Selection of low-toxic and highly efficient ionic liquids for the separation of palladium and platinum in acidic solution, and prediction of the metal affinity of ionic liquids

Abstract

Separation studies were conducted to separate palladium (Pd) or platinum (Pt) from acidic solutions using 15 hydrophobic ionic liquids (ILs) capable of forming a binary phase with acidic aqueous solution. This study considered two cases. The first was to find high-efficient ILs capable of separating metals from a single solution, and the second was to find ILs capable of selectively separating Pt from an acidic mixture solution of Pd and Pt. In addition, we also considered toxic effect of ILs for greener separation process. For this studies, we experimentally examined the separation efficiencies of the ILs, and discuss the properties with their toxicities, theoretically predicted by previously presented toxicity prediction model, were compared. The results revealed that the extraction efficiency of ILs for Pd and Pt from acidic solutions depends on the types of IL and metal concentration. In the separation study on Pd in a single metal solution, trihexyltetradecylammonium bromide [P6,6,6,14]Br and trioctylmethylammonium choride [N8,8,8,1]Cl efficiently separates Pd from acidic solutions, while the separation on Pt in its single metal solution, 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[IM1-1Bz][(CF3SO2)2N], 1-methyl-3-dodecylimidazolim bis(trifluoromethylsulfonyl)imide[IM12][(CF3SO2)2N], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[IM16][(CF3SO2)2N], [P6,6,6,14]Br and [N8,8,8,1]Cl led to high efficiencies. Moreover, in the study to selectively separate Pt from a mixed solution of Pd and Pt, [IM1-1Bz][(CF3SO2)2N] and [IM16][(CF3SO2)2N] have significant separation efficiency. When comparing the efficiencies and toxicities of the two ILs, [IM1-1Bz][(CF3SO2)2N] is more efficient, but it is more toxic than [IM16][(CF3SO2)2N]. Thus, for sustainable development, [IM16][(CF3SO2)2N] can be considered as a better option. Moreover, in order to understand the partitioning mechanisms of ILs we developed quantitative structure–activity relationship models with R2 values greater than 0.91.