**Acknowledgements**

*Solvents, Ionic Liquids and Solvent Effects*

trimethylamine [17].

Perkin reaction [115].

solubility but poor cellulose solubility.

mable and also biodegradable [121].

smart solvents for tailored applications.

**6. Conclusions**

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

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

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

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, inflam-

Rising concerns over depletion of fossil reserves and the drive towards sustainable and responsible consumption of resources have presented a challenge and opportunity for scientist and engineers to work together in developing methods, technologies and processes for producing of bio-based products. These include commodity chemicals, fuels and materials of which many of the chemicals have solvent properties. As such, they have been widely used in extractions and in solvat-

ing a range of physiochemical processes and numerous consumer goods

ized and discussed some existing and emerging approaches in these areas.

(by either dissolving, stabilizing or modifying the latter's properties) across the manufacturing research and development landscapes. This review has contextual-

Even though most bio-solvents are still volatile like their petroleum counterparts, the bio-solvents have the advantage of being biodegradable, derived from renewable resources, and their production often results in CO2 emission savings. A novel array of ionic liquids and deep eutectic solvents based on inexpensive bio-derived components has been developed, and because these solvents have low volatility, it presents a solution to the volatile organic solvent concern. Additionally, they are usually recyclable after use, with the benefit of tunability to bring about

**16**

This work was supported by the Royal Society and African Academy of Sciences, the Future Leaders—African Independent Researchers (FLAIR) Scheme (Fellowship ref.: 191779), the National Research Foundation (NRF) of South Africa (Grant numbers: 105559, 112809 and 117989) and the University of Johannesburg's Centre for Synthesis and Catalysis.
