Experimental and theoretical studies of the thermal degradation of a phenolic dibenzodioxocin lignin model
Résumé
A large part of biphenyl structures in lignin are etherified by alpha- and beta-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-dibenzo[a,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 (alpha-O-4) will dissociate first since its barrier energy for scission is lower than the other one (beta-O-4), by roughly 12 kcal mol(-1) (a parts per thousand 50 kJ mol(-1)). This conclusion is likely applicable to thermal degradation of DBDO units in lignin polymer.