(392c) Computational Investigation of Bifunctional Catalysts for Bio-Oil Upgrade

In recent years there has been an increased focus on the lignin component of biomass for the production of bio-oils and value-added chemicals. While the fast pyrolysis producing bio-oil offers great potential as feedstock for transportation fuels, a catalytic upgrade route via hydrodeoxygenation (HDO) is needed to generate economically viable products. Bifunctional catalysts such as HZSM-5 supported Ni clusters containing metal and acid sites have shown high activities for HDO reactions such as the conversion of lignin-derived aromatics and carbonyl compounds to medium-chain hydrocarbons. In order to better understand the utility of these catalysts, we employ electronic structure calculations to elucidate the catalytic pathways by modeling a zeolite-supported nickel tetramer cluster (Ni₄-ZSM). Hydrogenation of acetone to isopropanol (IPA) followed by dehydration to propene have been investigated as model reactions on the metal and acid sites, respectively. Additionally, we also explore the alternate hydrogenolysis pathway for the conversion of IPA to propane. In the optimizations of the Ni₄-ZSM system, we observe a reverse hydrogen spillover, whereby the Brønsted hydrogen migrates from the zeolite active site to the metal cluster, and discuss how it facilitates the hydrogenation reaction in reference to the bare nickel cluster. Studies conducted on the dehydration reaction pathways indicate that the Brønsted acid catalysis in HZSM-5 is preferred over the metal catalyzed pathway in the Ni₄-ZSM system. Finally, the unique nature of these bifunctional catalysts is showcased by looking at the reaction mechanism for the one-step conversion of IPA to propane instead of the traditional two-step dehydration and hydrogenation reactions.