Investigation of Crystallization of Molecular Sieve AlPO4-5 by the Dry Gel Conversion Method

Crystallization of AlPO4-5 (AFI), a representative aluminophosphate-based molecular sieve with AFI topology, was examined under dry gel conversion, in particular, vapor phase transport (VPT) conditions. The VPT approach allows one to “freeze” crystallization at different stages and, therefore, to capture several reactive intermediates, which provide “snapshots” of the crystallization process. These intermediates were characterized by powder X-ray diffraction, Raman spectroscopy, and a combination of several solid-state NMR techniques. The results show that the reactive intermediates have two key features: (1) They bear certain structural characteristics of the target framework. (2) They are fragile and held together by weak bonding interactions. Hydrogen bonding apparently plays a key role in the formation of intermediates and their subsequent transformation to the final AFI structure. An AlPO4-5 crystallization mechanism under VPT conditions was proposed.