Highly efficient and targeted adsorption of Congo Red in a novel cationic copper-organic framework with three-dimensional cages

Metal-organic frameworks (MOFs) with a three-dimensional (3D) caged structure and charged networks are promising adsorbents for separation and purification applications. However, it is a challenge to synthesize the MOFs in crystalline structures with a targeted ability of efficiently removing and reutilizing the individual diazo dye from sewage. Here, we report the synthesis of a novel copper-organic framework (Cu-MOF) with a 3D caged stucture and cationic networks using CuCl2 and a triazole ligand (methyl-1H-1,2,4-triazole-3-carboxylate MTC). The novel Cu-MOF exhibited highly efficient and targeted adsorption, separation and recovery characteristics for Congo Red (CR). Specifically, the maximal adsorption capacity was 119.76 mg·g−1, and the adsorbed CR can be efficiently released in NaCl-DMF solution with a recovery rate of almost 100%. After suffered from three adsorption–desorption cycles, the Cu-MOF could still reach a adsorption rate of >85% and maintain an intact skeleton structure. The batch-equilibrium experiment combined with density functional theory calculation further revealed that the adsorption process was spontaneous and exothermic, following the pseudo-second-order kinetic equation with monolayer diffusion. The specific adsorption of Cu-MOF for CR was precisely generated through electronic interaction with the –NH2 and -SO3 groups, exhibiting the adsorption energies of −2.16 eV and −2.04 eV, respectively. The two-dimensional correlation spectroscopy analysis comfirmed that the energy distribution in CR adsorption was dominated by electrostatic interactions, followed by π-π stacking and van der Waals interactions. In summary, our work not only presents a promising strategy for the targeted separation and purification of CR from wastewater, but also contributes to the design of highly efficient MOF-adsorbents for other organic pollutants.