Elucidating the Role of Oxygen Species in Oxidative Coupling of Methane over Supported MnOx-Na2WO4-containing Catalysts

The present study of oxidative coupling of methane (OCM) over MnOx-Na2WO4/support catalysts demonstrated that the selectivity to C2H6 and C2H4 (C-2-hydrocarbons) is affected by the kind of support, co-fed water, and the kind of oxidant (O-2 vs. N2O). In addition to previous studies with MnOx-Na2WO4/SiO2, an enhancing water effect was obtained using catalysts based on TiO2- or ZrO2-containing supports. However, a negative effect on methane conversion was established for SiO2-Al2O3-supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnOx-Na2WO4 to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnOx-Na2WO4. Among the prepared catalysts, MnOx-Na2WO4/TiO2 was found to be promising for H2O-assisted OCM. The use of N2O instead of O-2 further increases the selectivity to C-2-hydrocarbons to 84 % at 6.8 % CH4 conversion due to the formation of predominantly monoatomic oxygen species from N2O that selectively convert CH4 into C2H6.