Catalytic performance, structure, surface properties and active oxygen species of the fluoride-containing rare earth (alkaline earth)-based catalysts for the oxidative coupling of methane and oxidative dehydrogenation of light alkanes

Abstract

The fluoride-containing catalysts, mostly of the rare earth (alkaline earth)-based system, demonstrate good catalytic performances for the oxidative coupling of methane and oxidative dehydrogenation of ethane, propane as well as iso-butane. The results of structural analysis of the catalysts for oxidative coupling of methane show that the promoting effects of the fluoride in the catalysts may be principally related to the phase–phase interaction between fluoride and oxide. Compared to the corresponding alkaline earth oxide promoted rare earth oxide catalyst system, an alkaline earth fluoride-promoted rare earth oxide catalyst system is less basic and will therefore be favorable to reduce CO 2 inhibition in the reaction of methane oxidative coupling. However, there is no simple correlation between the acidity/basicity of a methane oxidative coupling catalyst and its catalytic performance. In the experiments of in situ spectroscopic characterizations carried out at the temperature from 650°C to 800°C, O − 2 species was detected over five fluoride-containing rare earth (alkaline earth)-based catalysts for methane oxidative coupling reaction, and the reactions between O − 2 species and CH 4 to form C 2H 4 and the corresponding side-products were observed using in situ IR over four catalysts, which suggest that O − 2 is probably the active oxygen species for the methane oxidative coupling reaction over the corresponding catalysts.