Studies On The Fabrication Of Zeolites With Special Aggregative States

Molecular sieves, such as zeolites and mesoporous materials, have been widely used in the fields of catalysis, adsorption, ion-exchange, and are finding new applications in optics, electronics, magnetism and medicine, etc. Molecular sieves with different morphologies or aggregation states, such as fibers and spheres have been prepared to fulfill the need for various applications. For example, zeolite fibers as catalyst can decrease the back pressure which is caused by the narrow channels in zeolite framework; zeolite capsules with functional interiors are ideal catalyst in a lot of catalytic reaction due to their good selectivity, poison resistance and reusability. Conventional methods on the fabrication of zeolite fibers and capsules mainly focus on the using of macroscopic materials as hard templates. Zeolite nanoparticles are adsorbted on the surface of templates and a zeolitic shell is formed. Sometimes, the templates can be removed by extraction or calcination. But this process is relatively high cost, complicated and the morphologies of fibers or capsules rely on that of the templates, all of these severely limit their practical application in industrial production. So a versatile and straightforward method to prepare zeolite fibers and capsules is needed. In recent years, electrohydrodynamic technique, which employs electrostatic force to manipulate materials has been demonstrated to be an effective way to fabricate 0D and 1D micro/nanomaterials with special morphologies. In this thesis, we fabricate a series of zeolite or mesoporous materials with fibrous or spherical morphologies by electrohydrodynamic method and prominent applications of these materials are studied.A coaxial electrospinning (electrospray) method was employed to fabricate zeolite hollow fibers (capsules) with walls made of zeolite nanoparticles. Followed by selective removal of the organics, the zeolite hollow fibers possessed a hierarchical structure with different level of micro-, meso- and macroporosity. The channel diameter of the fibers could be tuned by simply changing the flow rate of inner fluid. Compared with zeolite powder, an increasing anti-coking stability was found in the alkylation reaction of benzene and isopropanol when zeolite hollow fibers acted as catalyst. These hierarchical materials will be particularly useful in the refining industry of petroleum. The diameter and wall thickness of zeolite capsules could be modified by changing the flow rate of outer and inner fluid. When a Ag nanoparticles dispersed liquid paraffin was used as inner fluid, functional zeolite capsules were prepared in a single-step. Combined with the nature of zeolite shell, these functionalized zeolite capsules might find potential applications in the field of sensor, catalysis and drug delivery, etc.TiO₂ is a conventional photocatalyst and has been widely used in the purification of water and air. But the poor adsorption properties limit TiO₂ to the best of its photoefficency. So some adsorbents are usually introduced to increase the photoefficency of titania. Zeolite is an ideal adsorbent because its regular channels and large surface area that can enrich the reactant around TiO₂. In this thesis, electrospinnign was employed to prepare TiO₂/zeolite composite fibers and the photocatalytic activities of composite fibers was evaluated by photodegradation of methylene blue dye in the reaction solution. Compared with pure TiO₂ fibers prepared by electrospinning and commercial P25, the zeolite/TiO₂ composite fibers exhibited the highest catalytic efficiency.By employing electrospinning technique, Si-Al mesoporous fibers were prepared using CTAB as template and TEOS as silica source in an aqueous acidic environment in the presence of ethanol. The 2D hexagonal mesopore oriented aligned along the axis of fibers. The surface area more than 1100 m2/g was the highest among all the reports on the fabrication of mesoporous fibers by electrospinning. There were tetrahedrally incorporated Al species in the Si-Al mesoporous fibers and the fibers with lewis acid sites were efficient heterogeneous catalyst in the acetalization of cyclohexanone with ethylene glycol. If other heteroatom, such as Fe and Ti was introduced into the framework of mesoporous fibers, the hybridized fiber could act as catalyst and be use in a series of redox reactions.The preparation of oriented zeolite thin film is also an important part of this dissertation. Compared with random-oriented zeolite film, the defect-free and oriented film can make the guest molecule diffuse along a particular direction of zeolite channels, thereby reducing the transmission path of guest molecule and improving separation efficiency. So there are growing interest in the preparation of defect-free and oriented zeolite film.A TiO₂ coating with thickness of several nanometers was formed on the glass slide through a sol-gel process. Continuous and oriented zeolite film was prepared on the TiO₂ coated substrate by in situ growth. Compared with growth situation of zeolite crystals on bare substrate and substrates with 1, 3, 5 modification cycles, respectively, we can conclude that the chemical modification promoted the growth of zeolite crystals. This method is not limited to the TiO₂ modification on glass substrate, modifications can be carried out on a series of substrates with hydroxyl groups but not suitable for zeolite crystal growth, and kinds of easily hydrolyzable metal alkoxides can be used to change the chemical composition of substrate surface in order to prepare oriented and defect-free zeolite film.