The selective oxidation of methane to C2-hydrocarbons over lithium-doped TiO2 catalysts

Abstract

The oxidative coupling of methane (OCM) to C2-hydrocarbons was studied on a series of lithiumdoped titania catalysts by cofeeding methane (PCH4 = 0.5 bar) and oxygen (Po2 = 0.07-0.16 bar) at 1 bar total pressure in the temperature range of 750-900°C. The degree of lithium doping was varied between 0 and 4 wt.-% on Li2O basis. Electrical conductivity measurements confirmed that doping (i.e. incorporation of the Li+ cations into the crystal structure of TiO2) did indeed occur. At 900°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 practically constant within a period of 72 h of continuous reaction. Transient kinetic studies of ethene and ethane reactions with the lattice (in the absence of gaseous oxygen) and adsorbed (in the presence of gaseous oxygen) oxygen species of undoped and Li+-doped TiO2 catalysts showed that adsorbed oxygen species are more reactive towards ethene than ethane, 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 selectivity correlates well with surface acidity and basicity characteristics of the Li+ -doped TiO2 catalysts. © 1994.