Effectiveness of using deep eutectic solvents as an alternative to conventional solvents in enzymatic biodiesel production from waste oils

Ionic liquids (ILs) have been proposed as a more benign replacement to toxic and volatile organic solvents in enzymatic biodiesel production, used to minimize methanol inhibition effect and enhance the stability of immobilized enzyme. Despite their several advantages, ILs are expensive, which renders the overall process unfeasible. In this work, the use of low-cost deep eutectic solvents (DESs) has been investigated as a new reaction medium for enzymatic biodiesel production from waste oils. A DES composed of chlorine-chloride and glycerol (ChCl:Gly) was tested and its effectiveness was compared to 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF₆], which has shown positive results. To decrease the viscosity of ChCl:Gly DES, ternary ChCl:Gly:water DESs with different water contents were prepared and tested. Despite lower cost compared to ILs, without successful repeated reuse of the DES-immobilized lipase system, their advantages remain theoretical. Therefore, the reusability of the DES-Novozym^®435 system in consecutive cycles was examined. It was shown that 34% biodiesel production yield could be achieved in ChCl:Gly at 1:2 molar ratio, compared to 23% when [bmim][PF₆] was used. The yield increased further, when the ternary DES of ChCl:Gly:water (1:2) with 3 wt% water was used, reaching 44%. By removing the by-product glycerol, the reusability of the DES-immobilized system was improved, with better results achieved using 1-butanol compared to using ethylene glycol.