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Synthesis and Properties

Karine De Oliveira Vigier and François Jérôme

Université de Poitiers, B1, ENSIP, IC2MP UMR CNRS 7285, 1 rue Marcel Doré, TSA 41105, 86073, Poitiers Cédex, France

1.1 Introduction

At the beginning of this century, deep eutectic solvents (DESs) appeared as a new class of green solvents [1] and they are considered as a new class of ionic liquids (ILs) due to their similar characteristics and properties. However, they are two different types of solvents. An ionic liquid is an association of a cation and an anion. In contrast, DES is a combination of two or more solids that form, through hydrogen bond formation, a eutectic liquid mixture at a temperature lower than the melting point of each compound that is part of the DES [2]. The intersection of the eutectic temperature and the eutectic composition gives the eutectic point (E in Figure 1.1). E is the point where the eutectic mixture is a unique composition of two or more nonmiscible phases of solid components that after association form a liquid at a defined temperature.

Figure 1.1 Representation of a eutectic point.

The interest in DESs increased considerably at industrial or academic level as shown by the number of publications dedicated to DESs. This is due to their great convenience of synthesis, their low production costs by using safe components, and their unusual reactivities near the eutectic point [3]. Moreover, the possibility to tune their properties makes them ideal candidates to be used for a wide range of applications. They are usually composed of Lewis or Brønsted acids and bases, which can contain a variety of anionic and/or cationic species. One of the components most used to produce DESs is choline chloride (ChCl). It is a cheap, biodegradable and nontoxic salt that can be extracted from biomass or produced from fossil carbon. Some of these mixtures present rather a glass transition temperature point than a eutectic point, and are therefore also called low-melting mixtures [4] or low-transition-temperature mixtures [5]. Just like ionic liquids, DESs often have a melting point close to RT, and exhibit low volatility and high thermal stability. However, DESs are biodegradable, cheap, and very easy to prepare. This chapter aims at a general presentation of the synthesis of these solvents and their physicochemical properties.

1.2 Synthesis

Generally, DESs can be prepared from two or more cheap and safe components through hydrogen bond interactions between the hydrogen bond donor (HBD) and the hydrogen bond acceptor (HBA) [6]. Practically, they are prepared by adding directly an appropriate amount of HBD and salt into a flask. After heating and stirring, a colorless liquid is formed. Obviously, the molar ratio corresponding to the eutectic point is variable in composition and also in temperature according to the nature of each component. The synthesis procedure of DESs is very simple and produces no waste products. Therefore, the synthesis of DESs is green and environmentally benign because their reaction has zero emissions, zero E-factor value. Moreover, the atom economy of the final formation of the DES is 100%, because all initial components are included in the final mixture. All of these factors make their ecological footprint minimal [7]. At an economic level, DESs are inexpensive and approximately 10-fold less expensive than the components of ILs [8].

The number of DESs that can be synthesized from the available chemicals has no limitation owing to the large number of quaternary ammonium, phosphonium, or sulfonium salts and HBDs that can be used to synthesize the DESs (Figure 1.2). Therefore, it is almost impossible to study all the combinations.

Figure 1.2 Structure of some HBDs and salts used in the formation of DESs.

DESs are composed of large, nonsymmetric ions with low lattice energy and thus low melting points owing to the charge delocalization occurring through hydrogen bonding between, for example, a halide ion and the hydrogen donor moiety. Typical DESs are composed of choline chloride, natural amino acids such as Lewis/Brønsted bases or urea, natural carboxylic acids, or polyalcohols such as Brønsted acids. One can note that they come from renewable sources. For example, ChCl is an additive in chicken food for accelerating their growth, and is simply produced from trimethylamine, hydrochloric acid, and ethylene oxide in a continuous, single-stream process. The toxicity of DESs is nonexistent or very low [9] and their biodegradability is extraordinarily high [10]. Moreover, the high solubility of DESs in water allows the separation of organic products that will precipitate or appear as a water-insoluble layer with the addition of water that dissolves the DES, avoiding the typical organic solvent extraction at the end of the reaction. DES can be recycled with the evaporation of water from the aqueous layer.

In 2007, Abbott et al. provided the general formula R1R2R3R4N+X-Y-[11] for DESs. DESs are classified depending on the nature of the complexing agent used [2, 12] (Table 1.1). Four types of DESs exist. DESs of Type I are composed of quaternary ammonium salt and metal chloride and can be considered as analogous to metal halide/imidazolium salt systems. Examples of Type I eutectics include chloroaluminate/imidazolium salt melts and DESs formed with imidazolium salts and various metal halides including FeCl2, AgCl, CuCl, LiCl, CdCl2, CuCl2, SnCl2, ZnCl2, LaCl3, YCl3, and SnCl4. DESs of Type II are composed of quaternary ammonium salt and metal chloride hydrate. The relatively low cost of many hydrated metal salts coupled with their inherent air/moisture insensitivity makes their use in industrial processes viable. DESs of Type III are composed of quaternary ammonium salt and HBD. In Type III, choline chloride and HBDs have been widely used for many applications such as metal extraction and organic synthesis [2, 6, 13]. Type IV DESs are composed of metal chloride and HBD.

Table 1.1 The fourth type of DESs.

These liquids are simple to prepare and relatively unreactive with water; many are biodegradable and have relatively low cost. The wide range of HBDs available indicates that this class of DESs is particularly adaptable. The physical properties of the liquid are dependent upon the HBD and can be easily tailored for specific applications.

1.3 Properties

Owing to their physicochemical and thermal properties (density, viscosity, surface tension, conductivity, freezing temperature [T f], miscibility, and polarity), which can be easily tuned by altering the components and their ratios, DESs have a big potential as solvents [2, 6]. Moreover, a high number of DESs can be obtained, making this new type of solvents even more designable.

1.3.1 Freezing Point (T f)

As mentioned previously, DESs are formed by mixing two solids capable of generating a new liquid phase via hydrogen bonds formation. This liquid phase is characterized by a lower freezing point than that of the individual constituents. This decrease of the freezing point comes from an interaction between HBD and the salt. Table 1.2 reports the freezing points of various DESs described in the literature. Although for all DESs reported in the literature the freezing point is lower than 150?°C, it should be pointed out that the number of DESs that are liquid at room temperature (RT) is still quite limited. Among DESs that are liquid at room temperature, we can cite the combination of glycerol or urea with ChCl, presumably due to their stronger ability to form hydrogen bond interactions with ChCl. It means that depending on the halide salt, the choice of HBDs is a critical point in the formation of a DES with a low freezing point. For instance, with ChCl as a salt, HBDs such as carboxylic acids (levulinic acid, malonic acid, phenylpropionic acid, etc.) or sugar-derived polyols (e.g. xylitol, D-isosorbide, and D-sorbitol) lead to room temperature liquid DESs. In the same way, for a defined HBD the nature of the halide salts (e.g. ammonium or phosphonium salts) also affects the freezing points of the corresponding DESs. For example, when urea is selected as HBD and mixed with different salts in a molar ratio of 2 : 1 (urea:salt), the obtained DESs exhibit very different freezing points, from -38 to 113?°C (Table 1.2). For a similar salt, the nature of the anion is also of importance for the...