Oxidative coupling of methane over La-promoted CaO catalysts: influence of precursors and catalyst preparation method

Abstract

The oxidative coupling of methane to C₂ hydrocarbons has been studied over a series of La-promoted CaO (La/Ca = 0.05) catalysts, prepared using different precursor salts for CaO and La₂O₃ (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 conditions (temperature: 700-850· C, CH₄/O₂ ratio: 4.0 and 8.0, and GHSV: 51360 cm³· g^-1· h^-1). The surface area and surface basicity/base 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 La₂O₃) and catalyst preparation method. Basicity (total and strong basic sites measured in terms of CO₂ chemisorbed at 50° C and 500° C, 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 C₂ hydrocarbons.