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

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). H-1 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 H-1 resonances to each water molecule was possible. H-1 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 H-1-C-13 heteronuclear correlation MAS NMR, high-field H-1 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.