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

Abstract

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.

Reaction Pathways for the Deoxygenation of Biomass‐Pyrolysis‐Derived Bio‐oil on Ru: A DFT Study using Furfural as a Model Compound

“…However, the formation of furan is thermodynamically favored on Pd surfaces, in agreement with experimental observations . Computational studies detailing the mechanism of furfural hydrodeoxygenation to furfuryl alcohol, furan, and MF have also been reported on Pt and Ru surfaces. Cai et al .…”

“…Recent studies by Mironenko et al. on Ru(0 0 1) surfaces report electronic energy barriers of 75 and 123 kJ mol −1 for the former and latter pathways . Also, (C 4 H 3 O)CHOH is 38 kJ mol −1 less stable than (C 4 H 3 O)CH 2 O, making (C 4 H 3 O)CHOH formation both thermodynamically and kinetically less favorable than (C 4 H 3 O)CH 2 O.…”

“…calculated activation barriers for furfural hydrogenation on flat Pt(1 1 1), stepped Pt(2 1 1), and Pt 55 clusters and concluded the hydrogenation to furfuryl alcohol to be kinetically favored at terrace and step sites whereas furan formation is favorable at corner sites. Recently, Mironenko et al . reported C−O scission and hydrogenation barriers for the formyl group of furfural on Ru(0 0 1) surfaces.…”

Synergetic Catalysis of Nickel Oxides with Oxygen Vacancies and Nickel Phosphide for the Highly Efficient Hydrodeoxygenation of Phenolic Compounds

“…pointed out that C−O bonds can be further weakened if biomass‐oxygenate is adsorbed on the oxygen vacancy sites of MoO 3 catalysts . Moreover, Mironenko and Vlachos noted that RuO 2 oxygen vacancies promote C−O band scission . Previously, we found that oxygen vacancies on RuO 2 /C catalysts favor the cleavage of O−H in water .…”

Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena‐Based Catalysts

“…O atoms are attracted by the coordinately unsaturated Mo. The oxidized sites are reduced sequentially by H 2 to close the redox cycle and the reduced oxygenate desorbs from the surface . It is most likely that more than one cycle leads to the formation of propylene.…”