Organosolv pretreatment of sorghum bagasse using a low concentration of hydrophobic solvents such as 1-butanol or 1-pentanol

BACKGROUND The primary components of lignocellulosic biomass such as sorghum bagasse are cellulose, hemicellulose, and lignin. Each component can be utilized as a sustainable resource for producing biofuels and bio-based products. However, due to their complicated structures, fractionation of lignocellulosic biomass components is required. Organosolv pretreatment is an attractive method for this purpose. However, as organosolv pretreatment uses high concentrations of organic solvents (>50 %), decreasing the concentration necessary for fractionation would help reduce processing costs. In this study, we sought to identify organic solvents capable of efficiently fractionating sorghum bagasse components at low concentrations. RESULTS Five alcohols (ethanol, 1-propanol, 2-propanol, 1-butanol, and 1-pentanol) were used for organosolv pretreatment of sorghum bagasse at a concentration of 12.5 %. Sulfuric acid (1 %) was used as a catalyst. With 1-butanol and 1-pentanol, three fractions (black liquor, liquid fraction containing xylose, and cellulose-enriched solid fraction) were obtained after pretreatment. Two-dimensional nuclear magnetic resonance analysis revealed that the lignin aromatic components of raw sorghum bagasse were concentrated in the black liquor fraction, although the major lignin side-chain (β-O-4 linkage) was lost. Pretreatment with 1-butanol or 1-pentanol effectively removed p-coumarate, some guaiacyl, and syringyl. Compared with using no solvent, pretreatment with 1-butanol or 1-pentanol resulted in two-fold greater ethanol production from the solid fraction by Saccharomyces cerevisiae. CONCLUSIONS Our results revealed that a low concentration (12.5 %) of a highly hydrophobic solvent such as 1-butanol or 1-pentanol can be used to separate the black liquor from the solid and liquid fractions. The efficient delignification and visible separation of the lignin-rich fraction possible with this method simplify the fractionation of sorghum bagasse.