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.