Artificial Cation–Chloride Co‐Transporters for Chloride‐Facilitated Lithium/Magnesium Separation

Inspired by nature, many artificial ion sieving materials have been developed, shedding light on the next-generation ion, e.g., Li⁺, extraction applications. Artificial co-transporters remain difficult to construct since they have a much more complex ion-sieving property. For example, the cation-chloride co-transporters have both alkaline ion and chloride ion selectivity but no alkaline ion/chloride ion selectivity. We here demonstrate a method to construct artificial co-transporters, using a porous organic framework membrane which has a relatively disordered stacking structure and rich quaternary ammonium groups paired with counter-ions. This imparts the membrane with extremely narrow pores (∼0.3 nm) and almost no surface charge, enabling size-based high alkaline ion selectivity against other cations, high Cl^- selectivity against other anions, but almost no alkaline ion/Cl^- selectivity. Such synchronized sieving property allows us to enhance the extraction of high-value cations (Li⁺) by simply feeding excessive low-value anions (Cl^-). As a demonstration, we realized high-flux (0.44 mol m^-2 h^-1) and highly selective (selectivity: 185) Li⁺/Mg²⁺ separation by reversing the current industrial brine-based lithium extraction process, i.e., sieving Li⁺ before removing NaCl.