Molecular engineering of microporous crystals: (IV) Crystallization process of microporous aluminophosphate AlPO ₄-11

The products of microporous aluminophosphate AlPO ₄-11 crystallized for different periods of time were freeze-dried and characterized by XRD and NMR techniques. Six crystallographically distinct Al- and P-centered large fragments and 34 small fragments of dimer (Al-P), trimer (Al-P-Al and P-Al-P), tetramer (Al-P ₃ and P-Al ₃), pentamer (Al-P ₄ and P-Al ₄), and 4- and 6-membered rings were extracted from the framework of AlPO ₄-11. Each of the six large fragments contained two completed coordination layers surrounding the centered atom. The shielding tensors of the Al and P atoms of the fragments were calculated using the quantum mechanics density functional theory. The experimentally observed chemical shifts of Al or P from the well-crystallized AlPO ₄-11 were assigned to the shielding tensors of the center atoms of the six large fragments and were further used as references in the determination of the chemical shifts of the Al or P atoms in the small fragments. A comparison of the calculated chemical shifts of the Al and P atoms in the small fragments to the experimental data of the products isolated at different crystallization periods suggested that the fragments of dimers, trimers of the form Al-P-Al, tetramers of the form P-Al ₃, and pentamers may exist in the crystallization process of AlPO ₄-11. On the basis of these observations regarding the putative small fragments, a possible crystallization process of AlPO ₄-11 was proposed.