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

Rare-earth metal oxides (viz. La<SUB>2</SUB>0<SUB>3</SUB>, Ce0<SUB>2</SUB>, Sm<SUB>2</SUB>0<SUB>3</SUB>, Eu<SUB>2</SUB>0<SUB>3</SUB>, and Yb<SUB>2</SUB>0<SUB>3</SUB>) have been compared for their acid and base strength distribution (measured by stepwise thermal desorption of C0<SUB>2</SUB> from 323 to 1173 K and TPD of NH<SUB>3</SUB> 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<SUP>3</SUP>. g<SUP>-1</SUP> h<SUP>-1</SUP> and CH<SUB>4</SUB>/O<SUB>2</SUB> = 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<SUB>LC</SUB><SUP>n+</SUP>i c 0<SUB>LC</SUB><SUP>2-</SUP>, 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<SUB>3</SUB><SUP>- </SUP>(which interacts with M<SUB>LC</SUB><SUP>n+</SUP> .c) and (OH)<SUB>LC</SUB><SUP>-</SUP> and a transfer of electron from the carbanion to O<SUB>2</SUB>, resulting methyl radical and O z. Regeneration of basic sites and the possibility of formation of 0<SUP>-</SUP> species on the catalyst surface are discussed