Amidophosphonate ligands as cerium extractants in supercritical CO₂

Industrial-scale rare earth extraction is typically performed by liquid-liquid extraction with large amounts of solvents, acids and organophosphorus ligands. An alternative is to use these extractants in supercritical CO₂ (SC-CO₂), where the crucial operating parameter is their solubility. Different molecules with amidophosphonate backbones have been synthesized and investigated to improve their solubility in SC-CO₂ and their capacity to extract cerium. The CO₂-philicity of these extractants was adjusted by attaching branched or linear alkyl chains with four, six and eight carbons. Using a dynamic gravimetric setup, the solubility and cerium extraction capacity of these molecules were measured to vary from 1.56 to 2.47 47 mmol_ligand.mol_(CO2)^-1 and 0.25 to 0.46 mmol_Ce.mol_ligand^-1, respectively. The two properties were correlated and in both cases the highest values were obtained with a diisobutylamine chain. Solubility measurements between 313 and 331 K and between 13 and 28 MPa shows that optimal condition is (320 K; 28 MPa). (authors)