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 [Zn4(bpta)2(H2O)2] (H4bpta = 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, [Zn4(bpta)2(4,4′-bipy)2(H2O)2]·(DMF)3·H2O (1) (DMF = N,N′-dimethylformamide and 4,4′-bipy = 4,4′-bipyridine) reveals selective adsorption of H2 over N2 and O2 as the result of narrow pore size. [Zn4(bpta)2(azpy)2(H2O)2]·(DMF)4·(H2O)3 (2) and [Zn4(bpta)2(dipytz)2(H2O)2]·(DMF)4·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 N2 and O2 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.