**5. Deep eutectic solvents**

Deep eutectic solvents are analogous to ILs in the sense that they possess similar properties of low melting point and non-volatility, but they are, however, two different types of solvent systems. Unlike ILs, which are made up of one type

**15**

**Table 1.**

**Figure 11.**

*DES based on choline chloride and urea.*

*Bio-Solvents: Synthesis, Industrial Production and Applications*

of discrete cation and anion [17], DESs are formed from an eutectic mixture of a hydrogen bond acceptor (HBA), mostly quaternary ammonium salts, and a metal salt or hydrogen bond donor (HBD). The mixture forms a eutectic phase which has a lower melting point than the individual components [109]. There is a charge delocalization created through hydrogen bonding between, for example, a halide anion of the HBA and the HBD which causes the decrease in the melting point [18]. DESs are usually formed from relatively inexpensive bio-based components which makes them biodegradable and affordable [17]. Abbott et al. initially developed DESs based on choline chloride and zinc chloride as cheaper alternatives to imidazolium-based ionic liquids, which would make them readily accessible for bulk applications in syntheses. These new solvents were made by heating mixtures of the choline chloride and zinc chloride in molar ratios of 1:1, 1:2 and 1:3 until clear colorless liquids were obtained, with the 1:2 mixture giving the lowest freezing point of 25°C [110]. Choline is a provitamin which is derived from lecithin, found in plants and animal organs [111]. As such, choline-derived

**Hydrogen bond donor (HBD) ChCl/HBD ratio (mol/**

*Novel room temperature liquid DESs derived from renewable sources [119].*

**mol)**

1:2 32

1:1 96

1:1 99

1:2 62

**M.p. of HBD/°C**

*DOI: http://dx.doi.org/10.5772/intechopen.86502*

## *Bio-Solvents: Synthesis, Industrial Production and Applications DOI: http://dx.doi.org/10.5772/intechopen.86502*

*Solvents, Ionic Liquids and Solvent Effects*

tetraoctylammonium or methyltrioctylammonium chloride. The ILs were then successfully used as solvents in the extraction of metal ions from aqueous solutions [107]. Kwan et al. also synthesized ILs using lipids through alkylating a tertiary amine (**Figure 10**). The lipids used in this instance were methyl oleate and methyl stearate. In a third instance, cyclopropanated oleic acid methyl ester synthesized the reaction of the double bond of the oleate with diiodomethane, and diethylzinc was used to alkylate the tertiary amine. After reacting the obtained alkyl iodide with the imidazoles, anion exchange of the iodide with bistriflimide gave imidazolium

*Alkaloid-based ILs [(A) ampicillin-based ILs; (B) ephedrine-based IL; (C) nicotine-based ILs].*

Deep eutectic solvents are analogous to ILs in the sense that they possess similar properties of low melting point and non-volatility, but they are, however, two different types of solvent systems. Unlike ILs, which are made up of one type

**14**

bistriflimide ILs [108].

*Lipid-based imidazolyl ILs.*

**Figure 9.**

**Figure 8.**

**Figure 10.**

*Anions and cations used in the syntheses of lipid-based ILs.*

**5. Deep eutectic solvents**

of discrete cation and anion [17], DESs are formed from an eutectic mixture of a hydrogen bond acceptor (HBA), mostly quaternary ammonium salts, and a metal salt or hydrogen bond donor (HBD). The mixture forms a eutectic phase which has a lower melting point than the individual components [109]. There is a charge delocalization created through hydrogen bonding between, for example, a halide anion of the HBA and the HBD which causes the decrease in the melting point [18]. DESs are usually formed from relatively inexpensive bio-based components which makes them biodegradable and affordable [17]. Abbott et al. initially developed DESs based on choline chloride and zinc chloride as cheaper alternatives to imidazolium-based ionic liquids, which would make them readily accessible for bulk applications in syntheses. These new solvents were made by heating mixtures of the choline chloride and zinc chloride in molar ratios of 1:1, 1:2 and 1:3 until clear colorless liquids were obtained, with the 1:2 mixture giving the lowest freezing point of 25°C [110]. Choline is a provitamin which is derived from lecithin, found in plants and animal organs [111]. As such, choline-derived

**Figure 11.** *DES based on choline chloride and urea.*

**Table 1.** *Novel room temperature liquid DESs derived from renewable sources [119].*

DESs are bio-based solvents, particularly where it is paired up with bio-sourced HBDs such as lactic acid, levulinic acid, glycerol and sugars. On a large scale, choline is produced by a single-step reaction between HCl, ethylene oxide and trimethylamine [17].

DESs based on urea (**Figure 11**) were prepared by mixing choline chloride (m.p. 302°C) and urea (m.p. 133°C) in different molar ratios. This showed that a eutectic phase occurs at a choline to urea ratio of 2. This DES has a very low freezing point of 12°C, which was significantly lower than its constituents, thus giving a room temperature solvent [112]. The choline chloride-urea DES has been used as both a catalyst and solvent for the selective *N*-alkylation of various aromatic primary amines. This method avoids the complexity of multiple alkylations, which is a problem encountered when polar volatile organic solvents are used. After the reaction, DES recycling by simple biphasic extraction with ethyl acetate was carried out, and the DES was reused at least five times (with just a slight loss in activity) [113]. This same group have used the choline chloride-urea DES as a solvent and catalyst for the bromination of 1-aminoanthra-9,10-quinone [114] and for the Perkin reaction [115].

DESs based on choline chloride and glycerol or ethylene glycol have also been used as extraction solvents to remove excess glycerol from biodiesel [116–118]. Also, new choline chloride-based DESs with levulinic acid and sugar-based polyols as renewable hydrogen bond donors were synthesized. The best ratios of choline chloride (ChCl) and HBD which gave liquid DESs at room temperature are shown in **Table 1** [119]. DESs based on different bio-based HBD (oxalic, lactic and malic acids) and HBA (choline chloride, betaine, alanine, glycine, histidine, proline and nicotinic acid) have also been prepared and tested as solvents for the dissolution of lignin, cellulose and starch. Majority of the resulting solvents exhibited high lignin solubility but poor cellulose solubility.

This was advantageous since DESs can be used in separating lignin from cellulose [120]. DESs can be viewed as a more environmentally friendly alternative to volatile organic solvents—they are affordable, easily prepared and scalable, inflammable and also biodegradable [121].
