Vanadium Oxide Based Nanostructured Materials for Catalytic Oxidative Dehydrogenation of Propane: Effect of Heterometallic Centers on the Catalyst Performance

摘要

Catalytic properties of a series of new class of catalysts materials—[Co3(H2O)_(12)V_(18)O_(42) (XO4)].24H2O (VNM-Co), [Fe3(H2O)_(12)V_(18)O_(42)(XO4)].24H2O (VNM-Fe) (X = V, S) and [H6Mn3(H2O)_(12)V_(18)O_(42)(VO4)].30H2O for the oxidative dehydrogenation of propane is studied. The open-framework nanostructures in these novel materials consist of three-dimensional arrays of {V_(18)O_(42)(XO4)} (X = V, S) clusters interconnected by {-O-M-O-} (M = Mn, Fe, Co) linkers. The effect of change in the heterometallic center M (M = Mn, Co, Fe) of the linkers on the catalyst performance was studied. The catalyst material with Co in the linker showed the best performance in terms of propane conversion and selectivity at 350 °C. The material containing Fe was most active but least selective and Mn containing catalyst was least active. The catalysts were characterized by Temperature Programmed Reduction (TPR), BET surface area measurement, Diffuse Reflectance Infrared Fourier Transform Spectroscopy, and X-ray Absorption Spectroscopy. TPR results show that all three catalysts are easily reducible and therefore are active at relatively low temperature. In situ X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) studies revealed that the oxidation state of Co(II) remained unchanged up to 425 °C (even after pretreatment). The reduction of Co(II) into metallic form starts at 425 °C and this process is completed at 600 °C.