Understanding the mechanism(s) of ketone oxidation on VO_x/γ-Al₂O₃

The oxidative scission of carbonyl compounds is of interest in the production of carboxylic acids, and homogeneously-catalyzed pathways have been well-studied in condensed media. The reaction can also be performed in the gas phase over solid oxides; however, mechanistic insights about heterogeneous pathways are sparse. To this end, we investigate the oxidative scission of 3-methyl-2-butanone over γ-Al₂O₃ and VO_x/γ-Al₂O₃ using kinetic analysis and spectroscopic methods. We observe that the oxidative scission of 3-methyl-2-butanone can occur on reducible vanadium oxides and non-reducible aluminum oxides, and we consider the existence of separate pathways mediated by lattice oxygen and gas phase dioxygen. Fourier-transformed infrared (FTIR) and diffuse reflectance ultraviolet–visible (DR UV–Vis) spectra obtained under reaction conditions provide insights into the nature of surface species; the regimes where they form; and the degree of lattice reduction in those regimes. Based on these complementary insights, we propose a mechanism for the gas-phase oxidative scission of ketones on VO_x/γ-Al₂O₃. We include elements of Mars-van Krevelen and Eley-Rideal pathways, and we consider the significance of acid-base and redox steps in facilitating the oxidative scission of ketones.