Propane Dehydrogenation To Propylene In CO₂ Atomsphere On VO_x/Silicalite-1 Catalysts

Propylene is a basic chemical.Propane dehydrogenation(PDH)in CO2 atomsphere(CO2-PDH)to propylene is expected to increase the production of propylene and to contribute to CO2 recycling even its valorization.Although vanadium-based catalysts can effectively enhance propane dehydrogenation,there still exist challenges such as inadequate activity and durability.In this thesis,for raising the conversion and activity durability of CO2-PDH,Silicalite-1(S-1),which can avoid side reactions on acid-catalytically active sites,was selected as a carrier for the active vanadium species,and a series of catalysts were prepared.Suitable loading of VOx and reaction conditions were first investigated.Modification by Cu promotor and ethylenediamine treatment to S-1 surface were further conducted.The effects of VOx loading amount,Cu addition and alkali treatment on physicochemical properties and catalytic performance of the catalysts were studied.The structural properties and structureactivity relationship of the active species were revealed using multiple characterizations.For nVOx/S-1(n=5,10,15 wt%)catalysts prepared by ultrasound-assisted impregnation,10VOx/S-1 showed the highest activity at reaction conditions of C3H8/CO2/Ar=1/1/1.5,W/F=12.5 g·h/mol,550℃and 0.1 MPa,with propane conversion of 46.3%,CO2 conversion of 28.8%,and propylene selectivity of 87.6%,which was higher compared with other supported vanad-based catalysts at similar reaction conditions.The loadings affected the vanadium species and their acidity and basicity.10VOx/S-1 contained main phases of isolated or oligomeric vanadium species,exhibited high weak acid amount and the most moderate basicity sites,derived from isolated tetrahedral coordination VOx species and didontates formed on the V-O pairs sites,respectively,advantageous to adsorption-activation of propane and CO2.When reaction temperature was elevated to 600℃,conversion of propane and CO2 on 10VOx/S1 reached respectively 59.3%and 32.4%.Nevertheless,side reactions as propane cracking and propylene dehydrogenation were promoted at high temperature,either,decreasing propylene selectivity to 79.6%.Modified yCu-10VOx/S-1 with different Cu addition significantly improved the activity.0.5Cu10VOx/S-1(Cu content=0.5 wt%)increased propane conversion by 9.3%and CO2 conversion by 12.1%whereas propylene selectivity remained unchanged,due to the drive of the reverse water gas shift to PDH.Cu addition distinctly changed the acidity-basicity of the catalysts,increasing the weak acid amount and basic site number.Besides,Cu addition promoted the dispersion of vanadium species,with lowered percentage of polymerized vanadium species.It also affected the valence distribution of VOx.The highly dispersed V5+appeared in the catalyst while the percentage of inactive V3+was reduced.After 50 hours over time-on-stream the reaction maintained a high conversion of the feedgas.S-1 modification through ethylenediamine treatment at different temperatures and times increased the silica hydroxyl group on the surface and enhanced the interaction between VOx and S-1,stabilizing VOx.Under unchanged catalytic activity for CO2-PDH,the activity durability was improved.The catalyst using S-1 treated at 170℃ for 12 h showed the lowest deactivation factor of 1.2 h-1.Compared with untreated S-1 catalyst,the deactivation factor dropped by 79%.