Abstract
Chemical characterization of kraft lignin (KL) from mixed hardwoods (Acacia spp. from Vietnam and mixed hardwoods (mainly Quercus spp.) from Korea) was conducted for its future applications. To compare the structural changes that occurred in KL, two milled wood lignins (MWLs) were prepared from the same hardwood samples used in the production of KL. Elemental analysis showed that the MWL from acacia (MWL-aca) and mixed hardwood (MWL-mhw) had almost similar carbon content, methoxyl content, and C9 formula. KL had high carbon content but low oxygen and methoxyl contents compared to MWLs. The C9 formula of KL was determined to be C9H7.29O2.26N0.07S0.12(OCH3)1.24. The Mw of KL and MWLs was about 3000 Da and 12,000–13,000 Da, respectively. The structural features of KL and MWLs were investigated by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectrometry (1H, 13C NMR). The analyses indicated that KL underwent severe structural modifications, such as γ-carbon cleavage, demethylation, and polycondensation reactions during kraft pulping, which resulted in increased aromatic content and decreased aliphatic content. The main linkages in lignin, β-O-4 moieties, were hardly detected in the analysis as these linkages were extensively cleaved by nucleophilic attack of SH- and OH- during pulping.
Highlights
Lignin is the second-most abundant organic carbon source after cellulose
The total lignin content of acacia was 26.90% which was slightly higher than mixed hardwood (24.76%)
For kraft lignin (KL), a shoulder appeared at 1275 cm−1 and a weak absorption band was observed at 1030 cm−1. These results suggest that G and S units were present in KL but might have been significantly degraded or modified compared to milled wood lignins (MWLs)
Summary
Lignin is the second-most abundant organic carbon source after cellulose. The amount of lignin discharged is about 50–70 million tons annually [1,2]; it has been estimated that only a small amount 1–2%) is used in specialty products [3]. Lignin can be obtained by pulping and kraft pulping accounts for about 85% of the total lignin production [4]. The lignin recovered from the black liquor after kraft pulping is called kraft lignin. The majority of KLs are directly burned in pulp mills to generate energy and to recover the remaining pulping chemicals, while some KLs are used as rubber additives and phenolic resin adhesives [5,6]. Limitations on the utilization of KLs are due to their heterogeneity, modified structure, presence of sulfur from sodium sulfide, and the poor quality of the final product [7,8]
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