Unraveling the structural characteristics of lignin in hydrothermal pretreated fibers and manufactured binderless boards from Eucalyptus grandis
- Ling-Ping Xiao†1,
- Zhi Lin†2,
- Wan-Xi Peng2Email author,
- Tong-Qi Yuan1,
- Feng Xu1,
- Nian-Chun Li2,
- Qing-Song Tao3,
- Hang Xiang4 and
- Run-Cang Sun1, 5Email author
© Xiao et al.; licensee Chemistry Central Ltd. 2014
Received: 17 December 2013
Accepted: 4 April 2014
Published: 11 April 2014
Eucalyptus grandis is one of the most abundant biomass from plantation in many parts of the world. The binderless board were manufactured from hydrothermal pretreated fibers of Eucalyptus wood and characterized for the chemical analyses and mechanical strengths in order to assess the mechanism of self-bonding. To make clear the self-bonding mechanism of these binderless boards, the structural characteristics of cellulolytic enzyme lignin (CEL) isolated from Eucalyptus wood, its hydrothermal pretreated fibers, and binderless boards were thoroughly investigated by chemical and spectroscopic methods.
The result revealed that hydrothermal pretreatment and hot pressing process could change cellulose crystalline structures by disrupting inter/intra hydrogen bonding of cellulose chains. During the hydrothermal pretreatment of Eucalyptus wood, acid-catalyzed cleavage of β-O-4′ linkages and ester bonds were the major mechanisms of lignin cleavage. This degradation pathway led to a more condensed lignin which has a high average molecular weight and more phenolic hydroxyl groups than the control. The hot pressing process resulted in the binderless boards with reduced lignin contents and decreased the glass transition temperature, thus making the lignin more accessible to the fiber surface. CEL isolated from the binderless boards showed an increased syringyl to guaiacyl propane (S/G) ratio but a lower molecular weight than those of the untreated Eucalyptus wood and the hydrothermal pretreated fibers.
Based on the finding of this study, it is suggested that the combination of hydrothermal pretreatment and hot pressing process is a good way for conditioning hardwood sawdust for the production of binderless boards. The thermal softening of lignin, rich in phenolic hydroxyl groups, and increased condensed lignin structure contributed to the self-bonding formation of lignocellulosic materials.
KeywordsHydrothermal pretreatment Hot pressing Cellulolytic enzyme lignin Binderless board Self-bonding
The stringent environmental and human health safety regulations and mounting raw material costs have prompted research into reducing the amount of harmful and/or expensive adhesive components and replacing synthetic adhesives with more environmentally-friendly and safer alternatives . Recently, the authors introduced fully green nanotechnology as a gateway to beneficiation of natural cellulose fibers [2–5]. Thermal pretreatment has been conducted in moist environments using hot water or steam at temperatures up to 160 and 240°C, respectively, or in dry environments using inert gases at temperatures up to 240°C [6, 7]. Under these conditions, hemicelluloses are removed, crystallinity index of cellulose is increased, and cellulose degree of polymerization is reduced, while lignin is not considerably affected [7, 8]. It seems clear that a thermochemical pretreatment conducted above lignin melting temperature can cause lignin to coalescence, migrate, and redeposit on biomass cell walls matrix . The softening point of lignin (spherical droplets) after streaming treatment is believed to be lower than that of the original material and this makes it possible for a plastic flow to occur in situ . These lignin droplets were found by hot-water extraction or dilute acid pretreatment of lignocellulosic biomass [9, 11, 12]. Steam pretreatment can cause partial hydrolysis of hemicellulose for both hardwoods and softwoods which markedly increases the compressibility of wood and in turn significantly reduces the build-up of internal stresses in composites during hot pressing [13, 14]. This steam pretreatment process is a very effective method for producing dimensionally stable wood-based composites .
It has been proposed that the pressing temperature is one of the most important manufacturing parameters influencing the binderless boards’ properties [15–18]. Okuda et al. [15, 18] proposed that parts of lignin and hemicelluloses were decomposed during hot pressing process. Recently, the mechanism of self-bonding was summarized by Fahmy and Mobarak , who demonstrated that the chemical changes take place in two stages during hot pressing. The first stage is a hydrolysis stage, whereby acetic and formic acids and some sugars are liberated through the moisture found in the wood particles and then these acids hydrolyze the hemicelluloses in the lignocellulosic material. Some of the pentoses and hexoses produced during hydrolysis are further dehydrated to furfural and hydroxymethyl furfural, respectively. Simultaneously, the lignin becomes activated, i.e., a part of the lignin–carbohydrate bond becomes cleaved (cracked by the formed acids) resulting in the exposition of new functional groups or sites. The second stage is the recondensation reaction of the activated lignin molecules as well as lignin degradation products (such as phenol, which might be formed at high temperature above 155°C) with furfural. However, the self-bonding mechanism during hydrothermal treatment and hot pressing has not been completely elucidated so far.
Traditionally, milled wood lignin (MWL) is considered to be a representative source of native lignin and has been extensively used for the elucidation of native lignin structure. This is currently the most common procedure for the isolation of lignin from wood. However, the yield of MWL is relatively low as revealed by Björkman . Furthermore, lignin yield is dependent on milling time. Nevertheless, longer milling time leads to chemical modification of the lignin, such as increase in carbonyl content and phenolic hydroxyl content as well as decrease in molecular weight and cleavage of aryl ether linkages . Whiting and Goring concluded that MWL is not representative of the whole lignin in wood but primarily originates in the secondary wall of the cell according to the structural studies of dioxane-extracted lignin from compound middle lamella and secondary wall of spruce [21, 22]. To overcome these limitations, a preparative method combining physical and chemical treatments for lignin isolation and purification, such as milling and enzymatic attack, was recently introduced. Chang et al.  isolated lignin by extracting MWL first, and subsequently the residue was treated with cellulolytic enzymes followed by 96% aqueous dioxane extraction to isolate cellulolytic enzyme lignin (CEL). CEL has higher molecular weights and β-O-4′ inter-linkages, but a lower lignin condensation compared to MWL. The higher yield of CEL extracted from wood helps us understand the lignin structure as a whole. CEL is preferred over MWL for studies of lignin structure as it can be isolated in higher yield with less degradation due to ball milling [23, 24].
The objective of this study was to unravel the structural characteristics of lignins of Eucalyptus wood and their behaviors during hydrothermal pretreatment and hot pressing in the production of self-bonding binderless. Because polysaccharides and lignin strongly contribute to self-bonding. This would be also important to understand the differences in mechanical properties of boards from hydrothermal pretreated fibers prepared under various conditions. Moreover, an exhaustive chromatographic and spectroscopic characterization of the isolated CELs was provided as a preliminary investigation.
Solid yield and chemical composition of untreated Eucalyptus globule , hydrothermal pretreated fibers, and manufactured binderless boards from Eucalyptus grandis
Residence time (h)
Mechanical properties of the binderless boards
Binderless boards prepared from hydrothermal pretreated fibers by various process conditions and the obtained internal bond (IB)
Internal bond (MPa)
Bamboo (Phyllostachys pubescens)
Hot water extraction: 60°C, 6 h
Hot pressing: 200°C, 10 min
Aspen (Populus tremuloides Michx.) and pine (Pinus contorta Dougl.)
Streaming: 1.55 MPa, 3 min
Hot pressing: 205°C, 4 min
Norway spruce (Picea abies Karst) and Scots pine (Pinus sylvestris L.)
Hydro-thermolysis: 180°C, 30 min
Hot pressing: 180°C, 12 s
Stream explosion: 200°C, 7.5 min
Hot pressing: 210°C, 5 min
Oil palm trunk waste
Streaming: 120°C, 46 min
Hot pressing: 215°C, 29 min
Hot compressed water: 150°C, 8 h
Hot pressing: 160°C, 20 min
Hydrogen bond intensity and cellulose crystallinity ratios
Structural changes during hydrothermal pretreatment and hot pressing
The hydrogen bond intensity (HBI) of cellulose is closely related to the crystal system and the degree of intermolecular regularity, that is, crystallinity, as well as the amount of bound water . The ratio of the absorbance bands at 3400 and 1323 cm−1 was used to study the cellulose samples HBI [29, 30]. As quantitative indices for evaluation of the overall crystallinity of cellulose, the lateral order index (LOI) and total crystallinity index (TCI) were determined based on FT-IR spectra. Higher values of TCI and LOI indicate that the materials have a higher crystallinity and more ordered structure of cellulose [31, 32]. Low LOI values are indicative of less cellulose type I present . The results obtained are shown in Table 3.
It can be seen from Table 3 that the hydrothermal pretreated fibers and manufactured binderless boards from E. grandis showed higher numbers of HBI, TCI, and LOI than those of the control. This reveals that the cellulose of Eucalyptus wood has a highly ordered crystalline structure and the crystallinity of cellulose increases due to degradation of the amorphous cellulose, which provides a great stability to the cellulose chains and protects them against acid attack during hydrolysis. This finding was in consistent with the results reported in the literature . However, this pretreatment process was unable to completely break apart the inter- and intra- chain hydrogen bondings . The cellulose crystallinity index (CrI) values were also calculated from the corresponding X-ray diffraction (XRD) patterns (Additional file 1: Figure S2) and solid-state cross polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR) patterns (Additional file 1: Figure S3), and the data are listed in Table 3. As the data shown, they were consistent with the conclusion from the FT-IR analysis, and further confirmed that hydrothermal pretreatment and hot pressing can change cellulose crystalline structures by disrupting inter/intra hydrogen bonding of cellulose chains.
FT-IR spectra analysis of the isolated lignins
2D HSQC NMR of the isolated lignins
Assignments of main lignin and polysaccharide 13 C- 1 H correlation signals in the HSQC spectra the lignin fractions
Cβ-Hβ in phenylcoumaran substructures (C)
Cβ-Hβ in resinol substructures (B)
C-H in methoxyls (MeO)
Cγ-Hγ in β-O-4′ substructures (A)
Cγ-Hγ in p-hydroxycinnamyl alcohol end groups (F)
Cγ-Hγ in phenylcoumaran substructures (C)
71.7/3.81 and 4.18
Cγ-Hγ in resinol substructures (B)
Cα-Hα in β-O-4′ substructures (A)
Cβ-Hβ in β-O-4′ substructures linked to a G unit (A)
Cα-Hα in resinol substructures (B)
Cα-Hα in phenylcoumaran substructures (C)
Cβ-Hβ in β-O-4′ substructures linked to a S unit (A)
C2,6-H2,6 in etherified syringyl units (S)
C2,6-H2,6 in oxidized (Cα = O) phenolic syringyl units (S’)
C2-H2 in guaiacyl units (G)
G 5 /G 6
115.0/6.64 and 6.91; 119.0/, 6.82
C5-H5 and C6-H6 in guaiacyl units (G)
The main cross-signals observed in the aromatic region of the HSQC NMR spectra belong to the aromatic rings of lignin monomers. Correlations from S and G lignin units could be observed in the spectra of the CELs. Strong C2,6-H2,6 correlations for syringyl (S 2,6 ) and Cα-oxidized S-lignin units (S’) were observed at 103.9/6.69 ppm and 106.2/7.19 ppm, respectively. For guaiacyl (G) units, G 2 correlation was observed at 111.1/6.96 ppm and correlations for G 5 /G 6 were observed at 115.0/6.64 and 6.91 ppm, and 119.0/6.82 ppm.
Summary of changes in lignin structure as revealed by 2D HSQC NMR
Lignin structural characteristics from integration of 13 C- 1 H correlation signals in the HSQC spectra the lignin fractions
Lignin interunit linkages (%)
β-O-4′ aryl ether (A)
Resinol substructures (B)
Lignin aromatic units
It was found that the S/G ratio and the signal of Cα-oxidized S-lignin units (S’) increased. The CEL of the untreated sample E. grandis which had a proportion of S/G ratio at around 1.7 (Table 5), became a S-rich lignin after hydrothermal pretreatment and hot pressing. The result is consistent with our recent work in which we observed that the S/G ratio of shrub wood lignin, analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and 2D HSQC NMR, increased with the increment of hydrothermal pretreatment severity . Similar results have been reported by Okuda et al.  who found that the S/V ratio increased with increasing pressing temperature. This result could have been due to the preferential degradation of low molecular weight of no-condensed G-type lignin during the treatment. Higher thermal stability of the β-aryl ether linkages of syringyl present as compared to guaiacyl-type lignin in hardwood make the later one lost during hydrothermal pretreatment and hot pressing . Thus a more pronounced decrease in G units compared to S units likely accounted for the increased S/G ratio of lignin from 1.7 (Control) to 2.4 (F4-b). Moreover, a recent work has demonstrated that the presence of these oxidized S’ units in eucalypt wood might be an artifact of the milling process [45, 46]. However, all the samples in the present study were planetary ball milled under the same conditions, demonstrating that the variation was due to the differences in the composition of the biomass. After electron transfer and aryl cation radical formation, aromatic ring oxidation has been reported in model degradation by laccase-HBT, whereas the Cα attack followed by alkyl-aryl ether breakdown predominates, thus resulted in an increase in the amount of Cα-oxidized S-lignin [47, 48]. The proposed mechanism was through the Baeyer-Villiger reaction which may be similar to the mechanism of hot pressing in the present study. Furthermore, this can be rationalized due to the basis that non-phenolic lignin compounds was oxidized and cleaved during hot pressing process and that the action of hot pressing couple proceeds via Cα-hydrogen atom abstraction .
31P NMR spectra of the lignin polymers
Concentrations of functional groups and molecular weight distributions of the lignin fractions
Functional groups concentration (mmol/g)a
Molecular weight (g/mol)b
It can be seen from Table 6, a slight increase in aliphatic OH concentration was observed after hydrothermal pretreatment. This was because higher pretreatment severity led to greater reductions in β-O-4 structures, resulting increase in a higher amount of free phenolic groups . Moreover, a post-treatment with hot pressing process was accompanied by a slight decrease in the aliphatic OH but an increase of S-OH. This was most likely as a result of acid-catalyzed elimination reactions thus led to a cleavage of β-O-4 linakges . The fact that the more syringyl units are produced during hydrothermal treatment, such as steam explosion treatment, most likely as a result of scission of β-O-4 bonds, has already been supported by Granata and Argyropoulos . In addition, loss of carboxylic hydroxyl group can be observed in the resulting CELs obtained from the hydrothermal treatment and hot pressing.
Molecular weight distribution of the isolated lignins
The result showed that during the hydrothermal pretreatment of Eucalyptus wood, acid-catalyzed cleavage of β-O-4′ linkages and ester bonds were the major mechanisms of lignin cleavage. This degradation pathway led to a more condensed lignin that has a high average molecular weight and more phenolic hydroxyl groups than those of the control. The hot pressing process resulted in the binderless boards with reduced lignin contents and decreased the glass transition temperature, thus making the lignin more accessible to the fiber surface. CEL isolated from the binderless boards showed an increased syringyl to guaiacyl propane (S/G) ratio but a lower molecular weight than that of the untreated Eucalyptus wood and the hydrothermal pretreated fibers. Moreover, Eucalyptus wood has the potential to be used to manufacture binderless boards. Based on the finding of this study, it is suggested that the combination of hydrothermal pretreatment and hot pressing process is a good way for conditioning hardwood sawdust for the production of binderless boards. The thermal softening of lignin, rich in phenolic hydroxyl groups, and increased condensed lignin structure contributed to the self-bonding formation of lignocellulosic materials. Future work will be conducted to optimize the pretreatment conditions and improve the performance of binderless board to higher values and commercial uses.
Eucalyptus grandis x E. urophylla, five years old, was manually collected from Hezhou City, Guanxi province, China. The dried Eucalyptus stems were ground to pass a 40-mesh screen. All chemical reagents were analytical grade or best available. The acid xylanase and cellulase with declared activities of 100,000 international units (IU) per gram of protein and 145 filter paper units (FPU)/g were kindly provided by Shanghai Youtell Biochemical Co., Ltd. (Shanghai China).
Hydrothermal pretreatment of Eucalyptuswood
The wood meals of Eucalyptus (100 g dry basis per batch) were added into a batch reactor (1 litter volume). For hot water extraction, the sample was treated under desired temperature 150°C and digestion periods (1, 4, and 8 h). For steam pretreatment, the sample was treated under desired temperature 160°C and digestion periods (1.5, 3, 4.5, and 6 h). Defibrillated fibers were collected to prepare binderless boards and denoted as F1-a, F4-a, F8-a, Q1-a, Q2-a, Q3-a, and Q4-a, respectively.
Binderless board preparation and mechanical characterization of the boards
Each air-dried sample of powders was hand-formed into homogeneous single-layered mats using a forming box (50 × 50 cm2) and pressed at a pressure of 15 MPa at 160°C for 20 min, by adjusting the target board thickness to 5.0 mm and the density to 1.0 g/cm3. Two boards were prepared for each powder. The hydrothermal pretreated Eucalyptus fibers were dried to 10% moisture content under air-dried condition. The binderless boards were prepared on a laboratory sale by the standard techniques under controlled conditions. The manufactured binderless boards from the defibrillated fibers were denoted as F1-b, F4-b, F8-b, Q1-b, Q2-b, Q3-b, and Q4-b, respectively.
The properties of the binderless boards were evaluated in accordance with China National Standard for fiberboards [27, 54]. The five specimens of 25.4 × 25.4 × 1.25 cm3 were prepared. The internal bonding strength (IB) test, regarded as the index of self-bonding, was carried out by using a universal testing machine.
Isolation of CEL
To investigate the original lignin polymers of the untreated and pretreated Eucalyptus wood as well as the binderless board, cellulolytic enzyme lignin (CEL) were isolated according to the procedure developed by Chang et al.  and modified by Hu et al. . Specifically, ball milled wood (5 g) was suspended in citrate buffer (250 mL, pH 4.8) and incubated for 24 h at 50°C. The cellulase and acid xylanase were used at a loading of 50 FPU/g substrate and 200 IU/g substrate, respectively, to hydrolyze the cellulose and hemicelluloses. The enzyme treatment was repeated twice. The residue was collected by centrifugation, washed with hot distilled water (200 mL), centrifuged, and freeze-dried. Then the freeze-dried residue was extracted twice (2 × 24 h) with 100 mL of dioxane/water (96:4, v/v) under a nitrogen atmosphere. After each extraction, the supernatant was collected, combined and poured into a 250 mL volumetric flask. Subsequently, the solution was concentrated, precipitated in water, and freeze-dried to produce the CEL.
Physicochemical characterization of the Eucalyptuswood, hydrothermal pretreated fibers, and binderless boards
The chemical compositions of the extractive-free Eucalyptus wood, hydrothermal pretreated fibers and binderless boards were determined according to National Renewable Energy Laboratory Analytical Procedures . FT-IR spectra were acquired using a Thermo Scientific Nicolet iN10 FT-IR Microscope as previously described . The crystallinities of the samples were measured using a XRD-6000 instrument (Shimadzu, Japan) . CP/MAS 13C NMR spectra of the different samples were obtained using a Bruker AVIII 400 M spectrometer (Germany) .
Characterization of the lignin polymers
FT-IR spectra of the CELs were performed the same as raw material. The weight average molecular weight (Mw) and number average molecular weight (Mn) of the CELs were performed with GPC after acetylation according to previous publications [56, 57]. Briefly, about 20 mg of dry lignin was dissolved in a 1:1 mixture of acetic anhydride/pyridine (1.00 mL) and stirred at room temperature for 24 h. Ethanol (25.0 mL) was added to the reaction mixture, left for 30 min, and removed with a rotary evaporator. The addition and removal of ethanol was repeated until all traces of acetic acid were removed from the sample. The residue was dissolved in chloroform (2.0 mL) and drop-wise to diethyl ether (100.0 mL) followed by centrifugation. The precipitate was washed three times with diethyl ether and dried under vacuum prior to GPC analysis. The acetylated lignin was dissolved in tetrahydrofuran (THF) (2 mg/mL), and the solution was filtered through a 0.45 μm filter. The filtered solution (20.0 μL) was injected into the HPLC system and detected using an UV detector set at 240 nm. THF was used as the mobile phase and the flow rate was 0.5 mL/min. Standard narrow polystyrene samples were used for calibration, as described previously .
For 2D HSQC spectra, the Bruker standard pulse program hsqcetgpsi was used for HSQC experiments as previously described . 31P NMR spectra were acquired according to a previous paper with minor modifications . A total of 20 mg lignin was dissolved in 0.5 mL of anhydrous pyridine and deuterated chloroform (1.6:1, v/v) under stirring. This was followed by the addition of 0.1 μL of cyclohexanol (10.85 mg/mL) as an internal standard, and 0.1 μL of chromium(III) acetylacetonate solution (5 mg/mL in anhydrous pyridine and deuterated chloroform 1.6:1, v/v), as relaxation reagents. Finally, the mixture was reacted with 0.1 μL of phosphitylating reagent (2-chloro-1,3,2-dioxaphospholane) for 15 min and then was transferred into a 5 mm NMR tube for subsequent NMR analysis. Acquisition conditions were as described previously . The content of hydroxyl groups in lignin was obtained by integration of the following spectral regions as described by Cateto et al.  and Monteil-Rivera et al.  with miner modification: aliphatic hydroxyls (149.0 – 146.0 ppm), syringyl (S) phenolic hydroxyls (144.5 – 143.2 ppm), condensed phenolic units (difference between the integrals of the peaks at 143.2 – 142.2 ppm and those at 142.2 – 141.4 ppm, guaiacyl (G) phenolic hydroxyls (140.2 – 138.7 ppm), p-hydroxyphenyl (H) phenolic hydroxyls (138.2 – 137.1 ppm), and carboxylic acids (135.5 – 134.2 ppm).
Cellulolytic enzyme lignin
Cellulose crystallinity index
- CP/MAS NMR:
Cross polarization/magic angle spinning nuclear magnetic resonance
Filter paper unit
Fourier transform infrared spectroscopy
Gel permeation chromatography
Hydrogen bond intensity
Heteronuclear single quantum coherence
Lateral order index
Milled wood lignin
Weight average molecular weight
Number average molecular weight
Pyrolysis gas chromatography/mass spectrometry
Total crystallinity index
2–chloro–4, 4, 5, 5–tetramethyl–1, 3, 2–dioxaphospholane
The authors are grateful for the financial support from the State Forestry Administration (201204803), National 863 Project (863-2012AA023204), Major State Basic Research Projects of China (973-2010CB732204), and the Specific Programs in Graduate Science and Technology Innovation of Beijing Forestry University (BLYJ201314).
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