Urea-Introduced Ionic Liquid for the Effective Extraction of Pt(IV) and Pd(II) Ions

Abstract

A urea-introduced imidazolium-based ionic liquid (IL), 1-butyl-3-{3-(3-methyl-2H-imidazol-1-yl)­propyl}­urea bis­(trifluoromethylsulfonyl)­imide (L1 ), whose selectivity to Pt­(IV) and Pd­(II) was changed only by a pH region, was synthesized, and its Pt­(IV) and Pd­(II) extraction behaviors were investigated. In the conventional organic extraction system, an organic extractant with a urea group, 1-butyl-3-(2-ethylhexyl)­urea, in isooctane extracted Pt­(IV) and Pd­(II) in a similar pH range, the extraction equilibrium half-time (t 1/2) of Pd­(II) was 8.95 h. In contrast, in the IL extraction system, Pt­(IV) and Pd­(II) were extracted in the lower and higher pH regions, respectively, indicating the mutual separation of Pt­(IV) and Pd­(II) by pH-change. Furthermore, the extraction equilibrium t 1/2 of Pd­(II) with the IL extraction system was 0.18 h. Ultraviolet–visible and extended X-ray absorption fine structure spectroscopies of Pt­(IV) and Pd­(II) after extraction using the IL were conducted to determine the extraction mechanism. As a result, Pt­(IV) was extracted as [PtCl6]2– via anion-exchange (outer-sphere complex). On the other hand, Pd­(II) was extracted via a mixed reaction of ligand-exchange (inner-sphere complex) and anion-exchange. Finally, the IL extraction system did not produce a third phase even when loaded with a high concentration of Pt­(IV).

Alternative Extraction Systems for Precious Metals Recovery: Aqueous Biphasic Systems, Ionic Liquids, Deep Eutectic Solvents

“…An urea-based imidazolium IL, 1-butyl-3-{3-(3-methyl-2H-imidazol-1-yl)propyl} urea bis(trifluoromethylsulfonyl)imide ([C 4 UC 3 mim][Tf 2 N]), was synthesized and studied for Pt(IV), Pd(II) and Rh(III) separation [58]. Based on experimental data, the anion exchange mechanism was supposed for the extraction of Pt(IV) at pH = 1.13 and the inner-sphere coordination for Pd(II) extraction at pH = 5.45.…”