The lipids extraction from various resources is essential for production of biodiesel. Usually, the lipids extraction process uses large amounts of organic solvents, such as hexane, CHCl3, etc. which also results in significant losses and energy consump­tion during the solvent recycling process. Hexanes and related hydrocarbon extractants are also becoming an environmental and health concerns. Therefore, exploration of new extraction technologies has received much attention [21].

A new class of “switchable solvents” has been proposed [22, 23], which are based on an exothermic transformation from an organic base, an alcohol and an acid gas (e. g. CO2). These solvents are capable of changing composition reversibly under mild conditions to shift between molecular liquid and ionic liquid, in associated with switching properties, such as polarity and viscosity [24—27]. Thus, ‘switchable solvents’ have been tested to extract lipids from soybean flakes [28]. It was found that the combination of an amidine and excess water gave superior solvent/oil separation, adequate oil extraction. The contamination levels of residual amidine in the soy oil are very low. This method takes advantage of the fact that amidines can be made to switch their hydrophilicity by application or removal of CO2 in the system. However, the extraction efficiency is lower than those of traditional hexane, and ethanol.

Microalgae are one of the most important emerging resources for lipids. In 2010, Samori, et al. extended the switchable solvent to the lipid extraction from water- suspended and dried microalgae Botryococcus braunii. It was found that DBU/octanol exhibited the highest yields of extracted hydrocarbons from both freeze-dried and liquid algal samples (16 and 8.2 % respectively against 7.8 and 5.6 % with n-hexane) [29]. Their follow-up research demonstrated that a new switchable system based on the reversible reaction of N, N-dimethylcyclohexylamine (DMCHA) with water showed better performance in lipid extraction of wet algal samples or cultures (Fig. 7.2) [30].

The lipid extraction efficiency of the system applied to both of wet and dry biomass, was higher than that obtained through a typical extraction procedure with CHCl3-MeOH (Tables 7.1, 7.2). The FAMEs yield was very good for all of the tested algae, independent of the biomass/DMCHA ratio. The higher extraction content may be because DMCHA had access to structural lipids which are resistant to extraction with CHCl3-MeOH [30].

In 2010, Young et al. reported the ability of a co-solvent system composed of a hydrophilic IL 1-ethyl-3-methyl imidazolium methyl sulfate and methanol at a mass ratio of 1.2:1 to extract and auto partition lipids from various biomass. The extraction yields of lipids were summarized in Table 7.3. The results suggest that the ILs-methanol co-solvent is successful in complete extraction of the lipids from these biomass sources. The proposed IL-methanol co-solvent system differs from traditional organic co-solvent systems which both dissolve and solubilize the extracted lipid and thus suffer extraction efficiencies limited by the solvent’s carrying capacity [31].

Considering the main components (e. g. polysaccharide, protein, lipids) of algae, Teixeira investigated the use of traditional ILs to extract lipids and produced chemical feedstocks from algae without acids, bases or other catalysts, which is based on the fact that a dissolution and hydrolysis of wet alga biomass in ILs. Deconstruction reached completion in <50 min regardless of algae species, at 100-140 °C and atmospheric pressure. The fast rate of hydrolysis without acids or bases suggests the ionic liquid itself is acting as both a solvent and a catalyst. Depolymerization of algae cell wall polysaccharides could result in the deconstruc­tion of the cell wall, including the phospholipid membrane, and creation of a cell — free mixture that can be separated into constituent fractions, and result in a full separation of lipids from algae. This work presented a strategy of full utilization of algae biomass [32].

Table 7.2 Lipids fractionation of oils obtained through extraction of dried algae samples (CHCl3- MeOH) and wet algal biomass (DMCHA), expressed on algal dry weight basis (Reprinted with permission from [30]. Copyright © 2012 The Royal Society of Chemistry)