Hemicellulosic sugars to biobutanol via acid catalyzed pretreatment and acetone-butanol-ethanol fermentation

The aim of this study was to use acid catalyzed pretreatment for efficient solubilization of hemicellulosic sugars from lignocellulosic materials and test the fermentability of the liquid prehydrolysate via acetone-butanol-ethanol (ABE) fermentation. Three different lignocellulosic materials were chosen: barley straw (Hordeum vulgare), a willow species (Salix schwerinii), and a spruce species (Picea abies). The aim of the pretreatment was to clarify the most optimal conditions to liberate hemicellulosic sugars into a fermentable monomeric form without serious degradation and leave the cellulose as intact as possible. With the barley straw, xylan was completely extracted into the liquid prehydrolysate with the combined severity (CS) 1.27 (120°C, 1% H2SO4 and 60 min) and with willow, approximately 65% of xylan was extracted as monosaccharidic xylose with the CS 2.29 (0.1% H2SO4, 200°C, 30 min). Microwave pretreatment was shown to be effective with Norway spruce, with almost complete extraction of mannan, galactan, and xylan to the liquid prehydrolysate. Additionally, low concentrations of degradation products including furfural, HMF, formic acid, and levulinic acid were produced during acid-catalyzed pretreatments. On the other hand, results showed that the dilute acid catalyzed pretreatments tested gave incomplete enzymatic saccharification of the willow and Norway spruce pretreated solid materials. Results showed, however, that with the optimization of pretreatment conditions based on the lignocellulosic biomass used, hemicelluloses could be extracted more selectively to fermentable sugars and cellulose preserved for further biorefining. The liquid prehydrolysate of willow without detoxification but supplemented with starch was successfully fermented to butanol using Clostridium acetobutylicum, with butanol and ABE yields of 0.22 g/g and 0.35 g/g monosaccharide, respectively. It was also found that starch from barley grain ensured the essential nutrients for ABE fermentation. For efficient utilization of hemicellulose for butanol production, combining starch-containing sidestreams to the hemicellulosic side-streams would offer an option for industrial ABE production.