Multinuclear solid-state NMR studies on phase transition of mesostructured aluminophosphate

Abstract

The phase transition process from a hexagonal to a lamellar mesostructured aluminophosphate was studied by multinuclear solid-state NMR techniques in combination with low-angle XRD and elemental analysis measurements. The XRD result showed that after hydrothermal treatment for 9 h, the initial hexagonal phase began to transform to a lamellar phase. It was also found by NMR that some initially formed aluminophosphate was hydrolyzed during the earliest 2 h of the hydrothermal treatment, and the inorganic aluminophosphate framework became more and more condensed during the phase transition. The organic surfactant mainly has a gauche conformation in the hexagonal phase, whereas it mainly has a trans conformation in the lamellar phase and is packed more compactly. The Br− anion is found to be involved in the phase transition and likely enters into the interface to compensate the lost of negative charges that generated by the condensation of the aluminophosphate. Finally, a mechanism for the phase transition is proposed.