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 (6Li and 7Li) 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 ([Cn(B15C5)mim][NTf2], n = 6, 8, 10) are synthesized. The association constants of [Cn(B15C5)mim][NTf2] and Li+, Li+ partition, and the separation factor of 6Li and 7Li using [Cn(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 [Cn(B15C5)mim][NTf2] with Li+ increase with the elevation of the alkyl chain length due to the flexibility changes. However, [C8(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 [Cn(B15C5)mim][NTf2] enables sustainable and robust use in the separation process. Both molecular dynamics simulations and interfacial tension experiments confirm that [C8(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 6Li and 7Li.