Revisiting Alkaline Pretreatment of Lignocellulose: Understanding the Structural Evolution of Three Components

Abstract Herein, sodium hydroxide (NaOH) pretreatment of lignocellulose is performed to investigate the structural evolution of lignin, cellulose, and hemicellulose. It is found that most of hemicellulose and part of cellulose are hydrolyzed/converted to monosaccharide, alcohols, and carboxylic acids. Over half of cellulose is stabilized because the reducing end groups of cellulose convert to stable carboxylic groups. In addition, lignin is converted to aromatic monomers (predominately vanillin and acetovanillone) with a highest yield of 6.1 wt%, along with dimers and oligomers with M_w less than 10 000 Da. According to heteronuclear single quantum coherence analysis, no common interlinkages (β‐O‐4, β‐β, β‐5, etc.) appear in lignin oligomers due to lignin fragments condensed during the alkaline pretreatment. Notably, the acidification process is also the cause of lignin condensation since high aromatic monomers yield of 15 wt% (based on original lignin in pine) is achieved by a postoxidation of alkaline solution, while few no aromatic monomers generated by using the isolated lignin oligomers. The direct oxidation of lignocellulose in alkaline solutions to aromatic monomers demonstrates the potential application of lignocellulose alkaline pretreatment for high‐value chemicals production.