Supercritical Fluids in Ionic Liquids

Abstract

Ionic liquids and supercritical fluids are both alternative environmentally benign solvents, but their properties are very different. Ionic liquids are non-volatile but often considered highly polar compounds, whereas supercritical fluids are non-polar but highly volatile compounds. The combination of these two types of solvents has some unique features. It has been discovered that the solubility of supercritical carbon dioxide in several ionic liquids is very high but that the solubility of ionic liquids in supercritical carbon dioxide is negligibly low. Therefore, organic solutes can be extracted from an ionic liquid using supercritical carbon dioxide without any contamination by the ionic liquid. The phase behaviour of many binary or ternary (ionic liquid+supercritical carbon dioxide) systems was subsequently studied. Combined with the fact that ionic liquids are excellent reaction media for catalysed reactions, this led to the development of chemical processes where the reaction was carried out in the ionic liquid and the product was extracted afterwards with supercritical carbon dioxide. Newest developments include the multi-functional use of supercritical carbon dioxide as extraction medium, transport medium, and miscibility controller in these processes, resulting in higher reaction and separation rates.