Clean enzymatic preparation of oxygenated biofuels from vegetable and waste cooking oils by using spongelike ionic liquids technology

Abstract

The biocatalytic synthesis of oxygenated biofuels (fatty acid solketal esters, FASEs) and biodiesel (fatty acid methyl esters, FAMEs) was carried out by both the direct esterification of fatty acids (i.e., lauric, myristic, palmitic, and oleic acids, respectively) with solketal or methanol, and the transesterification of vegetable oils (i.e. sunflower, olive, cottonseeds, and waste cooking oil) with the same alcohols, in hydrophobic ionic liquids (ILs) based on cations with long alkyl side-chains (e.g., octadecyltrimethylammonium bis- (trifluoromethylsulfonyl)imide [C18tma][NTf2]). These hydrophobic ILs are temperature switchable ionic liquid/solid phases that behave as sponge-like system. As liquid phases, they are excellent monophasic reaction media for proposed biotransformations with all the assayed fat substrates, e.g. near to 100% yield of fatty acids solketyl esters (FASEs) and fatty acid methyl esters (FAMEs) in 6 h at 60 °C. By using waste cooking oil mixed with free fatty acids as substrate, green biofuels containing either both FAMEs and FASEs (e.g., aprox. 80% FAMEs and 20% FASEs, etc.) can easily be prepared. Moreover, the reaction mixture can be easily fractionated by iterative centrifugations at controlled temperature into three phases, i.e. solid IL, water, and FAMEs + FASEs mixture leading to a straightforward and clean approach allowing the full recovery of the biocatalyst/IL system for further reuse and the simple product isolation. Furthermore, the enzyme did not shown any loss in activity during reuse in these reaction systems after six operation cycles.