Insight into the Role of Na<sub>2</sub>WO<sub>4</sub> in a Low-Temperature Light-off Mn<sub>7</sub>SiO<sub>12</sub>–Na<sub>2</sub>WO<sub>4</sub>/Cristobalite Catalyst for Oxidative Coupling of Methane
Jiaqi Si, Guofeng Zhao, Tian Lan, Jiayong Ni, Weidong Sun, Ye Liu, Yong Lu
Abstract
The MnOx-Na2WO4-based catalysts show the most promise for practical application in the oxidative coupling of methane (OCM), but its working mechanism is still open for debate. Herein, we attempt to reveal the role of Na2WO4 in a low-temperature light-off Mn7SiO12–Na2WO4/Cristobalite catalyst system. Three catalysts with Mn7SiO12–Na2WO4, Mn7SiO12, and Na2WO4 supported on cristobalite were rationally tailored, tested in the OCM reaction, and deeply characterized by 17O-labeling solid state nuclear magnetic resonance and Raman combined with temperature-programmed reaction/desorption methods. The OCM reaction proceeds via the Na2WO4-modulating Mn7SiO12 ↔ MnSiO3 redox with CH4/O2. The Na2WO4 substantially reduces the trigger temperature of such a redox cycle and gets it running selectively instead of combustion. Neither direct redox of Na2WO4 nor oxygen spillover between Na2WO4 and Mn7SiO12 was observed. Nevertheless, oxygen exchange between O2 and Na2WO4 will still occur via the Td-WO4 ↔ Oh-WO6 tautomerization only in case of re-oxidizing MnWO4 (Oh-WO6) formed during reaction at >750 °C back to Mn7SiO12 and Na2WO4 (Td-WO4).