OXIDATIVE COUPLING OF METHANE OVER BAF2-PROMOTED RARE-EARTH-OXIDES WITH VARIABLE VALENCE

The catalytic behaviors for oxidative coupling of methane (OCM) over B aF2-promoted CeO2, Pr6O(11) and Tb4O7 catalysts have been studied. It was found that methane conversions, C-2 selectivities and C2H4/C2H6 ra tios over the BaF2-promoted catalysts were significantly higher than t hose over the corresponding oxides. The maximum C-2 selectivity (57.5% ) and yield (19.3%) were obtained over the BaF2/Pr6O11 (Ba/Pr=2) catal yst under the conditions of 1073 K, CH4:O-2=3:1 and GHSV=20 000 h(-1). XRD analysis indicated that partial ion exchange took place during th e catalyst preparation, leading to the contraction of BaF2 lattice and /or the formation of rare earth oxyfluorides. XRD, XPS and TPR measure ments 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 C-2 hydrocarbons and thus will result in an increase of C-2 selectivity. Since there is a certain correlati on between methane conversion and C-2 selectivity for an oxygen-limite d OCM reaction, methane conversion will also be found to increase alon g with the increase of C-2 selectivity. However, no direct relationshi p between the surface acidity/basicity and catalytic activity/selectiv ity was observed over these catalysts.