Oxidative Coupling of Methane: Examining the Inactivity of the MnOx-Na2WO4/SiO2 Catalyst at Low Temperature

Oxidative coupling of methane (OCM) catalyzed by MnOx-Na2WO4/SiO2 has great industrial promise to convert methane directly to C2-3 products, but its high light-off temperature is the most challenging obstacle to commercialization and its working mechanism is still a mystery. We report the discovery of a low-temperature active and selective MnOx-Na2WO4/SiO2 catalyst enriched with Q(2) units in the SiO2 carrier, being capable of converting 23 % CH4 with 72 % C2-3 selectivity at 660 degrees C. From experiments and theoretical calculations, a large number of Q(2) units in the MnOx-Na2WO4/SiO2 catalyst is a trigger for markedly lowering the light-off temperature of the Mn3+<-> Mn2+ redox cycle involved in the OCM reaction because of the easy formation of MnSiO3. Notably, the MnSiO3 formation proceeds merely through the SiO2-involved reaction in the presence of Na2WO4: Mn7SiO12+6 SiO2 <-> 7 MnSiO3+1.5 O-2. The Na2WO4 not only drives the light-off of this cycle but also gets it working with substantial selectivity toward C2-3 products. Our findings shine a light on the rational design of more advanced MnOx-Na2WO4 based OCM catalysts through establishing new Mn3+<-> Mn2+ redox cycles with lowered light-off temperature.