Functional UiO-66 Series Membranes with High Perm Selectivity of Monovalent and Bivalent Anions for Electrodialysis Applications

Metal–organic frameworks (MOFs) have a good separation potential due to their uniform pore size, easy functionalization, and adjustable structure. Therefore, we make use of this feature of MOFs for the separation of Cl–/SO42– systems with different charges and sizes. In this work, two kinds of UiO-66 membranes with different functional groups were synthesized and applied to the monovalent perm-selective separation of the Cl–/SO42– system in electrodialysis (ED). UiO-66-NH2 membranes were used to investigate the influence of current density and initial solution concentration on perm selectivity. A layer of UiO-66-NH2 was first grown on the anodic alumina oxide (AAO) disks. After that, short chains containing sulfonic acid groups were grafted onto the amino groups by the ring-opening reaction of 1,3-propanesulfonate. The structural stability of the MOF layer was confirmed by XRD analysis of the membrane surface after the ED test. Ion exchange capacity (IEC) and elemental mapping were used to confirm the comparison changes in the degree of sulfonation. It was found that UiO-66-NH2 membranes exhibited a higher monovalent anion perm selectivity with a peak value of 36.23 (initial mixture solution of 0.05 M NaCl and 0.05 M Na2SO4) at lower current density (2.5 mA·cm–2). The perm selectivity of UiO-66-SO3H membranes increased with running time under 24 h of sulfonation time. In particular, with low IEC value of 0.10–0.16, it still showed a good performance on perm selectivity. Compared with published polymer ion exchange membranes (perm selectivity < 5) for ED, it has higher perm selectivity, more convenient functionalization, and resistance to organic solvents.