In situ trapping of enol intermediates with alcohol during acid-catalysed de-polymerisation of lignin in a nonpolar solvent

Abstract

Acid-catalysed degradation of wood in toluene–MeOH yields the lignin monomers homovanillyl aldehyde dimethyl acetal and homosyringaldehyde dimethyl acetal selectively.

Abstract

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.