Radical chain reactions in pyrolytic cleavage of the ether linkages of lignin model dimers and a trimer

Abstract

β-Ether-type dimers, [1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol and 1-(3, 4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol], and an α, β-diether-type trimer [1-(4-(3, 4-dimethoxybenzyloxy)-3-methoxyphenyl) -2- (2-methoxyphenoxy) -1-propanol] were pyrolyzed in a closed ampoule reactor (N₂/250–400°C/2 min). 1-Phenylpropenes (Cα=Cβ) and 1-phenylpropanones (Cα=O) were obtained as the major β-ether-cleaved products. Radical chain mechanisms are proposed in which hydrogen abstraction at the phenolic O-H and Cα-Hs occurs, respectively. The former reaction which gives rise to three radical species was much more effective than the latter. As the effective reaction increases the radical concentration, cleavage of the β-ether linkage in the phenolic dimer is achieved at a much lower temperature (260°C) than that of the non-phenolic type (360°C). Radical chain reactions are initiated in the case of the trimer with a weak Cα-O bond at lower temperature (320°C) than those of the non-phenolic (methylated) dimer, since homolysis of the Cα-O bond produces the phenoxy type dimer and 3, 4-dimethoxybenzyl radicals as initiators. However, some of the dimer phenoxyl radical was stabilized by H-abstraction (to form dimer) or by recombination with a 3, 4-dimethoxybenzyl radical (to form C-benzylated products) so that the chain depolymerization via quinone methide intermediate was suppressed.