Evaluation of the Extraction Properties and Stability of Extracted Rare Earth Complexes in Ionic Liquid Extraction System Using β-Diketone

Abstract

ABSTRACTThe extraction behavior of rare earth (RE) species was investigated on benzoyltrifluoroacetone (Hbfa) /triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) system in this study. The stoichiometry of the extracted complex shown RE: ligands = 1:3 by slope analysis regardless of the kinds of acid media in the aqueous phase. The extraction mechanism in this system was revealed as [RE3+]aq + 3[Hbfa]IL [RE(bfa)3]IL + 3[H+]aq and [TFSA]− anion did not participate in the extraction mechanism. The result of Raman spectroscopy also supported the stoichiometry of the extracted complex. Moreover, the chelate formation constant (β) of [Nd(bfa)3] and [Dy(bfa)3] was also evaluated by the direct measurement of [bfa]− anion by ion chromatography. The hypersensitive peak (4I9/2→2G7/2*) in the UV-Vis-NIR spectrum also confirmed the formation of [Nd(bfa)3]. The average chelate formation constants (log β) of [Nd(bfa)3] and [Dy(bfa)3] were estimated to be 7.58 and 7.93, respectively. The value of log β(Dy) was larger than that of log β(Nd); thus, this result indicates that [Dy(bfa)3] was more stable than [Nd(bfa)3]. Furthermore, the optimized structure of the extracted complexes – [Nd(bfa)3(H2O)3] and [Dy(bfa)3(H2O)2] – were also evaluated by the density functional theory (DFT) calculation. On the optimized geometries, the average bond length of the centered Nd3+ ion or Dy3+ ion against the O atom in the 1-carbonyl group was 2.728 Å or 2.407 Å, respectively. This result suggested that the Dy3+ ion would be combined more strongly with the [bfa]− anion than the Nd3+ ion.