Methane-Selective Oxidation of Silica-Supported Molybdenum(VI) Catalysts

Abstract

Silica-supported molybdenum oxide catalysts have been prepared with variable surface concentrations and tested in the selective oxidation of methane. The nature of the supported molybdenum species has been studied by Laser Raman spectroscopy and their dispersion by FTIR of chemisorbed NO probe. It was found that polymolybdates are formed in the low Mo-content region (below 0.8 Mo·nm-2) while bulk MoO3 dominates at higher Mo-contents; the former catalysts being more selective for the production of formaldehyde. The impregnation pH seems to play an important role in the genesis of catalysts as it controls both the nature and anchorage of molybdates species on the silica carrier. For instance, at pH values slightly below the zero point charge of the carrier (ca. 2.0) the anionic Mo7O246- species interact more strongly than in neutral or basic media. Activity data also showed that formaldehyde and carbon dioxide are primary products while carbon monoxide is a secondary product. The deeper oxidation of formaldehyde to carbon monoxide can be reduced to a great extent by careful design of the reactor which minimizes the post reaction volume.