Insight into the Role of Na₂WO₄ in a Low-Temperature Light-off Mn₇SiO₁₂-Na₂WO₄/Cristobalite Catalyst for Oxidative Coupling of Methane

The MnO_x-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 T_d-WO₄ ↔ O_h-WO₆ tautomerization only in case of re-oxidizing MnWO₄ (O_h-WO₆) formed during reaction at >750 °C back to Mn₇SiO₁₂ and Na₂WO₄ (T_d-WO₄).