A novel insight into the bimolecular concerted interaction in lignin pyrolysis: Tautomer-assisted hydrogen transfer mechanism

Bimolecular concerted interaction mechanism plays a vital role in lignin pyrolysis. According to the recently proposed hydroxyl-assisted hydrogen transfer (AHT) mechanism, the primary pyrolytic products can assist the hydrogen transfer process of linkage decomposition with their hydroxy groups acting as the mediator. Herein, the concerted interaction between two hydrogen transfer processes was further studied for the β-O-4 linked lignin. A novel concerted interaction mechanism named tautomer-AHT was found and carefully confirmed by combining theoretical calculations, electronic analyses, pyrolysis and isotope experiments, with 2-phenoxy-1-phenylethanol (α-OH-PPE) as the β-O-4 lignin model compound. During this interaction, the tautomer can make the best of its saturated and unsaturated groups to carry out hydrogen-donating and hydrogen-receiving processes of the β-O-4 breakage, respectively, accompanied by the hydrogen transfer of the mediator itself. As a result, the β-O-4 breakage can be promoted and the unstable tautomer turns into its corresponding stable isomer, leading to the formation of stable phenolics and ketones. In the early pyrolysis stage, the tautomer-AHT reaction triggered by unstable tautomers is basically superior to the hydroxyl-AHT reaction due to the lower energy barrier. With the consumption of unstable tautomers and the accumulation of stable hydroxy products, hydroxyl-AHT interaction becomes predominant in the latter pyrolysis stage. The bimolecular tautomer-AHT mechanism builds a special relationship between hydrogen transfer reactions, and lays a theoretical foundation for exploring complex interactions in lignin pyrolysis.