Electrolytic membrane extraction enables production of fine chemicals from biorefinery sidestreams

Short-chain carboxylates such as acetate are easily produced through mixed culture fermentation of many biological waste streams, although routinely digested to biogas and combusted rather than harvested. We developed a pipeline to extract and upgrade short-chain carboxylates to esters via membrane electrolysis and biphasic esterification. Carboxylate-rich broths are electrolyzed in a cathodic chamber from which anions flux across an anion exchange membrane into an anodic chamber, resulting in a clean acid concentrate with neither solids nor biomass. Next, the aqueous carboxylic acid concentrate reacts with added alcohol in a water-excluding phase to generate volatile esters. In a batch extraction, 96 ± 1.6% of the total acetate was extracted in 48 h from biorefinery thin stillage (5 g L^-1 acetate) at 379 g m^-2 d^-1 (36% Coulombic efficiency). With continuously regenerated thin stillage, the anolyte was concentrated to 14 g/L acetic acid, and converted at 2.64 g (acetate) L^-1 h^-1 in the first hour to ethyl acetate by the addition of excess ethanol and heating to 70 °C, with a final total conversion of 58 ± 3%. This processing pipeline enables direct production of fine chemicals following undefined mixed culture fermentation, embedding carbon in industrial chemicals rather than returning them to the atmosphere as carbon dioxide.