Acidity/basicity of rare-earth oxides and their catalytic activity in oxidative coupling of methane to C 2-hydrocarbons

Abstract

Rare-earth metal oxides (viz. La 20 3, Ce0 2, Sm 20 3, Eu 20 3, and Yb 20 3) 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 NH 3 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 cm 3 · g −1 h −1 and CH 4/O 2 = 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 LC n+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 CH 3 − (which interacts with M LC n+ .c) and (OH) LC − and a transfer of electron from the carbanion to O 2, resulting methyl radical and O z. Regeneration of basic sites and the possibility of formation of 0 − species on the catalyst surface are discussed.