Currently, most commercial biodiesel is produced from plant lipids using homoge­neous basic catalysts such as NaOH or KOH [20]. Nevertheless, these catalytic systems have a number of drawbacks: (I) catalysts cannot be reused and have to be neutralized which produces wastewater; (II) formation of stable emulsions that makes FAMEs separation difficult; (III) glycerol is obtained as an aqueous solution with low purity; (IV) the process is sensitive to residual water and free fatty acids [33]. ILs are recognized as green solvents due to their special properties comparing with traditional organic solvents, such as tunability, non-detectable vapor point and performance benefits over molecular solvents. Their properties can be designed to suit a particular need by changing the structures of the cation, anion or both acidic or basic for special synthesis [34, 35]. The principle was widely used for biodiesel production from lipids [11—13].

In consideration of good solubility of ILs to inorganic and organic compounds, and tunable miscibility with organic solvents, the simplest way is to use ILs for biodiesel production as solvents to immobilize traditional acidic or basic catalysts, such as K2CO3, NaOH, hydroxide salts of ammonium cations, sodium methoxide, lithium diisopropylamide, and H2SO4 [36]. Usually, a two-phase system (a glycerol-methanol-ILs-catalyst phase and biodiesel phase) forms due to the immiscibility of biodiesel with ILs after the reaction is done. The catalytic system can be reused after decanting the biodiesel directly. For example, under basic conditions, the combination of 1-n-butyl-3-methylimidazolium bis(trifluoromethyl — sulfonyl)imide (BMI • NTf2), alcohols, and K2CO3 (40 mol%) results in production of biodiesel from soybean oil in high yield and purity. H2SO4 immobilized in BMI • NTf2 efficiently promotes the transesterification reaction of soybean oil and various primary and secondary alcohols. In this multiphase process the acid is almost completely retained in the IL phase, while the biodiesel forms a separate phase. The recovered IL containing the acid catalyst could be reused for six times without significant yield or selectivity loss [36].

It is known that the acidity and basicity of ILs can be tuned by changing the composition of cationic and anionic species. Some acidic or basic ILs have been used as both catalysts and solvents for the synthesis of biodiesel (Scheme 7.2) These ILs can be synthesized by introduction of acidic functional groups into either the cation or anion, or adding a Lewis acid catalysts in ILs to form a catalytic active Lewis acid ILs [12, 37]. No matter the use of ILs as solvents or catalysts, the processes are usually efficient and facile for biodiesel production (Table 7.4). Inexpensive materials such as non-edible oils and waste cooking oils contain high free fatty acid contents, which are not suitable for base-catalyzed biodiesel produc­tion process. Therefore, free fatty acids should be converted into FAMEs, for which acidic ILs have been better than traditional mineral acids. For example, the dicationic IL N, N,N, N-tetramethyl-N, N-dipropanesulfonic acid ethylenediammonium hydro­gen sulfate could be used as efficient and recyclable catalyst for the synthesis of biodiesel from long-chain free fatty acids or their mixtures [44]. The reaction was accomplished in a monophase at 70 °C for 6 h, while the products were separated by liquid/liquid biphase separation at room temperature with yields of 93-96 %. The work-up process was simple, and the catalysts could be reused for six times with little activity loss. This novel and clean procedure offered advantages including short reaction time, high yield, operational simplicity, and environmental friendliness. To achieve a better catalyst separation, acidic ILs-based catalysts have been covalently immobilized onto SBA-15. The immobilized catalysts displayed relatively high activity in esterification of oleic acid with short-chain alcohols because of the synergistic effects of both Lewis and Br0nsted acidic sites. Under the optimal reaction conditions (molar ratio of methanol to oleic acid 6:1,5 wt% catalyst loading, and 363 K for 3 h), the conversion of oleic acid reached 87.7 %. It was found that some metal chloride-based ILs could efficiently convert un-pretreated Jatropha oil with high-acid value (13.8 mg KOH/g) to biodiesel. For example, when FeCl3 was added to [1-butyl-3-methyl-imidazolium][CH3SO3], a biodiesel yield of 99.7 % was achieved at 120 °C [53].

The basic ILs can be designed by introducing a strong basic anion or an organic basic moiety. The principle was used widely to synthesize ILs for the transester­ification of lipids with methanol and ethanol. Most recently, three novel dicationic basic ILs were prepared for synthesis of biodiesel from soybean oil. Among them, 1,2-bis(3-methylimidazolium-1-yl)ethylene imidazolide showed the highest bio­diesel yield of 99.6 %. When the acidity of soybean oil was 0.49 mg KOH/g, the yield of biodiesel was 99.6 %. However, when it was 1 mg KOH/g, the yield of biodiesel dropped to 82.5 %. Thus, basic ILs had limited capacity to use high acidity feedstock for biodiesel production [54].

It is well recognized that organic bases (e. g. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), guanidine) are important catalysts for the transesterification of lipid with alcohols [27, 55—57]. As both organic bases and alcohols are important components in the reaction, a novel phase-switching homogeneous catalysis was devised for clean production of biodiesel and glycerol (Scheme 7.3). It was found that the FAMEs can be decanted from the system and the yields were up to 95.2 % [27].

The produced glycerol was extracted from FAMEs completely by the “switchable solvents”, and recovered with high purity after recycling DBU by another extraction process. This system has been tested for integrated production of biodiesel from cell mass of the oleaginous yeast Rhodosporidium toruloides Y4. While intracellular lipid was successfully extracted, only about 21.9 % of the lipid was converted into FAMEs. Nonetheless, such systems offered significant advantages.