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 degrees 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 C-2-C-3 hydrocarbons even at 700 degrees 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 degrees C whereas it can achieve reactivity (similar to 24% CH4 conversion and similar to 74% C-2-C-3 selectivity) comparable to the TiO2-doped one at 800 degrees 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.