Non-ionic hydrophobic eutectics – versatile solvents for tailored metal separation and valorisation†
In comparison with the well-described ionic eutectic mixtures, hydrophobic eutectic solvents (HESs) composed of two non-ionic compounds represent a relatively new class of eutectics. In this work, a number of non-ionic HESs liquid at room temperature were identified from a large initial screening of potential mixtures. Three new HESs based on thymol + TOPO (trioctylphosphine oxide), TOPO + capric acid and hydrocinnamic acid + capric acid were investigated as extracting media for the recovery and separation of platinum group and transition metals in HCl media. Full phase diagrams and physical properties including viscosities, densities, chemical stability and the influence of water were characterised, with these HESs presenting low viscosities and high hydrophobicity suitable for application as solvents for liquid–liquid extraction. By simple variation of the eutectic component the selectivity of the system for a given metal could be tuned, with the TOPO-based system displaying good to excellent selectivity towards Pt4+, Pd2+ and Fe3+ under a range of conditions. The extraction mechanism was found to vary due to a complex interplay between the HES composition, acid concentration and the predominant metal complex present. The observed extraction behaviour in HESs composed of two metal complexing ligands such as TOPO + capric acid, in which each respective component is responsible for metal extraction under given conditions, opens the possibility to design hydrophobic eutectic mixtures presenting synergistic effects. Finally, the HES phase following palladium extraction was used as the template for the formation of palladium nanoparticles. The results presented highlight the great potential of HESs as environmentally benign and tuneable media for the solvent extraction of metal ions.