Study on the mechanism of lignin and lignin-carbohydrate complex hydrolysis under alkaline conditions by density functional theory

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Lignin hydrolyzes during alkali-heat pretreatment of lignocellulose , due to the cleavage of covalent linkages between lignin and lignin-carbohydrate complex. Various structure of lignin monomers are responsible of the complicated hydrolysis mechanism of lignin and lignin-carbohydrate complex. This study used density functional theory to reveal the hydrolysis mechanism of lignin with various substituents and lignin-carbohydrates complex with different linkages. The results indicated that the S N 2 reaction was observed to take place in the lignin. Methoxy groups and methyl group had an insignificant influence on hydrolysis reaction pathway of lignin, whereas the presence of a methyl group on the α-carbon atom had a hindering effect on the nucleophilic attack during alkaline pretreatment . The lignin-carbohydrate complex with different linkage showed a significant difference in hydrolysis reaction pathway. Additionally, the reaction energy barrier of ester bond hydrolysis was lowest among the hydrolysis of ester bond, phenyl glycosidic bond , benzyl ether bond, γ-ester bond, coumarin ester bond and acetal bond under alkaline condition. This study provides valuable insights into the process control, reactor design, and performance enhancement for lignocellulose pretreatment .