Oxidative coupling of methane over La-promoted CaO catalysts:Influence of precursors and catalyst pr

The oxidative coupling of methane to C_2 hydrocarbons has been studied over a series of La-promoted CaO(La/Ca=0.05)catalysts,prepared using different precursor salts for CaO and La_2O_3(viz.acetates,carbonates,nitrates and hydroxides)and catalyst preparation methods(viz.physical mixing of precursors,co-precipitation using ammonium carbonate/sodium carbonate as a precipitating agent),under different reaction strength distribution of the catalysts have also been investigated.The surface properties and catalytic activity/selectivity of the La-promoted CaO catalysts vary from catalyst to catalyst depending on the catalyst precursors used and catalyst preparation method.The basicity/base strength distribution is strongly influenced by the precursors(for CaO and La_2O_3)and catalyst preparation method.Basicity(total and strong basic sites measured in terms of CO_2 chemisorbed at 50℃ and 500℃,respectively)observed for the catalyst prepared by co-precipitation method is higher than that of the catalysts prepared by physical mixing method.The catalysts prepared by the nitrates of La-and Ca-and coprecipitated by the solution of sodium carbonate and ammonium carbonate exhibit different catalytic performance in OCM.The finding that no direct relationship between the surface basicity and catalytic activity/selectivity in OCM exists indicates that basicity is not solely responsible for obtaining high selectivity to C_2 hydrocarbons.