New Insights into the Hydrogen Bond Network in Al-MIL-53 and Ga-MIL-53

Abstract

Metal−organic framework-type Al-, Ga-, and Ga(OH,F)-MIL-53 have been characterized by solid-state NMR and powder X-ray diffraction (PXRD). 1H 2D double-quantum–single-quantum (DQ–SQ) magic angle spinning (MAS) NMR experiments unambiguously evidence two inequivalent water molecules in Al-MIL-53_np_H2O. A careful reinvestigation of the XRD structure of hydrated Al-MIL-53 proves, for the first time, the doubling of the unit cell supporting the presence of two inequivalent water molecules. One type of water molecule is located in the type A channel, interacting with the aromatic protons of framework organic moieties, whereas the other type of water molecule is in the type B channel far away from aromatic protons. Assignment of the corresponding 1H resonances to each water molecule was possible. 1H 1D MAS NMR leads to the same conclusion (i.e., two inequivalent water molecules) for the isostructural gallium-based material thanks to the positioning of the hydrogen atoms by Rietveld refinement. Moreover, when Ga-MIL-53 is prepared with fluorine in the synthesis medium (giving Ga(OH,F)-MIL-53_np_H2O), the situation is different. 2D 1H–13C heteronuclear correlation MAS NMR, high-field 1H DQ–SQ, and radio-frequency-driven recoupling MAS NMR indicate that only one type of water molecule is found in this material, in agreement with the structure.