Innovative Applications of Ionic Liquids as ‘‘Green’’ Engineering Liquids
Chemistry Program, Savannah State University, Savannah, Georgia, USA
Over the past decade, ionic liquids (ILs) have become one of the fastest growing ‘‘green’’media for chemists and engineers due to their superb physicochemical properties. The applications of these remarkable salts in reactions and extraction processes have been extensively investigated and reviewed. This review, however, highlights recent advances of ILs as versatile ‘‘green’’ engineering liquids in a variety of industrial applications including heat transfer fluids, azeotrope-breakingliquids, lubricants, electrolytes, liquid crystals, supported IL membranes, plasticizers, and more. This review is not intended to be comprehensive, but rather to discuss the potentials of ILs for diverse industrial applications. Keywords Electrochemistry; Engineering fluid; Green technology; Heat transfer fluid; Industrial application; Ionic liquid
Ionic liquids (ILs) are a newclass of organic salts that are liquids at low temperature (usually 400 À5.1–> 400 11.29–457.04 À80–> 400 À80–400 or 450 À75–416
Data from (Lide, 2003). Data from manufacturer’s specifications. c Data from Van Valkenburg et al. (2005), where EMIM ¼ 1-ethyl-3-methylimidazolium, BMIM ¼ 1-butyl-3-methylimidazolium, DMPI ¼ 1,2dimethyl-3-propylimidazolium, Tf2N ¼ bis(trifluorosulfonyl)imide(CF3SO2)2N À . d Data from Holbrey et al. (2003), where C6MIM ¼ 1-n-hexyl-3-methylimidazolium. e Data from Moens et al. (2003), where C8MIM ¼ 1-n-octyl-3-methylimidazolium. f Data from Wu et al. (2001).
Ionic Liquids as ‘‘Green’’ Engineering Liquids
latent heat storage density, thermal conductivity, density, and viscosity (Wu et al., 2001; Holbrey et al., 2003; Van Valkenburget al., 2005). Table 1 indicates that ILs are more thermally stable and have large heat capacities. More excitingly, [EMIM][BF4] was found to possess a heat of fusion of 48.2 J=g and a latent heat storage density of 60.4 MJ=m3 (Van Valkenburg et al., 2005); [C8mim][PF6] has a latent heat storage density of 378 MJ=m3, much higher than that of current heat transfer fluids. These findingsdemonstrate that many properties of ILs are superior to those of current heat transfer fluids. ILs can store considerable amounts of heat, and they have a large liquid range, low vapor pressure, and high thermal stability. The challenges of employing ILs as heat transfer fluids for solar energy storage were explained by Moens et al. (2003) as IL availability, cost, purity, material compatibility,environmental safety=health issues, and issues of intellectual property. Azeotrope-Breaking Liquids The separation of azeotropic or close-boiling mixtures has been a challenging subject in distillation or membrane processes. This engineering problem is often solved by using extracting agents or entrainers. ILs as nonvolatile entrainers have more advantages than conventional entrainers (Jork et al., 2004,2005; Seiler et al., 2004): (1) ILs are high-boiling and thermally stable compounds and thus cause minimum distillate contamination (however, potential contamination might be due to the decomposition of products themselves at high temperatures); (2) ILs can offer high selectivities and capacities due to the large choices of possible ILs and their tailor-made properties; (3) the extractivedistillation may be energy efficient by lowering reflux ratios when favorable entrainers (such as ILs) are used; and (4) the regeneration of nonvolatile entrainers (ILs) could be achieved through stripping, evaporation, drying, or crystallization. Experimentally, ILs have exhibited the ability to separate close-boiling or azeotropic mixtures including ethanol=water, acetone=methanol, tetrohydrofuran...