Applications of phosphonium-based ionic liquids in chemical processes

Abstract

Among ionic liquids, phosphonium-based ionic liquids (PILs) are quite elegant. These categories of ionic liquids represent some merits over other types of ionic liquids such as imidazolium- and pyridinium-based ionic liquids. PILs have more thermal and chemical stability than other reported ILs. These influential characteristics connected with PILs make them as potential structures for varied applications in academic and industrial processes. In recent years, however, PILs become popular because of relatively low cost of their synthesis (the rate of phosphonium salt formation is faster than those of nitrogen-based salts, implying higher productivity and lower cost in industrial manufacturing of PILs) as well as their good thermal stability, beneficial for high-temperature operation. Room temperature ionic liquids (RTILs) have numbers of unique applications in electrochemical systems and among them, phosphonium room temperature ionic liquids (PRTILs) have been increasing for their considerable advantages such as chemical and thermal stabilities, relatively low viscosities and high conductivities when compared to the corresponding ammonium RTILs. PRTILs are yummy electrolysis solutions because of their wide electrochemical window. Determination of the electrochemical stability of the PRTILs is important for detection and application of these ionic salts as electrolytes in electrochemistry. In order to evaluate electrochemical stability of the phosphonium RTILs, various voltammetric techniques such as cyclic voltammetry, linear sweep voltammetry and square wave voltammetry have been used. PRTILs characterized by a wide electrochemical window have been regarded as attractive candidates for lithium-battery electrolytes because of their stability and safety aspects. Contrary to what is seen in conventional organic solvents, superoxide is stable in ionic liquids. PILs are an unprecedented class of electrolytes that can support the electrochemical generation of a stable superoxide ion and can offer many advantages such as low combustibility, ionic conductivity, low volatility and a wide electrochemical window. PILs have been intensively developed as new electrolytic mediator for various electrochemical devices such as supercapacitors and lithium-ion batteries. There is also a growing interest for their use in separation processes including metal ions extraction, extractive desulfurization, gas adsorption and dissolution or extraction of biologically relevant compounds and materials. In mentioned processes and other applications where PIL is the solvent, of particular interest are physicochemical properties (e.g., viscosity, density, surface tension, solubility, polarity and so on). Moreover, the quantum chemical method is invoked to interpret superior properties of PILs. Experimental works have also satisfied that the PILs fulfill the necessary requirement of being a good inhibitor of metal corrosion in different media because of surface active properties. Owning to special physicochemical properties, the PILs are emerging as possible candidates to improve surfactant-enhanced oil recovery methods. Because they have also shown great importance in a vast number of industrial and pharmaceutical applications, such as lubricants, electrolytes, or solvents/catalysts for organic reactions, ecotoxicity of these ILs was studied for environmental and human health risks assessments. PILs have been used as efficient solvents and/or catalysts for synthesis of various kinds of organic compounds. This review article presents an excellent puzzle that each of its pieces lead to the rational design, synthesis and applications of novel and task-specific PILs as multi-purpose materials.