Naohide Tsuzuki

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

Abstract 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 C 2+ 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 C 2+ selectivity of 74.1% at 1023 K under atmospheric pressure with a total gas flow-rate of 100 ml NTP/min ( CH 4 /O 2 molar 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 C 2+ 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 La 2 O 3 and La 3 NbO 7 . When sodium was added to La 2 O 3 with a La/Na ratio of 1, new base sites were produced, resulting in an increase in the C 2+ selectivity with a significant decrease in methane conversion. The methane conversion was increased by adding a small amount of niobium to the Na/La catalyst, resulting in a change of part of the La 2 O 3 to La 3 NbO 7 . However, when all of the La 2 O 3 was changed to La 3 NbO 7 , the C 2+ selectivity decreased due to the lack of new base sites. A good balance of both crystals (La 2 O 3 and La 3 NbO 7 ) may be obtained at a Nb/Na atomic ratio of 0.15–0.3, which optimizes the catalytic performance of the mixed oxides.

Oxidative coupling of methane over alkali halide- promoted perovskite oxide catalysts

Abstract 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;Pch4=10.1 kPa,PO2 = 5.1 kPa,PHe = 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 NaCI has been removed from the catalyst during the test. However, the same level of activity is recovered when NaCI 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.

Natural Calcium Compounds as Catalysts for Oxidative Coupling of Methane

Oxidative coupling of methane(OCM) over various natural calcium compounds was investigated. Some kinds of shells were found to be good catalysts for OCM. The study of surface properties of the catalysts by XPS revealed that a high performance of the shell catalysts was due to not only morphology but also high concentration of minor components such as sodium on the surface of the catalysts.