Nanoporous Metal-Organic Framework Adsorbent Constructed via Ligand Tailoring for Rare-Earth Metal Ion Recovery

Herein, a nanoporous metal-organic framework (UiO-66-H-1/H-2) was constructed, which was developed as a universal platform for rare earth (RE) ion adsorption. In detail, 1,2,4,5-benzenetetracarboxylic acid (H-2 linker) and 1,2,4-benzenetricarboxylic acid (H-1 linker) were simultaneously introduced to construct the mixed-linker UiO-66-H-1/H-2. Compared with UiO-66-H-2, the slightly reduced carboxyl groups of UiO-66-H-1/H-2 boost the specific surface area. This feature as well as the nanoscale particle promotes the diffusion and mass transfer efficiency of guest ions in the adsorbent channel. Meanwhile, the triangular-window channel shows an angstrom-level carboxyl-rich trap, whose size suits well with the diameters of the RE ions. Benefiting from these advantages, effective adsorption for various RE ions was achieved in the optimized UiO-66-H-1/H-2-a, with large capacities of 150-250 mg g(-1) and rapid kinetics (similar to 30 min for equilibrium). Ionization of carboxyl induces the negatively charged surface of UiO-66-H-1/H-2-a, which facilitates the diffusion of cationic RE ions via electrostatic interactions. Meanwhile, the strong chelation interaction from free carboxyl groups contributes largely to the tight capture of RE ions, as proved by experiments and theoretical calculations. These results suggest that UiO-66-H-1/H-2-a is an effective adsorbent for the capture and recovery of RE ions. We expect that this work can provide an interesting insight into the balance group number and porosity.