Abstract
The value-added utilization of tobacco stalk lignin is the key to the development of tobacco stalk resources. However, the serious heterogeneity is the bottleneck for making full use of tobacco stalk lignin. Based on this, lignin was separated from tobacco stalk through hydrothermal assisted dilute alkali pretreatment. Subsequently, the tobacco stalk alkaline lignin was fractionated into five uniform lignin components by sequential solvent fractionation. Advanced spectral technologies (FT-IR, NMR, and GPC) were used to reveal the effects of hydrothermal assisted dilute alkali pretreatment and solvent fractionation on the structural features of tobacco stalk lignin. The lignin fractions extracted with n-butanol and ethanol had low molecular weight and high phenolic hydroxyl content, thus exhibiting superior chemical reactivity and antioxidant capacity. By contrast, the lignin fraction extracted with dioxane had high molecular weight and low reactivity, nevertheless, the high residual carbon rate made it suitable as a precursor for preparing carbon materials. In general, hydrothermal assisted dilute alkali pretreatment was proved to be an efficient method to separate lignin from tobacco stalk, and the application of sequential solvent fractionation to prepare lignin fractions with homogeneous structural features has specific application prospect.
Highlights
Lignin is generally considered to be a polymer formed by free radical coupling dehydrogenation of three hydroxycinnamyl alcohols (p-coumaryl, coniferyl, and sinapyl alcohols), and it is the second most renewable natural terrestrial polymer (Huang et al, 2008; Mahmood et al, 2016)
The weight-average molecular weight of unfractionated tobacco stalk lignin (F0) was 3,155 g/mol, which was much lower than that of natural tobacco stalk lignin. This was because the lignin was degraded and some of the chemical bonds were broken during the hydrothermal-assisted alkaline pretreatment process (Wang et al, 2017a)
It could be found that the molecular weight gradually increased from F1 to F5, which proved that the lignin with small molecular weight had better solubility and was extracted first during the sequential solvent fractionation
Summary
Lignin is generally considered to be a polymer formed by free radical coupling dehydrogenation of three hydroxycinnamyl alcohols (p-coumaryl, coniferyl, and sinapyl alcohols), and it is the second most renewable natural terrestrial polymer (Huang et al, 2008; Mahmood et al, 2016). Lignin is often defined as a by-product of the pulping and biorefinery industries due to its complex structural (Wen et al, 2013b; Han et al, 2021a). The annual production of industrial lignin is up to 60 million tons, but it is often used for low-value thermal energy conversion, and the high value utilization rate is less than 2% (Aro and Fatehi, 2017). Lignin is gradually being regarded as inferior raw material for obtaining heat energy due to its high carbon dioxide emissions during combustion (Ogunkoya et al, 2015). The development of high-value commercial products with lignin as raw materials is the key to the sustainable development of lignocellulosic resources, which has tremendous economic and environmental benefits (Ma et al, 2021a; Han et al, 2021b).
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