Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K2WO4/SiO2 Catalyst Revealed by In Situ Characterization

Abstract

The oxidative coupling of methane (OCM) over a K2WO4/SiO2 catalyst was investigated using a CH4–O2 flow system. The addition of H2O to the reactant stream caused an increase in the CH4 conversion rate and C2 selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4 with OH radicals. The C2+ (a sum of C2 and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4 yield 17.0%) at a CH4 conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2 catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2 and KO2 species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2 species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4 maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4 previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2 yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.