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

The current solvent extraction method for refining platinum group metals (P GMs) creates contact between aqueous and organic phases and thus leads to c ross-contamination. We have therefore endeavored to employ liquid carbon di oxide as the organic phase in such extractions to eliminate the waste strea m deriving from organic contamination of the aqueous phase. Whereas traditi onal metal-binding ligands dissolve in CO2 at moderate pressures, the resul ting complexes [(LH+)(2)MClx2-] exhibit negligible solubility in CO2 at pre ssures up to 400 atm. Consequently, we have designed and synthesized a seri es of highly CO2-soluble analogues to the metal-binding ligands currently u sed by the PGM-refining industry. We have measured equilibrium efficiency f actors for various ligands and ligands bound to platinum. We have found tha t the structure of the CO2-philic component of the ligand affects the bindi ng constant and hence that fluoroalkyls are better extractants of platinum than fluoroethers. Finally, we have noted that the phase behavior of the li gand-metal complex is a function of both the ligand structure and the ident ity of the metal, opening the possibility for separations based on both pre ssure and ligand selectivity.