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 nu mber of other selective oxidation reactions, primarily on the basis of its selectivity-conversion behavior and the data is presented for a r ange of promoted vanadium oxide catalysts supported on silica. The rea ction mechanism involves activation of methane by an adsorbed oxygen s pecies with subsequent generation of a CH3 species. This species, in t urn, reacts with lattice oxygen to form formaldehyde. Selectivity is d etermined by the ability of the activating species to discriminate bet ween a C-H bond in methane and a similar, but much weaker C-H bond in formaldehyde. Conventional selective oxidation catalysts are not capab le 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 b onds in methane. The discriminating capacity of active sites on oxide catalysts has been exceeded in attempting to convert methane into form aldehyde, hence the poor selectivity observed. (C) 1998 Elsevier Scien ce B.V.