(72a) Mechanism of Furfural Hydrodeoxygenation on Metal/Metal Oxide Catalysts

Selective C-O bond activation at low temperatures is essential for oxygen removal to produce second generation biofuels and chemicals from biomass. Constraints, imposed by energy scaling relationships, make monofunctional catalysts (metals, oxides, and carbides) inefficient. At the same time, recently discovered multifunctional metal/metal oxide catalysts (e.g., Rh/MoO_x, Ir/VO_x) exhibit unusual activity and selectivity for C-O bond hydrogenolysis at temperatures < 200^oC. Herein, we utilize density functional theory calculations, first principles microkinetic modeling, and electronic structure analysis to shed light on the site cooperativity in the Ru/RuO₂ catalyst, which enables up to the 76% yield of 2-methyl furan in catalytic transfer hydrogenolysis of furfural. We find the facile C-O scission to be associated with a formation of a conjugation-stabilized furfuryl radical intermediate on RuO₂ oxygen vacancies. Three types of catalytic sites required for catalytic transfer hydrogenolysis, are identified. Furthermore, we elucidate electronic factors responsible for the high catalyst activity, and establish correlations between binding energy descriptors and the catalyst activity for a variety of reducible oxides. The mechanistic picture is fully consistent with isotopic labeling experiments and kinetic studies, and the insights gained might prove useful in catalyst selection and reaction pathway design for hydrodeoxygenation of biomass.