The Kagomé Topology of the Gallium and Indium Metal-Organic Framework Types with a MIL-68 Structure: Synthesis, XRD, Solid-State NMR Characterizations, and Hydrogen Adsorption

The vanadium-based terephthalate analogs of MIL-68 have been obtained with gallium and indium (network composition: M(OH)(O2C−C6H4−CO2), M = Ga or In) by using a solvothermal synthesis technique using N,N-dimethylformamide as a solvent (10 and 48 h, for Ga and In, respectively, at 100 °C). They have been characterized by X-ray diffraction analysis; vibrational spectroscopy; and solid-state 1H and 1H−1H radio-frequency-driven dipolar recoupling (RFDR), 1H−1H double quantum correlation (DQ), and 13C{1H} cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The three-dimensional network with a Kagomé-like lattice is built up from the connection of infinite trans-connected chains of octahedral units MO4(OH)2 (M = Ga or In), linked to each other through the terephthalate ligands in order to generate triangular and hexagonal one-dimensional channels. The presence of DMF molecules with strong interactions within the channels as well as their departure upon calcination (150 °C under a primary vacuum) of the materials has been confirmed by subjecting MIL-68 (Ga) to solid-state 1H MAS NMR. The 1H−1H RFDR and 1H−1H DQ spectra revealed important information on the spatial arrangement of the guest species with respect to the hybrid organic−inorganic network. 13C{1H} CPMAS NMR of activated samples provided crystallographically independent sites in agreement with X-ray diffraction structure determination. Brunauer−Emmett−Teller surface areas are 1117(24) and 746(31) m2 g−1 for MIL-98 (Ga) and MIL-68 (In), respectively. Hydrogen adsorption isotherms have been measured at 77 K, and the storage capacities are found to be 2.46 and 1.98 wt % under a saturated pressure of 4 MPa for MIL-68 (Ga) and MIL-68 (In), respectively. For comparison, the hydrogen uptake for the aluminum trimesate MIL-110, which has an open framework with 16 Å channels, is 3 wt % under 4 MPa.