Effect of CO2 on the structural variation of Na2WO4/Mn/SiO2 catalyst for oxidative coupling of methane to ethylene

Abstract

In this work, the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/SiO2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation method and characterized by XRD, Raman and XPS techniques. Appropriate amount of CO2 in the reactant gases enhanced the formation of surface tetrahedral Na2WO4 species and promoted the migration of O in MOx, Na, W from the catalyst bulk to surface, which were favorable for oxidative coupling of methane. When the molar ratio of CH4/O2/CO2 was 3/1/2, enriched surface tetrahedral Na2WO4 species and high surface concentration of O in MOx, Na, W were detected, and then high CH4 conversion of 33.1% and high C2H4 selectivity of 56.2% were obtained. With further increase of CO2 in the reagent gases, the content of active surface tetrahedral Na2WO4 species and surface concentration of O in MOx, Na, W decreased, while that of inactive species (MnWO4 and Mn2O3) increased dramatically, leading to low CH4 conversion and low C2H4 selectivity. It could be speculated that Na2WO4 crystal was transformed into MnWO4 crystal with excessive CO2 added under the reaction conditions. Pretreatment of Na2WO4/Mn/SiO2 catalyst by moderate amount of CO2 before OCM also promoted the formation of Na2WO4 species.