Catalytic Cracking Performance And Deactivation Of Low-carbon Olefins On SAPO Molecular Sieves With Small Pore

Propylene is one of the most important basic raw materials in petrochemical engineering.An increasing demand for propylene bring about a huge gap between its production and demand with the rapid economic development of China.Therefore,the development of high propylene-selective catalyst has become a research hotspot.The small pore SAPO molecular sieve catalyst has attracted extensive attention due to its many advantages.In this paper,SAPO molecular sieves with different silicon contents,including SAPO-34（CHA）,SAPO-18（AEI）and SAPO-34/18（CHA/AEI）intergrowth zeolite,were synthesized and characterized by XRD,NH3-TPD,BET and SEM.Three series of 8-rings SAPO molecular sieves were used to evaluate the catalytic performance of low carbon olefins with different carbon atomic number such as 1-butene,1-hexene and 1-octene on fixed-bed reactor.It can be inferred that the addition amount of silicon is directly related to the acid amount of SAPO molecular sieve.An appropriate addition in silicon content can increase the acid amount of SAPO molecular sieve,but excessive acid amount will lead to side reactions such as hydrogen transfer and coke which lead to the reduction of propylene’s selectivity and yield.Different reaction temperature and space velocity were examined for the SAPO-34/18intergrowth molecular sieve to catalyze three different low carbon olefin 1-butene,1-hexene,and1-octene.The results show that the intergrowth molecular sieves SAPO-34/18 catalytic 1-butene cracking of the optimum reaction temperature and reaction speed is:2.5 h^-11 520℃,1;1-hexene catalyzed cracking for optimum reaction temperature and reaction speed:500℃,2.5 h^-1;1-octene catalytic cracking for optimum reaction temperature and reaction speed:500℃,2.5 h^-1.In order to simplify calculation and analysis,the spatial structure of SAPO-18 was simulated by Materials studio molecular simulation software,which can form a simplified model of molecular sieve.The model can be used to investigate the reaction mechanism of catalytic cracking of1-butene,1-hexene and 1-octene.It is show that in the catalytic cracking of 1-butene,the basic cracking mechanism is followed by the key reaction step of olefin dimerization,isomerization and beta-site cracking.Specific cage structure in 8-membered ring SAPO molecular sieve is conducive to the generation of the special morphology C₈⁺carbocation and improve propylene selectivity as well as yield.Due to the high carbon atom number,direct crack preferred to be the main mechanism of 1-hexene and 1-octene.1-hexene breaks from the central carbon chain due to its special six carbon atom number,which further improves the yield and selectivity of propylene.However,1-octene is prone to pore adsorption catalysis due to its long carbon chain,which leads to an increasing yield of isomeric olefin in the reaction products.Addition,analysis instrument XRD,TGA and BET were used to compare the three series of SAPO molecular sieve SAPO-34,SAPO-18,SAPO-34/18 catalyst deactivation after catalyze1-butene cracking reaction and the situation of the coking.Compared to SAPO-34 and SAPO-18,the least amount of coke was produced when SAPO-34/18 intergrowth molecular sieve is used as the catalyst of catalyze 1-butene cracking reaction,smallest variation of XRD diffraction peak intensity and BET specific surface area and micropore volume can be detected after reaction.Therefore,SAPO-34/18 intergrowth molecular sieve has the best catalytic activity stability and relatively best coke deposition control ability in the catalytic cracking of 1-butene.Finally,SAPO-34/18 intergrowth molecular sieve was characterized and analyzed by TGA,BET,XRD after catalyze 1-hexene and 1-octene decomposition reaction.The results proves that1-octene is slightly better than 1-butene on the control of carbon deposition and catalytic activity stability,and 1-hexene showed excellent ability of carbon deposition control level and stability of the catalytic activity which provide the highest research value and the research prospect.