Insight into the Pyrolysis Mechanism of α-O-4 Linked Lignin: The Role of Alkali Metal Potassium

The inherent alkali and alkaline earth metals have certain influences on the pyrolysis of lignin. However, the microscopic pyrolysis mechanisms remain unclear, especially for the α-O-4 linked lignin. In this article, the α-O-4 lignin dimer was used as a model compound to investigate the effects of alkali metal ions (K+) on its pyrolysis process and products by using density functional theory (DFT) calculations and electronic structure analysis. The main binding modes of K+ with the lignin dimer were first identified, and then, the radical and concerted mechanisms under different binding modes were calculated. K+ shows a preference for binding to the nucleophilic benzene ring and O atoms. The varied binding sites of K+ result in different influences on the unimolecular/bimolecular radical and concerted reactions. Generally, reactions can be promoted when K+ bonds to the hydroxyl O or the O-terminal benzene ring, and the competitiveness of different pyrolysis reactions is altered by K+ due to its distinct binding sites and influence. The current work can contribute to the research of lignin pyrolysis.