Mechanistic insights into alkaline earth metal ion-catalyzed pyrolysis of lignin monomers: Pathways to CO and CO₂ evolution

In this study, sixteen types of lignin monomer model compounds with aldehyde and carboxyl substituents at the Cα or Cβ positions were selected, representing phenyl, p-hydroxyphenyl, guaiacyl, and syringyl structural units. The catalytic effects of alkaline earth metal ions (Ca2+ and Mg2+) on the formation of CO and CO2 during lignin pyrolysis were investigated by using density functional theory (DFT) calculations. The results revealed that Ca2+ and Mg2+ readily coordinate with oxygen-containing functional groups in lignin model compounds, forming stable complexes with lower energy states. When aldehyde, carboxyl, phenolic hydroxyl, and methoxy groups coexist, the optimal binding sites for Ca2+ and Mg2+ are O(phenolic hydroxyl)–Ca2+/Mg2+–O(methoxyl) configurations. During the pyrolysis process of lignin monomers, Ca2+ and Mg2+ selectively promote the decarbonylation of benzaldehyde-type and phenylethanal-type lignin monomer model compounds (excluding phenylethanal and 4-hydroxyphenylethanal), thereby enhancing the generation of CO. Conversely, except for the Ca2+-catalyzed decarboxylation of 4-hydroxy-3-methoxyphenylethanoic acid, Ca2+ and Mg2+ can suppress the decarboxylation of benzoic acid-type and phenylacetic acid-type lignin monomer model compounds, resulting in the reduction of CO2 formation.