Study on the Mechanism of Lignin Depolymerization during Lignocellulose Pretreatment in an Acid– n -Butanol System

The dilute acid, n-butanol system, a partially miscible biphasic medium with an upper critical solution temperature, exhibits a strong capability to depolymerize and separate lignin and hemicellulose. This study integrated density functional theory (DFT) calculations and experiments to systematically elucidate the mechanism of lignin depolymerization during dilute acid and n-butanol pretreatment. DFT calculations indicated that, compared with the acid-only system, n-butanol captures carbocations with a lower energy barrier (7.70 kcal/mol), facilitating the formation of α-etherified products, and the α-etherified structures exhibit a higher energy barrier for β-O-4 bond cleavage (35.28 kcal/mol). Thermodynamic and kinetic analyses indicated that the ether-bond cleavage of the α-etherified intermediate was the rate-limiting step during lignin depolymerization, and the corresponding reaction rate constant was 8.573 × 10–14. 2D HSQC NMR analysis showed that lignin obtained via acid–n-butanol pretreatment contained a relatively higher proportion of β-O-4 interunit linkages, indicating that its structural integrity was superior to that of acid-pretreated lignin. This observation was consistent with the results of 31P NMR analysis, which showed a lower total phenolic hydroxyl content in the acid–n-butanol-pretreated lignin. This study offers new mechanistic insights for the depolymerization of lignin during lignocellulose pretreatment.