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

摘要

β-Ether-type dimers, [1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol and 1-(3,4-dimethoxy-phenyl)-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 (N_2/ 250-400℃/2 min). 1-Phenylpropenes (C_α = C_β) and 1-phenylpropanones (C_α = 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 C_α-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 (3-ether linkage in the phenolic dimer is achieved at a much lower temperature (260℃) than that of the non-phenolic type (360℃). Radical chain reactions are initiated in the case of the trimer with a weak Cα-O bond at lower temperature (320℃) than those of the non-phenolic (methylated) dimer, since homolysis of the C_α-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 depoly-merization via quinone methide intermediate was suppressed.