Study On The Catalysts For Oxidative Dehydrogenation Of Propane To Propene

Propene is a very important raw material in chemical industry,and it has been widely used for the production of many chemical intermediates.The direct dehydrogenation of propane to propene has already achieved industrialization.However,this process is restricted by the reaction equilibrium and fast deactivation of the catalysts.In theory,the oxidative dehydrogenation of propane?ODP?to propene can solve these problems.However,the introduction of O2 leads to deep oxidation.It is very hard to obtain a high propene selectivity at a relatively high propane conversion.In order to address this issue,two approaches were proposed.One approach was to substitute CO2 for O2.CO2 was believed to be a mild oxidant and a greenhouse gas.The use of CO2 as oxidant could not only avoid deep oxidation,but provided an effective way to utilize the CO2 resource.The other approach was to introduce H2 to the O2-containing system to control the deep oxidation.This dissertation systematically studied the molybdenum-based catalysts in the ODP reaction using CO2 as oxidant.A series of characterization methods,such as XRD,BET,FT-IR,H2-TPR,NH3-TPD,CO2-TPO-MS and XPS,were employed to explore the role of CO2 and the reaction mechanism.Additionally,a primary research on platinum-based catalysts in the ODP reaction with O2 as oxidant was carried out.Support,loading amount,and reaction conditions could affect the catalytic performance significantly.Al₂O₃-supported catalyst showed the best catalytic acitivity,which was ascribed to the acidity of catalysts.The loading amount of Mo/Al₂O₃ could change the state of Mo species and catalyst acidity.20Mo/Al₂O₃ exhibited the best catalytic performance,and the propane conversion and propene yield could reach 34.8% and 24.7%,respectively.The optimal reaction conditions were as follows: reaction temperature = 600°C,GHSV = 900h-1-1200h-1,C3H8/CO2 = 1/1?molar ratio?.Mo/Al₂O₃ catalysts displayed completely different catalytic performance under N2 and CO2 atmospheres.When the reaction was conducted in N2,an induction period could be observed at the initial stage,during which the catalyst was reduced and some acitive Mo species?probably Mo2+?accounting for propene hydrogenolyis reaction were formed,leading to the production of a large amount of methane and ethane.In contrast,the dehydrogenation reaction was predominant in the whole reaction when it was performed in CO2.Although CO2 could not inhibit the reduction of Mo6+ and the formation of active Mo species for propene hydrogenolysis reaction,it could promote the propene desorption.Thus,the secondary reaction of propene was suppressed effectively.The Lewis acid on Al₂O₃ and the Mo species could exhibit good synergetic effect during the reaction,as reflected by the high activity of the physical mixture of MoO₃ and Al₂O₃.The Lewis acid on Al₂O₃ could help to activate the C-H bond of propane and form highly active intermediate species,which could participate in subsequent reactions.The introduction of H2 could dramatically improve the catalytic performance of platinum-based catalysts with O2 as oxidant.For instance,the propene yield and selectivity were still as high as 33.0% and 91.9% respectively even after 11 h on stream over Pt-Sn/MgAl2O4 catalyst,implying that the catalyst has excellent catalytic activity and stability.Hence,this catalytic system has a promising perspective in the industrial application.