ON THE NATURE OF ACTIVE-SITES OF SILICA-BASED OXIDE CATALYSTS IN THE PARTIAL OXIDATION OF METHANE TO FORMALDEHYDE

The partial oxidation of methane to formaldehyde with molecular O-2 ha s been investigated on various commercial bare SiO2 samples and silica supported MoO3 and V2O5 catalysts at 550-650 degrees C. Amongst the d ifferent SiO2 samples, the highest HCHO productivity (STYHCHO, g . kg( cat)(-1). h(-1)) is found with 'precipitated' silica, while 'fumed' Si O2 results in the least reactive silica. Incorporation of molybdena de presses the STYHCHO value for the 'precipitated' silica but enhances t he STYHCHO for bare 'fumed' silica. In contrast, addition of vanadia t o either 'precipitated' or 'fumed' silicas leads to higher STYHCHO val ues. On the basis of a series of experiments performed by continuous s canning of the reaction mixture with a quadrupole M.S., the participat ion of lattice oxygen in the main reaction pathway has been ruled out. A straight correlation between the density of reduced sites (rho, 10( 16) s(r) . g(cat)(-1)), evaluated in steady-state conditions by O-2 ch emisorption, and the reaction rate has been disclosed. MoO3 and V2O5 d opants modify the catalytic properties of SiO2 by affecting the proces s of oxygen activation on the catalyst surface.