Conjugation-Driven “Reverse Mars–van Krevelen”-Type Radical Mechanism for Low-Temperature C–O Bond Activation

C–O
bond activation on monofunctional catalysts (metals,
carbides, and oxides) is challenging due to activity constraints imposed
by energy scaling relationships. Yet, contrary to predictions, recently
discovered multifunctional metal/metal oxide catalysts (e.g., Rh/ReOx, Rh/MoOx, Ir/VOx) demonstrate unusually high C–O scission
activity at moderate temperatures. Herein, we use extensive density
functional theory calculations, first-principles microkinetic modeling,
and electronic structure analysis to elucidate the metal/metal oxide
synergy in the Ru/RuO2 catalyst, which enables up to 76%
yield of the C–O scission product (2-methyl furan) in catalytic
transfer hydrogenolysis of furfural at low temperatures. Our key mechanistic
finding is a facile radical-mediated C–O bond activation on
RuO2 oxygen vacancies, which directly leads to a weakly
bound final product. This is the first time the radical reduction mechanism is reported in heterogeneous catalysis at temperatures
<200 °C. We attribute the unique catalytic properties to the
formation of a conjugation-stabilized furfuryl radical upon C–O
bond scission, the strong hydroxyl affinity of oxygen vacancies due
to the metallic character of RuO2, and the acid–base
heterogeneity of the oxide surface. The conjugation-driven radical-assisted
C–O bond scission applies to any catalytic surface that preserves
the π-electron system of the reactant and leads to C–O
selectivity enhancement, with notable examples including Cu, H-covered
Pd, self-assembled monolayers on Pd, and oxygen-covered Mo2C. Furthermore, we reveal the cooperativity of active sites in multifunctional
catalysts. The mechanism is fully consistent with kinetic studies
and isotopic labeling experiments, and the insights gained might prove
useful more broadly in overcoming activity constraints induced by
energy scaling relationships.