Chemical and enzymatic oxidation using molecular oxygen as a means to valorize technical lignins for material applications: Dissertation

Oxidation by molecular oxygen (O2) is one of the lignin modification methods. O2 is active towards phenolic groups, which are particularly abundant in kraft and soda lignins. The main aim of this thesis was to apply oxidation by O2 to modify technical lignins to enhance their utilization for polymeric chemicals and material applications. O2 oxidation was aided by using either alkaline conditions or laccase enzyme as a catalyst. In addition, oxygen delignification of pulp was studied using kraft lignin as a model substrate to provide data for a mechanistic model for the process. Lignin oxidation mechanisms by O2 under alkaline conditions and laccase catalysis are discussed. A simple alkali-O2 oxidation method under high lignin content was developed to increase the water solubility of soda lignin, desirable for dispersing applications. Lignin characterization was done directly from the reaction solution. Both the negative charge and the molecular mass of the lignin were controlled by the oxidation parameters, and especially by pH. Oxidation without controlling the pH decrease caused condensation and an increase in molecular mass. Oxidation under a constant pH of 11.5 clearly hindered the condensation and increased the negative charge. Oxidation at constant pH of 13 decreased molecular mass. The results indicate that the organic hydroperoxide formed via coupling of a phenoxyl radical with superoxide (O2 -) is the key intermediate. The course of further reactions is dependent on the degree of protonation of this intermediate (pKa 12-13) and is thus pH dependent. The hydroperoxide anion rearranges leading to degradation. Below pH 12, the protonated form decomposes back to the phenoxyl radical, which spontaneously undergoes coupling and thus induces condensation. Under laccase catalysis conditions, O2 - is not present and thus the reaction paths described above do not function. Therefore, the formed phenoxyl radicals couple with each other rather than degrade. O2 has a significantly lower tendency to attach to the phenoxyl radical compared to O2 -. The oxidized soda lignin solutions were applied as ready-to-use products for concrete plasticizing. They were superior to commercial lignosulfonate and good in comparison to synthetic superplasticizers. The best performing lignin solution (oxidized at a constant pH of 11.5) also showed promising results in other concrete application tests. To enhance the utilization of kraft lignin in composite applications, both laccase- and alkali-catalyzed O2 oxidation were used to polymerize lignin-derived low-molecular phenolics for the reduction of VOCs. According to sensing and chemical analysis, the undesirable odor and the formation of VOCs under elevated temperatures were reduced to a greater extent by alkali- than by laccase-catalyzed oxidation. However, neither method led to adequate odor removal. In order to lower the glass transition temperature of lignin, functionalization with a hydrophilic phenolic compound was attempted. However, homogeneous polymerization of this compound was favored over coupling to lignin. The operating conditions of alkali-O2 oxidation could probably be optimized for targeted lignin characteristics, which would increase the further application potential of technical lignins. Laccase-catalyzed oxidation is best applied when lignin polymerization is desired.