Benzo-15-Crown-5 Functionalized Ionic Liquids with Enhanced Stability for Effective Separation of Lithium Isotopes: The Effect of Alkyl Chain Length

Crown ethers (CEs) are promising materials for lithium isotope ( 6 Li and 7 Li) separation for their unique cavity structure and size effect, but the low partitioning of Li + makes the practical implementation not applicable. The loss of CE molecules is also a critical problem. In this work, benzo-15-crown-5 functionalized imidazolium ionic liquids with different alkyl chain lengths ([C n (B15C5)mim][NTf 2 ], n = 6, 8, 10) are synthesized. The association constants of [C n (B15C5)mim][NTf 2 ] and Li +, Li + partition, and the separation factor of 6 Li and 7 Li using [C n (B15C5)mim][NTf 2 ] were investigated. Compared with the B15C5 monomer, the covalent bonding of B15C5 onto the cation of imidazolium ILs contributes to the dramatic increase of binding ability toward Li + . The association constants of [C n (B15C5)mim][NTf 2 ] with Li + increase with the elevation of the alkyl chain length due to the flexibility changes. However, [C 8 (B15C5)mim][NTf 2 ] delivers the most outstanding partition coefficient of Li + up to 7.59 with a separation factor of 1.033 (273 K), exceeding those reported for CE-based materials. The increased hydrophobicity of [C n (B15C5)mim][NTf 2 ] enables sustainable and robust use in the separation process. Both molecular dynamics simulations and interfacial tension experiments confirm that [C 8 (B15C5)mim][NTf 2 ] exhibits the best affinity to the interfaces of dichloroethane/water. The coordination environment of Li + can be tuned by regulating the length of the alkyl chains, resulting in different isotopic effects. This study offers a promising system for the effective and sustainable separation of 6 Li and 7 Li.