Cross-linking between lignin and polysaccharide in plant cell-wall determines physical, chemical, and biological features of lignocellulosic biomass. Since Erdmann’s first report in 1866, numerous studies have suggested the presence of a bond between hemicelluloses and lignin; however, no clear evidence for this interaction has been reported. We describe the first direct proof of covalent bonding between plant cell-wall polysaccharides and lignin. Nuclear magnetic resonance spectroscopy was used to observe the long-range correlations through an α-ether bond between lignin and the primary hydroxyl group of a mannose residue in glucomannan. Complete signal assignment of the cognate structural units was also achieved. Thus, we identified lignin–carbohydrate bonds by complete connectivity analysis from the phenylpropane unit to the carbohydrate moiety.

Processes that can de-polymerise lignin efficiently to yield low molecular mass compounds have received considerable attention in recent years owing to their potential in generating aromatic compounds from a renewable resource. Widely employed protocols for the acidolysis of lignocellulosic biomass can generate numerous products owing to the re-condensation of enol ether intermediates formed during the decomposition of structures containing β-O-4 linkages. Here, we show that the acid-catalysed degradation of lignin in Japanese cedar and Eucalyptus globulus wood samples in a hydrophobic solvent toluene, containing a small amount of methanol at 140 °C–170 °C by microwave heating, yields the lignin monomers homovanillyl aldehyde dimethyl acetal and homosyringaldehyde dimethyl acetal selectively. The maximum combined yield of lignin monomers and oligomers is >10%, based on the weight of the wood sample. Furthermore, when de-polymerisation is performed in hydrophobic media, hydrolysis and hydroxylation of cellulose and lignin fragments are suppressed, wherein methanol serves as an in situ trapping agent for the enol ether intermediate, generating oligolignols and the dimethyl acetal derivatives.

Lignin, the most abundant aromatic polymer in nature, is a potential renewable source for aromatic compounds in a post-oil era. Lignin is a prospective raw feedstock for epoxy resins because of its phenolic hydroxyl group, which offers a reaction point for glycidylation. However, precisely controlling the physical properties of a lignin-based epoxy resin based on the structure–property relationship is difficult because lignin is a heterogeneous macromolecule with unclarified substructures. The degradation of lignin into its monomers and their subsequent use as a building block for epoxy resins is another possible synthetic route, but this approach suffers from the formation of a wide variety of products by the chemical degradation of lignin. Herein, we successfully demonstrate that the selective degradation of lignin into the C2-acetal monomer by solvolysis in a hydrophobic solvent, i.e., toluene, containing methanolic H₂SO₄ and the subsequent synthesis of a cross-linked structure through the phenol units of C2-acetal enabled the rational control of the thermodynamic properties of a lignin-based epoxy resin. To introduce a flexible structure into the lignin-based epoxy resin, the transacetalization of C2-acetal with di(trimethylolpropane) (DTMP) was performed to obtain epoxy–DTMP–lignin. For comparison, we introduce a rigid structure by the annulation of C2-acetals to obtain epoxy-annulated lignin. The glass transition temperatures of epoxy–DTMP–lignin and epoxy-annulated lignin obtained by differential scanning calorimetry were 94 °C and 134 °C, respectively, demonstrating the achievement of the precise thermodynamic properties of lignin-based epoxy resins expected based on their structural design.