Methane oxidative coupling: II. A study of lithium-titania-catalyzed reactions of methane

Abstract

Methane conversion to higher hydrocarbons via oxidative coupling was studied on a series of lithium-doped titania catalysts by cofeeding methane and oxygen. The degree of lithium promotion was studied by varying the lithium loading from 0 to 31.7 wt% lithium on the rutile crystal structure of titania. Catalytic results indicate that lithium-doped titania catalysts are effective for oxidative coupling of methane with hydrocarbon product selectivities ranging from 20 to 85%. Increasing the lithium loading reduces the combustion activity of the catalyst, improves hydrocarbon selectivity, and increases space-time yields. Product yield reached a maximum for the 16.2-wt% loading; this catalyst had methane conversions of ca. 15% with hydrocarbon selectivities of ca. 75%. The 16.2-wt% lithium-titania catalyst was studied under various operating conditions and compared favorably with other catalytic results that have been reported. The observed activation energy of the 16.2% lithium-titania catalyst was dependent on the reactant partial pressure and ranged from 25.6 to 37.6 kcal/mole. X-ray diffraction, X-ray photoelectron spectroscopy, infrared analysis, and differential thermal analysis results of the catalysts indicate that increasing the degree of lithium promotion lowers the surface area and the surface concentration of oxygen, and creates different phases at elevated temperatures. Routes for surface-catalyzed reactions coupled with gas-phase reactions were studied by transient pulse and step-change experiments. The transient experiments suggest that adsorbed oxygen is responsible for the C 2 hydrocarbon activity of the lithium-titania catalyst and that lattice oxygen activates methane nonselectively.