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.     

Analysis of Lignin Aromatic Structure in Wood Based on the IR Spectrum

Abstract

A total of 17 softwoods and 48 hardwoods were analyzed by IR spectroscopy to examine if syringyl ratio (syringyl/(syringyl+guaiacyl)) calculated from nitrobenzene oxidation products can be precisely expressed by area ratios of characteristic peaks of lignin in IR spectrum. Area ratio of two peaks is referred to as that of two wavenumber domains, represented by “wavenumber 1/ wavenumber 2.” Examined peak area ratios were 1595/1509, 1509/1460, 1275/1220, 1130/1032, and 835/(855+815). Among these ratios, log(1595/1509) and log(1275/1220) showed significant linear relationship with the syringyl ratios with a correlation coefficient of 0.98 for all 65 woods. These two ratios could also be used to distinguish all the hardwoods from the softwoods.     

Influence of wood species on the properties of biopolyurethane prepared from liquefied wood with residue

Biopolyurethane prepared from liquefied wood with the residue of the liquefied wood product was investigated in this article. Previous results indicated that the residue of the liquefaction product was composed mostly of compounds originated from lignin. The chemical structures of lignin in softwood and hardwood are different. The influence of soft‐ and hardwood species on the chemical structure and mechanical properties of biopolyurethane prepared from liquefied wood with residue was investigated by tensile testing and Fourier transform infrared spectroscopy. The experimental results showed that the liquefaction of softwood occurs within a shorter time than that of hardwood and the biopolyurethane prepared from softwood was harder than that prepared from hardwood, which suggests that the properties of the liquefaction product and biopolyurethane are influenced by the chemical structure of the lignin. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Hemicellulose Composition in Different Cell Wall Fractions Obtained using a DMSO/LiCl Wood Solvent System and Enzyme Hydrolysis

In our previous studies on the relationships between lignin structure and hemicellulose composition in a wide range of hardwood species, we showed that the higher the syringyl ratio, the higher the xylan/mannan ratio. In this study, the relationship of the syringyl ratio and xylan/mannan ratio was further studied using fractions obtained by stepwise extraction of finely milled beech wood by use of aqueous dioxane and subsequent dimethyl sulfoxide (DMSO)/LiCl solvents with increasing LiCl concentration. Each soluble fraction obtained with a different LiCl concentration was compared with the corresponding insoluble residue, and it was shown that both the syringyl ratio and xylan/mannan ratio were always higher in the insoluble fraction. To analyze polysaccharides closely located with lignin, milled wood lignin (MWL), and cellulolytic enzyme lignin (CEL) obtained by enzyme treatment after DMSO/LiCl swelling were investigated. Although the xylan/mannan ratio of MWL was higher, that of CEL with more than 80% lignin yield was lower.     

Characteristics and Reactivity of Lignin in Acacia and Eucalyptus Woods

The chemical characteristics of lignin, including the Klason lignin content, acid-soluble lignin content, proportion of the aromatic ring types [syringyl ratio = syringyl/(syringyl + guaiacyl)], and proportion of the diastereomeric forms for the β-O-4 structure [erythro ratio = erythro/(erythro + threo)], and pulpability were investigated for 28 wood samples belonging to the genera Acacia and Eucalyptus. Although the lignin characteristics of these 28 woods varied widely, the chemical characteristics of the two genera could be categorized in two clearly distinguished groups on the basis of the syringyl ratio. Clear negative correlations were observed between both the syringyl and erythro ratios and the total lignin content (sum of the Klason and acid-soluble lignin contents) within each genus. In addition, the syringyl ratio correlated positively with the erythro ratio and acid-soluble lignin content, regardless of the genus. The existence of a clear, high correlation between the syringyl and erythro ratios supports the hypothesis that the aromatic ring type (syringyl ratio) is a decisive factor for controlling the diastereomeric forms of the β-O-4 structure (erythro ratio). Each of the wood samples was also subjected to Kraft pulping, and it was demonstrated that the woods with higher syringyl ratios were easier to delignify. This tendency is reasonably attributed to the high reactivity of the erythro-rich and syringyl-rich β-O-4 structures, and the low lignin content of these syringyl-rich woods.     

Condensation Reactions of Softwood and Hardwood Lignin Model Compounds Under Organic Acid Cooking Conditions

In order to examine the condensation reactions of softwood and hardwood lignin during organic acid cooking, mixtures of benzyl alcohol type lignin model compounds with guaiacyl and syringyl units as the sources of benzyl cations and creosol and 5-methoxycreosol as the sources of electron-rich aromatic carbons were cooked under acidic pulping conditions.

From the yield of the condensation products in the initial reactions, it was shown that the carbonium cations of guaiacyl nuclei were more reactive than those of syringyl nuclei.

Syringyl type aromatic ring carbons had higher reactivities than guaiacyl type ones.

The cleavage of the benzyl ether bond in syringyl compounds was slower than that of guaiacyl compounds.

The diphenylmethane structures formed by the condensation reaction between veratryl alcohol and 5-methoxycreosol were found to be unstable under the strong acidic cooking conditions.

It is concluded that the condensation reactions between benzylic cations from the guaiacyl compounds and the electron rich aromatic ring carbons of syringyl ones are very fast, but the condensation products are unstable under the strong acidic pulping conditions.

When the guaiacyl nuclei react as an electron-rich aromatic carbon, the reaction is slower but the condensation product is much more stable.

These differences in reactivities, and the stabilities of the condensation products, may contribute to the resistance of softwood toward complete delignification on acid pulping.     

Cold-setting wood adhesives from kraft hardwood lignin

The model compound used in this study, for hardwood kraft lignin, 3,5-dimethoxy-4-hydroxyphenyl ethane, was successfully hydroxymethylated at the position meta to the aromatic hydroxy group. The consequent expectation that kraft hardwood lignin syringyl units can, under optimum conditions, also be hydroxymethylated to a substantial extent was confirmed in the present work and made it possible to cross-link the kraft hardwood lignin to yield polymeric products that could be used as adhesives. © 1994 John Wiley & Sons, Inc.     

Characterization of soda hardwood lignin and the formation of lignin fibers by melt spinning

Lignin fibers were developed from a commercial available soda hardwood lignin (SHL) with a melt‐spinning approach. SHL showed spinnability to form the fine fibers when poly(ethylene oxide) was used as a plasticizer with lignin. The thermal properties of lignin provided valuable information to assist the processing steps of the lignin fiber formation. The guaiacyl/syringyl ratio in SHL was determined by ³¹P‐NMR because it had great influence on the thermal mobility of lignin. A suitable temperature profile for the melt spinning was predicted through rheological studies of lignin. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Spectroscopic studies and evaluation of thermorheological properties of softwood and hardwood lignin

Fourier transform infrared spectroscopy ( FTIR) was used to determine characteristic absorption peaks of softwood kraft lignin and soda hardwood lignin. Remarkable spectral differences were noticed between the different lignins due to their various chemical structures. Proton nuclear magnetic resonance spectrometry (¹H NMR) was employed to analyze the structure of lignin. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry ( MALDI‐TOF) provided important data regarding the molecular weight distribution of lignin. Thermal stability of softwood lignin was found to be remarkably higher than that of hardwood lignin. Softening temperature and glass transition temperature of lignin were measured by differential scanning calorimetry (DSC) which was useful in selecting an optimal temperature profile during extrusion process. Rheological studies provided valuable information about the viscosity of lignins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Experimental study of the charring rate of tropical hardwoods

This paper describes an experimental investigation of the charring rates of timber. An experimental procedure was designed for assessing the charring rate of 20 specimens exposed to fire in a single test, with one‐dimensional heat transfer conditions. Each test specimen was manufactured by gluing seven laminates together. Four thermocouples were inserted at different depths in four different laminates located in the middle of the test specimen. The test was conducted using a gas‐fired furnace and specimens were exposed to the standard ISO 834 fire. In order to evaluate the fire performance of tropical hardwoods, seven different species with densities ranging from 500 to 1000 kg/m³ were used. For the purpose of verifying the experimental procedure against existing data, two softwood species (spruce and fir) and one European hardwood species (oak) were also examined. Experimental results indicate that the test method and procedure can be used for assessing the charring rate of timber both for softwood and for hardwood species. They also show that the density of wood significantly affects the charring rate and that the values recommended in Eurocode 5 for high densities are somewhat too conservative. Copyright © 2004 John Wiley & Sons, Ltd.     

A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy

Hard and softwood and wood constituent polymers (cellulose and lignin) were studied using Fourier transform infrared (FTIR) spectroscopy. The hollocellulose‐to‐lignin ratio was estimated for some of the timber species. The structural difference between Klason lignin isolated from softwood (Pinus roxberghii and cupressus lusitanica) and hard wood (Acacia auriculaeformis and Eucalyptus tereticornis) species was studied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1969–1975, 1999     

Fractionation-Purification,IR, 1H 13C NMR Spectral and Property Studies of an Industrial Based Sludge Lignin

Abstract

A flow scheme was devised for the fractionation-purification of sludge lignin material (mixture of hardwood and softwood) used in the synthesis of a hardboard adhesive. Infrared spectroscopy, proton and carbon-13 nuclear magnetic resonance spectroscopy was used to assign the structure of the sludge lignin. The structural characterization indicated appreciable bond cleavage-rearrangement, and subsequent generation of additional acidic/ketone linkages, p-hydroxyphenyl units and saturated hydrocarbons. Additionally, the sludge lignin based extracts showed good adhesive bonding properties when used in a blend.     

Pulping with organic acids: 3-acetic acid pulping of Bagasse

Bagasse was subjected to acetic acid pulping; the effects of the addition of 0.5% H₂SO₄ to the pulping solutions (50, 70 and 90% acetic acid) and varying the liquor ratio, on pulp yield, delignification %, pentosan content and strength properties of the pulps obtained were evaluated. Addition of H₂SO₄ improved the delignification but at the expense of yield and pentosan content. The breaking length and burst deteriorated, whereas the tear was slightly improved. Raising the liquor ratio from 5:1 to 10:1 also reduced the strength properties of the pulps. Delignification of Casuarina was easier than bagasse. IR studies of the acetic acid lignins obtained from both raw materials showed that Casuarina lignins contained more syringyl nuclei than bagasse lignin; the latter resembled softwood lignins. Addition of H₂SO₄ to bagasse changed the syringyl-deficient to guaiacyl-deficient lignin. A relationship was found between the ease of delignification for both raw materials and the relative intensity of the bands corresponding to syringyl groups.     

Investigation and Characterization of Lignin Precipitation in the LignoBoost Process

Lignin of high purity can be separated from black liquor using the LignoBoost process, of which the overall efficiency is largely dependent on the precipitation yield of lignin, which depends on the properties of black liquor and process conditions. In this paper, the influences of process conditions on the precipitation yield of lignin from mixed hardwood/softwood black liquor were investigated. The Klason and standard UV method were used to determine lignin concentration. The chemical and structural properties of lignin were also analyzed. The results showed that the precipitation yield of lignin increased along with a decrease in pH and temperature, or with an increase in the ion strength of black liquor, and the yield was lower when mixed softwood/hardwood black liquor was used. It also showed that at a higher precipitation yield the precipitated lignin had a lower average molecular weight but had higher methoxyl and phenolic hydroxyl content.     

不同年生慈竹木素特性及分布的研究

本文研究了不同年生慈竹木素在厚壁纤维细胞中的分布,发现纤维细胞壁的木素含量最低,而纤维细胞角隅的木素含量最高。在1—3年生的竹纤维细胞中,复合胞间层的木化速度较次生壁快,而三年生之后则相反。对不同年生慈竹磨木木素的分析表明,慈竹木素的相对分子质量较软木木素小,而酚羟基含量较高。慈竹木素中有共轭酯键存在,并有较高的紫丁香基结构单元,同时有一定量的对羟苯丙烷单元存在,尤其是一年生慈竹木素含有很高的对羟苯丙烷单元,慈竹木素中主要的残余糖分是阿拉伯糖和木糖。     

The Role of Carbohydrates in Alkali Anthraqutnone Pulping

The reactions of carbohydrates with anthraquinones in alkaline pulping processes are reviewed. AQ reacts mainly with the short-lived intermediates that are formed in the degradation reactions of wood polysaccharides. Oxidation of the reducing end groups of polysaccharides to stable aldonic acid end groups is marginal and corresponds to less than 1 % of all reactions of AQ. The stabilization of the polysaccharides may be enhanced by the use of salts of alkaline earth metals which increase the relative oxidation rate of sugar enediols and promote the hydride shift reaction of the intermediate aldos-2-ulose end groups to stable hexonic acid end groups. Analysis of the monomeric carbohydrate-derived oxidation products indicates that hardwood lignin probably contains more structures that react with anthrahydroquinone than does softwood lignin. This difference may partly explain the more facile delignification of hardwoods.     

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.     

Oxidation of Various Kraft Lignins with a Bacterial Laccase Enzyme

Modification of kraft lignin (KL), traditionally uses harsh and energy-demanding physical and chemical processes. In this study, the potential of the bacterial laccase CotA (spore coating protein A) for oxidation of KL under mild conditions was assessed. Thereby, the efficiency of CotA to oxidize both softwood and hardwood KL of varying purity at alkaline conditions was examined. For the respective type of wood, the highest oxidation activity by CotA was determined for the medium ash content softwood KL (MA_S) and the medium ash content hardwood KL (MA_H), respectively. By an up to 95% decrease in fluorescence and up to 65% in phenol content coupling of the structural lignin units was indicated. These results correlated with an increase in viscosity and molecular weight, which increased nearly 2 and 20-fold for MA_H and about 1.3 and 6.0-fold for MA_S, respectively. Thus, this study confirms that the CotA laccase can oxidize a variety of KL at alkaline conditions, while the origin and purity of KL were found to have a major impact on the efficiency of oxidation. Under the herein tested conditions, it was observed that the MA_H KL showed the highest susceptibility to CotA oxidation when compared to the other hardwood KLs and the softwood KLs. Therefore, this could be a viable method to produce sustainable resins and adhesives.     

A new softening agent for melt spinning of softwood kraft lignin

Kraft lignin obtained from the pulping of wood is an interesting new precursor material for carbon fiber production because of its high carbon content and ready availability. However, continuous spinning of softwood kraft lignin (SKL) has been impossible because of its insufficient softening characteristics and neat hardwood kraft lignin (HKL) has required extensive pretreatments to enable fiber formation. Softwood kraft lignin permeate (SKLP) and hardwood kraft lignin permeate (HKLP), fractionated by membrane filtration, were continuously melt spun into fibers. To improve the spinnability of SKL and HKL, HKLP was added as a softening agent. SKL‐ and HKL‐based fibers were obtained by adding 3–98 wt % HKLP. A suitable temperature range for spinning was 20–85°C above the T_g of the lignin samples, and this range gave a flawless appearance according to the SEM analysis. Smooth, homogeneous fibers of SKLP, HKLP, and SKL with HKLP were successfully processed into solid carbon fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Improved Protocol for Alkaline Nitrobenzene Oxidation of Woody and Non-Woody Biomass

The protocol of alkaline nitrobenzene oxidation was investigated to improve its ability to identify the different lignin structures for both woody and non-woody biomass. The survival factors of all six oxidation products—syringaldehde (Sr), vanillin (V), p-hydroxybenzaldehyde (B) and their corresponding acids, syringic acid (Sa), vanillic acid (Va), and p-hydroxybenzoic acid (Ba)—were studied at 170, 180, and 190°C for several residence times. Under similar conditions, various lignin model compounds—a softwood (loblolly pine), a hardwood (red maple), and a non-wood raw material (corn stover)—were oxidized. Molar yields of oxidation products were determined and the ratios of (Sr+Sa)/(V+Va), (Sr/V), and B/(V+Va) (B/V) were calculated. All oxidation products were relatively stable at 170 and 180°C but showed some degradation at 190°C, especially at long residence time. In all cases, p-hydroxybenzoic acid was barely detectable. While yields of oxidation products reach a maximum at 170°C for pine and maple, maximal yields of corn stover require 190°C. Consequently, we recommend that nitrobenzene oxidation be carried out at 170°C for 2.5 h for softwood and hardwood, but at 190°C and 4 h with correction for the survival factors for corn stover and other non-woody biomass. Alternatively, a protocol of oxidation at two temperatures is recommended for non-woody biomass.