Nb−V Mixed Oxide with a Random Assembly of Pentagonal Units: A Catalyst for Oxidative Dehydrogenation of Ethane and Propane

Abstract

AbstractHigh‐dimensionally structured (HDS) mixed oxides of vanadium with metals (M) (e. g., Nb, Mo, and W; denoted as HDS‐MVO) were constructed by {M6O21}12− pentagonal units and {MO6} (M=Nb, Mo, W, or V) octahedra as linkers. The materials were synthesized using a hydrothermal method and rod‐shaped solids. The random assembly of the pentagonal units and octahedra in the cross‐sectional plane of the rods facilitated the formation of micropore channels along the long axis of the rods. Micropore formation was directly observed in the cross‐section by HAADF‐STEM. These structural features are common to HDS‐NbVO, HDS‐MoVO, and HDS‐WVO. The catalytic activity of these three HDS‐MVOs with V/Mo ratios in the range 0.35–0.39 was tested for the oxidative dehydrogenation of ethane and propane. The reaction rates per surface area for ethane oxidation and propane oxidation over the HDS‐MoVO and HDS‐WVO catalysts were comparable, whereas the HDS‐NbVO catalyst showed an appreciable difference between the two reaction rates. Both HDS‐MoVO and HDS‐WVO exhibited higher selectivity for olefin formation during ethane oxidation than propane oxidation. Interestingly, the olefin selectivity over the HDS‐NbVO catalyst was found to be almost independent of the alkane substrate. These catalytic features were discussed on the basis of V−O−V or V−O−Mo redox coupling and pore structure effects in HDS‐MoVO and HDS‐WVO and also of isolated and valence stable surface V in HDS‐NbVO.