Post-Synthesis And Modification Of Zeolites And Their Applications

Zeolites are an important family of crystalline microporous materials with three-dimensional framework constructed by corner-sharing TO₄ tetrahedrons(T refers to tetrahedrally coordinated framework atoms Si,Al,or other heteroatoms).The different ways of TO₄ tetrahedra connections result in a rich variety of crystalline microporous structures.Zeolites have been widely used as catalysts and catalyst supports in petroleum and chemical industries,as ion-exchangers in detergents,and as sorbents in separation processes.The inexhaustible post-synthetic options to tailor zeolite properties have been and will continue to be indispensable to realize emerging and to improve conventional applications.In this thesis,we focused on optimizing the post-treatment methods of typical zeolite to prepare high-performance catalysts or sorbents and systematically explored the effects of chemical composition,acid sites,pore structure and crystal morphology on the catalytic or adsorption properties.Specially,it was divided into four parts as follows:In the first part,hierarchical core/shell ZSM-5 zeolite was hydrothermally postsythesized in the solution of Na OH and diammonium surfactant via a dissolution-reassembly strategy to solve the problem of low yield in the process of dissolving silicon.The silica and alumina species were firstly dissolved partially from the bulky ZSM-5 crystals and then were in situ reassembled into the MFI-type nanosheets with the structure-directing effect of diammonium surfactant,attaching to the out-surface of ZSM-5 core crystals.The mesopores thus were generated in both the core and shell part,giving rise to a micropore/mesopore composite material.The micropore volume and the acidity of the resultant hybrid were well-preserved during this in situ recrystallization process.Possessing the multiple mesopores and enlarged external surface area,the core/shell ZSM-5 zeolite exhibited higher activity in the ketalation and acetalization reactions involving bulky molecules in comparison to the pristine ZSM-5.In the second part,hydrogen transfer and aromatization and other side reactions are easy to occur on the surface of zeolite in the 1-butene cracking and methanol conversion and other acid catalytic reactions.Therefore,it is necessary to passivate the surface of zeolite to inhibit these side reactions,so as to improve the selectivity of target products.It is an effective strategy to grow all-Si shell on the surface of zeolite.However,the hydrothermal stability of mesoporous shell material grown on the surface of zeolite is poor,and the pure microporous shell is not conducive to the diffusion.In this work,the core/shell structure zeolite with all-Si layered MFI as shell and ZSM-5 as core were successfully prepared with adding silicon source.The nanosheet structure of core/shell ZSM-5 zeolites has abundant mesoporous structure,which has little effect on the diffusion of materials.However,the acid properties of core/shell ZSM-5 zeolites are basically unchanged by NH₃-TPD and pyridine-IR characterization.The core/shell ZSM-5 catalyst after secondary crystallization showed higher propene selectivity in 1-butene cracking and methanol conversion compared with pristine ZSM-5.In the third part,partially removing the framework Al atoms from low-silica aluminosilicates is an old but still challenging subject,because a high framework Al content generally induces lower acid strength and stability as well as fast deactivation.An inappropriate dealumination method may cause structural collapse.The organic structure directing agent(OSDA)-free Beta zeolite is expected to serve as a green solid acid catalyst,but the Al content is needed to decreased to enhance the stability and acid strength before catalytic application.Through a cannibalistic strategy,Al₂(SO₄)₃ was used to extract effectively the framework Al species from Al-rich OSDA-free Beta(Si/Al=4.3)under mild conditions,forming a natroalunite phase.The natroalunite phase was then selectively washed away with diluted Na OH solution,resulting in dealuminated zeolite(De Al Beta)with a pure*BEA phase and higher Si/Al ratios of 7.6–12.The dealuminated Beta zeolites were revealed to possess abundant mesopores while maintaining high crystallinity.The decrease of the total acid amount in De Al Beta zeolites was accompanied by the enhancement of acid strength,resulting in higher selectivity of deeper cracking products and lower deactivation rate in the 1,3,5-triisopropylbenzene cracking reaction.The De Al Beta also showed a high catalytic activity comparable to commercial nanosized Beta in the liquid-phase Friedel–Crafts acylation,which was much higher than the parent Al-rich Beta.In the fourth part,in order to solve the problem that a large number of hydroxyl groups are easily produced in the conventional steam treatment of zeolite Y,and the corresponding hydrophobicity of zeolite Y is poor.We first use aluminum sulfate as the dealumination agent to stabilize low-silica Na-Y zeolite,and the natroalunite phase was completely washed away with diluted Na OH solution to get the pure stabilized Y zeolite.Then,the zeolite is roasted at high temperature and dealuminated with low concentrated acid solution for several times,finally treated with reflux pickling.The hydrothermal stability and hydrophobic properties of the zeolites were investigated in detail.The resulting high silica Y zeolite with high crystallinity and abundant mesopores and high hydrophobicity showed excellent adsorption properties in the adsorption of VOCs.