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.

Revealing the Structural Influence on Lignin Phenolation and Its Nanoparticle Fabrication with Tunable Sizes

“…In-depth structural characterization to reveal the structural changes of lignins before and after phenolation was characterized by 2D HSQC NMR (Figure ). Signals of β–O–4 (A), β–5/α–O–4 (B), and β–β (C) substructures were observed in the side chain region (δ C /δ H 50–105/2.5–6.0 ppm) of both CEL and KL . Compared to CEL, substructures of secoisolariciresinol (SR) and arylglycerol (Aly) appeared in C–H crosslinking signals in the spectra of KL. , Polysaccharide signals (colored in gray) were also detected in KL, which might be originated from the residual hemicellulose that was linked to lignin, namely, lignin-carbohydrate complexes, due to the difficulty of complete removal in the kraft process. , As the softwood lignins, G2, G5, and G6 were observed in the aromatic region (δ C /δ H 105–130/6.0–8.0 ppm).…”

“…For the 2D HSQC NMR analysis, ∼ 80 mg of lignin or ∼ 10 mg of the model compound was placed in the NMR tube, and then ∼ 0.5 mL of DMSO- d 6 was added to dissolve it. The experimental parameters used were according to the literature. ,, The central peak of dimethyl sulfoxide (DMSO) (δ C /δ H , 39.51/2.49 ppm) was used as an internal reference. 31 P NMR spectra were recorded according to the literature. , First, the lignin samples were fully reacted with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, and then the samples were scanned according to the following experimental parameters: 90° pulse angle, 10 s relaxation delay, 256 scans, and 61.9 ppm scan width.…”

“…The experimental parameters used were according to the literature. 24,44,47 The central peak of dimethyl sulfoxide (DMSO) (δ C /δ H , 39.51/2.49 ppm) was used as an internal reference. 31 P NMR spectra were recorded according to the literature.…”

“…31 P NMR spectra were recorded according to the literature. 47,48 First, the lignin samples were fully reacted with 2chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, and then the samples were scanned according to the following experimental parameters: 90°pulse angle, 10 s relaxation delay, 256 scans, and 61.9 ppm scan width.…”

“…Signals of β−O−4 (A), β−5/α−O− 4 (B), and β−β (C) substructures were observed in the side chain region (δ C /δ H 50−105/2.5−6.0 ppm) of both CEL and KL. 47 Compared to CEL, substructures of secoisolariciresinol (SR) and arylglycerol (Aly) appeared in C−H crosslinking signals in the spectra of KL. 14,44 Polysaccharide signals (colored in gray) were also detected in KL, which might be originated from the residual hemicellulose that was linked to lignin, namely, lignin-carbohydrate complexes, due to the difficulty of complete removal in the kraft process.…”