Platinum group metals (PGMs) are of significant importance for modern technologies and advanced materials. However, the continuous high demand and scarcity of PGMs are critical issues. Correspondingly, we need to challenge recycling technologies to fill the gap between demand and supply. Conventional organic solvent extraction is widely applied to the practical recovery method of PGM ions. Recently, ionic liquids (ILs) are considered as an alternative green solvent for PGM extraction. In our previous study, we have reported a metal-selective deprotection-mediated Pd(II) extraction by use of ILs with tetrahydropyran-2H-yl-protected thiol moieties. 1) Also, the functionalization of ILs, regarded as task-specific ILs (TSILs), is a promising technique to introduce PGM extraction ability into ILs. In the present work, we have designed and synthesized novel ILs with amino moieties 2) in order to develop the selective recovery method of Pt, Pd, and Rh ions for industrial use.
All TSILs Cn (n means alkyl chain length of dialkylamines) forming a pale-yellow IL state at room temperature were immiscible in water. We next investigated the PGM extraction ability of Cn for Pd(II), Pt(IV), and Rh(III) ions. The initial PGM ion concentrations in the aqueous solutions were adjusted to 100 ppm. Influence of HCl concentration on the extractability was examined in the range from 0.30 M to 4.0 M. The extraction experiments of the PGM ions into the Cn-based IL phases (400 mg) from the aqueous phase (2.0 mL) was carried out by mixture of the solution in a vial, followed by vigorous shaking. The extractability was evaluated through the difference of PGMs concentration in the aqueous phase before and after extraction with ICP-AES measurements.
Both dihexylkamino-substituted C6 and dioctylamino-substituted C8 exhibited excellent extractability (> 80%) towards Pt(IV) and Pd(II) ions in HCl concentration range from 0.30 to 4.0 M. However, the extractability of dibutylamino-substituted C4 was significantly decreased with an increase in HCl concentration, possibly because of the instability of C4 at rather high HCl concentrations. The extractability to Rh ions reached at 73% at maximum for C8 in 3.0 M HCl solution, which exceeded that for C6, 51%. We considered that the high extractability of C8 was attributed to the high hydrophobicity. The extraction/back-extraction abilities of Cn were also investigated.
1) K. Funaki, S. Ma, S. Kawamura, A. Miyazaki, A. Sugie, A. Mori, A. Muramatsu, and K. Kanie, Chem. Lett., 46, 434-437 (2017).
2) S. Ma, K. Funaki, A. Miyazaki, A. Muramatsu, and K. Kanie, Chem. Lett., 46, 1422-1425 (2017).