Microscopic catalytic mechanisms of alkali and alkaline earth metals in the production of phenolic compounds during lignin pyrolysis

Lignin pyrolysis serves as a crucial pathway for producing high-value phenolic compounds. This process is significantly influenced by the inherent alkali and alkaline earth metals (AAEMs) in biomass, which can selectively regulate pyrolysis pathways and product distributions. However, the microscopic catalytic mechanisms of AAEMs remain unclear, hindering precise control over the formation of specific phenolic products such as guaiacyl (G-type), syringyl (S-type) and p-hydroxyphenyl (H-type) phenolics. Therefore, this study systematically investigates the effects of K, Na, Ca, and Mg on lignin pyrolysis characteristics. Using 2,6-dimethoxyphenol (syringol) as a model compound, pyrolysis reaction pathways were constructed and the microscopic mechanisms of AAEMs were elucidated through electronic structure analysis. The results indicated that the introduction of AAEMs promoted the formation of H-type phenolics. Among them, Ca exhibited the most pronounced catalytic effect, increasing the relative selectivity of H-type phenolics by 12.3% compared to non-catalysis. TG-FTIR analysis revealed enhanced release of CH 4 , further confirming that AAEMs facilitated demethylation reactions. DFT calculations demonstrated that the initial pyrolysis pathway of syringol involved the homolytic cleavage of a methoxy group to form a methyl radical (•CH 3 ). Subsequent reactions primarily included methoxy group rearrangement and radical-induced demethoxylation. For the radical-induced demethoxylation pathway, which proceeded via two transition states (radical attack and methoxy dissociation), AAEMs were found to promote demethoxylation by reducing the energy barrier of the methoxy dissociation step in the case of H radical-induced reactions. In contrast, for methyl radical-induced reactions, alkali metals (K and Na) also functioned by lowering the methoxy dissociation barrier, whereas alkaline earth metals (Ca and Mg) promoted demethoxylation by merging the two transition states into a single step through induced electron redistribution. This study systematically elucidates the influence of AAEMs on product distribution and the microscopic catalytic mechanisms during lignin pyrolysis. The findings provide a theoretical foundation for the targeted pyrolysis of lignin to produce specific high-value phenolic compounds. #