Separation of lithium from alkaline solutions with hydrophobic deep eutectic solvents based on β-diketone

Deep eutectic solvent (DES) is gaining much attention as a sustainable and green solvent for chemical separation. This work proposed a series of β-diketone based DES composed by benzoyl trifluoroacetone as hydrogen donor and compounds with different functional group as hydrogen acceptors. The newly prepared DES presented much higher lithium capacity than the conventional solvent extraction system. Due to the strong electron donating nature of phosphine oxide and hydrophobicity of longer carbon chains, TRPO and HBTA with molar ratio of 1/1 showed the best performance for lithium extraction efficiency and separation effect over sodium. The single crystal of the complex formed by lithium and diketone was obtained for the first time, and the stoichiometry of complex formed by HBTA/TRPO-DES and alkali metal was determined to be 1:1:1 by single crystal X-ray diffraction. The synthesized HBTA/TRPO-DES was used to recover lithium from mother liquor of lithium carbonate which was a complex solution with high molar ratio of Na/Li, the extraction of lithium at O/A = 1/3 could reach 90%, and the separation factor of Li/Na was 335 under that condition. After scrubbing and stripping, the DES could be used to next extraction cycle, and the extraction ability remained practically constant. The present work may provide a novel way for lithium recovery from alkaline solutions.