New Insights into the Hydrogen Bond Network in Al-MIL-53 and Ga- MIL-53 Guillaume Ortiz, † Ge ́ rald Chaplais,* ,† Jean-Louis Paillaud, † Habiba Nouali, † Joë l Patarin, † Jesus Raya, ‡ and Claire Marichal* ,† † Equipe Mate ́ riaux a ̀ Porosite ́ Contrô lé e (MPC), Institut de Science des Mate ́ riaux de Mulhouse (IS2M), UMR 7361, ENSCMu, Universite ́ de Haute Alsace (UHA), CNRS, 3 bis rue Alfred Werner, F-68093 Mulhouse, France ‡ Biophysique des membranes et RMN, UMR 7177, Universite ́ de Strasbourg, CNRS, 1 rue Blaise Pascal, F-67000 Strasbourg, France *S Supporting Information 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). 1 H 2D double- quantum−single-quantum (DQ−SQ) magic angle spinning (MAS) NMR experiments unambiguously evidence two inequivalent water molecules in Al-MIL-53_np_H 2 O. 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 1 H resonances to each water molecule was possible. 1 H 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_H 2 O), the situation is different. 2D 1 H− 13 C heteronuclear correlation MAS NMR, high-field 1 H 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. 1. INTRODUCTION Among the porous materials, metal−organic framework (MOF)-type materials are very interesting owing to their hybrid and most often crystalline framework consisting of metal species connected by organic linkers. By choosing properly the metal and the organic linker, various topologies can be prepared, allowing fine tuning of the physicochemical proper- ties of the resulting material. The MOFs are considered to be versatile porous materials for large potential applications such as drug storage/release, 1,2 gas storage, 3−8 separation, 8,9 catalysis, 10,11 and chemical sensing. 12 MIL-53-type materials constitute one of the most studied and flourishing families of MOFs since they exist with various trivalent metals such as Al, Ga, Sc, Cr, Fe, V, .... The aluminum form of MIL-53 is particularly interesting since this structure is capable of reversibly adsorbing water molecules. 13 The chemical stability of Al-MIL-53 appears to be intermediate between those of Cr-MIL-53 and V-MIL-47 toward acids, bases, and water, 14 and the formation of a γ-Al(O)(OH) shell has been evidenced after water reflux. 15 Nevertheless, the dehydrated form of Al-MIL-53 is remarkably stable upon heating until 500 °C, whereas Cr-MIL-53 or V-MIL-47 decomposes already at 350 °C. 14 As for Ga-MIL-53, even if it seems to be less stable than the aluminum counterpart, it has been shown to be a more efficient and selective catalyst than H- BEA or H-MOR for the alkylation of biphenyl. 16 One main difference between Al- and Ga-MIL-53 resides in the nature of the phase obtained after dehydration. Indeed, the removal of occluded water molecules from the aluminum hydrated form (Al-MIL-53_np_H 2 O; see Table 1 and Figure 1b) leads to a dehydrated and large-pore phase (Al-MIL- 53_lp_empty; see Figure 1a), in the same way as Cr-MIL-53, whereas, for the gallium counterpart containing a small amount of fluorine (Ga(OH,F)_MIL-53_np_H 2 O; see Figure 1g) or no fluorine (Ga-MIL-53_np_H 2 O_dc; see Figure 1e), this process leads to a dehydrated and narrow-pore form (Ga-MIL- 53_np_empty/Ga(OH,F)-MIL-53_np_empty; see Figure 1d). As described recently, this difference observed experimentally between Al- and Ga/Ga(OH,F)-MIL-53 has been rationalized using quantum chemistry calculations. 17 They suitably explain the higher np → lp transition temperature experimentally evidenced by variable-temperature X-ray diffraction (VT-XRD), differential scanning calorimetry (DSC), and FTIR in the 450− 520 K range 18 for Ga-MIL-53 versus the 150−325 K range for Al-MIL-53. 19 Even if Ga- and Ga(OH,F)-MIL-53 present many similarities, it is worth noting that Ga_MIL-53_np_H 2 O_dc crystallizes with a doubling of the unit cell volume compared to Received: June 13, 2014 Revised: September 2, 2014 Published: September 3, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 22021 dx.doi.org/10.1021/jp505893s | J. Phys. Chem. C 2014, 118, 22021−22029