**3. Esters**

#### **3.1 Acyclic esters**

Lactates are a class of non-toxic and green solvents obtained from treating lactic acid with various alcohols, such as ethanol, propanol and butanol. Lactic acid feedstock for making these solvents can be obtained *via* biochemical and thermochemical routes. The latter is economical but uses toxic hydrogen cyanide and gives a racemic mixture of lactic acid, while the biological process uses microbial fermentation technology and is more selective.

Ethyl lactate is the most common of the lactate esters, and Archer Daniels Midland Company, USA, operates a commercial production plant. Ethyl lactate has excellent physical properties, including a low vapor pressure and high boiling point (151–155°C) and solvent power (Kauri-butanol value > 1000). This makes it a good replacement for halogenated solvents, acetone and toluene and an excellent solvent for solubilizing resins and polymers. As such, it is used to dissolve plastics and to remove salts from circuit boards. Ethyl lactate also dissolves grease, inks and solder paste and strips paint [52].

Fatty acid methyl esters are produced by the transesterification of triglycerides, from vegetable oils or animal fats, with methanol for biodiesel applications. However, FAMEs can also be used as bio-based solvents and have been found to possess high solvent power. When mixed with ethyl lactate, evaporation is aided post utilization in dissolution or in cleaning industrial parts [53].

### **3.2 Glycerol carbonate and γ-valerolactone**

Glycerol carbonate can be synthesized via direct or indirect routes. With direct routes, carbon monoxide and oxygen [54] or carbon dioxide [55] is treated with glycerol in the presence of metal catalysts such as Pd and Sn, respectively. The use of CO2 is more desirable due to its abundance and lower toxicity in comparison to CO; however, this route is low yielding (7–35%). The indirect route involves carbonation of glycerol with an activated carbonation source such as urea, dimethyl carbonate or phosgene, and among these dimethyl carbonate is preferred from an industrial production perspective [56].

Due to its favorable properties which include high polarity, boiling point (110–115°C) and flash point (109°C) and low vapor pressure, glycerol carbonate has interesting applications as a polar protic solvent, electrolyte liquid carrier, detergent solvent, humectant and nail polish/gel stripper [57]. It has also been demonstrated to serve as a promising solvent in pretreatment of sugarcane bagasse [13].

γ-Valerolactone (GVL) can be produced from 5-hydroxymethylfurfural or furfural alcohol via their dehydration and hydrolysis, respectively [58]. The resultant levulinic acid can then be converted to GVL in the presence of hydrogen and a suitable catalyst [59, 60]. This therefore links the cost of GVL production to the production of hydrogen. This has resulted in the limited widespread production and use commercially. The increased number of hydrogen production plants by water electrolysis (or use of transfer hydrogenation techniques) should aid in improving the process and economics to favor GVL commercial-scale production in the near future.

**9**

**Figure 3.**

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

dominated by heterogeneous Pd catalysts [75, 76].

bond formation reactions [79].

alkanes as shown in **Figure 4** [80–83].

*Synthetic steps of Cyrene™ from cellulose.*

**3.4 Alkanes and aromatics**

*3.4.1 Alkanes*

The physical and chemical properties of GVL favor its application as a solvent and illuminating fuel. It has a low melting point, high boiling and flash point and low toxicity and has a characteristic herbal smell that can be used to identify leaks or spills [61]. GVL has been used as a solvent in many biomass conversion reactions. For example, it was used as a solvent in the pretreatment of lignocellulose, hydrolysis of lignocellulose and dehydration of carbohydrates to furans [61, 62]. It has also been used as solvent in crosscoupling reactions such as Sonogashira [63], Hiyama [64] and Mizoroki-Heck [65].

Cyrene (dihydrolevoglucosenone or 6,8-dioxabicyclo[3.2.1]octanone) is a ketone functionality containing solvent that can be prepared in a two-step process from cellulose [66]. The common starting point in its synthesis is from levoglucosenone (LGO), which can be obtained from a variety of cellulosic starting feedstocks such as Bilberry presscake [67], corn cob [68], poplar wood [69] or bagasse [70]. LGO has been successfully obtained from cellulose [71–73], by the patented Furacell™ process discovered by Circa Group Ltd., Australia. The process gives 40% yield of LGO and has consequently been scaled up to a 50 ton/year production [74]. The next step after the production of LGO is its hydrogenation to afford Cyrene™ (**Figure 3**). The hydrogenation of levoglucosenone to Cyrene has been largely

Concerns over the use of toxic solvents (such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and *N-*methyl-2-pyrrolidone (NMP)) in the industry have led to the exploration of Cyrene™ as a potential replacement. These dipolar aprotic solvents have very similar properties to those of Cyrene™ [77]. As a result, more examples of its use as solvent in syntheses are becoming popular. For example, metal organic frameworks that were previously prepared in DMF solvent have been successfully synthesized in Cyrene™ [78]. Cyrene™ has also been used in amide

Furfural and hydroxymethylfuran can react with acetone (derived from fermentative processes) resulting in aldol-condensed products with longer carbon chain lengths. Subsequent hydrodeoxygenation of the aldol condensation products using a supported Pd catalyst, H2 (plus acetic and/or Lewis acid cocatalysts), produces

Alkanes are useful for reactions requiring nonpolar medium because they are generally unreactive. Consequently, they have been used as reaction medium during

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

**3.3 Cyrene™**

The physical and chemical properties of GVL favor its application as a solvent and illuminating fuel. It has a low melting point, high boiling and flash point and low toxicity and has a characteristic herbal smell that can be used to identify leaks or spills [61]. GVL has been used as a solvent in many biomass conversion reactions. For example, it was used as a solvent in the pretreatment of lignocellulose, hydrolysis of lignocellulose and dehydration of carbohydrates to furans [61, 62]. It has also been used as solvent in crosscoupling reactions such as Sonogashira [63], Hiyama [64] and Mizoroki-Heck [65].
