A novel ion-pair strategy for efficient separation of lithium isotopes using crown ethers

Effective separation of lithium isotopes (6Li and 7Li) is of great importance for the development of nuclear energy. Crown ethers (CEs) are widely used and reported separation systems, but the distribution coefficient of Li (DLi) is relatively low by CEs (DLi, 10−2–10−5) and mainly depends on the anion type of Li salt. Herein, a novel and facile ion-pair strategy by introducing tetrachloridoferrate ([FeCl4]−) as counter anion is presented, which helps to overcome the Hofmeister bias and facilitates the efficient transfer of Li+ from extremely hydrophilic chlorides. An exceptionally high DLi of 54 is achieved for benzo-15-crown-5 (B15C5), surpassing those of solvent extraction from solely Li salt aqueous solution. This strategy also demonstrates good separation performance of 6Li/7Li. The maximum 6Li/7Li separation factors (α6Li/7Li) of 1.038 and 1.049 are obtained for B15C5 and benzo-12-crown-4 (B12C4) in dichloroethane at 273 K. The highly efficient phase transfer of Li+ can be attributed to the formation of stable [Li(B15C5)(H2O)][FeCl4] or [Li(B12C4)2][FeCl4] ion pairs. Theoretical studies based on density functional theory (DFT) calculations suggest that the coordination and electrostatic environment of Li+ plays an essential role in the separation of 6Li/7Li. Weakening the coordination and electrostatic interactions associated with Li+ can facilitate the separation of 6Li/7Li. Additionally, B15C5 can be easily recycled by using ultra-pure water as the stripping solution. This work provides an industrially promising system for 6Li/7Li separation.