The ambient hydration of the aluminophosphate JDF-2 to AlPO-53(A): insights from NMR crystallography

The aluminophosphate (AlPO), JDF-2 is prepared hydrothermally with methylammonium hydroxide (MAH⁺ HO^–, MAH⁺ = CH₃NH₃⁺), giving rise to a microporous AEN-type framework with occluded MAH⁺ and extra framework (Al-bound) HO^–. Despite the presence of these species within its pores, JDF-2 can hydrate upon exposure to atmospheric moisture to give AlPO-53(A), an isostructural material whose crystal structure contains one molecule of H₂O per formula unit. This hydration can be reversed by mild heating (such as the frictional heating from magic angle spinning). Previous work has shown good agreement between NMR parameters obtained experimentally and calculated from the (optimised) crystal structure of JDF-2. However, several discrepancies are apparent between the experimental NMR parameters for AlPO-53(A) and those calculated from the (optimised) crystal structure (e.g., four ¹³C resonances are observed, rather than the expected two). The unexpected resonances appear and disappear reversibly with the respective addition and removal of H₂O, so clearly arise from AlPO-53(A). We investigate the ambient hydration of JDF-2 using quantitative ³¹P MAS NMR to follow the transformation over the course of ~3 months. The structures of JDF-2 and AlPO-53(A) are also investigated using a combination of multinuclear solid-state NMR spectroscopy to characterise the samples, and first-principles density functional theory (DFT) calculations to evaluate a range of possible structural models in terms of calculated NMR parameters and energetics. The published structure of JDF-2 is shown to be a good representation of the dehydrated material, but modification of the published structure of AlPO-53(A) is required to provide calculated NMR parameters that are in better agreement with experiment. This modification includes reorientation of all MAH⁺ cations and partial occupancy of the H₂O sites.