MIL-96, a porous aluminum trimesate 3D structure constructed from a hexagonal network of 18-membered rings and μ3-oxo-centered trinuclear units

A new aluminum trimesate Al12O(OH)(18)(H2O)(3)(Al-2(OH)(4))[btc](6)(.)24H(2)O, denominated MIL-96, was synthesized under mild hydrothermal conditions ( 210 C, 24 h) in the presence of 1,3,5-benzenetricarboxylic acid ( trimesic acid or H 3 btc) in water. Hexagonal crystals, allowing a single-crystal XRD analysis, are grown from a mixture of trimethyl 1,3,5-benzenetricarboxylate (Me(3)btc), HF, and TEOS. The MIL-96 structure exhibits a three-dimensional (3D) framework containing isolated trinuclear mu(3)-oxo-bridged aluminum clusters and infinite chains of AlO4(OH)(2) and AlO2(OH)(4) octahedra forming a honeycomb lattice based on 18-membered rings. The two types of aluminum groups are connected to each other through the trimesate species, which induce corrugated chains of aluminum octahedra, linked via mu(2)-hydroxo bonds with the specific -cis-cis-trans- sequence. The 3D framework of MIL-96 reveals three types of cages. Two of them, centered at the special positions 0 0 0 and 2/3 1/3 1/4, have estimated pore volumes of 417 and 635 angstrom(3), respectively, and encapsulate free water molecules. The third one has a smaller pore volume and contains disordered aluminum octahedral species (Al(OH)(6)). The solid-state NMR characterization is consistent with crystal structure and elemental and thermal analyses. The four aluminum crystallographic sites are resolved by means of Al-27 3QMAS technique. This product is able to sorb both carbon dioxide and methane at room temperature (4.4 mmol(.)g(-1) for CO2 and 1.95 mmol(.)g(-1) for CH4 at 10 bar) and hydrogen at 77 K (1.91 wt % under 3 bar).