Structural Control And Performance Of Vanadiumbased Oxide Catalysts For Propane Dehydrogenation

In recent years,the demand for propylene in the world continues to increase,and gradually exceeds the traditional production capacity.At the same time,due to the exploration of shale gas,the production of propane is increasing year by year.In this situation,the technology of propane dehydrogenation to propylene has been widely concerned by academic researchers.Catalysts are the core of propane direct dehydrogenation technology development.At present,the catalysts commonly used in industrialization are Pt based catalysts and Cr based catalysts.The former is expensive and difficult to be regenerated,while the latter has serious environmental pollution,which limits the development of propane dehydrogenation technology.Compared with other transition metal oxides,supported vanadium catalysts have higher activity and selectivity,and have important development prospects.The activity of supported vanadium catalyst is greatly affected by the carrier.Starting from the oxide carrier,this study studied the influence of the carrier on the activity and stability of vanadium oxide propane dehydrogenation.The main research results are as follows:（1）The effects of calcination temperature on the structure and properties of VO_x/Al₂O₃ catalysts were revealed.By adjusting the calcination temperature of VO_x/Al₂O₃ catalyst,it is found that the addition of VO_x species can induce the local structure rearrangement of alumina at a relatively low temperature,and promote the transformation of alumina carrier from amorphousδ-Al₂O₃ toθ-Al₂O₃;DFT calculation found that the chemical state of the active V-O site in the catalyst could be regulated by changing the coordination environment of the ligand Al³⁺,that is,the octahedral Al³⁺is more conducive to enhancing the C-H bond breaking ability of the V-O site.The Al³⁺of octahedral coordination increases with the transformation of alumina from amorphousδ-Al₂O₃ toθ-Al₂O₃;Catalysts with highly dispersed vanadium species can reduce the formation of surface coke and appropriately improve the catalyst performance.The calcination temperature can improve the dispersion of vanadium species on the alumina support.（2）The influence of ZrO₂/SiO₂ mass ratio on the surface structure of ZrO₂-SiO₂support and the difference of dehydrogenation performance of VO_x/ZrO₂-SiO₂ catalyst in Ar and CO₂ environment were revealed.ZrO₂-SiO₂ support was prepared by sol-gel method,and VO_x/ZrO₂-SiO₂ catalyst was prepared by equal impregnation method.By adjusting the content of ZrO₂ in ZrO₂-SiO₂ material to control the mass ratio of ZrO₂ to SiO₂,it is found that when the mass ratio of ZrO₂ to SiO₂ is relatively low,ZrO₂ can be highly dispersed on the surface of SiO₂,and the presence of SiO₂ can inhibit the transformation of tetragonal ZrO₂ to monocline.The reduction degree of vanadium species on VO_x/ZrO₂-SiO₂ catalyst can be regulated by changing the mass ratio of ZrO₂/SiO₂ and vanadium loading capacity in VO_x/ZrO₂-SiO₂ catalyst.It was observed that CO₂ could improve the dehydrogenation activity of VO_x/ZrO₂-SiO₂ catalysts with low carrier acid content and high VO_x dispersion.By controlling ZrO₂/SiO₂ mass ratio and VO_x species loading in the catalyst,the acid content and VO_x species dispersion on the catalyst surface could be regulated.（3）The effect of Al/Zr ion ratio on the structure of VO_x/Al₂O₃-ZrO₂ catalyst and the dehydrogenation performance of VO_x/Al₂O₃-ZrO₂ catalyst in Ar and H₂environments was revealed.Al₂O₃-ZrO₂ carrier was prepared by sol-gel method and VO_x/Al₂O₃-ZrO₂ catalyst was prepared by equal amount impregnation method,the Al/Zr ion ratio is controlled by controlling Zr ion content in Al₂O₃-ZrO₂.Under certain V surface density,the interaction between VO_x species and the carrier and the degree of VO_x species polymerization can be regulated by changing Al/Zr ions in VO_x/Al₂O₃-ZrO₂catalyst and calcination temperature of the carrier.Deactivation of VO_x/Al₂O₃-ZrO₂catalysts is related to the surface active sites covered by carbon deposition.The coking rate of VO_x catalyst with high degree of polymerization was faster than that of VO_xcatalyst with high dispersion.The introduction of a small amount of H₂ in the dehydrogenation reaction can inhibit the generation of coke on the catalyst surface,which is often reflected on catalysts with a fast coking rate.