Regulation Of The Active Centers In Titanosilicate Zeolites TS-1 And Their Catalytic Performance

As a class of widely used inorganic microporous crystalline materials,zeolites play a crucial role in traditional industrial field,as well as emerging fields such as photocatalysis and bio-medicine.Titanosilicate-1（TS-1）zeolite is obtained by isomorphous substituting titanium atoms into the framework in pure silicon MFI zeolite.TS-1 zeolite exhibits excellent catalytic performance in the oxidation reactions using H₂O₂ as oxidant,such as epoxidation of olefins,hydroxylation of phenol,and ammoximation of ketones.Moreover,due to its green and pollution-free characteristics,this catalytic system has attracted considerable attention.The properties of framework titanium species in TS-1 zeolite are closely related to the catalytic performance.In generally,the content of the Ti O₄ species in framework is directly proportional to the catalytic activity of TS-1.However,an excessive input of titanium sources in the synthesis can lead to the formation of anatase,and the synthesis procedure of titanium-rich TS-1 is usually complex or time-consuming.Moreover,due to steric effects,the content of Ti O₄ in TS-1 zeolite is below 2 wt%（Si/Ti=40）,thus the construction of Ti species with different coordination forms favors to increase the number of active species in the TS-1 catalyst.Studies have shown that hexacoordinated Ti（OH₂）₂（OH）₂（OSi）₂ species exhibit superior catalytic activity compared to the traditional tetracoordinated Ti O₄,but their construction methods（such as post-treatment,seed-assisted,microwave irradiation,etc.）suffer from poor controllability,complex synthesis processes,and low content.These facts have inspired studies on crystal engineering to control the crystallization process and then to tailor the physicochemical properties and active site structure of zeolite catalysts,which requires in-depth research on the crystal nucleation and growth of zeolites.By now,research on the crystallization process is currently mainly focused on model zeolites（such as silicalite-1 and SSZ-13）,but limited research on TS-1 zeolites.In this thesis,we focus on regulating the active centers of TS-1 zeolite and controlling the physicochemical properties of the zeolite from the perspective of the crystallization process,realizing controlled preparation of TS-1 zeolite catalysts with high framework Ti content,high catalytic activity Ti species,and high accessibility of Ti species.The catalytic activity of the as-synthesized TS-1 zeolite in epoxidation of olefins and oxidative desulfurization reactions has been investigated.These studies provide suggestions for understanding the zeolite crystallization process and preparing titanosilicate zeolite with high catalytic performance,which proposes new possibilities for the industrial production and application of titanosilicate zeolite catalysts.The contents of the thesis are as follows:1.The development of simple and rapid methods for preparing Ti O₄-enriched TS-1 zeolites is an important direction in the research of titanosilicate zeolite catalysts.Herein,a high-efficiency strategy has been proposed by using 1,3,5-benzenetricarboxylic acid（H₃BTC）as a crystallization modifier to rapidly synthesize enhanced-titanium TS-1zeolite without extra-framework anatase Ti O₂.In such system,a mixed liquid-solid precursor phase is formed and TS-1 crystals can be produced within one day based on the special combination mechanism of liquid-phase and solid-phase conversion.Ti species enter into the TS-1framework by the transformation from the liquid phase to crystals as well as by the in-situ transformation of the solid phase,which makes the content of framework Ti increase significantly（Si/Ti=48.5）without any anatase.In the epoxidation of 1-hexene with H₂O₂ as an oxidant,the conversion of alkene over the synthesized TS-1 was improved by 8.01%compared to the traditional TS-1（18.11%vs.10.10%）.2.The construction of Ti species with different coordination forms can lead to a further increase in the overall quantity and activity of Ti active species within the catalyst.Herein,a polymer（polyacrylamide,PAM）was utilized to control the precursor structure evolution of TS-1 zeolite,so that the crystallization path was switched from a classical to a non-classical mechanism,which greatly accelerated nucleation and enriched active Ti sites.It was found that specific interactions between PAM and Si/Ti species promoted the assembly of colloidal precursors containing ordered structural fragments and stabilized Ti species in the precursors,leading to the construction of highly active Ti O₆ species,enriched Ti content in TS-1（Si/Ti=29.0）,and a 1.5-fold shortened crystallization time.The PAM-regulated TS-1 zeolite exhibited enhanced catalytic performance in oxidative reactions compared to conventional samples.3.In TS-1 zeolite,the exact structure,formation process,and catalytic mechanism of highly active Ti O₆ species remain unclear,and there is a lack of effective and controllable construction methods.Herein,a simple amide-assisted strategy（represented by acrylamide,AM）was proposed to synthesize hierarchical TS-1 zeolite with rich and homogeneously distributed Ti（OH₂）₂（OH）₂（OSi）₂ species.Strong interactions between AM and Si/Ti as well as self-polymerization of AM in synthetic gels changed the zeolite formation process,thus to effectively construct Ti（OH₂）₂(OH2)（OSi）₂ and form micro-mesopore structure.The advantages of Ti（OH₂）₂(OH2)（OSi）₂ over Ti O₄ in substrate adsorption and reactive intermediate formation for oxidative desulfurization were further proved by experiments and theoretical calculations.By adjusting AM amount to optimize the Ti（OH₂）₂(OH2)（OSi）₂ content and distribution,the obtained catalyst achieves 100%dibenzothiophene removal at 10 min(TOF=134.8 h^-1),and maintains>92%removal after seven cycles.