Degradation mechanism of a lignin model compound during alkaline aerobic oxidation: formation of the vanillin precursor from the β-O-4 middle unit of softwood lignin

Abstract

We have proposed plausible reaction pathways involved in the chemical conversion of softwood lignin to vanillin through alkaline aerobic oxidation.

Insights into Production of Hydroxybenzaldehydes via Lignin Oxidation

“…The oxidative degradation mechanism of the glycerol side-chain to the Ca-aldehyde has been studied by analyzing the detailed reaction behaviors of several model compounds, including VG and veratrylglycerol. Three oxidative pathways have been proposed, as shown in Figure 5: 1) oxidation of the a-OH group to a carbonyl group; 2) generation of the Cg-aldehyde intermediate from the Ca=O via keto-enol tautomerization; 3) generation of the Ca-aldehyde via a retro-aldol reaction 45) . However, the detailed elementary reaction mechanisms involved in this oxidative process, including the identity of the oxidants responsible for the oxidation of the a-OH group, remain unclear.…”

“…However, the detailed elementary reaction mechanisms involved in this oxidative process, including the identity of the oxidants responsible for the oxidation of the a-OH group, remain unclear. Furthermore, the role of this 38,39,45) .…”

“…Fig.5. Pathways involved in the aerobic oxidation of a non-phenolic mode compound, veratrylglycerol βguaiacyl ether VG38,39,45) .…”

Pathways for vanillin production through alkaline aerobic oxidation of a phenolic lignin model compound, guaiacylglycerol-β-guaiacyl ether, in concentrated aqueous alkali

“…The increased yield of vanillin in the presence of organic cations is hypothesized to result from the suppression of bimolecular reactions, such as the disproportionation reactions that occur in the vanillin end group in Scheme 1A. 19 Phenolic units, present in limited quantities in lignin and emerging through the cleavage of b-O-4 linkages in the intermediate units, generally exhibit higher reactivity in oxidation reactions with various oxidants compared to non-phenolic units. The pathway for vanillin production from the alkaline aerobic oxidation of these phenolic units is proposed in Scheme 1B, drawing on the mechanistic insights gleaned from various reactions in pulp bleaching.…”

Novel Mechanoenzymatic Strategy for Lignin Depolymerization

“…Among the various types of C–O bonds present in lignin, including β-O-4, β–β, β-5, β-1, 4-O-5, 5–5, and dibenzodioxins, the cleavage of the β-O-4 bond is the most commonly employed strategy for degradation. This is attributed to its relatively low bond dissociation energy (BDE) compared to other C–O bonds, making up approximately 45–60% of all degradation methods. − …”