Article pubs.acs.org/EF
Characterization of High-Boiling-Solvent Lignin from Hot-WaterExtracted Bagasse Qiang Wang,*,†,‡,§ Shanshan Liu,†,§ Guihua Yang,† and Jiachuan Chen† †
Key Laboratory of Pulp and Paper Science and Technology, Ministry of Education, Qilu University of Technology, Jinan, Shandong 250353, People’s Republic of China ‡ Jiangsu Provincial Key Laboratory of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, People’s Republic of China § Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada ABSTRACT: The complicated structures of bagasse hinder the bioconversion processes for the production of bioenergy and biomaterials. In this study, an integrated process of hot-water extraction followed by high-boiling-solvent cooking (HBS, i.e., 1,4butanediol) was demonstrated to fractionate bagasse into hemicellulose, lignin, and cellulose. The hot-water extraction resulted in the removal of hemicellulose, which facilitated the HBS cooking for the open fiber structure. As a result, 57−70% of lignin was isolated from the cooking spent liquor. Gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), 31P and 1H nuclear magnetic resonance (NMR), and thermogravimetry (TG) were employed for characterization of the organosolv lignin. Results showed that the organosolv lignin exhibited a chemical structure similar to enzymatic hydrolysis/mild acidolysis lignin (EMAL) but formed new phenolic OH groups (3−6-fold of EMAL). The high molecular weight and thermal stability would contribute to the potential application of lignin into value-added products. The proposed processes provide an efficient approach for fractionating the three main components of bagasse, especially lignin. alternative process to fractionate highly phenolic lignin.8,9 In addition, the organosolv lignin was considered to be relatively pure, i.e., low in carbohydrates and ash and free of sulfur. Therefore, a vast range of potential applications can be realized beyond the combustion fuel.10 Several organosolv processes have been suggested for lignin removal from biomass, such as ethanol processes,11,12 formic acid, acetone, and high boiling solvent (HBS). HBS (i.e., 1.4butanediol) was of great interest because of its high boiling temperature (232 °C), good lignin solubility, and low pressure generated in the reactor during cooking.13−15 A significant amount of lignin (60−80% on a lignin basis) can be removed using HBS cooking. During the cooking, the phenolic β−O−4 linkage of lignin was cleaved, whereas β−β and β−5 substructures were kept stable.16,17 Meanwhile, HBS cooking technology was applied on biomass to produce highly pure silica.18 It is crucial to exploit the full benefit from bagasse, especially for hemicellulose and lignin. Although the HBS delignification mechanism was reported extensively, the integrated hot-water extraction with HBS cooking has not been investigated and the properties of isolated lignin were scarcely characterized. Therefore, the hot-water extraction was initially performed to remove the hemicellulose of bagasse, and HBS cooking was subsequently conducted to fractionate lignin from extracted substrate. The dissolved lignin in organic spend liquor was recovered by adding water and centrifugation. The chemical compositions and polymeric properties of recovered lignin were
1. INTRODUCTION Biomass, in particular, agricultural residues, represents the most abundant renewable material for the production of bioenergy and bioproducts in an environmentally friendly way.1 One promising way is to fractionate biomass into its three main compositions (i.e., hemicellulose, lignin, and cellulose), which were subsequently used as chemicals, polymers and materials.2 Hemicellulose is a branched polymer composed of several monosaccharides, i.e., xylose, mannose, galactose, arabinose, glucose, etc.3 It can be removed effectively through a preextraction process, including steam explosion, alkali, dilute acid, and hot-water extraction. Among these technologies, the hotwater extraction has been adopted in a Kraft-based dissolving pulp production process for its environmentally friendly and flexibility manner.4 During hot-water extraction, acetic acid released from hemicellulose can enhance the extraction efficiency. Oligo-/mono-sugars of hemicellulose and part of lignin were dissolved into pre-hydrolysis liquor.5 The downstream use is being developed for value-added products, i.e., fuel, film, and chemicals. In addition to hemicellulose, lignin is an alternative feedstock for chemical production based on its functional groups, i.e., hydroxyl, methoxyl, carbonyl, and carboxyl. Lignin, as a threedimensional amorphous polymer, includes three main types of phenylpropane units of guaiacyl (G), syringyl (S), and phydoxyphenyl (H).6 Chemical cooking processes (sulfate and sulfite cooking processes) were well-established to separate lignin from the cellulosic fibers. However, chemical bonds of the lignin backbone were extensively cleaved by added chemicals, showing the properties different from naturally occurring lignin.7 The organosolv cooking mainly resulted in the cleaving of α−O−4 and β−O−4 bonds, providing an © 2014 American Chemical Society
Received: January 1, 2014 Revised: April 16, 2014 Published: April 23, 2014 3167
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studied. Spectroscopic methods, including Fourier transform infrared spectroscopy (FTIR) and 31P and 1H nuclear magnetic resonance (NMR) were employed to characterize the functional groups. Gel permeation chromatography (GPC) and thermogravimetry (TG) were performed to evaluate the molecular weight and thermal stability, respectively. In addition, the role of catalyst of citric acid was also included for enhancing the HBS cooking process.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Bagasse was collected from Nanning (China) and had been fragmented into pieces of 10−40 mm in length, stored in a polyethylene bag after air-dried. The reagents used for NMR analysis were purchased from SigmaAldrich (Shanghai, China). All of the other reagents were analyticalgrade and purchased from Qianhui Chemical Reagent Co. (Guangzhou, China) 2.2. Preparation of Enzymatic/Mild Acidolysis Lignin (EMAL). The bagasse was smashed into powder and extracted for 8 h in a Soxhlet extractor with a benzene/ethanol solution. After dried, the sample was ground in a vibratory ball mill for 72 h with zirconia balls. For the preparation of EMAL, a two-step of procedure was used according to the literature.19 2.3. Extraction and Cooking Processes. 2.3.1. Hot-Water Extraction. Bagasse was placed into a high-pressure stainless-steel vessel with a liquid/solid ratio of 6 (v/w) at 170 °C for 2 h. The desired temperature in the reactor was maintained with a thermostated oil bath. After the vessel cooled, solid residual was collected by filtration with an 80-mesh screen. 2.3.2. HBS (i.e., 1,4-Butanediol) Cooking. The extracted solid was placed into the same vessel with a liquid/solid ratio of 6 (v/w) at 190 °C for 90 min, and the cooking solvent was 80:20 (v/v) HBS/water. After cooking, the cellulose was filtrated from spent liquor by an 80mesh screen. 2.4. Isolation of Lignin from Spent Liquor. A 3 times equivalent volume of water was added to organic spent liquor, stirred for 15 min, and then centrifuged at 5000 rpm for 10 min. The solid lignin was thoroughly washed with water and subsequently freezedried for 24 h. In one experiment, HBS cooking was carried out by autocooking without catalyst. The isolated lignin was named AL. In another experiment, citric acid of 8 mmol L−1 was used as a catalyst in HBS cooking. The isolated lignin was denoted CL. 2.5. Characterization of Lignin. The content of cellulose, hemicellulose, and lignin of raw material was analyzed according to the TAPPI method.20 Supernatant solution was measured by ion chromatography (IC, Dionex ICS-3000) following the method described in the literature.21 Lignin acetylation was carried out using acetic anhydride/pyridine (1:1, v/v) at room temperature for 72 h.22 The molecular weight distribution of acetylated lignin was determined by GPC (Agilent 1100) using tetrahydrofuran (THF) as a solvent. The solution was filtered using a 0.45 μm filter, and then 20 μL of filtered solution was injected into a GPC system. FTIR spectra of lignin were performed on a Nexus 670 (Thermo Nicolet) instrument. Each spectrum was recorded in a frequency range of 500−4000 cm−1 using a potassium bromide (KBr) disc. KBr was previously oven-dried to reduce the interference of water. 31 P and 1H NMR analyses were performed on a Bruker 400 MHz spectrometer. The derivatization and determination of lignin followed the procedure described by Zhang et al.23 Thermal analysis was performed on a TGA Q500 (TA Instruments, Inc.) under a nitrogen atmosphere from room temperature to 700 °C at a heating rate of 20 °C min−1. Lignin weighted 8−10 mg and was continually flushed with nitrogen at a flow rate of 30 mL min−1.
Figure 1. Overall scheme of hot-water extraction and HBS cooking.
the integrated process of hot-water extraction followed by HBS cooking to fractionate bagasse into hemicellulose, cellulose, and lignin. Hot-water extraction not only transferred hemicellulose into hydrolysis liquor prior to cooking but also led to the fibrillation and deconstruction of the cell wall, thus promoting the subsequent HBS cooking. The extracted bagasse was then subjected to HBS cooking with/without catalyst. Meanwhile, the lignin was degraded and dissolved into the spend liquor. As a result, the three main compositions of bagasse were basically distributed in hemicellulose-rich prehydrolysis liquor, ligninrich organic spend liquor, and cellulose-rich residual. In addition, the HBS solvent can be reused after distillation and reduced environmental impacts. The chemical compositions of extracted bagasse and cooked pulp are listed in Table 1. Hot-water extraction significantly Table 1. Chemical Composition of Bagasse, Extracted Bagasse, and Pulpa hot-water extraction bagasse cellulose (g) Klason lignin (g) soluble lignin (g) hemicellulose (g) total sugarb(g) sum (g) total (g)
44.3 21.0 1.80 21.1 88.2 100
spent liquor
1.61 17.3 18.9 25.8
HBS cooking
solid
auto
catalyzed
40.1 17.5 0.20 2.20
37.1 3.61
34.5 1.22
1.90
1.30
60.0 74.2
42.6 45.3
37.0 40.0
Hot-water extraction conditions, 170 °C, 120 min, and liquor/solid ratio of 6; cooking conditions, 190 °C and 90 min. bSum of xylose, glucose, arabinose, mannose, and galactose of IC analysis. a
removed the hemicellulose but allowed the cellulose and Klason lignin content to be almost unchanged. These results provided direct evidence that hemicellulose was more sensitive to hot-water hydrolysis than cellulose. On the other hand, the soluble lignin was also substantially dissolved into hydrolysis liquor, which should be minimized in pre-hydrolysis liquor because of its negative effect on the hemicellulose use. The result agreed well with the observation in the literature that the conditions, such as 170 °C and 2 h, were optimized for higher hemicellulose solubilization and lower sugar degradation and about 50% of xylose was dissolved into pre-hydrolysis liquor when using hot water to extract hemicellulose composition from bagasse.24 Subsequently, HBS cooking was carried out to remove lignin from extracted substrate. About 83% of lignin was dissolved into spend liquor, while catalyzed cooking (using citric acid) further increased the lignin removal of up to 94%, as a result of the organic weak acid effect on enhanced delignification.
3. RESULTS AND DISCUSSION 3.1. Proposed Process of Hot-Water Extraction and HBS Cooking for Bagasse Fractionation. Figure 1 showed 3168
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Dominggus and Laszlo also investigated the catalyst effect on ethanol cooking, and they concluded that the desired catalyst exerted a positive effect on organosolv cooking.25 Yang et al.26 studied solid alkali cooking assisted with active oxygen (O2 and H2O2) to fractionate lignin from bagasse and found that 95% of lignin was removed into yellow spend liquor. 3.2. Chemical Composition and Molecular Weight of Lignin. The chemical composition and molecular weight of lignin are listed in Table 2. EMAL was prepared as a reference. Table 2. Chemical Compositions and Molecular Weight of Lignina chemical composition
molecular weight
lignin
EMAL
AL
CL
yield (%)b Klason lignin (%) xylose (%) glucose (%) arabinose (%) mannose (%) galactose (%) total sugar (%) ash (%) Mw (g mol−1) Mn (g mol−1) Mw/Mn
11.5 93.3 4.07 1.37 1.34 ND ND 6.78 0.80 28080 12680 2.21
13.1 96.9 2.17 NDc 0.98 ND ND 3.15 0.51 12770 6351 2.01
16.0 96.0 2.31 0.45 0.42 ND ND 3.18 0.46 11410 5638 2.02
Figure 2. FTIR spectra of lignin (EMAL refers to enzymatic hydrolysis/mild acidolysis lignin, and AL and CL refer to autocooking lignin and catalyzed cooking lignin, respectively).
vibrations of the phenyl propane skeleton). A similar chemical structure of lignin illustrated that the organosolv process did not change the core of lignin significantly. This agreed well with the literature results that there is no drastic change in the lignin structure when using dioxane to fractionate lignin from triploid.23 3.4. 31P NMR Analysis. 31P NMR was performed to analyze lignin in terms of aliphatic OH group, phenolic OH group, and carboxylic OH group, and the results are shown in Figure 3 and
a
EMAL refers to enzymatic hydrolysis/mild acidolysis lignin, and AL and CL refer to autocooking lignin and catalyzed cooking lignin, respectively. bOn the basis of oven-dried bagasse. cNot detectable.
It can be seen that the obtained lignin yield reached 57−70%, implying the efficient fractionation of the proposed process. In comparison to EMAL, the organosolv lignin (AL and CL) showed higher purity (higher Klason lignin and lower monosaccharide). This result can be mainly attributed to the substantial removal of hemicellulose during hot-water extraction. The ash content of AL and CL was lower than that of EMAL. The enzymatic hydrolysis/mild acidolysis process was also conducted by Wu and Argyropoulos27 to isolate lignin from wood, which lead to a lignin yield of 69.9−75.3% with a purity of 87.4−91.3%. The molecular weight is another important parameter for lignin because of the increasing polymerization chance. As seen, the molecular weights of AL and CL were lower than that of EMAL, which may be caused by fragmentation and side reactions of lignin during the severe cooking process. Lan et al.28 studied the fractionation of bagasse with ionic liquid treatment followed by alkaline extraction, which can yield lignin with a molecular weight (Mw) of 9090, while the Mw of milled wood lignin (MWL) was 13 720. 3.3. FTIR Spectroscopy. FTIR spectra of EMAL, Al, and CL lignin were recorded in Figure 2. The assignments of bands were in good agreement with the literature reports.2,6 The minor changes in peaks and absorption intensities indicated the analogous structure of lignin. The band at ∼3400 cm−1 could be attributed to phenolic and aliphatic OH groups. The bands at 1120, 833, and 1036 cm−1 indicated a typical structure of lignin with p-hydroxyphenyl propane (H), guaiacyl (G), and syringyl (S) units, giving signals for typical H, G, and S units of lignin. The presence of p-hydroxyphenyl propane (H) and guaiacyl (G) units in lignin confirmed that all lignin had a potential active site for polymerization. All lignin samples showed bands at ∼1600, 1513, and 1425 cm−1 (aromatic ring
Figure 3. 31P NMR spectra of lignin (EMAL refers to enzymatic hydrolysis/mild acidolysis lignin, and AL and CL refer to autocooking lignin and catalyzed cooking lignin, respectively).
Table 3. As seen, the organosolv lignin (AL and CL) had a higher phenolic OH group content and a lower aliphatic OH group content than that of EMAL. Furthermore, AL lignin had the highest phenolic OH group (5.63 mmol/g) of guiacyl and p-hydroxyphenyl units, followed by CL (3.64 mmol/g) and EMAL (3.12 mmol/g). The free phenolic OH groups in guiacyl and p-hydroxyphenyl units play more important roles in crosslinking in comparison to syringyl units, the latter of which had two methoxyl groups at positions 3 and 5. This suggested the potential of organosolv lignin (in particular, AL) for polymer use. In one study,6 several processes were used to fractionate lignin from biomass and the organosolv lignin (ethonal cooking) had high phenolic guiacyl (0.99 mmol/g) and phydroxyphenyl (0.34 mmol/g) units. In another study,23 the three-step dioxane extraction process was performed to isolate 3169
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Table 3. 31P NMR Results of Lignin (mmol g−1) signal (ppm)
letters in Figure 3
assignment
EMAL
AL
CL
145.0−150.0 144.9 144.6−143.6 142.4−140.2 143.6−142.4 140.2−138.6 138.6−137.0 136.0−134.0
A B C
aliphatic OH groups cholesterol standard condensed phenolic OH
11.98
10.82
6.84
0.55
1.84
1.14
D E F G
syringyl OH guaicyl OH p-hydroxyphenol OH carboxyl groups total phenolic
0.41 1.07 2.05 0.23 4.08
2.46 2.39 3.24 0.28 9.92
2.10 1.74 1.90 0.03 6.88
hydroxyls. These results agreed well with literature reports, in which 13C NMR was employed to analyze lignin structure changes during HBS cooking. The results showed that phenolic β−O−4 linkages were cleaved homolytically.16 In another study, 1H−13C correlation two-dimensional (2D) NMR was applied for understanding the delignification mechanism during HBS cooking, and the results showed that β−O−4 structures disappeared, while β−1, β−β, and β−5 structures remained in lignin.17 3.6. Thermal Stability of Isolated Lignin. The lignin thermal characteristic is also important with respect to polymer chemistry. A similar decomposition behavior should be performed for the mixing materials, as documented in the literature.19 As shown in Figure 5, the wide temperature ranges
lignin from bagasse, which produced much more phenolic OH groups (from 1.66 to 9.42 mmol/g of lignin). 3.5. 1H NMR Analysis. The chemical structure of various lignin samples was investigated by 1H NMR, as shown in Figure 4. The two sharp signals at 2.5 and 3.3 ppm were related to
Figure 4. 1H NMR spectra of lignin (EMAL refers to enzymatic hydrolysis/mild acidolysis lignin, and AL and CL refer to autocooking lignin and catalyzed cooking lignin, respectively).
DMSO-d6 solvent and water, respectively. The signal assignments were similar to the previous reference22 and listed in Table 4. As seen, all lignin samples displayed G-, S-, and H-type subunits, which were in agreement with 31P NMR results. The rapid decrease of proton in β−O−4 and β−1 stuctures of the organosolv lignin can be attributed to the extensive cleavage of the aryl−ether bonds during HBS cooking, especially for the sample with catalyst. The aliphatic hydroxyl content in the organosolv lignin was much lower than that in EMAL, assuming that an aryl−ether cleavage eliminated aliphatic
Figure 5. Thermogravimetric (TG and DTG) analysis of lignin (EMAL refers to enzymatic hydrolysis/mild acidolysis lignin, and AL and CL refer to autocooking lignin and catalyzed cooking lignin, respectively).
of 100−700 °C were covered for thermal degradation of lignin. When the temperature increased to 700 °C, about 28% (EMAL), 33% (AL), and 37% (CL) of residue still remained in
Table 4. 1H NMR Results of Lignin (mmol g−1) signal (ppm)
assignment
EMAL
AL
CL
7.80−7.20 7.20−6.50 6.50−6.20 6.20−5.75 5.75−5.25 5.25−4.90 4.90−4.00 4.00−3.50 2.15−1.90 1.90−1.70
aromatic H in H units aromatic H in G units aromatic H in S units Hα of β−O−4 and β−1 Hα of β−5 H of xylan residue Hα and Hβ of β−O−4 and Hα of β−β H of methoxyl groups H of aromatic acetates H of aliphatic acetates
1.00 1.84 0.36 0.20 0.43 0.94 1.08 3.77 0.18 0.39
1.00 2.69 0.45 0.12 0.30 0.37 1.02 5.70 0.25 0.14
1.00 2.23 0.48 0.08 0.28 0.36 1.01 3.94 0.21 0.07
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solid form. The initial weight lost (
