Milled Wood Lignin Research Articles

Investigation of structural modification and thermal characteristics of lignin after heat treatment

Milled wood lignin was subjected to heat treatment between 150 and 300°C to understand the pattern of its structural modification and thermal properties. When the temperature was elevated with interval of 50°C, the color of the lignin became dark brown and the lignin released various forms of phenols from terminal phenolic groups in the lignin, leading to two physical phenomena: (1) gradual weight loss of the lignin, up to 19% based on dry weight and (2) increase in the carbon content and decrease in the oxygen content. Nitrobenzene oxidation and 13C NMR analyses confirmed a cleavage of β-O-4 linkage (depolymerization) and reduction of methoxyl as well as phenolic hydroxyl group were also characteristic in the lignin structure during heat treatment. Simultaneously with lignin depolymerization, GPC analysis provided a possibility that condensation between lignin fragments could also occur during heat treatment. TGA/DTG/DSC data revealed that thermal stability of lignin obviously increased after heat treatment, implicating the structural rearrangement of lignin to reduction of β-O-4 linkage as well as accumulation of CC bonds.

Chemical Changes of Raw Materials and Manufactured Binderless Boards during Hot Pressing: Lignin Isolation and Characterization

Thermomechanical pulp (TMP) is used for fiber production in binderless boards industries. Milled wood lignin (MWL) and enzymatic mild acidolysis lignin (EMAL) isolated from raw material and from binderless boards (BB) were comparatively analyzed to investigate the effects of chemical changes on the bonding performance in BB. The results showed that acid-insoluble lignin of the BB were increased during the sodium silicate solution pretreatment after hot-pressing. The lignin fractions obtained were characterized by gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) spectroscopy, and 1H-13C correlation heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy. Results showed that 31.1% of EMAL (based on Klason lignin) with low molecular weight (Mw=1630 g/mol) was isolated from the BB. The increased total phenolic OH groups (3.97 mmol/g) of EMAL from sodium silicate solution pretreated BB indicated that there was degradation of lignin and cleavage of lignin-carbohydrate linkages during hot-pressing. In addition, the content of β-O-4' aryl ether linkages of EMAL from the BB increased to 69.2%, which was higher than that of the untreated sample (60.1%). It was found that S units (syringyl-like lignin structures) were preferentially condensed by hot pressing over G (guaiacyl-like lignin structures) units, and the S/G ratio increased after the hot-pressing process.

The structural changes of lignin and lignin–carbohydrate complexes in corn stover induced by mild sodium hydroxide treatment

Non-woody biomass such as corn stover is a very abundant and sustainable biofuel feedstock in the US whose technical hurdles for enzymatic hydrolysis have not been adequately addressed. There is very little useful data on the lignin and the lignin–carbohydrate complexes of corn stover and the impacts of them on bioconversion to fermentable sugars. The following principal tasks were addressed, which will help to develop the roadmap of effective saccharification of corn stover: (1) corn stover was separated into stem, cob, and leaf; (2) lignin (cellulolytic enzyme lignin, CEL) and lignin–carbohydrate complexes (milled wood lignin, MWLc) were isolated from the extractive-free and the alkaline-treated samples, respectively; and (3) the structural changes of lignin and lignin–carbohydrate complexes (LCCs) were characterized by alkaline nitrobenzene oxidation, 13C, and 1H–13C HSQC NMR. The results indicated: (1) a significant amount of p-coumarate and ferulate esters was identified and quantified; (2) lignin of the alkaline-treated sample was more condensed; (3) an unanticipated amount of LCCs was quantified in the extractive-free sample, however, the amount of LCCs decreased significantly with the alkaline treatment. Therefore, lignin and LCCs of the treated sample should be characterized to elucidate their effects on the enzymatic saccharification.

Hydrothermal Liquefaction of Cypress: Effect of Water Amount on Structural Characteristics of the Solid Residue

AbstractHydrothermal liquefaction of cypress was performed in an autoclave with various amounts of water. The obtained acid‐soluble and acid‐insoluble solid residues were characterized by Fourier transform infrared spectroscopy, X‐ray diffraction, sugar analysis, elemental analysis, and nuclear magnetic resonance to help understand the reaction mechanisms of this process. The characteristics of solid residues were significantly affected by the water amount. Cellulose was more stable at high water amount and hemicelluloses were much more reactive than cellulose. Comparison of nuclear magnetic resonance spectra of milled wood lignin and milled solid residues indicated a significant cleavage of the side chains. The components of milled solid residues were mainly derived from decomposition and repolymerization of lignin. The decomposition of the side chains was substantial for lignin depolymerization during hydrothermal liquefaction.

The influence of lignin–carbohydrate complexes on the cellulase-mediated saccharification I: Transgenic black cottonwood (western balsam poplar, California poplar) P. trichocarpa including the xylan down-regulated and the lignin down-regulated lines

The influence of the putative lignin–carbohydrate complexes (LCCs) on enzymatic saccharification was elucidated for the first time by examining two groups of transgenic black cottonwood (P. trichocarpa) comprised of the lignin down-regulated and the xylan down-regulated lines. Any adventitious contaminants that could affect the characterization of LCCs and the enzymatic saccharification were removed by performing a thorough extraction on the samples. The crude milled wood lignin was subsequently isolated from which the phenyl glycoside, benzyl ether and γ-ester linkages representative of the LCCs were identified and quantified with the combination of 13C and 1H–13C Heteronuclear Single Quantum Coherence (HSQC) NMR. The result indicated that the samples showed different levels of the three LCC linkages, depending on the xylan and/or lignin content. The correlation between the LCCs and enzymatic saccharification nearly conclusively demonstrated that the LCCs accounting for the recalcitrance of lignocellulosic biodegradation.

Mechanisms for the formation of monomers and oligomers during the pyrolysis of a softwood lignin

The formation of monomers and oligomers during the pyrolysis of softwood lignin has been studied with particular emphasis on the reactivity of coniferyl alcohol (CA), which is the anticipated product of the primary pyrolysis of lignin. Under the pyrolysis conditions (N2/250–350°C/5min), the lignin contained in Japanese cedar (Cryptomeria japonica) as well as the milled wood lignin (MWL) fraction gave the monomeric guaiacols, including CA, coniferyl aldehyde, dihydroconiferyl alcohol, isoeugenol and 4-vinylguaiacol. These products were similar to those obtained from the pyrolysis of CA, although the yields were much lower. The addition of an aprotic solvent, such as diphenoxybenzene (DPB), led to a substantial increase in the recovery of CA from its pyrolysis reaction, likely by inhibiting the formation of the quinine methide intermediates for the polymerization products, whereas the solvent effects were comparatively small for the lignin pyrolysis. Alternatively, the inclusion of a H-donor (1,2,3,10b-tetrahydrofluoranthene) to the DPB led to a substantial increase in the yields of the monomers and oligomers from lignin. Based on the present data, the pyrolytic formation of the monomers and oligomers from the softwood lignin is discussed at the molecular level.

Structural Variation of Bamboo Lignin before and after Ethanol Organosolv Pretreatment

In order to make better use of lignocellulosic biomass for the production of renewable fuels and chemicals, it is necessary to disrupt its recalcitrant structure through pretreatment. Specifically, organosolv pretreatment is a feasible method. The main advantage of this method compared to other lignocellulosic pretreatment technologies is the extraction of high-quality lignin for the production of value-added products. In this study, bamboo was treated in a batch reactor with 70% ethanol at 180 °C for 2 h. Lignin fractions were isolated from the hydrolysate by centrifugation and then precipitated as ethanol organosolv lignin. Two types of milled wood lignins (MWLs) were isolated from the raw bamboo and the organosolv pretreated residue separately. After the pretreatment, a decrease of lignin (preferentially guaiacyl unit), hemicelluloses and less ordered cellulose was detected in the bamboo material. It was confirmed that the bamboo MWL is of HGS type (p-hydroxyphenyl (H), vanillin (G), syringaldehyde (S)) associated with a considerable amount of p-coumarate and ferulic esters of lignin. The ethanol organosolv treatment was shown to remove significant amounts of lignin and hemicelluloses without strongly affecting lignin primary structure and its lignin functional groups.

Characterization of milled solid residue from cypress liquefaction in sub- and super ethanol

Cypress liquefaction in sub- and super ethanol was carried out in an autoclave at various temperatures. Milled solid residue (MSR) was isolated from solid residue remaining from the liquefaction process, and its chemical characteristics was comparatively investigated with milled wood lignin (MWL) of cypress by sugar analysis, elemental analysis, FT-IR analysis, gel permeation chromatography, and NMR analysis. Results showed that there were two reactions (de-polymerization and re-polymerization) during the cypress liquefaction in sub- and super ethanol and the re-polymerization reactions were the main reaction at 220-260°C. Considering the stability of side-chain, the stability of lignin side-chain in cypress during liquefaction process in ethanol could be sequenced as follows: β-5>β-β'>β-O-4'. The MSR were mainly from the decomposition and re-polymerization of lignin. This study suggests that characterization of MSR provides a promising method to investigate the mechanisms of cypress liquefaction in ethanol.

Multi-step degradation method for β-O-4 linkages in lignins: γ-TTSA method. Part 3. Degradation of milled wood lignin (MWL) from Eucalyptus globulus

Abstract Milled wood lignin (MWL) of Eucalyptus globulus has been treated by a selective degradation method for β-O-4 linkages, which lets the lignin carbohydrate complexes (LCC) intact. The method consists of four reaction steps: (1) γ-tosylation, (2) thioetherification, (3) sulfonylation and (4) mild alkali treatment (γ-TTSA method). Each step was followed by spectroscopies for chemical structural analysis; especially, the HSQC-NMR analysis was in focus. It was demonstrated that β-O-4 linkages were selectively and quantitatively cleaved by the γ-TTSA method while the β-β linkages in the MWL remained intact. The method leads to an enrichment of LCCs.

The influence of lignin–carbohydrate complexes on the cellulase-mediated saccharification II: Transgenic hybrid poplars (Populus nigra L. and Populus maximowiczii A.)

Twelve transgenic hybrid poplars (Populus nigra L. and Populus maximowiczii A.) were used to demonstrate the influence of the lignin–carbohydrate complexes (LCCs) on enzymatic saccharification. The samples have different levels of the syringaldehyde (S) to vanillin (V) ratio from 0.1 to 2.6 and the lignin content from 10.5% to 24.3%, compared to the control (the S/V ratio 1.7 and the lignin content 22.0%). Any adventitious contaminants that could affect the final enzymatic saccharification were removed by performing thorough extraction on the samples. The crude milled wood lignins were subsequently isolated from which the phenyl glycoside, benzyl ether and γ-ester linkages representative of the LCC were identified and quantified by 13C and 1H–13C HSQC NMR. It was found that the samples showed different levels of the three LCC linkages, depending on the lignin content and/or the S/V ratio. The correlation between the LCCs and enzymatic saccharification nearly conclusively demonstrated that the LCCs accounted for the recalcitrance of lignocellulosic biodegradation.

Catalytic Alkaline Oxidation of Lignin and its Model Compounds: a Pathway to Aromatic Biochemicals

Catalytic oxidation via the application of molecular oxygen and copper complexes is a useful pathway toward valuable low molecular mass compounds from in situ or waste stream lignins. In this study, two dimeric β-ether model compounds, one β-ether oligomer, and a milled wood lignin sample from Loblolly pine were catalytically oxidized. Yields and stability of the aromatic aldehyde and acid products were measured. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to monitor structure/composition and molecular mass changes of the lignin before and after catalytic oxidation to study the degree of depolymerization and structure of the residual lignin. Oxidized units appear to be derived from β-aryl ether, phenylcoumaran, and biphenyl ether components. To date, this method breaks down the lignin polymeric structure reasonably effectively, producing low molecular mass products; this work also highlights some of the issues that need to be overcome to optimize this approach.

Structural features and thermal degradation properties of various lignin macromolecules obtained from poplar wood (Populus albaglandulosa)

Four different lignins obtained from poplar wood (milled wood lignin: ML, organosolv lignin: OL, ionic liquid lignin: IL and Klason lignin: KL) were subjected to several types of chemical/thermal analyses to compare their structural features and thermal decomposition properties. The ML, OL, IL and KL yield from poplar wood was 5.5, 3.9, 5.8, 19.5 wt%, respectively. Functional group analysis revealed that during the OL and KL extraction processes, the condensation reaction involved with phenolic hydroxyl groups of lignins significantly prevailed, which led to a highly condensed OL and KL structure. Thermogravimetric analysis (TGA) results showed that OL and KL thermal stability was much higher than that of ML and IL. The derivatization followed by reductive cleavage (DFRC) data showed that the thermal stability was highly associated with the frequency of arylglycerol-β-aryl ether (β-O-4) linkages in the lignin polymers. Pyrolysis-GC/MS (Py-GC/MS) analysis confirmed that acetic acid and several types of phenolic compounds were the main lignin pyrolysis products. The maximum sum of ML (13.8 wt%), OL (9.9 wt%) and IL (11.8 wt%) pyrolysis products was obtained at the pyrolysis temperature of 600 °C, whereas KL (1.6 wt%) was significantly lower due to its high thermal stability and condensation degree. The S- and G-type pyrolysis products (S/G) ratio varied from 1.61 to 1.93 for ML, 2.28 to 5.28 for OL, 2.06 to 2.86 for IL and 1.40 to 2.20 for KL, depending on the pyrolysis temperature, which ranged between 400 °C and 700 °C.

Understanding of Formaldehyde Emissions from Solid Wood: An Overview

Wood is known to contain and emit volatile organic compounds including formaldehyde. The emission of formaldehyde from wood increases during its processing to lumber and wood-based panels (i.e., particleboard and fiberboard). This increased emission can be attributed to the processing procedure of wood, which includes drying, pressing, and thermo-hydrolysis. Formaldehyde is emitted from wood under very high heat and is not expected to be a significant source of the emissions from composite wood products during normal service. Formaldehyde is also detectable even if wood has never been heated as well as under more or less ambient conditions. The presence of formaldehyde in the emissions from wood that does not contain adhesive resin has been explained by thermal degradation of polysaccharides in the wood. The emission levels of formaldehyde depend on factors such as wood species, moisture content, outside temperature, and time of storage. Additionally, the pyrolysis of milled wood lignin at 450 °C yields benzaldehyde, and the pyrolysis of spruce and pinewood at 450 °C generate formaldehyde, acetaldehyde, 2–propenal, butanal, and butanone, which can be attributed to the breakdown of the polysaccharide fraction of the wood.

Structural Elucidation of the Lignins from Stems and Foliage of Arundo donax Linn.

As one of the potential energy crops, Arundo donax Linn. is a renewable source for the production of biofuels and bioproducts. In the present study, milled wood lignin (MWL) and alkaline lignin (AL) from stems and foliage of A. donax were isolated and characterized by FT-IR spectroscopy, UV spectroscopy, GPC, ³¹P NMR, 2D HSQC NMR, and DFRC. The results indicated that both stem and foliage lignins were HGS type lignins. The semiquantitative HSQC spectra analysis demonstrated a predominance of β-O-4' aryl ether linkages (71-82%), followed by β-β', β-5', β-1', and α,β-diaryl ethers linkages in the lignins. Compared to stem lignins, foliage lignins had less β-O-4' alkyl-aryl ethers, lower weight-average molecular weight, less phenolic OH, more H units, and lower S/G ratio. Moreover, tricin was found to incorporate into the foliage lignins (higher content of condensed G units) in significant amounts and might be alkaline-stable.

Structural characterization and deposition of stone cell lignin in Dangshan Su pear

Stone cell content is one determinant of pear fruit quality. Stone cells are formed by the deposition of lignin on primary cell walls, followed by the secondary thickening of cell walls. Studies of the structural features and deposition of pear lignin were rare. To understand the structural characteristics of pear fruit lignin, chemical functional groups and bond types of pear fruit lignin were studied, along with an analysis of the deposition of lignin in the stone cell. Lignin was extracted, purified and analyzed by Fourier-transform infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (1H NMR) spectroscopy. The deposition of lignin in the pear stone cell wall was detected by electron microscopy. Lignin in the pulp of Pyrus bretschneideri cv. Dangshan Su pear comprised guaiacyl-syringyl-lignin, with a relatively high amount of guaiacyl-lignin (G) compared to syringyl lignin (S). During the early development of Dangshan Su pear, lignin exhibited CO stretching in unconjugated ketoses, carbonyls and esters, but during later development, aliphatic CH stretching in CH3 (not in Ome) and phen OH occurred. The main linkages of the acetylated milled wood lignin structural units in Dangshan Su pear fruit are β-O-4, β-1, β-5 and β-β. β-O-4 was the major factor related to the solubility of lignin, with the presence of few β-5 linkages corresponding to the formation of fewer condensation-type structures, while the presence of fewer than 30% phenolic hydroxyl groups suggested that this lignin exhibited low levels of polymerization. In addition, a large number of hydrocarbon protons and side chains were present in Dangshan Su pear fruit lignin, suggesting this lignin is more easily dissolved than those with more condensation-type structures. Dangshan Su pear fruit lignin exhibited low levels of polymerization and was therefore easily hydrolyzed. Ultramicroscopy revealed that lignification extended from the corner of the primary cell wall to the remaining regions of the compound N middle lamella and the secondary wall, with lignin particles deposited along the cellulose microfibrils. Lignin particles and microfibrils were alternatively arranged until they filled up the entire cell cavity, culminating in the formation of stone cells.

Fractionation and characterization of lignin-carbohydrate complexes (LCCs) of Eucalyptus globulus in residues left after MWL isolation. Part II: Analyses of xylan-lignin fraction (X-L)

Abstract The residual wood meal left after milled wood lignin (MWL) isolation [milled wood residue (MWR)] of 5-year-old Eucalyptus globulus was fractionated to afford a xylan-lignin fraction (X-L) in 2.9% yield (based on MWR) by the method reported previously. X-L was further fractionated with the lignin solvent 1,4-dioxane/water (9:1, v/v) to give a soluble fraction (XL-F1; 24.0%) and an insoluble fraction (XL-F1-residue; 74.6%; both yields based on X-L). XL-F1-residue was further extracted with the good xylan solvent dimethyl sulfoxide and the soluble fraction was termed XL-F2 (43.0%; based on the XL-F1-residue). XL-F1 was mainly composed of lignin with a small amount of xylan and it is similar to purified MWL, whereas XL-F2 was mainly composed of xylan with some amount of lignin and it is similar to a fraction that was prepared by the extraction of crude MWL with acetic acid [lignin-carbohydrate complex (LCC)-AcOH]. The two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectra of XL-F1 and XL-F2 were interpreted that the former has α-ether-type lignin-carbohydrate (LC) linkages and the latter might have LC linkages of the phenyl glycoside type, which are different from those in LCC-AcOH.

Elucidation of structural condensation in lignin of eastern cottonwood

The chemical structure and condensation of eastern cottonwood (Populus deltoides) enzymatic lignin were investigated by wet-chemical and spectroscopic techniques. A cocktail of carbohydrate digesting enzymes were used to hydrolyze the polysaccharides of P. deltoides wood. The chemical structure of biochemical purified lignin was elucidated by 2D 1H–13C HSQC correlation NMR and quantitative 13C NMR analysis. The monomeric compositions of enzymatic lignin were analyzed by derivatization followed by a reductive cleavage method to allow a quantitative determination of different monomeric products in lignin. The results indicated that enzymatic lignin had more uncondensed structures compared to milled wood lignin which could be a consequence of lower oxidation during the extraction process. The contribution of H:G:S units in P. deltoides were in the order of 5:60:35, respectively. Also, the amount of β-O-4/α-OH structures as the prominent moieties of lignin in enzymatic lignin was determined as 0.43/C9.

Influence of Isolation Condition on Structure of Milled Wood Lignin Characterized by Quantitative 13C Nuclear Magnetic Resonance Spectroscopy

Milled wood lignin (MWL) was widely characterized to demonstrate the structure of native lignin by liquid state 13C NMR. As an isolated lignin, the structure of MWL was influenced by the isolation procedure performed. In this article, hardwood (sweetgum) and softwood (loblolly pine) were subjected to various isolation conditions to elucidate the effect of extracting temperature and milling time on the structure of MWL. Purification was also carried out on the crude MWL. The structure of the crude MWL and the purified MWL was identified and quantified by 13C NMR. Based on the yield and the lignin content of the crude MWL, the optimal isolation was achieved with 8 h milling and 20 °C extracting for hardwood. For softwood, the optimal isolation condition for crude MWL was 12 h milling and 20 °C extracting.

Pyrolysis of wood, cellulose, lignin–brominated epoxy oligomer flame retardant mixtures

The thermal decomposition of brominated epoxy oligomer (BrEpoxy)–natural polymer mixtures were examined by Py-GC/MS and TG/MS techniques in order to clarify the reactions between the components of the mixtures. The natural polymer components were: pine and beech wood, pine and beech milled wood lignin and two types of cellulose (Avicel and Whatman cellulose). It was found that the decomposition of both celluloses shifts to the temperature range of BrEpoxy decomposition. Significantly decreased formation of hydroxyacetaldehyde, hydroxypropanone and levoglucosan and increased yield of levoglucosenone was observed from cellulose–BrEpoxy mixtures. Drastically increased evolution of bromomethane was found from lignin or wood–BrEpoxy mixtures, indicating a radical scavenging reaction of lignin leading to the formation of bromomethane and phenoxy radicals.

Characterization of Lignins Isolated with Alkaline Ethanol from the Hydrothermal Pretreated Tamarix ramosissima

The objective of this study was to characterize the changes in lignin structure during hydrothermal pretreatment of shrub Tamarix ramosissima. Lignins in residual wood meal were isolated with alkaline ethanol solution and recovered with acid precipitation. A comparison between the recovered lignin fractions with milled wood lignin has been made in terms of yield, purity, gel permeation chromatography, Fourier transform infrared spectroscopy, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), 1D 13C and 2D heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR) spectroscopic techniques. Semiquantitative HSQC NMR showed that the relative amounts of β-O-4′ (around 76 % side chains) and resinol type substructures (16 %) of lignins were significantly modified during hydrothermal pretreatment. Py-GC/MS analyses brought direct evidences of these lignin samples with high S/G ratios ranging from 1.7 to 2.6. Moreover, the results indicated that an increase in the severity of the hydrothermal pretreatment enhanced the degradation of lignin unit side chains and the condensation of lignin and decreased the molecular weight of the recovered lignin fractions. This study demonstrated that the combination of autohydrolysis and alkaline ethanol process could potentially turn the recovered lignin fractions into value added products being in accordance with the “biorefinery” concept.