Molecular design of covalent−organic framework membranes for Li+/Mg2+ separation: Significant charge effect

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-NH 3 + ) and thus different charges are designed. It is found that the positively charged membrane (Tp-Tta-NH 3 + ) shows higher ion selectivity in mixed Li + /Mg 2+ ion solution, compared with the negatively charged and neutral membranes. In Tp-Tta-NH 3 + 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 Mg 2+ ions, the former is easier to dehydrate and enter the pore. The charge effect in Li + /Mg 2+ separation uncovered by molecular simulation is microscopically insightful toward the design of COF membranes and other organic framework membranes for high-performance ion separation. • Three types of COF membranes with positive, neutral, negative charges are constructed. • MD simulation is applied to elucidate Li + /Mg 2+ separation in COF membranes. • The positively charged COF membrane exhibits the highest selectivity. • The secondary confinement and differentiated dehydration provide high selectivity.