Structural organization of catalytic functions in Mo-based oxides for propane selective oxidation

Several single phasic Mosingle bondVsingle bondO-based mixed oxides, all of which have a layer structure in the direction of c-axis and a high dimensional arrangement of metal octahedra in a–b plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate structure effects on catalytic property and roles of constituent elements. It was clearly demonstrated that the catalyst with the particular arrangement of MO6 (M = Mo, V) octahedra forming slabs with pentagonal, hexagonal and heptagonal rings in (0 0 1) plane of orthorhombic structure was exclusively superior both in the propane oxidation activity and in the selectivity to acrylic acid to the other related Mo- and V-based layer oxide catalysts consisting of either pentagonal or hexagonal ring unit. The role of constituent elements was clarified by the comparison of catalytic performance of MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic structure. Mo and V, which were indispensable elements for the structure formation, were found to be responsible for the catalytic activity for propane oxidation. Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively, resulting in a high selectivity to acrylic acid. The introduced Nb occupied the same structural position of V and the resulting catalyst clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to COx was suppressed.