Optimizing butyrate production from methanol and CO₂ in microbial electrosynthesis

Microbial electrosynthesis (MES) enables the conversion of carbon dioxide (CO₂) into valuable chemicals utilizing renewable electricity. Acetate is often the main product but supplying a soluble electron donor facilitates upgrading acetate to butyrate via chain elongation. Compared to ethanol as the electron donor, methanol is a promising alternative as its production avoids the competition with food production. However, the optimal operation conditions for maximizing butyrate production rates in methanol assisted MES have not yet been determined. In this study, methanol assisted chain elongation process was first evaluated in batch bottles to set up the optimal scenario for butyrate production. The highest butyrate production rates and titers were achieved at 35 °C temperature, 1 atm headspace pressure, and a methanol/CO₂ ratio of 3 (on a carbon basis). These operational conditions were subsequently applied to a MES cell operated in fed-batch mode, obtaining a remarkable average butyrate production rate of 107.4 ± 19.7 g m^-2 d^-1 and a selectivity of 75.2 ± 1.1 %. In particular, temperature was a pivotal factor determining butyrate productivity. At 23 °C, regardless of changes in pressure or methanol/CO₂ ratio, acetate remained the main product (with selectivity over 80 %) in both fermentation batch bottles and in MES. In conclusion, this study not only provided a comprehensive multi-variable analysis to optimize key operational parameters for methanol assisted chain elongation but also demonstrated an effective strategy for butyrate production from CO₂ and methanol in MES.