Recovery of palladium from spent catalyst with supercritical CO2 and chelating agent

A system designed to efficiently recover palladium (Pd) from palladium-based catalysts using supercritical carbon dioxide (scCO2) and a chelating agent has been investigated. The scCO2 extraction of Pd directly from a spent Pd-based solid catalyst is reported. The scCO2 was modified with chelating agents in order to bind Pd from the solid matrix. The effects of pressure, temperature, extraction time, scCO2 density, and the type of chelating agent on the extraction efficiency of Pd were considered. Findings suggested that among the studied chelating agents of acetyl acetone (AA), tri-n-butyl phosphate/nitric acid complex (TBP/HNO3/H2O), and bis(2,2,4-trimethylpentyl)monothiophosphinic acid (Cyanex 302), Cyanex 302 was the best ligand for the extraction of Pd. At pressures ranging from 8 to 20 MPa and temperatures ranging from 313 to 353 K and a 10 min dynamic extraction time, Pd was successfully recovered from the solid matrix with an extraction efficiency of almost 100%. The extraction efficiencies for AA and TBP/HNO3/H2O were around 80 and 60%, respectively.