Artificial Cation-Chloride Co-transporters for Chloride-Facilitated Lithium/Magnesium Separation

Abstract

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 notoriously 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 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 further 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, driven by only concentration gradient) 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.