Oxidative coupling of methane on LaAlO3 perovskites partially substituted with alkali or alkali-earth ions

Methane coupling was investigated at 600-750 degreesC on LaAlO3, La1-xMxAlO3 (M = Na, K, Ca, Ba, x = 0.1) and LaAl1-xMxO3 (M = Li, Mg, x = 0.1) perovskite-type catalysts, prepared by calcining the citrate precursors at 800 degreesC for 5 h. The introduction of the alkali and alkali-earth metals produces oxygen vacancies and higher bond strength for both lattice and surface oxygen species. The substitution of Al3+ with Li+ and Mg2+ increases both catalytic activity and selectivity to C-2-hydrocarbons in comparison with unsubstituted LaAlO3 perovskite. A diffusional control is suggested for the oxidation to carbon oxides, whereas methane coupling should occur tinder kinetic control. The overall process should take place through a complex pattern of reactions. The results were rationalized on the basis of the structural properties of the catalysts and of their adsorptive behavior towards oxygen, investigated by means of temperature programmed desorption (TPD). (C) 2001 Elsevier Science BN. All rights reserved.