Porous Structure And Acidity Of Reconstructed USY Zeolite By Hydrothermal Post-synthesis

Y zeolite is widely used in catalytic cracking,hydrocracking and alkylation processes due to its advantages over amorphous silicon aluminum,high catalytic activity,easy adjustment of acid energy,good ring opening ability and selectivity.However,The Y zeolite synthesized by one-step hydrothermal synthesis contains rich microporous structure and poor mesoporous performance,which limits the diffusion of heavy oil reactants in the pores in terms of physical structure,prevents some macromolecules from entering the pores,makes some macromolecular reactants unable to enter the catalytic reaction center,and even stops the reaction in extreme cases.Therefore,the directly synthesized Y zeolite(silicon aluminum ratio of 2.5)needs steam treatment,acid washing and alkali washing to improve its hydrothermal stability and obtain mesoporous.However,the treatment industry caused a large amount of framework aluminum loss,and the specific surface area of micropores decreased in varying degrees,resulting in a great waste.The essence of the decline of microporous specific surface is the destruction of zeolite framework structure,the removal of framework aluminum atoms and the reduction of Br(?)nsted Acid Sites(BAS),which is not conducive to the demand for zeolite molecular sieves in catalytic cracking reaction.In this thesis,different types of recrystallized Y zeolite were obtained by adjusting the crystallization conditions with sodium hydroxide as the alkali source and CTABr as the mesoporous template.The phase analysis was carried out by XRD and nitrogen adsorption to investigate the crystallization law under different alkali sources,time,temperature and alkalinity.On the basis of certain crystallization law,the mechanism of mesoporous formation of recrystallized Y zeolite under two alkalinity was discussed,and the effects of hydrothermal stability of recrystallized Y zeolite and mesoporous formation mode on the number and distribution of acid centers were investigated.Low silicon multistage pore USY was prepared by changing the crystallization conditions,and its crystallization regularity was summarized.The results showed that the skeleton structure of zeolite was completely destroyed and became amorphous with the introduction of K~+.When Na OH is used as alkali source,the relative crystallinity increases with the increase of temperature.The optimum time and temperature of crystallization are 24h and 100℃,respectively.Skeleton dissolution will occur during recrystallization at room temperature,and the relative crystallinity will decrease significantly.The turning point of the change curve of silicon aluminum ratio occurs when the concentration of Na OH is 0.45M.When it is less than this concentration,the decline rate of silicon aluminum ratio is 20/mol.Above this concentration,the decrease rate of silicon aluminum ratio is 4.9/mol.The pore structure and acid energy of Y zeolite with 0.15m and 0.45M crystallization alkalinity were further explored.The results showed that the amount of weak acid and strong acid increased significantly after 0.45M crystallization,the amount of weak and medium strong B acid increased significantly,the amount of L acid increased significantly,the outer specific surface increased by 250%,and the mesopore volume increased by 150%.In addition,recrystallized Y zeolite has good thermal stability.When treated under boiling condition,its relative crystallinity,specific surface area and pore volume have not changed significantly.