Effective usage of sorghum bagasse: Optimization of organosolv pretreatment using 25% 1-butanol and subsequent nanofiltration membrane separation

Highlights

• Conditions for organosolv pretreatment of sorghum bagasse were optimized.

• A low concentration of butanol (<40%) was used.

• 25% butanol and 0.5% sulfuric acid were effective in recovering cellulose.

• Pretreatment at 200 °C for 60 min was effective in removing lignin.

• Membrane nanofiltration removed butanol in the liquid fraction to aid fermentation.

Abstract

We investigated the use of low concentrations of butanol (<40%, all v/v) as an organosolv pretreatment to fractionate lignocellulosic biomass into cellulose, hemicellulose, and lignin. The pretreatment conditions were optimized for sorghum bagasse by focusing on four parameters: butanol concentration, sulfuric acid concentration, pretreatment temperature, and pretreatment time. A butanol concentration of 25% or higher together with 0.5% or higher acid was effective for removing lignin while retaining most of the cellulose in the solid fraction. The highest cellulose (84.9%) and low lignin (15.3%) content were obtained after pretreatment at 200 °C for 60 min. Thus, pretreatment comprising 25% butanol, 0.5% acid, 200 °C, and 60 min process time was considered optimal. Enzymatic saccharification and fermentation by Saccharomyces cerevisiae produced 61.9 g/L ethanol from 200 g/L solid fraction obtained following pretreatment, and 10.2 g/L ethanol was obtained from the liquid fraction by xylose-utilizing S. cerevisiae following membrane nanofiltration to remove butanol.

Introduction

The use of plant biomass as a feedstock for the production of transportation fuels and chemicals will help alleviate global shortages of fossil fuels and supply renewable energy and materials (Tanaka and Kondo, 2015, Zhao et al., 2009). Sorghum (Sorghum bicolor L. Moench), a highly productive C4 photosynthetic plant, is an attractive feedstock for ethanol production because it requires less fertilizer and water than other sugar crops, is drought tolerant, and can produce a large amount of biomass a few months after sowing (Kim and Day, 2011, Rooney et al., 2007).

Each of the three major components of sorghum bagasse, i.e., cellulose, hemicellulose, and lignin, can be utilized as a source of valuable industrial materials (Kim and Day, 2011, Lan et al., 2011). However, the recalcitrance of sorghum bagasse (and lignocellulosic biomass in general) due to the protection of polysaccharides by lignin against degradation prevents the effective usage of biomass components (Kawaguchi et al., 2015, Capolupo and Faraco, 2016, Karimi and Takerzadeh, 2016). Achieving high sugar yields and product concentrations therefore requires fractionation of these components by disrupting the complex cellular structure, thus making the carbohydrate susceptible toward enzymatic attack (Ostovaren et al., 2015, Yang and Wyman, 2004). Organosolv pretreatment that utilizes organic or aqueous-organic solvent at temperatures of 100–250 °C, with or without the addition of catalysts, is a practical way of recovering cellulose as a solid fraction, with hemicellulose and lignin dissolving in the liquid fraction and organosolv fraction, respectively (Zhao et al., 2009, Huijgen et al., 2012, Zhang et al., 2016). However, organosolv pretreatment often requires the use of high concentrations (≥50%) of organic solvents (such as ethanol or acetone) (Taherzaden and Karimi, 2008, Agbor et al., 2011, Hu et al., 2011) and thus it was necessary to decrease the amount of organic solvent required in order to reduce pretreatment costs. In our previous study, organosolv pretreatment with a low concentration of butanol (12.5%) could generate three fractions from sorghum bagasse: a solid fraction rich in cellulose, a liquid fraction rich in hemicellulose-derived sugar, and an organosolv fraction rich in lignin (Teramura et al., 2016). However, some practical problems remain to be solved. First, the optimum pretreatment conditions using a low concentration of butanol (<40%) remain unclear. Identification of these conditions would provide a larger amount of glucose following enzymatic hydrolysis of the solid fraction and of ethanol following fermentation. Second, butanol present in the liquid fraction inhibits microbial fermentation, requiring a methodology to separate butanol from sugars in the liquid fraction (Okolo et al., 1987, Okolo et al., 1990). To date, there are no reports of the application of membrane processes to the liquid fraction obtained after organosolv pretreatment, although membrane processes are widely used in biorefineries (Abels et al., 2013).

The aims of this study were twofold. First, we investigated the optimum conditions for using low concentrations of butanol (<40%) as judged from the composition and recovery of cellulose, hemicellulose, and lignin in the solid fraction obtained after butanol-dilute acid pretreatment of sorghum bagasse. Four parameters were evaluated in detail: butanol concentration, sulfuric acid concentration, pretreatment temperature, and process time. Second, the liquid fraction was optimized for microbial fermentation by applying a membrane separation technology to the liquid fraction to remove butanol.