Investigation of reaction mechanisms of metal oxide catalyzed reactions for the production of monomers
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Polymers are ubiquitously used in most of our daily life products. Polymers are made of repeating chemical units called monomers and the demand for monomers is rapidly increasing as is their environmental impact. Therefore, it is imperative to develop renewable and energy-efficient monomer production routes that help in keeping up with the rising demand as well as protect the environment. For this, catalysts and processes are needed. To discover suitable catalysts, fundamental mechanistic studies play an important role. Ab-initio electronic structure calculations and microkinetic modeling coupled with detailed characterization and kinetic experiments could help unravel key reaction mechanisms, deduce rate expressions, and potentially identify reaction descriptors, all of which are useful for catalyst discovery and reactor modeling studies. ☐ In this thesis, we perform Density Functional Theory (DFT) and microkinetic modeling studies in collaboration with experimental work to unravel reaction mechanisms of the production of ethylene from shale gas-based ethane and conjugated dienes from biomass-based cyclic ethers. Chapter 2 of this thesis presents the study of the reaction mechanism of ethane dehydrogenation on Al2O3-supported Ga catalysts. In this work, we use DFT and microkinetic modeling coupled with kinetic experiments to unravel the role of Ga as an active site in the ethane dehydrogenation reaction. We find the grafted-Ga sites are catalytically inactive whereas dopant-Ga sites are more active than Al sites of pristine-Al2O3. The inclusion of surface hydroxylation effects in the models explains the experimentally observed relative rates between Al2O3 and Ga/Al2O3, and the apparent activation energies. H2O in the reaction environment preferentially blocks the AlIII sites, thereby enhancing the importance of Ga as active sites. ☐ In Chapters 3 and 4 of this thesis, the focus lies in understanding the mechanistic aspects of metal-oxide-catalyzed production of conjugated dienes from biomass-based cyclic ethers. In Chapter 3, we use DFT and microkinetic modeling to unravel the reaction mechanism of the production of butadiene from biomass-based tetrahydrofuran (THF) on ZrO2 and Al2O3 catalysts. We also attempt to explain why ZrO2 and Al2O3 show a stark contrast in product selectivity upon the reaction of THF. Our studies show that the hydroxylated models of ZrO2 and Al2O3 explain the experimentally observed product selectivity. The main reason behind the difference in product selectivity between ZrO2 and Al2O3 is the difference in local topology; unlike hydroxylated-Al2O3, the presence of neighboring Lewis-acidic metal sites on ZrO2 favors a facile butadiene formation mechanism, thus making butadiene the most favorable product on this surface. In Chapter 4, we use DFT studies to fundamentally explain the experimentally observed kinetic favorability of 1,3-pentadiene product upon the reaction of tetrahydropyran (THP) and 2-methyl tetrahydrofuran (2-MTHF) on ZrO2 catalyst. The facile isomerization of surface 4-pentenolate to 3-pentenolate intermediate, and the stable transition state of the dehydration reaction of 3-pentenolate when compared to that of 4-pentenolate are the reasons behind the kinetic favorability of 1,3-pentadiene.
Description
Keywords
Catalysis, Density Functional Theory, Mechanism, Metal oxides, Microkinetic modelling, Monomers