Rational Construction of 3D Pillared Metal-Organic Frameworks: Synthesis, Structures, and Hydrogen Adsorption Properties

In our efforts toward rational design and systematic synthesis of 'pillar-layer' structure MOFs, three porous MOFs have been constructed based on [Zn-4(bpta)(2)(H2O)(2)] (H(4)bpta = 1,1'-biphenyl-2,2',6,6'-tetracarboxylic acid) layers and three different bipyridine pillar ligands. The resulted MOFs show similar structures but different pore volume and window size depending on the length of pillar ligands which resulted in distinct gas adsorption properties. In the three MOFs, [Zn-4(bpta)(2)(4,4'-bipy)(2)(H2O)(2)]center dot(DMF)(3)center dot H2O (I) (DMF = N, N'-dimethylformamide and 4,4'-bipy = 4,4'-bipyridine) reveals selective adsorption of H-2 over N-2 and O-2 as the result of narrow pore size. [Zn-4(bpta)(2)(azpy)(2)(H2O)(2)]center dot(DMF)(4)center dot(H2O)(3) (2) and [Zn-4(bra)(2) (dipytz)(2)(H2O)(2)]center dot(DMF)(4)center dot H2O (3) (azpy =4,4'-azopyridine, dipytz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) reveal pore structure change upon different activation conditions. In addition, the samples activated under different conditions show distinct adsorption behaviors of N-2 and O-2 gases. Furthermore, hydrogen adsorption properties of activated 1-3 were studied. The results indicated that the activation process could affect the hydrogen enthalpy of adsorption.