Kinetics, Mechanism, and Active Site Requirements for Hydrodeoxygenation over Reducible Metal Oxides

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Bio-oils are obtained from the fast pyrolysis of biomass, and their conversion into fuels and chemicals represents a sustainable alternative to coal, natural gas, and petroleum sources. The critical step in upgrading bio-oil is the selective removal of oxygen atoms from the lignin-derived fraction under ambient pressures of hydrogen and in the presence of a catalyst (otherwise known as "hydrodeoxygenation"). The ideal catalyst for hydrodeoxygenation would be abundant, inexpensive and easy to handle, while also being active, selective, hydrogen efficient, stable, and regenerable. The goal of this research project is to develop a molecular-level understanding of the chemical and physical processes occurring during the hydrodeoxygenation of biomass-derived model compounds over a class of catalysts called reducible metal oxides. Primarily, the studies will focus on cerium oxide (ceria), a material widely used commercially as an oxygen storage component in automotive exhaust systems. The research project serves as a training ground for graduate and undergraduate students to perform cutting edge research. The researchers also will participate in outreach activities in the Houston area, including Energy Day and Chevron Girls Engineering the Future Day, and they will host high school students in the laboratory and provide them with hands-on research experience as part of Project ACS SEED.The fact that bulk ceria selectively cleaves carbon-oxygen bonds in biomass-derived model compounds raises interesting fundamental questions regarding the nature and density of oxygen vacancies in reducible metal oxides. The precise mechanism through which carbon-oxygen bond cleavage over ceria occurs will be studied via four experimental steps: (1) demonstration of the potential of bulk ceria as a hydrodeoxygenation catalyst; (2) elucidation of the mechanism of anisole hydrodeoxygenation over bulk ceria; (3) establishing the active site requirements for hydrodeoxygenation over cerium oxide; and (4) understanding the effect of doping on active site identity, density, and efficacy. This research project aims to optimize energy, cost, and atom-efficiency of biomass conversion processes by guiding the design of hydrodeoxygenation catalysts and clarifying the identity of the mechanistic steps involved in Mars-van Krevelen/reverse Mars-van Krevelen redox cycles, ubiquitous in the realm of bulk oxide catalysis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.