3. Results and discussion
3.1. Direct hydrolysis of sugarcane bagasse with the assistance of
GVL
As a polar aprotic solvent, GVL has attracted continuous interest due to its application in improving biomass conversion. In this study, liquid solutions containing 80% GVL and 20% water with dilute concentrations of sulfuric acid were used for the hydrolysis of sugarcane bagasse. The effects of sugarcane bagasse loading ranging from 1 to 5 wt% and different sulfuric acid concentrations on biomass degradation were investigated (Table 1). Hydrolysis of sugarcane bagasse was successfully achieved by using GVL/water solution. Higher sulfuric acid concentration of 50 mM resulted in no remarkable effects on the degradation rate of sugarcane bagasse in all experiments, which was in agreement with the results of levoglucosan production [5]. Hydrolysis of biomass by dilute acid is typically carried out with at least 0.5–2 wt% [18], while low concentration of 5 mM (∼0.05 wt%) was needed by using the GVL/water solution, which could avoid the recovery process of mineral acid and improve the economics of biomass-derived products. Dramatic decrease in the concentration of glucose, xylose, and arabinose were observed with the increase of biomass loading. After phase separation, total sugars reached the highest concentration of 24.08 g/L with the biomass loading of 1%, while the lowest concentration of 4.14 g/L was found with high biomass loading of 5%. It was interesting that when biomass loading increased from 1% to 3%, the concentration of glucose was reduced, while xylose was increased. GVL-assisted hydrolysis leads to enhanced yields by inhibiting the polymerization of levoglucosan, which is the primary sugar dehydration product. The maximum biomass degradation rate reached 67% and significantly decreased to 10% with the sugarcane bagasse loading increased to 5 wt%. Low degradation rate resulted in low concentration of total sugars; however, cellulose with 5 wt% loading could be dissolved in GVL solution and converted to levoglucosenone [5], suggesting that the complex structure of sugarcane bagasse might hinder the access of GVL/water solution to polysaccharides. Polysaccharides are crosslinked with the complex network of lignin in the plant cell walls [28]. Thus, removal of lignin might be a promising strategy to enhance the hydrolysis of sugarcane bagasse in GVL/water solution.
3.2. Removal of lignin enhances the hydrolysis of sugarcane bagasse in GVL/water solution
Several methods have been proposed for lignin removal from lignocellulosic biomass, during which microwave treatment is found to be an efficient method [3]. Therefore, microwave pretreatment is generally carried out at 400 W with the supplement of 1% NaOH (Fig. 1). Lignin was continuously removed with the increase of treatment time. The results showed that more than 90% of the initial lignin was removed from sugarcane bagasse after 8 min treatment and the residual contains only 0.7% lignin while the content of cellulose and hemicelluloses increased to 64.3% and 25.6%, respectively. Further increase the treatment time to 10 min resulted in no obvious decrease in lignin content. After removal of lignin, the obtained residual was used as the substrate for hydrolysis in GVL/water solution (Table 2). The lignin-removed sugarcane bagasse with 1% loading was almost completely dissolved in GVL/water solution while it only reached 67% without microwavealkali pretreatment. Total sugars of 9.4 g/L was obtained with a high sugar yield of 0.92 g/L, in which glucose, xylose and arabinose was 6.73, 2.56 and 0.11 g/L, respectively. The sugars were concentrated to 48.88 g/L after phase