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 (Li-6 and Li-7) 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][NTf2], n = 6, 8, 10) are synthesized. The association constants of [C-n(B15C5)mim][NTf2] and Li+, Li+ partition, and the separation factor of Li-6 and Li-7 using [C-n(B15C5)mim][NTf2] 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][NTf2] with Li+ increase with the elevation of the alkyl chain length due to the flexibility changes. However, [C-8(B15C5)mim][NTf2] 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][NTf2] enables sustainable and robust use in the separation process. Both molecular dynamics simulations and interfacial tension experiments confirm that [C-8(B15C5)mim][NTf2] 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 Li-6 and Li-7.