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

Oxidative coupling of methane (OCM) catalyzed by MnO_x-Na₂WO₄/SiO₂ has great industrial promise to convert methane directly to C_2–3 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 MnO_x-Na₂WO₄/SiO₂ catalyst enriched with Q² units in the SiO₂ carrier, being capable of converting 23 % CH₄ with 72 % C_2–3 selectivity at 660 °C. From experiments and theoretical calculations, a large number of Q² units in the MnO_x-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 O₂. The Na₂WO₄ not only drives the light-off of this cycle but also gets it working with substantial selectivity toward C_2–3 products. Our findings shine a light on the rational design of more advanced MnO_x-Na₂WO₄ based OCM catalysts through establishing new Mn³⁺↔Mn²⁺ redox cycles with lowered light-off temperature.