ScCO2/Green Solvents: Biphasic Promising Systems for Cleaner Chemicals Manufacturing

Solvents play a key role in the chemical industry; novel classes of solvents such as gas expanded liquids and switchable solvents have attracted great interest in recent years as their emergence and utilization in chemical processes hold many promises to develop benign environmental technologies. This perspective paper aims at reflecting on the state of the art concerning biphasic scCO(2)/green solvent systems. Rather than discussing the reactions that have been performed in each major type of scCO(2)/green solvent biphasic system, this paper is structured instead in terms of the problems or difficulties that these innovative systems help to solve in processes engineering by taking advantage of the unique advantages of these systems such as greenness, solubility enhancement, and pressure-tunable properties, allowing for overcoming issues concerning monophasic systems that the chemical engineer is confronted with when scCO(2) or a ReactionftepatatIon green solvent are used separately and that have limited their utilization at industrial scales. Among such issues, one can cite (1) difficulties in the solubilization of these compounds, (2) difficult separation of reactants, products, and catalysts, and (3) loss of the catalyst. These problems arise on one hand from the poor solvent power of scCO(2) when used in homogeneous phase and on another hand from the need for a decrease in the usage of distillation as a separation technique. However, their association in biphasic systems allows for overcoming these problems and provides unique opportunities and perspectives to develop future versatile, flexible, and atom economical chemical processes in full accordance with the principles of green sustainable engineering. The green solvents considered in this perspective paper are water, ionic liquids, biobased ionic liquids, and other green solvents such as glycerol and liquid polymers. Ionic liquids have been considered in this paper as they have been claimed as green because they allow the prevention of volatile emissions; however, some of them present toxic issues together with a high environmental impact because of their nonbiodegradability and high manufacturing costs.(1) We have then tried to attract special attention to biosourced ionic liquids that may have better toxicological and environmental properties and that may hold promise for their future use in chemical processes. This paper presents also the studies concerning phase equilibrium between scCO(2) and green solvents where biphasic systems can be obtained, as phase behavior control is an interesting tool for designing effective catalytic reactions and catalyst-product separation processes. The unique properties of green solvents that have been proposed as a homogeneous phase in previous studies have prompted us to include them in this perspective paper with the aim of interesting the scientific community in testing them in biphasic systems inducing scCO(2). Finally, some reflections about the next steps toward greener processes using scCO(2)/green solvent biphasic systems are presented and concern technical and scientific requirements to take full advantage of the capabilities of these systems.This perspective paper does not intend to be comprehensive but instead tries to attract attention on recent usages of these systems in order to stimulate future advances in the study and development of such systems.