• Kalliola, Anna

Abstract

Oxidation by molecular oxygen (O2) is one of the ligninmodification methods. O2 is active towards phenolicgroups, which are particularly abundant in kraft and sodalignins. The main aim of this thesis was to applyoxidation by O2 to modify technical lignins to enhancetheir utilization for polymeric chemicals and materialapplications. O2 oxidation was aided by using eitheralkaline conditions or laccase enzyme as a catalyst. Inaddition, oxygen delignification of pulp was studiedusing kraft lignin as a model substrate to provide datafor a mechanistic model for the process. Lignin oxidationmechanisms by O2 under alkaline conditions and laccasecatalysis are discussed.A simple alkali-O2 oxidation method under high lignincontent was developed to increase the water solubility ofsoda lignin, desirable for dispersing applications.Lignin characterization was done directly from thereaction solution. Both the negative charge and themolecular mass of the lignin were controlled by theoxidation parameters, and especially by pH. Oxidationwithout controlling the pH decrease caused condensationand an increase in molecular mass. Oxidation under aconstant pH of 11.5 clearly hindered the condensation andincreased the negative charge. Oxidation at constant pHof 13 decreased molecular mass. The results indicate thatthe organic hydroperoxide formed via coupling of aphenoxyl radical with superoxide (O2 -) is the keyintermediate. The course of further reactions isdependent on the degree of protonation of thisintermediate (pKa 12-13) and is thus pH dependent. Thehydroperoxide anion rearranges leading to degradation.Below pH 12, the protonated form decomposes back to thephenoxyl radical, which spontaneously undergoes couplingand thus induces condensation. Under laccase catalysisconditions, O2 - is not present and thus the reactionpaths described above do not function. Therefore, theformed phenoxyl radicals couple with each other ratherthan degrade. O2 has a significantly lower tendency toattach to the phenoxyl radical compared to O2 -.The oxidized soda lignin solutions were applied asready-to-use products for concrete plasticizing. Theywere superior to commercial lignosulfonate and good incomparison to synthetic superplasticizers. The bestperforming lignin solution (oxidized at a constant pH of11.5) also showed promising results in other concreteapplication tests. To enhance the utilization of kraftlignin in composite applications, both laccase- andalkali-catalyzed O2 oxidation were used to polymerizelignin-derived low-molecular phenolics for the reductionof VOCs. According to sensing and chemical analysis, theundesirable odor and the formation of VOCs under elevatedtemperatures were reduced to a greater extent by alkali-than by laccase-catalyzed oxidation. However, neithermethod led to adequate odor removal. In order to lowerthe glass transition temperature of lignin,functionalization with a hydrophilic phenolic compoundwas attempted. However, homogeneous polymerization ofthis compound was favored over coupling tolignin.The operating conditions of alkali-O2 oxidation couldprobably be optimized for targeted lignincharacteristics, which would increase the furtherapplication potential of technical lignins.Laccase-catalyzed oxidation is best applied when ligninpolymerization is desired.

Topics

• impedance spectroscopy
• compound
• Oxygen
• glass
• glass
• composite
• glass transition temperature
• lignin
• functionalization
• molecular mass