Insight into the pyrolysis mechanism of -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 alpha-O-4 linked lignin, has not been clearly revealed. Herein, unsubstituted alpha-O-4 lignin dimer benzyl phenyl ether (BPE) and C-alpha-substituted alpha-CH2OH-BPE, alpha-(CH2)(2)OH-BPE and alpha-CHOHCH2OH-BPE were employed to study the radical and concerted mechanisms of alpha-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 alpha-O-4 structure can only undergo unimolecular C-alpha-O homolysis without hydrogen transfer, while various substituents provide more hydrogen sites, leading to the occurrence of radical chain reactions and concerted reactions. C-beta-dehydrogenated radicals derived from substituted alpha-O-4 dimers can be easily generated by intermolecular C-beta-hydrogen abstraction, which tends to break the C-alpha-O bond. Similarly, hydroxyl-assisted hydrogen transfer (AHT) can promote C-alpha-O cleavage by inducing C-beta/C-gamma-hydrogen exchange of substituted hydroxy groups. Comprehensively, both radical hydrogen abstraction and concerted hydroxyl-AHT interactions have positive effects on the C-alpha-O decomposition of alpha-O-4 linkage. This study provides a more reasonable explanation for the initial decomposition of alpha-O-4 linked lignin and the preliminary generation of phenolic hydroxy and phenoxy radical derivatives.