Oxidative coupling of methane over ternary metal oxide catalysts consisting of Groups I, III and V elements in the Periodic Table Original Research Article

Ternary metal oxides consisting of Group I (alkali metals)/Group III/Group V metals with an atomic ratio of 1:1:0.3 were prepared and their catalytic performance for the oxidative coupling of methane as well as their physicochemical properties were investigated. When adding Group V metals to Group I/Group III binary catalysts, a noticeable increase in the conversions of methane and oxygen was observed without any change in C2+ selectivity. Of the ternary metal oxide catalysts, a Na/La/0.3Nb metal oxide showed the highest performance, viz. a methane conversion of 16.0% and a C2+ selectivity of 74.1% at 1023 K under atmospheric pressure with a total gas flow-rate of 100 ml NTP/min (CH4/O2molar ratio= 9). The stability test using a Na/La/0.2Nb catalyst, which was performed under the same conditions as above, showed that the catalyst was very stable with no decrease in methane conversion and C2+ selectivity for more than 100 h. The crystalline structure, the surface composition and properties of base sites of Na/La/Nb oxide catalysts were investigated using X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) of carbon dioxide. The catalytic performance of the Na/La/Nb mixed oxides depended on the ratio of the two crystalline phases of La2O3 and La3NbO7. When sodium was added to La2O3 with aLa/Na ratio of 1, new base sites were produced, resulting in an increase in the C2+ selectivity with a significant decrease in methane conversion. The methane conversion was increased by adding a small amount of niobium to theNa/La catalyst, resulting in a change of part of the La2O3 to La3NbO7. However, when all of the La2O3 was changed to La3NbO7, the C2+ selectivity decreased due to the lack of new base sites. A good balance of both crystals (La2O3 and La3NbO7) may be obtained at aNb/Na atomic ratio of 0.15–0.3, which optimizes the catalytic performance of the mixed oxides.