Lignin oxidation mechanisms under oxygen delignification conditions. Part 3. Reaction pathways and modeling

Abstract

A detailed kinetic model for the lignin oxidation chemistry is presented. It is based mainly on the mechanisms and kinetics presented in the literature. Parameters that could not be found in the literature were regressed against the experimental data obtained from oxidation experiments with softwood kraft lignin. In addition to the detailed model for the chemistry, acid-base equilibrium reactions and gas-liquid mass transfer were modeled. Most of the experimental observations could be reproduced with the developed model. The reasons behind the behavior of guaiacyl and condensed phenols are discussed. The reaction routes affecting lignin solubilization and chemical consumption are presented. Model-ing of acid-base equilibria proved to be important because acid-base pairs of reactants react differently. Carbon dioxide buffers the pH and in this way affects the chemical reactions through the pH. A similar model could also be developed for other lignin treatments; for example, waste water purification or chemicals production in new biorefinery concepts. The developed reaction scheme will be used as a part of oxygen delignification model.

Alkali-O2 oxidized lignin – A bio-based concrete plasticizer

“…The consumption of NaOH does not; however, necessarily reflect the real formation of carboxylic acids in the lignin polymer. Acidic, secondary oxidation products including CO 2 are formed and they contribute to pH Kuitunen et al, 2011) and consume NaOH. It is probably impossible to totally avoid the further oxidation of the primary oxidation products.…”

“…This leads to lignin depolymerization and introduction of acidic groups in lignin, both of which increase its water solubility (Chang and Gratzl, 1980;Sixta et al, 2006;Ji et al, 2009). The secondary oxidation of the primary oxidation products also takes place leading to small molecular degradation products (Gierer and Imsgard, 1977;Chang and Gratzl, 1980;Kuitunen et al, 2011). Below pH 12, the hydroperoxide intermediates tend to protonate and decompose homolytically back to phenoxyl radicals, which without further oxidation may spontaneously combine leading to lignin condensation (Chang and Gratzl, 1980;Kalliola et al, 2014).…”

Chemical characteristics and stability of eucalyptus kraft pulps bleached with tertiary amine catalyzed hypochlorous acid

“…H 2 O 2 is a nucleophile that is commonly applied for increasing the brightness of pulps especially in the end of bleaching sequences. H 2 O 2 reacts specifically with unconjugated and conjugated aldehydes and ketones, such as hydroxyquinones formed in other bleaching sequences (Kuitunen et al 2011). In peroxide bleaching (P stage) traces of transition metals may catalyze the decomposition of H 2 O 2 and formation of hydroxyl radicals that may lead to oxidation and depolymerization of hemicelluloses and cellulose similar to Z stage.…”

Radar Tool for Lignin Classification on the Perspective of Its Valorization

“…Therefore, these two coexistent features are important for V and Sy production by oxidation by O 2 : low frequency of condensed linkages and high β-O-4 structures without CαO. Structures carrying C O at Cα lead preferentially to the respective carboxylic acids, 32 VA and SA. These structures are always produced in the oxidation process with O 2 in alkaline medium, but it is preferred that they be at low levels in the initial lignin.…”