Lignin’s phenylpropane-based structure can, under suitable conditions, be broken down into smaller aromatic and aliphatic organic compounds. Because of this, lignin is generally regarded as a renewable alternative to some petroleum-derived chemical feedstocks. This page summarizes general research-literature background on lignin degradation pathways; it reflects published research findings, not a Green Agrochem process specification.
Chemical degradation routes
The main chemical approaches studied for degrading lignin are hydrolysis, pyrolysis and oxidative degradation. Depending on the route and the specific bonds broken within the lignin structure, reported degradation products include phenols and substituted phenols (from cleavage of β-O and α-O linkages), benzene and substituted benzenes (where the aromatic ring itself is cleaved), saturated or unsaturated hydrocarbons (from breakdown of the aliphatic carbon chains), and organic acids of varying molecular weight (from oxidative cleavage). These processes can also release lower-molecular-weight byproducts such as organic sulfur compounds, CO, CO2 and H2.
Chemical degradation of lignin remains an area of active research rather than a fully mature technology — the strength and variety of bonds within the lignin structure make selective degradation difficult, and only a limited number of small molecules can currently be obtained in this way at scale. One established example is vanillin, which can be produced from the carbohydrate-fermentation byproducts of softwood sulfite pulping liquor via alkaline treatment followed by oxidation. Degradation using solvents such as dimethyl sulfoxide or dimethyl ether has also been studied as a route to smaller lignin fragments.
Enzymatic degradation routes
Three classes of naturally occurring enzymes have been reported in the research literature as capable of degrading lignin: lignin peroxidase, manganese peroxidase and laccase (phenol oxidase). Lignin peroxidase and manganese peroxidase are reported to act by cleaving the Cβ–Cα bond in the lignin structure, generating phenoxy radical intermediates. Laccase has been reported to play a dual role in lignin chemistry — contributing to both degradation and polymerization — with some studies finding its polymerizing effect on wood-derived lignin more pronounced than its degrading effect.
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