Oxidative coupling of methane over BaF2-promoted rare earth oxides with variable valence Original Research Article

The catalytic behaviors for oxidative coupling of methane (OCM) over BaF2-promoted CeO2, Pr6O11 and Tb4O7 catalysts have been studied. It was found that methane conversions, C2 selectivities andC2H4/C2H6 ratios over the BaF2-promoted catalysts were significantly higher than those over the corresponding oxides. The maximum C2 selectivity (57.5%) and yield (19.3%) were obtained over theBaF2/Pr6O11 (Ba/Pr=2) catalyst under the conditions of 1073 K,CH4:O2=3:1 andGHSV=20 000 h−1. XRD analysis indicated that partial ion exchange took place during the catalyst preparation, leading to the contraction of BaF2 lattice and/or the formation of rare earth oxyfluorides. XRD, XPS and TPR measurements revealed that the concentrations of quadrivalent rare earth ions and reducible oxygen species decreased after adding BaF2 to the rare earth oxides. This will be favorable to decreasing the chance of deep oxidation of methyl radicals and C2 hydrocarbons and thus will result in an increase of C2 selectivity. Since there is a certain correlation between methane conversion and C2 selectivity for an oxygen-limited OCM reaction, methane conversion will also be found to increase along with the increase of C2 selectivity. However, no direct relationship between the surface acidity/basicity and catalytic activity/selectivity was observed over these catalysts.