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) (O (2) C-C (6) H (4) -CO (2) ), 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 degrees C). They have been characterized by X-ray diffraction analysis; vibrational spectroscopy; and solid-state (1) H and (1) H- (1) H radio-frequency-driven dipolar recoupling (RFDR), (1) H- (1) H double quantum correlation (DQ), and (13) C (1) H 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 MO (4) (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 degrees C under a primary vacuum) of the materials has been confirmed by subjecting MIL-68 (Ga) to solid-state (1) H MAS NMR. The (1) H- (1) H RFDR and (1) H- (1) H DQ spectra revealed important information on the spatial arrangement of the guest species with respect to the hybrid organic-inorganic network. (13) C (1) H 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) m (2) 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 A channels, is 3 wt % under 4 MPa.