Preparation Of Silicalite-1@ZSM-5 Core-shell Zeolites For Aromatization Of 1-hexene

Hydrodesulfurization-Octane Recovery Process is the key technology to upgrading fliud catalytic cracking?FCC?gasoline,and producing clean gasoline according with state VI.However,in the procss,there are some problems such as the rise of dry point of FCC gasoline due to alkylation of light aromatic hydrocarbons,which will reduce the yield of FCC gasoline and affect the technology economy.In this thesis,we propose Silicalite-1@ZSM-5 core-shell structure catalyst coupling separation and aromatization which can maximize the yield of aromatic hydrocarbons and virtually eliminate the occurrence of further alkylation of light aromatic hydrocarbons.This core-shell structure catalyst can optimize product distribution and avoid the rise of dry point of FCC gasoline,which is meaningful for the design of high-performance octane recovery catalysts.The nano-sized ZSM-5 with different Si/Al ratio was successfully synthesized by hydrothermal crystallization method.The effects of Si/Al ratio on the aromatization of1-hexene were investigated.The experimental results indicate that with the increase of Si/Al ratio,the yield of C₁-C₄ hydrocarbons increases and the selectivity of C₉⁺aromatic hydrocarbons decreases.The highest yield of aromatic hydrocarbons and BTX is obtained over HZ-5?50?with least coke deposition,because of more amount of B acid site and L acid site.The ZSM-5 zeolite with different Si/Al ratio was used as core.The hydrothermal conditions of core-shell zeolites was optimized.The results show that with fumed silica as silicon source,one day as synthesized time and 2 wt.%as solid-to-liquid ratio,a uniform and tight MFI layer completely overgrown ZSM-5 zeolite,and core-shell zeolite has higher relative crystallinity,intact morphology,and it's shell thickness is about 40 nm.However,due to the migration of aluminum from ZSM-5 core to the surface of the core-shell,the MFI membrane is high-silica ZSM-5 zeolite which adjust the acidity of ZSM-5 zeoites.After covered with high-silica shell,the yield of C₁-C₄ hydrocarbons over zeolite increases,but the yield of aromatic hydrocarbons decreases.The dealumination of the external surface of ZSM-5 zeolites had accomplished by acid leaching in HCl solution which can inhibit the migration of aluminum atom.And a pure-silica shell is covered around ZSM-5 zeolite.The characterization results confirm that a uniform and tight silicalite-1 layer has completely overgrown ZSM-5 zeolite,and the core-shell MFI/MFI zeolite with single crystal structure was synthesized.After coating with silicalite-1 zeolite membrane,the results of 1-hexene aromatization show that the selectivity of C₉⁺aromatic hydrocarbons and coke deposition on Silicalite-1@ZSM-5 core-shell zeolite is greatly decreased.When Si/Al ratio of core is 50,the selectivity of C₉⁺aromatics hydrocarbons decreased from 28 wt.%to 13 wt.%,and the amount of carbon deposition decreased from 7 wt.%to 2 wt.%after coating with silicalite-1 zeolite membrane.It is found that the alkylation side reaction on external surface and coke deposition can effectively suppressed by using pure silica zeolite as shell.And the stability of zeolites is improved.