CHAPTER 4: Catalysis in Ionic Liquid–Supercritical CO₂ Systems

The combination of ionic liquids (ILs) and supercritical CO₂ (scCO₂) allows efficient catalytic processes to be developed. Catalyst separation is generally a major challenge when enzymes or homogeneous organometallic catalysts are utilised for reactions, and IL–scCO₂ systems address these separation problems, facilitating the recycling or continual use of the catalyst. Typically these systems involve a catalyst being dissolved in an IL and this is where it remains during the process, with scCO₂ extracting the products from the IL (catalyst) phase. ILs and many catalysts are not soluble in scCO₂ and this facilitates the clean separation of products from the catalyst and IL. When the pressure is reduced in a collection chamber, the scCO₂ returns to CO₂ gas and products can be obtained without contamination of catalyst or solvents. It is possible to operate IL–scCO₂ systems in a continuous flow manner and this further improves the efficiency and industrial potential of these systems. This chapter will introduce the fundamental properties of these multiphase catalytic systems. It will also highlight key examples of catalytic processes from the academic literature which illustrate the benefits of utilising this combination of solvents for catalysis.