Effectiveness of dilute oxalic acid pretreatment of Miscanthus × giganteus biomass for ethanol production

• Miscanthus × giganteus biomass has been pretreated with dilute oxalic acid.
• Enzymatic hydrolysis showed a glucan conversion of 91.2%.
• SSF using Schefferomyces (Pichia) stipitis CBS 6054 reached 20.2 g L−1 of ethanol.
• Negative correlations have been found between xylan vs. glucan conversion and xylan vs. ethanol.
• Hemicellulose hydrolysate was fermented to ethanol with high yield (380 g kg−1).

In the present study the effect of temperature, reaction time and dilute oxalic acid (OA) concentration during steam-pretreatment of Miscanthus × gigantueus has been evaluated using the combined severity factor (CS). At the highest CS glucan and lignin content in the water insoluble fraction (WIF) increased, while xylan content decreased. While glucose recovery in the water soluble fraction (WSF) was found at low concentration when mild CS were used (≤5.0 g L−1 at CS ≤ 2.17), xylose and arabinose concentrations were higher at low-mild CS (1.58–2.17) with a concentration peak at CS 2.03 (39.9 and 3.2 g L−1 for xylose and arabinose, respectively). The decrease in pentoses coincided with inhibitory formation in the WSF, namely acetic acid, furfural, HMF and phenolic compounds. Glucan conversion rose from 46.1% at CS 1.54 to 91.2% at CS 2.76. Likewise, maximum ethanol concentration was achieved at CS 2.76, corresponding to 20.2 g L−1 and a volumetric ethanol productivity of 0.28 g L−1 h−1. Negative correlations have been found between xylan vs. glucan conversion and xylan vs. ethanol production, suggesting that decreasing the xylan content in WIF increases both saccharification rate and ethanol concentration (R2 0.91 and R2 0.93, respectively). On the other hand, a positive correlation was found between ethanol production and glucan conversion (R2 0.93). Fermentation of WSF by Scheffersomyces (Pichia) stipitis CBS 6054 at CS 1.54 produced 12.1 g L−1 of ethanol after 96 h incubation with a volumetric ethanol productivity of 0.13 g L−1 h−1.