Extraction and Esterification of Low‐Titer Short‐Chain Volatile Fatty Acids from Anaerobic Fermentation with Ionic Liquids

Ionic liquids can both act as a solvent and mediate esterification to valorize low‐titer volatile fatty acids and generate organic solvents from renewable carbon sources including biowaste and CO 2 . In this study, four phosphonium ionic liquids were tested for single‐stage extraction of acetic acid from a dilute stream and esterification to ethyl acetate with added ethanol and heat. The esterification proceeded with a maximum conversion of 85.9±1.3 % after 30 min at 75 °C at a 1:1 stoichiometric ratio of reactants. Extraction and esterification can be tailored using mixed‐anion ionic liquids; this is demonstrated herein using a common trihexyl(tetradecyl)phosphonium cation and a mixed chloride and bis(trifluoromethylsulfonyl)imide anion ionic liquid. As a further proof‐of‐concept, ethyl acetate was generated from an ionic liquid‐driven esterification of an acetic acid extractant generated using CO 2 as the only carbon source by microbial electrosynthesis.