Kinetics of methane oxidative coupling on li-doped TiO2 catalysts

Abstract

A kinetic study of methane conversion to C2-hydrocarbons was conducted by cofeeding methane and oxygen at 1 bar total pressure over a series of Li-doped TiO2 catalysts. The lithium dopant concentration was varied between 1 and 4 wt% Li2O. Electrical conductivity measurements confirmed the incorporation of Li+ into the crystal lattice of rutile TiO2, XPS measurements the enrichment of the surface with lithium, and XRD the presence of Li2TiO3 in the 4 wt% Li2O-doped TiO2 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 C2-hydrocarbons and COx (x = 1, 2) formation. An optimum in methane activity was observed in the range between 0.5 and 1.5 wt% Li2O dopant concentration. On the other hand, selectivity towards C2-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 C2-hydrocarbons andCOx formation showed dependence on Li+ dopant concentration and also on methane/oxygen ratio. CO2chemisorption followed by temperature-programmed desorption was used as a probe technique to characterize the basicity of the series of Li-doped TiO2 catalysts. It was found that the 1wt% Li2O-doped TiO2 exhibited the highest amount of CO2 uptake. A distribution in the strength ofbasic sites was also observed. Surface acidity measurements by amine titrations over the series of Li-doped TiO2 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 TiO2 catalysts. © 1993 Academic Press, Inc.