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

Dawson, Daniel M. and Walton, Richard I. and Wimperis, Stephen and Ashbrook, Sharon E. (2017) The ambient hydration of the aluminophosphate JDF-2 to AlPO-53(A):insights from NMR crystallography. Acta Crystallographica Section C: Structural Chemistry, 73 (3). pp. 191-201. ISSN 2053-2296

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Abstract

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+ 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 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 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 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 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 H2O sites.

Item Type:
Journal Article
Journal or Publication Title:
Acta Crystallographica Section C: Structural Chemistry
Additional Information:
# 2017 International Union of Crystallography
Uncontrolled Keywords:
/dk/atira/pure/subjectarea/asjc/2500/2505
Subjects:
ID Code:
86604
Deposited By:
Deposited On:
05 Jun 2017 12:40
Refereed?:
Yes
Published?:
Published
Last Modified:
31 May 2020 04:38