OXIDATIVE COUPLING OF METHANE OVER ALKALI HALIDE-PROMOTED PEROVSKITE OXIDE CATALYSTS

The catalytic performance of some alkali halide-promoted perovskite oxides as well as that of the perovskite oxides alone for the oxidative coupling of methane were investigated. Although the perovskite oxides alone showed relatively low catalytic activity, a noticeable increase in activity and C2 selectivity was observed when the oxides were promoted by alkali halides. Sodium chloride (NaCl) was the most effective promoter among the alkali halides. Of the perovskite oxides examined in the present study, the catalytic activity of PbTiO3 was most remarkably promoted by addition of NaCl, because it showed almost no activity for the reaction in the absence of NaCl. The maximum C2 yield of 28.2% was obtained for an NaCl (0.2 mol against 1 mol of PbTiO3) added PbTiO3 catalyst, where methane conversion and C2 selectivity were 50.7% and 55.7%, respectively, under the conditions; p(CH4) = 10.1 kPa, p(O2) = 5.1 kPa, pH, = 86.1 kPa, total gas flow-rate = 100 ml STP/min, temperature = 1023 K, W/F = 0.22 g min/ml. The stability test for the 0.2 mol NaCl/PbTiO3 catalyst suggests that the catalyst loses its high activity when most of the NaCl has been removed from the catalyst during the test. However, the same level of activity is recovered when NaCl is added again to the deactivated catalyst. The thermal and temperature-programmed desorption analyses suggest that both high activity and high C2 selectivity for the NaCl-promoted PbTiO3 catalyst can be ascribed to the generation of new basic active sites. These sites must be created through the interaction between NaCl and PbTiO3 on the surface of the catalyst.