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Experimental and theoretical studies of the thermal degradation of a phenolic dibenzodioxocin lignin model |
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| Authors: | Christian Gardrat Reinaldo Ruggiero Marie-Thérèse Rayez Jean-Claude Rayez Alain Castellan |
| Institution: | 1. Laboratoire de Chimie des Polymères Organiques, IPB/ENSCBP, Université Bordeaux 1, 16 avenue Pey-Berland, 33607, Pessac Cedex, France 2. Laboratoire de Chimie des Polymères Organiques, Centre National de la Recherche Scientifique, 33607, Pessac cedex, France 3. Laboratorio de Fotoquímica e Materiais Lignocelulósicos, Universidade Federal de Uberlandia, P.O. Box. 596, Uberlandia, Brazil 4. Institut des Sciences Moléculaires, Université Bordeaux 1, UMR 5255, 33405, Talence Cedex, France |
| Abstract: | A large part of biphenyl structures in lignin are etherified by α- and β-carbons of another phenylpropane unit to give an eight-member ring called dibenzodioxocin. The behavior of a phenolic dibenzodioxocin lignin model, 4-(4,9-dimethoxy-2,11-n-dipropyl-6,7-dihydro-5,8-dioxa-dibenzoa,c]cycloocten-6-yl)-2-methoxyphenol (DBDOH, 1), was studied by different mass spectrometry and thermal methods, leading to the conclusion that dibenzodioxocins are thermally unstable products. Both semi-empirical and density functional theory quantum calculations show that both C–O bonds, which connect the biphenyl part of the dibenzodioxocin molecule to the phenolic group, can be broken under increasing temperature. However, they do not play the same role since their dissociation occurs through different barrier heights. The C–O bond directly connected to the phenolic group (α–O-4) will dissociate first since its barrier energy for scission is lower than the other one (β–O-4), by roughly 12 kcal mol–1 (≈50 kJ mol?1). This conclusion is likely applicable to thermal degradation of DBDO units in lignin polymer. |
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