Novel class of crown ether functionalized ionic liquids with multiple binding sites for efficient separation of lithium isotopes

Lithium isotopes (6Li, 7Li) separation is an industrially important but challenging process due to their very similar chemical and physical properties. Crown ethers (CEs) have great potential in the isotopes separation for their typical cavity structures. However, the low distribution of Li+ has restricted their industrial use. In this work, a novel class of functionalized ionic liquids with grafted crown ether-moieties (CE-FILs) for efficient separation of 6Li and 7Li are first reported. The enhanced distribution of Li+ could be achieved by the synergistic effect of ion–dipole interaction, hydrogen bonding and coordination interaction. As a result, an exceptionally high distribution coefficient for Li+ (DLi, up to 136) is obtained using 0.3 M [C6(B15C5)mim][NTf2] with a satisfied separation factor of 1.034 (at 273 K) for 6Li/7Li selectivity from an extremely hydrophilic chloride solutions, which strikes an excellent balance between Li distribution and 6Li/7Li separation. Theoretical calculations and spectroscopic characterization confirm that [C6(B15C5)mim][NTf2] supplies as an ion pair receptor to bind Li+, forming a 1:1 ion-pair complex [C6(B15C5)mim][NTf2]…Li+FeCl4- with double cations and anions. The separation performance of 6Li/7Li mainly depends on the weak interactions associated with Li+. The CE-FILs pave a new way for Li isotopes separation.