The Morphology Control Of Zeolites And Their Catalytic, Adsorptive Function Studies

Zeolites are regarded as the most important heterogeneous acid catalyst in industry due to their rich channel structure, large surface area, narrow pore size distribution, high thermal and hydrothermal stabilities and high adsorption capacity. The service performance of zeolites are closely related to their morphology, for example, large size zeolites have an important application in studying the adsorption and diffusion mechanism of organic molecules or designing optical devices. Small size zeolites show excellent catalytic performance owes to good adsorptive and diffusive property of reactants and products, while in the fields of zeolites film, small and uniform zeolite crystals are necessary and these crystals need to be arranged in preferred orientation. Thus, the morphology control of zeolites is very meaningful not only for basic research but also for practical application.The present project try to obtain zeolites with different morphology by choosing different zeolites synthesis system and different additives and the synthesis conditions such as amount of additives, adding order of additives, crystallization temperature, crystallization time also need to be optimized. The crystallization mechanism and the catalytic and adsorptive properties of the products are also studied systematically in present project.Microporous titanosilicate zeolite with MFI structure (TS-1), discovered by Enichem Company in 1983, obtains great attention for its excellent catalytic activity and stability. Especially, TS-1 shows superior catalytic properties in a series of oxidations such as epoxidation of linear olefins, hydroxylation of aromatics, oxidations of linear alkanes, amines, alcohols, sulfur compounds and ethers in the presence of environmentally benign oxidant of H2O2. The catalytic activities of TS-1 zeolite is dramatically dependent on the crystal sizes. For example, TS-1 nanocrystals (less than 0.3μm) show good activity in the hydroxylation of phenol, while the crystals larger than 1μm are almost inactive. For TS-1 nanocrystals, the transmission and diffusion of reactants and products are much more easier and there are more active sites exposed. The preparation of TS-1 crystals with small sizes (<0.3μm) usually required a centrifugation route for the separation of TS-1 nanocrystals from a slurry system, which is not suitable for the production of TS-1 zeolite in a large scale for industrial process. To overcome these problems, the self-assembly of preformed titanosilicate precusors with triblock copolymers has been employed by Xiao F.-S., giving ordered mesoporous titanosilicates (MTS-9). In this case, MTS-9 was easily obtained from a normal industrial route of filtration. However, the thermal stability of titanium in MTS-9 is relatively low, which strongly influences its catalytic properties in a series of oxidations. Despite of encouraging progresses for the thermal stability of titanium species in recent years, the catalytic performances of the current titanosilicate materials are yet generally lower than those of TS-1 nanocrystals. Therefore, the preparation of TS-1 zeolite with good properties in catalytic oxidations by a simple filtration route is strongly desirable. We demonstrate here a facile filtration methodology for preparation of stable bulky-particles of TS-1 zeolite with good catalytic activities (Bulky-TS-1) in the presence of H2O2. Bulky-TS-1 exhibits comparable properties in catalytic phenol hydroxylation with TS-1 nanocrystals collected from a high-speed centrifugation.MFI-type zeolite possesses an anisotropic framework with two intersecting 10-membered ring channels including straight channels (5.3x5.6 A) parallel to b-axis and zig-zag channels (5.1x5.5 A) parallel to a-axis. The straight channels are favorable for diffusion and formation of relatively large products such as cyclohexanone oxime and p-xylene (kinetic diameter ca.5.8 A), while the zig-zag channels are difficult for the adsorption of these molecules due to the shape selectivity. The MFI zeolites with shorter b-orientation mean higher diffusive and adsorptive rates for molecules. Therefore, by controlling crystalline length along b-axis, catalytic and adsorptive properties over MFI crystals could be significantly adjusted. There are successful examples for preferential growth of ZSM-5 (aluminosilicate MFI-type) and TS-1 crystals. Hwang et al. show microwave fabrication of MFI zeolite crystals with a fibrous morphology and the product they got shows excellent catalytic and adsorptive property. Ryoo R et al produced MFI zeolites with very short b-axis by empoying a kind of special surfactant C22H45-N1(CH3)2-C6H12-N1(CH3)2-C6H13. The thickness of the product is as thin as two crystal cell and the short b-oriented length is favorable for the catalytic reaction. Despite of encouraging progresses for the zeolite morphology control in recent years, most need complex procedure and expensive template and these are not suitable for industrial process. Thus, the morphology control of zeolite by employing cheap raw materials and simple synthesis process is a meaningful work. We report an one-pot growth of TS-1 crystals with various b-oriented length by introducing organic additives in the starting titanosilicate gels. After addition of urea in the starting gel, TS-1 crystals with sheet-like morphology designated as TS-1-S have been successfully synthesized. After addition of surfactant FC-4, TS-1 with chain-like morphology designated as TS-1-C have also been successfully obtained. By optimizing the synthesis conditions,b-oriented length of the TS-1 crystals can be varied from 80 nm to 5μm. The TS-1 products we obtained exhibit quite distinguishable catalytic activities in the Beckmann rearrangement of cyclohexanone oxime and good adsorptive properties for xylenes.Zeolite Y possesses a three dimensional channel structure and high thermal and hydrothermal stability, in chemical industry especially for FCC reaction, zeolite Y shows excellent catalytic property. As a adsorbent, zeolite Y also shows good adsorptive property towards volatile organic compounds. The Si/Al ratio of framework is very important for zeolite Y. As increasing the Si/Al ratio, the stability of Y can be improved, this is favorable for FCC reaction. Generally, the crystallization temperature of Y is 100℃and the Si/Al ratio of the framework is about 5.0-5.3. So far, researchers have made lots of efforts to increase the Si/Al ratio of zeolite Y. Post treatment is a popular method for preparing zeolite Y with higher Si/Al ratio. For example, high temperature steam can be employed to dealuminize zeolite Y and the framework can be maintained, thus high Si/Al ratio Y can be obtained. Although post treatment method is effective to prepare high Si/Al ratio Y, the synthesis procedure is complex and the cost is high. If high Si/Al ratio Y can be synthesized by one-pot method, it will be a very meaningful work for industry. We know high crystallization temperature is favorable for synthesizing high Si/Al ratio Y, but high crystallization temperature sometimes leads to the phase transition of zeolite crystals. Up to now, crystallization temperature of Y zeolite is mainly lower than 110℃, although some reached 110-150℃, the expensive organic templates must be employed and the addition of template will make the synthesizing process more complex. Here, without using organic template, just by adding a little methyltriethoxysilane (MTS), we have successfully obtained sheet-like zeolite Y crystals in high crystallization temperature (140℃) through one-pot hydrothermal synthesis. Compared with conventional Y zeolite, Y product we got own higher Si/Al ratio of the framework, smaller crystal size and better adsorptive property towards organic volatile compounds.