Oxidative coupling of methane: MO_x-modified (M = Ti, Mg, Ga, Zr) Mn₂O₃-Na₂WO₄/SiO₂ catalysts and effect of MO_x modification

Mn₂O₃-Na₂WO₄/SiO₂ is considered as the most promising catalyst for the oxidative coupling of methane (OCM) process; however, it only has a better catalytic performance over 800 °C. To improve its low-temperature performance, an attempt has been made to modify the Mn₂O₃-Na₂WO₄/SiO₂ catalyst using TiO₂, MgO, Ga₂O₃, and ZrO₂. Among the synthesized catalysts, the TiO₂-modified Mn₂O₃-Na₂WO₄/SiO₂ catalyst shows markedly improved low-temperature OCM per-formance, achieving a high CH₄ conversion of ~23% and a good C₂-C₃ selectivity of ~73% at 700 °C (the catalyst bed temperature), along with promising stability for at least 300 h without signs of deactivation. In comparison with the unmodified Mn₂O₃-Na₂WO₄/SiO₂ catalyst, the TiO₂ modification results in significant improvement in the low-temperature activity/selectivity, whereas the MgO modification has almost no impact and the Ga₂O₃ and ZrO₂ modifications have a negative effect. The X-ray diffraction (XRD) and Raman results reveal that the formation of a MnTiO₃ phase and a MnTiO₃-dominated catalyst surface is crucial for the improvement of the low-temperature activity/selectivity in the OCM process.