Transformations of complex biomass molecules over multifunctional catalysts
Date
2022
Authors
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Publisher
University of Delaware
Abstract
To mitigate the accumulation of greenhouse gases and the climate change caused by using fossil fuels, we need the development of green technologies from renewable materials. Biomass is a sustainable, carbon-neutral alternative to fossil fuels. Unlike their simpler fossil-fuel feedstocks, biomass feedstocks are complex multifunctional oxygenates whose oxygen content needs to be reduced toward value-added chemicals. The central theme of this dissertation is the development of catalysts, catalytic processes, and structural-functional relationships to enable selective biomass deoxygenation for a low-carbon economy. ☐ Metal and metal oxide (M-MOx) multifunctional catalysts are suitable candidates for biomass transformations owing to their superior activity and selectivity. However, these catalysts are structurally complex, multifunctional, and dynamic. The lack of a fundamental understanding of the catalyst active sites hinders rational catalyst design. This dissertation presents an integrated approach to tackle essential biomass transformations by linking multiscale modeling conducted by collaborators, catalyst synthesis, spectroscopies, and reaction kinetics. ☐ In Chapter 2, we developed an efficient catalytic system for producing succinic acid from tartaric acid, available in the waste streams of winemaking. Molybdenum oxide (MoOx) supported on black carbon (BC) and hydrobromic acid (HBr) in acetic acid are selective in reducing the hydroxyl groups in tartaric acid without over-reducing the terminal carboxylic groups, resulting in an 87% succinic acid yield. Pre-reduction and X-ray characterization studies correlate the high catalyst activity of MoOx/BC with the formation of lower Mo oxidation states (+4 to 0). Since HBr is corrosive, Chapter 3 describes the development of a halide-free catalyst system and identifies suitable metal-metal oxide catalysts for future exploration. ☐ Chapter 4 introduced a strategy for selective C–O bond activation by doping the surface of moderately reducible oxides with an ultralow loading (0.04 wt%) of noble metals. We demonstrated the concept using highly dispersed Pt anchored onto TiO2 for furfuryl alcohol conversion to 2-methylfuran. Density functional theory calculations, microkinetic modeling, catalyst characterization, and kinetic experiments exposed substantial C–O activation rate enhancement with neither bulk catalyst reduction nor unselective ring hydrogenation. A site quantification methodology was introduced, revealing the redox sites as the most active and selective for C–O bond activation. ☐ M-MO inverse catalysts are at the other extreme, where metal oxide clusters reside on a metal core. They have also been successfully applied to many biomass reactions owing to their multifunctionality (Brønsted acid sites, redox sites, and metal sites). However, the nature and dynamics of Brønsted acid sites under working conditions remain poorly understood. Chapter 5 investigated the formation and the dynamics of Brønsted acid and redox sites on PtWOx/C under working conditions. DFT-based thermodynamic calculations and microkinetic modeling revealed a complex interplay between Brønsted acid and redox sites that drives interesting catalyst dynamics. Combining in situ characterization and probe chemistry, we demonstrated up to two orders of magnitude change in the density of Brønsted acid sites on PtWOx/C by tuning the reaction parameters. We elicited an order of magnitude increase in the average acid-catalyzed dehydration reaction rate by periodic hydrogen pulsing. ☐ Chapter 6 investigated the roles of MOx modifiers for the tetrahydrofurfuryl alcohol ring-opening (RO) reaction. We developed methods for synthesizing and ranking MOx-Pt (MOx = MoOx, WOx, ReOx, NbOx) inverse catalysts. The MOx surface coverage on Pt is a critical parameter to compare the various oxides. To establish reliable structure-activity relationships, a strategy, which removes the inactive WOx species from the carbon, was demonstrated. A dehydration probe reaction and the reduced RO activity upon acid site poisoning suggest the potential role of Brønsted acid sites in the RO reaction under working conditions.
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Keywords
Biomass, Catalysis, Brønsted acid sites, Accumulation of greenhouse gases, Biomass deoxygenation