Developing Catalytic Oxygen Delignification for Kraft Pulp:  Kinetic Study of Lignin Oxidation with Polyoxometalate Anions

Abstract

The effect of pH, ionic strength, and temperature on the kinetics of oxidation reactions between residual lignin and polyoxometalate (POM) anions ([AlMn III (OH 2 )W 11 O 39 ] 6and [SiMn III (OH 2 )-W 11 O 39 ] 5-) in a pulp suspension of unbleached softwood kraft pulp at varying conditions was investigated. According to the results, cation concentration and pH have a remarkable effect on reactivity of the POMs. At cation concentrations e0.10 mol L -1 , no reaction was observed during the first 5000 s; at 0.30 mol L -1 , the reactions were significantly more rapid, and the rate correlated strongly with increasing pH for both POMs. The activation energies (E a ) of the oxidation reactions at 0.30 mo L -1 cation concentration varied between ∼27 and 68 kJ mol -1 for [AlMn III (OH 2 )W 11 O 39 ] 6-, depending on pH and reaction time; the respective values for [SiMn III (OH 2 )W 11 O 39 ] 5were ∼17 to 82 kJ mol -1 . The reasons for the observed decrease in E a along with increasing pH and cation concentration are discussed.

Alternatives for lignocellulosic pulp delignification using polyoxometalates and oxygen: a review

“…38 The kinetics of redox reactions between residual lignin and POM anions [AlW 11 Mn III (H 2 O)O 39 ] 62 and [SiW 11 Mn III -(H 2 O)O 39 ] 52 in a suspension of unbleached softwood kraft pulp were studied, taking into account the effect of cation concentration (mainly potassium), pH, ionic strength and temperature. 43,44 According to the results, cation concentration and pH had a remarkable effect on the reactivity of the POMs toward the phenolic lignin structures. At cation concentrations lower than 0.10 mol L 21 , no reaction was observed during the first hour and 20 min; at a concentration of 0.30 mol L 21 , the reactions were significantly more rapid, and the rate correlated strongly with increasing pH for both POMs.…”

Catalytic Transformation of Lignin for the Production of Chemicals and Fuels

“…The addition of an electron-donating group in the ortho -phenol substituent not only increased the electron density of the aromatic ring, but also helped stabilize the intermediate phenoxy radical through resonance stabilization and delocalization. Kinetic study of lignin oxidation with POM anions ([AlMn III (OH 2 )­W 11 O 39 ] 6– and [SiMn III (OH 2 )­W 11 O 39 ] 5– ) showed that cation concentration and pH have a remarkable effect on the reactivity of the POMs.…”

Sustainable Productions of Organic Acids and Their Derivatives from Biomass via Selective Oxidative Cleavage of C–C Bond

“…The key issue is to avoid the overoxidation or repolymerizaiton of the products, especially in the presence of phenolic groups. Historically, in the paper industry, lignin is oxidatively degraded into waste in the delignification process. − It needs to develop more selective catalysts to convert lignin into valuable chemicals. Recent work has focused on the lignin models, as a “bottom-to-up” strategy due to the structure complexity of lignin.…”