Catalyst development and reaction engineering for chemical upgrading of emerging feedstocks
Date
2024
Authors
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Publisher
University of Delaware
Abstract
Catalysis is a cornerstone of the chemical manufacturing industry enabling the production of fuels, polymers, and commodity chemicals. Consequently, the development of new catalytic technologies is critical for transitioning petrochemical based industries to more sustainable feedstocks. My research has focused on the design and synthesis of catalyst architectures and reaction engineering to address major challenges in the conversion of cellulosic biomass to ethylene glycol. I have also applied the principles of reaction and reactor engineering to study radical pathways for polypropylene deconstruction. ☐ While cellulosic biomass has been studied extensively as a feedstock to produce ethylene glycol, all reported technologies suffer from challenges to selectivity, and harsh reaction conditions. A major contributor to this is the lack of hydrogenation selectivity for carbohydrate mixtures over supported Nickel (Ni) catalysts. Encapsulation of active Ni particles inside siliceous zeolites can improve hydrogenation selectivity towards ethylene glycol and expand the operating regime (temperature, catalyst loadings). A novel synthetic approach for Ni encapsulation, based on Ni dissolution, was developed, and implemented. Contrary to previously reported hydrothermal methods, size-selective hydrogenation catalysts can be synthesized from existing zeolites through impregnation, and post-synthetic treatments. Ex-situ physical characterization and reactivity tests were performed on the catalysts before implementation in the biomass system. ☐ The conversion of cellulose (or glucose) to ethylene glycol involves 2 reactions, occurring in a one-pot tandem system. The retro-aldol reaction fragments glucose into glycolaldehyde which can undergo hydrogenation to ethylene glycol. However, the retro-aldol reaction is equilibrium-limited and the subsequent hydrogenation is necessary to drive the equilibrium forward by consuming the glycolaldehyde. Molybdenum (Mo) and Tungsten (W) based oxides are known to be good catalysts for the retro-aldol reaction with Mo exhibiting superior performance at lower temperatures. However, initial investigations of Mo-based systems revealed poor performance in the tandem system due to the deactivation of Mo under the reducing reaction conditions. For this reason, W is chosen as the retro-aldol catalyst. Further characterization of the deactivated materials revealed the existence of specific Mo-Ni interactions responsible for the selective hydrodeoxygenation of biomass-derived oxygenates. Nonetheless, for biomass conversion, the primary focus is on W based retro-aldol catalysts and zeolite encapsulated Ni catalysts. ☐ By adjusting catalyst loadings, high yields of ethylene glycol were achieved while maintaining size selectivity and preventing sugar hydrogenation. The consequence of encapsulation is evident in the selectivity but also in the ability to reduce reaction temperatures by 50 °C. Additionally, these systems exhibit a unique regime where ethylene glycol selectivity is independent of catalyst composition (i.e. Ni:W ratio). This is the first example of high ethylene glycol yields from batch reactions of glucose at 170 °C. This performance was also extended to polysaccharide systems of starch and cellulose, with further tuning of reaction conditions for each substrate. ☐ An emerging field in the catalysis community is the deconstruction (and upcycling/upgrading) of polymer waste. Leveraging the thermodynamic implications of radical (de)polymerization can be an interesting avenue to produce high-value products such as alkenes from polypropylene waste streams. The strong entropic driving force for polymer deconstruction predicts that at elevated temperatures, initiated polypropylene chains should undergo chain scission reactions resulting in degraded polymers of lower molecular weight. To this end, a class of bibenzyl and substituted bibenzyl molecules are investigated as potential high-temperature initiators (150 – 250 °C). TGA-MS analyses of PP + initiator mixtures reveal the temperature ranges where initiation can occur. A modified Parr reactor with flow-thru capabilities was constructed and tested for this process. Proof-of-concept tests with PP and different initiators show a decrease in MW and the presence of vinylidene groups consistent with $\beta$-scission reactions. Radical isomerization and chain-transfer reactions impact the polymer tacticity and result in differences in the thermal properties of the solid residue. The changes in polymer MW, degree of unsaturation, and thermal properties can facilitate further processing and upgrading.
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Keywords
Biomass valorization, Encapsulated catalysts, Hydrogenation, Polypropylene deconstruction, Reaction engineering