Using Diglycolamide Extractants in an Imidazolium-Based Ionic Liquid for Rare Earth Element Extraction and Recovery

Growing demands for rare earth elements (REEs) have prompted sustainability concerns worldwide. Given the need for sustainable extraction methods amidst REEs, ionic liquids (ILs) have been investigated as tunable extraction substitutes for conventional organic solvents, offering negligible volatility and diverse physical and chemical properties. Recent reports have shown that the introduction of extractants, like N,N,N',N'-tetraoctyldiglycolamide (TODGA) or N,N-dioctyldiglycolamic acid (DODGAA), into ILs can provide high selectivity and affinity for REE capture. Precipitate formation has been observed in IL-extractant systems across several studies; however, the molecular interactions that drive this phenomenon have yet to be explored. This study investigates the coordination environment in the precipitate formed between [Yb³⁺], DODGAA, and the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM⁺][PF₆ ^-]) IL. The composition of the precipitate was confirmed using several spectroscopic techniques and revealed an underlying hydrogen bonding interaction between the fluorine atom of [PF₆ ^-] anion and -OH of the Yb-DODGAA complex. Computational studies were also conducted to examine the coordination environment of the Yb-TODGA and Yb-DODGAA complexes. The binding affinity of the extractants toward [Yb³⁺] is analyzed by calculating the associated binding energy values. The results clearly show a stronger binding affinity of the extractants toward [Yb³⁺], supporting the observed high extraction efficiencies of DODGAA and TODGA.