Characterization of the Interunit Bonds of Lignin Oligomers Released by Acid-Catalyzed Selective Solvolysis of Cryptomeria japonica and Eucalyptus globulus Woods via Thioacidolysis and 2D-NMR

摘要

Acid-catalyzed degradation of lignin in toluene containing methanol selectively yields C6-C2 lignin monomers and releases lignin oligomers, a potential raw feedstock for epoxy resins. We herein characterize the structures of the lignin oligomers by focusing on the changes in the interunit linkage types during solvolysis. The oligomeric lignin products were analyzed via thioacidolysis and 2D-HSQC-NMR. The results show that lignin oligomers ranging from monomers to tetramers are released through considerable cleavage of the beta-O-4 linkages. The lignin oligomers from Cryptomeria japonica (softwood) mainly comprise beta-5, beta-1, and tetrahydrofuran beta-beta linkages, whereas Eucalyptus globulus (hardwood) yields oligomers rich in beta-1 and syringaresinol beta-beta linkages. Both wood samples exhibit selective release of beta-beta dimers and a relative decrease in 5-5 and 4-O-5 bonds during solvolysis. The method presented for the separation of lignin oligomers without beta-O-4 linkages and with linkages unique to each wood species will be useful for the production of lignin-based materials.