Tungstate promoted CaMnO3-based core-shell redox catalysts for efficient chemical looping oxidative coupling of methane

Chemical looping oxidative coupling of methane (CLOCM) provides a promising route for methane conversion to high-valued olefins, in which oxygen carriers serve as oxygen donors and catalysts to facilitate methane activation and C–C coupling reactions. Herein, the perovskite CaMnO3 was used as the oxygen carrier for CLOCM due to the high catalytic activity of Mn, and tungstate was selected to modify CaMnO3 for improving the C2 product selectivity. The results showed that CaMnO3 with 6% Na2WO4 doping exhibited the highest activity at 800 °C with a C2 yield of 15.5% and good redox stability for at least 30 cycles. The coexistence of Na2WO4 with CaMnO3 synergistically controlled the methane conversion and C–C coupling. Further characterizations indicated that the core-shell structure with the inner layer of CaMnO3 and the outer layer of Na2WO4 effectively regulated the oxygen release and transformation. Na2WO4 modification resulted in an obvious decrease of nonselective oxygen species which were prone to the deep oxidation of methane while leading to a great increase of O2− species that could selectively oxidize methane to C2 hydrocarbons. These findings provide guidance for dopant-induced regulation of oxygen vacancies and selective oxygen species in oxygen carriers for improving CLOCM performance.