Catalytic Cracking Performance Of Different Olefins On SAPO-18 Molecular Sieves

As the raw material,propylene has an indispensable position in petrochemical enterprises.With the rapid development of industry,China has become the propylene production and consumption power in Asia and the world.However,the great gap between propylene supply and demand become obvious gradually which lead to the wild attention of the development and application of technology to produce propylene through the FCC procedure.ZSM-5(MFI)which exhibits good cracking activity is widely applied to the production of propylene in the FCC process.However,the selectivity of polycyclic aromatic hydrocarbons is high on ZSM-5 zeolite which leads to an unsatisfied propylene selectivity.It is obviously significant to search for a type of catalyst with high propylene selectivity for butene catalytic cracking process.SAPO-18 with different contents of Si were synthesized and characterized by XRD,SEM and NH3-TPD.Catalytic performance of 1-butene cracking reaction on H-ZSM-5(MFI),H-Beta(*BEA)and SAPO-18(AEI)which have different forms of structure has been investigated in a fixed-bed microreactor.It was found that the structure and acidity of molecular sieves play key roles in the results of 1-butene catalytic cracking process.These results showed that the high acidity of catalysts improved the 1-butene conversion but leaded to the decrease of the selectivity of propylene owing to the side reactions(such as hydrogen transfer,aromatization)which derived from additional acid sites.What is more,the pore dimension and channel structure determined the available maximum selectivity of propylene.SAPO-18-0.8 with eight-ring pore and pear-shaped cage achieved satisfactory propylene selectivity which approached 59.1 % with a 60.7 % conversion of 1-butene under the TOS of20 min condition.Meanwhlie,SAPO-18 has been investigated for 1-hexene and 1-octene cracking reaction in a fixed-bed microreactor to compare the catalytic product distribution of 1-butene,1-hexene and 1-octene.It showed that the length of carbon chain of olefins has significantly affect on catalytic cracking performance of SAPO-18 molecular sieve.1-butene and 1-hexene are easily diffuse into the cage of SAPO-18 molecular sieve to conduct cracking reaction.However,1-octene was strongly absorbed on the 8-ring orifice which caused the diffusion rate dropped significantly.The carbon deposition of catalysts were analysed by TG and the results indicated that the cracking ways of different olefins had great influence on the content of carbon deposition.The unimolecule cracking way adopted by SAPO-18 on 1-hexene and 1-octene catalytic cracking reaction can produce small molecules which diffuse more easily,causing the decrease of the probability for generating the carbon precursor,so the carbon deposition were less relatively in 1-hexene and 1-octene cracking reaction.The dimerization of dimolecular improved the propylene selectivity of 1-butene catalytic cracking reaction on SAPO-18.However,the large aromatic hydrocarbons produced in the cracking process cannot diffuse out from the SAPO-18 molecular sieve cage,leading to an increase of the coke amount.Last,the effect of reaction temperature and reaction space velocity on the catalytic properties of SAPO-18 in 1-butene,1-hexene and 1-octene catalytic cracking reactions were investigated.The results showed that SAPO-18 exhibited better catalytic performance when the reaction temperature was 500℃ and the reaction space velocity of 1-butene was 3.5h-1.And when the reaction temperature was 500℃,the ratio of 1-hexene and mixed gas was 0.7:1with a 20ml/min flow rate,SAPO-18 showed good catalytic performance on 1-hexene catalytic cracking reaction and SAPO-18 exhibited high catalytic property on 1-octene catalytic cracking reaction when the ratio of 1-octene and mixed gas was 0.5:1.