Effect of the addition of calcium hydroxide on the hydrothermal–mechanochemical treatment of Eucalyptus

• Ca(OH)2 enhanced cleavage of ester bonds between lignin and hemicellulose.
• Glucose yield was increased up to 90% with two-thirds lignin remaining.
• Specific surface area of substrate increased efficiently by milling with Ca(OH)2.

The effect of Ca(OH)2 addition on optimization of hydrothermal–mechanochemical pretreatment, which combines hydrothermal and milling treatments, was examined. The highest glucose yield of 90% was achieved in the ball-milled specimen previously treated at 170 °C in the presence of 20% Ca(OH)2 per substrate weight. The specific surface area of the substrate was closely correlated with glucose yield, and a larger specific surface area was obtained when treating the specimen at 170 °C in the presence of Ca(OH)2 compared to treatment at 170 °C without Ca(OH)2. Although the Ca(OH)2-treated specimen was relatively unaffected by delignification, the cleavage of the ester bonds between lignin and hemicellulose was confirmed by FT-IR. This suggests that Ca(OH)2 weakens the substrate structure by loosening the bonds between lignin and hemicellulose as the mechanism to increase the specific surface area regardless of the high lignin content, facilitating the fibrillation of fibers with mechanical milling.