Synthesis And Application Of A Hierarchical Lewis Acid Zeolite

Zeolites are crystalline materials containing uniform and well-arranged micropores.They have an open framework.Because they allow only molecules with a smaller diameter than their pore size to enter,the molecules in the mixture can be sieved by size.Titanosilciate molecular sieve is a high silica molecular sieve with Ti atoms in its framework.Its catalytic system combined with H₂O₂ as an oxidant can selectively oxidize organic molecules under mild conditions,which plays an important role in the green catalytic oxidation production industry.However,conventional titanosilicates have strict diffusion limitations in the catalytic process due to the small pore size.Therefore,it is necessary to introduce larger pores to solve the above problems.Among them,the introduction of mesopores into microporous molecular sieves to prepare hierarchically porous titanosilicates and the preparation of extra-large-pore titanosilicates are two effective ways to solve the mass transportation problem.In this thesis,a hierarchical titanosilicate molecular sieve was prepared by introducing mesopores into microporous titanium silicalite-1（TS-1）by adjusting the crystallization kinetic method.The influencing factors of hierarchical structure were investigated in detail.The formation mechanism of kinetic control was tentatively proposed.In addition,a 14-ring extra-large-pore titanosilicates was prepared for the first time using a fluoride ion system.Due to the better mass transportation,the prepared hierarchical TS-1 and extra-large-pore titanosilicates exhibit good catalytic performance in the catalytic oxidation of olefin.The thesis contains the following two parts:（1）During the synthesis of hierarchical titanosilicate molecular sieves,polyacrylic acid（PAA）is introduced into the conventional synthesis system to adjust the crystallization kinetics of the molecular sieves.The hierarchical titanosilicates can be synthesized by adjusting Si/Ti molar ratio,the amount of PAA and the synthesis temperature.All the samples were characterized by XRD,TEM,IR,UV,and nitrogen adsorption.The prepared hierarchical titanosilicate has a flower-like morphology with100-200 nm in size,and each particle has a single-crystal structure.The mesopores of5-20 nm are distributed therein.It is concluded that PAA forms a composite template with the organic template in the synthesis process,which limits the growth of nanocrystals,mainly restricts the surface ion migration,effectively prevents the Oswald ripening process,and the small crystal grains are maintained due to the composite template.The distance between these small grains is brought closer.After orientation growth,small grains grow together to form a large particle,and the gaps between the small grains form mesopores.In the epoxidation reaction of 1-hexene,the performance of the synthesized hierarchical TS-1 titanosilicate is significantly higher than that of the TS-1 molecular sieve synthesized under the conventional conditions.Subsequently,the PAA system was further expanded to synthesize Ti-free pure-silica molecular sieve.Since PAA can adjust the synthesis system to near neutrality,the Si-OH defect of the synthesized zeolite can be greatly reduced,thereby improving the pore volume and hydrophobicity of pure-silica molecular sieve.Therefore,it exhibits good adsorption performance in volatile organic compound（VOC）capture even under high temperature（120℃）conditions.（2）An extra-large-pore titanosilicates with CFI-type topology was directly synthesized under a fluoride ion system at the first time.Since the conventional method of synthesizing extra-large-pore CFI-type titanosilciate molecular sieves uses LiOH as accelerator to cause a large amount of extra-framework titanium formation,thereby reducing the catalytic activity.The further post-treatment activation operations are cumbersome and complicated.Here,the use of fluoride ion system can effectively avoid the use of LiOH.The influence on the Ti-CFI molecular sieve were studied such as the fluorine source,the concentration of the starting material and the seed additive.The extra-large-pore Ti-CFI from the fluoride ion system shows better catalytic performance than that from the conventional synthetic system.