Reduction Of Carboxylic Acids To Alcohols Using Syngas And Clostridium Ljungdahlii As Biocatalyst

Abstract

Short-chain carboxylic acids generated by various mixed- or pure-culture fermentation processes have been considered valuable precursors for production of bioalcohols. While conversion of carboxylic acids into alcohols is routinely performed via catalytic hydrogenation or with strong chemical reducing agents, biological conversion routes are being explored. Reduction of nbutyric acid by pure cultures using glucose as the source of energy and electrons have been studied. However, the high cost of sugars has lead to the searching for cheaper sources. Syngas produced via biomass gasification, which is a blend of carbon monoxide, hydrogen and carbon dioxide, was studied as an economical source of energy and electrons with pure cultures of Clostridium ljungdahlii as a biocatalyst for the carboxylic acids reduction. Acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, and n-caproic acid were successfully converted into their corresponding alcohols. Furthermore, biomass yields and fermentation stoichiometry from our experimental data enabled us to amend thermodynamic calculations with the goal to evaluate how much metabolic energy C. ljungdahlii can generate during fermentation of carbon monoxide. Our results show ATP yield of 0.42 ATP per carbon monoxide molecule consumed for fermentation of syngas, and 0.38 ATP per carbon monoxide molecule consumed when n-butyrate is added to the growth medium. The ratio of protons pumped across the cell membrane vs. electrons transferred from ferredoxin to NAD+ via the RNF complex is suggested to be 1.0. The results obtained in this work suggest that the production of bioalcohols, based on the reduction of carboxylic acids, may be an attractive alternative industrial process.