Study On The Supported Sn Catalyst For Catalytic Conversion Of Propane To Propene

With the increasing shortage and heaviness of petroleum resources,naphtha steam cracking and catalytic cracking alone have been unable to meet the increasing demand for propylene.Propane catalytic dehydrogenation can convert low value-added propane into high value-added propylene,which is of great significance for improving the comprehensive utilization of oil and gas.Although the commercial Cr-based and Pt-based catalysts show excellent dehydrogenation performance,they still suffer from rapid deactivation due to coke deposition.In addition,Cr O_x is highly toxic to the environment,while Pt is expensive,and its application is limited.Therefore,the development of a new type of low-cost and environmentally friendly alkane dehydrogenation catalyst is the key to further popularizing alkane dehydrogenation technology.Silica-supported metallic tin（Sn/Si O₂）catalyst is proven to be an effective catalyst for propane dehydrogenation.However,the melting point of tin（232 ^oC）is much lower than the reaction temperature required for propane dehydrogenation（>550 ^oC）,The tin will volatilize over the course of PDH,causing irreversible deactivation of the Sn/Si O₂ catalyst.In this paper,the catalytic stability of Sn/Si O₂ catalysts with different loadings in the 50 h long-period propane dehydrogenation reaction was investigated in detail,and XRD,H₂-TPR,BET,and carbon analysis were used to study the deactivation of the catalyst.It was found that in addition to the loss of metallic tin,the blockage of the catalyst pore structure is another important reason for the irreversible deactivation of the Sn/Si O₂ catalyst.Due to the low melting point of tin,it not only has volatility in the high-temperature dehydrogenation reaction,but also has high fluidity.It will continue to migrate into the pores of the catalyst during the reaction,causing the blockage of the pore structure of the catalyst and making the reactant molecules unable to diffuse to the active sites in the pores,causing irreversible deactivation of the catalyst.The inherent properties of metallic tin with a low melting point determine that it is difficult to maintain stable catalytic performance over the course of high-temperature dehydrogenation reactions,but metallic tin is prone to form high-melting alloy phases with various metals.Therefore,in order to further improve the dehydrogenation stability of the catalyst,the second metal element M（M=Cu,Co and Ni）was introduced into the Sn/Si O₂catalytic system.Studies have found that Ni can form an Ni₃Sn₂ alloy phase with a melting point higher than Sn,but the melting point of the alloy phase formed by Cu,Co and Sn is still lower than the reaction temperature required for propane dehydrogenation,and Ni and Sn have a synergistic effect.Sn splits Ni atoms and inhibits the cracking reaction of propane on Ni metal particles.Ni and Sn form a high melting point alloy.The phase stabilizes the metal Sn and inhibits the loss and migration.The catalytic stability of the Ni-Sn alloy in the long-period propane dehydrogenation reaction is better than that of the Sn catalyst alone.Further studies have found that the catalytic activity of Ni₃Sn₂ alloy is related to its dispersibility.The higher the dispersibility,the higher the propane dehydrogenation activity.In recent years,silica-supported isolated metal oxides（Zn O_x,Ga O_x,Fe O_x,Cr O_x）have been shown to have excellent propane dehydrogenation activity,but the propane dehydrogenation performance of silica-supported Sn O_x has not been reported in detail.Therefore,Sn O_x/Si O₂ catalysts with different loadings were prepared by the conventional impregnation method.The study found that when the loading of Sn O₂ was less than 1.7 wt%,the Sn O_x species on the surface of the Si O₂ support existed in an isolated state.Research on propane dehydrogenation performance shows that isolated Sn O_x has excellent stability and regeneration performance.XRD,UV-vis,TPR and XPS indicate that there is a strong interaction between the isolated Sn O_x and the Si O₂ support.This strong interaction inhibits the reduction of Sn O_x to metallic state and hence improves the stability of the catalyst.The Sn-doped Sn-MCM-41 catalyst sample was synthesized by a two-step method.It was found that compared with the traditional impregnation method and hydrothermal synthesis method,under the same Si/Sn molar ratio,the Sn in samples prepared by the two-step method mainly exists in the framework of MCM-41.However,there are also some non-framework nano-Sn O₂ species in the samples prepared by impregnation method and hydrothermal synthesis method.The XPS characterization of the sample after the reaction showed that the framework Sn species can be stabilized in the oxidation state during the reaction,while the non-framework Sn species are reduced to the metal state.In addition,the samples prepared by the two-step method contain the highest acidity and therefore have higher dehydrogenation activity.