Kinetic-Regulated Crystallization Of Titanosilicate Zeolites And Their Catalytic Performance

Zeolites are an important class of inorganic microporous crystalline materials widely used in ion exchange,gas adsorption,and heterogeneous catalysis.In 1983,the titanosilicate TS-1 zeolite constructed by incorporating Ti atoms into the MFI zeolite framework by isomorphous substitution,opened the door to the microporous zeolite as a highly efficient shape-selective catalyst for oxidation reactions.TS-1 has excellent catalytic performance in the oxidation reactions with H₂O₂ as the oxidant under mild reaction conditions,and is soon used in industrial production,such as propylene epoxidation to prepare propylene oxide,phenol hydroxylation to prepare bisphenol and cyclohexane ammoximation of ketones.The substitution position,coordination state and microstructure of Ti atoms in the MFI framework of TS-1 zeolite have been the subject of in-depth research by many scientists.In addition to the typical four-coordinate titanium species in the zeolite framework,the researchers found that the titanium species with a six-coordinate structure has a higher catalytic activity.However,most of the currently reported titanium species with high catalytic activities are constructed by post-treatment methods,which increases the difficulty of synthesis and the cost of preparation.How to achieve the controllable preparation of highly active titanium species is one of the most important challenges faced by researchers.In addition,the relatively narrow pore openings of MFI have strict restrictions on the size of molecules involved in oxidation,which weakens the successful industrial applications of TS-1.In the past few decades,scientists have been committed to improving the catalytic activity of TS-1 by applying advanced synthetic methods.The introduction of the hierarchical structure increases the accessibility of the active sites in the microporous TS-1 crystals,shortens the length of the molecular diffusion path,and avoids the continuous reaction of the products in the crystals.However,how to construct a multi-level pore structure conveniently and efficiently while realizing the construction of highly active Ti species still needs to be further explored.In this thesis,we focus on the dynamic control of the crystallization process of titanosilicate zeolites,and realize the controllable preparation of zeolite precursors containing highly catalytically active titanium species and hierarchical TS-1 zeolite catalysts,and explore their catalytic activity and products selectivity in oxidative desulfurization reaction and the olefin epoxidation reaction,establishing the structure-activity relationship of the active titanium species and the molecular mass transfer in the crystals with the catalytic activities.This research work will provide guidance for the preparation of TS-1 zeolite catalysts with high catalytic activities,and will also open the possibility to further realize the industrial production and application of new titanosilicate zeolite catalysts.The contents of the thesis are as follows:1.Titanosilicate zeolites are catalysts of interest in the field of chemical industries.However,the generation and accessibility of active sites in the titanosilicate materials for catalyzing reactions with large molecules remains a challenge.We have prepared titanosilicate zeolite precursors with open zeolitic structures,tunable pore sizes,and controllable Si/Ti ratios through a hydrothermal crystallization strategy by using quaternary ammonium templates.A series of quaternary ammonium ions are discovered as effective organic templates.The prepared amorphous titanosilicate zeolites with some zeolite framework structural order have extra-large micropores and abundant octahedrally coordinated isolated Ti species,confirmed by UV Raman spectra and X-ray adsorption spectra,thus leading to superior catalytic performance in the oxidative desulfurization of DBT and epoxidation of cyclohexene.It is anticipated that the amorphous prezeolitic titanosilicates will benefit the catalytic conversion of bulky molecules in a wide range of reaction processes,providing useful guidance for the preparation of porous titanosilicate catalysts with high catalytic activities and selectivities.2.The exploration of highly efficient catalysts based on nano-sized hierarchical titanosilicate zeolites with controllable active titanium species is of great importance in promoting the oxidative desulfurization of bulky organosulfur compounds.Here we report the anatase-free nano-sized hierarchical TS-1 zeolite prepared by regulating the kinetic crystallization of zeolites through the seed-assisted hydrothermal method.Interestingly,it is found that the intermediately crystallized zeolite phase exhibits better performance in oxidative desulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene,with turnover number up to 17.9 and 18.0 at 30 min of the reaction,respectively,almost two folds higher than the final completely crystallized zeolite phase(8.8and 6.7,respectively).This superior activity is attributed to the existence of abundant inter-and intracrystalline mesopores,the surface enriched active titanium species,as well as the absence of anatase species within nano-sized intermediate phase,which is beneficial for exposing more accessible active sites,enhancing the mass transfer efficiency,as well as avoiding the inefficient decomposition of oxidants.In addition,we have also applied this strategy of regulating the crystalline state of zeolites to the synthesis of microporous TS-1 molecular sieve and microporous Ti-Beta.The results show that the intermediate crystalline zeolite catalysts exhibit a higher external specific surface area and larger mesoporous volume,and free of anatase species,showing better catalytic activity than the completely crystallized counterparts in the oxidative desulfurization reaction of dibenzothiophene.This intermediate crystallization method provides a useful strategy in exploring high-efficient zeolite catalysts.3.The classical crystallization mechanism based upon the addition of silica molecules is concomitant with the nonclassical route which is characterized by the attachment of silica nanoparticle precursors in the Silicalite-1(MFI)crystallization process.The factors that govern the preferences for each mechanism are still unclear.In this part of work,we examine the crystallization mechanism of titanium silicalite-1(TS-1)zeolite,and illustrate the changes in self-assembled precursor structure in the early crystallization stage,mediated by the Ti and H₂O,which could determine the preferred modes of crystal growth from classical to nonclassical crystallization mode.Furthermore,we have investigated the strong impact of the crystal growth mechanism on the catalytic activity of TS-1 for oxidation of 4,6-dimethyldibenzothiophene.This work paves new avenue for the preparation of high-performance heteroatom-doped zeolite catalysts.