A DFT study of the mechanism and kinetics of methane oxidation to formaldehyde occurring on silica-supported molybdena

A theoretical analysis was carried out of the mechanism and kinetics of methane oxidation to formaldehyde occurring on isolated molybdate species supported on silica. Both mono-oxo and di-oxo molybdate structures were used to represent the active centers. The energetics for each elementary reaction was determined from density functional theory calculations, and the entropy changes were determined from calculations based on statistical mechanics. The results of this analysis show that the mechanism based on di-oxo molybdate species agrees more closely with observed rates of methane oxidation than that based on mono-oxo molybdate species. It is also found that the formation of formaldehyde occurs via the reaction of methane with peroxide species formed via the adsorption of O2 on reduced MoIV centers. The extent of MoVI reduction to MoIV is well under 1% under reaction conditions, in good agreement with experimental observations.