Selective oxidation of methane over MoOx/SiO2: isolation of the kinetics of reactions occurring in the gas phase and on the surfaces of SiO2 and MoOx

The selective oxidation of methane to formaldehyde over MoOx/SiO2 was investigated to identify the contributions of methane and formaldehyde reactions occurring in the gas phase and on the surfaces of SiO2 and dispersed MoOx to the overall observed reaction rate. Experiments were conducted with 4.5 wt% MoO3/SiO2 at a nominal surface concentration of 0.44 Mo nm−2. Data were acquired for CH4/O2 ratios from 2 to 34 and temperatures from 848 to 923 K. Homogeneous oxidation of methane was negligible, and the homogeneous oxidation of formaldehyde contributed only to a small degree. The formation of formaldehyde was found to occur largely over MoOx and only to a limited degree over SiO2. By contrast, the rates of formaldehyde consumption over MoOx and SiO2 were comparable. For SiO2 the only process occurring was CH2O decomposition, which was independent of the CH4/O2 ratio, whereas in the case of MoOx, CH2O decomposition was accompanied by combustion of the H2 formed and by direct CH2O combustion principally to CO. A comprehensive model of the overall reaction kinetics was assembled from the kinetics determined for the reactions of methane and formaldehyde in the gas phase and on the surfaces of SiO2 and MoOx. This model describes the observed rates of methane conversion and the formaldehyde selectivity measured experimentally as functions of temperature and CH4/O2 ratio. This model is then used to determine the maximum single-pass yield of CH2O for a given temperature and CH4/O2 ratio. It is predicted that the maximum single-pass yield increases with reaction temperature and decreasing CH4/O2 ratio.