The limiting selectivity of active sites on vanadium oxide catalysts supported on silica for methane oxidation to formaldehyde

The selective oxidation of methane to formaldehyde is compared to a number of other selective oxidation reactions, primarily on the basis of its selectivity-conversion behavior and the data is presented for a range of promoted vanadium oxide catalysts supported on silica. The reaction mechanism involves activation of methane by an adsorbed oxygen species with subsequent generation of a CH₃ species. This species, in turn, reacts with lattice oxygen to form formaldehyde. Selectivity is determined by the ability of the activating species to discriminate between a C-H bond in methane and a similar, but much weaker C-H bond in formaldehyde. Conventional selective oxidation catalysts are not capable of selectively activating a C-H bond in a reactant in the presence of a similar C-H bond in a product when the bond dissociation enthalpy of the product is weaker by more than 30-40 kJ mol^-1. The C-H bonds in formaldehyde are 75 kJ mol^-1 weaker than the corresponding C-H bonds in methane. The discriminating capacity of active sites on oxide catalysts has been exceeded in attempting to convert methane into formaldehyde, hence the poor selectivity observed.