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Rui Wang, Chen-Loung Chen, Josef S. Gratzl

Ozonation of pine kraft lignin in alkaline solution. Part 1: Ozonation, characterization of kraft lignin and its ozonated preparations

Keywords: chemical composition, functional group analysis, molecular mass distribution, nitrobenzene-potassium permanganate oxidation, ozonation in alkaline solution, pine kraft lignin, reaction mechanisms, Ultrafiltration

Pine kraft lignin was purified to obtain a kraft lignin preparation (KL) with weight average molecular mass (M_w) of 5500. The KL was then ozonated with an ozone-air stream containing 2–2.5% of ozone in 0.1 M sodium hydroxide solution to prepare ozonated kraft lignin preparations with ozone consumption of 10, 25, 30 and 40% per KL; Oz-10-KL, Oz-25-KL, Oz-30-KL and Oz-40-KL, respectively. The pH of reaction mixture was decreased with increasing ozone consumption, while the carboxylic acid content and the M_w of resulting ozonated lignins increase with increasing ozone consumption. The KL and its ozonated preparations were then characterized by elemental composition, functional group analysis, molecular mass distribution and nitrobenzene-K₄MnO₄ oxidation. The results showed that the KL extensively undergoes oxidative cleavage of both side chains and aromatic moieties without decrease in the M_w as well as dehydrogenationive coupling of phenolic degraded fragments by active oxygen radical species, such as hydroperoxyl and hydroxyl radicals. The formation of these active oxygen radical species are produced by way of a series of reactions initiated by the reaction of ozone with hydroxide anions at pH range of 12.4–10.5, producing superoxide (^-O₂?) and hydroperoxyl (HOO?) radicals.

Holzforschung, Walter de Gruyter

Print ISSN: 0018-3830
Volume: 58, 10/2004
Pages: 622 - 630

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