TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst for oxidative coupling of methane: Solution combustion synthesis and MnTiO₃-dependent low-temperature activity improvement

The Mn₂O₃-Na₂WO₄/SiO₂ catalyst is the most promising one among the enormous catalysts for the oxidative coupling of methane (OCM) but only at above 800 °C. No doubt that lowering temperature of the OCM process is at the forefront of this catalysis field. A promising low-temperature active and selective TiO₂-doped Mn₂O₃ Na₂WO₄/SiO₂ catalyst, consisting of 6 wt% TiO₂, 6 wt% Mn₂O₃, 10 wt% Na₂WO₄ and SiO₂ in balance, is developed by solution combustion synthesis (SCS) method. This catalyst is capable of converting 20% CH₄ with 70% selectivity to C₂-C₃ hydrocarbons even at 700 °C (catalyst bed temperature) and is stable for at least 250 h without deactivation sign, for a feed gas of 50% CH₄ in air using a gas hourly space velocity of 8000 mL g_cat.⁻¹ h⁻¹. In contrast, the non-TiO₂-doped SCS catalyst is almost inactive at 700 °C whereas it can achieve reactivity (∼24% CH₄ conversion and ∼74% C₂-C₃ selectivity) comparable to the TiO₂-doped one at 800 °C. XRD and Raman results evidently reveal that the formation of MnTiO₃ during the OCM process appears to be important for the low-temperature OCM activity improvement by TiO₂-doping.