A Robust Co(II)-Based MOF for Highly Selective and Efficient Adsorption of Congo Red Dye

Abstract

A new heteroleptic Co(II)-based metal−organic framework (MOF), {[Co₂(L)(bib)_1.5(H₂O)]·2H₂O}_n (1), is synthesized using 5,5′-(1,3-phenylenebis(methyleneoxy)diisophthalate) (H₄L) and 4,4′-bis(imidazolyl)biphenyl (bib) as structurally complementary linkers and characterized. The structural analysis reveals that 1 adopts a robust 2-fold interpenetrated 3D framework with a (4,6)-connected fsc topology, whose highly interlocked architecture contributes to structural stability in aqueous media. The flexible V-shaped polycarboxylate ligand promotes extended connectivity, while the rigid nitrogen-rich biphenyl-imidazole linker enhances framework stability via strong coordination and π···π stacking interactions. Adsorption studies demonstrate that MOF 1 exhibits remarkable selectivity and efficiency toward the anionic dye Congo Red (CR), achieving 99.55% removal and a maximum adsorption capacity (q_max) of 5529 mg·g⁻¹ at 20 °C under mild conditions. Dosage optimization identifies 9 mg of MOF 1 as optimal, providing 98.94% removal and 5496 mg·g⁻¹ capacity under near-ambient conditions. pH-dependent studies show maximum uptake at pH 6, where electrostatic attraction, hydrogen bonding, and framework integrity act synergistically. Furthermore, MOF 1 maintains a high recovery efficiency of 87.50% over multiple cycles. The material retains an appreciable adsorption performance over repeated cycles, although some loss in capacity occurs during successive adsorption−desorption treatments. The combination of high adsorption capacity, rapid dye removal, and structural stability positions MOF 1 as a promising candidate for sustainable wastewater dye remediation.