Novel open-chain crown ether bridged diphosphates as chelating ligands for lanthanides extraction in supercritical carbon dioxide

A series of highly CO 2 -soluble open-chain crown ether bridged diphosphates were designed and prepared as chelating ligands which were used to extract lanthanide ions from cellulose paper into supercritical CO 2 (ScCO 2 ). These ligands were designed by introducing different ester side chains and joining the two phosphorus groups with different length of ethylene oxide group which could provide enhanced solubility and complexation ability. The solubility data of these synthesized diphosophates in ScCO 2 were measured at 313.15 K and 10.65 MPa–13.69 MPa. Lanthanides metal ions including La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Gd 3+ , Er 3+ , and Yb 3+ were successfully removed from the solid matrix by supercritical fluid extractions using these chelating ligands at 313.15 K and 20 MPa. The extraction efficiency of metal ion was affected by chelating ligands’ side chain, which increased from 68% to 94% mainly driven by the increased solubility capability.