Design of ligands for the extraction of PtCl62- in introduction to liquid CO2

The current solvent extraction method for refining platinum group metals (PGMs) creates contact between aqueous and organic phases and thus leads to cross-contamination. We have therefore endeavored to employ liquid carbon dioxide as the organic phase in such extractions to eliminate the waste stream deriving from organic contamination of the aqueous phase. Whereas traditional metal-binding ligands dissolve in CO(2) at moderate pressures, the resulting complexes [(LH(+))(2)MCl(x)(2-)] exhibit negligible solubility in CO(2) at pressures up to 400 atm. Consequently, we have designed and synthesized a series of highly CO(2)-soluble analogues to the metal-binding ligands currently used by the PGM-refining industry. We have measured equilibrium efficiency factors for various ligands and ligands bound to platinum. We have found that the structure of the CO(2)-philic component of the ligand affects the binding constant and hence that fluoroalkyls are better extractants of platinum than fluoroethers. Finally, we have noted that the phase behavior of the ligand-metal complex is a function of both the ligand structure and the identity of the metal, opening the possibility for separations based on both pressure and ligand selectivity.