Hydroxyl-Assisted Hydrogen Transfer Interaction in Lignin Pyrolysis: An Extended Concerted Interaction Mechanism

The lignin pyrolysis process involves complex interaction reactions, where the bimolecular concerted interactions have been rarely studied. In the present work, an extended concerted interaction mechanism, hydroxyl-assisted hydrogen transfer (hydroxyl-AHT), was proposed based on the pyrolysis of the typical nonsubstituted and α-hydroxyl-substituted β-O-4 type lignin models (2-phenylethyl phenyl ether, PPE, and 2-phenoxy-1-phenylethanol, α-OH-PPE). This hydroxyl-AHT mechanism was investigated and confirmed with the combined fast pyrolysis experiments and quantum chemistry modeling. The results indicate that phenolic, watery, alcoholic, and enolic hydroxy compounds all can act as mediators for the concerted hydrogen transfer process in lignin pyrolysis and have similar assistant effects. Whereas, the hydroxyl-AHT has a great influence on the competitiveness of different concerted reactions to break the Cβ–O, Cα–Cβ, and Cα–OH bonds, respectively. To compare the pyrolysis of PPE and α-OH-PPE, the α-hydroxyl substituent leads to a significant promotion to the Cβ–O cleavage based on hydroxyl-AHT because there are more hydroxyl mediators and more concerted reaction types that can be involved in the bimolecular hydrogen transfer in α-OH-PPE pyrolysis. Hence, more phenol derived from the Cβ–O breakage was detected in the pyrolysis experiment of α-OH-PPE than that of PPE. In addition, this bimolecular hydroxyl-AHT interaction mechanism is universal in the concerted pyrolysis processes of lignin, also accelerating the cleavage of α-O-4 linkage, β-1 linkage, etc.