CO₂-triggered switchable solvent for lipid extraction from microalgal biomass

This study proposed a novel and energy-efficient method using switchable polarity solvents for lipid recovery from microalgae. Different from the existing methods, use of switchable polarity solvents does not require the fractional distillation for separation of lipid and solvent by only converting the polarity of the solvent after lipid extraction. When a non-polar amine solvent reacts with CO₂, amino group (NH) can be transformed to a polar form, i.e. carbamate (NHCOO^-). Nuclear magnetic resonance (NMR) spectrum indicated that only secondary amines are convertible to a polar compound of carbamate after CO₂ treatment. The polarity switching potential of each amine candidate was quantitatively evaluated by normalized polarity energy (E_T^N). Dipropylamine (DPA) showed the greatest change in E_T^N from 0.452 to 0.789 kcal/mol (E_T^N of water = 1.0 kcal/mol) before and after CO₂ treatment. DPA is a potential polarity switchable solvent capable of achieving an excellent lipid extraction yield of 7.51% from tested microalgal biomass (Chlorella vulgaris) with 9.16% of total lipid content and 95.5% fatty acid methyl esters (FAMEs) content. Furthermore, the used solvent could be recovered at the high efficiency of 84.0%. With a significant polarity switchability from nonpolar amine to carbamate in the presence of CO₂, DPA, a secondary amine, could be suggested as a suitable solvent used for both extraction of lipids with a higher FAMEs content from microalgae and separation of lipid by only adding CO₂.