Structural Control And Performance Of VO_x/Molecular Sieve Catalysts For Propane Dehydrogenation

Propylene is the basic raw material of three major synthetic materials.In recent years,the traditional propylene production method can’t meet the rapid growth of demand for propylene at home and abroad.At the same time,the shale gas revolution has provided abundant and cheap propane,making propane dehydrogenation（PDH）an important technical means to increase propylene production.Compared with Pt-based catalysts and Cr O_x-based catalysts,supported VO_x base catalysts have cost advantages,but they still face the prominent problems of low activity and poor stability when applied to PDH.In this study,the VO_x molecular structure on the molecular sieve surface and its catalytic performance of propane dehydrogenation were modified by incorporating Zr element in the molecular sieve framework and pore structure.The specific research results are as follows:（1）The effect of Zr incorporating on the structure of VO_x/MCM-41 molecular sieve catalyst and propane dehydrogenation performance was investigated.It was found that when the Si/Zr ratio was greater than 25,Zr was dispersed in the molecular sieve framework in the form of tetrad coordination,and the presence of Zr in the framework significantly improved the support acidity and surface hydroxyl activity of Zr-MCM-41molecular sieve.Further study showed that Zr atoms in MCM-41 framework could improve the catalyst stability by increasing the dispersion state of VO_x species on the surface,and improve the acidity and oxidation reduction of VO_x species,thus affecting the PDH performance.The experimental and theoretical results show that the structure Zr atoms and surface VO_xspeciation of V-O-Zr active center bridge oxygen atoms have stronger electronegativity than V-O-Si,and thus have higher PDH dehydrogenation activity.In addition,it was found that CO₂ further increased the catalyst activity by dissociating from the active center to produce more active lattice oxygen.（2）The effects of preparation conditions of V/ZSM-5 catalytic system on the molecular structure of VO_x and its catalytic performance of propane dehydrogenation were investigated.In situ Raman and Uv-vis studies showed that the dispersion of VO_xspecies on the surface of V/ZSM-5 catalyst increased with the increase of calcination temperature（550-600℃）.The calcination temperature can induce the migration and dispersion of VO_x on the ZSM-5 molecular sieve surface,and also affect the activity and stability of the catalyst.With the increase of vanadium loading,highly polymerized VO_xspecies and V₂O₅ crystals were formed on the catalyst surface.It was found that the intrinsic activity and stability of V/ZSM-5 catalyst decreased with the increase of vanadium loading due to the high polymerization of surface VO_x species to form V₂O₅crystals.When the vanadium loading was 0.75 wt%,the catalyst showed high activity(TOF=3.01×10^-3(S^-1))and stability（10-hour deactivation rate was 6.23%）.（3）The effect of pore structure on VO_x species on molecular sieve surface and catalytic dehydrogenation of propane was studied.By adding silane reagent（PHAPTMS）into the hydrothermal synthesis reaction,ZSM-5 molecular sieve was silylated to obtain multistage porous ZSM-5 molecular sieve support with mesoporous ratio up to 44%.When the vanadium loading was 1 wt%,the dehydrogenation activity of hierarchical V/ZSM-5 catalyst was twice that of conventional V/ZSM-5 catalyst,and the deactivation rate of the catalyst was about 7%in 4 h（conventional V/ZSM-5 catalyst was 11%）.BET,Raman,Uv-vis and TEM results show that hierarchical ZSM-5molecular sieve supports with larger specific surface area and mesoporous ratio can greatly improve the dispersion degree of VO_x species,and make them exist in the form of isolated tetrad coordination on the surface,thus improving the dehydrogenation activity and stability.