THE SELECTIVE OXIDATION OF METHANE TO C2-HYDROCARBONS OVER LITHIUM-DOPED TIO2 CATALYSTS

The oxidative coupling of methane (OCM) to C2-hydrocarbons was studied on a series of lithium-doped titania catalysts by cofeeding methane ( P(CH4) = 0.5 bar) and oxygen (P(O2) = 0.07-0.16 bar) at 1 bar total pr essure in the temperature range of 750-900-degrees-C. The degree of li thium doping was varied between 0 and 4 wt.-% on Li2O basis. Electrica l conductivity measurements confirmed that doping (i.e. incorporation of the Li+ cations into the crystal structure of TiO2) did indeed occu r. At 900-degrees-C, a C2-hydrocarbons selectivity of the order of 70% , at a methane conversion level of 12%, was obtained over the 1 wt.-% Li2O-doped TiO2 catalyst. This catalytic performance stayed practicall y constant within a period of 72 h of continuous reaction. Transient k inetic studies of ethene and ethane reactions with the lattice (in the absence of gaseous oxygen) and adsorbed (in the presence of gaseous o xygen) oxygen species of undoped and Li+-doped TiO2 catalysts showed t hat adsorbed oxygen species are more reactive towards ethene than etha ne, while lattice oxygen is more reactive towards ethane than ethene. These results explain well the C2H4/C2H6 product ratio obtained under OCM reaction conditions. It was also found that C2-hydrocarbons select ivity correlates well with surface acidity and basicity characteristic s of the Li+-doped TiO2 catalysts.