Acidity/basicity of rare-earth oxides and their catalytic activity in oxidative coupling of methane to C₂-hydrocarbons

Abstract

Rare-earth metal oxides (viz. La₂0₃, Ce0₂, Sm₂0₃, Eu₂0₃, and Yb₂0₃) have been compared for their acid and base strength distribution (measured by stepwise thermal desorption of C0₂ from 323 to 1173 K and TPD of NH₃ from 323 to 1223 K, respectively) and their catalytic activity/ selectivity in oxidative coupling of methane to C2-hydrocarbons at 973-1123 K (space VELOCITY = 108,000 cm³. g^-1 h^-1 and CH₄/O₂ = 4 and 8). The catalyst activity and selectivity showed dependence on both the surface acidity and basicity. However, the relationship of the catalytic activity/ selectivity with the surface acidity /basicity is not straightforward; it is quite complex. Stronger acid sites are found to be harmful for the selectivity. There is a possibility of an involvement of an acid-base pair (M_LCⁿ⁺i c 0_LC^2-, where subscript LC denotes low coordination) on the surface in the abstraction of the H-atom from adsorbed methane molecule by its polarization followed by heterolytic C-H bond rupture to form CH₃^- (which interacts with M_LCⁿ⁺ .c) and (OH)_LC^- and a transfer of electron from the carbanion to O₂, resulting methyl radical and O z. Regeneration of basic sites and the possibility of formation of 0^- species on the catalyst surface are discussed.