Mechanistic investigation of solvent extraction based on anion-functionalized ionic liquids for selective separation of rare-earth ions

In this study, solvation has been found to be a dominant mechanism in an ionic liquid (IL)-based extraction system for rare earth elements (REEs). Trioctylmethylammonium di(2-ethylhexyl)phosphate ([TOMA][DEHP]), an anion-functionalized IL extractant, was used in 1-alkyl-3-methylimidizolium bis[(trifluoromethylsulfonyl)]imide ([C_nmim][NTf₂], n = 4, 6, 8, 10) and 1-alkyl-3-methylimidizolium bis(perfluoroethanesulfonyl)imide ([C_nmim][BETI], n = 4, 6, 8, 10) for the separation of REEs. Surprisingly, a very similar extraction behavior was observed even as the carbon chain length on the IL cation increased from butyl (C4) to decyl (C10). This behavior is in sharp contrast to that exhibited by the conventional molecular extractants, whose extraction efficiencies are strongly dependent on the hydrophobicity of IL cations. Furthermore, the addition of IL cations ([C _nmim]⁺) in [C_nmim]Cl form or IL anions ([NTf₂]^- or [BETI]^-) in Li[NTf₂] or Li[BETI] form to the aqueous phase had a minor effect on the extraction behavior of the above extraction system, ruling out the strong involvement of the ion-exchange mechanism associated with traditional IL-based extraction systems. Results showed that the extractabilities and selectivities of REEs using [TOMA][DEHP] in [C₁₀mim][NTf₂]/[BETI] are several orders of magnitude better than those achieved using a conventional organic solvent, diisopropylbenzene (DIPB). This study highlights the potential of developing a comprehensive IL-based extraction strategy for REEs separations via ionic extractants.