A porous framework polymer based on a zinc(II) 4,4′-bipyridine-2,6, 2′,6′-tetracarboxylate: Synthesis, structure, and "zeolite-like" behaviors

The robust metal-organic framework compound {[Zn₂(L)] ·4H₂O}_∞ I has been synthesized by hydrothermal reaction of ZnCl₂ and 4,4′-bipyridine-2,6, 2′,6′-tetracarboxylic acid (H₄L). Compound I crystallizes in a chiral space group, P4₂2₁2, with the chirality generated by the helical chains of hydrogen-bonded guest water molecules rather than by the coordination framework. Removal of guest water molecules from the crystal affords the porous material, [Zn₂(L)]_∞ (II), which has very high thermal stability and is chemically inert. The N ₂ isotherm of II at 77 K suggests a uniform porous structure with a BET surface area of 312.7 m²/g and a remarkably strong interaction with N₂ molecules (βE₀ = 29.6 kJ mol^-1). II also exhibits significant gas storage capacities of 1.08 wt % for H ₂ at 4 bar and 77 K and 3.14 wt % (44.0 cm³/g, 67 v/v) for methane at 9 Bar at 298 K. The adsorption behavior of II toward organic solvent vapors has also been studied, and isotherms reveal that for different solvent vapors adsorption is dominated by two types of processes, absorbate-absorbate or absorbate-absorbent interactions. The adsorption and desorption kinetic processes in II are determined mainly by the molecular size of the guest species and their interaction with the host.