Abstract
In this study, molecular simulation is utilized to explore the charge effect in COF membranes on separation performance. Three types of COF membranes with different functional groups (Tp-Tta-COOH, Tp-Tta-COO− and Tp-Tta-NH3+) and thus different charges are designed. It is found that the positively charged membrane (Tp-Tta-NH3+) shows higher ion selectivity in mixed Li+/Mg2+ ion solution, compared with the negatively charged and neutral membranes. In Tp-Tta-NH3+ membrane, the effective pore size is reduced by the secondary confinement effect of Cl− ions. The orderly arranged anions (Cl−) reduce the actual pore size and generate the secondary confinement effect, which fortifies the pore-entrance sieving effect, and due to the large difference in hydration energies of Li+ and Mg2+ ions, the former is easier to dehydrate and enter the pore. The charge effect in Li+/Mg2+ separation uncovered by molecular simulation is microscopically insightful toward the design of COF membranes and other organic framework membranes for high-performance ion separation.
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