SELECTIVE OXIDATION OF METHANE TD C1 OXYGENATES CUER MOLYBDENA-SILICA CATALYSTS

The selective oxidation of methane to C1 oxygenates, in a single catalytic step, over silica-supported molybdena (MoO3 < 1.7 wt %) catalysts has been studied. A laboratory fixed-bed reactor, cofed with a CH4:O2 = 9:1 molar ratio) and operated at atmospheric pressure in the temperature range 550-650-degrees-C has been used for this purpose. The increase of the reaction temperature led to an increase of the CH4 conversion and the parallel decrease of formaldehyde selectivity. In addition, the formation of oligomerization products (mainly ethane) was favoured by increasing the reaction temperature. The catalysts were characterized by UV-visible difuse reflectance spectroscopy (DRS), isothermal reduction in hydrogen and X-ray photoelectron spectroscopy (XPS). The DR spectra of catalysts showed the Mo6+ ions to possess both O(h) and T(d) coordinations, but the relative proportion being dependent on the MoO3 loading. Apart from this, the MoO3 was found to be highly dispersed on the Aerosil surface, essentially in monolayer as revealed by the I(Mo3d)/I(Si2p) ratios measured in the photoelectron spectra. Only for MoO3 loadings above 1.0 wt % a slight decrease of dispersion was observed. All these results have shown that both coordination and dispersion of the MoO3 phase control to a great extent formation of C1 oxygenates.