Morphological Control And Functionalization Of Zeolite And Mesoporous Silica Materials

Inorganic porous materials have long been a highlight in the research region of materials science. Zeolite and mesoporous silica, which possess ordered pore structures, have been paid special attention. Recently, with the rise and development of nanotechnology, nanosized inorganic porous materials (e.g. nanozeolites and mesoporous silica nanoparticles) have aroused wide interest. The small size of these materials leads to relatively short pore channels, which may accelerate the diffusion of guest molecules into and out of the interior and endow these materials with a bright future in the applications such as catalysis and adsorption. However, the nanoscale particle size of these materials would also bring about some inconvenience of manipulation and separation in practical applications. At the present time, a feasible way to solve this problem is to pre-assemble the nanosized inorganic porous materials on an appropriate substrate through various approaches. As a result, we only need to manipulate the substrate in the practical applications, which may avoid the troublesome centrifugation and ultrasonication steps. Layer-by-layer and aqueous electrophoretic deposition are two frequently used methods in the assembly of nanozeolites, but they have some defects such as low controllability and low efficiency, which need to be overcome. Notably, a high disperibility of nanozeolites was the premise of some applications, where the assembly route was no longer applicable and new stategies are desired. Except the direct applications, nanozeolites could behave as the seeds to fabricate a variety of ultrathin zeolite films and hierarchically porous zeolite materials through hydrothermal growth. The resulted materials have some characters whi(?)h are distinguished from the traditional zeolite materials. How to make full us(?)hese characters to realize the applications of zeolites in a wider range or (?) the performance of zeolites in the current applications is also an important task lor the materials scientists.In 1992, Mobil researchers firstly synthesized M41S mesoporous silica, which set up a milestone in the history of materials science. After more than ten years of research, great progress has been achieved in the synthesis technique of mesoporous silica. The pore structure and pore size could be controlled through changing the surfactant type and/or adding some special reagents. In the recent years, the morphological control of mesoporous silica has been paid more and more attention,because the morphology has an important effect on the performance of mesoporous silica in practical applications. Moreover, such research work may also help to explain the formation mechanism of the peculiar morphologies of biosilica. Main research objects of morphological control are the mesoporous silica templated by cationic surfactants and block copolymers, while the synthesis system with neutral amine as the template received less attention. Notably, in the research work of morphological control, the synthesis of mesoporous silica nanoparticles has become a highlight because of their potential advantages in practical performance. However, they would also meet the similar difficulties in practical applications to nanozeolites, which need to be resolved.Based on the research status and problems of zeolite and mesoporous silica, this thesis mainly focuses on two topics: synthesis, assembly and functionalization of nanozeolites and morphological control of mesoporous silica. Meanwhile, the syntheses of hierarchically porous zeolite materials and mesoporous silica nanoparticles are also involved in the detailed work. The research work is categorized and discussed in the seven chapters of this thesis.Chapter 2 involves the preparation of hierarchically porous zeolite materials, and the main work is synthesizing iron-containing zeolite ZSM-5 microspheres from Jilin diatomite.The utilization of natural minerals has always attracted widespread attention. Diatomite is a suitable raw material for the synthesis of zeolites due to its chemical composition. I found that the morphology of zeolite product could be controlled by adding sodium chloride to the synthesis system and zeolite microspheres composed of submicrometer-sized crystallites were obtained. The intercrystalline pores between zeolite crystallites may facilitate the diffusion process of molecules in the catalytic reactions. Moreover, the silica, n'umina and ferric oxide components in diatomite were made full use of and the re(?)ed iron-containing zeolites may be applied in the environmentally catalytic rea(?); nvolving nitrogen oxides.Chapter 3 discusses the synthesis and assembly of nanozeolites as well as the fabrication of ultrathin zeolite films through seed-film method with nanozeolites as the seeds. The content of this chapter constructs the basis of Chapters 4 to 6, which involve the research work on nanozeolites. The synthesis conditions of nanozeolites are normally rigorous and very sensitive to the chemical reagents. The synthesis and preparation methods provided in this chapter were well reproducible because they were based on the methods in the literatures and improved through repeated tries onthe available reagents in the laboratory.Chapter 4 studies the electrophoretic deposition of nanozeolites in the organic media such as isopropanol and acetyl acetone. The results showed that the elctrophoretic deposition in organic media was a high-efficiency assembly method of nanozeolites. The traditional layer-by-layer method and aqueous electrophoretic deposition have been widely applied in the assembly of nanozeolites. However, the thickness of the resulted zeolite coating was normally on the submicrometer scale and the assembly time ranged from tens of minutes to several hours. When electrophoretic deposition was carried out in organic media, the assembly time could be reduced to tens of seconds and the thickness of zeolite coating could be controlled from submicrometer to micrometer scale. The zeolite coating was affected by the silicon/aluminum ratio of nanozeolites, physical/chemical properties of media, concentration of nanozeolite suspension, and electrophoretic voltage/time.Chapter 5 involves the rational design and fabrication of a nanosilver/zeolite film/copper grid catalyst that showed well activity and selectivity in the selective oxidation reaction of alcohols. The traditional electrolytic silver catalyst was widely used in the catalytic oxidation reaction in industry. However, it led to a large amount of lean oxidation products at relatively low temperatures due to its low activity. On the other hand, reactants tended to be over-oxidized at high reaction temperatures. The catalyst developed in our work had a very high catalytic activity and could convert the alcohols to the desired moderate oxidation products at a relatively low temperature and a relatively high space velocity. The exothermic character of oxidation reaction was fully considered in the design of catalyst. Accordingly, copper grid was applied as the substrate and ultrathin zeolite film was used as the support of silver catalyst, in order to quickly conduct the reaction heat.Chapter 6 discusses the syni(?)si (?) appli(?)tion of magnetically separable nanozeolites. The previous reseaic (?), of our group showed that the conventional nanozeolites possessed a high adsorption amount for proteins and a proper interaction with bio-macromolecules, which made them promising candidates in bio-applications. However, the troublesome centrifugation and ultrasonication steps would become the inevitable obstacles in practical applications. I introduced magnetite nanoparticles into the synthesis system and obtained superparamagnetic magnetite/zeolite composite nanoparticles through the adjustment of synthesis conditions. Except the careful characterization of the product, the performance of the magnetically separablenanozeolites in the immobilization of enzymes was also tested. It was found that the product had a high adsorption amount for enzymes, and the immobilized enzymes showed a high stability and a well catalytic activity. Because of the advantages of magnetically separable nanozeolites in manipulation and separation over the conventional nanozeolites, they are expected to greatly expand the applicable range of zeolite materials in bio-technology.Chapter 7 involves the morphological control of mesoporous silica. I synthesized a kind of mesoporous silica flakes with neutral amine surfactant as the template. The product could basically retain the advantages of mesoporous silica nanoparticles in the diffusion of guest molecules, because it had nanoscale short pore channels in one dimension. On the other hand, the product could be easily separated from the system through filtration due to its micrometer size in the other two dimensions. Guest-molecule adsorption experiments showed that the mesoporous silica flakes possessed a higher initial adsorption rate and a higher saturated adsorption amount than their microspheric analogues. Through further investigation of the synthesis system, a morphological transformation was discovered. With the variation of synthesis conditions, the product morphology transformed from flakes, to microspheres, and to nanoparticles. Moreover, a "clear point" was observed under certain condition. Taking advantage of this phenomenon, the size of mesoporous silica nanoparticles could be well controlled through accurately adjusting the solvent composition.Chapter 8 discusses an extension work, which involves the biomimetic synthesis of calcium carbonate. Utilizing the hydrolysis reaction of carbonate ester in alkaline solution to slowly release carbonate ions, the precipitation and crystallization process of calcium carbonate could be controlled. Therefore, the organ(?) reagents had enough time to interact with the calcium carbonate crystal and (?)y controlled its morphology. The approach itself showed no discrir(?) . the meta-stable polymorphs of calcium carbonate and could lead to any polymorphs by adding different organic controlling agents. For example, calcite crystals with specific morphology and color could be obtained with negatively charged organic dye molecules as the organic controlling agents.