Study of the Reaction Mechanism of Selective Photooxidation of Cyclohexane over V2O5/Al2O3

The selective oxidation of a hydrocarbon to a ketone over an orthovanadate-like (V=O)O-3 species in the presence of O-2 consists of five elementary steps: (1) photoactivation of orthovanadate-like (V=O)O-3 species, (2) adsorption of hydrocarbon, (3) formation of an alkoxide intermediate, (4) evolution of ketone and water, and (5) desorption of the ketone. In this study, the alkoxide intermediate was isolated and characterized by in situ FT-IR spectroscopy. By using C6D12, we found that the rate-determining step involved the elimination of a proton. A kinetic study was carried out on the basis of our proposed photocatalytic cycle to identify the rate-determining step. Further, the rate of evolution of cyclohexanone produced by the photooxidation of cyclohexane was measured under various conditions to determine the reaction order. The rate-determining step was found to be the elimination of a proton from the adsorbed hydrocarbon under photoirradiation.