MECHANISTIC EVIDENCE OF THE PARTIAL OXIDATION OF METHANE TO FORMALDEHYDE OVER SILICA-BASED OXIDE CATALYSTS BY TEMPERATURE-PROGRAMMED REACTION STUDIES

The catalytic behaviour of SiO2 supported MoO3 and V2O5 catalysts in the partial oxidation of methane to formaldehyde with O-2 (MPG) in the range 400-800 degrees C has been investigated by temperature programmed reaction (TPR) tests. Both the sequence of the onset temperature of product formation and the product distribution patterns signal that MPO on silica based oxide catalysts occurs mainly via a consecutive reaction path: CH4 --> HCHO --> CO --> CO2. At T greater than or equal to 700 degrees C a parallel surface assisted gas-phase reaction pathway leads to the formation of minor amounts of C-2 products both on SiO2 and MoO3/SiO2 catalysts. The redox properties of MoO3/SiO2 and V2O5SiO2 catalysts have been systematically evaluated by H-2 and CH4 temperature programmed reduction (H-2-TPR, CH4-TPR) measurements. H-2-TPR results do not account for the reactivity scale of oxide catalysts in the MPO. CH4-TPR measurements indicate that the enhancement in the specific activity of the silica is controlled by the capability of MoO3 and V2O5 promoters in providing 'active' lattice oxygen species.