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.

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

“…Our research group is conducting analyses on the reaction behaviors of various lignin model compounds under alkaline aerobic oxidation conditions to elucidate the vanillin generation pathway from native lignin in sowood. [17][18][19] Scheme 1A illustrates the proposed formation of vanillin from b-O-4 linkages in the middle units of native lignin polymers, where b-O-4 (b-ether) constitutes approximately 50% of the interunit linkages in native lignin. 17 The analysis of oxidative degradation behaviors of non-phenolic b-O-4 type model compounds, representing b-O-4-type intermediate residues in lignin, indicate that vanillin production initiates with the alkaline hydrolysis of the b-ether bond, forming a glycerol end group.…”

“…It involves the oxidation of the a-hydroxy group into a carbonyl group, translocation of the Ca-carbonyl group to the g-position, and side-chain fragmentation via a retro-aldol reaction. 19 The retro-aldol reaction starting from the thermodynamically more stable a-carbonyl moiety also occurs. However, the activation barrier for this reaction is larger than that from the g-carbonyl moiety, rendering the retro-aldol reaction from the a-carbonyl structure kinetically unfavorable.…”

“…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. − …”