1 Mechanism of Polymer Thermal Degradation.................................................29 2.5.2 Mechanism of Catalytic Cracking Reactions...................................................30 2.5.2.1 Cracking of a Carbon-Carbon Bond .........................................................32 2.5.2.2 Isomerization.............................................................................................32 2.5.2.3 Hydrogen Transfer....................................................................................33 2.6 Past Work Involving Methods of Recycling Plastic Waste................................34 2.7 Catalysis..............................................................................................................37 2.7.1 Basics of Catalysis ...........................................................................................37 2.7.2 Different Types of Catalysts ............................................................................39 2.8 Zeolite based catalysts ........................................................................................40 2.8.1 Silicalite and ZSM-5....................................................................................43 2.8.2 Ultrastable Y Zeolite (US-Y).......................................................................45 2.8.3 Commercial Cracking Catalysts ..................................................................46 2.8.4 Secondary Reactions and Over-Cracking ....................................................48 2.9 Silylation (Poisoning) .........................................................................................50 2.10 Temperature Programme Desorption (TPD) ....................................................52 2.11 Thermo-Gravimetric Analysis (TGA) ..............................................................54 2.12 Deactivation of Catalysts by Coking ................................................................55 Chapter Three ............................................................................................................57 Experimental ...........................................................................................................57 3.1 Introduction.........................................................................................................57 3.2 Materials .............................................................................................................58 3.3 Semi Batch Reactor.............................................................................................59 3.3.1 Equipment....................................................................................................59 6 3.3.2 Experimental procedure...............................................................................60 3.3.3 Experimental Calculations...........................................................................64 3.3.4 Boiling Point Distribution............................................................................67 3.4 Thermo-gravimetric Analysis Equipment (TGA)...............................................72 3.5 Temperature Programmed Desorption (TPD) ....................................................73 3.6 Chemical Liquid Deposition (CLD) ...................................................................74 Chapter Four..............................................................................................................75 Role of external sites on catalytic polymer degradation performance...............75 4.1 Introduction.........................................................................................................75
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