Microwave processing of lignin in green solvents: A high-yield process to narrow-dispersity oligomers

Abstract

Functional narrow-dispersity kraft lignin oligomers in high-yields were produced by a combined microwave-assisted extraction/degradation process. The process took place under mild non-catalytic conditions, in green solvents, and moderate temperatures (80−160 °C). High yields, i.e., 63 % and 64 %, of lignin oligomers with narrow dispersity were obtained after only 40 min microwave processing in methanol and ethanol, respectively. The ethanol-soluble yield increased substantially after microwave processing as compared to samples extracted by liquid-solid solvent-extraction. This increase was mainly ascribed to the cleavage of β-O-4 linkages, as indicated by semiquantitative 2D-HSQC NMR. Under the corresponding conditions in methanol, the yields after microwave processing were similar to those obtained by liquid-solid solvent-extraction. The difference is likely ascribed to the different reactivities of the solvents as O-alkylation agents. Furthermore, the obtained lignin fractions had high functionality, as shown by hydroxyl-group quantification through 31P-NMR. This, along with the narrow dispersity and rich aromatic structure, makes these oligomers interesting precursors for future thermoset syntheses.