Amide-functionalised phosphonium-based ionic liquids as ligands for rhodium(iii) extraction

Abstract

Seven new amide-functionalised phosphonium-based ionic liquids (APILs) with chloride anions are synthesised and applied to extraction of rhodium(iii) from HCl solution. The effects of structural modification of the APILs on the extraction performance are examined by liquid-liquid extraction using toluene as a diluent and the results compared with those for trihexyltetradecylphosphonium chloride ([P66614][Cl]), a typical commercial extractant. The performance of the APILs as rhodium(iii) extractants is influenced by three main factors: (1) the length of the alkyl chains attached to the P atom; (2) the length of the linker between the amide and phosphonium moiety; and (3) the type of amide group. A novel ligand, [3°C2P444][Cl], had outstanding performance in the effective recovery of rhodium(iii). Extraction of rhodium(iii) from a 1.0 mol dm−3HCl solution with 0.5 mol dm−3[3°C2P444][Cl] proceeded quantitatively (>98%) and the extraction efficiency was higher than that of the commercial extractant [P66614][Cl]. The mechanism of rhodium(iii) extraction by [3°C2P444][Cl] was investigated by slope analysis, UV-vis, and FT-IR spectroscopy. These results indicate that [RhCl4(H2O)2]−in aqueous solution is extracted by [3°C2P444][Cl] through an anion-exchange mechanism and slowly converted into a dimer, [Rh2Cl9]3−, in the organic phase.