Examining the self-assembly of microporous material AIPO4-11 by dry-gel conversion

Crystallization of a representative aluminophosphate microporous material, namely AlPO4-11 with AEL structure, has been investigated by dry-gel conversion (DGC) including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The crystallization under SAC and VPT conditions appears to follow the same pathway: the initial amorphous material is converted to a semicrystalline intermediate, which transforms into AlPO4-1 1. The semicrystalline intermediate appears to have a three-dimensional structure that bears some similarity to AEL structure. Raman spectrum suggests that it contains the 10-ring channel. XRD patterns indicate that the structure seems to be only ordered in the layer associated with the unit cell ab plane of AlPO4-1 1. The layers are stacked together via H-bonding and van der Waals interactions. However, the layers are not perfectly aligned as they are in the AEL structure, resulting in disordering along the 10-ring channel direction. Quantitative P-31 MAS NMR spectra confirm the amorphous to semicrystalline to AEL phase transition sequence. Since the semicrystalline phase is held together by weak intermolecular interactions, washing with water results in the conversion back to amorphous phase. The weak bonding forces in the semicrystalline phase are important, because they provide flexibility for reorganization of local bonding environment to form a three-dimensional covalent AEL framework. The VPT study provides unambiguous evidence that the formation of the semicrystalline phase is under the direction of di-n-propylamine.