Construction of Local‐Ion Trap in Phase‐Reversed Mixed Matrix COF Membranes for Ultrahigh Ion Selectivity

Abstract Artificial molecular/ion traps afford grand potential in membrane‐based separation processes. However, the existing trap‐based architectures often confer over‐strong binding forces, which severely impede the release of bound solutes during their transmembrane diffusion processes. Herein, we propose an unprecedented local‐ion trap bearing moderate binding force and additional repulsion force in a type of phase‐reversed mixed matrix covalent organic framework (PRCOF) membrane. By implementing COF as a continuous phase and polymer as a dispersed phase at the molecular level, the local‐ion trap is formed in the COF channels equipped with free amino groups from polyethyleneimine (PEI). This unique local‐ion trap built by electronegative COF nano‐domains and electropositive PEI nano‐domains offers appropriate interaction toward Li + , which allows the precise recognition and rapid transport of Li + in the membrane channels. By tuning the microenvironments of local‐ion trap, the optimum PRCOF‐1 membrane exhibits considerably high actual selectivity of 190 along with a rapid Li + permeation rate of 0.262 mol h −1 m −2 in dealing with a Li + /Mg 2+ binary mixture. This work provides in‐depth insights into the design of high‐performance membranes with appropriate chemical interactions.