| Since aluminophosphate (AlPO4)-based molecular sieves are extensively used in various fields, a better understanding of their crystallization is very important. However, the mechanisms are still not completely understood because the conventional hydrothermal synthesis used for their preparation is an extremely complicated process.;This study demonstrated that DGC can be used as an effective approach to study the crystallization of AlPO4-based molecular sieves. It allows one to "freeze" the reactions for capturing reactive intermediates. The frozen intermediates can subsequently be characterized by multiple techniques. The crystallization can then be resumed by using the same intermediate. Our results further indicated that it is necessary to carefully examine both washed and unwashed intermediate gel samples. A comparison between washed and unwashed samples can provide critical information.;Keywords. Molecular sieves, Aluminophosphates, Dry gel conversion (DGC), Steam assisted conversion (SAC), Vapour phase transportation (VPT), Self-assembly, Crystallization mechanism.;This thesis describes an investigation of the crystallization of AlPO 4-based molecular sieves by dry gel conversion (DGC) methods, including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The study consists of the preparation and characterization of a series of dry gel samples to follow the crystallization of several representative molecular sieves, including AlPO4-5, AlPO4-11, and AlPO4-18. For AlPO4-5, two different pathways exist in the crystallization process. In VPT, a layered phase with 12-ring-like large pore forms when template molecules are transported via vapor phase onto the surface of the amorphous dry gel. The layered phase eventually evolves into AlPO4-5 through a precursor with a 3-D structure similar to AFI framework. In SAC, AlPO 4-5 crystallizes from an amorphous phase directly. For AlPO4-11, both SAC and VPT processes follow the same self-assembly mechanism, involving the formation of a semi-crystalline phase with 10-ring channel system. This intermediate phase is only ordered in the layer perpendicular to the channel direction but disordered along the channel direction. For AlPO4-18, a layered phase forms as well under SAC conditions, which then transforms into AlPO4-18 via a series of structural reorganizations. The three inequivalent P sites of AlPO4-18 are found to develop at different rates with the P1, and P3 sites developing faster than the P2 site. Lastly, we have explored the synthesis of several Si and Co substituted AlPO4-based molecular sieves with AFI, AEL, and CHA structures under DGC conditions by using layered AIPO precursors as the starting materials. |
