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Nvestigating the Kinetics of Anisole: a Simple Lignin Model Compound

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Nvestigating the Kinetics of Anisole: a Simple Lignin Model Compound INVESTIGATING THE KINETICS OF ANISOLE: A SIMPLE LIGNIN MODEL COMPOUND by Yogesh Koirala 66 A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Chemical Engineering). Golden, Colorado Date__________________ Signed:____________________________ Yogesh Koirala Approved: ____________________________ Dr. Anthony M. Dean Chemical Engineering Department Thesis Advisor Golden, Colorado Date__________________ ____________________________ Dr. David W.M. Marr Professor and Department Head Chemical and Biological Engineering Department ii ABSTRACT Thermochemical conversion of biomass is a potentially important process for providing a re- newable source of fuels and chemicals. One hurdle to an economically viable process is better con- trol of the pyrolysis products of the biomass component lignin. Development of a kinetic model that could predict the impact of various operating conditions on the conversion and product yield could substantially accelerate commercial development. It is essential that the kinetic model de- scribe the reactions at the molecular level with theoretically consistent rates, to confidently explore the various regions of potential operating parameters. In this thesis, anisole (C6H5OCH3) is used as a model compound since it has the weak phenolic-carbon linkage so prevalent in lignin. Anisole pyrolysis experiments were performed under conditions similar to those proposed for fast pyrolysis of biomass: T= 525-650 °C, t~0.4 s, P ~0.82 atm. For one set of experiments the initial anisole mole fraction was 0.75% with the balance inerts. In a second set, 5% H2 was added to shift the mole fractions of some of the important radicals in the system. A detailed kinetic mechanism was developed by combining an anisole model from the lit- erature with an extensively validated CSM hydrocarbon pyrolysis model. A combination of sensi- tivity analysis and rates of production analysis demonstrated that the recombination of methyl rad- icals and phenoxy (the products of the initial dissociation of anisole) was a critical reaction. This recombination is complicated since the phenoxy radical assumes three different resonant struc- tures, resulting in three different recombination products (anisole, o-methylcyclohexadienone , and p- methylcyclohexadienone ). This reaction network (the C7H8O potential energy surface), was characterized using CBS-QB3 electronic structure calculations. The resulting surface was signifi- cantly different from earlier estimates and led to improved predictions. Similar electronic structure calculations were performed for many other reactions involving oxygenated species. All of these iii reactions were added to the mechanism and a sensitivity analysis was performed to refine the mod- el. The only change that could easily be justified was an increase to the barrier for o-cresol for- mation on the C7H8O potential energy surface by 2 kcal/mol to improve the cresol predictions. No other adjustments were made and the predictions of this model were then used to compare to the collected data in this work as well as several earlier studies reported in the literature. These comparisons of the model predictions to these multiple data sets suggest that this fundamentally-based mechanism has led to an improved description of anisole pyrolysis and has identified specific issues that need to be addressed to further improve the kinetic description. The biggest remaining issue is to identify theoretically consistent reaction pathways that will divert some of the cresol product to phenol. iv TABLE OF CONTENTS ABSTRACT ............................................................................................................................................. iii TABLE OF CONTENTS .......................................................................................................................... v LIST OF FIGURES .................................................................................................................................. x LIST OF TABLES ............................................................................................................................... xviii LIST OF REACTIONS .......................................................................................................................... xxi LIST OF EQUATIONS ........................................................................................................................ xxv ACKNOWLEDGEMENTS ................................................................................................................. xxvi 1. INTRODUCTION ............................................................................................................................. 1 1.1 Methods to derive liquid fuel from biomass .................................................................................. 2 1.2 Lignin characterization is a first step toward improved fast pyrolysis process ............................. 5 2. LITERATURE REVIEW OF ANISOLE KINETICS ....................................................................... 8 2.1 Anisole pyrolysis experiments and kinetic modeling .................................................................... 8 2.2 Important reactions in the anisole reaction mechanism ............................................................... 14 2.2.1 Unimolecular decomposition of anisole ...................................................................... 14 2.2.2 Phenoxy radical chemistry ........................................................................................... 15 2.2.3 Methyl radical reactions ............................................................................................... 21 3. EXPERIMENTAL AND MODELING TECHNIQUES ................................................................. 23 3.1 Anisole pyrolysis experiment ...................................................................................................... 23 3.1.1 Vaporizer design and stability test ............................................................................... 23 3.1.2 Detection Technique .................................................................................................... 28 v 3.1.3 Stability test with anisole ............................................................................................. 31 3.2 General Modeling ........................................................................................................................ 33 3.2.1 Pressure dependent reactions ....................................................................................... 34 3.2.2 Role of CHEMKIN in mechanism development ......................................................... 42 3.2.3 CHEMKIN modeling ................................................................................................... 43 3.2.4 Rate of Production and Sensitivity Analysis ............................................................... 44 4. EXPERIMENTAL RESULTS AND DISCUSSION ...................................................................... 45 4.1 Pyrolysis of anisole ...................................................................................................................... 45 4.1.1 Reproducibility check .................................................................................................. 45 4.1.2 Calibration.................................................................................................................... 46 4.1.3 Product Distribution and Atom balances ..................................................................... 47 4.2 Pyrolysis of anisole in H2 environment ....................................................................................... 53 4.3 Impact of added hydrogen on anisole pyrolysis .......................................................................... 55 5. ANISOLE KINETIC MECHANISM DEVELOPMENT PART I .................................................. 61 5.1 Development of current anisole kinetic scheme .......................................................................... 61 5.1.1 Reactions on C7H8O Potential Energy Surface (PES) formed from the phenoxy + CH3 radicals ................................................................................................................. 64 5.1.2 H abstraction reactions from o/p-MecyHDOE ............................................................ 73 5.1.3 Reactions involved on the C6H6O potential energy surface formed from the phenoxy + H radicals ................................................................................................... 78 5.1.4 Unimolecular decomposition of phenoxy and phenoxy methyl radicals ..................... 82 5.1.5 Unimolecular reactions of o-MePhOJ and its subsequent reaction chemistry ............ 84 5.1.6 Recombination reactions of o/p-MePhOJ with other radicals .................................... 90 vi 5.1.7 Formation of benzofuran from 2-ethylphenol.............................................................. 93 5.1.8 o/p-quinone methide (o/p-QM) .................................................................................... 97 5.1.9 Reaction pathway analysis to reduce o/p-methyl anisole .......................................... 101 5.1.10 Abstraction reaction from anisole .......................................................................... 101 5.1.11 Cyclopentadienyl reaction chemistry ....................................................................
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