Enhancement of Gas Sorption and Separation Performance via Ligand Functionalization within Highly Stable Zirconium-Based Metal–Organic Frameworks

By using a modulated synthetic strategy, three isostructural Zr-based metal organic frameworks Zr6O4(OH)4(EDDC)6 (JLU-Liu34), Zr6O4(OH)4(EDDC-NO2)6 (JLU-Liu35), and Zr6O4(OH)4(EDDC-NH2)6 (JLU-Liu36) (H2EDDC = (E)-4,4′-(ethene-1,2-diyl)dibenzoic acid, H2EDDC-NO2 = (E)-2,2′-dinitro-4,4′-(ethene-1,2-diyl)dibenzoic acid, H2EDDC-NH2 = (E)-2,2′-diamino-4,4′-(ethene-1,2-diyl)dibenzoic acid) which possess different substituent groups have been successfully synthesized. Significantly, the framework and crystallinity of the functionalized Zr-metal–organic frameworks (Zr-MOFs) are well-retained even though the nature of the functional group is different (NO2 is electron-withdrawing, whereas NH2 is electron-releasing). All of the three analogues display 12-connected fcu topology, and the classical Zr6 clusters in the porous crystal make the whole framework approach high chemical and thermal stability. As a result of the functionalization effect, the three analogues show different sorption and separation abilities to small gases, especially for JLU-Liu34 which exhibits a significant C3H8 uptake capacity of 303 cm3 g–1 at 273 K under 1 bar. Although JLU-Liu36 shows lower Brunauer–Emmett–Teller surface area than JLU-Liu34, it possesses enhanced CO2 adsorption enthalpy and selectivity for CO2 over CH4 influenced by the additional amino groups. Ligand modification provides an efficient strategy to design and synthesize porous MOFs materials for small gases sorption and separation.