A. N. Mitrofanova

Lignin transformations and reactivity upon ozonation in aqueous media

The reaction of ozone with lignin in aqueous acidic solutions is investigated. The Danckwerst model is used to describe the kinetics of gas/liquid processes occurring in a bubble reactor. The efficient ozonation rate of a soluble lignin analog, sodium lignosulfate, is determined. The main lines of the reaction between ozone and lignin are revealed on the basis of kinetic analysis results and IR and UV spectroscopy data.

The ozonization of sodium lignosulfonate in the presence of hydrogen peroxide

The kinetics of ozonization of sodium lignosulfate (LS) in the presence of H2O2 was studied. The effective rate constants for the oxidation of LS and the total ozone consumption were determined. The keff = 30 ± 8 M−1 s−1 value was found to be independent of the concentration of H2O2. The total ozone consumption decreased as the concentration of H2O2 increased from 1.0 × 10−4 to 1.0 × 10−3 M because of the participation of the radicals generated in the O3 + H2O2 reaction in LS transformations. The kinetic and UV and IR spectroscopy data allowed the conclusion to be drawn that the destruction of the LS aromatic system in the LS + O3 + H2O2 reaction was caused by the interaction of LS with O3, whereas radicals generated in this system contributed to deeper destruction of LS in the interaction of aliphatic LS macromolecule fragments with low-molecular-weight polymer oxidation products. The depth of polymer oxidation could be changed by varying the content of hydrogen peroxide in the system for LS ozonization.

Mechanism of the catalytic ozonization of lignin in the presence of Mn(II) ions

The reaction between ozone and lignin in aqueous solutions catalyzed by Mn(II) ions is studied. The rate of destruction for aromatic structures of lignin is found to increase in the presence of Mn(II) ions. However, the greatest catalytic effect is observed upon the transformation of aliphatic acids that are difficult to oxidize with ozone. The introduction of catalyst raises the total consumption of ozone from 3 to 7 mol per each structural unit of lignin. A scheme is proposed for the transformation of phenol fragments of lignin using ozone with the participation of Mn(II) ions: at the initial stage, we observe the ozone oxidation of lignin and Mn(II) to Mn(III) ions stabilized with products of lignin oxidation and accompanied by the formation of chelate complexes, and the Mn(III) chelate complexes act as low-molecular mediators, attacking phenol structures and initiating radical processes.

The ozonization of model lignin compounds in aqueous solutions catalyzed by Mn(II) ions

The influence of Mn(II) ions on the rate of the reaction between ozone and model lignin compounds, guaiacol and veratrole, was studied. The catalyst did not influence the rate of the destruction of the aromatic ring and intermediate ozonolysis products, compounds with conjugated double bonds, in acid media but substantially increased the rate of oxidation of saturated carboxylic acids, ketoacids, and aldehydes. Ozone consumption then increased from 2 to 5 moles per mole of the transformed substrate. A mechanism of the catalytic action of Mn(II) in reactions between ozone and the compounds studied was suggested.