Kinetics of Methane Oxidative Coupling on Li-Doped TiO2 Catalysts

Abstract

Abstract A kinetic study of methane conversion to C 2 -hydrocarbons was conducted by cofeeding methane and oxygen at 1 bar total pressure over a series of Li-doped TiO 2 catalysts. The lithium dopant concentration was varied between 1 and 4 wt% Li 2 O. Electrical conductivity measurements confirmed the incorporation of Li + into the crystal lattice of rutile TiO 2 , XPS measurements the enrichment of the surface with lithium, and XRD the presence of Li 2 TiO 3 in the 4 wt% Li 2 O-doped TiO 2 catalyst. It was found that the overall activation energy for methane conversion was independent of the Li + dopant concentration ( E = 45 kcal mol −1 ), a result opposite to that for C 2 -hydrocarbons and CO x ( x = 1, 2) formation. An optimum in methane activity was observed in the range between 0.5 and 1.5 wt% Li 2 O dopant concentration. On the other hand, selectivity towards C 2 -hydrocarbons showed a rather monotonic increase with Li + dopant concentration over a wide range of temperatures and partial pressures of methane and oxygen. The rates of C 2 -hydrocarbons and CO x formation showed dependence on Li + dopant concentration and also on methane/oxygen ratio. CO 2 chemisorption followed by temperature-programmed desorption was used as a probe technique to characterize the basicity of the series of Li-doped TiO 2 catalysts. It was found that the 1 wt% Li 2 O-doped TiO 2 exhibited the highest amount of CO 2 uptake. A distribution in the strength of basic sites was also observed. Surface acidity measurements by amine titrations over the series of Li-doped TiO 2 catalysts revealed that total acidity decreases with increasing Li + dopant concentration. Surface basicity and acidity results seem to be related to the catalysis of the oxidative coupling of methane reaction over Li-doped TiO 2 catalysts.