Differences in the Mechanisms of MnO₂Oxidation between Lignin Model Compounds with thep-Hydroxyphenyl, Guaiacyl, and Syringyl Nuclei

Abstract

The purpose of this study was to examine how the rate and mechanism of MnO 2 oxidation differ between the phydroxyphenyl (H), guaiacyl (G), and syringyl (S) types of simple nonphenolic lignin model compounds as well as the p-ethylphenyl (E) type compounds. The oxidation was conducted using an excess amount of MnO 2 in a sulfate buffer solution at a pH value of 1.5 at room temperature. MnO 2 oxidized at least the G and S nuclei, although it commonly oxidizes alcohols present at the benzyl position. The oxidation rates of the benzyl alcohol derivatives were in the order of G-> S-≫ H-> E-type, which suggests that the rates are determined by the electronic effects of their methoxy and ethyl functional groups on not only their benzyl positions but also their aromatic π-electron systems. The kinetic isotope effect was observed in the MnO 2 oxidations of the same derivatives deuterated at their benzyl hydroxymethyl groups. The observed magnitudes were in the order of E-≫ H-> G-≫ S-type, suggesting that the contribution of oxidation of their aromatic nuclei, which is another reaction mode of the oxidation of their benzyl positions, increases in the reverse order.

Effect of side-chain length in lignin model compound on MnO2 oxidation: comparison of oxidations between C6-C2- and C6-C1-type compounds

“…3) and their deuterated compounds at the benzyl positions (H-type (V H ), G-type (V G ), S-type (V S ), and E-type (V E ), Fig. 3), were oxidized by MnO 2 under the same conditions as in our recent report [4]. The obtained results were compared with those in our recent report to examine the effect of the presence of the β-methyl groups and whether the contribution of each reaction mode differs in the MnO 2 oxidations between the C 6 -C 1 -(II and III) and C 6 -C 2 -type (IV and V) compounds.…”

“…1), and the corresponding benzyl alcohol derivatives deuterated at the benzyl positions (H-type (III H ), G-type (III G ), S-type (III S ), and E-type (III E ), Fig. 1) at a pH of 1.5 and room temperature in our recent report [4]. Compound I G or I S is oxidized by MnO 2 to afford the corresponding benzoquinone derivative (B G or B S , respectively, Fig.…”

“…Although our recent report describes why the E-type compounds, which are not lignin models, were applied [4], this reason is included again here. The Hammett's substituent constant (σ value) of the ethyl group present at the para-positions of the benzyl carbons in the E-type compounds, − 0.151, is almost the same as the sum of those of the two methoxy groups present at the…”

Oxygen Limitation Accelerates Regeneration of Active Sites on a MnO2 Surface: Promoting Transformation of Organic Matter and Carbon Preservation

“…Among the commonly found active minerals, Fe and Mn oxides are ubiquitous in soil and sediment and involved in a variety of chemical processes. , In previous works, considerable attention has been paid to the roles of Fe oxides in the transformation and preservation of soil organic carbon . Compared to Fe oxides, although their contents are lower, Mn oxides demonstrate stronger oxidizing ability mainly owing to their higher redox potential. − Consequently, a variety of organic molecules were transformed on the surface of manganese oxides by the direct redox reaction. , As a major process in the mineralogy and geochemistry of manganese, interactions between manganese dioxide and OM frequently occur in aerobic/anaerobic environments. For instance, fulvic acid (FA) was oxidized by MnO 2 to lower-molecular-weight substances through adsorption and electron transfer under anaerobic conditions, and a part of δ-MnO 2 with a layered structure was converted into MnOOH with a tunnel crystal phase. , …”

“…5−7 Consequently, a variety of organic molecules were transformed on the surface of manganese oxides by the direct redox reaction. 8,9 As a major process in the mineralogy and geochemistry of manganese, interactions between manganese dioxide and OM frequently occur in aerobic/anaerobic environments. For instance, fulvic acid (FA) was oxidized by MnO 2 to lower-molecular-weight substances through adsorption and electron transfer under anaerobic conditions, 10 and a part of δ-MnO 2 with a layered structure was converted into MnOOH with a tunnel crystal phase.…”