Milled Wood Lignin Research Articles

A new insight of lignin pyrolysis characteristics based on dehydrogenation polymers (DHPs)

Understanding the pyrolysis mechanism of native lignin is essential for improving the conversion efficiency of fast pyrolysis technology. However, due to the complex and various nature of lignin interunit linkages, the pyrolysis mechanism of lignin has not been clearly revealed so far, especially for native lignin. In this work, two types of guaiacyl-dehydrogenation polymers (G-DHPs) were synthesized using two different methods in vitro, namely endwise polymerization (G-DHPs-endwise) and bulk polymerization (G-DHPs-bulk), which were used as native lignin models. The chemical structures of the synthesized G-DHPs along with natural G-lignin (i.e., milled wood lignin, MWL) were analyzed by using 2D HSQC NMR, and their pyrolysis characteristics were investigated by using a fixed bed reactor. In comparison with G-DHPs-bulk, G-DHPs-endwise had more similar structure and pyrolysis characteristics with that of MWL, which suggested that G-DHPs-endwise was a more suitable model for G-type lignin. Further mechanical study revealed that the pyrolysis of G-DHPs-endwise underwent an end-breaking pathway, by which the branched carbon of the coniferyl alcohol structure ends were cleaved at the early stage to depolymerize into guaiacol. This study can thus enhance the understanding of the thermal depolymerization of native lignin and provide the evidence for revealing pyrolysis mechanism.

Fast pyrolysis of guaiacyl-syringyl (GS) type milled wood lignin: Product characteristics and CH4 formation mechanism study

It is anticipated that the insight into the demethylation and mechanism of CH4 formation from natural lignin using in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ FTIR) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) and density functional theory (DFT) calculation analysis would contribute to a deeper insight of bond cleavage mechanism of lignin pyrolysis. Herein, GS-type lignin (poplar MWL) was characterized by Fourier transform infrared spectroscopy (FTIR) and heteronuclear Single-Quantum Correlation Nuclear Magnetic Resonance (HSQC), and its pyrolysis at different temperatures was performed in a lab-scale fixed-bed reactor. The biochar, gaseous and liquid products were qualitative, and quantitative analysis of gases and bio-oil is demonstrated using gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). The key of CH4 formation is the homolytic cleavage of the methoxyl functional group generating methyl radical and further verified via in-situ FTIR combined with 2D-PCIS and DFT calculation. The study established a new methodology based on multiple factor analysis to evaluate the CH4 formation mechanism in GS-type milled wood lignin at the molecular level, which is of positive significance for increasing lignin valorization and improving the environment.

Structural characterization of lignin and lignin-carbohydrate complex (LCC) of sesame hull

In the present study, lignin and lignin-carbohydrate complex (LCC) constituting the cell wall structure of sesame hulls were investigated to explore novel techniques of dehulling. Milled wood lignin (MWL), Björkman LCC, and acid-soluble LCC (LCC-AcOH) were extracted from sesame hulls and characterized by carbohydrate composition analysis, molecular weight analysis, UV–vis spectroscopy, FT-IR, thermal analysis, Py-GC/MS, 2D HSQC NMR, and 31P NMR. The results showed that rhamnose accounted for the largest proportion of the lignin and LCC fractions, followed by glucose. Björkman LCC had the largest molecular weight, MWL had the smallest molecular weight, and LCC-AcOH had the largest polydispersity index. The lignin of sesame hulls consisted of syringyl (S), guaiacyl (G), p-hydroxyphenyl (H), and caffeyl alcohol (C) units. The most abundant monomer was guaiacyl (G), followed by caffeyl alcohol (C). C-type lignin is a new type of lignin that is different from the three traditional lignin monomers. The major lignin-linked bonds in the MWL and LCC-AcOH were β-O-4′ and β-β′, and β-5′ bonds were present in the Björkman LCC. The major LCC chemical bonds in the three fractions were PhGly. These findings will provide the factual basis for exploring different dehulling methods to enhance the quality of sesame products.

Deciphering the linkage type and structural characteristics of the p-hydroxyphenyl unit in Pinus massoniana Lamb compressed wood lignin

To reveal the existence of p-hydroxyphenyl (H) units in compressed wood lignin, four different milled wood lignins were extracted using Pinus massoniana Lamb compressed wood, Pinus massoniana Lamb normal wood, and sugarcane bagasse as raw materials. Then, three dehydrogenation polymers (DHPs) were synthesized using coniferyl/p-coumaryl alcohol as raw materials to reveal the interunit linkages of H units. The lignin and DHP samples were systematically characterized by 1H, 13C, 2D HSQC, and 31P NMR techniques. Compared with the opposite wood milled wood lignin (OW-MWL) and the normal wood milled wood lignin (NW-MWL), the compressed wood milled wood lignin (CW-MWL) contained a large amount of H units, and the H/G ratio and the p-hydroxyphenyl OH group contents were 0.15 and 1.09 mmol/g, respectively. Through the characterization of CW-MWL and DHPs, it was revealed that p-hydroxyphenyl units mainly coupled with other units by β-O-4, β-β, and β-5 linkages. Compared to the sugarcane bagasse milled wood lignin, it was clearly demonstrated that the H unit rather than p-coumarate ester occurred in CW-MWL. This study comprehensively explored the structural characteristics and linkages of H units in compress wood lignin, and provided useful information for revealing the participation of H units in the construction of lignin macromolecules.

Revealing the sodium storage behavior of biomass-derived hard carbon by using pure lignin and cellulose as model precursors

Lignin and cellulose are dominant components in biomass and hold the key for preparing hard carbons. Identifying the sodium storage behaviors of sole lignin/cellulose-derived hard carbons is significant for choosing optimal biomass precursors. Herein, milled-wood lignin and microcrystalline cellulose are used as model precursors to prepare hard carbons and the corresponding sodium storage performances are investigated to understand the contribution of each biomass component. Compared with lignin-derived carbon, cellulose-derived carbon enables a larger initial Coulombic efficiency of 87.1%, a higher reversible capacity of 343.3 mA h g−1 at 0.02 A g−1 and a good rate capability of 49.2 mA h g−1 at 1 A g−1 owing to larger La, lower ID/IG values and higher sp2C, CO contents with the benefit of enhancing the conductivity, plateau capacity, and the rapid diffusion of sodium ions. The excellent performance of cellulose-derived carbon provides guidance on the choice of biomass precursors for high-performance sodium-ion batteries.

A modified ionization difference UV–vis method for fast quantitation of guaiacyl-type phenolic hydroxyl groups in lignin

An ionization difference UV–Vis method (Δε-spectrum method) is the most potentially simple method for fast quantitation of phenolic hydroxyl groups (ph-OH) in lignin. However, the underestimated results were calculated from the conventional Δε-spectrum method using one- or two-point wavelengths measurement. In this study, a modified Δε-spectrum method using multi-point wavelengths measurement was developed and the negative absorbance was also considered. Four main typical lignin models, e.g. vanilla alcohol, 5-5 biphenyl, stilbenoid and vanillin, were applied as the guaiacyl-type (G-type) phenolic models for the determination of ph-OH by the modified Δε-spectrum method. The 2-methoxyethanol/water/acetic acid = 8/2/0.2 (V/V/V) was used as the acidic solvent system and the 2-methoxyethanol/0.2 M NaOH solution = 1/9 (V/V) was used as the alkaline solvent system. The ph-OH contents in the spruce milled wood lignin (SMWL) and the spruce Kraft lignin (SKL) were respectively quantified by the modified Δε-spectrum method as 1.078 and 4.348 mmol/g, which were comparable to the counterparts determined by 31P Nuclear Magnetic Resonance Spectroscopy (31P NMR). The results revealed that the modified Δε-spectrum method can provide more accurate and reliable results compared to the conventional method.

Structural analysis of light-colored separated lignin (lignocresol) and its antioxidant properties

The use of lignin is limited by its heterogeneity and complexity. This study was processed using different methods to obtain spruce milled wood lignin (SMWL), spruce kraft lignin (SKL), and spruce lignocresol (SLC) for comparative analysis of the structure and antioxidant activity. SMWL has a complete softwood lignin side-chains structure and lignin carbohydrate complexes. SKL contains fewer ether bonds, while more conjugate structures and condensed structures contribute to the color. However, the α-position of the lignin side chain eliminates most of the hydroxyl and ether bonds (β-O-4/α-OH, phenylcoumaran, and dibenzodioxocine structure) and effectively grafts p-cresol in the phase separation reaction. It not only inhibits the self-condensation of lignin, but also forms the 1,1-diarylpropane unit while protecting β-O-4 linkages from not breaking. Importantly, SLC has few conjugate structures that result in the lightest color among all lignin isolated. Besides, SLC has a high yield and contains trace carbohydrates, indicating that the phase separation method can achieve great amounts of purity separated lignin. The antioxidant activity of lignin was evaluated, results show that 85% of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals were scavenged at the end of 60 min. Owing to its unique color, structural properties, and continuous antioxidant activity, SLC has the potential to manufacture antioxidant cosmetics.

The Effect of Ball Milling Time on the Isolation of Lignin in the Cell Wall of Different Biomass.

Ball milling technology is the classical technology to isolate representative lignin in the cell wall of biomass for further investigation. In this work, different ball milling times were carried out on hardwood (poplar sawdust), softwood (larch sawdust), and gramineous material (bamboo residues) to understand the optimum condition to isolate the representative milled wood lignin (MWL) in these different biomass species. Results showed that prolonging ball milling time from 3 to 7 h obviously increased the isolation yields of MWL in bamboo residues (from 39.2% to 53.9%) and poplar sawdust (from 15.5% to 35.6%), while only a slight increase was found for the MWL yield of larch sawdust (from 23.4% to 25.8%). Importantly, the lignin substructure of ß-O-4 in the MWL samples from different biomasses can be a little degraded with the increasing ball milling time, resulting in the prepared MWL with lower molecular weight and higher content of hydroxyl groups. Based on the isolation yield and structure features, milling time with 3 and 7 h were sufficient to isolate the representative lignin (with yield over 30%) in the cell wall of bamboo residues and poplar sawdust, respectively, while more than 7 h should be carried out to isolate the representative lignin in larch sawdust.

Synergistic effects during co-pyrolysis of milled wood lignin and polyolefins at the gas phase and liquid/solid phase contacting modes

Co-pyrolysis of biomass and waste plastics is the preferred technology for enhancing the production of fuels and valuable chemicals. Here, we studied the co-pyrolysis of milled wood lignin (MWL) with polyethylene (PE) and polypropylene (PP) at various weight ratios by thermogravimetry (TG), evolved gas analysis–mass spectrometry (EGA–MS), and product recovery tests using a fixed-bed reactor. Two reaction modes, liquid/solid phase (LSP) and gas phase (GP) contacting modes, were applied to investigate the physical prevention of melted polymers on the interactions between the vapored intermediates. The TG results showed that the weight losses were slightly altered in the MWL/PE and the decomposition was delayed in MWL/PP blends, whereas the EGA-MS analysis indicated that more vaporized products from the blends were detected by MS. The fixed-bed experiments showed that the yield of liquid was largely enhanced by the higher proportion of PE or PP in the blends, primarily due to the formation of lower oligomeric hydrocarbons from polyolefins. The yields of gasoline and kerosene were significantly increased in the GP mode from the co-pyrolysis of MWL with PE and PP, respectively. These results suggest that the MWL pyrolysates could enhance the cracking of PE intermediates in the GP reaction mode, whereas the rearrangement reactions of PP intermediates may be inhibited in the LSP reaction mode, converting more wax products into liquid. This paper provides a better understanding of the detailed synergistic effects during biomass and plastics co-pyrolysis.

A feasibility study on green biorefinery of high lignin content agro-food industry waste through supercritical water treatment

This work discusses hydrolysis of defatted grape in supercritical water (SCW) at 380 °C and 260 bar from 0.18 s to 1 s focusing attention to sugars recovery in the liquid phase of the product and detailed characterization of remaining solid phase enriched in polyaromatics (e.g. lignin, flavonoids, etc.). After the longest reaction time of 1 s, 56% of carbohydrates could be recovered in the liquid phase, as a result of carbohydrate hydrolysis. The high content of insoluble lignin in biomass (36%), acts as a mass transfer limitation and presents an important feature in the hydrolysis process, slowing down the conversion of carbohydrate fraction, as after the maximum time of 1s, 10% of carbohydrates still remained in the solid phase. Milled wood lignin, extracted from biomass and dioxane extract from the solid phase were characterized in order to understand the main structural changes during the SCW hydrolysis process. Dioxane (80%) extraction of solids produces a very complex mixture of lipophilic extractives, flavonoids and lignin with a certain amount of chemically linked carbohydrates. 2D NMR analysis of dioxane extract shows remarkably subtle changes in the amounts of main lignin moieties (β-O-4′, β-β’ (resinol) and β-5 (phenylcoumaran)). This subtle change of the main lignin structures is an important feature in the further valorisation of this sulfur-free lignin residue.

Catalytic Hydrogenolysis of Lignin: The Influence of Minor Units and Saccharides.

The precise elucidation of native lignin structures plays a vital role for the development of “lignin first” strategies such as reductive catalytic fractionation. The structure of lignin and composition of the starting material has a major impact on the product yield and distribution. Here, the differences in structure of lignin from birch, pine, reed, and walnut shell were investigated by combining detailed analysis of the whole cell wall material, residual enzyme lignin, and milled wood lignin. The results of the 2D heteronuclear single quantum coherence NMR analysis could be correlated to the product from Ru/C‐catalyzed hydrogenolysis if monomeric products from ferulate and p‐coumaryl and its analogous units were also appropriately considered. Notably, residual polysaccharide constituents seemed to influence the selectivity towards hydroxy‐containing monomers. The results reinforced the importance of adequate structural characterization and compositional analysis of the starting materials as well as distinct (dis)advantages of specific types of structural characterization and isolation methods for guiding valorization potential of different biomass feedstocks.

Unmasking radical-mediated lignin pyrolysis after benzyl hydroxyl shielding

Whether lignin benzyl hydroxyl shielding could promote its pyrolysis to phenolic compounds was investigated in this paper. Lignin benzyl hydroxyl was first preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and stabilized by propionaldehyde respectively, then pyrolysis was conducted with milled wood lignin as a control. Organic stable radicals in pyrolytic chars were further detected to reveal lignin pyrolysis chemistry. Results showed that benzyl hydroxyl shielding process weakened lignin thermal stability, and decreased liquid yields regardless of the frequency of lignin β-O-4 linkages. In addition, char yield grew after benzyl hydroxyl shielding. Radical concentration was inversely proportional to β-O-4 content which indicated the non-negligible impact of shielded benzyl hydroxyl on lignin pyrolysis. Furthermore, gases from propionaldehyde stabilized lignin quenched its radicals. This work confirmed that lignin β-O-4 linkages and shielded benzyl hydroxyl both played the great role in radical-mediated pyrolysis, but the enhancement of liquid products could not be achieved via benzyl hydroxyl shielding.

Separation and characterization of biomass components (cellulose, hemicellulose, and lignin) from corn stalk

Representative hemicellulose, milled wood lignin (MWL), and cellulose were directly separated from corn stalk, and their main chemical content was determined using NREL methods. The chemical elements, chemical groups, and molecular structure of corn stalk biomass components (hemicellulose, MWL, and cellulose) were analyzed by elemental analysis, Fourier transform infrared, and nuclear magnetic resonance spectroscopy analyses．The results showed that the purity of the biomass components separated from corn stalk was high, the degree of damage was relatively small, and their own structural characteristics were relatively intact. The hemicellulose that was separated from corn stalk was mainly composed of L-arabino-β-(1→4)-D-glucuronoxylan units. There were also sugar residues attached to the main chain in the form of side chains, such as D-glucopyranose, galactose, glucuronic acid, and galacturonic acid. The isolated cellulose consisted of glucosyl linked by β-(1→4)-glucosidic bond. The MWL separated from corn stalk has a GSH-type of β-O-4 structure, and the contents were as follows, in order of more to less: guaiacyl (G), p-hydroxyphenyl (H), and syringyl (S) units. Biomass components with high purity were separated from corn stalk, and their respective structure and composition were understood, which provides a foundation for the subsequent high-value utilization of corn stalk.

Lignin Resists High-Intensity Electron Beam Irradiation.

The electron beam irradiation (EBI) of native lignin has received little attention. Thus, its potential use in lignin-based biorefineries is not fully understood. EBI was applied to selected lignin samples and the structural and chemical changes were analyzed, revealing the suitability, limitations, and potential purpose of EBI in wood biorefineries. Isolated milled wood, kraft, and sulfite lignin from beech and eucalyptus were subjected to up to 200 kGy of irradiation. The analysis included gel permeation chromatography for molar masses, heteronuclear single quantum coherence (HSQC)- and 31P NMR and headspace gas chromatography-mass spectrometry for functional groups, and thermogravimetric analysis for thermal stability. Most samples resisted irradiation. Subtle changes occurred in the molecular weight distribution and thermal stability of milled wood lignin. EBI was found to be a suitable pretreatment method for woody biomass if the avoidance of lignin condensation and chemical modification is a high priority.

Extraction of Milled Wood Lignin from Coffee Husk (Coffea Arabica L.) and the Analysis of Its Potential as a UV-Protective Component of Lotion and Sunscreen

This study aims to determine the effect of adding milled wood lignin (MWL) to the sun protection factor (SPF) of commercially available lotion and sunscreen. The MWL was isolated from Coffea Arabica L. husks using the Björkman method. FT-IR spectroscopic analysis showed two characteristic peaks located at around 1200 cm-1 and 1300 cm-1, which indicates the presence of guaiacyl and syringyl groups. UV-Vis spectroscopic analysis showed two peaks located at around 278 nm and 325 nm, which indicates the presence of guaiacyl and some conjugated phenolic groups, respectively. The isolated MWL was then added to commercially available sunscreen and lotion at various concentrations. The sun protection factor (SPF) was then determined using the Mansur method and UV-Vis Spectroscopy. Results have shown that from 0% to 40 % by mass of MWL added, there was an observable average increase from 0.90 to 13.86 SPF for lotion, while there was an increase from 26.47 to 34.00 SPF for sunscreen. Results suggest that for lotion the SPF significantly increased if MWL was added from 0% up to 40% while for sunscreen the SPF significantly increased if MWL was added from 0% up to 20%.

Comparison of structure, thermal stability, and pyrolysis products of lignin extracted with ChCl-formic acid/lactic acid systems

The selection of pretreatment conditions has important meaning for the high-value utilization of lignocellulose. This study explored the influence of different acid-deep eutectic solvent (DES) systems on the extraction of lignin. It was found that the lignin samples extracted with ChCl (Choline chloride)-formic acid (FDES) had a greater degree of damage, and they had a lower molecular weight (Mw < 2000) than milled wood lignin (MWL). The side-chain structure of lignin was also damaged, but the thermal stability of the lignin sample was better than that extracted with ChCl-lactic acid (ADES), and the pyrolysis products mostly came from the functional groups directly connected to the benzene ring. After ADES treatment, the carbohydrate content of lignin was higher (>10%) and the structure was less damaged, but with increasing reaction time, the polymerization effect of lignin became more obvious. This research is meaningful for exploring the process of acid DES separation of lignin and the green transformation of biomass.

Chemical Characterization of Kraft Lignin Prepared from Mixed Hardwoods.

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.

Revealing the Sodium Storage Behavior of Biomass-Derived Hard Carbon by Using Pure Lignin and Cellulose as Model Precursors

Lignin and cellulose are dominant components in biomass and hold the key for preparing hard carbons. Identifying the sodium storage behaviors of sole lignin/ cellulose-derived hard carbons is significant for choosing optimal biomass precursors. Herein, milled-wood lignin and microcrystalline cellulose are used as model precursors to prepare hard carbons and the corresponding sodium storage performances are investigated to understand the contribution of each biomass component. Compared with lignin-derived carbon, cellulose-derived carbon enables a larger initial coulombic efficiency of 87.1 %, a higher reversible capacity of 343.3 mA h g-1 at 0.02 A g-1 and a good rate capability of 49.2 mA h g-1 at 1 A g-1. That’s attributed to large La, IG/ID values and high sp2C, C=O contents, which enhance the conductivity, plateau capacity, and the rapid diffusion of sodium ions. The excellent performance of cellulose-derived carbon provides guidance on the choice of biomass precursors for high-performance sodium-ion battery.

Understanding the effects of different residual lignin fractions in acid-pretreated bamboo residues on its enzymatic digestibility

BackgroundDuring the dilute acid pretreatment process, the resulting pseudo-lignin and lignin droplets deposited on the surface of lignocellulose and inhibit the enzymatic digestibility of cellulose in lignocellulose. However, how these lignins interact with cellulase enzymes and then affect enzymatic hydrolysis is still unknown. In this work, different fractions of surface lignin (SL) obtained from dilute acid-pretreated bamboo residues (DAP-BR) were extracted by various organic reagents and the residual lignin in extracted DAP-BR was obtained by the milled wood lignin (MWL) method. All of the lignin fractions obtained from DAP-BR were used to investigate the mechanism for interaction between lignin and cellulase using surface plasmon resonance (SPR) technology to understand how they affect enzymatic hydrolysisResultsThe results showed that removing surface lignin significantly decreased the yield for enzymatic hydrolysis DAP-BR from 36.5% to 18.6%. The addition of MWL samples to Avicel inhibited its enzymatic hydrolysis, while different SL samples showed slight increases in enzymatic digestibility. Due to the higher molecular weight and hydrophobicity of MWL samples versus SL samples, a stronger affinity for MWL (KD = 6.8–24.7 nM) was found versus that of SL (KD = 39.4–52.6 nM) by SPR analysis. The affinity constants of all tested lignins exhibited good correlations (r > 0.6) with the effects on enzymatic digestibility of extracted DAP-BR and Avicel.ConclusionsThis work revealed that the surface lignin on DAP-BR is necessary for maintaining enzyme digestibility levels, and its removal has a negative impact on substrate digestibility.

Upgrade the torrefaction process of bamboo based on autohydrolysis pretreatment

Torrefaction is a routine and effective method for recovering energy from biomass. In this study, autohydrolysis pretreatment was proposed to upgrade the torrefaction process of moso bamboo, and their relationship was evaluated. Results showed that about 80.24 % of bamboo hemicellulose were dissolved during autohydrolysis pretreatment. Compared with main chain, arabinan side chain of hemicellulose was much easier to be degraded, and more uniform hemicellulose could be obtained at 160 °C. The O/C values of autohydrolysis solid residues and the mass yield of torrefied bamboo decreased gradually with the enhancement of reaction severity (autohydrolysis and/or torrefaction), while the HHV values increased. The higher torrefaction temperature or autohydrolysis pretreatment severity was, the better thermostability of torrefied bamboo could be observed. 2D-HSQC of milled wood lignin revealed that autohydrolysis pretreatment can effectively accelerate the degradation of β-O-4 linkage in lignin and increase G-type lignin content during torrefaction process. Compared with direct torrefaction, prehydrolysis can greatly facilitate the torrefaction process and enhance the quality of final products.