Kinetic Coupling of Redox and Acid Chemistry in Methanol Partial Oxidation on Vanadium Oxide Catalysts

Kinetic, isotopic, and spectroscopic studies establish the active site requirements for three kinetically coupled catalytic cycles catalyzed by redox, Brønsted, and Lewis acid–base sites, which occur during methanol and oxygen reactions on titania-supported vanadium oxide catalysts. The initial activation of methanol during its oxidative dehydrogenation to formaldehyde restricts the overall turnovers─this reaction proceeds via CH3OH dissociative adsorption followed by a kinetically relevant C–H bond scission of the CH3O intermediate on V–O redox site pairs found at the interface of VOx and TiO2. The Gibbs free energy change of these two steps (ΔGads and ΔG⧧, respectively) both decrease as V–O–V coordination decreases and V–O–Ti coordination increases, leading to higher turnovers per surface vanadium as VOx dispersion increases, except the extreme case of isolated VO4. Coupled kinetically with the oxidative dehydrogenation cycle is the Brønsted acid-catalyzed cycle that forms dimethoxymethane─this cycle proceeds via methanol- and formaldehyde-derived CH3OCH2OH adsorption to Brønsted sites at the VOx–TiO2 interface, followed by its kinetically relevant C–O bond scission. Its turnovers also increase with VOx dispersion following the same trend as the oxidative dehydrogenation cycle but at a much faster rate, so the reaction can readily approach chemical equilibrium. Alongside the other two catalytic cycles is the Tishchenko reaction that forms methyl formate from two formaldehyde molecules, produced by the methanol oxidative dehydrogenation cycle, on exposed Ti4+–O2– Lewis acid–base pairs uncovered by VOx─this cycle proceeds via kinetically relevant intermolecular C–O bond formation followed by a rapid 1,3-hydride shift. The interplay of coexisting redox, Brønsted, and Lewis sites, each with its unique catalytic roles, leads to different rates and yields of the three products. The active site structure and mechanistic knowledge established here allow us to optimize the product ratios required for downstream synthesis of larger oxygenates.