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

Abstract

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)-α-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.