| Propylene is one of the most important raw materials in the chemical industry and is mostly used in the production of polymers(e.g.polyethylene and polypropylene),oxygenated compounds(e.g.glycol,acetaldehyde,acetone and propylene oxide),etc.The demand for propylene is also increasing year by year due to the widespread use of propylene downstream chemical products.Steam cracking and fluid catalytic cracking of naphtha,light diesel and other petroleum by-products are common processes to obtain propylene,however,the high energy consumption and low selectivity of these processes for propylene production,coupled with many factors such as decreasing oil reserves and rising oil prices,need to drive the petrochemical industry to find greener and more efficient propylene production processes.In recent years,the discovery and exploitation of shale gas has led to a continuous increase in propane production,and the production of propylene from low-cost propane not only saves production costs,but also meets the huge market demand for propylene products.Among them,the use of CO2 oxidation of propane to dehydrogenate propylene(CO2-ODHP)can improve the equilibrium conversion rate of propane and propylene yield through reverse water gas conversion(RWGS),and can consume CO2 to promote the "double carbon" goal.Cr/Al2O3 catalysts are usually used in CO2-ODHP industry,which are prone to side reactions such as cracking and coking on the strong acid sites on the alumina surface,resulting in low propane conversion and propylene selectivity.Therefore,attempts have been made to use new carrier materials to improve the performance of Cr-based catalysts,for example,mesoporous molecular sieves with large specific surface area and pore volume,which provide higher dispersion of the active component;High hydrothermal stability of microporous molecular sieves,which can improve product selectivity;The close contact of microporous-mesoporous composite molecular sieve can form the kinetic effect of pore continuum,accelerate the diffusion of reactants and reaction products from pores,hinder the secondary reaction of products,and prevent carbon deposition.Therefore,in this thesis,MCM-48 molecular sieve and xCr/MCM-48 catalysts with unique three-dimensional mesoporous network structure were synthesized by hydrothermal method and impregnation method respectively,based on which,MCM-48/ZSM-5(hereinafter referred to as"MZ")composite molecular sieve and 5CrxCo/MZ catalysts with mesoporous pore channel and micropore wall structure were prepared respectively,and their catalytic performance for the dehydrogenation of propylene from CO2 oxidized propane was investigated:1.The 3Cr/Al2O3,3Cr/TiO2,3Cr/CeO2 and 3Cr/MCM-48 catalysts were prepared by the impregnation method,and their activities were evaluated,among which the 3Cr/MCM-48 catalyst had the best performance.Characterization revealed that the surface of MCM-48 contained more silica hydroxyl groups,which led to a higher dispersion of active components,and the large specific surface area and pore volume made it more capable of adsorption and desorption of C3H8 and CO2.Similarly,xCr/MCM-48 catalysts with different Cr loadings(3 wt%,5 wt%,7 wt%)were prepared and evaluated for activity,and it was found that the catalyst with 5 wt%Cr loading had the best activity with 25%conversion of C3H8,24.8%conversion of CO2,and over 80%selectivity of C3H6.After that,the effects of reaction temperature and airflow air velocity on the reaction were investigated,and the optimal conditions for the reaction of 5Cr/MCM-48 catalyst were concluded to be 550℃ and 480 h-1 for GHSV.2.To increase the hydrothermal stability of the carrier,the composite carrier MZ was prepared and characterized to contain MCM-48 type cubic Ia3d structure and ZSM-5 type MFI topology,which effectively improved the hydrothermal stability of the catalyst and reduced the carbon deposition;the large surface area and pore volume,as well as the Lewis acid property,gave it both structural and functional properties.3.To further improve the conversion of C3H8 and CO2,5CrxCo/MZ catalysts with different Co contents(0 wt%,1 wt%,2 wt％,3 wt%)were prepared.It was shown that the best performance of the catalyst loaded with 2 wt%Co at 550℃ and GHSV of 480 h-1 led to an increase in C3H8 conversion from 25%to 37.8%,CO2 conversion from 24.8%to 45%,and a more stable selectivity of C3H6.The characterization revealed that:firstly,the addition of Co increased the dispersion of Cr3+and promoted the production of Cr6+in the active center;secondly,the loading of Co reduced the strong interaction between Cr and the carrier MZ,which led to an increase in its reducibility and also led to an increase in the lattice oxygen concentration;thirdly,Co significantly increased the amount of weak acid in the catalyst and increased the amount of carbon deposition in the catalyst. |
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