Revisiting Activity- and Selectivity-Enhancing Effects of Water in the Oxidative Coupling of Methane over MnOx-Na2WO4/SiO2 and Proving for Other Materials

The oxidative coupling of methane (OCM) to C-2-hydrocarbons (C2H4 and C2H6) is attractive both from fundamental (selective C-H bond activation) and applied viewpoints. The main drawback hindering its commercialization is the low selectivity to C-2-hydrocarbons due to their further oxidation to carbon oxides. In this respect, we focused on elucidating fundamentals of the previously reported positive effect of water on the activity and selectivity in the OCM reaction over MnOx-Na2WO4/SiO2 by means of steady-state kinetic and mechanistic tests at ambient pressure as well as temporal analysis of products with sub-millisecond resolution and isotopic tracers in vacuum. The obtained results cannot be rationalized by the earlier developed concepts explaining the role of water in the OCM reaction over this catalyst system. In addition, the selectivity-enhancing water effect was determined for MnOx/SiO2, MnOx/Al2O3, Na2WO4/SiO2, and Na2WO4/Al2O3, as well as SiO2-based materials with supported PbOx, ZrO2, or La2O3. No rate-improving effect was established for MnOx/Al2O3 and ZrO2/SiO2, while positive or negative effects were determined for PbOx/SiO2, MnOx/SiO2, and Na2WO4/SiO2 or La2O3/SiO2 and Na2WO4/Al2O3, respectively. Thus, testing other catalysts in the water presence seems to be promising in view of improving the OCM performance to an industrially attractive level. Owing to the developed protocol for catalyst testing under alternating water-free and water-containing OCM feeds, both reversible and irreversible water effects on the selectivity to C-2-hydrocarbons over MnOx-Na2WO4/SiO2 were identified. The irreversible effect was related to water-induced redispersion of MnOx. Mechanistic insights into the reversible rate- and selectivity-improving effects were derived from kinetic analysis of the rates of methane conversion into individual reaction products at different water partial pressures and temperatures. Selectivity-conversion relationships enabled us to understand the specific reaction pathways in the course of the OCM reaction affected by water. Water was established to increase the rates of methane conversion into C-2-hydrocarbons, CO, and CO2 with the strongest effect being determined for CO formation. For all the rates, the strength of the positive water effect decreases with an increase in temperature from 750 to 825 degrees C. In addition to the acceleration of methane conversion, water also helps to transform nonselective molecular-adsorbed oxygen species responsible for the direct oxidation of methane to carbon dioxide.