A novel interaction mechanism in lignin pyrolysis: Phenolics-assisted hydrogen transfer for the decomposition of the β-O-4 linkage

Abstract

Abstract Pyrolysis of lignin involves complex unimolecular decomposition and bimolecular interaction mechanisms. Unimolecular decomposition of lignin model compounds has been widely investigated, while bimolecular interactions are far from well elucidated. Currently, only two radical reaction-based interactions, i.e., hydrogen abstraction and hydrogen-bond-induced homolysis mechanisms, have been proposed to explain the interactions in the lignin pyrolysis process, which ignores the concerted reaction-based interactions involving the pyrolytic products of lignin. Particularly, a large number of phenolics are produced in the pyrolysis of lignin, which can further affect the concerted pyrolysis reactions of lignin. In the present work, a novel interaction mechanism, phenolics-assisted hydrogen transfer (phenolics-AHT) process for the concerted reactions, is confirmed and carefully investigated. Phenylethyl phenyl ether (PPE), a typical β-O-4 lignin dimer, is used as the model compound to reveal the phenolics-AHT mechanism in the lignin pyrolysis system. Density functional theory (DFT) calculation, electrostatic potential (ESP) analysis, and fast pyrolysis experiment results are combined to reveal that the phenolic products participate in the hydrogen transfer process of the concerted Maccoll elimination reaction by means of their phenolic hydroxyl structures and significantly reduce the energy barrier, thus promoting the cleavage of the β-O-4 linkage. On the contrary, phenolics-AHT has no obvious effect on the concerted retro-ene fragmentation. At medium pyrolysis temperatures, the hydrogen transfer processes induced by phenolic products play a more significant role in the dissociation of the β-O-4 linkage than the hydrogen abstraction induced by radicals, due to the limited amount of free radicals. In addition, DFT calculation results further suggest that decarbonylation, decarboxylation, dehydration, tautomerism, and other concerted reactions can also be promoted by phenolics-AHT.