Role of methoxyl group in char formation from lignin-related compounds

Although char formation from lignin is an important process in wood pyrolysis, the molecular mechanism has not been clarified yet. In this study, the vapor-phase char formation of various lignin-related compounds was studied under the pyrolysis conditions of N2/600 °C/80 s. The methoxyl groups were found necessary for char formation. Interestingly, ethoxyl group was not effective in such char formation. Guaiacol and 2-ethoxyphenol gave pyrocatechol (formed through thermal homolysis of the O–R bonds) and the ether-rearranged products (o-cresol, 2-ethylphenol, xanthenes and 2,3-benzofuran). The latter products are likely formed via an o-quinone methide intermediate, initiated from the H-abstraction of the phenolic hydroxyl groups. 2-Ethoxyphenol gave larger amounts of 2,3-benzofuran and less xanthenes and char than guaiacol, with yields of other products similar. Such reactivity difference is explainable with the different reactivities of the o-quinone methide intermediates. Based on the results, o-quinone methide is proposed as an important intermediate for lignin char formation during pyrolysis.