Theoretical Study on the Mechanism of Lignin Depolymerization by Choline Amino Acid Ionic Liquids

Addressing the challenge of efficient lignin depolymerization, this study focuses on environmentally friendly choline amino acid ionic liquids ([Ch][AA] ILs) and systematically investigates the microscopic mechanism of β-O-4 bond cleavage in a lignin model compound (GG) using density functional theory (DFT). This study systematically compares three potential pathways: ionic liquid acid catalysis, base catalysis, and stepwise acid-base synergistic catalysis, proposing for the first time a highly advantageous "stepwise acid-base synergistic catalytic mechanism". The results demonstrate that compared to single base and acid catalysis, which exhibit excessively high energy barriers, the synergistic pathway effectively lowers the energy barrier of the rate-determining step through the tautomerism of key intermediates. The results indicate that the synergistic pathway exhibits a relative energetic advantage. Whereas, accounting for computational accuracy, the acid-catalyzed pathway may be considered as a competing reaction pathway under experimental conditions. Furthermore, the study reveals the impact of anion structure on catalytic activity: on the one hand, polar groups on the anion side chain can construct a strong hydrogen bond network and ion-dipole interactions, significantly reducing the rate-determining step energy barrier; on the other hand, long alkyl side chains effectively stabilize key transition states by enhancing noncovalent interaction. This study not only elucidates the key roles of multisite synergistic effects and van der Waals interactions in lignin depolymerization at the molecular level but also provides theoretical support for the design of efficient, low-toxicity catalytic systems for lignin valorization.