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Licensed Unlicensed Requires Authentication Published by De Gruyter May 7, 2009

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

Toshihiro Watanabe, Haruo Kawamoto and Shiro Saka
From the journal

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.


Corresponding author. Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan Phone/Fax: +81-75-753-4737

Received: 2008-11-12
Accepted: 2009-2-16
Published Online: 2009-05-07
Published Online: 2009-05-07
Published in Print: 2009-07-01

©2009 by Walter de Gruyter Berlin New York