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.