Application of two resinols as lignin dimer models to characterize reaction mechanisms during pyrolysis

Two lignin dimer models, pinoresinol and syringaresinol, were subjected to analytical pyrolysis to elucidate the cleavages of resinol-type linkage, as well as the reactions of pendant groups on aromatic compounds. Nineteen products from pinoresinol and twenty two products from syringaresinol were identified from pyrolysis GC/MS, respectively. Four products comprising side chains derived from dimer structures were detected from pinoresinol pyrolysis, which indicated that the pyrolysis reaction occurred on the resinol side chain. Moreover, similar products were also identified from syringaresinol pyrolysis, and there were no differences between the side chain reactions observed in the two resinols. The formation of these products was initiated by homolytic cleavage of the C alpha-O or C gamma-O ether following the elimination of the C beta-C beta, C beta-C gamma, or C alpha-C beta bond. Elimination and rearrangement reactions of the pendant groups on the aromatic rings occurred frequently during pyrolysis. In the case of pinoresinol pyrolysis, demethoxylation from the guaiacyl ring occurred to form phenols, and rearrangement from the methoxyl group to the methyl group occurred to form o-cresols. In contrast, demethylation from the methoxyl group in syringaresinol occurred to form methoxyl-catechols, some of which were further subjected to demethoxylation to form catechols. Increasing the pyrolysis temperature was observed to accelerate the secondary reactions, including the decomposition of the methylfuran side chain structure, production of C alpha-carbonyl compounds, and reactions of methoxyl groups. (C) 2016 Elsevier B.V. All rights reserved.