Design of a Ɛ-caprolactam-based ionic liquids for the extraction of lipids from starved Parachlorella kessleri microalgae for sustainable aviation fuels (SAF)

Sustainable and efficient microalgae lipid extraction methods for the advancement of third-generation biofuels is crucial to propel the decarbonization of the transport sector. Within this context, COCPIT European project is working on the decarbonization of the aviation and marine transports through the exploration of different biofuel production routes from microalgae. The present study aimed to design, synthesize, and validate four bio-based ionic liquids based on ε-caprolactam, combined with various organic anions (lactate, acetate, formate, and citrate), to extract lipids from starved Parachlorella kessleri for the production of sustainable aviation fuels (SAF). The Brønsted-type ionic liquids were synthetised using acid-base neutralization reactions. The lipid extraction efficiency of each ionic liquid was first evaluated, with ε-caprolactam-lactate (CPL-LAC) leading to the highest recovery, 89.80 %. The ionic liquids performance was validated with COSMO-RS calculations to have a more solid rationale for ionic liquid selection. Although caprolactam-lactate ionic liquid ranked third in the COSMO-RS affinity predictions, it was selected for response surface design optimization due to its experimental performance and the lack of significant differences between the ionic liquids. Caprolactam-lactate ionic liquid, in turn, exhibited outstanding physicochemical properties, such as high thermal stability (TGA: 198.60 °C), low melting point (DSC: −3.75 °C), and a strong ability to facilitate molecular interaction, as shown in NMR and FTIR spectroscopy. The lipid extraction process with caprolactam-lactate ionic liquid was optimized through surface response analysis, which required a total of 31 experiments. The optimized conditions (42 °C, biomass : ionic liquid ratio 1:11, 8 h of stirring, 19 min of sonication) led to extraction efficiencies within 89.60 % and 91.80 %. The recovered lipids were esterified and analysed through gas chromatography-FID in terms of fatty acid methyl esters (FAMEs) composition. The most predominant FAMEs were palmitic (C16:0), stearic (C18:0), and oleic (C18:1 cis + tras) acids, suggesting the suitability of the obtained extract in biofuel applications. • Designed four ε-caprolactam-based ionic liquids for lipid extraction • Caprolactam-lactate achieved 89.8 % lipid recovery from Parachlorella kessleri . • COSMO-RS validated ionic liquid–lipid affinity predictions. • Optimized extraction reached up to 91.8 % efficiency. • FAMEs profile supports suitability for sustainable aviation fuels.