Studies On The Formation Mechanism Of LTA, NaY And EMT Zeolites By NMR Techniques

Zeolites such as LTA and NaY have been widely used in many scientific fields due to their outstanding properties of adsorption, ion-exchange and catalysis. Therefore, the development of zeolites with unique function or novel structures has attracted much attention. However, the synthetic mechanism of zeolites was still unclear. Nowadays, design and synthesis of new zeolites based on the intensive understanding of formation mechanism. As a result, there is an urgent call for further investigation of formation mechanism on zeolites.The formation of zeolites usually contains a lot of complex process, for example, the polymerization of silicates anions, polymerization of aluminosilicates anions, formation and transformation of sol, formation of gel, structure directing, growth of zeolites crystal, metaphase transformation and so on. Among these processes, the polymerization states and their structures in liquid phase have been intensively investigated through various techniques. Typically, TMS-GC and ²⁹Si-NMR were considered as powerful characterizations. Because most of zeolites were aluminosilicates, the research on the polymerization and structure of aluminosilicates is very important for the synthesis mechanism on zeolites. However, there are still some problems that should be addressed. (i), the interaction between silicates with aluminates usually results in the formation of gel products rather than sol products; (ii), low concentration of aluminosilicate anions in the samples, which is difficult to analyze these species by NMR and other techniques. Due to the above problems, the research on aluminosilicates species is very difficutl yet.In this work, we have used ²⁹Si-and ²⁷Al-NMR to characterize the aluminosilicate solutions. In the previous studies, the description of aluminates in organic and inorganic alkali solutions is unclear. For example, chemical shift of Q1 in TMAS solution is 75.2 ppm, however, the chemical shift of Q1 in sodium silicate is about 70⁷2 ppm. In our results, the intensive signals of chemical shift appear at 75.8 ppm in sodium aluminosilicates solutions, indicating that previous results were not correct. In fact, there is not any significant difference between the NMR signals of aluminium in sodium aluminosilicates and organic alkali solutions. We carefully investigated the change of ²⁷Al-NMR peaks, and find that Q2 signal reveal a much broader peak than other ones, which was explained as the quadrupolar interaction of cyclic aluminosilicates in grades of electric field.Additionally, this work is also to calculate the unknown chemical shift of aluminosilicate anions by using known ones, and it is effective. There are some precious report on chemical shift of cyclic tetramer and double cyclic tetramer of silicates. In addition, the synthesis of single crystal D4R of aluminosilicates was also reported, in which Si chemical shift of Si-Al double cyclic tetramer was obtained. Then we could calculate the chemical shift of Si in the cyclic tetramer of aluminosilicates by using above three factors according to the method reported in Nature. Typically, we got the chemical shift of Si in the cyclic tetramer is -75.27 ppm from calculation. In the experimental results, chemical shift at -75.1 ppm was detected, in good agreement with that from the calculation. Moreover, it is obtained two possibilities for connecting D6R, one is resulted from the connection of three S4R, and another is obtained from the connection of two S6R. The two connections are well consistent with experimental chemical shifts of NaY and EMT zeolite. UV-Raman spectra also confirm the two connections. Therefore, it is suggested the conclusions in the following: a, There are double cyclic hexamers formed by three times of single cyclic tetramers in the crystallization of FAU zeolite, and the highest atom ratio Si/Al is 3; b, There are a lot of double cyclic hexamers formed by two times of single cyclic hexamers in the crystallization of EMT zeolite, and the highest atom ratio Si/Al is 5.We also investigated the seeds solution of NaY zeolites by ²⁹Si-NMR, and find out that there are crystalline nuclei of NaY in the seeds solution, D6R attached FAU cages. The larger nuclei with smaller D6R result in the decrease of seeds solution for crystallization of NaY zeolite.In this thesis, author successfully explain the polymerization of aluminosilicate anions in the solutions, understanding the crystallization process of LTA, FAU, EMT zeolites. This work would become a new methodology for investigating formation mechanism of zeolites.