Study of the Effect of Gas Space Time on the Combination of Methane Gas-Phase Oxidation and Catalytic Oxidative Coupling over Mn/Na2WO4/SiO2 Catalyst

The effects of gas space time on the product distribution in noncatalytic gas-phase methane oxidation and in the combination reaction of noncatalytic and catalytic oxidative coupling of methane (OCM) over Mn/Na2WO4/SiO2 catalyst were studied. For the gas-phase reaction, methane and oxygen conversions and syngas selectivity increase drastically with increasing gas space time (tg) from 6.2 to 7.7 s; however, the selectivity of C2 hydrocarbons decreases sharply in this range and the concentration of C2H4 in the producing mixture is much lower than that of CO and H2. For the combination reaction, there is a minimum of methane conversion and C2 hydrocarbon selectivity with tg increasing from 3 to 5 s resulting from the variation of the mixture composition reaching the catalyst bed. The dilution of the catalyst bed with silica chips in the combination reaction partially restricts the sequential oxidation of products and raises the selectivities of ethylene and hydrogen. Surface enrichment of Na, W, and Mn species is observed on the catalysts used both in the sole catalytic OCM and in the combination reaction. The combination reaction makes the enrichment of Na and W species more evident, and the aggregation of Mn species less obvious, than the OCM reaction.