Characterization of kraft lignin prepared from mixed hardwoods by 2D HMQC and 31P NMR analyses

Abstract

This study was conducted to determine the lignin substructures, hydroxyl (phenolic + aliphatic) contents, and carboxyl contents in kraft lignin (KL) prepared from mixed hardwoods by using 2D Heteronuclear Multiple Quantum Coherence Nuclear Magnetic Resonance (HMQC NMR) and 31P NMR techniques. Based on 2D HMQC NMR analysis of KL, stilbene and vanillin substructures were present in the aromatic region, while trace amounts of β-O-4 and β-β moieties were detected in the oxygenated aliphatic region. The total hydroxyl content calculated from 31P NMR was 5.24 mmol/g KL. The aliphatic hydroxyl content was 1.04 mmol/g KL, and phenolic hydroxyl content was 4.20 mmol/g KL. Of the phenolic hydroxyl groups, the contribution of syringyl (S) and guaiacyl (G) was 1.02 and 0.97 mmol/g KL, respectively. The S/G molar ratio of KL calculated from 31P NMR was 1.05. The carboxyl content was 0.44 mmol/g KL.

Thermal characterization of kraft lignin prepared from mixed hardwoods

“…According to Py-GC/MS at 675 °C, the S/G ratio of KL was calculated as 1.07. The S/G ratio obtained from Py-GC/MS was in agreement with the S/G ratios obtained from 1 H NMR (1.13) and 31 P NMR spectroscopy (1.05) (Mun et al 2021;Mun et al 2022). Stilbene moieties are thermally degraded into 4-vinylguaiacol 9 and 4-vinylsyringol 21 while styrene produces 4-methylguaiacol 5 and 4-methylsyringol 15.…”

“…The thermal characteristics of KL were compared alongside with milled wood lignins (MWLs) from acacia (MWL-aca) and mixed hardwoods (MWL-mhw) to gain insights on the structural changes that took place in KL during kraft pulping. Moreover, this study is an extension of previous works (Mun et al 2021;Mun et al 2022) on the characterization of KL produced in South Korea. The outcomes obtained in this study will aid the selection of chemical modifications, functionalization, and applications that are compatible with KL.…”

Potential of hardwood kraft lignin as a bio-based dye for cotton fabrics

“…After kraft pulping, lignin was separated and purified from the black liquor using acid precipitation method with sulfuric acid. Detailed information on pulping conditions, purification process, as well as chemical, structural, and thermal characterization of KL can be found in previous works (Mun et al 2021(Mun et al , 2022Pe et al 2023). The thoroughly dried KL was stored at ambient temperature and was used as dye material.…”