Multinuclear Solid-State NMR Studies on the Formation Mechanism of Aluminophosphate Molecular Sieves in Ionic Liquids

Abstract

In the present work, multinuclear solid-state NMR techniques together with power X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to monitor the crystallization process of aluminophosphate AlPO4-11 molecular sieves in 1-ethyl-3-methylimidazolium bromide ([emim]Br) ionic liquids (ILs). The local environments of the selected solid samples were probed by one-dimensional 27Al, 31P, and 19F MAS NMR experiments, and more information was obtained from two-dimensional 27Al→31P heteronuclear correlation (HETCOR) and 27Al triple-quantum MAS (3Q MAS) experiments. It is found that a large amount of amorphous aluminophosphates was formed with the F–Aloct–O–Ppar and Altet–O–Ppar structures in the initial stage of aging. With increasing crystallization time, the partially condensed framework phosphorus species disappeared gradually and became fully condensed. Meanwhile, the octahedral Al was transformed into the pentahedral and tetrahedral Al species with the structures of F–Alpenta–O–Pful and Altet–O–Pful in the AlPO4-11 frameworks. During the crystallization process, [emim] cations acting as the structure-directing agents were occluded into the channels, and F– anions were connected with pentahedral Al in the final AlPO4–11 phase. All of these findings can allow proposing a possible formation mechanism for the synthesis of AlPO4-11 in ionic liquids.