The ambient hydration of the aluminophosphate JDF-2 to AlPO-53(A) : insights from NMR crystallography
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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.
Dawson , D M , Walton , R I , Wimperis , S & Ashbrook , S E 2017 , ' The ambient hydration of the aluminophosphate JDF-2 to AlPO-53(A) : insights from NMR crystallography ' Acta Crystallographica Section C Structural Chemistry , vol C73 , no. 3 , pp. 191-201 . DOI: 10.1107/S2053229617000377
Acta Crystallographica Section C Structural Chemistry
© 2017, International Union of Crystallography. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1107/S2053229617000377
The authors would like to thank EPSRC for computational support through the Collaborative Computational Project on NMR Crystallography (CCP-NC), via EP/M022501/1 and EP/J501510/1. SEA would like to thank the Royal Society and Wolfson Foundation for a merit award. The research data (and/or materials) supporting this publication can be accessed at doi:10.17630/bad7af52-94cc-4a85-8aed-09fb761b894c.
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