Partial oxidation of methane to formaldehyde over W/SiO2 catalysts

Two samples of silica with different surface area have been used in the pre paration of silica-supported tungsten oxide catalysts with W loading rangin g from 0.8 to 15.8 wt%. Both catalyst series have been tested in the partia l oxidation of methane to formaldehyde with molecular oxygen at atmospheric pressure. It has been shown that both surface area and tungsten loading af fect the nature and dispersion of the tungsten oxide species and, therefore , the catalytic behavior of the samples. Poor dispersion and high reducibil ity of the tungsten oxide phase is observed for the low surface area cataly sts, thus leading to a high activity per unit area in methane conversion. H igh surface catalysts show a well-dispersed and hardly reducible tungsten o xide overlayer that covers the active sites of the silica surface. Conseque ntly, a negative effect on the catalytic activity is observed. Formaldehyde production has been related to the density and activity of three types of active sites present on the W/SiO2 catalysts: active sites of silica surfac e, terminal W=O sites and W-O-W functionalities. These results agree with t hose previously reported by the authors for W/HZSM-5 catalysts. (C) 1999 El sevier Science B.V. All rights reserved.