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Green fractionation and a comprehensive overview of lignin molecular structures during the DES (deep eutectic solvent) pretreatment are very important for lignin valorization and the whole biorefinery process. Herein, intractable woody biomass (poplar wood) was pretreated with five types of carboxylic-acid-based DESs (acetamide served as an HBA (hydrogen bond acceptor), propanedioic acid, tartaric acid, malic acid, glutaric acid, and succinic acid served as HBDs (hydrogen bond donors)) under the optimized pretreatment conditions. Results showed that the optimal delignification ratio was achieved for tartaric-acid-based DES at 140 °C for 20 min under microwave-assisted heating. Two-dimensional HSQC NMR data demonstrated that the isolated poplar DES lignin consisted mainly of β-β, β-O-4 (normal and acylated forms), β-5, and esterified p-hydroxybenzoates (PBs) in different contents. Especially, the contents of β-O-4 in the isolated DES lignin fractions varied based on the pretreatment temperature and different chemical compositions of the DES. The antioxidant activity (2,2-diphenyl-1-picrylhydrazyl, DPPH analysis), microstructure (scanning electron microscope, SEM), and molecular weights (gel permeation chromatography, GPC) of the DES lignin fractions demonstrated that the DES delignification promoted the rapid assembly of lignin nanoparticles (LNPs) and could yield homogeneous lignin (1.23 < PDI < 1.58) with controlled nanometer size (30–170 nm) and good antioxidant activity. This study will improve the knowledge of structural changes of lignin during the different carboxylic-acid-based DES pretreatments and maximize the lignin valorization.

Elucidating the chemical and structural characteristics of hemicelluloses and lignin in the p-coumarate 3-hydroxylase (C3H) down-regulated poplar wood will be beneficial to the upstream gene validation and downstream biomass conversion of this kind of transgenic poplar. Herein, the representative hemicelluloses and lignin with unaltered structures were prepared from control (CK) and C3H down-regulated 84K poplars. Modern analytical techniques, such as 13C NMR, 2D-HSQC NMR, and gel chromatography (GPC), were performed to better delineate the structural changes of hemicelluloses and lignin caused by transgenesis. Results showed that both the hemicelluloses (H-CK and H-C3H) extracted from control and C3H down-regulated poplar wood have a chain backbone of (1→4)-β-D-Xylan with 4-O-Me-α-D-GlcpA as side chain, and the branch degree of the H-C3H is higher than that of H-CK. With regarding to the lignin macromolecules, NMR results demonstrated that the syringyl/guaiacyl (S/G) ratio and dominant substructure β-O-4 linkages in C3H down-regulated poplar were lower than those of control poplar wood. By contrast, native lignin from C3H down-regulated poplar wood exhibited higher contents of p-hydroxybenzoate (PB) and p-hydroxyphenyl (H) units. In short, C3H down-regulation resulted in the chemical and structural changes of the hemicelluloses and lignin in these poplar wood. The identified structures will facilitate the downstream utilization and applications of lignocellulosic materials in the biorefinery strategy. Furthermore, this study could provide some illuminating results for genetic breeding on the improvement of wood properties and efficient utilization of poplar wood.

Lignin is the most abundant aromatic biomacromolecule on the earth, which is an attractive raw material for producing bio-based chemicals, materials, and fuels. However, the complexity, heterogeneity, and variability of the lignin structure always hinders the value-added application of different sources of raw materials. In this study, double enzymatic lignin (DEL) was isolated from balsa grown for different lengths of time to understand the structural variations of lignin macromolecules during the growth of balsa for the first time. Confocal Raman microscopy and component analysis were used to monitor the lignin accumulation in balsa. Meanwhile, the structural characteristics and chemical reactivity of DELs were synthetically characterized by advanced 2D-HSQC and 31P-NMR techniques. It was found that the balsa lignin is a typical hardwood lignin and it is overwhelmingly composed of C–O bonds (i.e., β-O-4 linkages), whose content is elevated with increasing tree-age. Interestingly, carbon–carbon linkages (e.g., β-β and β-5) in these DELs isolated from 3-and 5-year-old balsa are gradually disappearing. Considering the increasing molecular weight of DELs with tree-age, it was concluded that lignin macromolecules in balsa wood were gradually polymerized within the growth period. Furthermore, abundant C–O linkages with less C–C linkages in the DELs from 3 and 5-year-old balsa wood suggested that these feedstocks are promising in current lignin-first biorefinery and will facilitate the conversion of aromatic chemicals from the lignin macromolecule. In short, a comprehensive understanding of native lignin during the growth of balsa wood will not only advance the understanding of biosynthetic pathways of lignin biopolymer, but also facilitate the deconstruction and value-added applications of this kind of feedstock.

Abstract In this paper, identifying the initial value for high dimension heat equation with inhomogeneous source on a spherical symmetric domain is investigated. The truncation regularization method is a powerful technique for solving this inverse problem. We prove the convergence estimates between the regularization solution and the exact solution under the prior and the posterior regularization parameter choice rulers. A numerical example is presented to validate the effectiveness of this method.

Abstract In this paper, we investigate an inverse problem to determine an unknown source term that has a separable-variable form in the time-fractional diffusion equation, whereby the data is obtained at a certain time. This problem is ill-posed, and we use the Landweber iterative regularization method to solve this inverse source problem. Two kinds of convergence rates are obtained by using an a priori and an a posteriori regularization parameters choice rules, respectively. Numerical examples are provided to show the effectiveness of the proposed method.

In this study, lignin with fine structures and facile enzymatic saccharifying residue were successively dissociated based on the lignin-first biomass deconstruction strategy. In the lignin-first process, aldehyde-protected lignin fractions were firstly isolated by acid-catalyzed dioxane extraction in the presence of formaldehyde (FA) and acetaldehyde (AA) and then analyzed by advanced nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The optimized hydrogenolysis of the extracted lignin (LFA and LAA) resulted in a high yield (42.57% and 33.00%) of lignin monomers with high product selectivity (mainly 2,6-dimethoxy-4-propylphenol) (39.93% and 46.61%). Moreover, the cellulose-rich residues were saccharified into fermentable glucose for bioethanol production. The glucose yield of the substrate (RAA) reached to 75.12%, which was significantly higher than that (15.4%) of the substrate (RFA). In short, the lignin-first biomass deconstruction by adding AA is a promising and sustainable process for producing value-added products (energy and fine chemicals) from lignocellulosic biomass.