The role of oxygen vacancies in biomass deoxygenation by reducible zinc/zinc oxide catalysts†
Selective removal of oxygen is the key challenge in the upgrading of biomass-derived molecules, and reducible metal oxides have shown the ability to catalytically remove oxygen even at low exogenous H2 pressures. As opposed to a traditional surface reaction, it is possible that oxygen vacancies provide the active sites in such catalysts, in a Mars–van Krevelen (MvK)-like mechanism. In this work, we use a combination of theoretical calculations and experimental measurements to provide conclusive evidence that this reaction proceeds through such a vacancy-driven mechanism, for the example system of acetic acid deoxygenation to acetaldehyde over reducible zinc oxide surfaces. Density functional theory (DFT) calculations suggest that the catalyst without an oxygen vacancy on the surface was relatively unreactive, due to a strong energetic penalty of breaking the C–O bond, while the presence of the vacancy provides the reducing power to facilitate this elementary step. Experimentally, to examine the role of vacancies directly, we compared the rates and selectivities of acetic acid deoxygenation for standard hydrodeoxygenation conditions with rates in H2-free conditions starting with reduced Zn metal, in which presumably the only active sites are vacancies created through diffusion. We found a striking similarity in the product distribution, suggesting a common mechanism in both cases. Specifically, metallic zinc with numerous vacant sites on the surface showed a high activity in promoting the deoxygenation reaction, while oxidized zinc (without oxygen vacancies) was fully deactivated. This study suggests that a unique vacancy-driven mechanism is responsible for the reactivity of reducible metal oxide catalysts.