Wood Lignin Research Articles

The lignin-degrading abilities of Gelatoporia subvermispora gat1 and pex1 mutants generated via CRISPR/Cas9.

White-rot fungi efficiently degrade wood lignin; however, the mechanisms involved remain largely unknown. Recently, a forward genetics approach to identify several genes in Pleurotus ostreatus (Agaricales) in which mutations cause defects in wood lignin degradation was used. For example, pex1 encodes a peroxisome biogenesis factor and gat1 encodes a putative Agaricomycetes-specific DNA-binding transcription factor. In this study, we examined the effects of single-gene mutations in pex1 or gat1 on wood lignin degradation in another white-rot fungus, Gelatoporia (Ceriporiopsis) subvermispora (Polyporales), to investigate conserved and derived degradation mechanisms in white-rot fungi. G. subvermispora pex1 and gat1 single-gene mutant strains were generated from a monokaryotic wild-type strain, FP-90031-Sp/1, using plasmid-based CRISPR/Cas9. As in P. ostreatus, Gsgat1 mutants were nearly unable to degrade lignin sourced from beech wood sawdust medium (BWS), while Gspex1 mutants exhibited a delay in lignin degradation. We also found that the transcripts of lignin-modifying enzyme-encoding genes, mnp4, mnp5, mnp6, mnp7, and mnp11, which predominantly accumulate in FP-90031-Sp/1 cultured with BWS, were greatly downregulated in Gsgat1 mutants. Taken together, the results suggest that Gat1 may be a conserved regulator of the ligninolytic system of white-rot fungi and that the contribution of peroxisomes to the ligninolytic system may differ among species.

Comparative Production of Bio-Oil from In Situ Catalytic Upgrading of Fast Pyrolysis of Lignocellulosic Biomass

Catalytic upgrading of fast pyrolysis bio-oil from two different types of lignocellulosic biomass was conducted using an H-ZSM-5 catalyst at different temperatures. A fixed-bed pyrolysis reactor has been used to perform in situ catalytic pyrolysis experiments at temperatures of 673, 773, and 873 K, where the catalyst (H-ZSM-5) has been mixed with wood chips or lignin, and the pyrolysis and upgrading processes have been performed simultaneously. The fractionation method has been employed to determine the chemical composition of bio-oil samples after catalytic pyrolysis experiments by gas chromatography with mass spectroscopy (GCMS). Other characterization techniques, e.g., water content, viscosity, elemental analysis, pH, and bomb calorimetry have been used, and the obtained results have been compared with the non-catalytic pyrolysis method. The highest bio-oil yield has been reported for bio-oil obtained from softwood at 873 K for both non-catalytic and catalytic bio-oil samples. The results indicate that the main effect of H-ZSM-5 has been observed on the amount of water and oxygen for all bio-oil samples at three different temperatures, where a significant reduction has been achieved compared to non-catalytic bio-oil samples. In addition, a significant viscosity reduction has been reported compared to non-catalytic bio-oil samples, and less viscous bio-oil samples have been produced by catalytic pyrolysis. Furthermore, the obtained results show that the heating values have been increased for upgraded bio-oil samples compared to non-catalytic bio-oil samples. The GCMS analysis of the catalytic bio-oil samples (H-ZSM-5) indicates that toluene and methanol have shown very similar behavior in extracting bio-oil samples in contrast to non-catalytic experiments. However, methanol performed better for extracting chemicals at a higher temperature.

Research progress on the isolation and purification of ligninase

Lignin is a natural polymer substance with a phenolic structure, widely present in woody plants, herbs, vascular plants, is the second largest natural polymer material in nature after cellulose in quantity, and is the only aromatic compound that can be obtained from renewable resources in industry. Lignin is widely used in building materials industry, petroleum industry, light industry, and agriculture. Lignin is also an important part of human dietary fibres (DF), which can change the activity of microorganisms in the intestinal system, reduce the content of cholesterol and blood sugar in the blood, and have the effect of preventing cardiovascular disease; Lignin also has antioxidant activity and inhibition of cancer cell activity. As a natural antioxidant active substance, the anti-free radical activity of lignin in wood and crop straw has been extensively studied, while lignin in dietary fiber such as vegetables and fruits has not been studied much Extracting lignin from carrots and studying its structure and antioxidant activity is expected to obtain a natural, non-toxic side effect antioxidant active substance, which has a wide application prospect in the food industry, cosmetics, health products and other aspects.

Reductive Catalytic Fractionation of Abies Wood into Bioliquids and Cellulose with Hydrogen in an Ethanol Medium over NiCuMo/SiO2 Catalyst

Noble metal-based catalysts are widely used to intensify the processes of reductive fractionation of lignocellulose biomass. In the present investigation, we proposed for the first time using the inexpensive NiCuMo/SiO2 catalyst to replace Ru-, Pt-, and Pd-containing catalysts in the process of reductive fractionation of abies wood into bioliquids and cellulose products. The optimal conditions of abies wood hydrogenation were selected to provide the effective depolymerization of wood lignin (250 °C, 3 h, initial H2 pressure 4 MPa). The composition and structure of the liquid and solid products of wood hydrogenation were established. The NiCuMo/SiO2 catalyst increases the yield of bioliquids (from 36 to 42 wt%) and the content of alkyl derivatives of methoxyphenols, predominantly 4-propylguaiacol and 4-propanolguaiacol. A decrease in the molecular mass and polydispersity (from 1870 and 3.01 to 1370 Da and 2.66, respectively) of the liquid products and a threefold increase (from 9.7 to 36.8 wt%) in the contents of monomer and dimer phenol compounds were observed in the presence of the catalyst. The solid product of catalytic hydrogenation of abies wood contains up to 73.2 wt% of cellulose. The composition and structure of the solid product were established using IRS, XRD, elemental and chemical analysis. The data obtained show that the catalyst NiCuMo/SiO2 can successfully replace noble metal catalysts in the process of abies wood reductive fractionation into bioliquids and cellulose.

Study on Dissociation and Chemical Structural Characteristics of Areca Nut Husk.

From the perspective of full-component utilization of woody fiber biomass resources, areca nut husk is an excellent woody fiber biomass feedstock because of its fast regeneration, significant regeneration ability, sustainability, low cost, and easy availability. In this study, fiber cell morphologies, chemical compositions, lignin structures, and carbohydrate contents of areca nut husks were analyzed and compared with those of rice straw, and the application potentials of these two materials as biomass resources were compared. We found that areca nut husk fibers were shorter and wider than those of rice straw; areca nut husk contained more lignin and less ash, as well as less holocellulose than rice straw; areca nut husk and rice straw lignin were obtained by ball milling and phase separation, and areca nut husk lignin was found to be a typical GHS-type lignin. Herein, the yield of lignocresol was higher than that of milled wood lignin for both raw materials, and the molecular size was more homogeneous. Tricin structural monomers were discovered in the lignin of areca nut husk, similar to those present in other types of herbaceous plants. Structures of areca nut husk MWL (AHMWL) and AHLC were comprehensively characterized by quantitative NMR techniques (that is, 1H NMR, 31P NMR, and 2D NMR). The molecular structure of AHLC was found to be closer to the linear structure with more functional groups exposed on the molecular surface, and the hydroxyl-rich p-cresol grafting structure was successfully introduced into the lignin structure. In addition, the carbohydrate content in the aqueous layer of the phase separation system was close to the carbohydrate content in the raw material, indicating that the phase separation method can precisely separate lignin from carbohydrates. These experimental results indicate that the phase separation method as a method for lignin utilization and structure study has outstanding advantages in lignin structure regulation and yield, and areca nut husk lignin is suitable for application in the same phase separation systems as short-period herbs, such as rice straw and wheat grass, and has the advantages of low ash content and high lignification degree, which will provide guidance for the high-value utilization of areca nut husk in the future.

Statistical evaluation of DPPH, ABTS, FRAP, and Folin-Ciocalteu assays to assess the antioxidant capacity of lignins

This research studies in detail four different assays, namely DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), FRAP (ferric ion reducing antioxidant potential) and FC (Folin-Ciocalteu), to determine the antioxidant capacity of standard substances as well as 50 organosolv lignins, and two kraft lignins. The coefficient of variation was determined for each method and was lowest for ABTS and highest for DPPH. The best correlation was found for FRAP and FC, which both rely on a single electron transfer mechanism. A good correlation between ABTS, FRAP and FC, respectively, could be observed, even though ABTS relies on a more complex reaction mechanism. The DPPH assay merely correlates with the others, implying that it reflects different antioxidative attributes due to a different reaction mechanism. Lignins obtained from paulownia and silphium have been investigated for the first time regarding their antioxidant capacity. Paulownia lignin is in the same range as beech wood lignin, while silphium lignin resembles wheat straw lignin. Miscanthus lignin is an exception from the grass lignins and possesses a significantly higher antioxidant capacity. All lignins possess a good antioxidant capacity and thus are promising candidates for various applications, e. g. as additives in food packaging or for biomedical purposes.

Exploring how lignin structure influences the interaction between carbohydrate-binding module and lignin using AFM

Nonproductive adsorption of cellulase onto the residual lignin in substrate seriously hinders the enzymatic hydrolysis. To understand how lignin structure affects lignin-cellulase interaction, the carbohydrate-binding module (CBM) functionalized atomic force microscope tip was used to measure CBM-lignin interaction by single-molecule dynamic force spectroscopy in this work. The results showed that sulfonated lignin (SL) has the greatest adhesion force to CBM (4.74 nN), while those of masson pine milled wood lignin (MWL), poplar MWL and herbaceous MWLs were 2.85, 1.03 and 0.27–0.61 nN, respectively. It provides direct quantitative evidence for the significance of lignin structure on lignin-cellulase interaction. The CBM-MWLs interaction decreased sharply to 0.054–0.083 nN while SL was added, indicating the primary mechanism of SL promoting lignocellulose hydrolysis was significantly reducing the nonproductive adsorption of substrate lignin on cellulase. Finally, the “competitive adsorption” mechanism was proposed to interpret why SL effectively promotes the enzymatic hydrolysis of lignin-containing substrates.

Strong, flexible and UV-shielding composite polyvinyl alcohol films with wood cellulose skeleton and lignin nanoparticles

The development of high-performance composite films using biomass materials have become a sought-after direction. Herein, a green method to fabricate strong, flexible and UV-shielding biological composite film from wood cellulose skeleton (WCS), lignin nanoparticles (LNPs) and polyvinyl alcohol (PVA) was described. In the work, WCS and LNPs were prepared by chemical treatment of wood veneer and Enzymatic lignin, respectively. Then, WCS was infiltrated with the LNPs/PVA mixtures and dried to obtain composite films. WCS enhanced the mechanical properties of the composite films, the tensile stress reached to 85.8 MPa and the tensile strain reached to 6.39 %. The composite films with LNPs blocked over 98 % of UV-light, the water absorption decreased by 30 %, and the thermal stabilities were also improved. These findings would provide some references for exploring high quality biological composite films.

Application of Wavelength Selection Combined with DS Algorithm for Model Transfer between NIR Instruments

This study aims to realize the sharing of near-infrared analysis models of lignin and holocellulose content in pulp wood on two different batches of spectrometers and proposes a combined algorithm of SPA-DS, MCUVE-DS and SiPLS-DS. The Successive Projection Algorithm (SPA), the Monte-Carlo of Uninformative Variable Elimination (MCUVE) and the Synergy Interval Partial Least Squares (SiPLS) algorithms are respectively used to reduce the adverse effects of redundant information in the transmission process of the full spectrum DS algorithm model. These three algorithms can improve model transfer accuracy and efficiency and reduce the manpower and material consumption required for modeling. These results show that the modeling effects of the characteristic wavelengths screened by the SPA, MCUVE and SiPLS algorithms are all greatly improved compared with the full-spectrum modeling, in which the SPA-PLS result in the best prediction with RPDs above 6.5 for both components. The three wavelength selection methods combined with the DS algorithm are used to transfer the models of the two instruments. Among them, the MCUVE combined with the DS algorithm has the best transfer effect. After the model transfer, the RMSEP of lignin is 0.701, and the RMSEP of holocellulose is 0.839, which was improved significantly than the full-spectrum model transfer of 0.759 and 0.918.

Physical and Morphological Changes in Heat-Treated and Densified Fast-Growing Timber Material

Heat treatment is a modification method that can alter the polymeric components of wood (cellulose, hemicellulose and lignin). Densification technology has emerged as one of the promising technologies capable of improving the properties of low-density wood. In this study, the effects of heat treatment and densification on moisture content, density, and morphological features of low-density Paraserianthes falcataria laminas were examined. Laminas were heat-treated (100°C, 120°C and 140°C for 1 hour) and compressed at 50% compression ratio. Non-heat-treated laminas, on the other hand, were compressed at 40-60% compression ratios. The changes in pores area, moisture content and density of the heat-treated and densified laminas were identified. The lowest moisture content for heat-treated laminas was at 120°C. Laminas with 60% compression ratio were observed to have the highest deformed pores, where it increased the density of the laminas. In summary, heat treatment and densification affected the properties of the laminas. Heat treatment at high temperatures resulted in decreased density and moisture content, while increasing the compression ratio during densification increased the density. The results suggest that combining heat treatment and densification could be a viable method for improving the properties of low-density wood.

Visualisation of sulphur on single fibre level for pulping industry

In the pulp and paper industry, about 5 Mt/y chemithermomechanical pulp (CTMP) are produced globally from softwood chips for production of carton board grades. For tailor making CTMP for this purpose, wood chips are impregnated with aqueous sodium sulphite for sulphonation of the wood lignin. When lignin is sulphonated, the defibration of wood into pulp becomes more selective, resulting in enhanced pulp properties. On a microscopic fibre scale, however, one could strongly assume that the sulphonation of the wood structure is very uneven due to its macroscale size of wood chips. If this is the case and the sulphonation could be done significantly more evenly, the CTMP process could be more efficient and produce pulp even better suited for carton boards. Therefore, the present study aimed to develop a technique based on X-ray fluorescence microscopy imaging (µXRF) for quantifying the sulphur distribution on CTMP wood fibres. Firstly, the feasibility of µXRF imaging for sulphur homogeneity measurements in wood fibres needs investigation. Therefore, clarification of which spatial and spectral resolution that allows visualization of sulphur impregnation into single wood fibres is needed. Measurements of single fibre imaging were carried out at the Argonne National Laboratory’s Advanced Photon Source (APS) synchrotron facility. With a synchrotron beam using one micrometre scanning step, images of elemental mapping are acquired from CTMP samples diluted with non-sulphonated pulp under specified conditions. Since the measurements show significant differences between sulphonated and non-sulphonated fibres, and a significant peak concentration in the shell of the sulphonated fibres, the proposed technique is found to be feasible. The required spatial resolution of the µXRF imaging for an on-site CTMP sulphur homogeneity measurement setup is about 15 µm, and the homogeneity measured along the fibre shells is suggested to be used as the CTMP sulphonation measurement parameter.

Investigation of the Interaction Mechanism between Lignin Structural Units and Enzyme

The effect of lignin structural units on enzymatic hydrolysis of lignocellulosic biomass was investigated, especially the inhibitory role of lignin in non-productive adsorption with enzymes. Milled wood lignin (MWL) was isolated from different hardwoods of poplar, eucalyptus and acacia. The isolated lignin samples were characterized by elemental analysis, gel permeation chromatography, nitrobenzene oxidation and fourier infrared spectroscopy. The mechanism of lignin structural units on enzymatic hydrolysis of cellulose was studied by quartz crystal microbalance (QCM). The results showed that different structural units of lignin had different adsorption capacity for enzymes. The results of nitrobenzene oxidation indicated that the S/G ratio (S: syringyl-like lignin structures; G: guaiacyl-like lignin structures) of lignin of poplar was 0.99, that of eucalyptus was 1.92 and that of acacia was 1.34. According to the results of QCM, the adsorption capacity of the three lignin films was as follows: Poplar MWL (S/G ratio 0.99) < Acacia MWL (S/G ratio 1.34) < Eucalyptus MWL (S/G ratio 1.92). Eucalyptus MWL with higher degree of condensation and S/G ratio showed stronger affinity to enzymes and more non-productive adsorption with enzymes, resulting in less adsorption between enzymes and cellulose, and lower enzymatic hydrolysis efficiency.

Enhanced depolymerization of beech wood lignin and its removal with peroxidases through continuous separation of lignin fragments

A study highlighting the benefits associated with the continuous separation of MnP- (or LiP-) depolymerized lignin fragments from the reaction medium using a membrane bioreactor.

Cas9/gRNA-Mediated Mutations in PtrFLA40 and PtrFLA45 Reveal Redundant Roles in Modulating Wood Cell Size and SCW Synthesis in Poplar

Fasciclin-like arabinogalactan proteins (FLAs) play an important role in plant development and adaptation to the environment. However, the roles of FLAs in wood formation remain poorly understood. Here, we identified a total of 50 PtrFLA genes in poplar. They were classified into four groups: A to D, among which group A was the largest group with 28 members clustered into four branches. Most PtrFLAs of group A were dominantly expressed in developing xylem based on microarray and RT-qPCR data. The roles of PtrFLA40 and PtrFLA45 in group A were investigated via the Cas9/gRNA-induced mutation lines. Loss of PtrFLA40 and PtrFLA45 increased stem length and diameter in ptrfla40ptrfla45 double mutants, but not in ptrfla40 or ptrfla45 single mutants. Further, our findings indicated that the ptrfla40ptrfla45 mutants enlarged the cell size of xylem fibers and vessels, suggesting a negative modulation in stem xylem cell size. In addition, wood lignin content in the ptrfla40fla45 mutants was increased by nearly 9%, and the lignin biosynthesis-related genes were significantly up-regulated in the ptrfla40fla45 mutants, in agreement with the increase in wood lignin content. Overall, Cas9/gRNA-mediated mutations in PtrFLA40 and PtrFLA45 reveal redundant roles in modulating wood cell size and secondary cell wall (SCW) synthesis in poplar.

Exploring Alkyl-O-Alkyl Ether Structures in Softwood Milled Wood Lignins.

Recent studies have suggested that there are significant amounts of various alkyl ether (Alk-O-Alk; Alk = alkyl) moieties in a spruce native lignin preparation, milled wood lignin (SMWL). However, the comprehensive NMR assignment to these moieties has not been addressed yet. This study focused on investigating different types of Alk-O-Alk structures at the α- and γ-positions of the lignin side chain in an heteronuclear single-quantum coherence (HSQC) spectrum of SMWL using experimental NMR data of lignin and synthesized model compounds. Ambiguous structural features were predicted by computer simulation of 1H and 13C NMR spectra to complement the experimental NMR data. As a result, specific regions in the HSQC spectrum were attributed to different Alk-O-Alk moieties of Alk-O-Alk/β-O-4 and Alk-O-Alk/β-β' structures. However, the differences between the specific regions were rather subtle; they were not well separated from each other and some major lignin moieties. Furthermore, SMWL contained a large variety of Alk-O-Alk moieties but in minute individual amounts, resulting in rather broad, superimposing resonances. Thus, evaluation did not allow assigning individual types of Alk-O-Alk moieties from the HSQC spectra; instead, they were quantified as total (α- and γ-linked) Alk-O-Alk based on the balance of structural units in the 13C NMR spectra. At last, potential formation mechanisms of various Alk-O-Alk ether structures in lignin biosynthesis, lignin aging, and during ball milling of wood were hypothesized and discussed.

Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp

The residual lignin content of unbleached maritime pine (Pinus pinaster Aiton) Kraft pulps was assessed by analytical pyrolysis (Py-lignin) and the results were compared to the Klason lignin content and kappa number. Thirty samples, each from an individual tree, were delignified under identical conditions. The residual lignin content of the pulps varied widely as assessed by Py-lignin (5.9%–9.2%), Klason (8.2–15.1), and kappa number (59–112). Despite a systematic difference between Py-lignin and Klason, they were strongly correlated (R2 = 0.90). The H/G ratio of the residual pulp lignin ranged from 0.145 to 0.195, with a mean of 0.165, which is more than two times the average H/G ratio of Maritime pine wood lignin (0.064). The results show that Kraft pulping, which selectively degrades lignin with more labile inter-unit links, changed the pattern of pyrolysis products of pulp lignin considerably and, consequently, its structure. This pattern shows an enrichment in H-lignin-derived products, namely phenol, p-cresol, and m-cresol, and in some G-lignin-derived products such as guaiacol and 4-mthylguaiacol, and a decrease in coniferylaldehyde, homovanillin, and eugenol. Principal component analysis (PCA) of the G- and H-lignin-derived pyrolysis products shows that pulps are distributed along PC 1 based on their residual lignin content. The loadings plot shows that this separation is mainly due to a small number of G-lignin products, including 4-methyl guaiacol, 4-vinyl guaiacol, isoeugenol (trans), and guaiacol, which are more abundant in pulps with higher residual lignin content. The obtained results show that analytical pyrolysis is an appropriate method for quantifying the residual lignin content and H/G ratio of unbleached Kraft pulps and provide information regarding how lignin is degraded during the pulping process.

Thermal characterization of kraft lignin prepared from mixed hardwoods

Thermal characterization of kraft lignin (KL) prepared from mixed hardwoods was conducted to determine their thermal and thermo-oxidative stability, glass transition temperature (Tg), pyrolysis products composition, and syringyl/guaiacyl ratio. Two milled wood lignins (MWLs) from acacia (MWL-aca) and mixed hardwoods (MWL-mhw), the same hardwood species employed in the production of KL, were also characterized for comparison with KL. Thermogravimetric analysis (TGA) indicated that KL had higher thermal and thermo-oxidative stability than MWLs. Based on differential scanning calorimetry (DSC), the Tg of KL was 63.2 °C, while MWLs gave similar values near 165 °C. The syringyl (S), guaiacyl (G), p-hydroxyphenyl (H) composition of KL acquired from pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) at 675 °C was 48:44:8 (S/G ratio 1.07), while MWL-aca and MWL-mhw were 31:60:9 and 46:48:6, respectively.

Altitude-specific differences in tree-ring δ2H records of wood lignin methoxy in the Qinling mountains, central China

The Qinling Mountains (Qinling Mts.) are characterized as a distinct transition of bioclimatic zones along the north-south geographic boundary in China. Although ongoing global warming may affect the growth of most tree species across elevation gradients, the effect of modern warming on the stable hydrogen isotopes of tree-ring wood is still unknown. In this study, we developed early- and late-wood δ2H chronologies from 1900 to 2018 CE of lignin methoxy groups (δ2HLM) at 700-m intervals from 1700 to 3100 m a.s.l. along the elevation gradient in Mount Taibai (Mt. Taibai), the peak of the Qinling Mts. The δ2HLM values of the earlywood were relatively enriched in 2H compared to the latewood, and an elevation-dependent depleting trend along the altitudinal gradient was observed. Comparing the δ2HLM values with modelled δ2H values of precipitation (δ2HPre) of the sample sites, similar isotopic patterns can be obtained. The reconstructions of δ2HPre values confirm the strong coherence between the isotopic composition of the source water and the methoxy groups in the growing season. The consistency of δ2HLM chronologies between the early- and late-wood at the higher sites was stronger than that at the lower sites. The additional lagged effect and climate response revealed significant seasonal and altitudinal differences. At the timberline site, δ2HLM values of earlywood were mainly related to the January and February temperatures, whereas δ2HLM values of latewood correlated most strongly with temperature from July to September, and both values were controlled by the amount of total precipitation. In addition, the combined new δ2HLM chronologies, based on the lag-1 autocorrelation and the weighted average of the current year earlywood and previous latewood, could be used to merge the corresponding temperature signals from the middle- and low-elevation sites. Altitudinal correlations of elevation offset between the annual δ2HLM chronologies and interpolations of precipitation and temperature suggested that the δ2HLM values at higher elevations better represent regional climatic changes. Therefore, topographic effects such as elevation differences should be considered in large-scale applications of δ2HLM values in future studies.

Synergy of ball milling, microwave irradiation, and deep eutectic solvents for a rapid and selective delignification: walnut shells as model for lignin-enriched recalcitrant biomass

The combination of ball milling (BM), microwave irradiation (MI), and deep eutectic solvents (DES) results synergistic for an efficient, selective, and very rapid (10 min) delignification of materials with high lignin content (ca. 50 wt%) such as walnut shells (WS). Lignin is dissolved in the DES, whereas the polysaccharide fractions remain suspended with limited degradation, due to the rapid pretreatment. After ball milling procedure (3 h), biomass loadings in the range of 100–200 g L−1 are selectively delignified in 10 min at 150 °C by using choline chloride:formic acid DES (1:2 molar ratio), rendering lignin yields of 60–80% (ca. ~ 40–60 g lignin L−1). Ball milling, microwave irradiation, and DES systems are much more efficient than ball milling, conventional heating, and DES system. The obtained lignins exhibited similar Fourier-transform infrared spectroscopy (FTIR) profile to that of milled wood lignin (MWL), indicating minimal functional group changes.Graphical abstract

The role of acetone-fractionated Kraft lignin molecular structure on surface adhesion to formaldehyde-based resins

One of the key strategies for valorizing kraft lignin (KL) into value-added products such as bio-based adhesives is to perform solvent fractionation of KL to produce lignin with improved homogeneity. Understanding the structure and properties of fractionated KL will aid in the selection of the best samples for certain applications. In this study, acetone-fractionated KL from softwood and hardwood was characterized to understand its chemical structure, elemental composition, molecular weight, and thermal properties. The results revealed that acetone-insoluble KL (AIKL) fractions from softwood and hardwood have greater molecular weight, polydispersity, glass temperature, carbohydrate content, aliphatic hydroxyl groups, and a variety of native wood lignin side chains. In contrast, acetone-soluble KL (ASKL) fractions have a significantly lower molecular weight and polydispersity, a lower glass-transition temperature, a more condensed structure, more aromatic hydroxyl groups, and fewer native wood lignin side chains. In addition, the ASKL samples demonstrated stronger adhesive force and work of adhesion toward phenol–formaldehyde (PF) and urea–formaldehyde (UF) resins than the AIKL samples, regardless of the lignin source. These findings suggest that ASKL has great potential as a substitute for phenol in PF resins and as a green additive to reinforce UF resins.