Heat Treatment of Industrial Alkaline Lignin and its Potential

Research Article pubs.acs.org/journal/ascecg

Heat Treatment of Industrial Alkaline Lignin and its Potential Application as an Adhesive for Green Wood−Lignin Composites Yun Zhang,† Jian-Quan Wu,‡ Hui Li,§ Tong-Qi Yuan,*,† Yun-Yan Wang,*,∥ and Run-Cang Sun† †

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No. 35 Tsinghua East Road Haidian District, Beijing 100083, P. R. China ‡ National Paper Quality Supervision Testing Center, China National Pulp and Paper Research Institute, No. 4 Qiyang Road Chaoyang District, Beijing 100102, P. R. China § Composites Materials and Engineering Center, Washington State University, Pullman, Washington 99164, United States ∥ Center for Renewable Carbon, Department of Forestry, Wildlife, and Fisheries, University of Tennessee Institute of Agriculture, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: An industrial alkaline lignin preparation and poplar particles were mixed and hot pressed under different conditions. The alkaline lignin and the lignin isolated from the poplar particles were thoroughly investigated by quantitative nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and elemental analysis techniques. For the first time, it was found that the content of β-O-4′ linkages decreased accompanying with the formation of β-β′, β-5′, and β-1′ linkages at mild heat treatment temperatures (130−170 °C). However, it should be noted that most of the β-O-4′, β-β′, β-5′, and β-1′ linkages nearly disappeared at a higher temperature (180 °C). Cross-linking reactions were predominant during the hot-pressing process as the molecular weight of lignin increased at elevated temperature. Owing to the self-bonding between lignin fragments during the hot-pressing process, a green poplar wood−lignin composite was successfully prepared with poplar particles and a small amount of alkaline lignin (∼20%, w/w). Internal bond strength with 0.47 MPa was surprisingly achieved under the conditions of pressing temperature 160 °C, pressure 5 MPa. An in-depth understanding of the concerted reactions between fragmentation and cross-linking reaction in lignin during hot pressing was beneficial to a better development of self-bonding green wood−lignin composites in future. KEYWORDS: Self-bonding, Wood−lignin composites, Lignin, Quantitative NMR, DSC

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combusted to generate energy,7 but both of these industries are keenly interested in developing new revenue streams for it.8 An inspiring attempt sparked the idea that the full utilization of the renewable-resource lignin in wood composites could replace conventional synthetic adhesives; this could be an intelligent win−win solution. In recent decades, various techniques using different kinds of lignin for wood composites have been extensively investigated.9,10 The simplest way to use lignin as an adhesive was to use industrial lignins, such as, lignosulfonates, kraft lignin, and alkaline lignin, in various percentages under different hot pressing conditions to make wood composites.11−13 Another approach was to activate the lignin on the surface of the wood fibers coming from thermomechanical pulping by chemical and enzymatic means, which can give lignin the functionality of a self-bonding adhesive.14−16 Recently, steam explosion was

INTRODUCTION In the conventional production of wood-based composites, synthetic adhesives such as urea- and phenol-formaldehyde resins are used in combination with hot pressing in order to obtain high-performance products.1 These adhesives are associated with emission of a carcinogenic gas, formaldehyde.2 The “green adhesives” for bonding of woods providing both environmentally friendly and economic advantages are thus imperative for future development of the wood industry. In nature, wood composed of cellulose, hemicelluloses, and lignin is the best example of a natural composite.3 The functions of lignin are considered to connect cells to one another and to make cell walls of xylem tissues hard and repellent to water.4 Lignin, in woody parts, acts as permanent bonding agents between cells to provide the composite structure with outstanding resistant not only physically toward impact, compression, and bending but also chemically toward many types of aging or biological reactions.5 However, industrial lignins are served as byproducts of the pulping industries and biorefinery.6 Most recovered lignins are © 2017 American Chemical Society

Received: May 11, 2017 Revised: June 18, 2017 Published: June 23, 2017 7269

DOI: 10.1021/acssuschemeng.7b01485 ACS Sustainable Chem. Eng. 2017, 5, 7269−7277

Research Article

ACS Sustainable Chemistry & Engineering recognized as an efficient technique to pretreat lignocellulosic materials in the production of boards and composites as the steam explosion pretreatment can hydrolyze most of the hemicelluloses and separate the fibers well. Meanwhile, the lignin was exuded from the cell wall to the fiber surface. With this pretreatment, fibers can be easily hot-pressed to products without any additional synthetic binders.17,18 However, previous fundamental research has been focused on the manufacturing parameters of the binder-free wood composites. Unfortunately, limited efforts have been paid to the investigation of the structural transformation of lignin macromolecules during the hot-pressing process. It was difficult to obtain excellent representative lignin samples from the wood composites, and the analytical techniques were constrained.19 Hereby, clarification of the self-bonding mechanism is considered to be important for designing better manufacturing conditions that could further improve binder-free wood composite performance and promote a more environmentally friendly, less expensive alternative for bonding of wood in the wood composites industry. Fortunately, with the fast development of nuclear magnetic resonance (NMR) technology in the extensive application of lignin structure characterization,20,21 we are able to unveil the self-bonding mechanism of lignin during the hot-pressing process. Simultaneously, based on previous endeavor, an adept methodology of separation and multiple perspectives analysis of lignin macromolecules have been conducted.22,23 In our previous study, a series of optimum process parameters was conducted and the changes of lignin isolated from the corresponding binder-free wood composites after the hot pressing process have been discussed in detail.24 In the present study, the mixture of an industrial alkaline lignin and poplar particles was hot pressed under different temperatures. The structural changes of lignin during hot-pressing were thoroughly investigated by quantitative NMR (two-dimensional heteronuclear single-quantum coherence (2D-HSQC) and 13 C), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and elemental analysis techniques. Furthermore, the poplar−lignin composite was prepared to verify the possibility of lignin functioning as adhesive.

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Figure 1. Overall preparation scheme of lignin samples. 160 °C-hot-pressed poplar particles (PP-160), and the 160 °C-hotpressed wood−lignin composite (WLC-160) composed of poplar wood and corn cob alkaline lignin (AL/poplar mass ratio 0.2) were titled as Lpp, Lpp‑160, and LWLC‑160, respectively. The hot-pressed ALs were ground and sieved to obtain 80−120 mesh particles for later analysis. The obtained five lignin samples were labeled as AL130, AL150, AL160, AL170, and AL180 according to their corresponding hot pressing temperature. Characterization of the Lignin Samples. The carbohydrate moieties associated with all lignin samples were determined by sugar analysis.22,26 The weight-average (Mw) and number-average (Mn) molecular weights of lignin samples were determined by gel permeation chromatography (GPC) with an ultraviolet detector (UV) at 280 nm.22 To guarantee the perfect dissolution of lignin, the mobile phase was changed into tetrahydrofuran and dimethyl sulfoxide (1:1, V/V). NMR spectra were recorded on a Bruker AVIII 400 MHz spectrometer at 25 °C. For the quantitative two dimensional heteronuclear single quantum coherence (2D-HSQC) spectra, approximately 60 mg of lignin was dissolved in 0.5 mL of DMSOd6.27 Functional groups of the lignin samples were determined by 31P NMR spectra according to previous publications.24,25 For the quantitative 13C NMR experiments, 140 mg of lignin was dissolved in 0.5 mL of DMSO-d6 in the presence of 20 μL of chromium(III) acetylacetonate (0.01 M). Elemental analysis of the lignins was carried out using an elemental analyzer Vario EL III (Elementar, Hanau, Germany).28 To measure carbon, hydrogen, and nitrogen contents, 2− 4 mg sample was encapsulated in a tin container. The oxygen content was deduced from the difference with respect to the total sample. The C900 formula and degree of unsaturation was calculated. The glass transition temperature (Tg) of lignin samples were determined on a DSC-Q100 instrument (TA, USA) heating from 40 to 200 °C at 10 °C/min with an injection of N2 at 15 mL/min. Preparation of the Wood−Lignin Composite. The wood− lignin composite (WLC) was manufactured by mixing poplar particles with AL in different proportions based on their oven-dried weights. The mixture was shaped to a 5 mm-thick mat by using a 300 mm × 300 mm forming box. The thickness of the mat was controlled by the thickness gauge with a target density of 0.8 g/cm3. Upon forming, the mat was subjected to hot press under different conditions. The pressing time was constantly kept at 20 min. The 300 mm × 300 mm × 5 mm mats composed of pure AL were hot-pressed respectively at 130, 150, 160, 170, and 180 °C under 5 MPa for 20 min. Each sample was carried out in triplicate. Internal Bonding Strength of the Wood−Lignin Composite. The internal bonding strength (IBS) of the composite was tested according to the China National Standard GB/T 17657-2013.29 Prior to evaluating the mechanical and physical properties, the wood−lignin composite was conditioned at 25 °C and 65% relative humidity (RH) until reaching equilibrium moisture content. Six 50 mm × 50 mm specimens were prepared for IBS testing.

MATERIALS AND METHODS

Material. The poplar wood was obtained from Shandong province, China. The wood sample was ground and sieved to obtain 40−100 mesh particles. The major chemical compositions of the poplar wood were cellulose (43.1%), hemicelluloses (22.8%), acid-insoluble lignin (19.0%), and acid-soluble lignin (4.5%). The alkaline lignin (AL) used in this study was a byproduct of bioethanol production from corn cob at Shandong Longlive Biotechnology Co., Ltd., China. The AL consisted of 90.81% Klason lignin, 3.61% acid-soluble lignin, 0.63% sugars, 2.16% ash, and 2.79% others.25 Both poplar and lignin samples were dried at 60 °C in an oven for 16 h and stored at 5 °C before use. Heat Treatment of the Lignin Samples. The overall scheme26 for the preparation of lignin samples was presented in Figure 1. The poplar particles, poplar particles hot-pressed at 160 °C, 5 MPa for 20 min, and poplar wood−lignin composites (AL/poplar mass ratio 0.2) hot-pressed at 160 °C, 5 MPa for 20 min, were ground and sieved to obtain 80−120 mesh ground samples, respectively. Approximately 15 g of the ground sample was milled (5 h) in a Fritsch planetary ball mill (Germany). The ball milled sample was extracted with dioxane/water (96:4, v/v) for 24 h. The supernatant was concentrated to ca. 30 mL. The dissolved lignin was precipitated in 300 mL of acidified water (pH 2.0, adjusted by 6 M HCl). The precipiates were collected by centrifugation and, then, washed with the pH 2.0 water before freezedrying. The lignin samples isolated from the poplar particles (PP), the 7270


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ACS Sustainable Chemistry & Engineering

Figure 2. 2D-HSQC NMR spectra of the Lpp, Lpp‑160, and LWLC‑160 samples.

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RESULTS AND DISCUSSION 2D-HSQC Spectra of Lpp, Lpp‑160, and LWLC‑160. The twodimensional HSQC NMR has been applied to distinguish the featured signals of lignin macromolecules in the crowded spectral regions and quantify various structural units.22 The corresponding cross-peaks were assigned according to the recent literature.22,26 Various substructures linked by ether and C−C bonds, and basic aromatic units were depicted in Figure S1. It was observed that the spectra of Lpp, Lpp‑160, and LWLC‑160 (Figure 2) were similar except for several signals. The spectrum of Lpp embodies the featured cross-peaks that can be found in native poplar lignin without any heat treatment. However, the 160 °C hot-pressing brought about significant changes in macromolecular lignin structure of poplar particles as shown in the spectrum of Lpp‑160. For example, the cross-peaks corresponding to C2,6−H2,6 correlations in Cα-oxidized S′ units were observed at δC/δH 106.3/7.25 (S′2,6), which indicated the oxidation reaction occurred in the sidechain of S units during hot pressing at 160 °C. Compared to the pure hot-compressed poplar particles, new cross-peaks can be observed in the spectrum of LWLC‑160, such as C2,6−H2,6 correlations of H units at δC/δH 127.3/7.19 (H2,6), C2−H2, C6−H6, and C8−H8 correlations of ferulate (FA) at δC/δH 110.8/7.35 (FA2), 123.1/7.16 (FA6), and 116.6/6.43 (FA8), respectively. The free p-coumarate acid (PCA) was also confirmed by the obvious signals at δC/δH 143.9/7.51 (PCA7), 115.0/6.24 (PCA8). Generally, hardwood poplar is

exclusive of the FA and PCA units which are normally found in the gramineous plants.30 Therefore, these new cross peaks representing FA and PCA in the spectrum of the LWLC‑160 sample arise from the corn cob AL (Figure 4). The correlations at δC/δH 129.8/7.52 (PB2,6/PCA2,6) should be attributed to C2,6−H2,6 linkages of both p-hydroxybenzoate (PB) and PCA units. Although carbohydrate contents in the obtained samples (Lpp, Lpp‑160, and LWLC‑160) were low (Table S1), LWLC‑160 isolated from hot-compressed blend of AL and poplar particles was difficult to analyze quantitatively compared to Lpp and Lpp‑160. Therefore, to unveil the self-bonding mechanism of lignin during hot pressing, the pure AL was hot pressed in a molding under the different temperatures. 2D-HSQC Spectra of the AL and Hot-Pressed AL Samples. The detailed chemical transformations of lignin macromolecules during hot pressing were further investigated by using a 2D NMR technique. The main structural characteristics of lignins, including basic aromatic units and various substructures can be observed in the 2D-HSQC spectra (Figures 3 and 4). The signal assignments and main identified lignin substructures in these lignin samples are listed in Table S2 and Figure S1, respectively. In the sidechain regions (Figure 3), the prominent correlating signals observed in all spectra were the β-O-4′ ether linkages (A). Specifically, the signals at δC/δH 71.6/4.88 (Aα), δC/δH 82.8−85.7/4.13−4.46 (Aβ(G) and Aβ(S)), and δC/ δH 59.5−59.7/3.40−3.63 (Aγ) belong to the Cα−Hα, Cβ−Hβ, and Cγ−Hγ correlations of the β-O-4′ ether substructures, respectively. The Cβ−Hβ correlations attached to G or S unit of 7271


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Figure 3. Sidechain in the 2D-HSQC NMR spectra of the AL and hot-pressed AL samples.

β-O-4′ aryl ether substructures (A) disappeared in the AL180 sample; however, the Cα−Hα and Cγ−Hγ correlations were still preserved. It implied that the β-O-4′ aryl ether bonds were vulnerable to cleavage at elevated temperature. The first obvious change between the spectra of crude and hot-pressed AL was the appearance of β-1′ substructures (D) as the intensive signals of Cα−Hα (δC/δH 81.8/4.89) (Da) was found in AL130. When the hot-pressing was increased beyond 130 °C, more C−C bonds were formed. For example, correlations of Cα−Hα (Ca) of β-5′ substructures (C) and Cγ−Hγ (δC/δH 71.3/4.37) (Bγ) of β-β′ substructures (B) were clearly present in AL150 and became more intensive as hot-pressing temperature increased. It implied that due to the weaker steric hindrance, the free C1 embraced the higher reactivity than C5 and Cβ in the lignin phenyl propane (C9) units. Interestingly, the signals of β-β′ substructures (B) and β-1′ substructures (D) receded in the spectrum of L180 as the C−C bonds were hard to form at higher temperature. In the aromatic region (Figure 4), C−H correlations corresponding to S, G, and H units could be observed in all the spectra. Especially, the typical signals of FA were observed at δC/δH 110.8/7.35, 123.1/7.16, and 116.6/6.43 corresponding to C2−H2, C6−H6, and C8−H8, respectively. PCA was confirmed by the obvious signals presented at the δC/ δH 129.8/7.52, 143.9/7.51, and 115.0/6.24, respectively. The 2D-HSQC NMR method provides explicit structural evolution of lignin during hot pressing at different temperatures. The relative content of different linkages in these lignin samples is listed in Table 1. The content of β-O-4′ linkages

tended to decrease in the hot-pressed samples compared to that of the AL (78.2%). The percentage of β-O-4′ linkage obtained by using semiquantitative HSQC during hot-pressing decreased during hot-compressing, moreover the results of quantitative 13 C NMR (Table 2) displayed a similar variation that supported the actual cleavage of β-O-4′ linkages. Taking the performance of the wood−lignin composites into consideration, it can be concluded that the IBS of the wood− lignin composites was mainly attributed to the formation of new C−C bonds during hot-pressing, i.e., if the content of C− C bonds reduced, then the IBS would decrease. In practice, the maximum content of C−C bonds was obtained in L160, and the best internal bonding strength was also acchieved at 160 °C. As depicted in Figure 6, the homolytic cleavage of aryl ether bond, e.g, β-O-4′ linkages, releases an ether radical (structure 3) and a phenolic radical (structure 4). The couplings in between them and the quinonmethide intermediates (structure 9 and 10) give new β-O-4′ substructures (structures 11 and 15) along with C− C bonds such as β-5′ (structure 12), β-β′ (structure 13), and β1′ (structure 14). In addition, another factor that must be considered was the abundance of free phenolic hydroxyl groups in the lignin structure: in their radical form, they are susceptible to intermolecular coupling that results in further polymerization.28 Another important parameter, S/G/H ratio, was also determined to trace the changes of lignin. The distribution of S/G/H presented a slightly increase of G and H units in these hot-pressed lignin samples as compared to the control AL (43/ 7272


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Figure 4. Aromatic ring in the 2D-HSQC NMR spectra of the AL and hot-pressed AL samples.

both C3 and C5 positions (ortho to the phenolic hydroxyl) unsubstituted on the aromatic ring; on the other hand, in the S unit, both C3 and C5 positions are linked to a methoxy group, resulting in low reactivity.31 From this point of view, low S/G/ H ratio will favor intermolecular self-bonding by formation of C−C bonds. Quantitative 13C NMR Spectra of the AL and HotPressed AL Samples. Quantitative 13C NMR was used to investigate the structural changes in the carbon skeleton of macromolecular lignin during hot pressing. The amount of specific substructures were estimated as equivalences per aromatic ring (equiv/Ar), and the chemical shifts were assigned based on previous reports.26,32 It should be noted that these lignin samples did not show signals between 90.0 and 102.0 ppm, which indicates the absence of carbohydrates. It was observed that the six spectra were similar except for the intensities of specific signals in Figure S2. PCA was confirmed by the obvious signals at 164.1, 159.7, 144.3, 130.2, 125.3, 115.8, and 115.4 ppm, derived from C9, C4, C7, C2/C6,

Table 1. Quantification of the AL and Hot-Pressed AL Samples by the 2D-HSQC NMR Method

a

samples

β-O-4′

β-β′

β-5′

β-1′

S/G/H

AL AL130 AL150 AL160 AL170 AL180

78.2 77.5 70.0 53.2 54.7 66.8

NDa ND 4.9 22.3 22.1 ND

21.8 17.6 13.5 7.3 9.2 18.9

ND 5.0 11.6 17.2 14.0 14.3

43/25/32 38/29/33 37/29/34 33/29/38 35/30/35 37/27/36

Not detected.

25/32). This implied that demethoxylation probably occurred on S units leading to an increase in the content of the G unit. In the proposed mechanism of demethoxylation (Figure 6), the free radical on structure 3 could relocate to the ortho- and paramethylene sites (structures 9 and 10 in Figure 6) and lead to the cleavage of methoxy group to a G unit (structure 8). In general, the G unit have a free C5 position and H units have

Table 2. Assignment and Quantification of the Signals of the 13C-NMR Spectraa

a

δ, ppm

assignment

AL

AL130

AL150

AL160

AL170

AL180

155.0−140.0 140.0−124.0 124.0−102.0 61.3−58.0

aromatic C−O aromatic C−C aromatic C−H β-O-4′ linkages

1.69 1.88 2.43 0.22

1.66 1.89 2.45 0.26

1.76 1.93 2.31 0.19

1.76 1.95 2.29 0.20

1.69 1.93 2.38 0.15

1.60 1.94 2.46 0.16

Results expressed per aromatic ring. 7273


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Table 3. Functional Groups of the AL and Hot-Pressed AL Samples As Determined by the Quantitative 31P-NMR Method (mmol/g) syringyl OH

a

guaiacyl OH

samples

aliphatic OH

Ca

NCb

C

NC

p-hydroxy phenyl OH

carboxylic group

total phenolic OH

AL AL130 AL150 AL160 AL170 AL180

1.77 0.99 1.39 1.18 1.22 1.00

0.10 0.21 0.19 0.19 0.22 0.22

0.55 0.53 0.49 0.51 0.54 0.52

0.22 0.27 0.25 0.28 0.31 0.30

0.42 0.96 1.01 1.01 1.05 1.05

0.65 0.73 0.77 0.75 0.80 0.74

1.09 1.10 1.52 1.32 1.33 1.10

1.94 2.70 2.71 2.74 2.92 2.83

Condensed. bNoncondensed.

A comparison of the integration values of different regions can provide more detailed information on the evolution of lignin macromolecules. As shown in Table 2, the contents of the β-O-4′ linkages in hot-pressed samples (except for L130) were decreased slightly with the increasing hot pressing temperature. However, the integration of the condensed aromatic carbons (C−C bonds) presented an increasing trend. Simultaneously, the decreased aromatic C−H linkages also indicated the occurrence of cross-linking reactions. The increase in the content of aromatic C−H linkages at 180 °C suggests that the substitution reaction on the aromatic ring was suppressed. This phenomenon was consistent with the result of 2D-HSQC NMR and the performance of WLC. 31 P NMR Spectra of the AL and Hot-Pressed AL Samples. Further evaluation of the functional groups was performed by using a quantitative 31P NMR technique, which involves phosphorylation of OH groups followed by quantitative analysis in the presence of an internal standard.32 The 31P NMR spectra of these lignin samples are shown in Figure S3. In Table 3, the content of aliphatic OH in the hot-pressed AL samples (AL130−AL180) decreased as compared to that of the AL sample. It suggested that the OH groups in the sidechain of lignin were fragmented and eliminated as dehydration reaction probably occurred during the hot-pressing process.33 On the other, additional free phenolic OH units were released at elevated hot-pressing temperature, and the highest content phenolic OH was obtained at 170 °C, a 150% increase compared to crude AL. Besides the aliphatic OH, the content of S-type OH was less than that of corresponding G-type OHs. This suggested that most of the S-type lignin units were involved in the formation of the β-O-4′ linkages in these lignins and only a small amount of free S−OH could be detected by the 31P-spectra. The content of SC and GC phenolic OH both embodied a remarkable elevation. Meanwhile, the content of GNC phenolic OH greatly increased from 0.42 to 1.05 mmol/g. The content of H-type phenolic OH was slightly increased from 0.65 to 0.80 mmol/g, which was probably due to the demethoxylation at 3-position of G units or 3,5-positions of S units at higher temperature.28 The reactivity of lignins is essentially influenced by the number of phenolic OH. This is due to the activation of the aromatic ring in the o-position and the fact that units with free phenolic hydroxyl groups are able to form quinonmethide intermediates, which are susceptible to nucleophilic reaction at the benzylic carbon atom. The presence of this group therefore tends to increase the reactivity of lignin toward the adjoining similar activated carbon atom.34 Elemental Analysis. Table 4 shows the elemental composition and C900 formula as well as the degree of unsaturation in the control and hot-pressed lignin samples. The

Table 4. Elemental Analysis and C900 Empirical Formula of the AL and Hot-Pressed AL Samples samples

C (%)

H (%)

O (%)

C900 formula

degree of unsaturation

AL AL130 AL150 AL160 AL170 AL180

60.55 63.33 63.63 64.50 64.45 65.03

5.23 5.45 5.26 5.30 5.26 5.27

34.22 31.22 31.11 30.13 30.17 29.70

C900H932O382 C900H929O333 C900H893O330 C900H887O315 C900H881O316 C900H874O308

435 436 454 457 460 464

Table 5. Weight-Average (Mw) and Number-Average (Mn) Molecular Weights and Polydispersity Indices (Mw/Mn) of the AL and Hot-Pressed AL Samples samples

AL

AL130

AL150

AL160

AL170

AL180

Mw Mn Mw/Mn

2260 1090 2.07

2320 1260 1.84

2470 1400 1.76

2830 1740 1.63

2980 1930 1.54

3030 1990 1.52

Figure 5. DSC thermograms of the AL and hot-pressed AL samples.

C1, C3/C5, and C8-position, respectively. Apart from the signals represented for PCA, the S, G, and H units were also clearly distinguished according to some publications.25,30 It was found that some signals in the spectrum of L180 were differences as follows: (1) The subdued signals for PCA were probably caused by the cleavage of the hanging molecules (PCA). (2) The fact that the signals for β-O-4′ linkages and etherified S3/5 were reduced suggests that some β-O-4′ linkages were cleaved and degraded at this temperature. (3) Condensation reaction occurred during hot pressing as the peak of 104.3 ppm (S2,6 correlated signals) became broad.28 7274


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Figure 6. Proposed self-bonding mechanism of lignin.

Figure 7. Result of single-factor tests.

percentage of carbon content increased gradually with that decreasing oxygen content. From the C900 formula, it was found that the degree of unsaturation slightly increased with the increasing of hot pressing temperature, which suggested that the lignin samples after hot pressing have more unsaturated bonds, such as unsaturated double bonds in the sidechain of the

lignin. This was also in line with the results of 2D-HSQC analysis in this study. Molecular Weight and Glass Transition Temperature. Changes in molecular weight of lignin (Table 5) can provide important insights into fragmentation and cross-linking reactions during hot pressing process.32 The weight-average molecular weight (Mw) of the control lignin AL was 2260 g/ 7275


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ACS Sustainable Chemistry & Engineering mol. After hot pressing, Mw increased as higher temperature was applied, in particular, at 160 °C, Mw of the L160 presented an abrupt augment. Generally, the cleavage of linkages, such as β-O-4′ linkages, can result in a decrease of the molecular weight of lignin; on the other hand, condensation reactions usually lead to an increase in the molecular size.35 The β-O-4′ linkages embraced lower bond energy as compared to the C−C bond, and they were susceptible to cleavage at higher heating treatment.36 The increasing molecular weight of these hotpressed AL samples was a good evidence that the cross-linking reactions between lignin fragments were predominant during hot pressing. Another evidence of cross-linking was observed in DSC analysis (Figure 5): the glass transition temperature of lignin samples increased from 105.7 to 144.1 °C accompanying the increasing molecular weight during hot pressing. Interestingly, homogeneous lignin fragments with narrower polydispersity index (Mw/Mn = 1.52−1.84) as compared to crude AL (2.07) were obtained at higher hot-pressing temperature (Table 5). It seems that the cross-linking reactions (Figure 6) at elevated temperature may be governed by template effect arising from strong intermolecular interactions between lignin fragments.37 Performance of the Wood−Lignin Composite. After lignin structures based upon heat treatment were thoroughly elucidated, it is speculated that lignin could perform as bonding agent (adhesive) for fabricating wood−lignin composite. Therefore, we successfully prepared the poplar wood−lignin composites with the similar temperature range applied in AL heat pretreatment. In general, the best IBS (0.47 MPa) was obtained at the temperature of 160 °C, pressure of 5 MPa, and AL/P mass ratio of 0.2 (Figure 7). Further study is underway in terms of optimizing the processing condition and other mechanical and physical properties, as well as other types of lignin. In summary, the heat treatment of an industrial alkaline lignin was investigated to elucidate the feasibility of selfbonding for the wood−lignin composites. The NMR and GPC results revealed a decrease of β-O-4′ aryl ether linkage and an increase in molecular weight during hot pressing. The cleavage of aryl ether linkages in the lignin backbone exposed more active sites on lignin aromatic ring that facilitate the formation of new C−C bonds. The changes of Tg at various hot-pressing temperature also confirmed the cross-linking reaction. Overall, the fragmentation and cross-linking were concerted reactions during the hot-pressing process. The comprehensive investigation of the reactivity and chemical transformations of lignin macromolecules during hot pressing provides valuable methodology for their further utilization in the wood composites industry.

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pressed AL samples. Figure S3. Quantitative 31P NMR spectra of the AL and hot-pressed AL samples (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-62336903. Fax: +86-10-62336903. E-mail: [email protected] (T.-Q.Y.). *E-mail: [email protected] (Y.-Y.W.). ORCID

Tong-Qi Yuan: 0000-0002-1854-938X Run-Cang Sun: 0000-0003-2721-6357 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this research from the Fundamental Research Funds for the Central Universities (2015ZCQ-CL-02), National Natural Science Foundation of China (31670587 and 31430092), and Program of International S&T Cooperation of China (2015DFG31860).

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REFERENCES

(1) Effendi, A.; Gerhauser, H.; Bridgwater, A. V. Production of renewable phenolic resins by thermochemical conversion of biomass: A review. Renewable Sustainable Energy Rev. 2008, 12 (8), 2092−2116. (2) Kim, K. H.; Jahan, S. A.; Lee, J. T. Exposure to formaldehyde and its potential human health hazards. J. Environ. Sci. Heal. C 2011, 29 (4), 277−299. (3) Bergander, A.; Salmén, L. Cell wall properties and their effects on the mechanical properties of fibers. J. Mater. Sci. 2002, 37 (1), 151− 156. (4) De Micco, V.; Aronne, G. Anatomical features, monomer lignin composition and accumulation of phenolics in 1-year-old branches of the Mediterranean Cistus ladanifer L. Bot. J. Linn. Soc. 2007, 155 (3), 361−371. (5) Chiang, V. L.; Harding, S. A.; Lung, J.; Popko, J. L.; Ralph, J.; Stokke, D. D.; Tsai, C. J.; Hu, W.-J. Repression of lignin biosynthesis promotes cellulose accumulation and growth in transgenic trees. Nat. Biotechnol. 1999, 17 (8), 808−812. (6) Austin, A. T.; Ballaré, C. L. Dual role of lignin in plant litter decomposition in terrestrial ecosystems. Proc. Natl. Acad. Sci. U. S. A. 2010, 107 (10), 4618−4622. (7) Saxena, R. C.; Adhikari, D. K.; Goyal, H. B. Biomass-based energy fuel through biochemical routes: a review. Renew. Sust. Renewable Sustainable Energy Rev. 2009, 13 (1), 167−178. (8) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311 (5760), 484−489. (9) Wang, C.; Kelley, S. S.; Venditti, R. A. Lignin-Based Thermoplastic Materials. ChemSusChem 2016, 9 (8), 770−783. (10) Khan, M. A.; Ashraf, S. M.; Malhotra, V. P. Development and characterization of a wood adhesive using bagasse lignin. Int. J. Adhes. Adhes. 2004, 24 (6), 485−493. (11) Hoareau, W.; Oliveira, F. B.; Grelier, S.; Siegmund, B.; Frollini, E.; Castellan, A. Fiberboards based on sugarcane bagasse lignin and fibers. Macromol. Mater. Eng. 2006, 291 (7), 829−839. (12) Westin, M.; Simonson, R.; Ö stman, B. Kraft lignin wood fiberboards−The effect of kraft lignin addition to wood chips or board pulp prior to fiberboard production. Eur. J. Wood Wood Prod. 2001, 58 (6), 393−400. (13) Calvo-Flores, F. G.; Dobado, J. A. Lignin as renewable raw material. ChemSusChem 2010, 3 (11), 1227−1235. (14) Felby, C.; Hassingboe, J.; Lund, M. Pilot-scale production of fiberboards made by laccase oxidized wood fibers: board properties

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01485. Table S1. Yield and carbohydrate content of lignin isolated from Lpp, L160, and LWLC‑160. Table S2. Assignment of main lignin 13C−1H cross-signals in the HSQC spectra of lignin. Figure S1. Main classical substructures, involving different interunit linkages and aromatic units identified by 2D-NMR. Figure S2. Quantitative 13C NMR spectra of the AL and hot7276


Research Article

ACS Sustainable Chemistry & Engineering and evidence for cross-linking of lignin. Enzyme Microb. Technol. 2002, 31 (6), 736−741. (15) Widsten, P.; Kandelbauer, A. Adhesion improvement of lignocellulosic products by enzymatic pre-treatment. Biotechnol. Adv. 2008, 26 (4), 379−386. (16) Felby, C.; Thygesen, L. G.; Sanadi, A.; Barsberg, S. Native lignin for bonding of fiber boardsevaluation of bonding mechanisms in boards made from laccase-treated fibers of beech (Fagus sylvatica). Ind. Crops Prod. 2004, 20 (2), 181−189. (17) Quintana, G.; Velasquez, J.; Betancourt, S.; Gañań , P. Binderless fiberboard from steam exploded banana bunch. Ind. Crops Prod. 2009, 29 (1), 60−66. (18) Velásquez, J. A.; Salvadó, F.; Ferrando, F. Binderless fiberboard from steam exploded Miscanthus sinensis: Optimization of pressing and pretreatment conditions. Wood Sci. Technol. 2003, 37 (3−4), 279−286. (19) Hatfield, R.; Fukushima, R. S. Can lignin be accurately measured? Crop Sci. 2005, 45 (3), 832−839. (20) Wen, J. L.; Sun, S. L.; Xue, B. L.; Sun, R. C. Recent advances in characterization of lignin polymer by solution-state nuclear magnetic resonance (NMR) methodology. Materials 2013, 6 (1), 359−391. (21) Mansfield, S. D.; Kim, H.; Lu, F.; Ralph, J. Whole plant cell wall characterization using solution-state 2D NMR. Nat. Protoc. 2012, 7 (9), 1579−1589. (22) Yuan, T. Q.; Sun, S. N.; Xu, F.; Sun, R. C. Structural characterization of lignin from triploid of Populus tomentosa Carr. J. Agric. Food Chem. 2011, 59 (12), 6605−6615. (23) Yelle, D. J.; Wei, D.; Ralph, J.; Hammel, K. E. Multidimensional NMR analysis reveals truncated lignin structures in wood decayed by the brown rot basidiomycete Postia placenta. Environ. Microbiol. 2011, 13 (4), 1091−1100. (24) Sun, Y. C.; Lin, Z.; Peng, W. X.; Yuan, T. Q.; Xu, F.; Wu, Y. Q.; Yang, J.; Wang, Y. S.; Sun, R. C. Chemical changes of raw materials and manufactured binderless boards during hot pressing: Lignin isolation and characterization. BioResources 2013, 9 (1), 1055−1071. (25) Yang, S.; Wen, J. L.; Yuan, T. Q.; Sun, R. C. Characterization and phenolation of biorefinery technical lignins for lignin−phenol− formaldehyde resin adhesive synthesis. RSC Adv. 2014, 4 (101), 57996−58004. (26) Wu, J. Q.; Wen, J. L.; Yuan, T. Q.; Sun, R. C. Integrated hotcompressed water and laccase-mediator treatments of Eucalyptus grandis fibers: Structural changes of fiber and lignin. J. Agric. Food Chem. 2015, 63 (6), 1763−1772. (27) Rico, A.; Rencoret, J.; Del Río, J. C.; Martínez, A. T.; Gutiérrez, A. Pretreatment with laccase and a phenolic mediator degrades lignin and enhances saccharification of Eucalyptus feedstock. Biotechnol. Biofuels 2014, 7 (1), 6. (28) Wen, J. L.; Sun, S. L.; Yuan, T. Q.; Xu, F.; Sun, R. C. Understanding the chemical and structural transformations of lignin macromolecule during torrefaction. Appl. Energy 2014, 121, 1−9. (29) Test methods of evaluating the properties of wood-based panels and surface decorated wood-based panels; China National Standard, GB/T 17657, 2013. (30) Wen, J. L.; Sun, S. N.; Yuan, T. Q.; Sun, R. C.; Xu, F. Fractionation of bamboo culms by autohydrolysis, organosolv delignification and extended delignification: Understanding the fundamental chemistry of the lignin during the integrated process. Bioresour. Technol. 2013, 150, 278−286. (31) Del Río, J. C.; Speranza, M.; Gutiérrez, A.; Martínez, M. J.; Martínez, A. T. Lignin attack during eucalypt wood decay by selected basidiomycetes: a Py-GC/MS study. J. Anal. Appl. Pyrolysis 2002, 64 (2), 421−431. (32) Wen, J. L.; Yuan, T. Q.; Sun, S. L.; Xu, F.; Sun, R. C. Understanding the chemical transformations of lignin during ionic liquid pretreatment. Green Chem. 2014, 16 (1), 181−190. (33) Wahyudiono; Sasaki, M.; Goto, M.; Kanetake, T. Decomposition of a lignin model compound under hydrothermal conditions. Chem. Eng. Technol. 2007, 30 (8), 1113−1122.

(34) Du, X.; Li, J.; Lindström, M. E. Modification of industrial softwood kraft lignin using Mannich reaction with and without phenolation pretreatment. Ind. Crops Prod. 2014, 52, 729−735. (35) Faravelli, T.; Frassoldati, A.; Migliavacca, G.; Ranzi, E. Detailed kinetic modeling of the thermal degradation of lignins. Biomass Bioenergy 2010, 34 (3), 290−301. (36) Pu, Y.; Hu, F.; Huang, F.; Davison, B. H.; Ragauskas, A. J. Assessing the molecular structure basis for biomass recalcitrance during dilute acid and hydrothermal pretreatments. Biotechnol. Biofuels 2013, 6 (1), 15. (37) Chen, Y. R.; Sarkanen, S. Macromolecular replication during lignin biosynthesis. Phytochemistry 2010, 71 (4), 453−462.

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