Preparation And Characterization Of Propane Dehydrogenation PtSn Nanoalloy Catalysts

Propylene is an important and basic chemical raw material,widely used in the production of high value-added products such as polypropylene,acrylonitrile and propylene epoxide.In recent years,with the growing demand for downstream derivatives,the demand for propylene has risen far beyond that of ethylene,changing the traditional pattern of olefin supply.At the same time,mature shale gas mining technology will further reduce the production cost of propylene and improve the economic benefits of the product.Therefore,the dehydrogenation of propane to propylene has become a hot topic in current research.In this paper,with Na₂PtCl₆·6H₂O and SnCl₄·5H₂O as precursors of Pt and Sn and CH₃COONa·3H₂O as alkali sources,PtSn metal carbonyl compounds were synthesized by metal carbonyl compound method under CO atmosphere.Then high purity,single crystal phase,high activity and stability of PtSn nano-alloy catalyst were prepared by decarbonylation and H₂reduction.The structure-activity relationship of the catalyst was investigated by XRD,BET,TEM,NH₃-TPD and other characterization methods.The correlation between the structural properties of PtSn nano-particles and their catalytic properties was preliminarily established.The catalyst dehydrogenation performance were optimized and the role of the second additives was analyzed purposely.The differences among different carriers were analyzed and compared,and the process conditions of catalyst propane dehydrogenation were optimized.Loading capacity of Pt was investigated,the results showed that the catalyst exhibited good dehydrogenation performance,when the Pt loading was 0.5%,the reaction temperature was580°C,and the conversion and selectivity of PtSn/Al₂O₃ reached 32.58%and 91.29%,respectively.By continuously adjusting the Pt/Sn ratio?molar ratio?,Pt₃Sn and PtSn nanocrystalline alloys with high purity and single crystal phase were synthesized.The support effect on the dehydrogenation performance of PtSn alloy catalysts was investigated by analyzing and comparing four different supports of?-Al₂O₃,ZSM-5,SaPO-34 and SiO₂.Due to the best coordination match between the metal centers and the acid sites on?-Al₂O₃,finally?-Al₂O₃ is determined as the best carrier.The effects of different kinds of additives on the dehydrogenation performance of PtSn/?-Al₂O₃ were analyzed by introducing the second additives Na,Ca,La,In,Ce and Zn,and the modification effects of different loadings of Zn on PtSn/?-Al₂O₃ dehydrogenation performance was mainly studied at high temperature of 600°C.Alkali metal Na and alkaline earth metal Ca have strong basicity,which can neutralize the acidic sites of the support well,reduce the occurrence of side reactions such as cracking and hydrogenolysis,and improve the selectivity of the reaction,the final selectivity of the reaction at a high temperature of 600°C is more than96%.However,the alkalinity of Na is too strong and the main reaction is inhibited greatly.The addition of Ce alters the pore structure of the support,improves the dispersibility of the active component,reduces the number of acidic sites on the surface of the support,further enhances the interaction between the metal and the support,and promotes the coke migration from the metal surface to the support surface.The modification effect of Zn on the catalyst is mainly attributed to the"geometric effect"and"electronic effect"of Zn to Pt,the improvement of the dispersion of Pt,the reduction of the particle size of Pt,the increase of the electron density of Pt,the rapid desorption of propylene,improvement of the reaction selectivity.At the same time,when the loading of Zn is 1.0%,the initial conversion and selectivity of propane reach 39.15%and 94.54%,respectively.The yield of the product reaches 37.02%.Finally,the exploration of propane dehydrogenation reaction conditions was carried out.The different reaction temperature,mass space velocity of propane,hydrogen to hydrocarbon ratio and reaction atmosphere were discussed.The optimal reaction conditions were as follows:the reaction temperature was 580°C,the pressure was atmospheric pressure,the mass space velocity of propane was 4^-1h^-1,propane/hydrogen feed ratio was 4:1.The initial propane conversion and selectivity of propene can reach 32.58%?94.42%respectively.