Mechanistic Insights into Alkaline Earth Metal Ion-Catalyzed Pyrolysis of Lignin Monomers: Pathways to CO and CO2 Evolution

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.