Effect of pillar modules and their stoichiometry in 3D porous frameworks of Zn(II) with [Fe(Cn)₆]^3–: high CO₂/N₂ and CO₂/CH₄ selectivity

Abstract

We report the synthesis, single-crystal structural characterization, and selective gas adsorption properties of three new 3D metal–organic frameworks of Zn(II), {[Zn₃(bipy)₃(H₂O)₂][Fe(CN)₆]_2·2(bipy)·3H₂O} _n (1), {[Zn₃(bipy)][Fe(CN)₆]₂·(C₂H₅OH)·H₂O}_n (2), and {[Zn₃(azpy)₂(H₂O)₂][Fe(CN)₆]_2·4H₂O} _n (3) (bipy = 4,4′-bipyridyl and azpy = 4,4′-azobipyridyl), bridged by [Fe(CN)₆]^3- and exobidentate pyridyl-based linkers. Compounds 1–3 have been successfully isolated by varying the organic linkers (bipy and azpy) and their ratios during the synthesis at RT. Frameworks 1 and 3 feature a biporous-type network. At 195 K, compounds 1–3 selectively adsorb CO₂ and completely exclude other small molecules, such as N₂, Ar, O₂, and CH₄. Additionally, we have also tested the CO₂ uptake capacity of 1 and 3 at ambient temperatures. By using the isotherms measured at 273 and 293 K, we have calculated the isosteric heat of CO₂ adsorption, which turned out to be 35.84 and 35.53 kJ mol^-1 for 1 and 3, respectively. Furthermore, a reasonably high heat of H₂ adsorption (7.97 kJ mol^-1 for 1 and 7.73 kJ mol^-1 for 3) at low temperatures suggests strong interaction of H2 molecules with the unsaturated Zn(II) metal sites and as well as with the pore surface. Frameworks 1 and 3 show high selectivity to CO₂ over N₂ and CH₄ at 273 K, as calculated based on the IAST model. The high values of ΔH_CO2 and ΔH_H2 stem from the preferential electrostatic interaction of CO₂ with the unsaturated metal sites, pendent nitrogen atoms of [Fe(CN)₆]^3-, and Π-electron cloud of bipyridine aromatic rings as understood from first-principles density functional theory based calculations.