Studies On The Crytallization Process Of Aluminophosphate Molecular Sieves Templated By Diethylamine And Diisopropylamine

Studies on the crystallization mechanism of aluminophosphate molecular sieves templated by diethylamine and diisopropylamine Bin Zhang Supervisor:Prof. Jihong Yu,Prof. Wenfu Yan Major:Inorganic ChemistryZeolites and related microporous crystalline materials, which have periodic three-dimensional framework and well-defined pore structures, have attracted much interest due to their wide applications in catalysis, ion-exchange, chemical separation, adsorption, host/guest chemistry, microelectronic devices, optics, and membranes. In 1982, Wilson and co-workers reported a new class of microporous crystalline aluminophosphates AIPO4-n(n denotes a structure type) with neutral zeolite-like open-frameworks, a high degree of structural diversity, and a wide range of pore sizes and volumes. Even though numerous zeolitic structures have been successfully synthesized via empirical findings or the exploration of synthesis parameters by trial and error, it is of significance to rationally design and synthesize more zeolitic structures with desired architectures and properties, which requires a clear understanding of their formation mechanism and crystallization process. So far, different mechanisms related to the specific synthesis systems have been discussed regarding nucleation and crystal growth, based on experimental evidence obtained with various methods such as X-ray diffraction and scattering, solid-state NMR spectroscopy, atomic force microscopy, and electron microscopy. However, the understanding on the formation mechanism of zeolitic structures is still very limited due to the intrinsic complexity of the synthesis system and imaging the initial stage of zeolite formation has not previously been possible.In this thesis, we investigated the crystallization mechanism of AlPO4-11 templated by diisopropylamine and the templating effect of diethylamine in the synthesis of AlPO4-11, AlPO4-41, and JDF-2. In the studies of crystallization mechanism of AlPO4-11, we investigated the influence of the concentration of reactants on the crystallization chemistry and crystallization pathway. It was found that a low concentration of reactants resulted in the formation of crystalline AlPO4-5 intermediate prior the crystallization of AlPO4-11 and a high concentration of reactants led to the direct crystallization of AlPO4-11 from the initial mixture. The NMR data clearly demonstrated that some fragments formed in the initial mixture and the early stage of crystallization contained some special P atoms possessing the environment which is same or very similar to that in the well crystallized AlPO4-11.AlPO4-11, AlPO4-41, and JDF-2 can be synthesized from a reaction system of Al2O3-P2O5-diethylamine-H2O by varying the content of diethylamine only. The samples crystallized for different periods were isolated and characterized by XRD and NMR techniques. The results showed that the coordination number of Al and P atoms in the initial mixture was controlled by the concentration of diethylamine, which determined the structure type of the final phase. Thus, the structure directing effect of an organic amine is determined not only by the nature but also by the concentration of the molecule.