The swelling induced choline alkali-urea (SICAU) process for sustainable crop straw upgrading

Upgrading lignocellulosic biomass into value-added product forms has been attractive but challenged by its structural recalcitrance and heterogeneity. Prevalent fractionation techniques employ aggressive chemical solvents or catalysts to cleave lignin-carbohydrates bonds with the purpose of dissolution ability enhancement. This work tailored the swelling induced choline alkali-urea (SICAU) process to fractionate rice straw with the powerful choline alkali-urea (ChOH-Ur) deep eutectic solvents (DES) with the preservation of lignin-carbohydrates bonds, thus the super high yield of high-value lignin-carbohydrates complexes (LCCs) was co-produced together with highly hydrolysable cellulose and ethanol-insoluble xylooligosaccharides (XOSs) fractions. Results showed that rather high cellulose recovery (87.3 %-100 %) was obtained with nearly 100 % cellulose to glucose conversion. Optimized LCCs yield of 34.3 % was achieved with 50 % choline alkali-urea concentration and 4 h cooking time, along with the corresponding yield of 10.2 % XOSs during the DES conversion process. The LCCs linkages were 0.06–0.16/100 Ar depending on cooking conditions. The proposed integrated SICAU process realized the complete upgrading of rice straw into three value-added products, which showed a very different working profile with prevalent systems that usually fractionated lignocellulose into cellulose, lignin, and hemicellulose. Therefore, the SICAU process could extract higher value from lignocellulose towards a more sustainable lignocellulose biorefinery.