Oxidative coupling of methane over BaF 2-promoted rare earth oxides with variable valence

Abstract

The catalytic behaviors for oxidative coupling of methane (OCM) over BaF 2-promoted CeO 2, Pr6O 11 and Tb 4O 7 catalysts have been studied. It was found that methane conversions, C 2 selectivities andC 2H 4/C 2H 6 ratios over the BaF 2-promoted catalysts were significantly higher than those over the corresponding oxides. The maximum C 2 selectivity (57.5%) and yield (19.3%) were obtained over theBaF 2/Pr 6O 11 (Ba/Pr=2) catalyst under the conditions of 1073 K,CH 4:O 2=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 BaF 2 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 BaF 2 to the rare earth oxides. This will be favorable to decreasing the chance of deep oxidation of methyl radicals and C 2 hydrocarbons and thus will result in an increase of C 2 selectivity. Since there is a certain correlation between methane conversion and C 2 selectivity for an oxygen-limited OCM reaction, methane conversion will also be found to increase along with the increase of C 2 selectivity. However, no direct relationship between the surface acidity/basicity and catalytic activity/selectivity was observed over these catalysts.