Dissolved Lignin and Other Aromatic Substances in Thermomechanical Pulp Waters

Abstract

Dissolved aromatic substances in water suspensions of unbleached and peroxide‐bleached spruce thermomechanical pulp (TMP) were isolated by sorption on XAD‐8 resin and fractionated into lignin and oligomeric aromatic substances (OAS). The fractions were characterized by chromatographic (GC and HP‐SEC) and spectrometric (FT‐IR, ¹³C‐NMR, GC‐MS) methods, as well as by GC after direct on‐line pyrolysis and thermally assisted hydrolysis and methylation. The dissolved lignin in unbleached TMP water was structurally similar to spruce milled‐wood lignin (MWL), but its average molar‐mass was lower and the mass distribution more narrow. The oligomeric aromatic substances included phenolic dimers, trimers, and tetramers that were structurally different from those of MWL. After peroxide bleaching the amount of dissolved semipolar (MTBE) extractives and oligomeric aromatic substances was much lower. The amount of dissolved lignin from wood fibers was, on the contrary, much higher. The lignin released from TMP after bleaching was extensively oxidized and had a slightly higher average molar‐mass than lignin released from unbleached TMP.     

Lignin and Other Aromatic Substances Released from Spruce Wood During Pressurized Hot-Water Extraction,Part 2: Structural Characterization

Extraction of ground spruce sapwood with pressurized hot water in an accelerated solvent extractor (ASE) at 170°C during 20, 60, and 100 min resulted in isolation of galactoglucomannans and aromatic substances, including lignin. The isolated lignin preparations were characterized by spectrometric (UV, FT-IR, ¹H NMR, liquid and solid-state ¹³C NMR), chromatographic (RP-HPLC, HP-SEC, GC-FID, and GC-MS), conventional pyrolysis, thermally assisted hydrolysis, and methylation techniques in tandem with GC-MS, and classical wet chemistry (methoxyl groups, total and phenolic hydroxyl groups, derivatization followed by reductive cleavage—DFRC). The content of β-O-4 bonds in isolated lignins was similar to that in MWL and their proportion decreased with extraction time. The oxidation of isolated lignins and content of total hydroxyl groups were significantly increased with extraction time. The lignin structure underwent condensation and demethylation reactions during hot-water extraction. The induction of new phenylcoumaran substructures was proposed in isolated lignins.     

How Well Do MWL and CEL Preparations Represent the Whole Hardwood Lignin?

Hardwood lignin preparations were isolated using classical milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) protocols. Furthermore, we managed to produce a lignin preparation of a very high yield, above 90%, with high purity and minimal structural degradation. This was achieved by dissolution and regeneration of milled wood prior to enzymatic hydrolysis, along with the use of 80% dioxane for lignin extraction. This preparation (RCEL-80) yield was about 4.5 and 2.5 times higher than the yields of the traditional MWL and CEL preparations, correspondingly, at the same milling time. The preparations obtained were comprehensively analyzed with state-of-the-art quantitative NMR techniques and wet chemistry methods. CELs were representative preparations for hardwood lignins as the S/G ratios in the CELs were equivalent to those in lignin in situ. Degradation of the main lignin subunits was very low. Importantly, the structures of CELs were independent of the preparation yield and were very similar to the structure of the high-yield lignin, RCEL-80. In contrast, the structures of MWLs were noticeably dependent on the preparation yield, especially when the yield was below 15–20% of total wood lignin. In particular, the S/G ratio increased with increasing MWL yield, but was still lower than the S/G ratios of the whole lignins in situ, even at high MWL yields. The amounts of β-O-4 lignin units in MWL were lower than those in the corresponding CEL preparations. It has been concluded that CEL preparations were representative of the whole wood lignin whereas MWLs represent a fraction with a lower S/G ratio.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

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

Abstract

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.     

Structural Characterization of Pine Kraft Lignin: BioChoice Lignin vs Indulin AT

BioChoice lignin (BCL) is a newly commercialized pine kraft lignin from Plymouth Mill of Domtar, which is precipitated from black liquor of bleachable-grade pulp. Indulin AT is a pine kraft lignin commercialized by Meadwestvaco for the past 60 years, which is precipitated from black liquor of linerboard-grade pulp. Thus, the two technical lignins are produced under quite different conditions in terms of alkali charge and pulping time/temperature. While the chemical structure of Indulin AT is well documented, that of BCL is totally unknown. In this study, chemical structures of BCL and Indulin AT are characterized using modern analytical techniques and compared with those of pine milled wood lignin (MWL) in order to elucidate the structural changes that occur during kraft pulping and the structural differences, if any, between BCL and Indulin AT. Both BCL and Indulin AT are structurally very different from the native lignin (MWL) in wood, indicating drastic structural modification during the kraft pulping process. Surprisingly, BCL and Indulin AT are structurally very similar, in spite of the fact that they are produced under different process conditions. However, there are subtle structural differences between BCL and Indulin AT. BCL has higher phenolic hydroxyl, catechol, enol ether, and stilbene contents, but lower methoxyl and β-O-4 contents. These differences are explained by the different pulping conditions under which the two technical lignins are produced.     

The Lignin of the Diffuse Porous Angiosperm Tree Triplochyton scleroxylon K. Schum with Low Syringyl Content

“Milled wood lignin” was isolated from Triplochyton scleroxylon tree (wawa MWL) and subjected to elemental analysis before and after acid treatment, and to analytical pyrolysis. Both FT-IR and ¹H NMR spectra were recorded. For comparison, lignins from spruce and beech isolated by the MWL, dioxane/HCl (DIL), and Cuene (CUL) techniques were also characterized. The residual carbohydrate content of the lignins was estimated. Wawa MWL is a GS lignin with 88% G and 12% S content. The utility of analytical pyrolysis and FT-IR for lignin classification is discussed. FT-IR spectroscopy, calibrated with data of 50 MWLs (PLS approach), gives reliable results with a minimum of experimental work. The yields of S phenols obtained by pyrolysis has to be multiplied with a factor around 0.5 for correction if the S content of a lignin is low. The cross-linking indices and OH_phen/OH_aliph ratios of lignins increase in the order MWL < DIL < CUL. ¹H NMR spectroscopy is also helpful to recognize the S/G ratio of a lignin.     

FT–Raman Investigation of Milled-Wood Lignins: Softwood,Hardwood, and Chemically Modified Black Spruce Lignins

Abstract

Raman spectroscopy is being increasingly applied to study wood and other lignin-containing biomass/biomaterials. Lignin's contribution to the Raman spectra of such materials needs to be understood in the context of various lignin structures, substructures, and functional groups so that lignin-specific features could be identified and the spectral information could be interpreted usefully. Additionally, to enhance the utility of Raman as a characterization tool, an understanding of chemical-treatment-induced changes to the lignin spectrum is important. In the present work, Raman spectra of four milled-wood lignins (MWLs)—black spruce, loblolly pine, aspen, and sweetgum—were compared, and using black spruce MWL, spectral changes brought about by alkaline hydrogen peroxide bleaching, hydrogenation, acetylation, and methylation reactions were analyzed. The band intensity changes depended upon the nature of the chemical treatments.     

Preparation of Enzymatically Liberated Lignin From Naturally Brown-Rotted Wood

Abstract

Brown rots from various conifer species were sieved (&lt60 mesh) and alcohol and water extracted to yield crude preparations of “naturally enzymatically liberated lignin” (NEL) containing 61.4–91.7Z of Klason plus soluble lignin and 0.5–2.6% ash. The <200 mesh fractions were generally best represented in <60 mesh preparations and contained highest Klason plus soluble lignin percentages (86.6–92.2Z). Carbohydrates varied in these fractions between 3.7 and 8.0% and contained glucose, mannose, galactose, xylose, and arablnose, decreasing generally in that order. Methoxyl contents were lower and oxygen contents higher than in milled wood lignin (MWL) suggesting some oxidative demethylation. Treatment with Cuoxaa increased Klason plus soluble lignin content to 93.0–95.8%, decreased carbohydrates to 1.7–5.0%, increased methoxyl and decreased oxygen contents by removal of a part of cellulose and hemicelluloses and of more degraded lignin fractions. Infrared spectra of the preparations showed a small increase in carboxyls and possibly in phenyl conjugated double bonds and a decrease in aromatic structures, as compared with MWL. In thermo-gravimetric analysis the curves for MWL and Cuoxam treated NEL preparations (CuNEL) were very similar when run in nitrogen. In air, however, MWL lost weight appreciably slower. In differential scanning calorimetry in air and oxygen, MWL exhibited less intensive exotherms below 500°C than NEL and left higher amounts of char. The different behaviour of MWL in thermal analysis in oxidative atmosphere was explained by its lower molecular weight and corresponding low glass transition temperature (Tg). It was concluded that preparation of enzymatically liberated lignin from natural brown rot represents a convenient procedure where larger amounts of lignin are required; such lignins are somewhat more degraded than MWL in terms of functional groups present, but are possibly closer to protolignin in terms of molecular weight.     

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.     

The methanogenic toxicity of wastewater lignins and lignin related compounds

Lignin derivatives are major components of wastewater streams generated in the chemical processing of wood. The objective of this study was to evaluate the inhibitory effect of various lignins isolated from forest industry wastewaters, and selected lignin model compounds, on methanogenic bacteria. The methanogenic inhibition was determined at 30°C in standard toxicity assays, utilizing anaerobic granular sludge as inoculum. The wastewater lignins differed considerably in their inhibitory activity. Some lignin samples were nontoxic, whereas others caused 50% inhibition at concentrations ranging from 3320 to 5950 mg COD dm^?3. Experiments with ultrafiltered lignins revealed that the toxicity of the inhibitory lignin samples originated from the low molecular weight fraction. In additional studies with low molecular weight lignin model compounds, it was observed that the inhibitory activity of these compounds was related to the functional groups on the aromatic ring. Compounds with aldehyde groups or apolar substituents were highly toxic, whereas those with carboxylic groups only caused significant inhibition at high concentrations. These results indicate that low molecular weight lignin derivatives in forest industry wastewaters are potential inhibitors of anaerobic treatment systems.     

Comparison of different methods for lignin determination as a basis for calibration of near-infrared reflectance spectroscopy and implications of lignoproteins

Three commonly employed methods for lignin determination, i.e., the thioglycolic acid (TGA), the acetylbromide (AB), and the acid detergent fiber (ADF) method, were compared using leaves and xylem tissue from five species (Nicotiana tabacum, Populus x canescens, Fagus sylvatica. Quercus robur, and Picea abies). In each case, cell walls were isolated before lignin determination. Each of the three methods estimated a different lignin concentration in a given tissue, except for spruce wood. The lignin concentration determined with the AB method was strongly dependent on whether or not the cell walls were subjected to alkaline hydrolysis to remove covalently bound aromatic nonligneous components before lignin determination. Lignin concentrations determined in hydrolyzed cell walls of different tissues and species by the AB method showed a good correlation with those obtained by the TGA method and, thus, were convertible. In contrast, gravimetrically estimated ADF lignins did not or only moderately correlate with lignins measured with methods based on the UV absorbance of the solubilized lignin degradation products. Leaves of a given species generally contained higher ADF-lignin concentrations than the corresponding stem tissue. Both ADF and TGA lignin data of beech were used to calibrate near-infrared reflectance spectra (NIRS) for lignin prediction. Both NIRS calibration procedures gave good statistical fits with correlation coefficients close to 1, indicating that TGA and ADF lignin concentrations of beech can be estimated by NIRS with high accuracy. However, the two calibrations were based on different empirical terms, showing that TGA and ADF lignins did not share the same physical basis for calibration. C/N analysis revealed the presence of 3.1 and 1.4% nitrogen in ADF lignins of beech leaves and wood, respectively. The major fraction of this nitrogen was recovered in amino acids, which corresponded to 14% and 3% protein in ADF lignins of leaves and wood, respectively. These results show that ADF lignins contain significant concentrations of lignin-bound proteins, which renders this method unsuitable to determine genuine lignin.     

Reactions of Conventional Kraft and Superbatch Pulp Residual Lignins with Peroxyformic Acid

Abstract

The residual lignins KRL and SRL, isolated from a conventional kraft pulp and a SuperBatch pulp respectively, were reacted with peroxyformic acid (PFA). The reagent consumptions were determined and the products fractionated according to their solubility. Both lignins consumed a roughly equal amount of PFA, which was nearly 50% higher than that consumed by kraft lignin. The undissolved fractions of PFA-treated residual lignins (yield >80%) contained less aromatic units, phenolic hydroxyl and methoxyl groups, and considerably more carboxyl groups (including those of formate esters) than the untreated lignins. The extent of these modifications was approximately equal for KRL and SRL, which together with the similar PFA consumptions and product yields of KRL and SRL indicates that the two residual lignins showed no essential difference in reactivity toward PFA. The effect of fiber wall morphology on delignification efficiency during PFA delignification is suggested to be small because of the cleavage of lignin-carbohydrate linkages occurring under acid conditions. The part of KRL remaining insoluble after PFA-treatment had a 40% higher molar mass than KRL while in the case of SRL there was hardly any such difference in molar mass. The lignins solubilized during the PFA treatments represented the most highly degraded part of the lignins, having very low molar masses and being richer in carboxyl groups and lower in aromatic units, phenolic hydroxyl, and methoxyl groups than the undissolved lignins.     

Analytical Pyrolysis of Hardwood and Softwood Lignins and Its use in Lignin-Type Determination of Hardwood Vessel Elements

Abstract

Pyrolysis-gas chromatography-mass spectrometry was performed on milled wood lignins (MWLs) and wood samples. Among the major pyrolysis products identified from loblolly pine lignin were guaiacol, 4-methylguaiacol, 4-vinylguaiacol, vanillin, coniferal-dehyde, and coniferyl alcohol. White oak lignin pyrolysis products also included guaiacol, 4-methylguaiacol, and vanillin and additionally 2,6-dimethoxyphenol, 4-methyl-2,6-dimethoxyphenol, syringaldehyde, and sinapaldehyde. By identification of these pyrolysis products from either wood or MWL it is possible to classify lignins as either guaiacyl-type or syringyl/guaiacyl-type. Pyrolysis of isolated vessel elements from white oak, white birch, and American elm indicated that vessel lignin is of the syringyl/ guaiacyl-type.     

Structural Characterization of the Lignins from the Green and Yellow Bamboo of Bamboo Culm (Phyllostachys pubescens)

Milled wood lignins were isolated from green and yellow bamboo (Phyllostachys pubescens), termed MWLg and MWLy, respectively. The structural features of the lignin preparations and the remaining LCC were characterized by quantitative ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy and two-dimensional heteronuclear signal quantum coherence NMR (2D HSQC NMR) spectroscopy. The results indicated that the main substructures in MWLg and MWLy were β-O-4 alkyl-aryl ether, resinol, phenylcoumaran, spirodienone, and α,β-diaryl ether, and their abundances per 100 Ar units were 38.2 and 39.8, 6.9 and 6.3, 3.8 and 2.9, 1.7 and 2.1, 0.4 and 0.3, respectively. The S:G:H ratios of MWLg and MWLy were estimated to be 40:54:6 and 51:44:5, respectively. The γ-acylation of green bamboo lignin was 17.22%, lower than that of yellow bamboo lignin (21.12%). Moreover, a flavonoid compound (tricin) was also detected in the MWLg and MWLy. Some carbohydrates remained in the purified MWL preparations, which were considered as the lignin-carbohydrate complexes (LCC).     

Direct liquefaction of different lignocellulosics and their constituents: 1. Fractionation,elemental composition

Oils have been produced thermochemically from the following lignocellulosic materials by applying constant hydrogenolytic conditions in an aqueous phase with palladium as catalyst : spruce and birch wood, spruce and birch holocellulose, cellulose, pine bark, spruce and bagasse organosolv lignins, and birch Willstätter lignin. The liquid products were separated into water-, acetone- and dichloromethane-soluble (oil) fractions. The highest oil yield was obtained from spruce organosolv lignin (64%) and the lowest from carbohydrates and pine bark (20–31%). The oils were separated into neutral, strongly acidic and weakly acidic fractions by a liquid/liquid extraction procedure. Sequential elution by solvent chromatography was used for additional comparative characterization of the oils. Calorific values of the feedstocks and oils were calculated based upon the data of elemental analysis.     

木质素结构及热解特性

为研究结构类型和提取方法对木质素热解特性的影响,本研究采用Klason方法预处理得到的棉杆、核桃壳Klason木质素和贝克曼方法预处理得到的核桃壳磨木木质素（MWL）,并结合傅里叶红外光谱仪（FT-IR）和快速热解仪-气相色谱/质谱联用（Py-GC/MS）对其化学结构和热解特性进行分析。FT-IR表明碱木质素为G型木质素,棉杆木质素属于GS类型木质素,而核桃壳木质素具有 HGS木质素的特征;Py-GC/MS分析发现原料的变化以及提取方法对木质素热解产物的组成有明显的影响,在棉杆Klason木质素中邻苯二酚类产物含量为28.18%,而在核桃壳Klason和磨木木质素的含量分别为18.12%和35.11%。同时研究表明经两种方法处理的木质素结构有明显的不同,其中Klason木质素结构中苯丙酸侧链上的α-和β-醚键明显断裂,而贝克曼处理主要体现在苯基芳基醚键的断裂。     

Fractionation of Lignocellulosic Materials for the Biorefinery: Separation and Characterization of Lignin from Calamagrostis angustifolia Kom

Dewaxed Calamagrostis angustifolia Kom was pretreated with hot water at 60 and 90°C for 3 h, and then the residue obtained was successively treated with 70% ethanol, and 70% ethanol containing 0.2%, 1.0%, 2.0%, 4.0%, and 8.0% NaOH at 80°C for 3 h. The dissolved components were subjected to further separation to get eight lignin fractions, which were characterized by gel permeation chromatography, Fourier transform infrared, and sugar analysis. All the lignin fractions had small weight-average molecular weights between 810 and 2580 g/mol. Two typical lignins, L₃ (prepared with 70% ethanol) and L₅ (prepared with 70% ethanol containing 1.0% NaOH), were further analyzed using ¹H, ¹³C NMR and HSQC spectroscopy. Signals from guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units observed in aromatic/olefinic region of HSQC spectra indicated that the lignin from Calamagrostis angustifolia Kom could be classified as “GSH” lignin. In aliphatic-oxygenated region, β-O-4′ together with small amounts of β-5′, β-β′, and p-hydroxycinnamyl alcohol end group were the main interunit linkages observed. Aqueous ethanol, which could avoid the cleavage of ether bonds in lignin at neutral condition, was more effective than water on lignin extraction.     

The Characteristics and Reactivity of Acid-Labile Aryl Ether Units in Wood Lignin

Wood lignin contains significant amounts of acid-labile aryl ether units, which play a significant role in lignin modification or delignification processes. We have evaluated the rate and reaction kinetics on the acid-catalyzed cleavages of aryl ether structures for wood lignin in situ based on the formation of phenolic hydroxyl groups. The content of acid-labile aryl ether units was quite uniform for a variety of softwood wood lignins (～4% per C₉ unit) and it varied appreciably among hardwood species, ranging from 4% for aspen to 9% for beech wood lignin. These variations, however, appear to be related to the content of syringyl units in wood lignin. The reactivity of these reactive aryl ether structures was noticeably higher for the spruce than for the aspen wood lignin. This difference in reactivity, based on the behavior of lignin model compound reactions, can be attributed to the influence of syringyl moieties in aspen wood lignin. It appears that most of the acid-labile aryl ether units in hardwood were associated with the syringy moiety being present as a benzyl unit, which is much less reactive than the corresponding guaiacyl moiety.     

Two-Stage Hot-Water Extraction of Galactoglucomannans from Spruce Wood

In order to preserve the polymeric structure and the acetylation degree of extracted galactoglucomannans and, at the same time, achieve high yield, ground spruce wood was subjected to a series of sequential two-stage extractions with an Accelerated Solvent Extraction (ASE) apparatus using plain water at 170°C. The total combined extraction time was one hour in all the extractions. The total yield of the dissolved material after 1 h extraction was almost the same, about 25% of the wood, irrespective of the time ratios between the first and the second extractions. The yield of hemicellulose high polymers with the weight average molar mass of 8–10 kDa during the first extraction had a maximum at 20 min extraction time, amounting to about 7% on dry wood basis, and comprising about half of the total extract. Along with the progress of the extraction, the molar mass of the hemicelluloses decreased and hemicellulose-derived low polymers with the weight average molar mass of 6–2 kDa became dominating. The extracted substances were fractionated, mainly according to their molar mass, by sequential precipitation with ethanol, acetone, and methyl tert-butyl ether (MTBE). The hemicelluloses with some amount of pectins comprised 83–90% of the precipitated polymeric material and the content of galactoglucomannans was about 80%.