Na₂WO₄-doped Mn₂O₃-TiO₂ oxygen carrier catalyst for chemical looping OCM: Redox catalysis and mechanistic insight

Intrinsic safety chemical looping-oxidative coupling of methane (CL-OCM) is an attractive method for direct conversion of methane to ethylene, but well-qualified oxygen carrier catalyst still remains challenging. Herein, we report a promising Na₂WO₄-doped Mn₂O₃-TiO₂ catalyst toward high-efficiency CL-OCM process. The standout Na₂WO₄/2Mn1Ti (Mn₂O₃/TiO₂ weight ratio of 2/1) catalyst, with good cycling performance, achieves a high space time yield of 27.8 (or 22.9) g_C2-C3 kg_cat ⁻¹h⁻¹ with 19.5 % (or 15.7 %) CH₄ conversion and 80 % (or 82.7 %) C₂-C₃ selectivity at 740 (or 720) ^oC and a low catalyst/CH₄ weight ratio of 13.5. The TiO₂ substantially lowers the CL-OCM reaction temperature, due to the establishment of a low-temperature light-off “Mn₂O₃ + TiO₂ ↔ MnTiO₃” redox cycle with CH₄/O₂. The enabling role of Na₂WO₄ in modulating the reaction of Mn₂O₃ with CH₄ into OCM instead of combustion is theoretically unveiled, which in nature benefits the *CH₃ transfer and spontaneous desorption to form ∙CH₃ radicals.