In‐Depth Identification of Phenolics Fractionated from Eucalyptus Kraft Lignin

Abstract Kraft lignin, the principal by‐product of the pulping industry, is an abundant yet highly underutilized aromatic resource. It is useful due to the vast scales involved as a source of fuels, chemicals, and functional materials when suitable postprocessing is applied. It is deemed to be a renewable alternative to fossil sources. Because of the extreme complexity of kraft lignin, in‐depth composition analysis and chemical structural identification are crucial for the development of effective valorization strategies. This work embarks on the characterization and identification of lignin‐derived phenolic compounds in representative hardwood (Eucalyptus) kraft lignin after selective fractionation using a single solvent. Advanced 2D heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR), ³¹P NMR, and mass spectrometry in combination with synthetic authentic compounds are used for the structural identification of the low‐molecular‐weight fraction of kraft lignin. A total of 20 phenolic compounds, including several novel monomers (catechols), dimers (diarylethanes, diarylethenes, etc.), and trimers with triphenyl structures are systematically identified in the Eucalyptus kraft lignin. In addition, polysulfide is detected in the low‐molecular‐weight fraction, indicating that the oxidative polymerization of S²⁻/HS⁻ occurrs during the kraft pulping process. An improved understanding of kraft lignin, especially the low‐Mw fraction, is expected to provide essential information for the selective fractionation and valorization of lignin.