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

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⁺/Mg²⁺ binary mixture. This work provides in-depth insights into the design of high-performance membranes with appropriate chemical interactions.