Reactions of Lignin with Cyanamide Activated Hydrogen Peroxide. Part 1. The Degradation of Lignin Model Compounds

Abstract

Monomeric lignin model compounds representing various functional groups found in residual lignins were reacted with cyanamide activated hydrogen peroxide. Non-phenolic lignin model compounds failed to react even under extreme reaction conditions (90 °C), while phenolic lignin model compounds underwent rapid oxidation. Reaction optimization studies reveal that these reactions are strongly dependent on pH and the cyanamide-to-peroxide ratio, while temperature is not a factor. Through the use of electron paramagnetic resonance (EPR) spectroscopy free radical involvement has been demonstrated. In contrast to previous literature, a predominately free-radical mechanism exists, and on the basis of the reactions carried out in the presence of Superoxide dismutase, the presence of ·Ο 2 -has been determined.

Reactions of Lignin with Cyanamide Activated Hydrogen Peroxide. Part 2. The Degradation Mechanism of Phenolic Lignin Model Compounds

“…Superoxide anion (·Ο 2~) appears to play an important role and is derived from the reaction of alkaline hydrogen peroxide with cyanamide. Under alkaline conditions, the activation of hydrogen peroxide with cyanamide likely proceeds via the generation of a peroxyimidic acid intermediate, I (1) (Sturm 1990;Reisner and Teichmann 1992;Sturm and Kuchler 1993;Hamilton et al 1996;Kadla et al 1998).…”

“…The net result is the generation of a series of oxidative radicals, which quickly react with available substrates. However, the cyanamide-peroxide system: (i) does not react with non-phenolic lignin model compounds even in extreme reaction conditions, and (ii) shows decreased reactivity in the presence of a radical scavenger 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) (Kadla et al 1998). Therefore, the contribution of hydroxyl radicals (eqs.…”

“…(2) and (3 a)) are likely not significant in this system. Furthermore, the addition of Superoxide dismutase (SOD), which results in a dramatic reduction in the degradation of the lignin model compounds clearly indicates that superoxide/perhydroxyl radical is important to the reaction system (Kadla et al 1998).…”

“…In the first part of this series it was shown that the cyanamide-hydrogen peroxide reaction system was very reactive towards lignin model compounds (Kadla et al 1998). Reaction optimization studies revealed that only the free-phenolic compounds underwent oxidative degradation.…”

Catalysis and Activation of Oxygen and Peroxide Delignification of Chemical Pulps: A Review

“…In contrast to the proposition of Sturm and Kuchler (92), recent work by Kadla and co-workers, on the reactions of lignin and lignin model compounds with cyanamide activated peroxide, demonstrated the involvement of superoxide anion radicals during the reaction (94). The data provided evidence for two competing reaction systems, radical and ionic, in which the majority of the chemistry proceeds via a radical mechanism.…”