Benzo-12-crown-4 and β-diketone bi-functionalized ionic liquids for highly selective lithium extraction from alkaline solution

Indeed, the efficient and sustainable recovery of lithium from highly concentrated sodium-containing mother liquors of Li 2 CO 3 and salt-lake brines remains a significant challenge. Herein, two typical Li + recognition receptors, namely benzo-12-crown-4 and thenoyltrifluoroacetone, were separately introduced into the cationic and anionic parts to successfully synthesize novel bi-functionalized task-specific ionic liquids [(B12C4)C n im][TTA] (n = 4, 6, 8). A new, green, and high-efficiency extraction system has been developed by dissolving [(B12C4)C 6 im][TTA] in commercial room-temperature ionic liquid 1-hexyl-3-methyl-imdazolium bis(trifluromethylsulfonyl)imide [C 6 mim][NTf 2 ], without the addition of any other co-extractants. The newly developed [(B12C4)C 6 im][TTA]-[C 6 mim][NTf 2 ] extraction system exhibited a wide pH working range (4.0–11.0), high lithium extraction efficiency (95.4 %), as well as superior separation factors of Li/Na (β Li/Na  = 2154.0) and Li/K (β Li/K =757.2). The extraction mechanism involving cation exchange was determined through slope analysis, ion chromatography, UV–vis spectroscopy and FT-IR spectra measurement, indicating that one [(B12C4)C 6 im][TTA] molecule can interact with two lithium ions and form a 1:2 complex. Moreover, the [(B12C4)C 6 im][TTA]-[C 6 mim][NTf 2 ] extraction system also demonstrated fast extraction kinetics, facile stripping and regeneration, along with good cycling performance. Furthermore, a multi-stage countercurrent extraction process to extract lithium from the simulated mother liquor of Li 2 CO 3 was evaluated. When the volume ratio (O/A) of oil phase (O) to water phase (A) was 1.0, only two theoretical extraction stages were needed to extract 91.4 % of lithium into the organic phase. Thus, the [(B12C4)C 6 im][TTA]-[C 6 mim][NTf 2 ] extraction system is promising for lithium extraction from mother liquor of Li 2 CO 3 and salt-lake brines. This study also opens the avenue for designing cationic and anionic bi-functionalized ionic liquids for metal ion separation.