Structural Characteristics of Lignin Macromolecules from Different Eucalyptus Species

Understanding of the fundamental chemistry of lignin macromolecules in lignocellulosic biomass will facilitate the further development of renewable biofuel and biomaterial production. In the present study, to thoroughly delineate the detailed structural characteristics of the native lignin in the three major Eucalyptus woods, MWL, CEL, and EHL preparations were sequentially isolated from the diverse Eucalyptus wood. Parallelly, a modified enzyme lignin (DEL) based on double ball-milling and enzymatic hydrolysis was also prepared with superhigh yield (102.5–109.5%) from same Eucalyptus wood. The structural characteristics of all the lignin preparations were elaborately investigated by HPAEC, GPC, and NMR techniques (2D-HSQC and 31P NMR). Results showed that the contents of different substructures (β-O-4, β-β, β-5, and β-1) and S/G ratios of lignin macromolecules varied among these Eucalyptus species. As compared to the β-O-4 content in the extracted lignins (MWLs and CELs, 44.5–60.2/100Ar), the residual lignins (EHLs and DELs) exhibited a higher content of β-O-4 linkages (59.1–64.3/100Ar). DEL preparations can be served as “lignin preparation” to develop a depolymerization strategy of lignin due to the higher content of β-O-4 linkages and abundant syringyl units. Moreover, the sufficient understanding of lignin will facilitate subsequent process of these Eucalyptus species for producing of biochemicals, bioenergy, and biomaterials.