Insight into the pyrolysis mechanism of α-O-4 linked lignin: The role of bimolecular interactions and substituents

Pyrolysis can convert lignin into various high-value phenolic derivatives, whereas the pyrolysis mechanism of lignin, particularly the α-O-4 linked lignin, has not been clearly revealed. Herein, unsubstituted α-O-4 lignin dimer benzyl phenyl ether (BPE) and Cα-substituted α-CH2OH-BPE, α-(CH2)2OH-BPE and α-CHOHCH2OH-BPE were employed to study the radical and concerted mechanisms of α-O-4 type lignin pyrolysis by density functional theory (DFT) calculation and electronic structure analysis. A pyrolysis system was constructed to carefully consider unimolecular and bimolecular reactions, and the influence of substituent groups was investigated. The results indicate that unsubstituted α-O-4 structure can only undergo unimolecular Cα–O homolysis without hydrogen transfer, while various substituents provide more hydrogen sites, leading to the occurrence of radical chain reactions and concerted reactions. Cβ–dehydrogenated radicals derived from substituted α-O-4 dimers can be easily generated by intermolecular Cβ–hydrogen abstraction, which tends to break the Cα–O bond. Similarly, hydroxyl-assisted hydrogen transfer (AHT) can promote Cα–O cleavage by inducing Cβ/Cγ–hydrogen exchange of substituted hydroxy groups. Comprehensively, both radical hydrogen abstraction and concerted hydroxyl-AHT interactions have positive effects on the Cα–O decomposition of α-O-4 linkage. This study provides a more reasonable explanation for the initial decomposition of α-O-4 linked lignin and the preliminary generation of phenolic hydroxy and phenoxy radical derivatives.