Enhanced Breathing Effect of Nanoporous UIO-66-DABA Metal-Organic Frameworks with Coordination Defects for High Selectivity and Rapid Adsorption of Hg(II)

Herein, the diamino-functionalized UIO-66-DABA is constructed by introducing coordination defects of 3,5-diaminobenzoic acid (DABA) as the metal-organic framework (MOF) linkers, which are systematically characterized by scanning electron microscopy, ¹H nuclear magnetic resonance, and Brunauer-Emmett-Teller analysis. The powder X-ray diffraction and thermogravimetric analysis results show that it exhibits excellent thermal stability and acid stability. Importantly, the adsorption experiments show that UIO-66-DABA has high selectivity and excellent adsorption performance (713 mg/g) for Hg²⁺. The adsorption data, including isotherms and kinetics, are well-matched with both Langmuir and pseudo-second-order models. Thermodynamic analysis reveals that the adsorption process is spontaneous, disordered, and exothermic. It is observed that the adsorption of a low concentrations of Hg²⁺ (20 μg/L) can reach drinking standards within 8 h. The recyclable usage of UIO-66-DABA for the removal of Hg²⁺ makes it potentially useful for industrial applications. Furthermore, the density functional theory results and molecule dynamics simulations further explore the interactions and conformational relationships between Hg²⁺ and MOFs (UIO-66, UIO-66-(NH₂)₂, and UIO-66-DABA). Among these, the lone electron pair on the amino nitrogen plays the key role in the selective adsorption for Hg²⁺. Additionally, the DABA ligand’s large vibrational amplitudes induce an increased breathing effect within the MOF structure, thereby facilitating the rapid entry of Hg²⁺ into the pores. As such, our work provides a novel strategy that can regulate the adsorption selectivity and adsorption efficiency of heavy metal ions by MOFs via introducing coordination defects.