Linker engineering in mixed-ligand metal–organic frameworks for simultaneously enhanced benzene adsorption and benzene/cyclohexane separation†

Abstract

The development of metal–organic frameworks (MOFs) that simultaneously possess high adsorption capacity and selectivity for benzene (Bz)/cyclohexane (Cy) separation is a formidable challenge. In this study, we employ the mixed-ligand approach to construct two novel isoreticular MOFs, designated DZU-72 and -73, to regulate Bz adsorption and Bz/Cy separation performances. Guided by the linker engineering, we have incorporated distinct dicarboxylate ligands, benzene-1,4-dicarboxylic acid (H₂BDC) and naphthalene-2,6-dicarboxylic acid (H₂NDC) with different aromatic rings, as second ligands into the frameworks of two MOFs, respectively. Vapor adsorption tests demonstrate that DZU-73 featuring naphthalene ring units exhibits superior uptake for Bz (6.92 mmol g⁻¹) compared to DZU-72 (Bz: 4.30 mmol g⁻¹) with benzene ring units. The calculated selectivity for Bz/Cy separation shows that DZU-73 has an IAST selectivity value of about 28.1, nearly 2.5 times that of DZU-72 (11.3). Breakthrough experiments further reveal that DZU-73 can effectively separate Bz/Cy mixed vapors with an interval time of 17.6 min g⁻¹. Density functional theory (DFT) calculations indicate that the synergistic effects of optimal pore environments and host–guest interactions from the naphthalene ring are pivotal in significantly enhancing the Bz adsorption capacity and Bz/Cy selectivity of DZU-73. This work highlights that linker engineering in mixed-ligand MOFs is a powerful strategy for tuning the Bz adsorption and Bz/Cy separation.