Synthesis And Characterization Of Molecular Sieves In Fluoride Media

In broad terms, those materials that have the function of sieving molecules are generally called molecular sieves. Because of their special pore systems,molecular sieves play an important role in ion-exchange, industrial catalysis and adsorption and therefore they have received extensive attentions from researchers for such a long time. Among that, molecular sieves that contain new structures and chemical compositions become one of the even hotter areas of research. However, new molecular sieves are always synthesized by new organic templates or critical reaction conditions, and solving the structure requires for more pure samples with high crystallinity, which makes synthesis of new molecular sieves becoming one of the most challenging researches.Since nanosized molecular sieves have larger specific surface areas and shorter diffusion path, they have irreplaceable advantages in industrial application.Even though molecular sieves with crystal size smaller than 100 nm have already been reported, their low crystallinity has been an unsolved problem so far.Studies show that F- ions can direct the formation of double 4-ring, which makes it easier to synthesis molecular sieves with opening structure and low density. Meanwhile, when F- ions act as the mineralizers, reactions can take place in a near-neutral condition, which helps to enhance the thermalstability of quaternary ammonium cations by avoiding the Hoffman degraded reaction. In addition, F- ions can maintain in small cages and balance the positive charges of templates, which effectively reduces the defects in high-silica molecular sieves and increases their crystallinity, but the particle sizes are usually at micron scale. Based on the characteristics of fluoride media mentioned above,germanosilicate molecular sieves with new structures and nanosized silicalite-1 molecular sieves with high crystallinity were synthesized by hydrothermal method and detailed studies on the characteristics of their structure as well as the influences of factors during synthesis process were talked about.By adjusting the three factors of GeO2/SiO2?H2O/( GeO2+SiO2) and temperature, ITQ-17 molecular sieves with Ge/Si ratio about 1.5 times of what has been reported were successfully synthesized under the condition of extremely low water content and mild temperature. This is because that the influence of water content on system pressure is tremendous, especially for concentrated solution and the high Ge-containing structure can only be stable at relatively lower pressure. Meanwhile, GeO2/SiO2 was found to have some influence on the length-diameter ratio of ITQ-17 molecular sieves with stick morphology, which was caused by the order of Ge-substitution in different sites of BEC-type framework and therefor the crystal preferred orientation was expressed to a greater degree.Besides, by adjusting the factors mentioned above, two new molecular sieves were synthesized. One of them is BUCT-1 molecular sieves with orthorhombic crystal system. Its cell parameters were a=14.1253 A, b=12.9242 A, c=7.7153 A, ?=?=?=90° obtained by indexing the XRD data and further confirmed by RED. It can be deducted from the analysis of IR and STA that BUCT-1 molecular sieves didn't have apertures lager than 6-ring and the structure contained Ge7 or Ge7 cluster. BUCT-1 molecular sieves can be obtained under the conditions of 120-150 ? , H2O/( GeO2+SiO2)?1.5-3,GeO2/SiO2=15-50. Since the synthesis range of BUCT-1 molecular sieves was very narrow, their particle sizes and crystallinity were influenced by the synergistic effect of GeO2/SiO2, H2O/(GeO2+SiO2) and temperature.Another one is BUCT-2 molecular sieves and it also has orthorhombic crystal system. The structure model of BUCT-2 molecular sieves was obtained by XRD. It belongs to layered molecular sieves and each layer has the same structure with STI-type molecular sieves along [001] direction (called sti layer).Its cell parameters were a=13.9007A,b=12.5280 A, c=10.6669 A, ?=?=?=90?and its space group is CMMA. The conditions of 140 ? and H2O/(GeO2+SiO2)=3-4.5 was thought to be suitable for the synthesis of BUCT-2 molecular sieves and in the structure, Si atoms can be totally substituted by Ge atoms. Therefore,the range of Ge contents in ITQ-17, BUCT-1 and BUCT-2 framework structure had the tendency of increasing. The phenomenon that molecular sieves synthesized with the same raw materials and under similar conditions of temperature and water content have different structures caused by the continuous substitution of Si by Ge gives us a new idea for the synthesis of new molecular sieves. Besides, after calcined in the air at 240 ?, it transferred into another new molecular sieves, T-BUCT-2. However, it still has not been sure whether T-BUCT-2 molecular sieves are obtained by direct condensation between sti layers.The key point of synthesizing nanosized molecular sieves in fluoride media is obtaining nanosized molecular sieves with high crystallinity at lower temperature and the key point of synthesizing molecular sieves with high crystallinity at lower temperature is using active silica source. When tetraethoxysilane was used to synthesize silica by sol-gel method, the most active silica with the largest amount of Si-OH in the surface was obtained by calcined at different temperatures. Though adjusting the fluoride source,systematic alkalinity, water content and reacting temperature and time and using the active silica as silica source, silicalite-1 molecular sieves with nanoscaled particle size was synthesized. Its particle size was about 50 nm confirmed by XRD method, external surface areas method and DLS method. If the crystallinity of industrial ZSM-5 molecular sieves was set as 100%, the relative crystallinity of our silicalite-1 molecular sieves was 120.3%. Meanwhile, its microporous surface areas is as high as 434 m2/g, which means its aperture is ordered and lacks of defects. Besides, its yield is 98.5%, which brings better economic benefits. For the thermal stability, the temperature of structure collapsion of the obtained silicalite-1 molecular sieves was 166? higher than that synthesized without F- ions, but their crystal sizes were almost the same.This result shows that by effective adjusting of the condition, the yield and crystallinity of molecular sieves can be significantly increased in fluoride media and the nanoscaled particle size can almost be maintained.During the reaction, the influences of fluoride source and systematic alkalinity were related to each other. NH4 was chosen as the fluoride source for its little influence on the pH value of reaction system, and with the help of tetrapropylammonium hydroxide and tetrapropylammonium bromide, the pH value was controlled near 10, which was suitable for the crystallization of silicalite-1 molecular sieves. As for the control of particle size, it can be realized by adjusting the water content, which has influences on the solubility of reactants and the degree of supersaturation of the solution and optimizing the reacting time and temperature. In the solution, nucleation takes place at lower temperature and then crystallization can complete at higher temperature, which makes it possible to effectively control the particle size without influencing the crystallinity. When compared with other available silica source with similar particle size, it was found that the activity of silica, that is, the quantity of Si-OH in the surface was also an important factor that could influence the crystallinity of samples. In the end, the methods of synthesizing silica and controlling reaction conditions mentioned in this thesis can be extend to the synthesis of other nanosized high-silica molecular sieves.