Efficient and selective oxidative coupling of methane over a Na₃PO₄/Mn₂O₃-TiO₂ catalyst via lattice oxygen looping

A promising oxygen-carrier catalyst of Na₃PO₄/Mn₂O₃-TiO₂ was developed for the chemical looping-oxidative coupling of methane (CL-OCM) process by facile impregnation of a Mn₂O₃-TiO₂ mixture with Na₃PO₄. A volcano-shaped evolution of C₂ selectivity against the mass content of Na₃PO₄ (x = 0.3∼9.6, wt%) was observed in the xNa₃PO₄/2Mn₂O₃-1TiO₂ (2Mn₂O₃-1TiO₂, Mn₂O₃/TiO₂ mass ratio of 2/1) catalysts. The oxygen consumption rate via the ‘Mn₂O₃ + TiO₂ → MnTiO₃ + O’ was dependent on the content of Na₃PO₄. An optimum Na₃PO₄ content of 0.6 wt% was identified, which demonstrated the ability to suppress the excessive oxidation of methane and C₂ products due to the lower surface oxygen potential. The 0.6Na₃PO₄/2Mn₂O₃-1TiO₂ was always highly selective over the whole temperature range (720 to 800 °C) with C₂ yields ranging from 5.1% to 18.2%, for example, achieving 87.9% (or 75.8%) C₂ selectivity with 5.8% (or 24.0%) CH₄ conversion at 720 °C (or 800 °C) and a low catalyst/CH₄ weight ratio of 13.5. Note that a high ethylene selectivity of > 54% could be achieved. As experimentally and theoretically revealed, a suitable amount of Na₃PO₄ enabled the reaction of PO₄³⁻ with surface Mn²⁺ ions to form an appropriate amount of Mn₂P₂O₇, effectively shielding the sites conducive to deep oxidation, while stabilizing the Na⁺ ions preventing their reaction with TiO₂ to form Na₂Ti₉O₁₉ (consuming a large amount of TiO₂). As a result, a shift of the redox cycle from ‘Mn₂O₃ + TiO₂ ↔ MnTiO₃ + O’ to ‘Mn₂O₃ ↔ Mn₃O₄ + O’, which would result in a marked deterioration of the CL-OCM performance, was avoided.