Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on Vanadia-Titania and Vanadia-Titania-Sulfate Catalysts

Density functional theory calculations were carried out using both cluster and slab models to investigate the catalytic reaction network and the effect of sulfate promoter on methanol selective oxidation using the V2O5/TiO2 catalyst. The hemiacetal mechanism was reaffirmed to be the most favorable reaction pathway for the formation of methyl formate (MF) by the prediction of another reaction pathway involving formic acid. Molecular oxygen was found to assist the desorption of the reaction products from the reduced catalyst active site, both formaldehyde and methyl formate (MF). The mechanism of catalyst regeneration was elucidated based solely on first principles calculations, which involve the conversion of another methanol molecule to formaldehyde over a peroxo species. This leads to our formulation of a complete catalytic reaction network for methanol selective oxidation to formaldehyde and MF on the V(2)O5(/)TiO(2) catalyst. In addition, the detailed mechanism for the formation of formaldehyde and MF was also predicted on the sulfate-promoted V2O5/TiO2 catalyst. Based on the calculated reaction networks, the preferred formation of MF on the V2O5/TiO2-based catalysts was attributed to the lower energy barrier of the oxidative dehydrogenation (ODH) of the CH3OCH2O* intermediate than that of CH3O*. Furthermore, our calculated energy barriers also suggest that the sulfate promoted V2O5/TiO2 catalyst has not only higher catalytic activity for methanol conversion but also higher selectivity of MF over CH2O, consistent with previous experimental observations. The sulfate promoter was found to increase the positive charge at the V site, leading to a lower energy barrier for the ODH of CH3O* than the unpromoted catalyst.