Quaternary Ammonium and Phosphonium Ionic Liquids in Chemical and Environmental Engineering

Quaternary ammonium salts (quats) are an economically advantageous class of industrial compounds. They have surface-active properties, possess anti-microbial activity and are known to be bioactive (Boethling & Lynch, 1992; Juergensen et al., 2000). Contrary, reports regarding low melting tetraalkylphosphonium salts were relatively rare in the literature during the last decades (Bradaric et al., 2003). Although ILs based on quaternary ammonium cations have been known and produced for years, and also numerous phosphonium based ILs have been produced even in ton-scale, these groups of ionic liquids have been more or less “neglected” in the literature comparing to their imidazolium or pyridinium based counterparts. However, during the last decade significant work has been done, pointing out the advantages and broad application spectrum of these types of ionic liquids. Their improved thermal and chemical stability in comparison to e.g. pyridinium and imidazolium based ILs, their unique miscibility behaviour and solvating properties advances their use in specific applications (Rogers et al., 2002; Plechkova & Seddon, 2007; Chowdhury et al., 2007; Wasserscheid & Welton, 2008). Several ILs based on these classes of cations are already commercially available and have been successfully applied as phase-transfer catalysts, solvents, lubricants, gas capture agents, coating materials, or chemical sensors (Bradaric et al., 2003; Weng et al., 2006; Pernak et al., 2006; Yuan et al., 2007; Fraser & MacFarlane, 2009; Werner et al., 2010). Tricaprylylmethylammonium chloride (trade name Aliquat® 336), produced in ton-scale by Henkel, is nowadays widely used as metal extraction agent, phase transfer catalyst, surfactant, or antistatic agent. In contrast, trihexyl(tetradecyl)phosphonium chloride (trade name Cyphos® IL 101) was developed by Cytec Industries as a thermally stable phase transfer catalyst (Bradaric et al., 2003). Due to the relatively simple preparation route, Cyphos® IL 101 is produced in ton-scale and therefore represents a fairly cheap IL. Hence, due to their commercial availability and low prices, both Aliquat® 336 and Cyphos® IL 101 can be used themselves or represent favorable precursors for a series of ionic liquids. The goal of this chapter is to give some insight into the fascinating and fast-growing field of