Synthesis And Catalytic Performance Of Hierarchical TS-1 And Beta Zeolites

Zeolites with tunable acidity,large surface area,high thermal/hydrothermal stability,and unique pore structures,are widely used in adsorption/separation,ion exchange,catalysis,and some emerging fields.Among them,TS-1 zeolites(MFI topology)with three-dimensional 10-membered ring channels and Beta zeolites(*BEA topology)with three-dimensional 12-membered ring channels are two types important industrial catalysts.However,the traditional zeolite catalysts are limited to the size of the micropores(<2 nm),which make the mass transfer process affected by the kinetics effect,usually causing side reactions or carbon deposition.And carbon deposition may cover the active sites of the zeolite catalysts or block the micropores of zeolites,resulting in the deactivation of zeolite catalysts.Due to the micropore limitation,many inside active sites of zeolites are not accessible,only part of the zeolite outer shell is used,which seriously influencs the catalytic performance.The construction of highly active zeolite catalysts is crucial solution to improve catalytic performance.To obtain highly active zeolite catalysts,on the one hand,we can introduce highly active sites into zeolites,which could affect the reaction path to enhance the catalytic efficiency of zeolites.On the other hand,it is essential to improve the mass transfer efficiency by introducing the hierarchical structures into zeolite frameworks or synthesizing nanosized zeolites,accelerating the molecular diffusion in catalysts to enhance the catalytic performance.In this thesis,we focus on synthesizing highly active zeolite catalysts by modulating the growth kinetics of zeolites.The TS-1 zeolite catalysts with highly active six-coordinated titanium species were synthesized by L-lysine assisted method coupled with two-step crystallization strategy.The single-crystalline hierarchical Beta zeolite catalysts with mulberry-like morphology were synthesized by the mesoporogen-free strategy.Sn-Beta zeolites with enriched framework Sn species were prepared by post-treatment method.1-Hexene oxidation reaction was employed to evaluate the catalytic performance of as-synthesized TS-1 zeolites.We evaluated the catalytic performance of Beta zeolites with cracking reaction of 1,3,5-trimethylbenzene,ethylation reaction of benzene,and esterification of lactic acid to produce lactide.The Baeyer-Villiger(BV)oxidation reaction of cyclohexanone was employed to evaluate the as-prepared Sn-Beta zeolites.The main results are as follows:1.In the synthesis of TS-1 zeolites,the introduction of amino acids is beneficial to forming six-coordinated titanium species.By exploring the TS-1 zeolite evolution process,the formation mechanism of six-coordinated titanium species is proposed.L-Lysine possesses functional groups,such as amino and carboxyl,which may interact with Ti species to form complex.Under TS-1 zeolite crystallization process,L-Lysine could protect Ti species,slowing down the generation of four-coordinated Ti species,avoiding the formation of anatase,and favoring the formation of six-coordinated Ti species.The six-coordinated Ti species are less restricted by the TS-1 zeolite framework.In the system of hydrogen peroxide as the oxidant,the six-coordinated Ti species are easier to form active transition-state intermediates with hydrogen peroxide.This transition-state active intermediate with less steric hindrance exhibits a higher catalytic performance in the epoxidation of 1-hexene.The as-synthesized TS-1 zeolites afford an improved 1-hexene conversion compared to conventional counterparts(33%vs23%).After five cycles,the catalysts still maintain highly catalytic activity,and the structure of TS-1 zeolites remains stable.This synthetic strategy effectively tailors the amounts and distributions of Ti species in TS-1 zeolites,which is vital to meet the growing demands for high feedstock conversions and high product selectivities in various oxidative catalytic processes.2.Single-crystalline hierarchical Beta zeolites are synthesized by using amino acid as growth modifier to control the growth kinetics of Beta zeolites.In the synthesis system,amino acids may coordinate with Si and Al species to form complex,and induce a non-compact packing of aluminosilicate intermediates,oriented coalescence of pre-formed crystallites,and preferential occurrence of intraparticle ripening process.It is beneficial to simultaneously construct single-crystalline feature and hierarchical structure into Beta zeolites.The as-synthesized single-crystalline hierarchical Beta zeolite with mulberry-like morphology possesses noticeable mesopores ant its mesopores volume value is 0.79 cm³ g^–1.Notably,the single-crystalline hierarchical Beta zeolite exhibits highly catalytic activity in the cracking reaction of 1,3,5-trimethylbenzene,the ethylation reaction of benzene,and the conversion of lactic acid to produce lactide.We have for the first time successfully synthesized single-crystalline hierarchical Beta zeolites in the absence of mesoporogens via an L-lysine-assisted kinetic regulation method,promising their industrial applications in conversion of various bulky feedstocks into value-added products.3.Based on previous work,methanol was used as the solvent to disperse tin source that was mixed with dealuminated hierarchical Beta zeolite to prepare Sn-Beta zeolites.XPS,UV-vis,and UV Raman characterizations were performed to investigate the tin species,revealing that abundant framework tin species in as-prepared Sn-Beta zeolites.Due to its larger surface area,the hierarchical Beta zeolite could provide enough defect to capture tin favorably.Moreover,because of its abundant framework tetracoordinate tin species and short diffusion path length,the as-prepared Sn-Beta zeolite exhibits higher conversion of reactants(33%)than conventional samples in the Baeyer–Villiger oxidation of cyclohexanone.This strategy provides reference and guidance for the preparation of other heteroatom-doped zeolites.