Oxidative Coupling of Methane: Examining the Inactivity of the MnO<sub><i>x</i></sub>‐Na<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub> Catalyst at Low Temperature
Jiaqi Si, Guofeng Zhao, Weidong Sun, Jincun Liu, Cairu Guan, Yong Yang, Xue‐Rong Shi, Yong Lu
Abstract
Abstract Oxidative coupling of methane (OCM) catalyzed by MnO x ‐Na 2 WO 4 /SiO 2 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 ‐Na 2 WO 4 /SiO 2 catalyst enriched with Q 2 units in the SiO 2 carrier, being capable of converting 23 % CH 4 with 72 % C 2–3 selectivity at 660 °C. From experiments and theoretical calculations, a large number of Q 2 units in the MnO x ‐Na 2 WO 4 /SiO 2 catalyst is a trigger for markedly lowering the light‐off temperature of the Mn 3+ ↔Mn 2+ redox cycle involved in the OCM reaction because of the easy formation of MnSiO 3 . Notably, the MnSiO 3 formation proceeds merely through the SiO 2 ‐involved reaction in the presence of Na 2 WO 4 : Mn 7 SiO 12 +6 SiO 2 ↔7 MnSiO 3 +1.5 O 2 . The Na 2 WO 4 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 ‐Na 2 WO 4 based OCM catalysts through establishing new Mn 3+ ↔Mn 2+ redox cycles with lowered light‐off temperature.