Milled Wood Lignin Research Articles

Solid/liquid- and vapor-phase interactions between cellulose- and lignin-derived pyrolysis products

Solid/liquid- and vapor-phase interactions between cellulose- and lignin (Japanese cedar milled wood lignin)-derived pyrolysis products were studied under the conditions of N 2/600 °C/40–80 s. A dual-space closed ampoule reactor was used to eliminate the solid/liquid-phase interactions, and careful comparison of the resulting data with those of the pyrolysis of the mixed samples gave some insights into the solid/liquid- and vapor-phase interactions separately. With the solid/liquid-phase interactions, the tar yields from both cellulose and lignin increased with the decreasing yields of the char fractions in a short pyrolysis time of 40 s (primary pyrolysis stage). Most of the identified tar components from cellulose and lignin increased in their yields. The vapor-phase interactions were significant at a longer pyrolysis time of 80 s (secondary reaction stage) when the methoxyl groups of the lignin-derived volatiles were cleaved homolytically. The vapor-phase interactions accelerated the gas formation from the cellulose-derived volatiles with suppressing the vapor-phase char formation of the lignin-derived volatiles. The yields of methane and catechols from lignin also increased greatly instead of the formation of o-cresols. Most of these influences are explained with a proposed interaction mechanism, in which the cellulose-derived volatiles act as H-donors while the lignin-derived volatiles (radicals) act as H-acceptors.

Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: Comparison of different lignin fractions formed during water prehydrolysis

Abstract The effect of autohydrolysis of Eucalyptus globulus wood was studied with regard to conditions applied in a prehydrolysis-kraft process on the physico-chemical properties of lignin obtained in both the wood residue and hydrolysate. As a reference, milled wood lignin (MWL) was isolated from native wood and compared to three lignin fractions formed during prehydrolysis: 1) lignin from the wood residue isolated as MWL, 2) lignin precipitated from the prehydrolysate during cooling and separated by centrifugation, and 3) lignin degradation products soluble in the prehydrolysate extracted with ethylacetate. All lignin fractions were subjected to Fourier transform infrared (FTIR) spectroscopy, methoxy group determination, elemental analysis, size exclusion chromatography and quantitative nuclear magnetic resonance (NMR) spectroscopy. The results indicate that extensive lignin degradation occurs during prehydrolysis through homolytic cleavage of the aryl-ether bonds resulting in a substantial molecular weight loss of the residual lignin in the treated wood and in the lignin fractions isolated from the prehydrolysate. The aryl-ether cleavage is coupled with a strong increase in phenolic hydroxyl groups and a decrease in aliphatic hydroxyl groups. Indication for condensation reactions were found by NMR spectroscopy.

Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 2: Influence of autohydrolysis intensity

Abstract Three different levels of autohydrolysis intensity, expressed as the prehydrolysis (P)-factor, were applied to Eucalyptus globulus wood at a liquor/wood ratio of 5:1. Lignin fractions were isolated from the wood residue as milled wood lignin (MWL), from the hydrolysate by centrifugation (insoluble fraction) and by ethyl acetate extraction (soluble fraction), and from the reactor wall as precipitate. With increasing autohydrolysis duration, a decrease in the content of aliphatic hydroxyl groups and of β-O-4 structures was detected in all lignin fractions, whereas the content of phenolic hydroxyl groups increased in the same order. MWL isolated from wood residue after autohydrolysis at the highest P-factor contained only half the β-O-4 structures contained in native lignin. Molecular weight distribution measurements revealed that fragmentation reactions dominated over condensation reactions in all lignin samples investigated. However, low-molecular-weight lignin dissolved in autohydrolysate exhibited extremely high reactivity towards acid-catalysed condensation reaction, which inevitably leads to the formation of sticky precipitates during storage at elevated temperature under the acid conditions prevailing.

Monolignol acylation and lignin structure in some nonwoody plants: A 2D NMR study

Lignins from three nonwoody angiosperms were analyzed by 2D NMR revealing important differences in their molecular structures. The Musa textilis milled-wood-lignin (MWL), with a syringyl-to-guaiacyl (S/G) ratio of 9, was strongly acylated (near 85% of side-chains) at the γ-carbon by both acetates and p-coumarates, as estimated from 1H– 13C correlations in C γ-esterified and C γ–OH units. The p-coumarate H 3,5–C 3,5 correlation signal was completely displaced by acetylation, and disappeared after alkali treatment, indicating that p-coumaric acid was esterified maintaining its free phenolic group. By contrast, the Cannabis sativa MWL (S/G ∼0.8) was free of acylating groups, and the Agave sisalana MWL (S/G ∼4) showed high acylation degree (near 80%) but exclusively with acetates. Extensive C γ-acylation results in the absence (in M. textilis lignin) or low abundance (4% in A. sisalana lignin) of β–β′ resinol linkages, which require free C γ-OH to form the double tetrahydrofuran ring. However, minor signals revealed unusual acylated β–β′ structures confirming that acylation is produced at the monolignol level, in agreement with chromatographic identification of γ-acetylated sinapyl alcohol among the plant extractives. In contrast, resinol substructures involved 22% side-chains in the C. sativa MWL. The ratio between β–β′ and β -O-4′ side-chains in these and other MWL varied from 0.32 in C. sativa MWL to 0.02 in M. textilis MWL, and was inversely correlated with the degree of acylation. The opposite was observed for the S/G ratio that was directly correlated with the acylation degree. Monolignol acylation is discussed as a mechanism potentially involved in the control of lignin structure.

Analytical pyrolysis study of biodelignification of cloned Eucalyptus globulus (EG) clone and Pinus pinaster Aiton kraft pulp and residual lignins

This study centred on the analysis of lignin in situ of cloned eucalypt and pine kraft pulps. Trametes versicolor laccase-violuric acid system (LMS) delignifications were performed on a softwood ( Pinus pinaster) and a hardwood ( Eucalyptus globulus) conventional kraft pulp with an initial kappa number of 34.5 and 15.5, respectively. The LMS treated pulps were then subjected to alkaline extraction stages (E). The kappa number data show that LMS is effective at biodelignifying both softwood and hardwood kraft pulps. However, under the conditions employed in this study, a greater level of biodelignification was obtained with LMS E. globulus (hardwood) than with LMS P. pinaster (softwood), but the amount of lignin removed was higher for the softwood pulp. The original milled wood samples, kraft pulps, biodelignified kraft pulps, and isolated residual lignin and milled wood lignins from the two wood samples have been characterized by pyrolysis-gas chromatography/mass spectrometry. The analysis of the pyrograms indicates that the lignin compositions of the two wood species and corresponding pulps are very different, as expected; however, the knowledge of the chemical mechanisms of delignification is very limited and requires additional work. Analytical pyrolysis is one the few degradative methods for the analysis of biopolymers that has shown a sufficient degree of success.

Effects of Two-Stage Dilute Acid Pretreatment on the Structure and Composition of Lignin and Cellulose in Loblolly Pine

A standard two-step dilute sulfuric acid pretreatment was performed on Loblolly pine to enhance the overall efficiency of enzymatic deconstruction of woody biomass to monomeric sugars. The structure of milled wood lignin and cellulose isolated from the untreated and acid-treated biomass was studied in detail. Solid-state 13C NMR spectroscopy coupled with line shape analyses has been employed to elucidate cellulose crystallinity and ultrastructure. The results indicate an increase in the degree of crystallinity and reduced relative proportion of less ordered cellulose allomorphs following the acid pretreatment. This increase was attributed to a preferential degradation of amorphous cellulose and less ordered crystalline forms during the high temperature pretreatment. Milled wood lignin structural elucidation by quantitative 13C and 31P NMR reveals an increase in the degree of condensation of lignin due to the pretreatment. The increase in degree of condensation is accompanied by a decrease in β-O-4 linkages which were fragmented and recondensed during the high temperature acid-catalyzed reactions.

Comparison between 10,000-year old and contemporary spruce lignin

Wood from white spruce Picea glauca that had been preserved by rapid burial in lake sediments 10,000 years ago, was investigated and compared to a contemporary reference white spruce wood. The 10,000-year old sample appeared to have an intact primary cell wall and middle lamella, whereas the carbohydrate monomer distribution, and microscopic images showed that the secondary wall was at least partially removed, indicating that this structure had been selectively attacked by bacteria. The Klason lignin amount in the aged spruce was found to be 60%. The relative lignin monomer content in the aged spruce was 9% lower than that of the reference wood, showing that there were fewer β-O-4′ linkages in the aged sample. This finding was supported by SEC analysis of the thioacidolysed samples as a larger proportion of lignin oligomers were observed in the aged spruce than in the reference material. This indicates a somewhat greater number of condensed bonds in the aged spruce than in the reference spruce sample. Quantitative 13C NMR analysis and HSQC techniques applied on milled wood lignins (MWL) revealed no significant structural differences between the aged spruce and the reference.

Secondary decomposition of levoglucosan in pyrolytic production from cellulosic biomass

Levoglucosan (1,6-anhydro-β- d-glucopyranose) is a major pyrolysis product from cellulose. To understand the secondary decomposition behavior in pyrolytic production of levoglucosan from cellulosic biomass, influences of the pyrolysis vapor from wood or wood constituent polymers on the recovering process of levoglucosan were studied at 400 °C (30 mmHg). Japanese cedar ( Cryptomeria japonica, a softwood) and Japanese beech ( Fagus crenata, a hardwood) wood samples, cellulose, xylan (a hemicellulose) and the milled wood lignins isolated from the wood samples were used as the substances for production of the pyrolysis vapor. Since levoglucosan and other substances were placed separately in a dual-space reactor to eliminate their direct contact, the interaction is expected in vapor-phase. Solid/liquid-phase interaction is also possible after condensation of the vapor on the reactor wall with lower temperature. The cellulose- and lignin-derived pyrolysis vapors reduced the recovery of levoglucosan substantially, while the influences of the vapors from xylan and the substances containing xylan were comparatively very small. Such decomposition induced by the cellulose- and lignin-derived vapors was inhibited in the presence of the xylan-derived pyrolysis vapor.

Structural characterization of milled wood lignins from different eucalypt species

AbstractThe chemical structure of milled-wood lignins fromEucalyptus globulus,E. nitens,E. maidenii,E. grandis, andE. dunniiwas investigated. The lignins were characterized by analytical pyrolysis, thioacidolysis, and 2D-NMR that confirmed the predominance of syringyl over guaiacyl units and only showed traces ofp-hydroxyphenyl units.E. globuluslignin had the highest syringyl content. The heteronuclear single quantum correlation (HSQC) NMR spectra yielded information about relative abundances of inter-unit linkages in the whole polymer. All the lignins showed a predominance of β-O-4′ ether linkages (66–72% of total side-chains), followed by β-β′ resinol-type linkages (16–19%) and lower amounts of β-5′ phenylcoumaran-type (3–7%) and β-1′ spirodienone-type linkages (1–4%). The analysis of desulfurated thioacidolysis dimers provided additional information on the relative abundances of the various carbon-carbon and diaryl ether bonds, and the type of units (syringyl or guaiacyl) involved in each of the above linkage types. Interestingly, 93–94% of the total β-β′ dimers included two syringyl units indicating that most of the β-β′ substructures identified in the HSQC spectra were of the syringaresinol type. Moreover, three isomers of a major trimeric compound were found which were tentatively identified as arising from a β-β′ syringaresinol substructure attached to a guaiacyl unit through a 4-O-5′ linkage.

Secondary reactions of lignin-derived primary tar components

Secondary reactions of lignin-derived primary tar components were studied with a closed ampoule reactor under the conditions of N 2/600 °C/40–120 s. The tar fractions from Japanese cedar milled wood lignin and model compounds were characterized with the GPC, 1H NMR and GC/MS analyses. Changes in the tar composition indicated the following pathways; unsaturated side-chain (>C C<) and phenolic aromatic ring, both of which are the major structures of the primary tar fraction, are the most reactive structure and would be converted to the condensation products; Then, the side-chains are cracked to form the low MW aromatics with saturated alkyl groups or –H; decarbonylation of the aldehyde groups and change in the substitution pattern of the aromatic ring also occur along with this cracking reaction; the guaiacyl (4-hydroxy-3-methoxyphenyl)-type aromatic ring in the primary tar fraction is converted to the catechol-, o-cresol- and phenol-type aromatic rings. Although the pure catechol-type compounds were stable, these compounds became reactive in the presence of the guaiacol-type compounds. Radical chain reaction through the H-abstraction from the phenolic hydroxyl groups would activate the catechol-type compounds.

The fate of phosphorus in the bleaching of spruce TMP with the new bleaching agent – THPS

Abstract Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) is a newly discovered, phosphorus-based bleaching agent that has been successfully tested in a pulp mill trial. The fate of phosphorus (P) and the chemistry in the course of THPS bleaching of a spruce TMP have been studied. The amount of P introduced into the THPS-bleached pulp depends on THPS dose and the bleaching conditions and it correlates well with the brightness gain. The P introduced into THPS-bleached pulp is covalently bonded to the pulp. The THPS-bleached and tetraethylphosphonium chloride-treated pulps have been additionally treated with calcium chloride before P analysis, and the results of these experiments support the covalent bonding hypothesis. Bleaching with 0.1 and 3.0% THPS (% based on oven-dried pulp) at 60 and 110°C leads to pulps with 0.13 and 2.17 mg P g-1 pulp, respectively, which corresponds to 0.08 and 1.4% THPS. More precisely, P is covalently bonded to lignin chromophores, such as lignin coniferaldehydes. This was found by P-content analysis, UV-visible spectroscopy, and 31P{1H} NMR studies of a crude milled wood lignin (crude MWL) and a partially purified MWL isolated from a THPS-bleached pulp.

Synthesis of β-O-4-type artificial lignin polymers and their analysis by NMR spectroscopy

We describe the synthesis and NMR spectroscopic analysis of three artificial lignin polymers containing only the beta-O-4 substructure: syringyl-type homopolymer, p-hydroxyphenyl-type homopolymer and guaiacyl/syringyl-type heteropolymer. Using gel permeation chromatography, the weight-average degree of polymerization (DP(w)) of the three polymers was determined as 19.2, 38.6, and 13.9, respectively. The polymers were prepared based on the synthetic methodology of guaiacyl-type homopolymer, and were fully characterized using (1)H-, (13)C-, and (1)H-(13)C NMR spectroscopy of the acetylated and non-acetylated forms. The spectra of guaiacyl/syringyl-type heteropolymers were in good agreement with those of the beta-O-4 substructure of milled wood lignin obtained from the hardwood of Japanese white birch.

A new and facile method for isolation of lignin from wood based on complete wood dissolution

Abstract A new method for the isolation of lignin in high yield from wood and pretreated wood is presented, avoiding the liquid-solid extraction step of the classical milled wood lignin (MWL) isolation. Dissolved wood lignin (DWL) was obtained by total dissolution of ball milled wood in dimethylsulfoxide and N-methylimidazole (DMSO/NMI) followed by precipitation in dioxane/water in the course of which lignin and carbohydrate fractions were separated. The lignin fraction was purified. High lignin yields and the low number of separation steps belong to the advantages of the described method. DWL lignin was isolated from beech wood (Fagus sylvatica L.) and the sample was compared to MWL obtained by the classical Björkman method. Elemental analyses, methoxyl group content, potassium permanganate oxidation, infrared spectroscopy, and 1D and 2D nuclear magnetic resonance spectroscopy indicated that both lignin preparations are quite similar. However, MWL contained significantly more phenolic hydroxyl groups than DWL. The results indicated that MWL contains higher amounts of smaller fractions with broken β-O-4 linkages than DWL.

Pyrolysis behavior of Japanese cedar wood lignin studied with various model dimers

Pyrolysis behavior of the milled wood lignin (MWL) isolated from Japanese cedar ( Cryptomeria Japonica) wood was studied in N 2 at 200–400 °C, as compared with the reactivities of various guaiacyl-types of lignin model dimers. The model dimer reactivity varied significantly depending on the structure including phenolic, methylated and linkage types. The C β O cleavage to cinnamyl alcohol-type structure (β-ether linkage) and C γ-elimination to stilbene-type structure (β-aryl linkage) were the major pyrolytic pathways in both dimer and MWL. With increasing the pyrolysis temperature of MWL, condensation, depolymerization and carbonization (formation of a multiple aromatic ring system) started at 250, 350 and 400 °C, respectively, in 1 min pyrolysis. Although methylation of the hydroxyl groups in MWL substantially reduced the condensation reactivity, depolymerization temperature did not change by methylation. By comparing the model dimer reactivity, roles of the linkage-types between phenylpropane-units in lignin were suggested; the phenolic end-group, which would form some conjugated C α C β structures, is important for low-temperature condensation; effective depolymerization of lignin macromolecule is related to the cleavage of the α-ether-linkage in lignin ether chain; reaction of the aromatic ring itself, which includes the O CH 3 homolysis, relates to the carbonization.

An NMR Comparison of the Whole Lignin from Milled Wood, MWL, and REL Dissolved by the DMSO/NMI Procedure

Lignins isolated from pine milled wood, milled wood lignin (MWL), and residual enzyme lignin (REL) were compared using modified thioacidolysis, modified DFRC, gel permeation chromatography (GPC), two‐dimensional Heteronuclear Multiple Quantum Coherence (HMQC) NMR, and quantitative 13C NMR. Dissolution of the lignin for solution‐state NMR was accomplished by utilizing the recently reported DMSO/N‐methylimidazole/acetic anhydride solvent system. Contrary to previous reports, comparison of the lignin preparations by thioacidolysis indicated that REL was more structurally similar to the lignin in the milled wood and Wiley wood meal than MWL. Total monomer yields indicated that the MWL was lower in β‐aryl ether content than the other preparations, and this was verified by quantitative 13C NMR. NMR analysis indicated that the inter‐unit linkages present in all the lignin preparations are consistent with the present knowledge about lignin biosynthesis. The contribution of minor end group structures in the MWL are further decreased in the milled wood, indicating that they are preferentially isolated as low molecular weight material, possibly generated during the milling process. All other structural moieties were similar in all preparations. GPC data indicated that the milled wood and REL both contain a portion of lignin with a molecular weight of 55,000 g/mol. Data indicate that the inefficiency of the DFRC method may be related to molecular mobility or accessibility in higher molecular weight portions of the lignin polymer.

Polyoxometalate (POM) Oxidation of Milled Wood Lignin (MWL)

The chemical structural changes of Lodgepole pine milled wood lignin (MWL) before and after polyoxometalate (POM) oxidation were investigated using gel permeation chromatography (GPC), Fourier Transform infrared spectroscopy (FTIR), and solution‐state nuclear magnetic resonance spectroscopy (NMR). 13C NMR spectroscopic data revealed an approximately 28% decrease in α‐OH/β‐O‐4 inter‐unit linkages after POM treatment. This was accompanied by an increase in carbonyl and phenolic hydroxyl content. These results suggest POM oxidation involved side chain (such as α‐OH/β‐O‐4) oxidation and/or degradation of some of the inter‐unit linkages. Quantitative 13C NMR along with GPC analysis revealed an increase in the degree of condensation of the MWL as a result of POM treatment, suggesting radical coupling as a major reaction pathway.

Occurrence of Naturally Acetylated Lignin Units

This work examines the occurrence of native acetylated lignin in a large set of vascular plants, including both angiosperms and gymnosperms, by a modification of the so-called Derivatization Followed by Reductive Cleavage (DFRC) method. Acetylated lignin units were found in the milled wood lignins of all angiosperms selected for this study, including mono- and eudicotyledons, but were absent in the gymnosperms analyzed. In some plants (e.g., abaca, sisal, kenaf, or hornbeam), lignin acetylation occurred at a very high extent, exceeding 45% of the uncondensed (alkyl-aryl ether linked) syringyl lignin units. Acetylation was observed exclusively at the gamma-carbon of the lignin side chain and predominantly on syringyl units, although a predominance of acetylated guaiacyl over syringyl units was observed in some plants. In all cases, acetylation appears to occur at the monomer stage, and sinapyl and coniferyl acetates seem to behave as real lignin monomers participating in lignification.

Photostabilizing Milled Wood Lignin with Benzotriazoles and Hindered Nitroxide†¶

Photostabilizing Milled Wood Lignin with Benzotriazoles and Hindered Nitroxide†¶

Lignin Modification during Eucalyptus globulus Kraft Pulping Followed by Totally Chlorine-Free Bleaching: A Two-Dimensional Nuclear Magnetic Resonance, Fourier Transform Infrared, and Pyrolysis−Gas Chromatography/Mass Spectrometry Study

Chemical modification of eucalypt lignin was investigated during kraft pulping and chlorine-free bleaching by comparing milled wood lignin, kraft lignin, and pulp enzymatic residual lignins. The syringyl-to-guaiacyl ratio (S/G) from analytical pyrolysis slightly changed during pulping and bleaching (S/G, 3-4) but was higher in the kraft lignin. Semiquantitative heteronuclear single quantum correlation (HSQC) nuclear magnetic resonance (NMR) showed that the relative amount of beta-O-4' (around 80% side chains) and resinol type substructures (15%) was slightly modified during pulping and oxygen delignification. However, a decrease of resinol substructures (to only 6%) was found after alkaline peroxide bleaching. The relative amount of surviving linkages in the highly phenolic kraft lignin was dramatically modified; resinols were predominant. Oxygen delignification did not change interunit linkages, but a relative increase of oxidized units was found in the HSQC aromatic region, in agreement with the small increase of pyrolysis markers with oxidized side chains. NMR heteronuclear multiple bond correlations showed that the oxidized units after oxygen delignification bore conjugated ketone groups.

Quantitative lignin analysis based on permanganate oxidation

Qualitative lignin analysis relies rather much on studies of lignin degradation products. As concerns precise quantification of lignin’s composition such studies in general have obvious limitations. Aromatic acids obtained on permanganate oxidation of pretreated lignins (cleavage of ethers and alkylation of phenolic groups) offer a possibility to estimate the amounts of differently substituted aromatic units in lignins. An equation is derived for the calculation of the gross composition of lignins based on the yields of methoxy-substituted aromatic acids obtained on permanganate oxidation of lignins with methylated phenolic groups. The equation could also be used for the calculation of the phenolic content in a lignin sample based on permanganate oxidation data, provided that such data are available for a similar lignin sample with known phenolic content. Literature data for milled wood lignin from spruce are used to exemplify the calculations.