Highly CO₂-soluble 5-Amido-8-hydroxyquinoline Chelating Agents
for Extraction of Metals in Sc-CO₂

摘要

Novel CO2-soluble 8-hydroxyquinoline (8-HQ) chelating agents were synthesized and evaluated for solubility and metal ion extraction ability in supercritical CO2 (Sc-CO2). Among them, secondary amide-containing 8- HQ derivatives cannot be dispersed well into Sc-CO2, but tertiary amide-containing derivatives can dissolve completely in Sc-CO2 even at low CO2 pressures, perhaps owing to the predominant intermolecular interaction between the chelating agent and the CO2 molecule. Based on 8-HQ chelating agent solubility data, we investigated the extraction of metal ions (Co2+, Cu2+, Sr2+, Cd2+, and Zn2+) using two highly CO2-soluble 8-HQ derivatives (4d, 4e) in Sc-CO2. The extraction efficiency of tertiary amide-containing 8-HQ ligands, both fluorinated and non-fluorinated forms, was dramatically increased in the presence of diethyl amine (organic base). We suggest that diethyl amine could play an important synergistic role in the stronger metal binding ability of 8-HQ through an in situ deprotonation reaction in Sc-CO2 medium.