Supercritical CO 2 : a twenty-first century solvent for the chemical industry

Abstract

Half a glass of water may be described as half full or half empty, but if the water is in a supercritical state we have to agree that the glass is filled with something that is neither a gas nor a liquid. Steam and boiling water when in equilibrium are two different phases that are clearly separated by a phase boundary, but if the temperature and pressure are raised, after a point the phase boundary disappears and water becomes a supercritical fluid (SCF). While by appropriate changes in temperature and pressure the supercritical phase for many other substances could also be reached, from the point of view of applications, supercritical carbon dioxide (scCO 2 ) is of special interest. The main impetus for the wide-scale application of scCO 2 and other SCFs comes from the desire to develop environment-friendly chemical processes 1―3 . The environmental impact of a chemical process can be measured by its E-factor (amount of waste per unit mass of desired product). The E-factor has an inverse relationship with its environment-friendliness and includes solvent loss in the process 4 . Many industrial solvents are not only hazardous (toxic, flammable, etc.), but their full recovery and recycling often are not easy, or economically viable. An appropriate example is the pharmaceutical industry where approximately only about 1 % (w/v) product comes out of the solvent used, with about 50% solvent recovery in the whole process 5 . Furthermore even a non-hazardous hydrocarbon solvent in the long run is converted to CO 2 and adds to the carbon cycle. Therefore, from the point of view of the overall environment-friendliness, replacement of common organic solvent is an important objective. Although the main motivation for the wide-scale application of scCO 2 in recent times originated from environmental considerations, studies on scCO 2 have also led to many important scientific findings 6―8 . In this review we present some of the recent scientific and technological advances associated with the use of SCF in the industry. The focus is mainly on the chemical aspects and the underlying principles of SCF-based technologies, but technologies other than chemical are also mentioned.