Kraft GL-ethanol pretreatment on sugarcane bagasse for effective enzymatic hydrolysis

• An increase in pretreatment temperature facilitated the release of lignin.
• The structure of lignin fractions from pretreatment liquor was characterized.
• The hydrophily of sugarcane bagasse was significantly increased.
• The improvement of glucose yield after low temperature pretreatment was high.
• The maximum glucose yield of the pretreated sugarcane bagasse reached 98.26%.

Kraft green liquor (GL) mainly consisting of sodium carbonate and sodium sulfide are available from Kraft mills. Kraft GL combined with ethanol (Kraft GL-ethanol) was chosen to pretreat sugarcane bagasse for effective enzymatic hydrolysis. And the obtained lignins and cellulose rich fractions were characterized in detail. The highest lignin removal of 95.3% was observed at a pretreatment temperature of 160 °C, and approximately 70.8% of lignin was removed at the pretreatment temperature of 100 °C. The lignin removal with Kraft GL-ethanol pretreatment was higher than that with Soda GL-ethanol pretreatment at 140 °C. An FT-IR study of the isolated lignins showed that an increase in pretreatment temperature resulted in the cleavage of ester bonds. GPC results revealed that the molecular weight decreased with increasing pretreatment temperature. The 2D HSQC NMR data showed that the cleavage of β-O-4 became more severe with increasing pretreatment temperature from 80 to 160 °C. Fewer condensation reactions between lignin units occurred when the pretreatment temperature was above 100 °C. Moreover, the enzymatic hydrolysis efficiency of sugarcane bagasse increasing with increasing pretreatment temperature. The glucose yield reached 65.13% at a pretreatment temperature of 80 °C. The maximum glucose yield (98.26%) of sugarcane bagasse after 72 h of enzymatic hydrolysis was achieved at a pretreatment temperature of 160 °C. When lignin content was low in samples after pretreatment at 140–160 °C, the enzymatic hydrolysis efficiency of the substrate increased because the number of crystalline cellulose regions decreased. Scanning electron microscopy showed that the surfaces of Kraft GL-ethanol pretreated sugarcane bagasse were separated into individual fibers. In addition, contact angle analysis showed that the Kraft GL-ethanol pretreated sugarcane bagasse was more hydrophilic than untreated sugarcane bagasse.

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