The purification of biodiesel is an essential process towards the production of high quality fuel. The main task of which is to remove glycerol and residual catalyst. Glycerol has low solubility in FAMEs and can be separated by settling or centrifugation. The presence of residual glycerides can cause deposition of bio­diesel in internal combustion engine injectors (carbon residue) [78]. In addition, residual glycerol can initiate settling problems in the engine and, on the long term, affect human or animal health by the emission of hazardous acrolein into the environment. The presence of catalysts in biodiesel can form deposits (carbon residue) in fuel injection system, poison the emission control system, and weaken the engine [79].

Ionic liquids’ abbreviation and lull name: [С;.тіт][ТГО]: l-Ethyl-3-methylimidazolium trifluoromethanesulfonate; [C4mim][NTf;.]: l-/)-butyl-3-methylimidazolium N-bistrifluoromethanesulfonyl)imidate; [C;.mim][BF4]: l-ethyl-3-methylimidazolium tetralluoroborate; [C4mim][BF4]: l-butyl-3-methylimidazolium

tetralluoroborate; [Ci8mim][NTf4]: l-methyl-3-octadecylimidazolium te(trifluoromethylsulfonyl)imide; [Ci6mim][NTlV]: l-hexadecyl-3-methylimidazolium bis (trilluoromethylsulfonyl)imide; [C4mim][PF6]: l-butyl-3-methylimidazolium hexalluorophosphate; [C8mPy][BF4]: l-octyl-3-methyl-pyrdininium tetralluoroborate

As glycerol and methanol are highly soluble in water, water washing was widely used to remove excess contaminations (e. g. glycerol, alcohols, residual metal salts, soaps, fatty acids). However, the presence of water brings many disadvantages, including increased costs and production time, and generation of waste water [80]. Traditionally, several other methods have been used to remove glycerol from biodiesel, such as adsorption over silica, membrane reactors, and the addition of lime and phosphoric acid. Yet, technical problems remain for biodiesel produc­tion at an industrial scale [81]. To develop better process for byproduct removal, some classes of deep eutectic solvents based on mixtures of quaternary ammonium salts and compounds with hydrogen bond-donating group, have been applied in biodiesel production from rapeseed and soybean oil [82]. Deep eutectic solvents are inexpensive, non-toxic, and environmentally benign. While pure quaternary ammo­nium salts alone were inefficient, the quaternary ammonium salt-glycerol mixture solvents were successful for extraction of glycerol from biodiesel production mixtures, and a glycerol/salt molar ratio of 1:1 was found most effective. Of those salts studied, choline chloride, ClEtNMe3Cl and EtNH3Cl showed high efficiency for glycerol removal.

Deep eutectic solvents have also been used to extract glycerol from palm oil-based biodiesel production in order to meet the EN 14214, and ASTM D6751 standards. The extraction process involved different compositions of a quaternary ammonium salt to glycerol as the solvent, and a ratio of 1:1 was found most efficient. Moreover, the ratio of biodiesel to deep eutectic solvent was more important than the ratio of quaternary ammonium salt to glycerol. The used solvent can be recovered by crystallization [83].

Deep eutectic solvents based on methyl triphenyl phosphonium bromide and different hydrogen bond donors (e. g. glycerol, ethylene glycol, and triethylene glycol) were employed to remove glycerol from palm-oil-based biodiesel [84]. It was found that the solvents including ethylene glycol or triethylene glycol were successful in removing free glycerol to below the ASTM standards. These sol­vents were able to reduce the content of monoacylglycerides (MGs) and diacylglycerides (DGs), but DGs were removed more effectively than MGs. Choline chloride and methyltriphenylphosphonium bromide based deep eutectic solvents could also be used to remove residual KOH efficiently from palm oil-based biodiesel [85].