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

Abstract

• A novel and facile ion-pair strategy for 6 Li/ 7 Li separation is presented. • An exceptionally high distribution of Li + is achieved for CEs using this strategy. • The strategy shows good separation performance of 6 Li/ 7 Li. • Weakening the interactions related to Li + can facilitate the separation of 6 Li/ 7 Li. Effective separation of lithium isotopes ( 6 Li and 7 Li) 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 ( D Li ) is relatively low by CEs ( D Li , 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 ([FeCl 4 ] − ) 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 D Li 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 6 Li/ 7 Li. The maximum 6 Li/ 7 Li separation factors ( α 6 L i / 7 L i ) 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)(H 2 O)][FeCl 4 ] or [Li(B12C4) 2 ][FeCl 4 ] 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 6 Li/ 7 Li. Weakening the coordination and electrostatic interactions associated with Li + can facilitate the separation of 6 Li/ 7 Li. Additionally, B15C5 can be easily recycled by using ultra-pure water as the stripping solution. This work provides an industrially promising system for 6 Li/ 7 Li separation.