Optimization of cation selectivity based on channel chemistry of covalent organic framework under confined size

XR Chu and JB Miao and QQ Ge and FM Sheng and R Xia and B Wu and JS Qian, JOURNAL OF MEMBRANE SCIENCE, 722, 123860 (2025).

DOI: 10.1016/j.memsci.2025.123860

Porous materials with regular pore structure have been widely used in membrane separation technologies, but there are still great challenges in the face of accurate selective ion separation. The adjustment of channel size and inner wall chemistry can optimize the ion transport behavior, achieving efficient selective separation of monovalent ions. However, the effect of cavity chemical microenvironment on ions migration under confined size needs to be further explored. Herein, three kinds of covalent organic frameworks (COFs)-based cation perm- selective membranes with different pore properties were constructed by selecting amino-containing ligands with sulfonic acid (-SO3-) group and methyl (-CH3) group. With the introduction of functional groups, the pore size of original COFs (TpPa) was further confined from similar to 2 nm to similar to 1 nm. Benefiting from the enhancement of cavity chemistry under confinement effect, the TpPa-SO3Na/PAN with a negatively charged structure achieved Na+/Mg2+ selectivity of up to similar to 480, more than similar to 80 times better than the TpPa-CH3/PAN with hydrophobic channel. Molecular dynamics simulation further verified that the dual-effect of hydrogen bonding and Coulomb interaction from -SO3- group can more efficiently peel off the outer water molecules of Na+ ions, resulting in the fastest migration. Interestingly, the ion distribution density map in the channel visually described that the synchronization of ion transport conditions drove the efficient migration of Na+ ions along a given location. The design of differentiated pore structure provides a platform for in-depth analysis of the influence of confined environment on ions transfer and separation mechanism.

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