Recovery of pure products from ionic liquids using supercritical carbon dioxide as a co-solvent in extractions or as an anti-solvent in precipitations

In this paper two advanced methods for separation and purification of products from ionic liquids by using supercritical carbon dioxide as a co-solvent in extractions or as an anti-solvent in precipitations are demonstrated. As an example, the recovery of the product N-acetyl-(S)-phenylalanine methyl ester (APAM) from the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) was studied experimentally. APAM is the product of the asymmetric hydrogenation of methyl-(Z)-a-acetamidocinnamate (MAAC). For extraction of the product, the solubility of APAM in CO2 should be sufficiently high. This solubility is 1.78 g kg-1 at 12.0 MPa and 323 K, whereas [bmim][BF4] has a negligible solubility in scCO2. The extracted product was found to contain no detectible amount of ionic liquid. The solubility of the reactant MAAC in scCO2 is five times lower than the solubility of APAM, which means that a selectivity towards extraction of APAM exists. The product APAM was also precipitated out of the ionic liquid phase using scCO2 as an anti-solvent, enabled by the lower solubility of APAM in ionic liquid/scCO2 mixtures compared to the solubility in the pure ionic liquid at atmospheric conditions (650 g l-1). For example, the solubility of APAM in ionic liquid + scCO2 (1:1.34 g g-1) at 313 K and 18.0 MPa is 162 g l-1. After precipitation the formed crystals can be washed using CO2 to obtain pure product