Preparation Of Chrome-zinc Oxide Catalyst And A Study Of Its Propane Dehydrogenation Performance

At present,the industrial demand for propylene is increasing year by year,and most propylene is prepared by cracking petroleum products.The efficiency of this propylene production process is low and cannot meet the increasing global consumption demand for propylene.Propane dehydrogenation process is considered to have great potential because of its high atomic economy.Currently,the main active components of catalysts that have successfully realized industrial production are Cr O_xand Pt.In addition,many other additives,such as Ni,Zn,Sn,K,Na and so on.,also show certain potential in enhancing the activity of the components,improving the stability and reducing the cost.Accordingly,in order to enhance its catalytic activity,this thesis introduced metal additives to modify Cr O_y/Al₂O₃and prepared a chrome-zinc oxide catalyst with a good activity performance and stability.The work completed are as following:Firstly,zinc species was introduced into Cr O_y/Al₂O₃ by impregnation to form Cr Zn_xO_y/Al₂O₃,which formed the modified catalyst and was the main research object of this thesis.The reaction performance and physicochemical properties of catalysts were investigated by means of catalytic activity test and various characterization methods.The optimal Zn/Cr mole ratio of the catalyst was determined to be 0.5 by the activity evaluation.The deactivation of the catalyst of the afore mentioned ratio was slower than that without Zn added.The addition of Zn increases the acid sites of the catalyst,and the acid sites of the catalyst are mainly weak acid sites under the optimal Zn/Cr ratio,which is conducive to the reaction.At the same time,the catalytic behavior of different Cr loads under the same Zn/Cr ratio was investigated,and the optimal Cr loading was determined to be 5wt.%.Under this loading,a higher proportion of monomeric and low-polymeric Cr（VI）is formed,and the activity test showed that its catalytic activity is higher.In addition to coke formation,the generation of inactive Cr（III）species was also the reason for the deactivation of catalysts with high loading.The catalyst with the optimal Cr/Zn ratio has an initial conversion rate of about 50%and a selectivity over 93%.Compared with the industrial catalyst,the lower Cr loading reduces the toxicity and environmental burden.Secondly,K,Na and Mg were used to modify the previously obtained catalyst in terms of acidity,and the optimal catalytic group was determined to be1.5K5Cr Zn_0.5O_y/Al₂O₃.K and Na were loaded on the alumina carrier together with the active component.For Mg,the carrier was modified by direct mixing method and loading method,and then the active component was introduced.The results showed that the catalyst deactivation could be significantly reduced by adding proper amount of K and Na.Under the optimal addition of 1.5wt%K,the yield of propylene increased from～20%to～29%after 4 h reaction.On the one hand,the doping of K neutralizes some acidic sites,and promotes the generation of active Cr（VI）to a certain extent.The effect of loading was better than direct mixing for Mg.Under the condition that Al is not excessive,the total amount of Cr（VI）produced on Mg-Al spinel is inhibited.Finally,two modified catalysts,1.5K5Cr Zn_0.5O_y/Al₂O₃ and5Cr Zn_0.5O_y/（Mg O/4A1₂O₃）,were selected to investigate their catalytic behavior under different reaction conditions,and the kinetic equation and activation energy were obtained to investigate their effects in potential practical application.The activity test result under different conditions shows that the 1.5K5Cr Zn_0.5O_y/Al₂O₃ catalyst has better stability than 5Cr Zn_0.5O_y/（Mg O/4A1₂O₃）with temperature and space velocity varying.Hence it was determined this catalyst has the potential to be further improved and used in the propane dehydrogenation process.It can achieve the purpose of maintaining high activity and reducing regeneration frequency.