Cost-effective 17O enrichment and NMR spectroscopy of mixed-metal terephthalate metal-organic frameworks
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17O solid-state NMR spectroscopy is employed to investigate the cation disorder in metal–organic frameworks containing two different types of metal cations. Although NMR offers exquisite sensitivity to the local, atomic-scale structure, making it an ideal tool for the characterisation of disordered materials, the low natural abundance of 17O (0.037%) necessitates expensive isotopic enrichment to acquire spectra on a reasonable timescale. Using dry gel conversion and a novel steaming method we show that cost-effective and atom-efficient enrichment of MOFs is possible, and that high-resolution 17O NMR spectra are sensitive both to the structural forms of the MOF and the presence of guest molecules. For mixed-metal forms of MIL-53, NMR can also provide information on the final composition of the materials (notably different to that of the initial starting material) and the preference for cation clustering/ordering within the MOFs. For Al, Ga MIL-53, the distribution of cations results in a mixed-pore form upon exposure to water, unlike the different structures seen for the corresponding end members. This work shows that as good levels of enrichment can be achieved at reasonable cost, 17O NMR spectroscopy should be an invaluable tool for the study of these important functional materials.
Bignami , G P M , Davis , Z H , Dawson , D M , Morris , S A , Russell , S E , McKay , D , Parke , R E , Iuga , D , Morris , R E & Ashbrook , S E 2017 , ' Cost-effective 17 O enrichment and NMR spectroscopy of mixed-metal terephthalate metal-organic frameworks ' Chemical Science , vol In press . DOI: 10.1039/C7SC04649A
Copyright 2017 the authors. This Open Access Article is licensed under a Creative Commons Attribution 3.0 Unported Licence
The authors thank the ERC (EU FP7 Consolidator Grant 614290 “EXONMR”), and EPSRC (EP/K025112/1, EP/L014475/1 and EP/M506631/1 (for GPMB)). SEA would like to thank the Royal Society and the Wolfson Foundation for a merit award. The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC (contract reference PR140003), as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).
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