Effects of pretreatment temperature and alkali concentration on the composition of alkali-soluble lignins from wheat straw

A new rapid method of two step precipitation for isolation of alkali-soluble lignins with relatively free of polysaccharides from wheat straw was proposed in this study. The isolated alkali-soluble lignin fractions LA contained rather low amounts of neutral sugars (0.7–1.0%) and had low average molecular weights (960–1440 Da). The isolated lignin fractions LA contained roughly equal amounts of guaiacyl (G) and syringly (S) units with relatively fewer p-hydroxyphenyl (H) units, and appeared to be closely associated to phenolic acids and glucuronic acid or 4-O-methylglucuronic acid. The chemical compositions in each of the fractions are reported. The physico-chemical properties and structure features of these lignin fractions LA were characterized by UV-, IR-, ¹³C-NMR-spectroscopy, and alkaline nitrobenzene oxidation. © 1996 John Wiley & Sons, Inc.     

Structural characterization of residual lignins isolated with cyanamide-activated hydrogen peroxide from various organosolvs pretreated wheat straw

The posttreatment of various organosolvs pretreated wheat straw with cyanamide-activated hydrogen peroxide was studied. About 44–80% of the total residual lignin and 38–85% of the total residual hemicelluloses were released or degraded during the posttreatment with 1.8% H₂O₂–0.18% cyanamide at 50°C under pH 10.0 for 4 h from different aqueous organic acids or alcohols pretreated straw. The seven degraded residual lignin preparations were subjected to a comprehensive physicochemical and structural characterization by UV, FTIR, and ¹H and ¹³NMR spectroscopy, and GPC. The nitrobenzene oxidation method was also applied to the in situ lignins. It was found that the seven residual lignin preparations contained large amounts of noncondensed syringyl and guaiacyl units, together with fewer noncondensed p-hydroxyphenyl units, esterified p-coumaric acid, and mainly etherified ferulic acid. All of the lignin fractions are free of associated polysaccharides and had molecular-average weights ranging between 2980 and 3820 g mol⁻¹. Analysis of these low molecular weight degradation products revealed an oxidation of residual lignin had occurred. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Chemical Factors Underlying the More Rapid β-O-4 Bond Cleavage of Syringyl than Guaiacyl Lignin under Alkaline Delignification Conditions

This study aimed to clarify why the β-O-4 bond cleavage of syringyl lignin is more rapid than that of guaiacyl lignin under alkaline pulping conditions. We examined whether or not three chemical factors, acidity of the α-hydroxy group, nucleophilicity of the generated α-alkoxide, and leaving ability of the leaving phenoxide, are different between syringyl and guaiacyl lignins and control the rate of the alkaline-induced β-O-4 bond cleavage, employing dimeric non-phenolic β-O-4-type lignin model compounds (LMCs) and novel methods for estimating these three factors. The results indicated that the α-hydroxy groups of syringyl-type LMCs are relatively more acidic than those of guaiacyl-type and that syringyl nucleus is a better leaving group than guaiacyl nucleus in the β-O-4 bond cleavage. These factors result in the β-O-4 bond of syringyl lignin being more prone to the alkaline-induced cleavage than that of guaiacyl lignin.     

BEHAVIOR OF EUCALYPTUS GLOBULUS LIGNIN DURING KRAFT PULPING. II. ANALYSIS BY NMR,ESI/MS,AND GPC

ABSTRACT

Residual and dissolved lignin from different phases of kraft delignification of Eucalyptus globulus wood were isolated and characterized by 1D and 2D ¹H NMR, ¹³C NMR, Electrospray Ionization Mass Spectrometry (ESI/MS), and gel permeation chromatography (GPC). During the temperature rise period, below 70°C, about 20% of the lignin was dissolved without significant structural changes. Above 70°C, the lignin suffered significant degradation/fragmentation in the cell wall prior to dissolution. The lignin ether-linked syringyl units were the most susceptible to alkaline degradation. Through the course of pulping, the residual lignin (RL) revealed a gradual increase of aliphatic moieties of unknown structure, as well as a decrease of native structures such as phenylcoumaran and pino-/syringaresinol lignin units. A significant decrease of the β-O-4 structures content in RL was detected only at the final cooking temperature. The lignin dissolved in the black liquor (BL) consisted of highly branched oligomers with rather low molecular weight (average mass 800–1000 u). A part of BL (about 30%) was chemically linked to carbohydrates and possessed a large molecular weight distribution (500–4000 u). BL showed a progressive decrease in β-O-4 and pino-/syringaresinol structures and formation of enol ether and stilbene structures. The GPC analyses showed a continuous decrease of the molecular weight of both the residual and dissolved lignins during the pulping process, particularly in the residual stage.     

Separation and Structural Characterization of Lignin from Hybrid Poplar Based on Complete Dissolution in DMSO/LiCl

In this paper, the physicochemical properties and structural features of six lignin preparations separated with a total dissolution of ball-milled wood in dimethyl sulfoxide and lithium chloride (DMSO/LiCl) followed by extraction with ethanol/water were investigated. These isolated lignin fractions were characterized using wet chemical analysis, FT-IR, and ¹H and¹³C-NMR techniques. Experimental results showed that separated lignin preparations were relatively free of associated polysaccharides. These lignin fractions were classified as guaiacyl-syringyl lignin type: mainly composed of guaiacyl units with noticeable amounts of syringyl units and fewer p-hydroxyphenyl units. The molar ratio of non-condensed guaiacyl units to syringyl units (G/S) decreased as the ratio of LiCl to poplar weight increased. The results also showed that these lignin preparations consisted mainly of β-O-4 ether bonds combined with small quantities of β-β′ and β-5 carbon-carbon linkages. Furthermore, considerable amounts of esterified p-hydroxybenzoic acids and minor amounts of esterified p-coumaric acid were also detected in these lignin fractions.     

Fractionation and Characterization of Water-soluble Hemicelluloses and Lignin from Steam-exploded Birchwood

Abstract

Two crude hemicellulosic fractions, obtained by extraction of steam-exploded birchwood with hot water, were treated with 8% NaOH at 20[ddot]C for 1 and 4h, and subsequently sub-fractionated into four lignin and four hemicellulosic fractions. Acid hydrolysis, alkaline nitrobenzene oxidation, ultraviolet (UV), gel-permeation chromatography (GPC), Fourier transform infrared (FT-IR), and carbon-13 nuclear magnetic resonance (NMR) spectroscopies were used to investigate the chemical compositions and structural features of the fractionated hemicelluloses and lignins. The sugar analyses indicated that xylose was the predominant sugar component in the four hemicellulosic fractions. Due to the autohydrolysis at elevated temperature and lower in acidity during the steam treatment processes, all the four hemicellulosic fractions showed a low degree of polymerization (DP, 38–41), with molecular-average weights between 5620 and 6160. Assignments of all the signals in the NMR spectrum led to the conclusion that the four lignin fractions, which differ from the organosolv lignins obtained from steam-exploded aspen wood meals, are still mainly composed of β—O—4 ether bonds, together with small amounts of less common β—β and β—1 carbon-carbon linkages between the lignin structural units. The weight-average molecular weights were found to be 2250–2620 with the polydispersity of 1.5.     

Quantitative Phosphorus-31 NMR Analysis of Six Soluble Lignins

By using a set of lignin samples, which have been subjected to thorough analyses by the international wood chemistry community, the recently developed quantitative method of ³¹P NMR spectroscopy was comprehensively examined. The values of total phenolic hydroxyl groups and those of total hydroxyl groups were found to favourably compare with those obtained by other laboratories, applying independent methods of analysis. Furthermore, the application of quantitative ³¹P NMR spectroscopy offered additional detailed structural information for the examined lignins which was in accord with literature accounts for similar wood species and lignin preparations. More specifically, the steam explosion lignins from aspen and yellow poplar woods and that produced by ball milling/enzyme hydrolysis of cottonwood were found to contain relatively high amounts of β-O-4 structures. In contrast, the kraft, organosolv, and the acid hydrolysis processes were found to induce significant chain scission on the resulting lignins. Ball milled cottonwood lignin contained the highest frequency of β-O-4 bonds and the lowest amount of free phenolic hydroxyls. The erythro form of β-O-4 structures were invariably predominant in the lignins from aspen, yellow poplar and cottonwood, in accord with the conclusions of previous reports on hardwood lignins. Thus, the application of quantitative ³¹P NMR spectroscopy offered the detailed chemical composition of the examined lignins.     

Some physical properties of acetosolv lignins from bagasse

Acetosolv-solubilized lignins were characterized by their solubility in different organic solvents, Fourier transform infrared, ¹³C-NMR, UV, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Solvents having a solubility parameter in the range of 10–12.7 and a hydrogen-bonding parameter in the range of 3.6–5 were considered good solvents for acetosolv lignins. Fourier transform infrared spectra of the lignins were typical for lignins containing p-hydroxy phenylpropane (H), guaiacyl (G), and syringyl (S) units. The lignins contained more conjugated and fewer nonconjugated CO groups, and the guaiacyl groups were etherified and condensed. ¹³C-NMR confirmed partial acetylation of the lignins and the presence of β-O-4 and β-5 linkages. Acetosolv lignins also showed the typical UV spectrum of annual plants. The effects of the acetic acid concentration and pulping time on the molecular weights of the lignins were explained with the presieving and condensation concepts. The thermal behavior of the acetosolv lignins was also studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Extraction and characterization of lignins from maize stem and sugarcane bagasse

Lignins were isolated from maize stem and sugarcane bagasse by using mild dioxane or acidic dioxane solution. The result of nitrobenzene oxidation of the isolated lignins shows that there is a high proportion of p‐hydroxyphenyl alcohol in the lignins of maize stem and sugarcane bagasse. The lignins isolated from maize stem and sugarcane bagasse have relatively same value of the weight‐average (M _w = 3405–3868 g mol⁻¹) and number‐average (M _n = 1411–1612 g mol⁻¹) molecular weights, and polydispersity (M _w/M _n = 2.24–2.51). Acidic dioxane treatment did attack the β‐aryl ether structures in lignins, in particular for β‐aryl syringyl ethers, and broke the ester bonds between arabinose and ferulic acid that etherified to lignins, and it also cleaved lots of bonds in hemicellulosic polymer. The proportion of β‐O‐4 (threo) guaiacyl units is higher than that of β‐O‐4 (erthreo) guaiacyl units. The phenyl glycoside and benzyl ether linkages between lignin and hemicelluloses are also demonstrated in NMR analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Hydroxymethylation of technical lignins from South American sources with potential use in phenolic resins

This work investigates the valorization of sodium lignosulfonate, kraft, and organosolv lignins from South America. A detailed characterization of the lignins and their chemical modification by hydroxymethylation through its reaction with formaldehyde were performed. The characterization included measurements of moisture, ash, carbohydrate contents, elemental and thermogravimetric analysis, and functional groups, molar mass distributions by Fourier transform infrared spectroscopy, and size exclusion chromatography, respectively. Also, reactive aromatic hydrogens ( H_Ar) were quantified by the measurement of phenolic hydroxyl groups (P-OH) content by UV–Vis spectroscopy. The different initial formaldehyde/lignin weight ratios (0.07, 1.47), temperatures (40, 50, and 70 °C), and pHs (9, 11); and the following of hydroxymethylation reactions by UV–Vis spectroscopy were investigated. All lignins resulted attractive for the use as replacement of phenol in phenolic resins, but sodium lignosulfonate was the most appropriate due to its water solubility. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47712.     

Molecular Mass Distribution of Lignin from the Alkaline Pulping of Hardwood,Softwood, and Wheat Straw

Abstract

The behavior of lignin during kraft (hardwood, softwood, and wheat straw) and soda-AQ (wheat straw) pulping was studied, mainly in terms of delignification degree and molecular mass distribution (MMD). In the initial delignification phase (at 140°C for 15–60 min), a prominent part of the dissolved softwood kraft lignin (18–25 g/L, MM mostly > 3,000 Da) was found in the liquid phase of chip cavities, rather than in the external bulk black liquor (5–7 g/L, MM mostly < 3,000 Da). The maximum weight average MM values ( _w) of the soluble lignin under conventional cooking conditions were detected for the kraft softwood (4,100 Da), and kraft birch (3,400 Da) when the degree of delignification was 65–75%, corresponding to a residual lignin content of 5–10%. The maximum _w of the dissolved wheat straw kraft (5,050 Da) and soda-AQ (5,900 Da) lignins was clearly higher than that of wood-based kraft lignins (2,950–4, 100 Da).     

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。     

Physicochemical characterization of lignins from rice straw by hydrogen peroxide treatment

Extraction of the dewaxed rice straw with 1% NaOH at 55°C for 2 h and following treatment without and with 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0% hydrogen peroxide (H₂O₂) at 45°C for 12 h at pH 11.5 resulted in a dissolution of 68.3, 85.4, 89.4, 92.3, 92.3, 94.3, and 95.1% of the original lignin, respectively. Meanwhile, the two‐stage treatment together solubilized 67.2, 77.2, 78.7, 83.7, 85.5, 87.3, and 88.5% of the original hemicelluloses and degraded 2.5, 9.8, 11.8, 12.1, 15.6, 16.4, and 17.8% of the original cellulose under the conditions given, respectively. Analyses of these lignins revealed that alkali‐soluble lignin fractions did not suffer sever oxidation, but nearly 60% of the original lignin was dissolved out during the first stage of alkali treatment. In the second stage of alkaline peroxide treatment, the residual lignins were substantially released and enriched in oxidized carbonyl and carboxyl groups. In comparison, the isolated eight pure lignin samples were further characterized by both destructive methods such as alkaline nitrobenzene oxidation and nondestructive techniques such as ultraviolet (UV), Fourier transform infrared (FTIR), and carbon‐13 magnetic resonance spectroscopy (¹³C‐NMR) as well as gel permeation chromatography (GPC), and the results are reported. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 719–732, 2001     

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part II. Organosolv Fractionation of Lignins in a Flow-Through Reactor

Various spruce mechanical pulps were subjected to delignification with ethanol-water (1 : 1, v / v ) containing 0.1 M acetic acid at 175°C in a flow- through reactor. A thermomechanical pulp (TMP) and the corresponding samples derived from its bleaching (BTMP) and yellowing ( YBTMP ) treatments were delignified to a similar extent, about 70% of delignification degree, as compared with spruce wood. A series of five successive lignin fractions was recovered from each pulp sample and then characterized by various analytical methods. Large structural variations were observed within these fractions. The number of phenolic structures appeared to be an important factor influencing the dissolution of lignins in the ethanol-water medium. These lignin fractions were found to be different in the amount of β-aryl ether structures and in the relative importance of main carbon-carbon bonding patterns. The results are discussed in relation to the lignin fractionation in the flow-through reactor.     

Effect of ultrasound on the physicochemical properties of organosolv lignins from wheat straw

The extractability and physicochemical properties of the wheat straw lignins were comparatively studied by using extraction methods with 0.5M NaOH in 60% aqueous methanol with and without application of ultrasonic irradiation. The results showed that applying sonication for 5, 10, 15, 20, 25, 30, and 35 min solubilized 67.4, 68.6, 74.4, 77.3, 77.3, 77.9, and 78.5% of the original lignin, whereas the treatment with 0.5M NaOH in 60% aqueous methanol at 60°C for 2.5 h without ultrasound assistance released 61.0% of the original lignin. The lignin preparations isolated by ultrasound‐assisted extractions showed slightly lower molecular weights, associated polysaccharides, and thermal stabilities during the initial stage of decomposition. More important, there were no significant differences in the primary structural features between the lignin preparations. Ultrasound‐assisted extractions under the alkaline organosolv extractions did not affect the overall structure of the lignin from wheat straw. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2512–2522, 2002     

Enhancing water resistance of welded dowel wood joints by acetylated lignin

Low molecular mass acetylated organosolv lignin from wheat straw and from depolymerised low sulphur organosolv wood lignin have been shown to markedly improve both the water resistance and the mechanical performance of welded dowel wood joints. The acetylated oligomers distribution and extent of acetylation of the two lignins were determined by Matrix assisted laser desorption ionization-time-of-flight mass spectrometry. Extensive acetylation was confirmed by CP-MAS ¹³C NMR spectrometry. Force–displacement measurements on welded dowel joints to which acetylated wood lignins were added showed a ductile behaviour. This is due to the interpenetration of the elastic acetylated lignin network into the more rigid composite network of the welded interphase.     

Graft polymerization of acrylonitrile onto wheat straw

Acrylonitrile was graft polymerized onto ground, water-washed wheat straw using Fe²⁺-H₂O₂ as initiator. Reaction conditions were selected to minimize homopolymer formation and maximize the amount of polyacrylonitrile (PAN) grafted to straw. Polymerizations typically yielded straw-g-PAN containing 30–35% PAN. A two-step fractionation scheme was developed for determining the relative amounts of PAN grafted to cellulose, hemicellulose, and lignin. This scheme involved (1) delignification of straw-g-PAN with sodium chlorite followed by removal of lignin-grafted PAN by extraction with dimethylformamide (DMF), and (2) hydrolysis of the hemicellulose component with 1 N trifluoroacetic acid followed by DMF extraction of hemicellulose-grafted PAN. Product remaining after these two treatments was assumed to be cellulose-g-PAN. When relative amounts of PAN grafted to cellulose, hemicellulose, and lignin were compared with relative percentages of these components present in wheat straw, the percentage of total PAN grafted to lignin was less than its relative percentage in straw, whereas that grafted to hemicellulose was considerably more. Although the use of Ce⁴⁺ as initiator gave little or no polymer with whole, water-washed straw, grafted polymerization occurred when delignified straw was used as substrate. Relative amounts of PAN grafted to cellulose and hemicellulose were not greatly different from those observed with Fe²⁺-H₂O₂ initiation onto whole straw.     

Characterization of Residual Lignins Isolated from Unbleached and Semibleached Softwood Kraft Pulps

Abstract

Residual lignins in an unbleached and a semibleached softwood (Pinus taeda L.) kraft pulp were isolated by enzymatic hydrolysis of polysaccharides in the pulps. After purification, the residual lignins were characterized. A dissolved lignin was also isolated from the alkaline extraction spent liquor by acidification and characterized.

Results of the characterization indicate that extensive degradation of residual lignin in kraft pulp occurred during the first two stages of bleaching. The results also strongly support the previous finding that stable covalent linkages between residual lignins and carbohydrates in pulp any be the most probable cause for the residual lignins to resist delignification during kraft pulping and prebleaching.