Enhanced Enzymatic Hydrolysis of Wheat Straw to Improve Reducing Sugar Yield by Novel Method under Mild Conditions

Wheat straw is a suitable source material for bioethanol production. Removing lignin and hemicellulose in wheat straw to improve enzymatic hydrolysis efficiency is essential because of its complex structure. Deep eutectic solvents (DESs) have become substitutes for ionic liquids (ILs), with the characteristics of good biocompatibility, simple synthesis procedure and low cost. However, the process of removing lignin and hemicellulose using present DESs requires a high operation temperature or long operation time. Therefore, we studied a novel method under mild conditions for screening a series of novel DESs based on an inorganic base to remove lignin and hemicellulose in wheat straw. In this work, the effect of DES type, the pH of the DESs, the operation temperature and operation time for enhancing enzymatic hydrolysis, and the crystal structure and the chemical structure and surface morphology of wheat straw were investigated. In particular, Na:EG exhibited the most excellent solubility for wheat straw under mild conditions, removing 80.6% lignin and 78.5% hemicellulose, while reserving 87.4% cellulose at 90 °C for 5 h, resulting in 81.6% reducing sugar produced during hydrolysis for 72 h. Furthermore, XRD, FT-IR and SEM analysis verified the lignin and hemicellulose removal. Hence, DESs based on an inorganic base used for removing lignin and hemicellulose will enhance enzymatic hydrolysis, and thus promote the industrial application of wheat straw to produce bioethanol.