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. (C) 1998 Elsevier Science B.V.