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The ambient hydration of the aluminophosphate JDF‐2 to AlPO‐53(A): insights from NMR crystallography
Author(s) -
Dawson Daniel M.,
Walton Richard I.,
Wimperis Stephen,
Ashbrook Sharon E.
Publication year - 2017
Publication title -
acta crystallographica section c
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.304
H-Index - 17
ISSN - 2053-2296
DOI - 10.1107/s2053229617000377
Subject(s) - microporous material , crystal structure , crystallography , density functional theory , isostructural , chemistry , magic angle spinning , crystal (programming language) , hydrate , molecule , solid state nuclear magnetic resonance , nuclear magnetic resonance spectroscopy , monoclinic crystal system , materials science , nuclear magnetic resonance , computational chemistry , stereochemistry , organic chemistry , physics , computer science , programming language
The aluminophosphate (AlPO) JDF‐2 is prepared hydrothermally with methylammonium hydroxide (MAH + ·HO − , MAH + = CH 3 NH 3 + ), giving rise to a microporous AEN‐type framework with occluded MAH + cations and extra‐framework (Al‐bound) HO − anions. 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 H 2 O 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 the NMR parameters obtained experimentally and those calculated from the (optimized) crystal structure of JDF‐2. However, several discrepancies are apparent between the experimental NMR parameters for AlPO‐53(A) and those calculated from the (optimized) crystal structure ( e.g. four 13 C resonances are observed, rather than the expected two). The unexpected resonances appear and disappear reversibly with the respective addition and removal of H 2 O, so clearly arise from AlPO‐53(A). We investigate the ambient hydration of JDF‐2 using quantitative 31 P 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 characterize 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 the MAH + cations and partial occupancy of the H 2 O sites.