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Structural features of lignin macromolecules extracted with ionic liquid from poplar wood
Bioresource Technology 102 (2011) 9020–9025
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Structural features of lignin macromolecules extracted with ionic liquid from poplar wood Jae-Young Kim a, Eun-Jin Shin b,1, In-Yong Eom a, Keehoon Won b,1, Yong Hwan Kim c,2, Donha Choi d,3, In-Gyu Choi a, Joon Weon Choi a,⇑ a
Department of Forest Sciences and Research Institute for Agriculture and Life Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-921, Republic of Korea Department of Chemical and Biochemical Engineering, Dongguk University, 26 Pil-dong 3-ga, Jung-gu, Seoul 100-715, Republic of Korea Department of Chemical Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-gu, Seoul 139-701, Republic of Korea d Division of Forest Bioenergy, Korea Forest Research Institute, 57 Hoegi-ro, Dongdaemun-gu, Seoul 130-712, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 20 July 2011 Accepted 20 July 2011 Available online 28 July 2011 Keywords: 1-Ethyl-3-methylimidazolium acetate Ionic liquid lignin Milled wood lignin b-O-4 linkage Thermogravimetric analysis
a b s t r a c t 1-Ethyl-3-methylimidazolium acetate ([Emim][CH3COO]) was used for the extraction of lignin from poplar wood (Populus albaglandulosa), which was called to ionic liquid lignin (ILL) and structural features of ILL were compared with the corresponding milled wood lignin (MWL). Yields of ILL and MWL were 5.8 ± 0.3% and 4.4 ± 0.4%, respectively. The maximum decomposition rate (VM) and temperature (TM) corresponding to VM were 0.25%/°C and 308.2 °C for ILL and 0.30%/°C and 381.3 °C for MWL. The amounts of functional groups (OMe and phenolic OH) appeared to be similar for both lignins; approximately 15.5% and 6.7% for ILL and 14.4% and 6.3% for MWL. However, the weight average molecular weight (Mw) of ILL (6347 Da) was determined to be 2/3-fold of that of MWL (10,002 Da) and polydispersity index (PDI: Mw/Mn) suggested that the lignin fragments were more uniform in the ILL (PDI 1.62) than in the MWL (PDI 2.64). Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Lignin is one of the main components of all terrestrial plants that accounts for about 15–20% of the mass, and is the second most abundant polymeric material in nature next to cellulose. Lignin is a three dimensional amorphous phenolic polymer composed of three C6C3 types of monolignols (p-coumaryl, coniferyl, and sinapyl alcohol), which are heterogeneously connected by various interunit linkages such as b-O-4, b–b, b-5, b-1, 5-5 and 4-O-5, etc. (Fengel and Wegener, 1984; Lai and Sarkanen, 1971). Lignin generally features an irregular structure with a highly condensed cross-linked polymer network providing the biomass with mechanical strength as well as rigidity to resist external forces (Nimz, 1974). Lignin is known to bind physically/chemically to cellulose/hemicellulose by covalent bonding such as benzyl-ether, benzyl-ester, and phenyl-glycoside bonds, forming lignin–carbohydrate complexes (LCC) in plant cell walls (Yaku et al., 1981). Ionic liquids (ILs), organic salts that melt below 100 °C, are composed of cation/anion pairs (Huddleston et al., 1998). ILs are ⇑ Corresponding author. Tel.: +82 2 880 4788; fax: +82 2 873 2318. 1 2 3
E-mail address: [email protected] (J.W. Choi). Tel.: +82 2 2260 8922; fax: +82 2 2268 8729. Tel.: +82 2 940 5675; fax: +82 2 941 1785. Tel.: +82 2 961 2745; fax: +82 2 961 2788.
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.07.081
so-called designed solvents because their properties are tunable depending on the selection of anion and cation. Furthermore, ILs have eco-friendly characteristics, being nonflammable and recyclable with low volatility and high thermal stability (Earle and Seddon, 2000; Van Rantwijk and Sheldon, 2007). Recently, ILs have attracted significant attention as promising green solvents for the pretreatment of lignocellulosic biomass. Many ILs are able to dissolve wood biomass and its major components, cellulose, hemicellulose, and lignin (Fu et al., 2010; Li et al., 2009; Murugesan and Linhardt, 2005; Sun et al., 2011; Swatloski et al., 2002; Zavrel et al., 2009). In this regard, it was previously proposed that ILs could be utilized as cleaning chemicals for biomass pretreatment to reduce both the crystallinity and lignin content thereby allowing accessibility of cellulolytic enzymes to the biomass (Lee et al., 2009; Samayam and Schall, 2010). During pretreatment of lignocellulosic biomass with ILs, some lignins as well as carbohydrate derivatives may also be produced as by-products. For the total utilization of biomass, the lignins (ionic liquid lignin; ILL) are used as raw materials for value-added products. Characterization of the structural and chemical features of ILL is needed for this purpose; however, very few studies on ILL have been performed to date. Therefore, the aim of this study was to characterize the residual lignin from biomass pretreatment with ILs for the first time. Poplar (Populus albaglandulosa) was used as the raw material, and
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Sigma–Aldrich (USA) and were used without further purification. ILL was prepared according to the procedure of a previous study (Sun et al., 2009), and this extraction process is briefly described in Fig. 1. The wood powder (1.0 g) was added to 1-ethyl-3-methylimidazolium acetate (20 g) in a glass vial. The mixture was stirred at 110 °C for 16 h to dissolve the suspended poplar powder. An equi-volume solution of acetone and water (200 ml) was added to the brown solution with stirring, and the resultant cellulosic materials were filtered using 0.45 lm filter paper. The filtrate was then heated at 80 °C until the acetone was removed and insoluble solid ILL precipitated. The crude ILL was washed several times with distilled water and was finally dried in a vacuum desiccator. MWL was isolated from the same wood species according to the procedure of Björkman (Björkman, 1956). The yields of ILL and MWL were 5.8 ± 0.3% and 4.4 ± 0.4%, respectively.
2.2. Structural and chemical analysis 2.2.1. Elemental analysis and functional groups Elemental analysis was performed to determine the carbon, hydrogen, oxygen, and nitrogen content in lignin samples using US/CHNS-932 (LECO Corp., USA). The essential functional groups attached to the lignin, methoxyl and phenolic hydroxyl moieties, were determined by GC and aminolysis methods, respectively (Baker, 1996; Mansson, 1983).
Fig. 1. Flow chart of the lignin extraction processes.
2.2.2. Thermogravimetric analysis (TGA) Thermal decomposition characteristics of MWL and ILL were determined with a Q-5000 IR instrument (TA Instruments, USA). TG analysis was performed at a constant heating rate of 10 °C/ min up to 800 °C. An inert atmosphere was maintained with a 25 ml/min N2 flow.
1-ethyl-3-methylimidazolium acetate ([Emim][CH3COO]), which is known to selectively extract lignin from wood and to be one of the best solvents for lignocellulosic materials among the ILs, was employed as the ionic liquid (Mäki-Arvela et al., 2010; Sun et al., 2009). In addition, [Emim][CH3COO] is one of the most promising candidates for industrial applications due to its low viscosity, low melting point, non-toxicity, and biodegradability (Mäki-Arvela et al., 2010; Sun et al., 2011). For comparative analysis, milled wood lignin (MWL) was used as a representative native lignin, extracted from the same wood species, and served as a control. The chemical properties of ILL were determined by several analyses including elemental analysis, functional group analysis, gel permeation chromatography (GPC), and derivatization followed by reductive cleavage (DFRC). Furthermore, thermo-decomposition behaviors as well as structural features were characterized by thermogravimetric analysis (TGA) and spectroscopic analyses including FT-IR, 1H- and 13C-NMR. On the basis of these results, the chemical characteristics of ILL and MWL were extensively compared.
2.2.3. Gel permeation chromatography (GPC) The lignin was acetylated with acetic anhydride/pyridine (1:1 v/ v) at 70 °C for 6 h, and the derivatized lignin was dissolved in tetrahydrofuran (THF) and analyzed by a GPC max instrument (ViscotekRImax, Viscotek, UK) equipped with a PLgel 5 lm MIXED-C column (300 7.5 mm, VARIAN, Inc.) and PLgel 5 lm guard column (50 7.5 mm, VARIAN, Inc.) using UV–Vis detection (VE3210, Viscotek). To determine the molecular weight of the effluent, polystyrenes with a mass range between 580 Da and 3250,000 Da were used to create a calibration curve.
2.2.4. Derivatization followed by reductive cleavage (DFRC method) To determine the frequency of arylglycerol-b-aryl ether linkages (b-O-4), ILL and MWL were subjected to the DFRC method (Lu and Ralph, 1997). Essential DFRC products, acetylated coniferyl and sinapyl alcohol, were quantitatively determined by GC analysis (Agilent 7890A) on an instrument equipped with a capillary column DB-5 (30 m 0.32 mm 0.25 lm, Agilent) using the following temperature program: injection and detector temperatures were set to 220 °C and 300 °C, respectively, and the oven temperature was increased at a rate of 5 °C/min from 100 °C to 280 °C,
2. Methods 2.1. Preparation of ionic liquid lignin Air-dried poplar (P. albaglandulosa) wood xylem was ground to a fine powder (less than 0.64 mm particle size). 1-Ethyl-3-methylimidazolium acetate ([Emim][CH3COO]), produced by BASF (P90%) and acetone (P99.5%) were purchased from
Table 1 Elemental composition, functional groups, and calculated C9 formula of ionic liquid lignin (ILL) and milled wood lignin (MWL).
a
Lignin
Yields (%)a
Poplar ILL Poplar MWL
5.8 ± 0.3 4.4 ± 0.4
Elemental composition (%)
Functional groups (%)
C
H
N
O
OMe
Phe-OH
60.1 58.6
5.9 5.8
1.2 0.1
32.9 35.5
15.5 ± 1.5 14.4 ± 0.1
6.7 ± 0.2 6.3 ± 0.2
Based on oven dried weight of poplar wood.
C9 formula
Mw/C9 (g/mol)
C900H790O232N15(OH)79(OCH3)100 C900H812O272(OH)76(OCH3)94
199.7 201.9
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Fig. 2. Proposed scheme for lignin extraction with ionic liquids from native lignin: (A) Structure of native lignin macromolecular in poplar wood; (B) Ionic liquid (1-ethyl-3methylimidazolium acetate); (C) Ionic liquid lignin model.
Table 2 Average molecular weight and polydispersity index (Mw/Mn) of ionic liquid lignin (ILL) and milled wood lignin (MWL). Lignin
Mw (Daltons)
Mn (Daltons)
Mw/Mn
Poplar ILL Poplar MWL
6347 10,002
3908 4060
1.62 2.46
Table 3 Quantitative analysis of DFRC products of ionic liquid lignin (ILL) and milled wood lignin (MWL). Lignins
Poplar ILL Poplar MWL
Fig. 3. Thermogravimetric and differential thermal analyses of ionic liquid lignin (ILL) and milled wood lignin (MWL).
DFRC (lmol/g sample)a G– CH@CHCH2OAc (Acetylated CA)
S– CH@CHCH2OAc (Acetylated SA)
Total
S/Gb
564.6 ± 4.13 560.6 ± 18.64
699.0 ± 46.89 745.7 ± 22.23
1263.7 ± 42.76 1306.3 ± 40.87
0.8 ± 0.06 0.7 ± 0.01
a
Data are means of triplicate analyses ± S.D. Average molar ratios of sinapyl alcohol/coniferyl alcohol formed by the cleavage of b-O-4 linkages. b
and then the final temperature was maintained for 8 min. The split ratio was 20:1. 2.2.5. Spectroscopic analysis (FT-IR, 1H- and 13C-NMR) FT-IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, USA) operating in the wavelength range of 4000–650 cm 1 with a resolution of 8 cm 1. Each sample was coated by potassium bromide (KBr) and formed into a pellet for analysis. 13C-NMR and 1H-NMR spectra were obtained with a Bruker AVANCE 600 spectrometer (Bruker, Germany) operating at 60 °C from 20,000 (13C-NMR) and 128 (1H-NMR) scans, respectively, with 150 mg of lignin sample dissolved in DMSO-d6. 3. Results and discussion 3.1. Elemental analysis and functional groups in ionic liquid lignins (ILL) Recently pretreatment of woody biomass with ILs attracts worldwide attention in the field of cellulose based bioethanol production due to its feature of environmental friendliness. However,
in spite of lots of intensive researches, the optimal condition for the most efficient pretreatment of woody biomass, using ionic liquids, are not yet established. In general, higher temperature and longer time help to dissolve woody biomass into ionic liquid, which resulted in higher pretreatment efficiency (Kilpelainen et al., 2007; Fort et al., 2006). Sun et al. (2009) found that higher temperature and longer time demand higher energy consumption as well as has risk of more degradation of cellulose, so IL pretreatment under the temperature of 110 °C and a time of 16 h were chosen as a compromise, between enhanced dissolution and higher energy demand. Table 1 shows the elemental composition, functional groups and C9 formula of the two lignin samples, extracted with ionic liquid ([Emim][CH3COO]) by Sun’s methodology (Sun et al., 2009) and purified by the Björkman procedure (Björkman, 1956), respectively. The elemental carbon content of ILL (60.1%) was determined to be slightly higher than that of MWL (58.6%), whereas the oxygen content of ILL (32.9%) was lower than that of MWL (35.5%). The
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Fig. 4. Spectroscopic analyses of ionic liquid lignin (ILL) and milled wood lignin (MWL). (A) FT-IR spectra, (B)
relatively high nitrogen content (1.2%) of ILL was particularly notable. The small amount of nitrogen in MWL was derived from protein bound to the lignin (Pan and Sano, 2000). Since the ionic liquid ([Emim][CH3COO]) utilized in this study has two moles of nitrogen per IL molecule, most of the nitrogen in the ILL probably came from the solvent.
13
C-NMR spectra, (C) 1H-NMR spectra.
The structure of the ionic liquid ([Emim][CH3COO]) contains an imidazolium-based salt and two alkyl groups. As shown in Fig. 2, due to the cationic nitrogen in the imidazolium, the ionic liquid may be physically or chemically associated with lignin at the electron rich oxygen, such as b-O-4 and a-O-4, and b–b linkages. It could be supposed that partial fragmentation of lignin could
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occur during this extraction process. Comparing the OMe contents of the two lignin samples, the OMe content (15.5%) of ILL was ca. 7% higher than that of MWL (14.4%). For the phenolic OH content, ILL had a 6% higher phenolic hydroxyl content than that of MWL, which indicates that ILL consists of more free-phenolic moieties that are probably released by partial fragmentation of lignin during the ionic liquid extraction. The fictive summative formulae (C900 formulas) of both lignins were also calculated based on fundamental analyses (elemental composition and determination of OMe and phenolic OH) (Table 1). 3.2. Thermal behavior of ionic liquid lignin (ILL) Thermogravimetric and differential thermal analysis (TGA and DTG) of ILL and MWL are described in Fig. 3. As seen in the DTG curves, the maximum decomposition rate (VM) of ILL was 0.25%/ °C while that of MWL was 0.3%/°C, and the temperature (TM) corresponding to VM shifted to a lower temperature region (308.2 °C) compared with the MWL TM (381.9 °C). Furthermore, the active pyrolysis region for ILL was from 100 °C to 308.2 °C, whereas for MWL, it was from 100 °C to 381.9 °C. The active pyrolysis region was defined as the thermal degradation region from about 100 °C (dehydration step) to TM as determined from VM in the DTG curve (Hodgson et al., 2011). These results suggest that MWL was more thermally stable than ILL, as evident from the higher TM for MWL. According to Sun et al. (2000), the thermal stability of lignins increased with incrementally higher Mw. With a greater degree of branching and condensation of the lignin macromolecule, the thermal energy needed for bond cleavage increased. Moreover, Sun et al. (2000) also found that TM corresponding to VM shifted to a higher region with increasing Mw. 3.3. Characteristics of the ionic liquid lignin (ILL) molecular structure 3.3.1. The average molecular weights (Mw and Mn) and polydispersity index of ILL The average molecular weights (Mw and Mn) and polydispersity index of both lignin samples were determined by GPC analysis, and the results are given in Table 2. The Mw of ILL was determined to be 6347 Da, which was only 2/3 of the Mw of MWL (10,002 Da). Together with the high content of phenolic OH, this lower Mw value for ILL is good evidence for lignin fragmentation by the ionic liquid. In contrast, the Mn of both ILL and MWL had similar values of about 4000 Da. The polydispersity index (Mw/Mn) of ILL and MWL was calculated to be 2.46 and 1.62, respectively. This result indicates that compared to MWL, ILL was formed with a relatively uniform lignin fragment size and a lower number of condensed linkages (Tan et al., 2009). 3.3.2. The frequency of arylglycerol-b-aryl ether linkage (b-O-4) in ILL In general, the arylglycerol-b-aryl ether linkage (b-O-4) is considered to be a typical linkage type in lignin macromolecules. The frequency of the b-O-4 linkage accounts for 40–65% of the various coupling patterns and greatly affects the chemical and physical properties of the lignin (Kishimoto et al., 2006; Nimz, 1974). Since derivatization followed by reductive cleavage (DFRC) analysis selectively cleaves the b-O-4 linkage and releases acetylated C6C3 type monomers [acetylated coniferyl (G type) and sinapyl alcohol (S type)] as DFRC products, the frequency of the b-O-4 linkage in the lignin can be indirectly evaluated by quantification of the DFRC products. Table 3 lists the quantitative amounts of DFRC products from ILL and MWL. Considering the total DFRC products, ILL gave rise to ca. 1264 lmol/g of total DFRC products, which was slightly lower than that of MWL (1306 lmol/g), and the S/G ratio of ILL (1.24) was slightly lower than that of MWL (1.34), indicating
that the G-type lignin was more easily extracted than the S-type lignin during treatment of the biomass with ionic liquid. 3.3.3. Spectroscopic analyses (FT-IR, 1H- and 13C-NMR analysis) For the in depth elucidation of structural features of lignin molecules, ILL and MWL were also subjected to various spectroscopic analyses (FT-IR, 1H- and 13C-NMR analysis), and representative spectra are shown in Fig. 4. As shown in Fig. 4 (Top), the FT-IR spectra for both lignins appear to be almost identical, and typical lignin peaks, such as O–H stretching (3412–3460 cm 1), C–H stretching in methyl/methylene groups (3000–2842 cm 1), aromatic skeletal vibrations (1505–1515 cm 1), and aromatic C–H in plane deformation (1200–1100 cm 1) were visible in both lignin spectra. For the 13 C-NMR spectra (Fig. 4; Bottom), most of the lignin signals, such as the aromatic carbons at 100 ppm–155 ppm, aliphatic side chain carbons at 86–60 ppm, and methoxyl carbon at 55 ppm appeared equally in both the ILL and MWL spectra. However, the signal at 62–63 ppm was only observed in MWL. This signal is assigned to the Cc with Ca@O originated from side chain oxidation via homolytic cleavage during extensive ball milling process (El Hage et al., 2009). Furthermore, a few differences are seen only in the 1H-NMR spectra. Unlike the MWL spectrum, two signals at 2.0 ppm and 3.2 ppm were not visible in the ILL spectrum. The signal at 2.0 ppm originated from aliphatic acetates (acetyl groups in xylan). In particular, the signal at 3.2 ppm is normally assigned to the Hb in b–b structures. As mentioned above for Fig. 2, reduction of the signal intensity at 3.2 ppm is also good evidence for cleavage of b–b structures during lignin extraction with ionic liquid (Lundquist, 1979). 4. Conclusions This study compares structural features of two different lignins; one is a ionic liquid lignin (ILL) purified with 1-ethyl-3-methylimidazolium acetate ([Emim][CH3COO]) from poplar wood and the other is the corresponding milled wood lignin (MWL). According to diverse spectroscopic analyses structure of ILL seems to be quite similar to that of MWL. However, GPC data revealed that average molecular weight of ILL is only 2/3-fold of the MWL. Furthermore, the lower S/G ratio of ILL (1.24) than that of MWL (1.34) indicates G-type lignin was more extractable than S-type lignin during the treatment of biomass with ionic liquid. Acknowledgements This research was supported by a grant from the Korea Research Foundation funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea (KRF-2008-331-F00027). References Baker, S., 1996. Rapid methoxyl analysis of lignins using gas chromatography. Holzforschung 50, 573–574. Björkman, A., 1956. Studies on finely divided wood. Part I. Extraction of lignin with neutral solvents. Svensk Papperst. 59, 477–485. Earle, M., Seddon, K., 2000. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 72, 1391–1398. El Hage, R., Brosse, N., Chrusciel, L., Sanchez, C., Sannigrahi, P., Ragauskas, A., 2009. Characterization of milled wood lignin and ethanol organosolv lignin from miscanthus. Polym. Degrad. Stab. 94, 1632–1638. Fengel, D., Wegener, G., 1984. Wood – Chemistry, Ultrastructure, Reactions. De Gruyter, Berlin. Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P., Moyna, G., Rogers, R.D., 2006. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methyl imidazolium chloride. Green Chem. 9, 63– 69. Fu, D., Mazza, G., Tamaki, Y., 2010. Lignin extraction from straw by ionic liquids and enzymatic hydrolysis of the cellulosic residues. J. Agric. Food. Chem. 58, 2915– 2922.
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Structural features of lignin macromolecules extracted with ionic liquid from poplar wood Jae-Young Kim a, Eun-Jin Shin b,1, In-Yong Eom a, Keehoon Won b,1, Yong Hwan Kim c,2, Donha Choi d,3, In-Gyu Choi a, Joon Weon Choi a,⇑ a
Department of Forest Sciences and Research Institute for Agriculture and Life Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-921, Republic of Korea Department of Chemical and Biochemical Engineering, Dongguk University, 26 Pil-dong 3-ga, Jung-gu, Seoul 100-715, Republic of Korea Department of Chemical Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-gu, Seoul 139-701, Republic of Korea d Division of Forest Bioenergy, Korea Forest Research Institute, 57 Hoegi-ro, Dongdaemun-gu, Seoul 130-712, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 20 July 2011 Accepted 20 July 2011 Available online 28 July 2011 Keywords: 1-Ethyl-3-methylimidazolium acetate Ionic liquid lignin Milled wood lignin b-O-4 linkage Thermogravimetric analysis
a b s t r a c t 1-Ethyl-3-methylimidazolium acetate ([Emim][CH3COO]) was used for the extraction of lignin from poplar wood (Populus albaglandulosa), which was called to ionic liquid lignin (ILL) and structural features of ILL were compared with the corresponding milled wood lignin (MWL). Yields of ILL and MWL were 5.8 ± 0.3% and 4.4 ± 0.4%, respectively. The maximum decomposition rate (VM) and temperature (TM) corresponding to VM were 0.25%/°C and 308.2 °C for ILL and 0.30%/°C and 381.3 °C for MWL. The amounts of functional groups (OMe and phenolic OH) appeared to be similar for both lignins; approximately 15.5% and 6.7% for ILL and 14.4% and 6.3% for MWL. However, the weight average molecular weight (Mw) of ILL (6347 Da) was determined to be 2/3-fold of that of MWL (10,002 Da) and polydispersity index (PDI: Mw/Mn) suggested that the lignin fragments were more uniform in the ILL (PDI 1.62) than in the MWL (PDI 2.64). Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Lignin is one of the main components of all terrestrial plants that accounts for about 15–20% of the mass, and is the second most abundant polymeric material in nature next to cellulose. Lignin is a three dimensional amorphous phenolic polymer composed of three C6C3 types of monolignols (p-coumaryl, coniferyl, and sinapyl alcohol), which are heterogeneously connected by various interunit linkages such as b-O-4, b–b, b-5, b-1, 5-5 and 4-O-5, etc. (Fengel and Wegener, 1984; Lai and Sarkanen, 1971). Lignin generally features an irregular structure with a highly condensed cross-linked polymer network providing the biomass with mechanical strength as well as rigidity to resist external forces (Nimz, 1974). Lignin is known to bind physically/chemically to cellulose/hemicellulose by covalent bonding such as benzyl-ether, benzyl-ester, and phenyl-glycoside bonds, forming lignin–carbohydrate complexes (LCC) in plant cell walls (Yaku et al., 1981). Ionic liquids (ILs), organic salts that melt below 100 °C, are composed of cation/anion pairs (Huddleston et al., 1998). ILs are ⇑ Corresponding author. Tel.: +82 2 880 4788; fax: +82 2 873 2318. 1 2 3
E-mail address: [email protected] (J.W. Choi). Tel.: +82 2 2260 8922; fax: +82 2 2268 8729. Tel.: +82 2 940 5675; fax: +82 2 941 1785. Tel.: +82 2 961 2745; fax: +82 2 961 2788.
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.07.081
so-called designed solvents because their properties are tunable depending on the selection of anion and cation. Furthermore, ILs have eco-friendly characteristics, being nonflammable and recyclable with low volatility and high thermal stability (Earle and Seddon, 2000; Van Rantwijk and Sheldon, 2007). Recently, ILs have attracted significant attention as promising green solvents for the pretreatment of lignocellulosic biomass. Many ILs are able to dissolve wood biomass and its major components, cellulose, hemicellulose, and lignin (Fu et al., 2010; Li et al., 2009; Murugesan and Linhardt, 2005; Sun et al., 2011; Swatloski et al., 2002; Zavrel et al., 2009). In this regard, it was previously proposed that ILs could be utilized as cleaning chemicals for biomass pretreatment to reduce both the crystallinity and lignin content thereby allowing accessibility of cellulolytic enzymes to the biomass (Lee et al., 2009; Samayam and Schall, 2010). During pretreatment of lignocellulosic biomass with ILs, some lignins as well as carbohydrate derivatives may also be produced as by-products. For the total utilization of biomass, the lignins (ionic liquid lignin; ILL) are used as raw materials for value-added products. Characterization of the structural and chemical features of ILL is needed for this purpose; however, very few studies on ILL have been performed to date. Therefore, the aim of this study was to characterize the residual lignin from biomass pretreatment with ILs for the first time. Poplar (Populus albaglandulosa) was used as the raw material, and
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Sigma–Aldrich (USA) and were used without further purification. ILL was prepared according to the procedure of a previous study (Sun et al., 2009), and this extraction process is briefly described in Fig. 1. The wood powder (1.0 g) was added to 1-ethyl-3-methylimidazolium acetate (20 g) in a glass vial. The mixture was stirred at 110 °C for 16 h to dissolve the suspended poplar powder. An equi-volume solution of acetone and water (200 ml) was added to the brown solution with stirring, and the resultant cellulosic materials were filtered using 0.45 lm filter paper. The filtrate was then heated at 80 °C until the acetone was removed and insoluble solid ILL precipitated. The crude ILL was washed several times with distilled water and was finally dried in a vacuum desiccator. MWL was isolated from the same wood species according to the procedure of Björkman (Björkman, 1956). The yields of ILL and MWL were 5.8 ± 0.3% and 4.4 ± 0.4%, respectively.
2.2. Structural and chemical analysis 2.2.1. Elemental analysis and functional groups Elemental analysis was performed to determine the carbon, hydrogen, oxygen, and nitrogen content in lignin samples using US/CHNS-932 (LECO Corp., USA). The essential functional groups attached to the lignin, methoxyl and phenolic hydroxyl moieties, were determined by GC and aminolysis methods, respectively (Baker, 1996; Mansson, 1983).
Fig. 1. Flow chart of the lignin extraction processes.
2.2.2. Thermogravimetric analysis (TGA) Thermal decomposition characteristics of MWL and ILL were determined with a Q-5000 IR instrument (TA Instruments, USA). TG analysis was performed at a constant heating rate of 10 °C/ min up to 800 °C. An inert atmosphere was maintained with a 25 ml/min N2 flow.
1-ethyl-3-methylimidazolium acetate ([Emim][CH3COO]), which is known to selectively extract lignin from wood and to be one of the best solvents for lignocellulosic materials among the ILs, was employed as the ionic liquid (Mäki-Arvela et al., 2010; Sun et al., 2009). In addition, [Emim][CH3COO] is one of the most promising candidates for industrial applications due to its low viscosity, low melting point, non-toxicity, and biodegradability (Mäki-Arvela et al., 2010; Sun et al., 2011). For comparative analysis, milled wood lignin (MWL) was used as a representative native lignin, extracted from the same wood species, and served as a control. The chemical properties of ILL were determined by several analyses including elemental analysis, functional group analysis, gel permeation chromatography (GPC), and derivatization followed by reductive cleavage (DFRC). Furthermore, thermo-decomposition behaviors as well as structural features were characterized by thermogravimetric analysis (TGA) and spectroscopic analyses including FT-IR, 1H- and 13C-NMR. On the basis of these results, the chemical characteristics of ILL and MWL were extensively compared.
2.2.3. Gel permeation chromatography (GPC) The lignin was acetylated with acetic anhydride/pyridine (1:1 v/ v) at 70 °C for 6 h, and the derivatized lignin was dissolved in tetrahydrofuran (THF) and analyzed by a GPC max instrument (ViscotekRImax, Viscotek, UK) equipped with a PLgel 5 lm MIXED-C column (300 7.5 mm, VARIAN, Inc.) and PLgel 5 lm guard column (50 7.5 mm, VARIAN, Inc.) using UV–Vis detection (VE3210, Viscotek). To determine the molecular weight of the effluent, polystyrenes with a mass range between 580 Da and 3250,000 Da were used to create a calibration curve.
2.2.4. Derivatization followed by reductive cleavage (DFRC method) To determine the frequency of arylglycerol-b-aryl ether linkages (b-O-4), ILL and MWL were subjected to the DFRC method (Lu and Ralph, 1997). Essential DFRC products, acetylated coniferyl and sinapyl alcohol, were quantitatively determined by GC analysis (Agilent 7890A) on an instrument equipped with a capillary column DB-5 (30 m 0.32 mm 0.25 lm, Agilent) using the following temperature program: injection and detector temperatures were set to 220 °C and 300 °C, respectively, and the oven temperature was increased at a rate of 5 °C/min from 100 °C to 280 °C,
2. Methods 2.1. Preparation of ionic liquid lignin Air-dried poplar (P. albaglandulosa) wood xylem was ground to a fine powder (less than 0.64 mm particle size). 1-Ethyl-3-methylimidazolium acetate ([Emim][CH3COO]), produced by BASF (P90%) and acetone (P99.5%) were purchased from
Table 1 Elemental composition, functional groups, and calculated C9 formula of ionic liquid lignin (ILL) and milled wood lignin (MWL).
a
Lignin
Yields (%)a
Poplar ILL Poplar MWL
5.8 ± 0.3 4.4 ± 0.4
Elemental composition (%)
Functional groups (%)
C
H
N
O
OMe
Phe-OH
60.1 58.6
5.9 5.8
1.2 0.1
32.9 35.5
15.5 ± 1.5 14.4 ± 0.1
6.7 ± 0.2 6.3 ± 0.2
Based on oven dried weight of poplar wood.
C9 formula
Mw/C9 (g/mol)
C900H790O232N15(OH)79(OCH3)100 C900H812O272(OH)76(OCH3)94
199.7 201.9
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Fig. 2. Proposed scheme for lignin extraction with ionic liquids from native lignin: (A) Structure of native lignin macromolecular in poplar wood; (B) Ionic liquid (1-ethyl-3methylimidazolium acetate); (C) Ionic liquid lignin model.
Table 2 Average molecular weight and polydispersity index (Mw/Mn) of ionic liquid lignin (ILL) and milled wood lignin (MWL). Lignin
Mw (Daltons)
Mn (Daltons)
Mw/Mn
Poplar ILL Poplar MWL
6347 10,002
3908 4060
1.62 2.46
Table 3 Quantitative analysis of DFRC products of ionic liquid lignin (ILL) and milled wood lignin (MWL). Lignins
Poplar ILL Poplar MWL
Fig. 3. Thermogravimetric and differential thermal analyses of ionic liquid lignin (ILL) and milled wood lignin (MWL).
DFRC (lmol/g sample)a G– CH@CHCH2OAc (Acetylated CA)
S– CH@CHCH2OAc (Acetylated SA)
Total
S/Gb
564.6 ± 4.13 560.6 ± 18.64
699.0 ± 46.89 745.7 ± 22.23
1263.7 ± 42.76 1306.3 ± 40.87
0.8 ± 0.06 0.7 ± 0.01
a
Data are means of triplicate analyses ± S.D. Average molar ratios of sinapyl alcohol/coniferyl alcohol formed by the cleavage of b-O-4 linkages. b
and then the final temperature was maintained for 8 min. The split ratio was 20:1. 2.2.5. Spectroscopic analysis (FT-IR, 1H- and 13C-NMR) FT-IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, USA) operating in the wavelength range of 4000–650 cm 1 with a resolution of 8 cm 1. Each sample was coated by potassium bromide (KBr) and formed into a pellet for analysis. 13C-NMR and 1H-NMR spectra were obtained with a Bruker AVANCE 600 spectrometer (Bruker, Germany) operating at 60 °C from 20,000 (13C-NMR) and 128 (1H-NMR) scans, respectively, with 150 mg of lignin sample dissolved in DMSO-d6. 3. Results and discussion 3.1. Elemental analysis and functional groups in ionic liquid lignins (ILL) Recently pretreatment of woody biomass with ILs attracts worldwide attention in the field of cellulose based bioethanol production due to its feature of environmental friendliness. However,
in spite of lots of intensive researches, the optimal condition for the most efficient pretreatment of woody biomass, using ionic liquids, are not yet established. In general, higher temperature and longer time help to dissolve woody biomass into ionic liquid, which resulted in higher pretreatment efficiency (Kilpelainen et al., 2007; Fort et al., 2006). Sun et al. (2009) found that higher temperature and longer time demand higher energy consumption as well as has risk of more degradation of cellulose, so IL pretreatment under the temperature of 110 °C and a time of 16 h were chosen as a compromise, between enhanced dissolution and higher energy demand. Table 1 shows the elemental composition, functional groups and C9 formula of the two lignin samples, extracted with ionic liquid ([Emim][CH3COO]) by Sun’s methodology (Sun et al., 2009) and purified by the Björkman procedure (Björkman, 1956), respectively. The elemental carbon content of ILL (60.1%) was determined to be slightly higher than that of MWL (58.6%), whereas the oxygen content of ILL (32.9%) was lower than that of MWL (35.5%). The
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Fig. 4. Spectroscopic analyses of ionic liquid lignin (ILL) and milled wood lignin (MWL). (A) FT-IR spectra, (B)
relatively high nitrogen content (1.2%) of ILL was particularly notable. The small amount of nitrogen in MWL was derived from protein bound to the lignin (Pan and Sano, 2000). Since the ionic liquid ([Emim][CH3COO]) utilized in this study has two moles of nitrogen per IL molecule, most of the nitrogen in the ILL probably came from the solvent.
13
C-NMR spectra, (C) 1H-NMR spectra.
The structure of the ionic liquid ([Emim][CH3COO]) contains an imidazolium-based salt and two alkyl groups. As shown in Fig. 2, due to the cationic nitrogen in the imidazolium, the ionic liquid may be physically or chemically associated with lignin at the electron rich oxygen, such as b-O-4 and a-O-4, and b–b linkages. It could be supposed that partial fragmentation of lignin could
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occur during this extraction process. Comparing the OMe contents of the two lignin samples, the OMe content (15.5%) of ILL was ca. 7% higher than that of MWL (14.4%). For the phenolic OH content, ILL had a 6% higher phenolic hydroxyl content than that of MWL, which indicates that ILL consists of more free-phenolic moieties that are probably released by partial fragmentation of lignin during the ionic liquid extraction. The fictive summative formulae (C900 formulas) of both lignins were also calculated based on fundamental analyses (elemental composition and determination of OMe and phenolic OH) (Table 1). 3.2. Thermal behavior of ionic liquid lignin (ILL) Thermogravimetric and differential thermal analysis (TGA and DTG) of ILL and MWL are described in Fig. 3. As seen in the DTG curves, the maximum decomposition rate (VM) of ILL was 0.25%/ °C while that of MWL was 0.3%/°C, and the temperature (TM) corresponding to VM shifted to a lower temperature region (308.2 °C) compared with the MWL TM (381.9 °C). Furthermore, the active pyrolysis region for ILL was from 100 °C to 308.2 °C, whereas for MWL, it was from 100 °C to 381.9 °C. The active pyrolysis region was defined as the thermal degradation region from about 100 °C (dehydration step) to TM as determined from VM in the DTG curve (Hodgson et al., 2011). These results suggest that MWL was more thermally stable than ILL, as evident from the higher TM for MWL. According to Sun et al. (2000), the thermal stability of lignins increased with incrementally higher Mw. With a greater degree of branching and condensation of the lignin macromolecule, the thermal energy needed for bond cleavage increased. Moreover, Sun et al. (2000) also found that TM corresponding to VM shifted to a higher region with increasing Mw. 3.3. Characteristics of the ionic liquid lignin (ILL) molecular structure 3.3.1. The average molecular weights (Mw and Mn) and polydispersity index of ILL The average molecular weights (Mw and Mn) and polydispersity index of both lignin samples were determined by GPC analysis, and the results are given in Table 2. The Mw of ILL was determined to be 6347 Da, which was only 2/3 of the Mw of MWL (10,002 Da). Together with the high content of phenolic OH, this lower Mw value for ILL is good evidence for lignin fragmentation by the ionic liquid. In contrast, the Mn of both ILL and MWL had similar values of about 4000 Da. The polydispersity index (Mw/Mn) of ILL and MWL was calculated to be 2.46 and 1.62, respectively. This result indicates that compared to MWL, ILL was formed with a relatively uniform lignin fragment size and a lower number of condensed linkages (Tan et al., 2009). 3.3.2. The frequency of arylglycerol-b-aryl ether linkage (b-O-4) in ILL In general, the arylglycerol-b-aryl ether linkage (b-O-4) is considered to be a typical linkage type in lignin macromolecules. The frequency of the b-O-4 linkage accounts for 40–65% of the various coupling patterns and greatly affects the chemical and physical properties of the lignin (Kishimoto et al., 2006; Nimz, 1974). Since derivatization followed by reductive cleavage (DFRC) analysis selectively cleaves the b-O-4 linkage and releases acetylated C6C3 type monomers [acetylated coniferyl (G type) and sinapyl alcohol (S type)] as DFRC products, the frequency of the b-O-4 linkage in the lignin can be indirectly evaluated by quantification of the DFRC products. Table 3 lists the quantitative amounts of DFRC products from ILL and MWL. Considering the total DFRC products, ILL gave rise to ca. 1264 lmol/g of total DFRC products, which was slightly lower than that of MWL (1306 lmol/g), and the S/G ratio of ILL (1.24) was slightly lower than that of MWL (1.34), indicating
that the G-type lignin was more easily extracted than the S-type lignin during treatment of the biomass with ionic liquid. 3.3.3. Spectroscopic analyses (FT-IR, 1H- and 13C-NMR analysis) For the in depth elucidation of structural features of lignin molecules, ILL and MWL were also subjected to various spectroscopic analyses (FT-IR, 1H- and 13C-NMR analysis), and representative spectra are shown in Fig. 4. As shown in Fig. 4 (Top), the FT-IR spectra for both lignins appear to be almost identical, and typical lignin peaks, such as O–H stretching (3412–3460 cm 1), C–H stretching in methyl/methylene groups (3000–2842 cm 1), aromatic skeletal vibrations (1505–1515 cm 1), and aromatic C–H in plane deformation (1200–1100 cm 1) were visible in both lignin spectra. For the 13 C-NMR spectra (Fig. 4; Bottom), most of the lignin signals, such as the aromatic carbons at 100 ppm–155 ppm, aliphatic side chain carbons at 86–60 ppm, and methoxyl carbon at 55 ppm appeared equally in both the ILL and MWL spectra. However, the signal at 62–63 ppm was only observed in MWL. This signal is assigned to the Cc with Ca@O originated from side chain oxidation via homolytic cleavage during extensive ball milling process (El Hage et al., 2009). Furthermore, a few differences are seen only in the 1H-NMR spectra. Unlike the MWL spectrum, two signals at 2.0 ppm and 3.2 ppm were not visible in the ILL spectrum. The signal at 2.0 ppm originated from aliphatic acetates (acetyl groups in xylan). In particular, the signal at 3.2 ppm is normally assigned to the Hb in b–b structures. As mentioned above for Fig. 2, reduction of the signal intensity at 3.2 ppm is also good evidence for cleavage of b–b structures during lignin extraction with ionic liquid (Lundquist, 1979). 4. Conclusions This study compares structural features of two different lignins; one is a ionic liquid lignin (ILL) purified with 1-ethyl-3-methylimidazolium acetate ([Emim][CH3COO]) from poplar wood and the other is the corresponding milled wood lignin (MWL). According to diverse spectroscopic analyses structure of ILL seems to be quite similar to that of MWL. However, GPC data revealed that average molecular weight of ILL is only 2/3-fold of the MWL. Furthermore, the lower S/G ratio of ILL (1.24) than that of MWL (1.34) indicates G-type lignin was more extractable than S-type lignin during the treatment of biomass with ionic liquid. Acknowledgements This research was supported by a grant from the Korea Research Foundation funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea (KRF-2008-331-F00027). References Baker, S., 1996. Rapid methoxyl analysis of lignins using gas chromatography. Holzforschung 50, 573–574. Björkman, A., 1956. Studies on finely divided wood. Part I. Extraction of lignin with neutral solvents. Svensk Papperst. 59, 477–485. Earle, M., Seddon, K., 2000. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 72, 1391–1398. El Hage, R., Brosse, N., Chrusciel, L., Sanchez, C., Sannigrahi, P., Ragauskas, A., 2009. Characterization of milled wood lignin and ethanol organosolv lignin from miscanthus. Polym. Degrad. Stab. 94, 1632–1638. Fengel, D., Wegener, G., 1984. Wood – Chemistry, Ultrastructure, Reactions. De Gruyter, Berlin. Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P., Moyna, G., Rogers, R.D., 2006. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methyl imidazolium chloride. Green Chem. 9, 63– 69. Fu, D., Mazza, G., Tamaki, Y., 2010. Lignin extraction from straw by ionic liquids and enzymatic hydrolysis of the cellulosic residues. J. Agric. Food. Chem. 58, 2915– 2922.
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