Study On The Active Sites Of Supported Vanadium-Based Catalysts In Propane Oxidative Dehydrogenation

Oxidative dehydrogenation of light alkanes to olefins is an important way of using natural gas. Metal oxide catalysts show a good catalytic property in the reaction, especially vanadium-based catalyst. Different species can be formed on the supported vanadium catalyst, such as isolated, polymeric or crystalline structure. Introducing the second metal can help to form new surface species, which may play different roles in the reaction.Raman spectroscopy is an important tool to characterize the structure of metal oxide catalysts. Visible laser is used as excitation source in the conventional Raman spectroscopy. UV Raman spectroscopy is a new progress in the spectrum characterization. Previous studies suggest that some innovational results were obtained from UV Raman due to fluorescence free and resonance effect.In this paper Visible and UV Raman spectrometer, combined with other means of characterization, were used to inspect the structure of sole and binary vanadium-based catalysts, which are controlled by the preparation methods. When the loading is 0.5-15 wt % two-dimensional polymerized vanadium species are formed on the V₂O₅/γ-Al₂O₃ catalyst, and the crystalline V₂O₅ appears when the loading reaches 20 wt %. Two-dimensional polymerized vanadium species are propitious to the selectivity of alkane ODH. The molybdenum and vanadium species on the V₂O₅-MoO₃ catalyst form a polymeric species in the sub monolayer region. When the loading reaches 30%wt, crystalline Al₂(MoO₄)₃, AIVMoO₇,AIV₃O₉, MoO₃ and V₂O₅ appear on the surface, depending on different vanadium molybdenum ratio. Through the study of situ raman and TPR, it was found that crystalline speciecs are propitious to the selectivity of alkane ODH. The AIVMoO₇ is easier to be reduced than Al₂(MoO₄)₃.V-O-Mo bond is easier to lose lattice oxygen and harder to be replenished than Mo=O bond. Adding Mg changes the surface structure of the catalyst. Mg₂V₂O₇ is formed with increasing the Mg amount. The appearance of Mg₂V₂O₇ changes the redox properties of the catalyst, thus affacting the catalytic properties for ODH reaction.