A review of ionic liquids towards supercritical fluid applications

Ionic liquids (ILs), considered to be a relatively recent magical chemical due their unique properties, have a large variety of applications in all areas of the chemical industries. The areas of application include electrolyte in batteries, lubricants, plasticizers, solvents and catalysis in synthesis, matrices for mass spectroscopy, solvents to manufacture nano-materials, extraction, gas absorption agents, etc. Non-volatility and non-flammability are their common characteristics giving them an advantageous edge in various applications. This common advantage, when considered with the possibility of tuning the chemical and physical properties of ILs by changing anion–cation combination is a great opportunity to obtain task-specific ILs for a multitude of specific applications. There are numerous studies in the related literature concerning the unique properties, preparation methods, and different applications of ILs in the literature. In this review, a general description of ILs and historical background are given; basic properties of ILs such as solvent properties, polarity, toxicology, air and moisture stability are discussed; structure of ILs, cation, anion types and synthesis methods in the related literature are briefly summarized. However, the main focus of this paper is how ILs may be used in the chemicals processing industries. Thus, the main application areas are searched and the basic applications such as solvent replacement, purification of gases, homogenous and heterogeneous catalysis, biological reactions media and removal of metal ions are discussed in detail. Not only the advantages of ILs but also the essential challenges and potentials for using ILs in the chemical industries are also addressed. ILs have become the partner of scCO2 in many applications and most of the reported studies in the literature focus on the interaction of these two green solvents, i.e. ILs and scCO2. The chemistry of the ILs has been reviewed in numerous papers earlier. Therefore, the major purpose of this review paper is to provide an overview for the specific chemical and physical properties of ILs and to investigate IL–scCO2 systems in some detail. Recovery of solutes from ILs with CO2, separation of ILs from organic solvents by CO2, high-pressure phase behavior of IL–scCO2 systems, solubility of ILs in CO2 phase, and the interaction of the IL–scCO2 system at molecular level are also included.