| Sn-Beta has attracted much attention for its excellent catalytic performances in selective oxidation of various organic compounds with dilute H2O2as the oxidant under mild conditions. Up to date, the synthesis of Sn-Beta still relies on the traditional fluoride method which reported by corma et al. using TEOS as the silica sources. Seeds are often required and are prepared by dealumination of nano-sized Al-Beta zeolites with concentrated nitric acid. The zeolite crystallization is time-consuming (at least ten days), and the use of hydrofluoric acid as mineralizer brings unwanted pollution and health hazard, that lead to poor synthesis reproducibility and high production costs. Thus, the process is energy intensive and difficult to scale-up.The dissertation was focused on developing facile, rapid and efficient synthesis Sn-Beta method, which is expected to provide an economical and scalable for the industrial application. A series of new strategies and methods has been used, such as hydrothermal crystallization, two-step post synthesis, steam assisted conversion (SAC) and two-step hydrothermal crystallization method. The properties of the as-prepared samples were characterized by XRD, SEM, FT-IR. UV-Vis. UV Raman. ICP and N2adsorptions. The main contents of the dissertation are as follows:1. Sn-Beta zeolite was successfully synthesized by hydrothermal crystallization method, using fumed silica as the silica source, SnCl4·H2O as the tin source and NH4F as the assisting agent. The effects of the molar ratio of n (H2O)/n (SiO2), the crystallization temperature and the amount of ethanol on the formation of the Sn-Beta zeolite frameworks, were investigated in detail. It is shown that using fumed silica as the silica source, can avoid the hydrolysis of the TEOS process and keep the molar ratio of n (H2O)/n (SiO2) in the final gel precisely, which simplify the synthesis process effectively. With the increase of n (H2O)/n (SiO2) molar ratio in the gel. the crystalline is decreased. The addition of ethanol into the prepared gel was helpful for improving the viscosity and fluidity of the initial gel. The as-synthesized sample exhibits no better, but comparable activity compares with Sn-Beta prepared by conventional hydrothermal method using TEOS as the silica source in cyclohexanone B-V oxidation reaction. 2. Sn-Beta zeolite was prepared by a two-step post synthesis method. The procedure consists of first dealumination of Al-Beta zeolite with nitric acid and then incorporation tin into the framworks by calcination at varying temperatures. The effect factors such as the number times of dealumination and calcination temperature in the preparation Sn-Beta was optimization. The results show that, tin content is increased with the amount of vacant T-sites in the precursor. The higher calcination temperature is benefited for incorporation of tin into the zeolite framework. However, when the calcination temperature is too high, the framework will collapse; Optimal synthesis conditions:Dealumination Al-Beta (Si/Al=21) with nitric acid (6M HNO3) two times, calcination temperature of773K. the highest Sn content in the as-prepared samples is3.92wt%(SnO2). The Cyclohexanone conversion is23.4%, the Lactone Selectivity is70%。3. Sn-Beta zeolite was prepared by a rapid and clean SAC method from the dry stannosilicate gel. The effects of the crystallization temperature and time, the gel drying temperature and the amount of water in the bottom of autoclave on the formation of the Sn-Beta zeolite frameworks, were investigated in detail. The results show that, the amorphous gel was converted to highly crystalline Sn-Beta within3-12h at mild reaction temperature of453K. Compared with the researchers, we have get a different results on the synthesize parameter of water content. In the previous literatures, all the researchers agreed that" the amount of water in the bottom of the autoclave seriously affects the formation of zeolite"; we found that the inferior design of the internal structure of the conventional SAC synthesis autoclave is the real reason. When the amount of water in the bottom of the autoclave is enough, the vapor-liquid balance could be kept in the autoclave all the time during the crystallization. The condensation of steam into water inside the autoclave would take place and the condensed water can drop back into the gel. This leads to the change of the synthesis composition. In order to avoid this phenomenon, an improved SAC autoclave was designed and used in this study; it is shown that, the amount of water in the bottom of the autoclave cannot significantly affect the formation of Sn-Beta zeolites. The higher crystallization temperature is benefited for crystal formation; Higher Sn content required longer crystallization time; the dry gel with SiO2/SnO2ratio less than75,could not be transformed into zeolite phase even after200h of crystallization. Optimized synthesis conditions of high quality of Sn-Beta zeolites are the gel drying temperature of333-373K and the crystallization temperature of433-473K. The prepared Sn-Beta zeolites are active and selective for cyclohexanone B-V oxidation reaction. 4. A novel strategy, CTAB-assisted two-step hydrothermal crystallization method, has been designed for the synthesis of hierarchical Sn-Beta. In the first step, dealuminated Al-Beta and the Sn (IV) precursor were dissolved in TEAOH (40%aqueous solution) and formed a mixture. Then, the mixture formed Sn-Beta zeolite primary structural unit by hydrothermal synthesis. In second step, primary crystal of Sn-Beta will be converted to be a hierarchical Sn-Beta by CTAB-assisted through self-assembled. The material shows greatly increased catalytic activity and a strongly prolonged lifetime in the Baeyer-Villiger oxidation cyclohexanone, as compared to conventional zeolite Sn-Beta. This excellent catalytic performance of mesoporous Sn-Beta is closely related to its hierarchical micro/meso-structure, and high hydrothermal stability against mesostructural collapse and Sn leaching during reactions. |
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