Study On Chromium Based And Zinc Based Catalysts For Propane Dehydrogenation

Propylene is an important chemical raw material,propylene has broad application prospects.At present,propane direct dehydrogenation technology is considered as one of the most promising methods for propylene production.Among the catalysts for propane dehydrogenation,Pt and Cr based catalysts are mostly studied.In terms of cost and catalyst stability,Cr-based catalysts have more advantages.However,Cr-based catalysts are environmentally unfriendly and produces carbon deposition.Therefore,reducing the content of Cr and improving the ability of resisting carbon deposition are the goals of Cr-based catalysts in propane dehydrogenation in the future.Researchers have found that Zn-based catalysts with low price and no toxicity are active for propane dehydrogenation and have the ability to resist carbon deposition,but the research is still immature and needs further exploration.In this paper,Cr-based catalysts with low Cr content and Zn-based catalysts were studied for direct dehydrogenation of propane.（1）Our research group independently developed low content Cr-based catalyst for direct dehydrogenation of propane.When the particle size of the catalyst is 40～80 mesh and the propane flow rate is 40～100 ml/min,the reaction performance of the catalyst is basically unaffected by internal and external diffusion.Under the conditions of pressure of 0.1 MPa,temperature of 595℃and space velocity of 600 h^-1,the catalyst has the best performance for propane direct dehydrogenation,with propane conversion of 45.9%and propylene selectivity of 80.7%.The catalyst can maintain its catalytic activity for a long time,which is obviously better than that of industrial catalysts,but the problem of carbon deposition exists in the catalyst.（2）In order to solve the problem of carbon deposition,Zn-based catalyst was prepared by equal volume impregnation method for direct dehydrogenation of propane.The effects of five different supports on the catalytic activity were investigated.The results showed that Zn-based catalyst had the best activity with activated carbon as the support.The results of optimization of process conditions show that the 3%Zn/AC catalyst had better catalytic effect under the conditions of pressure of 0.1 MPa,temperature of 565℃and space velocity of 400 h^-1,the conversion rate of propane is 33.0%,and the selectivity of propylene is 75.1%.The effect of Zn loading in the catalyst on the direct dehydrogenation of propane was investigated.The results showed that the optimal Zn loading was 6%with activated carbon as support.The 6%Zn/AC catalyst before and after the reaction was tested by thermogravimetry,and the results showed that the 6%Zn/AC catalyst had the ability to resist carbon deposition.（3）ZIF-8-based highly dispersed Zn-N-C catalyst was prepared for direct dehydrogenation of propane.The effects of the different reagent molar ratios of ZIF-8 and the different carbonization temperatures of ZIF-8 on catalytic activity were investigated.According to the catalytic performance,the optimum molar ratio of the reagent is Zn（NO₃）₂·6H₂O:2-m Im:PVP=4:32:4,and the best carbonization temperature is 1000℃.Under the conditions of pressure of 0.1 MPa,temperature of 595℃and space velocity of600 h^-1,the catalyst has the best performance,with propane conversion of 34.4%and propylene selectivity of 74.1%.ZIF-8-based highly dispersed Zn-N-C catalyst and Zn/AC catalyst with the same Zn content were used to test their lifetime.According to the test results,the catalytic activity of the former is better than that of the latter after 30 h reaction.With the same reaction time,the maximum difference of propane conversion can reach9.80%.This shows that ZIF-8-based highly dispersed Zn-N-C catalyst has certain advantages in direct dehydrogenation of propane.