Preparation Of VO_x/Al₂O₃ And Its Surface Vanadium Species And Propane Oxidative Dehydrogenation Reaction Performance

Revealing the structure-function relationship between surface vanadia species(VOx)of vanadium-based catalyst and oxidative dehydrogenation of propane(ODHP)reaction is significant for rational design and development of industrial catalyst with high performance,which is still,however,a challenging undertaking.In general,catalysts prepared via traditional method form a complicated surface that is composed of isolated,oligomeric,highly polymerized and V2O5 crystallites,resulting in a poor understanding in structure-function relationship between surface vanadia species and catalytic performance.In this thesis,VO,/Al2O3 catalysts were prepared via pyrolyzing metal-organic frameworks(MIL-47)and incipient wetness impregnation methods,respectively.Specifically,the effects of precursor pyrolysis temperature and catalyst calcination temperature on vanadia distribution and status were investigated.Moreover,the structure-function relationship between surface vanadia species and catalytic performance was established.The details are follows:(1)MIL-47 as precursor,VO,/Al2O3 catalysts with different vanadium loading(1.2-14.0 wt.%)were prepared via solid grinding and pyrolysis.Combining catalyst characterization with evaluation results,the correlation of surface vanadia species and catalytic performance indicates that V2O5 crystallites are directly formed via migration and aggregation of V-O-clusters under pyrolysis temperature of 500 ?,without formation of monolayer coverage on catalyst surface.In the case of 600 ?,the fusion and spontaneous dispersion of V2O5 crystallites over support surface results in formation of isolated and oligomeric vanadia species,and then V2O5 crystallites are reformed until monolayer coverage is done.Moreover,it is observed that those isolated and oligomeric vanadia species,as well as V2O5 microcrystal formed directly on support surface show higher activity(TOF=0.018 s'1)and propylene selectivity,whereas highly polymerized vanadia and V2O5 crystallites formed after monolayer cover show lower activity and propylene selectivity.(2)VOx/Al2O3 catalysts with different vanadium loading(1.2-14.0 wt.%)were prepared via incipient wetness impregnation.Combining catalyst characterization with evaluation results,the correlation of surface vanadia species and catalytic performance points out that the threshold value for formation of V2O5 crystallites is related to calcination temperature.Specifically,high temperature(600 ?)is more favorable to formation of V2O5 crystallites than low temperature(500 ?).The evolution of vanadia species on support surface follows from isolated,oligomeric,highly polymerized,to monolayer coverage,and to formation of three-dimensional V2O5 crystallites.Moreover,those isolated and oligomeric vanadia species show a comparable activity(TOF=0.017 s-1).As increasing surface vanadium density,TOF and propylene selectivity decreases,whereas selectivity of combustion products(COx)increases.