Abstract
Lignin is the most abundant aromatic polymer in nature, that makes it an attractive raw material for producing chemicals, materials, and fuels that are currently obtained from fossil resources. However, the structural complexity, heterogeneity, and variability of the lignin macromolecules hinder the development of efficient valorization technologies for different sources of raw materials. In this study, double enzymatic lignin (DEL) was isolated from different tree-age balsa tree to understand the structural variations of lignin macromolecules during the growth of balsa for the first time. Confocal Raman microscopy (CRM) and component analysis were used to monitor the accumulation of lignin in the plant cell wall. Meanwhile, the structural characteristics and chemical reactivity of DELs were synthetically characterized by advanced NMR techniques. Results showed that the lignin is almost composed of β-O-4 linkages and its content is elevated as the increasing tree-age. Interestingly, carbon-carbon linkages (e.g., β-β and β-5) in these DELs isolated from 3 and 5-year balsa are gradually disappeared. Considering the increasing molecular weight of DELs with the tree-age, it was concluded that lignin macromolecules in balsa wood was gradually polymerized with the increasing growth years. Furthermore, abundant C-O linkages with less C-C linkages in the DELs from 3 and 5-years balsa wood suggested that these feedstocks are promising feedstock in current lignin-first biorefinery scenario. Meanwhile, these lignin fractions from these feedstocks are beneficial to the downstream conversion of lignin into aromatic chemicals. In short, understanding the structural changes of lignin during the growth of balsa wood will facilitate the deconstruction and value-added applications of this kind of feedstock.
Highlights
Lignin is the most abundant natural biopolymer in the plant cell wall of the lignocellulosic biomass
GPC results showed that Mw of double enzymatic lignin (DEL) gradually elevated from 8460 g/mol (1-year-old balsa) to 8800 g/mol (3year-old balsa) and further climbed to 10,050 g/mol (5-yearold balsa). These results suggested that polymerization degrees of lignin macromolecules increase gradually as balsa wood grows
NMR results showed that the DELs from different growth years of balsa trees have abundant β-O-4 linkages (66.33–68.81/100Ar) and increasing
Summary
Lignin is the most abundant natural biopolymer in the plant cell wall of the lignocellulosic biomass. It is a heterogeneous and aromatic macromolecule polymer, which is mainly comprised of three kinds of component units, syringyl (S) units, guaiacyl (G) units, and p-hydroxyphenyl (H) units by biosynthesizing under the regulation of different kinds of enzymes. Structural Characterization of Balsa Lignin (Ralph et al, 2004; Rinaldi et al, 2016; Baruah et al, 2018) These units are linked by ether (e.g., β-O-4) and carbon–carbon (β-5, β-β, etc.) bonds via free radical coupling reactions (Ralph and Landucci, 2010). Detailed investigation of microscopic distribution of lignin in cell wall and molecular structures of lignin in the lignocellulosic biomass will facilitate deconstruction and value-added applications of the plant cell walls in the current biorefinery (Ragauskas and Yoo, 2018)
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