Solubility of Pd(hfac)2 and Ni(hfac)2·2H2O in supercritical carbon dioxide pure and modified with ethanol

Abstract

The solubility of palladium hexafluoroacetylacetonate [Pd(hfac)2] and nickel hexafluoroacetylacetonate dihydrate [Ni(hfac)2·2H2O] in supercritical CO2 (scCO2) has been measured using a high-pressure variable volume view cell. At 313.2K and 8.9MPa, the solubility of Pd(hfac)2 is above 3.6wt% (3.15×10−3 mole fraction). Solubility measurements of Ni(hfac)2·2H2O were also carried out at 313.2, 323.2 and 333.2K in the 9.4–25.1MPa pressure range. Ni(hfac)2·2H2O mole fraction solubility values vary from 2.95×10−5 to 2.02×10−4. For a given temperature, the solubility increases with pressure due to the higher density of the solvent. At constant pressure, however, a crossover has been observed. At mole fractions below ca. 1.2×10−4, solubility decreases with temperature due to the decrease in density of the fluid. On the contrary, at higher concentrations, solubility increases with temperature due to the increase in the solute vapour pressure. Ni(hfac)2·2H2O solubility data were satisfactorily correlated to semi-empirical equations. The solubility of Ni(hfac)2·2H2O in scCO2 was more than 2 orders of magnitude lower than the solubility of Pd(hfac)2 at similar conditions. This is related to the presence of water molecules bound to Ni in the precursor which allow the formation of intermolecular hydrogen bonds and may cause the formation of a molecules network. Adding a small amount of a polar modifier such as ethanol (2–6.5mol%) to scCO2 increased substantially the solubility of the precursor. At EtOH concentrations close to 5mol%, the solubilisation pressures were rather independent of the Ni(hfac)2·2H2O concentration in the 1–6×10−3 mole fraction range. The large solubility increase observed in scCO2 modified with EtOH may be related to the rupture of the hydrogen bonded Ni(hfac)2·2H2O network due to the better solvation of the complex in the CO2/EtOH mixture possibly by hydrogen bonding between Ni(hfac)2·2H2O and EtOH.