A comparison of BaF₂/La₂O₃ and BaBr₂/La₂O₃ catalysts for the oxidative coupling of methane

The 10 mol% BaBr₂/La₂O₃ catalyst is superior to the 10 mol% BaF₂/La₂O₃ catalyst in CH₄ conversion but slightly inferior in C₂ selectivity. At 750°C, the former has a CH₄ conversion of 44.7% and a C₂ selectivity of 40.5% (C₂ yield = 18.1%); whereas the latter has a CH₄ conversion of 27.0% and a C₂ selectivity of 43.7% (C₂ yield = 11.8%). The halide ions behave very differently in the two catalysts. The Br^- ions disperse on La₂O₃ while the F^- ions diffuse into the La₂O₃ lattice, leading to the formation of hexagonal LaF₃ and rhombohedral LaOF. In situ Raman studies of the two catalysts confirmed the generation of BaCO₃ and La₂O₂CO₃ during OCM reactions. The BaBr₂/La₂O₃ catalyst has higher CO₂ adsorption capacity and is superior to BaF₂/La₂O₃ in forming the crystal phase of hexagonal La₂O₂CO₃ as well as forming O₂ ^n- (1 < n < 2)ions in oxygen. We conclude that the dispersion of Br^- ions on the surface has enhanced the basicity of the catalyst. Consequently, both H-abstraction ability and CO₂ adsorption capability of the catalyst increase. The novel role of Br^- ions is to destabilize the La₂O₂CO₃ being formed. Influenced by the presence of Br^- ions, the La₂O₂CO₃ formed in OCM conditions decomposes at 690°C instead of 790°C. We speculate that the weakening of C-O bonds in CO₃ ^2- is due to the delocalization of Br^- electron into the π* orbital of CO₃ ^2-. During OCM reactions above 690°C, decomposition and formation of CO₃ ^2- occur simultaneously and randomly in the crystal frame of hexagonal La₂O₂CO₃, creating active sites which are beneficial for methane activation.