Chapter 28 - Supercritical fluids as green solvents

Abstract

In every chemical process industry, solvent plays a very important role mainly, in separation, by extraction, and also as a reaction intermediator, and promoter. The conventional organic solvents still in use in many industries are volatile organic compounds (VOCs) which are responsible for environmental pollution and health hazards to public. Replacing, reducing and restricting their use is one of most important initiative to be taken. Extensive research on alternate solvents and processes which are environmentally benign is the need of the day. Some of these are ionic liquids, natural extracts like plant sap, ester based, renewable feedstock-based solvents etc., and supercritical fluids (SCFs) which do not release toxic vapours into the atmosphere. Supercritical fluids which have low tunable densities varied between those of gas and liquid phases in critical conditions have been extensively studied for the past two decades to study their fundamental properties as solvents. By changing the density of the fluid, their solvation power can be tailored. These SCFs are comparatively chemically inert, eco-friendly, low-cost and have critical point under ambient conditions. Supercritical water (scH2O) has been used as an industry-attractive approach for catalytic reactions in aqueous biphasic systems. Similarly, in the sense of green chemistry and catalytic applications in various mono and biphasic systems, supercritical carbon dioxide (scCO2) is an important reaction medium. This chapter will mainly focus on green supercritical fluids solvents, solvents extracted from plants, ester solvents and briefly on room temperature ionic liquids. The content includes phase diagrams of these solvents, description of physicochemical properties and their dependence on temperature and pressure conditions, and finally their applications in different processes.