Synthesis,Mechanism And Catalytic Application Of Single-Crystalline Hierarchicalyl-Porous MFI Zeolites With Inter-connected Mesoporous Channels

As an important catalytic material,molecular sieves are widely used in the synthesis of petrochemicals and fine chemicals,and they act as important catalysts or catalytic carriers in many large-scale industrial catalysis.However,as a microporous material with a pore size of less than 2 nm,it is quite easy for molecular sieves to cause diffusion restriction during the catalysis;and a large number of active centers inside the molecular sieves framework cannot participate in catalysis,causing the waste of active centers;and molecular sieves are easy to cause the inactivation of coke deposits because of their long and narrow pores,and the too small pore size can not achieve macromolecular catalysis.All these reasons make the molecular sieves unable to be further applied.Hierarchical pore molecular sieve is a molecular sieve material with a dual-modal pore size distribution.It is composed of molecular sieve micropores and secondary pores（mesopores or macropores）.The introduction of secondary pores has greatly improved the performance defects of molecular sieves,not only the diffusion limit and the frequency of coke deposition.The first part introduces the synthesis of monocrystalline hierarchical structure titanium silicon molecular sieve,by introducing carbon dioxide to convert the molecular sieve raw material into a solid hydrogel,and then high-temperature crystallization,through this solid phase conversion method to synthesize hierarchical structure TS-1-CO_2.Compared catalysis with the catalytic results of ordinary molecular sieves,it reflects the influence of the introduction of secondary pores on the catalytic results.Through XRD,SEM,TEM,FTIR and other means to characterize the various time periods of the crystallization process,the mechanism of the formation of the multi-level structure is summarized.In the second part,the solid-state conversion method was extended to the synthesis of ZSM-5 molecular sieve,and the multi-stage MFI structure aluminum-silica molecular sieve Z-3 was successfully synthesized,and samples with different carbon dioxide input amounts were compared,summarizing the impact of carbon dioxide input on hierarchical structure.The hierarchical structure MFI molecular sieve Z-3 is applied to the polyethylene degradation.In the catalytic degradation of high-density polyethylene（HDPE）and low-density polyethylene（LDPE）,the liquid yield has reached the world leading level.The third part adopts a post-processing method to selectively etch the inside of each smaller nanocrystal based on the TS-1-CO₂ matrix by using a hollow cutting strategy to form hollow nanocrystals,thereby forming a short microporous channel.The dual-mode mesoporous hierarchical zeolite forms large cavities inside the molecular sieve,and there are penetrating pores between the nanocrystals,that is,the dual-mode mesoporous hierarchical zeolite.This zeolite has a shorter pore channel,which is conducive to the diffusion of the reactants during the catalysis,making the reactants easier to contact with the active sites,improving the catalytic efficiency,and the1-hexene conversion rate is the highest among the currently reported materials.