Promoter Effect On Structure Of Cr-based Catalysts And Dehydrogenation Efficiency Of Isobutane

Nowadays,as the demand of methyl tertiary butyl ether?MTBE?,which used as gasoline additive,keeping increasing,the production limit of its feedstock isobutene makes it necessary to find another way to enhance the output.One effective method is the dehydrogenation of isobutane.The isobutane use in LPG can be applied in this case since the use of LPG in urban gas decreases gradually these years.The dehydrogenation process in fixed-bed reactor has advantages of simple flowsheet,low investment and low energy consumption.Pt-based catalysts and Cr-based catalysts is commonly used in this process and Cr-based catalysts was chosen in our project due to its relatively low price and simple feed gas requirement compared with Pt-based catalysts.The catalyst carrier in this paper was prepared by mixing kneading and the surface acidity was adjusted to improve the catalytic performance.The effects of as precursors of Ba-promoter,the amount of KNO₃ and H₃BO₃ and calcination temperature of carrier were studied on preparention process.Based on the characterization results of XRD,BET,NH₃-TPD and O2-TPO,the optimum promoter loading was found to be 3.0 w% Ba?CH₃COO?2,6.0 w% KNO₃ with 1.5 w% H₃BO₃ and optimum calcination temperature was 500 ?.Surface acidity of the catalyst was decreased effectively when Ba and K was utilized as alkaline promoters.The dispersion of active components,Cr,was improved and the number of active center Cr3+ was increased by adding Ba.Both K and B can promote the selectivity of iso-butylene and reduce the carbon deposit on catalyst surface.Active components were loaded on the carrier by incipient impregnation.Catalysts prepared under different calcination temperatures,CrO₃ and Na2Cr2O7·2H2O loadings were investigated.Characterization data of XRD,BET,NH₃-TPD and O2-TPO is analysed.The optimized preparation conditions is: Na₂Cr₂O₇·2H₂O/ CrO₃=1:3?the total mass of Cr in 6.0 w% CrO₃ loaded on the carrier?and calcination temperature 600 ?.Activity of catalysts is influenced by the load of active components Cr and increased along with Cr loading when Cr was less than 6 w%.However,the stability of catalysts decreased and this can be solved by adding Na2Cr2O7·2H2O.Finally,reaction temperature,feeding space velocity and height-diameter ratio of the reactor were studied in iso-butane dehydration reaction using prepared catalysts.The optimal operation conditions were reaction temperature 560 ?,feeding space velocity 200 h^-1300 h^-1 and height-diameter ratio 4.0.