Production of Hexanol as the Main Product Through Syngas Fermentation by Clostridium carboxidivorans P7

Abstract

The production of hexanol from syngas by acetogens has gained attention as a replacement for petroleum-derived hexanol, which is widely used in the chemical synthesis and plastic industries. However, acetogenic bacteria generally produce C2 compounds (e.g., acetate and ethanol) as the main products. In this study, the gas fermentation conditions favorable for hexanol production were investigated at different temperatures (30–37°C) and CO gas contents (30–70%) in batch gas fermentation. Hexanol production increased from 0.02 to 0.09 g/L when the cultivation temperature was lowered from 37 to 30°C. As the CO content increased from 30 to 70%, the CO consumption rate and hexanol production (yield, titer, and ratio of C6 compound to total products) increased with the CO content. When 70% CO gas was repeatedly provided by flushing the headspace of the bottles at 30°C, the total alcohol production increased to 4.32 g/L at the expense of acids. Notably, hexanol production (1.90 g/L) was higher than that of ethanol (1.20 g/L) and butanol (1.20 g/L); this is the highest level of hexanol produced in gas fermentation to date and the first report of hexanol as the main product. Hexanol production was further enhanced to 2.34 g/L when 2 g/L ethanol was supplemented at the beginning of 70% CO gas refeeding fermentation. Particularly, hexanol productivity was significantly enhanced to 0.18 g/L/day while the supplemented ethanol was consumed, indicating that the conversion of ethanol to acetyl-CoA and reducing equivalents positively affected hexanol production. These optimized culture conditions (gas fermentation at 30°C and refeeding with 70% CO gas) and ethanol supplementation provide an effective and sustainable approach for bio-hexanol production.

Characterizing Novel Acetogens for Production of C2–C6 Alcohols from Syngas

“…Strain P21 also produced similar amounts of hexanol from syngas reported in a previous study using strain P7 with temperature variance [45]. However, strain P21 showed slightly lower hexanol production from syngas compared to strain P7 in previous studies, particularly those utilizing extractive syngas fermentation [32,[46][47][48].…”

A perspective on downstream processing performance for recovery of bioalcohols

“…21 Accordingly, concentrations of ethanol in the fermentation broth that microorganisms can tolerate (5-6 wt% 23 from syngas fermentation or 5-12 wt % from glucose fermentation) 24 are higher than non-harmful concentrations of isopropanol (4-5 wt% from syngas fermentation), 25 isobutanol (∼1.6 wt% from glucose fermentation) 26 and 1-hexanol (∼0.2 wt% from glucose fermentation). 27 Besides the nature of the fermentation product, pH, substrate type, microbe type, temperature, aeration, nutrients and additives also influence the extent of inhibition. 21 The concurrent alcohol fermentation and recovery (CARAF) may mitigate the endproduct inhibition problem by allowing continuous product removal during fermentation.…”

“…Realistically achievable concentrations of alcohols in the fermentation broth were assumed, as follows: 6.14 wt% ethanol, 23 5.01 wt% isopropanol including small amounts of acetone, 25 1.61 wt% isobutanol 26 and 0.24 wt% hexanol. 27 Nonetheless, it should be noted that the achievable concentrations of alcohols may vary depending on the type of fermentation process, which would significantly affect the performance of the downstream process. For example, higher concentrations of ethanol (about 5-12 wt%) 24 are achievable in fermentation from glucose compared to fermentation from syngas (about 5-6 wt%).…”

Syngas Fermentation: Cleaning of Syngas as a Critical Stage in Fermentation Performance

“…Hexanol production was enhanced to 2.34 g/L when ethanol was supplemented [192] Ethanol: 1.40-1.50 Butanol: 0.40-0.50 Hexanol: 0.10-0.20 1 Evaluating the effect of oxygen presence in syngas [193] Ethanol: 2.28 Butanol: 0.74 Optimizing medium and reducing costs [194] Table 3. Cont.…”