Oxidative Coupling of Methane over Mixed Metal Oxide Catalysts Including Group 5 Metals.

Oxidative coupling of methane (OCM) over binary and ternary catalytic systems including Group 5 metals (V, Nb, Ta) is reviewed from a point of the relation between catalytic performance and physicochemical properties, especially crystalline structure. Various mixed metal oxides of Group 1/Group 5, Group 1/Group 2/Group 5 and Group 1/Group 3/Group 5 were investigated as catalysts. Group 1 metals or Group 5 metals show relatively low catalytic performance when used as a single oxide or carbonate for the OCM reaction. However, when both metals were mixed at an appropriate atomic ratio, a remarkable increase in the performance, especially selectivity to C₂+ hydrocarbons (ethane, ethylene and higher hydrocarbons), was observed. The structure of catalysts with an atomic ratio of Group 1/Group 5 metals=1:1 was meta-type salts of Group 5 metallic acids, whereas the structure of catalysts with the atomic ratio=1:0.3 was ortho-type salts, the latter showed C₂+ selectivity higher than that of the former.<br>A higher catalytic performance was observed when Group 2 or Group 3 metals were added to the Group 1/Group 5 binary catalysts. Of Group 1/Group 3/Group 5 ternary catalysts, the effect of addition of niobium to Na/La catalyst (atomic ratio=1:1) on the catalytic performance was investigated in detail. The increase in the content of niobium up to a Nb/Na atomic ratio of 0.3 in this catalytic system resulted in the increase in the catalytic performance, but above the Nb/Na ratio of 0.3 the C₂+ selectivity decreased. The highest catalytic performance was obtained at the Nb/Na ratio range of 0.15-0.3 in which the coexistence of La₂O₃ and La₃NbO₇ phases was observed, and the amount of the strong base sites that were also observed in the Na/La catalyst increased. However, at the Nb/Na ratio of >0.4, only La₃NbO₇ and no La₂O₃ were detected in the catalyst and the strong base sites had disappeared. It is supposed that the addition of an appropriate amount of niobium increases the amount of the strong base sites which increase the amount of oxygen species active for the OCM reaction, viz. O₂^2-, hence the catalytic performance is improved.