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Cellulose extraction from wood chip in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl)
Bioresource Technology 102 (2011) 7959–7965
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Cellulose extraction from wood chip in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) Xuejing Wang a,b,1, Huiquan Li a,⇑, Yan Cao a,1, Qing Tang a a b
Key Laboratory of Green Process Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 16 March 2011 Received in revised form 19 May 2011 Accepted 22 May 2011 Available online 30 May 2011 Keywords: Ionic liquid Dissolution Extraction Wood chip Cellulose
a b s t r a c t In the present study, 1-allyl-3-methylimidazolium chloride (AmimCl), an ionic liquid (IL), was used to extract cellulose from pine, poplar, Chinese parasol, and catalpa wood chips. Results show that pine is the most suitable wood species for cellulose extraction with ILs. Its cellulose extraction rate can reach as high as 62% under optimized conditions and its cellulose content is as high as 85% when DMSO/water is used as the precipitant. The dissolution process can be clearly observed by hot stage optical microscopy, and the reaction time can be significantly reduced by microwave irradiation. 13C CP/MAS NMR, FTIR, XRD, and SEM were used to analyze the cellulose-rich extracts of pine. Results show that IL dissolves pine wood by destroying inter and intramolecular hydrogen bonds between lignocelluloses. The major component of pine extract is cellulose with a homogeneous and dense structure. After extraction, AmimCl can be easily recycled and reused. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fossil-based feedstock is a widely used energy source worldwide. However, the depletion of it makes the determinations of non-fossil energy sources are of utmost importance in the 21st century. Compared with energy production from fossil fuels, energy production from biomass is a new and important alternative energy feedstock that has distinct advantages, such as it is available in large reserves, renewable and there is no CO2 emission. Potential biomass sources include crops (corn, sugarcane), agricultural wastes, forest products, grasses, and algae (Lucas et al., 2010). Wood is the most abundant lignocellulosic resource on Earth. The main component of wood is a macromolecular substance with 4050% cellulose, 1525% hemicellulose, and 1530% lignin (Voitl et al., 2010). Among these, cellulose, being the most abundant renewable biopolymer in the world, has attracted much attention due to its fascinating structure and promising properties (Klemm et al., 2005). Cellulose is traditionally used as a raw material for the production of paper, paperboard, fiberboard, and other similar products. Its excellent characteristics, which include hydrophilicity, chirality, biodegradability, capacity for broad chemical modification, and ability to form semicrystalline fiber morphologies, have drawn considerably increased interest and encouraged ⇑ Corresponding author. Tel.: +86 010 82544825, fax: +86 010 62621355. 1
E-mail address: [email protected] (H. Li). These authors equally contributed to this work.
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.05.064
worldwide interdisciplinary research on cellulose and its products over the past few decades (Heinze and Liebert, 2001; Zhu et al., 2006). Wood remains the most important raw material source of cellulose. However, the structure of wood is highly complex because its lignin, which is a three-dimensional polymer network, binds with its carbohydrates (hemicellulose and cellulose) to form a tight and compact structure (Adler, 1977). The hardness of wood biomass is particularly challenging because it is impossible to dissolve wood in conventional solvents in its native state. Traditionally, cellulose is extracted from wood through the Kraft pulping process, which involves the semi-chemical degradation of the lignin/hemicellulose matrix by treatment with solutions of sodium hydroxide and sodium sulfide at high temperatures and pressures. The use of these toxic chemicals and the intensive processing conditions involved in the pulping procedure pose environmental hazards in the air and water (Istek and Gonteki, 2009; Stirling et al., 2010; Yang et al., 2007). Therefore, efficient utilization of this renewable resource with no hazards emission is necessary. Ionic liquids (ILs) are molten salts with glass transition or melting temperatures below 100 °C. In contrast to traditional volatile organic compounds, ILs have many attractive properties, including good chemical and thermal stability, non-flammability, immeasurable low vapor pressure, and recyclability (Rogers and Seddon, 2003). Recent research studies focused on the application of ILs in wood processing (Fort et al., 2007; Lucas et al., 2010; Yuan et al., 2010), resulting in the identification of several ILs that may be capable of dissolving wood. Myllymaki and Akasela (2005) first
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Nomenclature Rd Rr Re Mo Mr
the the the the the
dissolution rate regeneration rate cellulose extraction rate mass of the original wood added mass of the residue
invented a method to dissolve wood in 1-butyl-3-methylimidazolium chloride (BmimCl) and separate cellulose from the solution, but they did not describe in detail how the cellulosic fiber was recovered. Kilpelainen et al. (2007) demonstrated that BmimCl can fully dissolve ball-milled wood dust, sawdust, and thermo mechanical pulp (TMP) under gentle conditions. However, the preparation of sawdust, TMP, or ball-milled wood dust consumed considerable amounts of energy. Rogers’ group conducted a study on the dissolution of wood chips in ILs and discovered that wood chips can be partially dissolved in BmimCl (Fort et al., 2007) and nearly fully dissolved in 1-ethyl-3-methylimidazolium acetate (EmimAc) (Sun et al., 2009). Furthermore, they successfully recovered cellulosic fiber from the IL solution by the addition of a variety of precipitating solvents. Zavrel et al. (2009) showed that, compared with other imidazolium chloride RTILs, such as BmimCl, 1allyl-3-methylimidazolium chloride (AmimCl) is the most effective IL for dissolving wood chips using high-throughput systems. As well, the synthesis of AmimCl was more efficient. The conversion rate of the imidazolium reached up to 80% after a reaction time of 3 h, and was almost 100% after 6 h, because of the high reactivity of the allyl chloride (Zhang et al., 2005). AmimCl also showed a lower melting point and a considerably lower viscosity than BmimCl. Recently, ILs have been found to present excellent microwave irradiation (MI) conductivity as they consist only of anions and cations. Thus, MI has been used to accelerate organic reactions that use ILs as solvents, thereby reducing reaction times from hours to minutes (Myllymaeki et al., 2005; Satge et al., 2002; Torgovnikov and Vinden, 2010), at the same time decreasing the serious degradation of wood bring by the conventional oil bath heating under long reaction times. In the present paper, the dissolution and regeneration of wood chips in AmimCl, and the effects of wood species, dissolution time, temperature, microwave assistance, initial wood concentration, and types of precipitant on the efficiency of cellulose extraction are discussed. The recycle and reuse of AmimCl is also determined to ensure the cost efficiency of the use of ILs to recover cellulose. The research shows that the use of ILs for extraction of cellulose from wood avoids the use of toxic and hazardous chemicals, and can be carried out under mild conditions.
Me Wo-cel We-cel
the mass of the extract the cellulose content in the original wood the cellulose content in the extract
received. AmimCl was synthesized as described in the previous work (Zhang et al., 2005). 2.2. Wood dissolution Dimethyl sulfoxide (DMSO) (16 wt%) was added to AmimCl to form a clear solution with stirring. In a typical dissolution, approximately 10.0 g of dried wood powder was dispersed into 190.0 g of AmimCl/DMSO in a three-necked flask. The mixture was mechanically stirred at the desired temperature for a desired length of time in an oil bath (300 rpm) or with microwave heating (Sineo MAS-I, China, 500 W output, 400 rpm) in the open atmosphere. A dark, amber-colored, viscous wood suspension with about 5 wt% wood concentration was obtained. The partial dissolution of wood powder in AmimCl/DMSO was observed using a polarizing microscope with a digital camera (Olympus BX51, Japan) and a hot plate (Linkam THMS600, UK). 2.3. Cellulose extraction and residue separation After dissolution, the suspension was filtered through a stainless steel filter to remove the undissolved residue. Water was added to the resulting clear liquors, and a cellulose-rich extract was reconstituted from the liquor. This extract and the undissolved residue were respectively washed with water/DMSO and water until the IL was completely removed, and then dried overnight in an oven at 100 °C prior to use. The percentage of wood dissolved and regenerated, as well as the cellulose extraction rate in the material, was calculated according to Eqs. (1)–(3):
Rd ð%Þ ¼
Mo Mr 100% Mo
ð1Þ
Rr ð%Þ ¼
Me 100% Mo
ð2Þ
Re ð%Þ ¼ Rr
W e-cel 100% W o-cel
ð3Þ
2.4. Recycling of AmimCl 2. Methods 2.1. Materials and reagents Pine, poplar, Chinese parasol, and catalpa wood samples were obtained from Shandong Province, China. These were first dried in sunlight, and then cut into small pieces. Wood chips were ground into powder using a muller, and then pass through a 40– 60 mesh sieve (0.45–0.65 mm). The wood powders were dried overnight in an oven at 105 °C before using. Microcrystalline cellulose (MCC) with a degree of polymerization (DP) of 200–340 was purchased from Acros Organics, USA. All other chemical reagents were purchased from commercial sources in China and used as
After wood dissolution and cellulose extraction, recovery of the IL was accomplished by evaporating water from the precipitation liquid. The purity of the IL was determined by 1H NMR spectroscopy. The recycled IL was used to extract cellulose from the wood chip as described in Sections 2.2 and 2.3. 2.5. Characterization 2.5.1. FTIR The samples were ground into powder and vacuum dried for 24 h. The IR spectra of the samples were recorded using a Fourier Transform IR spectrometer (PE-2000, USA). Test specimens were prepared using the KBr-disk method.
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2.5.2. WAXD The cellulose-rich extracts, as well as reference samples, were vacuum dried for 24 h, and then ground into powder before their X-ray diffraction (XRD) patterns were determined with Cu Ka radiation (k = 1.5406) at 40 kV and 200 mA. Patterns were recorded in the range of 2h = 590° using an X’pert PRO X-ray diffractometer (PANalytical B.V., The Netherlands). 2.5.3. FE-SEM SEM images were obtained with a JSM6700F field emission scanning electron microscope (FE-SEM) operated at 5 kV accelerating voltage. The extract film was frozen in liquid nitrogen, immediately snapped, and then dried under vacuum. The free and fracture surfaces of the film were sputtered with gold, and then observed and photographed. 2.5.4. NMR Solid-state cross-polarization/magic angle spinning (CP/MAS) 13 C NMR spectra were obtained from the samples at 100 MHz using a Bruker AVANCE-III 400 NMR spectrometer with a 4 mm MAS probe at a 13C frequency of 100.37 MHz. The structures of the fresh and recycled AmimCl were determined by 1H NMR spectroscopy (Bruker ADVANCE 400) at room temperature in DMSO-d6 with a 5 mm BBO probe at a 13C frequency of 100.61 MHz. 3. Results and discussion 3.1. Dissolution of wood in AmimCl The dissolution of pine, poplar, Chinese parasol, and catalpa wood powders in AmimCl was studied. In each sample, a 5 wt% wood chip/IL suspension was prepared by the addition of 16 wt% DMSO to reduce the viscosity of the mixture and facilitate filtration. The addition of DMSO had no obvious effect on the solubility of the wood chips in AmimCl. The wood chips swelled shortly after they were added to the AmimCl/DMSO solvent. After heating for several hours at 100 °C under vigorous mechanical stirring, the colors of the solutions became dark and their viscosities distinctively increased, indicating that the dissolution of wood occurred. A clear, dark brown solution was obtained after filtration, and the brown gel was reconstituted by the addition of water; after washing several times with water/DMSO, the gel became white, indicating that its main components were carbohydrates. A hot stage polarizing microscope was used to observe the dissolution process of wood chips in AmimCl. Result shows that the wood samples were in their original states and fibrous structures
25
15 10 5
3
6
3.2.1. Effect of wood species Different types of wood result in various dissolution profiles in IL. In this work, pine was found to be more suitable for dissolution and regeneration in IL than the three other types of wood. Fig. 1(A) and (B) show that the respective dissolution and regeneration rates of pine can reach as high as 67% and 56%; these only reach 30% and 10%, respectively, for the other types of wood. Interestingly, the cellulose content in regenerated pine was nearly 20% higher than that in the regenerated poplar, Chinese parasol and catalpa chips (Table 1). This may be because pine is the softest wood among the four wood species. Softwood species are easier to be dissolved in ILs than hardwood because the former has lower densities and hardness compared with the latter (Kilpelainen et al., 2007).
B
Pine Polar Chinese parasol Catalpa
20
0
3.2. Evaluation of process variables
Dissolution/Extraction rate (wt%)
Dissolution/Extration rate (wt%)
A 30
can be observed below 50 °C. Swelling occurred at 70 °C, and dissolution can be observed at 100 °C. The dissolution rate increased at 120 °C; at this temperature, most of the fibrous material disappeared after only 10 min and the visual field became dark. After 30 min at 120 °C, no further changes were observed; a fully black field was not obtained even when the reaction time was increased to 4 h. In general, the experiments consistently indicate that AmimCl can only partially dissolve wood chips. ILs are capable of dissolving complex macromolecules and polymeric materials with high efficiency by breaking the extensive hydrogen bonding network of polysaccharides and promoting their dissolution (Swatloski et al., 2002). This mechanism has been observed to proceed as follows: The ion pairs in AmimCl dissociate into individual Cl and Amim+ ions. Free Cl ions associate with cellulose hydroxyl protons, and free Amim+ cations associate with cellulose hydroxyl oxygen groups. These disrupt hydrogen bonding in cellulose and cause its dissolution (Cao et al., 2009). It is presumed that a similar mechanism is followed in the case of the dissolution of wood in AmimCl. Among the three macromolecular components of wood, cellulose is recognized as the basic structural element of the cell wall. Lignin and hemicellulose are distributed through the cell wall in a random manner (Myllymaeki et al., 2005). Lignin, which is an intercellular substance that acts as a cementing agent bonded to hemicellulose, is the main obstacle that limits the dissolution of wood. Anions of AmimCl have the ability to break the lignocellulosic complex, especially the amorphous network of cross-linked phenylpropanoid units of lignin, and the inter and intramolecular hydrogen bonding interactions in the wood matrix. This leads to a decrease in cellulose crystallinity and dissolution of the wood.
Time (h)
15
24
70
Pine Poplar Chinese parasol Catalpa
60 50 40 30 20 10 0
90
100
110
Temperatureo(C)
120
Fig. 1. Dissolution and regeneration rates of different wood samples in AmimCl/DMSO as a function of time (A) and temperature (B) under oil bath heating, 5 wt% wood concentration.
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Table 1 Cellulose contents in the wood extracts of different wood samples as a function of time under oil bath heating at 100 °C, 5 wt% wood concentration. Dissolution time (h)
3 6 15 24
Cellulose content in wood extracts (%) Pine
Poplar
Chinese parasol
Catalpa
70 76 78 71
49 48 49 47
40 52 54 49
25 49 50 56
3.2.2. Effect of time The kinetics of the wood chips dissolved in AmimCl was studied as a function of dissolution time. Fig. 1(A) shows that the dissolution and regeneration rates increased with increasing reaction time for all wood samples except catalpa, which had nearly constant rates of about 20% and 6% respectively, because it was the only hardwood among the four types of wood investigated. Rate increments for pine were the most obvious. For example, from 6 to 24 h, the dissolution and regeneration rates for pine increased from 19% to 26% and 9% to 19%, respectively. When the dissolution time was extended to 43 h, the dissolution and regeneration rates can reach up to 40% and 25%, respectively (data not shown). Compared with the value obtained at 24 h for pine in Fig. 1(A), the dissolution rate increased by 14%, whereas the regeneration rate only increased by 6%, indicating that the polymer degradation occurs as Fort reported (Fort et al., 2007). The full dissolution of wood powder in ILs cannot be observed with increasing time because AmimCl cannot effectively solvate the aromatic character of lignin. The cellulose content changed minimally when the treatment time was increased, as shown in Table 1. However, since the regeneration rate increased, the following conclusions can be drawn: First, the cellulose extraction rate also increased with time. Second, although the cellulose content may be as high as 78% for pine, the highest extraction rate for its cellulose was only 27.5% (based on the average cellulose content of original pine, which is 49%) under the temperature of 100 °C, because of the low regeneration rate. The extraction rates for the other three wood powders were even lower.
3.2.3. Effect of temperature It is determined that the dissolution of wood in ILs is dependent on the applied temperature. Fig. 1(B) shows that the dissolution and regeneration rates increase when the temperature increases from 90 to 120 °C. The results of this analysis are similar with those in the time dependent curve in that, although the rates increased with temperature, the increments for poplar, Chinese parasol and catalpa were less obvious than those of pine. In fact, no regeneration product was obtained at 120 °C for catalpa, indicating that it has been degraded. The pine sample showed sharp increases in rates at 110 °C; even sharper rates were observed at 120 °C. Its dissolution and regeneration rates did not show any change when the temperature was increased from 90 to 100 °C; these remained stable at 18% and 9%, respectively. The dissolution and regeneration rates quickly increased to 38% and 21%, respectively, however, after the temperature was increased to 110 °C. These findings are consistent with those in Brandt et al.’s (2010) study, which concluded that swelling and dissolution in ILs is temperature-dependent, and that better dissolution and regeneration rates are obtained at temperatures beyond 100 °C. Further increases in temperature to 120 °C, may yield dissolution and regeneration rates as high as 67% and 54%, respectively. However, the cellulose content in the regenerated pine wood was only 66% at 120 °C, indicating the occurrence of degradation. When the temperature was further in-
creased to 140 °C, the wood powder fully dissolved in the IL and formed a dark black solution, but its regeneration rate was lower than 18%; this indicated that most of the dissolved pine degraded, similar to catalpa at 120 °C. 3.2.4. Effect of microwave irradiation MI was employed to dissolve the wood and extract cellulose. The use of MI in biomass chemistry has quickly developed over the years, and compared with conventional oil bath heating, it shows many advantages, such as non-contact heating, better control over the heating process, rapid heating, high rate of reactions and high product yields. ILs can efficiently absorb microwave radiation because they are only composed of anions and cations; thus MI may suitably be applied to IL solutions to dissolve wood (Satge et al., 2002; Torgovnikov and Vinden, 2010). In the experiments, it is also discovered that wood dissolution in IL can be efficiently carried out using MI. Table 2 shows that, while the same levels of dissolution and regeneration are achieved by both processes, the reaction time decreases significantly under microwave heating compared with oil bath heating. For example, the dissolution and regeneration rates can reach 26% and 16% at 100 °C after 15 h of oil bath heating. In contrast, only 2 h of microwave heating was necessary to achieve similar levels without decreasing the cellulose content of the products. Such quick and effective dissolution is due to the increment in collision frequency between the anions and cations of IL and the wood macromolecules brought by MI. Thus, breakages in the amorphous network of cross-linked phenylpropanoid units of lignin and the inter and intramolecular hydrogen bonding interactions in the wood matrix by Cl anions are intensified and more rapid dissolution occurs. Therefore, microwave heating is an efficient heating method for processing wood in ILs. 3.2.5. Effect of initial wood concentration The effect of initial wood concentration was investigated in the present study as shown in Table 3. Because the viscosity of the IL and the molecular weight of the three macromolecular components in wood are relatively high, these may result in filtration and separation difficulties, particularly at higher wood concentrations (e.g., 8%). Low concentrations favor the dispersion of molecules in solution and lead to high dissolution and regeneration rates. For example, increasing the wood solution from 1 wt% to 5 wt%, the dissolution rate decreased from 35% to 26%. Depending on the initial wood concentration, the regeneration rates also show the same trend. 3.2.6. Effect of precipitant In general, regenerated wood can be obtained by adding water to the filtrate. In Fort et al.’s (2007) study, the reconstitution of cellulosic materials was achieved through the use of a 1:1 acetone– water solution as the solvent. Off-white powdery flocs formed
Table 2 Dissolution and regeneration rates of pine in AmimCl/DMSO as a function of time under MI and oil bath heating at 100 °C, 5 wt% wood concentration. Heating style
Dissolution time (h)
Dissolution rate (%)
Regeneration rate (%)
Oil bath
3 6 15 24
16 19 26 26
6 9 16 19
MI
0.5 1 2 4
13 19 25 22
6 10 17 18
X. Wang et al. / Bioresource Technology 102 (2011) 7959–7965 Table 3 Dissolution and regeneration rates for pine in AmimCl/ DMSO as a function of concentration of wood under oil bath heating for 15 h at 100 °C. Concentration of wood in IL (%)
Dissolution rate (%)
Regeneration rate (%)
1 2 3 4 5
35 34 32 27 26
25 20 19 17 16
shortly after the coagulant was mixed with the wood liquors, and the regenerated wood was nearly free of lignin, which is soluble in acetone, as verified by 13C NMR and FTIR spectra. The findings in the experiment were significantly different because acetone cannot dissolve the lignin extracted from pine, and, therefore, cannot contribute to cellulose extraction. Thus, DMSO was used in place of acetone. Table 4 shows that the cellulose content of the extract precipitated with acetone/water was 76% (Entry 2), the value of which is nearly identical to that of the extract precipitated with water (Entry 1). Cellulose contents of the extract precipitated with DMSO/water can reach as high as 85% (Entry 3), which is 9% higher than those of the extracts precipitated with water and acetone/ water. It is believed that differences in the dissolution profile of lignin may be attributed to differences in the sources of pine and structures of the IL selected. The cellulose content of the extracts obtained by the IL method is quite close to that obtained by Kraft pulping (90%). The difference between the two cellulose extracting methods is that the cellulose extraction rate is lower for the IL method than for Kraft pulping because wood has dissolution limits in ILs. However, in light of the serious environmental pollution that Kraft pulping causes (Istek and Gonteki, 2009; Stirling et al., 2010; Yang et al., 2007), the IL method appears to be a more feasible approach for cellulose extraction. Thus, determination of a solution that can increase the dissolution rate of wood in ILs, such as enhancement of the reaction temperature or time as discussed above or the design of a new structure of ILs, among others, is necessary. 3.2.7. Mass balance of the cellulose from original wood As discussed above, 2 h reaction time and 3 wt% wood concentration at 110 °C under MI were chosen as optimal dissolving conditions because of the higher dissolution rate and lower degradation observed for pine wood under these conditions. In general, the results obtained under these conditions adequately match expected values. 10.0 g pine is processed by 323 g of AmimCl/DMSO. After filtration, 3.6 g of regenerated cellulose-rich material and 5.0 g of residue were obtained. There is also some loss, accounting for about 14% of the original wood, during this process. This loss has two components: loss from water-soluble low molecular matter that results from the little degradation of wood and DMSO-soluble lignin from filtration, and loss that occurs in the process of washing and filtration steps; the latter is considered the major source of mass loss. The cellulose content of the
Table 4 Cellulose content of the pine extracts obtained under different precipitants from MI heating for 2 h at 100 °C, 5wt% wood concentration. Entry
Precipitants
Cellulose (wt%)
1 2 3
Water Acetone–water DMSO–water
76 76 85
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regenerated cellulose-rich material is about 85%, whereas the unextracted cellulose that remains in the residue is believed to be about 25% (based on the residue mass). Considering the 49% cellulose content of the original pine wood, the extraction rate can reach 62%, indicating that the IL process is an effective method for extracting cellulose from wood. 3.3. Analysis of the regenerated cellulose-rich extract and undissolved residues 3.3.1. NMR spectroscopy Solutions containing five parts of pine wood and ninety five parts of AmimCl/DMSO were cooked at 100 °C for 2 h by MI. After filtration, the cellulose-rich extract was precipitated with water/ DMSO; undissolved residues were repeatedly washed with water and dried overnight in a vacuum oven at 100 °C before they were subjected to 13C CP/MAS NMR analysis. Original pine and lignin obtained from pine using traditional acid–base technologies and analytical microcrystalline cellulose (MCC) were selected for comparison (see Supplementary Data, Fig. S1). Result shows the lignin obtained from pine has characteristic peaks at 148, 133, 122, 113, and 56 ppm, while MCC shows characteristic peaks at 105 (C1), 90 (C4, crystalline), 85 (C4, amorphous), 78 (C2), 74 and 75 (C3, C5), 66 (C6, crystalline), and 60 ppm (C6, amorphous). The characteristic peaks of hemicellulose, whose structure is quite similar to and overlaps with cellulose, are 76 (C2), 75 (C3), 72 (C4), and 63 ppm (C5) (Boonstra et al., 1996; Gilardi et al., 1995; Jebrane and Sebe, 2008). The spectrum of original pine contains all the characteristic peaks of lignin and cellulose mentioned above. After processing by the IL, however, cellulose crystalline peaks for C4 at 89 ppm and C6 at 65 ppm disappeared in the residue and regenerated cellulose-rich extract, leaving only cellulose amorphous peaks for C4 at 84 ppm and C6 at 62 ppm, which can be regarded as evidence that the complexity of the wood powder was completely transformed into an amorphous structure. Furthermore, the spectrum of the regenerated cellulose-rich extract shows only the peaks of cellulose/hemicellulose; peaks for lignin cannot be observed, indicating that the principal component of the extract is cellulose. The spectra for both residue and original wood are similar to each other, but the spectra of the residue shows better resolution than the characteristic peaks for lignin (133, 125, and 113 ppm) when separated clearly. Unlike in the original wood, the three peaks overlapped and formed a broad peak, which is further proof that ILs not only disrupt the hydrogen-bonding interaction of crystalline cellulose in wood but also interact with and solvate the aromatic characters of lignin, making them more sensitive to analysis. The data presented above show that partial dissolution of wood samples occurs in the AmimCl/DMSO solvent system. Furthermore, the system is capable of efficient delignification and cellulose extraction from wood. The principal substance of the extract is cellulose; small amounts of hemicellulose and lignin are also found. 3.3.2. XRD analysis Fully amorphous materials were obtained after the regeneration of pine from ILs; this is clearly visible in the X-ray spectra. The untreated wood sample shows two prominent peaks near 2h of 15° and 22°, indicating the characteristic diffraction pattern of cellulose I. The regenerated cellulose-rich extracts display a slightly broad amorphous diffraction peak near 2h of 25°, which is the characteristic diffraction pattern of cellulose II (Mansikkamaki et al., 2005). Compared with the diffraction pattern of the original pine wood, the intensities of the diffraction peaks in the cellulose-rich extracts are smaller, revealing that ILs destroy the inter and intramolecular hydrogen bonds among lignocelluloses, leading to lower
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Table 5 Dissolution and regeneration rates for pine in AmimCl/DMSO obtained with fresh and recycled AmimCl under MI heating for 2 h, 5 wt% wood concentration. IL
Temperature (°C)
Dissolution rate (%)
Regeneration rate (%)
Fresh Recycled Fresh Recycled
100 100 110 110
25 27 40 40
17 17 25 26
crystallinity in the cellulose-rich extract (Wang et al., 2009). The results here are consistent with the 13C CP/MAS NMR analysis. 3.3.3. FTIR analysis The IR spectra of the cellulose-rich extract obtained from fresh and recycled IL, pine residue, MCC, and the original pine wood are compared. The cellulose-rich extracts are comparable to MCC. Absorbance peaks at 3321–3434 (OH stretching), 2891–2909 (CH stretching), 1732, 1644, 1370–1309, 1382, 1169–1012, 898, and 679–557 cm1 are all associated with cellulose or hemicellulose, while the characteristic peaks of lignin are observed at 1514/1504 (C@C stretching vibrations), 1458 (asymmetric bending in CH3), and 1261 (guaiacyl/syringyl ring) (Sun et al., 2009). The original pine and pine residue spectra show both cellulose and lignin peaks. After processing by IL, the lignin peaks become less noticeable in the cellulose-rich extracts compared with the original pine, indicating that most of the lignin is reserved in the pine residue and that the main component of the extract is cellulose. 3.3.4. Morphological investigation of original pine, the cellulose-rich extract, and pine residue FE-SEM images of the original pine wood, the corresponding cellulose-rich extract, and pine residue treated by AmimCl/DMSO are analyzed. Compared with the original wood, which has a fascicular texture, the pine residue shows a highly porous structure; this indicates that AmimCl effectively disrupts the intricate network of non-covalent interactions within lignocelluloses, leading to the dissolution of wood. The morphology of the regenerated cellulose-rich extracts obtained from both fresh and recycled IL is homogeneous and dense, and presents uniform macrostructures; thus, they may be utilized to further benefit wood processing for lignocelluloses products and their derivatives. 3.4. Recycling of the IL Compared with other organic solvents, one of the predominant benefits of ILs is that they are easy to recycle and reuse. Although the relatively high price of ILs prohibits their wide-scale application in the industry, their effective recover and large-scale production could avert this problem. In the present experiment, AmimCl was recovered by evaporating the collected filtrate under reduced pressure at 100 °C to remove water. The recycled IL was then placed in a vacuum oven at 80 °C for 48 h before reusing for wood dissolution. The dissolution and regeneration rates of pine obtained from both the fresh and recycled IL were close to each other under comparable reaction conditions, as shown in Table 5. The structure of cellulose-rich extracts obtained from the recycled IL is also similar to that obtained using fresh AmimCl, as verified by the 13C CP/MAS NMR, FTIR, XRD, and SEM results. 4. Conclusion The present study not only realizes an environmentally friendly method of cleanly extracting cellulose from wood under mild con-
ditions, but also provides a new approach for utilizing biomass resources. However, the cellulose extraction method presented here shows some limitations such as, it is not effective for all kinds of wood; The toxicity and corrosion profile of Cl is an obstruction for future large-scale industry application; more specific control under MI, and variations in the quantity of DMSO used were not investigated. Thus, further studies could be performed. The results presented here make the proposed technology highly promising for future applications. Acknowledgements This work was supported by the Major Project of Chinese National Programs for Fundamental Research and Development (973 Program) (Grant No. 2009CB219901) and the Chinese National Natural Science Foundation (No. 21006118). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2011.05.064. References Adler, E., 1977. Lignin chemistry: past, present and future. Wood Sci. Technol. 11, 169–218. Boonstra, M.G., Pizzi, A., Tekely, P., Pendlebury, J., 1996. Chemical modification of Norway spruce and Scots pine: a 13C NMR CP-MAS study of the reactivity and reactions of polymeric wood components with acetic anhydride. Holzforschung 50, 215–220. Brandt, A., Hallett, J.P., Leak, D.J., Murphy, R.J., Welton, T., 2010. The effect of the ionic liquid anion in the pretreatment of pine wood chips. Green Chem. 12, 672–679. Cao, Y., Wu, J., Zhang, J., Li, H.Q., Zhang, Y., He, J.S., 2009. Room temperature ionic liquids (RTILs): a new and versatile platform for cellulose processing and derivatization. Chem. Eng. J. 147, 13–21. Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P., Moyna, G., Rogers, R.D., 2007. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methylimidazolium chloride. Green Chem. 9, 63– 69. Gilardi, G., Abis, L., Cass, A.E.G., 1995. C-13 CP/MAS solid-state NMR and FT-IR spectroscopy of wood cell-wall biodegradation. Enzyme Microb. Tech. 17, 268– 275. Heinze, T., Liebert, T., 2001. Unconventional methods in cellulose functionalization. Prog. Polym. Sci. 26, 1689–1762. Istek, A., Gonteki, E., 2009. Utilization of sodium borohydride (NaBH4) in kraft pulping process. J. Environ. Biol. 30, 951–953. Jebrane, M., Sebe, G., 2008. A new process for the esterification of wood by reaction with vinyl esters. Carbohyd. Polym. 72, 657–663. Kilpelainen, I., Xie, H., King, A., Granstrom, M., Heikkinen, S., Argyropoulos, D.S., 2007. Dissolution of wood in ionic liquids. J. Agric. Food Chem. 55, 9142–9148. Klemm, D., Heublein, B., Fink, H.P., Bohn, A., 2005. Cellulose: fascinating biopolymer and sustainable raw material. Angewandte Chemie-International Edition 44, 3358–3393. Lucas, M., Macdonald, B.A., Wagner, G.L., Joyce, S.A., Rector, K.D., 2010. Ionic liquid pretreatment of poplar wood at room temperature: swelling and incorporation of nanoparticles. Appl. Mater. Interf. 2, 2198–2205. Mansikkamaki, P., Lahtinen, M., Rissanen, K., 2005. Structural changes of cellulose crystallites induced by mercerisation in different solvent systems; determined by powder X-ray diffraction method. Cellulose 12, 233–242. Myllymaeki, V., Aksela, R., Myllymaki, V., 2005. Method for dissolving lignocellulosic material, such as wood, involves mixing lignocellulosic material with ionic liquid solvent under microwave irradiation and/or pressure in absence of water for complete dissolution. pp. 1654307A1654301, Kemira Oyj (Kemh). Rogers, R.D., Seddon, K.R., 2003. Ionic liquids-solvents of the future? Science 302, 792–793. Satge, C., Verneuil, B., Branland, P., Granet, R., Krausz, P., Rozier, J., Petit, C., 2002. Rapid homogeneous esterification of cellulose induced by microwave irradiation. Carbohydr. Polym. 49, 373–376. Stirling, R., Bicho, P., Daniels, B., Morris, P.I., 2010. Kraft pulping of wood treated with carbon-based preservatives. Holzforschung 64, 285–288. Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H., Rogers, R.D., 2009. Complete dissolution and partial delignification of wood in the ionic liquid 1ethyl-3-methylimidazolium acetate. Green Chem. 11, 646–655. Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogers, R.D., 2002. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc. 124, 4974–4975.
X. Wang et al. / Bioresource Technology 102 (2011) 7959–7965 Torgovnikov, G., Vinden, P., 2010. Microwave wood modification technology and its applications. For. Prod. J. 60, 173–182. Voitl, T., Nagel, M.V., Rohr, P.R., 2010. Analysis of products from the oxidation of technical lignins by oxygen and H3PMo12O40 in water and aqueous methanol by size-exclusion chromatography. Holzforschung 64, 13–19. Wang, Z.G., Yokoyama, T., Chang, H.M., Matsumoto, Y., 2009. Dissolution of beech and spruce milled woods in LiCl/DMSO. J. Agric. Food Chem. 57, 6167–6170. Yang, R.D., Chen, K.F., Liu, Y.L., Chen, Q.F., 2007. Shear-thinning properties of nonwood kraft pulping waste liquor. J. Cent. South Univ. Technol. 14, 522–525. Yuan, T.Q., Sun, S.N., Xu, F., Sun, R.C., 2010. Homogeneous esterification of poplar wood in an ionic liquid under mild conditions: characterization and properties. J. Agric. Food Chem. 58, 11302–11310.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Cellulose extraction from wood chip in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) Xuejing Wang a,b,1, Huiquan Li a,⇑, Yan Cao a,1, Qing Tang a a b
Key Laboratory of Green Process Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 16 March 2011 Received in revised form 19 May 2011 Accepted 22 May 2011 Available online 30 May 2011 Keywords: Ionic liquid Dissolution Extraction Wood chip Cellulose
a b s t r a c t In the present study, 1-allyl-3-methylimidazolium chloride (AmimCl), an ionic liquid (IL), was used to extract cellulose from pine, poplar, Chinese parasol, and catalpa wood chips. Results show that pine is the most suitable wood species for cellulose extraction with ILs. Its cellulose extraction rate can reach as high as 62% under optimized conditions and its cellulose content is as high as 85% when DMSO/water is used as the precipitant. The dissolution process can be clearly observed by hot stage optical microscopy, and the reaction time can be significantly reduced by microwave irradiation. 13C CP/MAS NMR, FTIR, XRD, and SEM were used to analyze the cellulose-rich extracts of pine. Results show that IL dissolves pine wood by destroying inter and intramolecular hydrogen bonds between lignocelluloses. The major component of pine extract is cellulose with a homogeneous and dense structure. After extraction, AmimCl can be easily recycled and reused. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fossil-based feedstock is a widely used energy source worldwide. However, the depletion of it makes the determinations of non-fossil energy sources are of utmost importance in the 21st century. Compared with energy production from fossil fuels, energy production from biomass is a new and important alternative energy feedstock that has distinct advantages, such as it is available in large reserves, renewable and there is no CO2 emission. Potential biomass sources include crops (corn, sugarcane), agricultural wastes, forest products, grasses, and algae (Lucas et al., 2010). Wood is the most abundant lignocellulosic resource on Earth. The main component of wood is a macromolecular substance with 4050% cellulose, 1525% hemicellulose, and 1530% lignin (Voitl et al., 2010). Among these, cellulose, being the most abundant renewable biopolymer in the world, has attracted much attention due to its fascinating structure and promising properties (Klemm et al., 2005). Cellulose is traditionally used as a raw material for the production of paper, paperboard, fiberboard, and other similar products. Its excellent characteristics, which include hydrophilicity, chirality, biodegradability, capacity for broad chemical modification, and ability to form semicrystalline fiber morphologies, have drawn considerably increased interest and encouraged ⇑ Corresponding author. Tel.: +86 010 82544825, fax: +86 010 62621355. 1
E-mail address: [email protected] (H. Li). These authors equally contributed to this work.
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.05.064
worldwide interdisciplinary research on cellulose and its products over the past few decades (Heinze and Liebert, 2001; Zhu et al., 2006). Wood remains the most important raw material source of cellulose. However, the structure of wood is highly complex because its lignin, which is a three-dimensional polymer network, binds with its carbohydrates (hemicellulose and cellulose) to form a tight and compact structure (Adler, 1977). The hardness of wood biomass is particularly challenging because it is impossible to dissolve wood in conventional solvents in its native state. Traditionally, cellulose is extracted from wood through the Kraft pulping process, which involves the semi-chemical degradation of the lignin/hemicellulose matrix by treatment with solutions of sodium hydroxide and sodium sulfide at high temperatures and pressures. The use of these toxic chemicals and the intensive processing conditions involved in the pulping procedure pose environmental hazards in the air and water (Istek and Gonteki, 2009; Stirling et al., 2010; Yang et al., 2007). Therefore, efficient utilization of this renewable resource with no hazards emission is necessary. Ionic liquids (ILs) are molten salts with glass transition or melting temperatures below 100 °C. In contrast to traditional volatile organic compounds, ILs have many attractive properties, including good chemical and thermal stability, non-flammability, immeasurable low vapor pressure, and recyclability (Rogers and Seddon, 2003). Recent research studies focused on the application of ILs in wood processing (Fort et al., 2007; Lucas et al., 2010; Yuan et al., 2010), resulting in the identification of several ILs that may be capable of dissolving wood. Myllymaki and Akasela (2005) first
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Nomenclature Rd Rr Re Mo Mr
the the the the the
dissolution rate regeneration rate cellulose extraction rate mass of the original wood added mass of the residue
invented a method to dissolve wood in 1-butyl-3-methylimidazolium chloride (BmimCl) and separate cellulose from the solution, but they did not describe in detail how the cellulosic fiber was recovered. Kilpelainen et al. (2007) demonstrated that BmimCl can fully dissolve ball-milled wood dust, sawdust, and thermo mechanical pulp (TMP) under gentle conditions. However, the preparation of sawdust, TMP, or ball-milled wood dust consumed considerable amounts of energy. Rogers’ group conducted a study on the dissolution of wood chips in ILs and discovered that wood chips can be partially dissolved in BmimCl (Fort et al., 2007) and nearly fully dissolved in 1-ethyl-3-methylimidazolium acetate (EmimAc) (Sun et al., 2009). Furthermore, they successfully recovered cellulosic fiber from the IL solution by the addition of a variety of precipitating solvents. Zavrel et al. (2009) showed that, compared with other imidazolium chloride RTILs, such as BmimCl, 1allyl-3-methylimidazolium chloride (AmimCl) is the most effective IL for dissolving wood chips using high-throughput systems. As well, the synthesis of AmimCl was more efficient. The conversion rate of the imidazolium reached up to 80% after a reaction time of 3 h, and was almost 100% after 6 h, because of the high reactivity of the allyl chloride (Zhang et al., 2005). AmimCl also showed a lower melting point and a considerably lower viscosity than BmimCl. Recently, ILs have been found to present excellent microwave irradiation (MI) conductivity as they consist only of anions and cations. Thus, MI has been used to accelerate organic reactions that use ILs as solvents, thereby reducing reaction times from hours to minutes (Myllymaeki et al., 2005; Satge et al., 2002; Torgovnikov and Vinden, 2010), at the same time decreasing the serious degradation of wood bring by the conventional oil bath heating under long reaction times. In the present paper, the dissolution and regeneration of wood chips in AmimCl, and the effects of wood species, dissolution time, temperature, microwave assistance, initial wood concentration, and types of precipitant on the efficiency of cellulose extraction are discussed. The recycle and reuse of AmimCl is also determined to ensure the cost efficiency of the use of ILs to recover cellulose. The research shows that the use of ILs for extraction of cellulose from wood avoids the use of toxic and hazardous chemicals, and can be carried out under mild conditions.
Me Wo-cel We-cel
the mass of the extract the cellulose content in the original wood the cellulose content in the extract
received. AmimCl was synthesized as described in the previous work (Zhang et al., 2005). 2.2. Wood dissolution Dimethyl sulfoxide (DMSO) (16 wt%) was added to AmimCl to form a clear solution with stirring. In a typical dissolution, approximately 10.0 g of dried wood powder was dispersed into 190.0 g of AmimCl/DMSO in a three-necked flask. The mixture was mechanically stirred at the desired temperature for a desired length of time in an oil bath (300 rpm) or with microwave heating (Sineo MAS-I, China, 500 W output, 400 rpm) in the open atmosphere. A dark, amber-colored, viscous wood suspension with about 5 wt% wood concentration was obtained. The partial dissolution of wood powder in AmimCl/DMSO was observed using a polarizing microscope with a digital camera (Olympus BX51, Japan) and a hot plate (Linkam THMS600, UK). 2.3. Cellulose extraction and residue separation After dissolution, the suspension was filtered through a stainless steel filter to remove the undissolved residue. Water was added to the resulting clear liquors, and a cellulose-rich extract was reconstituted from the liquor. This extract and the undissolved residue were respectively washed with water/DMSO and water until the IL was completely removed, and then dried overnight in an oven at 100 °C prior to use. The percentage of wood dissolved and regenerated, as well as the cellulose extraction rate in the material, was calculated according to Eqs. (1)–(3):
Rd ð%Þ ¼
Mo Mr 100% Mo
ð1Þ
Rr ð%Þ ¼
Me 100% Mo
ð2Þ
Re ð%Þ ¼ Rr
W e-cel 100% W o-cel
ð3Þ
2.4. Recycling of AmimCl 2. Methods 2.1. Materials and reagents Pine, poplar, Chinese parasol, and catalpa wood samples were obtained from Shandong Province, China. These were first dried in sunlight, and then cut into small pieces. Wood chips were ground into powder using a muller, and then pass through a 40– 60 mesh sieve (0.45–0.65 mm). The wood powders were dried overnight in an oven at 105 °C before using. Microcrystalline cellulose (MCC) with a degree of polymerization (DP) of 200–340 was purchased from Acros Organics, USA. All other chemical reagents were purchased from commercial sources in China and used as
After wood dissolution and cellulose extraction, recovery of the IL was accomplished by evaporating water from the precipitation liquid. The purity of the IL was determined by 1H NMR spectroscopy. The recycled IL was used to extract cellulose from the wood chip as described in Sections 2.2 and 2.3. 2.5. Characterization 2.5.1. FTIR The samples were ground into powder and vacuum dried for 24 h. The IR spectra of the samples were recorded using a Fourier Transform IR spectrometer (PE-2000, USA). Test specimens were prepared using the KBr-disk method.
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2.5.2. WAXD The cellulose-rich extracts, as well as reference samples, were vacuum dried for 24 h, and then ground into powder before their X-ray diffraction (XRD) patterns were determined with Cu Ka radiation (k = 1.5406) at 40 kV and 200 mA. Patterns were recorded in the range of 2h = 590° using an X’pert PRO X-ray diffractometer (PANalytical B.V., The Netherlands). 2.5.3. FE-SEM SEM images were obtained with a JSM6700F field emission scanning electron microscope (FE-SEM) operated at 5 kV accelerating voltage. The extract film was frozen in liquid nitrogen, immediately snapped, and then dried under vacuum. The free and fracture surfaces of the film were sputtered with gold, and then observed and photographed. 2.5.4. NMR Solid-state cross-polarization/magic angle spinning (CP/MAS) 13 C NMR spectra were obtained from the samples at 100 MHz using a Bruker AVANCE-III 400 NMR spectrometer with a 4 mm MAS probe at a 13C frequency of 100.37 MHz. The structures of the fresh and recycled AmimCl were determined by 1H NMR spectroscopy (Bruker ADVANCE 400) at room temperature in DMSO-d6 with a 5 mm BBO probe at a 13C frequency of 100.61 MHz. 3. Results and discussion 3.1. Dissolution of wood in AmimCl The dissolution of pine, poplar, Chinese parasol, and catalpa wood powders in AmimCl was studied. In each sample, a 5 wt% wood chip/IL suspension was prepared by the addition of 16 wt% DMSO to reduce the viscosity of the mixture and facilitate filtration. The addition of DMSO had no obvious effect on the solubility of the wood chips in AmimCl. The wood chips swelled shortly after they were added to the AmimCl/DMSO solvent. After heating for several hours at 100 °C under vigorous mechanical stirring, the colors of the solutions became dark and their viscosities distinctively increased, indicating that the dissolution of wood occurred. A clear, dark brown solution was obtained after filtration, and the brown gel was reconstituted by the addition of water; after washing several times with water/DMSO, the gel became white, indicating that its main components were carbohydrates. A hot stage polarizing microscope was used to observe the dissolution process of wood chips in AmimCl. Result shows that the wood samples were in their original states and fibrous structures
25
15 10 5
3
6
3.2.1. Effect of wood species Different types of wood result in various dissolution profiles in IL. In this work, pine was found to be more suitable for dissolution and regeneration in IL than the three other types of wood. Fig. 1(A) and (B) show that the respective dissolution and regeneration rates of pine can reach as high as 67% and 56%; these only reach 30% and 10%, respectively, for the other types of wood. Interestingly, the cellulose content in regenerated pine was nearly 20% higher than that in the regenerated poplar, Chinese parasol and catalpa chips (Table 1). This may be because pine is the softest wood among the four wood species. Softwood species are easier to be dissolved in ILs than hardwood because the former has lower densities and hardness compared with the latter (Kilpelainen et al., 2007).
B
Pine Polar Chinese parasol Catalpa
20
0
3.2. Evaluation of process variables
Dissolution/Extraction rate (wt%)
Dissolution/Extration rate (wt%)
A 30
can be observed below 50 °C. Swelling occurred at 70 °C, and dissolution can be observed at 100 °C. The dissolution rate increased at 120 °C; at this temperature, most of the fibrous material disappeared after only 10 min and the visual field became dark. After 30 min at 120 °C, no further changes were observed; a fully black field was not obtained even when the reaction time was increased to 4 h. In general, the experiments consistently indicate that AmimCl can only partially dissolve wood chips. ILs are capable of dissolving complex macromolecules and polymeric materials with high efficiency by breaking the extensive hydrogen bonding network of polysaccharides and promoting their dissolution (Swatloski et al., 2002). This mechanism has been observed to proceed as follows: The ion pairs in AmimCl dissociate into individual Cl and Amim+ ions. Free Cl ions associate with cellulose hydroxyl protons, and free Amim+ cations associate with cellulose hydroxyl oxygen groups. These disrupt hydrogen bonding in cellulose and cause its dissolution (Cao et al., 2009). It is presumed that a similar mechanism is followed in the case of the dissolution of wood in AmimCl. Among the three macromolecular components of wood, cellulose is recognized as the basic structural element of the cell wall. Lignin and hemicellulose are distributed through the cell wall in a random manner (Myllymaeki et al., 2005). Lignin, which is an intercellular substance that acts as a cementing agent bonded to hemicellulose, is the main obstacle that limits the dissolution of wood. Anions of AmimCl have the ability to break the lignocellulosic complex, especially the amorphous network of cross-linked phenylpropanoid units of lignin, and the inter and intramolecular hydrogen bonding interactions in the wood matrix. This leads to a decrease in cellulose crystallinity and dissolution of the wood.
Time (h)
15
24
70
Pine Poplar Chinese parasol Catalpa
60 50 40 30 20 10 0
90
100
110
Temperatureo(C)
120
Fig. 1. Dissolution and regeneration rates of different wood samples in AmimCl/DMSO as a function of time (A) and temperature (B) under oil bath heating, 5 wt% wood concentration.
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Table 1 Cellulose contents in the wood extracts of different wood samples as a function of time under oil bath heating at 100 °C, 5 wt% wood concentration. Dissolution time (h)
3 6 15 24
Cellulose content in wood extracts (%) Pine
Poplar
Chinese parasol
Catalpa
70 76 78 71
49 48 49 47
40 52 54 49
25 49 50 56
3.2.2. Effect of time The kinetics of the wood chips dissolved in AmimCl was studied as a function of dissolution time. Fig. 1(A) shows that the dissolution and regeneration rates increased with increasing reaction time for all wood samples except catalpa, which had nearly constant rates of about 20% and 6% respectively, because it was the only hardwood among the four types of wood investigated. Rate increments for pine were the most obvious. For example, from 6 to 24 h, the dissolution and regeneration rates for pine increased from 19% to 26% and 9% to 19%, respectively. When the dissolution time was extended to 43 h, the dissolution and regeneration rates can reach up to 40% and 25%, respectively (data not shown). Compared with the value obtained at 24 h for pine in Fig. 1(A), the dissolution rate increased by 14%, whereas the regeneration rate only increased by 6%, indicating that the polymer degradation occurs as Fort reported (Fort et al., 2007). The full dissolution of wood powder in ILs cannot be observed with increasing time because AmimCl cannot effectively solvate the aromatic character of lignin. The cellulose content changed minimally when the treatment time was increased, as shown in Table 1. However, since the regeneration rate increased, the following conclusions can be drawn: First, the cellulose extraction rate also increased with time. Second, although the cellulose content may be as high as 78% for pine, the highest extraction rate for its cellulose was only 27.5% (based on the average cellulose content of original pine, which is 49%) under the temperature of 100 °C, because of the low regeneration rate. The extraction rates for the other three wood powders were even lower.
3.2.3. Effect of temperature It is determined that the dissolution of wood in ILs is dependent on the applied temperature. Fig. 1(B) shows that the dissolution and regeneration rates increase when the temperature increases from 90 to 120 °C. The results of this analysis are similar with those in the time dependent curve in that, although the rates increased with temperature, the increments for poplar, Chinese parasol and catalpa were less obvious than those of pine. In fact, no regeneration product was obtained at 120 °C for catalpa, indicating that it has been degraded. The pine sample showed sharp increases in rates at 110 °C; even sharper rates were observed at 120 °C. Its dissolution and regeneration rates did not show any change when the temperature was increased from 90 to 100 °C; these remained stable at 18% and 9%, respectively. The dissolution and regeneration rates quickly increased to 38% and 21%, respectively, however, after the temperature was increased to 110 °C. These findings are consistent with those in Brandt et al.’s (2010) study, which concluded that swelling and dissolution in ILs is temperature-dependent, and that better dissolution and regeneration rates are obtained at temperatures beyond 100 °C. Further increases in temperature to 120 °C, may yield dissolution and regeneration rates as high as 67% and 54%, respectively. However, the cellulose content in the regenerated pine wood was only 66% at 120 °C, indicating the occurrence of degradation. When the temperature was further in-
creased to 140 °C, the wood powder fully dissolved in the IL and formed a dark black solution, but its regeneration rate was lower than 18%; this indicated that most of the dissolved pine degraded, similar to catalpa at 120 °C. 3.2.4. Effect of microwave irradiation MI was employed to dissolve the wood and extract cellulose. The use of MI in biomass chemistry has quickly developed over the years, and compared with conventional oil bath heating, it shows many advantages, such as non-contact heating, better control over the heating process, rapid heating, high rate of reactions and high product yields. ILs can efficiently absorb microwave radiation because they are only composed of anions and cations; thus MI may suitably be applied to IL solutions to dissolve wood (Satge et al., 2002; Torgovnikov and Vinden, 2010). In the experiments, it is also discovered that wood dissolution in IL can be efficiently carried out using MI. Table 2 shows that, while the same levels of dissolution and regeneration are achieved by both processes, the reaction time decreases significantly under microwave heating compared with oil bath heating. For example, the dissolution and regeneration rates can reach 26% and 16% at 100 °C after 15 h of oil bath heating. In contrast, only 2 h of microwave heating was necessary to achieve similar levels without decreasing the cellulose content of the products. Such quick and effective dissolution is due to the increment in collision frequency between the anions and cations of IL and the wood macromolecules brought by MI. Thus, breakages in the amorphous network of cross-linked phenylpropanoid units of lignin and the inter and intramolecular hydrogen bonding interactions in the wood matrix by Cl anions are intensified and more rapid dissolution occurs. Therefore, microwave heating is an efficient heating method for processing wood in ILs. 3.2.5. Effect of initial wood concentration The effect of initial wood concentration was investigated in the present study as shown in Table 3. Because the viscosity of the IL and the molecular weight of the three macromolecular components in wood are relatively high, these may result in filtration and separation difficulties, particularly at higher wood concentrations (e.g., 8%). Low concentrations favor the dispersion of molecules in solution and lead to high dissolution and regeneration rates. For example, increasing the wood solution from 1 wt% to 5 wt%, the dissolution rate decreased from 35% to 26%. Depending on the initial wood concentration, the regeneration rates also show the same trend. 3.2.6. Effect of precipitant In general, regenerated wood can be obtained by adding water to the filtrate. In Fort et al.’s (2007) study, the reconstitution of cellulosic materials was achieved through the use of a 1:1 acetone– water solution as the solvent. Off-white powdery flocs formed
Table 2 Dissolution and regeneration rates of pine in AmimCl/DMSO as a function of time under MI and oil bath heating at 100 °C, 5 wt% wood concentration. Heating style
Dissolution time (h)
Dissolution rate (%)
Regeneration rate (%)
Oil bath
3 6 15 24
16 19 26 26
6 9 16 19
MI
0.5 1 2 4
13 19 25 22
6 10 17 18
X. Wang et al. / Bioresource Technology 102 (2011) 7959–7965 Table 3 Dissolution and regeneration rates for pine in AmimCl/ DMSO as a function of concentration of wood under oil bath heating for 15 h at 100 °C. Concentration of wood in IL (%)
Dissolution rate (%)
Regeneration rate (%)
1 2 3 4 5
35 34 32 27 26
25 20 19 17 16
shortly after the coagulant was mixed with the wood liquors, and the regenerated wood was nearly free of lignin, which is soluble in acetone, as verified by 13C NMR and FTIR spectra. The findings in the experiment were significantly different because acetone cannot dissolve the lignin extracted from pine, and, therefore, cannot contribute to cellulose extraction. Thus, DMSO was used in place of acetone. Table 4 shows that the cellulose content of the extract precipitated with acetone/water was 76% (Entry 2), the value of which is nearly identical to that of the extract precipitated with water (Entry 1). Cellulose contents of the extract precipitated with DMSO/water can reach as high as 85% (Entry 3), which is 9% higher than those of the extracts precipitated with water and acetone/ water. It is believed that differences in the dissolution profile of lignin may be attributed to differences in the sources of pine and structures of the IL selected. The cellulose content of the extracts obtained by the IL method is quite close to that obtained by Kraft pulping (90%). The difference between the two cellulose extracting methods is that the cellulose extraction rate is lower for the IL method than for Kraft pulping because wood has dissolution limits in ILs. However, in light of the serious environmental pollution that Kraft pulping causes (Istek and Gonteki, 2009; Stirling et al., 2010; Yang et al., 2007), the IL method appears to be a more feasible approach for cellulose extraction. Thus, determination of a solution that can increase the dissolution rate of wood in ILs, such as enhancement of the reaction temperature or time as discussed above or the design of a new structure of ILs, among others, is necessary. 3.2.7. Mass balance of the cellulose from original wood As discussed above, 2 h reaction time and 3 wt% wood concentration at 110 °C under MI were chosen as optimal dissolving conditions because of the higher dissolution rate and lower degradation observed for pine wood under these conditions. In general, the results obtained under these conditions adequately match expected values. 10.0 g pine is processed by 323 g of AmimCl/DMSO. After filtration, 3.6 g of regenerated cellulose-rich material and 5.0 g of residue were obtained. There is also some loss, accounting for about 14% of the original wood, during this process. This loss has two components: loss from water-soluble low molecular matter that results from the little degradation of wood and DMSO-soluble lignin from filtration, and loss that occurs in the process of washing and filtration steps; the latter is considered the major source of mass loss. The cellulose content of the
Table 4 Cellulose content of the pine extracts obtained under different precipitants from MI heating for 2 h at 100 °C, 5wt% wood concentration. Entry
Precipitants
Cellulose (wt%)
1 2 3
Water Acetone–water DMSO–water
76 76 85
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regenerated cellulose-rich material is about 85%, whereas the unextracted cellulose that remains in the residue is believed to be about 25% (based on the residue mass). Considering the 49% cellulose content of the original pine wood, the extraction rate can reach 62%, indicating that the IL process is an effective method for extracting cellulose from wood. 3.3. Analysis of the regenerated cellulose-rich extract and undissolved residues 3.3.1. NMR spectroscopy Solutions containing five parts of pine wood and ninety five parts of AmimCl/DMSO were cooked at 100 °C for 2 h by MI. After filtration, the cellulose-rich extract was precipitated with water/ DMSO; undissolved residues were repeatedly washed with water and dried overnight in a vacuum oven at 100 °C before they were subjected to 13C CP/MAS NMR analysis. Original pine and lignin obtained from pine using traditional acid–base technologies and analytical microcrystalline cellulose (MCC) were selected for comparison (see Supplementary Data, Fig. S1). Result shows the lignin obtained from pine has characteristic peaks at 148, 133, 122, 113, and 56 ppm, while MCC shows characteristic peaks at 105 (C1), 90 (C4, crystalline), 85 (C4, amorphous), 78 (C2), 74 and 75 (C3, C5), 66 (C6, crystalline), and 60 ppm (C6, amorphous). The characteristic peaks of hemicellulose, whose structure is quite similar to and overlaps with cellulose, are 76 (C2), 75 (C3), 72 (C4), and 63 ppm (C5) (Boonstra et al., 1996; Gilardi et al., 1995; Jebrane and Sebe, 2008). The spectrum of original pine contains all the characteristic peaks of lignin and cellulose mentioned above. After processing by the IL, however, cellulose crystalline peaks for C4 at 89 ppm and C6 at 65 ppm disappeared in the residue and regenerated cellulose-rich extract, leaving only cellulose amorphous peaks for C4 at 84 ppm and C6 at 62 ppm, which can be regarded as evidence that the complexity of the wood powder was completely transformed into an amorphous structure. Furthermore, the spectrum of the regenerated cellulose-rich extract shows only the peaks of cellulose/hemicellulose; peaks for lignin cannot be observed, indicating that the principal component of the extract is cellulose. The spectra for both residue and original wood are similar to each other, but the spectra of the residue shows better resolution than the characteristic peaks for lignin (133, 125, and 113 ppm) when separated clearly. Unlike in the original wood, the three peaks overlapped and formed a broad peak, which is further proof that ILs not only disrupt the hydrogen-bonding interaction of crystalline cellulose in wood but also interact with and solvate the aromatic characters of lignin, making them more sensitive to analysis. The data presented above show that partial dissolution of wood samples occurs in the AmimCl/DMSO solvent system. Furthermore, the system is capable of efficient delignification and cellulose extraction from wood. The principal substance of the extract is cellulose; small amounts of hemicellulose and lignin are also found. 3.3.2. XRD analysis Fully amorphous materials were obtained after the regeneration of pine from ILs; this is clearly visible in the X-ray spectra. The untreated wood sample shows two prominent peaks near 2h of 15° and 22°, indicating the characteristic diffraction pattern of cellulose I. The regenerated cellulose-rich extracts display a slightly broad amorphous diffraction peak near 2h of 25°, which is the characteristic diffraction pattern of cellulose II (Mansikkamaki et al., 2005). Compared with the diffraction pattern of the original pine wood, the intensities of the diffraction peaks in the cellulose-rich extracts are smaller, revealing that ILs destroy the inter and intramolecular hydrogen bonds among lignocelluloses, leading to lower
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Table 5 Dissolution and regeneration rates for pine in AmimCl/DMSO obtained with fresh and recycled AmimCl under MI heating for 2 h, 5 wt% wood concentration. IL
Temperature (°C)
Dissolution rate (%)
Regeneration rate (%)
Fresh Recycled Fresh Recycled
100 100 110 110
25 27 40 40
17 17 25 26
crystallinity in the cellulose-rich extract (Wang et al., 2009). The results here are consistent with the 13C CP/MAS NMR analysis. 3.3.3. FTIR analysis The IR spectra of the cellulose-rich extract obtained from fresh and recycled IL, pine residue, MCC, and the original pine wood are compared. The cellulose-rich extracts are comparable to MCC. Absorbance peaks at 3321–3434 (OH stretching), 2891–2909 (CH stretching), 1732, 1644, 1370–1309, 1382, 1169–1012, 898, and 679–557 cm1 are all associated with cellulose or hemicellulose, while the characteristic peaks of lignin are observed at 1514/1504 (C@C stretching vibrations), 1458 (asymmetric bending in CH3), and 1261 (guaiacyl/syringyl ring) (Sun et al., 2009). The original pine and pine residue spectra show both cellulose and lignin peaks. After processing by IL, the lignin peaks become less noticeable in the cellulose-rich extracts compared with the original pine, indicating that most of the lignin is reserved in the pine residue and that the main component of the extract is cellulose. 3.3.4. Morphological investigation of original pine, the cellulose-rich extract, and pine residue FE-SEM images of the original pine wood, the corresponding cellulose-rich extract, and pine residue treated by AmimCl/DMSO are analyzed. Compared with the original wood, which has a fascicular texture, the pine residue shows a highly porous structure; this indicates that AmimCl effectively disrupts the intricate network of non-covalent interactions within lignocelluloses, leading to the dissolution of wood. The morphology of the regenerated cellulose-rich extracts obtained from both fresh and recycled IL is homogeneous and dense, and presents uniform macrostructures; thus, they may be utilized to further benefit wood processing for lignocelluloses products and their derivatives. 3.4. Recycling of the IL Compared with other organic solvents, one of the predominant benefits of ILs is that they are easy to recycle and reuse. Although the relatively high price of ILs prohibits their wide-scale application in the industry, their effective recover and large-scale production could avert this problem. In the present experiment, AmimCl was recovered by evaporating the collected filtrate under reduced pressure at 100 °C to remove water. The recycled IL was then placed in a vacuum oven at 80 °C for 48 h before reusing for wood dissolution. The dissolution and regeneration rates of pine obtained from both the fresh and recycled IL were close to each other under comparable reaction conditions, as shown in Table 5. The structure of cellulose-rich extracts obtained from the recycled IL is also similar to that obtained using fresh AmimCl, as verified by the 13C CP/MAS NMR, FTIR, XRD, and SEM results. 4. Conclusion The present study not only realizes an environmentally friendly method of cleanly extracting cellulose from wood under mild con-
ditions, but also provides a new approach for utilizing biomass resources. However, the cellulose extraction method presented here shows some limitations such as, it is not effective for all kinds of wood; The toxicity and corrosion profile of Cl is an obstruction for future large-scale industry application; more specific control under MI, and variations in the quantity of DMSO used were not investigated. Thus, further studies could be performed. The results presented here make the proposed technology highly promising for future applications. Acknowledgements This work was supported by the Major Project of Chinese National Programs for Fundamental Research and Development (973 Program) (Grant No. 2009CB219901) and the Chinese National Natural Science Foundation (No. 21006118). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2011.05.064. References Adler, E., 1977. Lignin chemistry: past, present and future. Wood Sci. Technol. 11, 169–218. Boonstra, M.G., Pizzi, A., Tekely, P., Pendlebury, J., 1996. Chemical modification of Norway spruce and Scots pine: a 13C NMR CP-MAS study of the reactivity and reactions of polymeric wood components with acetic anhydride. Holzforschung 50, 215–220. Brandt, A., Hallett, J.P., Leak, D.J., Murphy, R.J., Welton, T., 2010. The effect of the ionic liquid anion in the pretreatment of pine wood chips. Green Chem. 12, 672–679. Cao, Y., Wu, J., Zhang, J., Li, H.Q., Zhang, Y., He, J.S., 2009. Room temperature ionic liquids (RTILs): a new and versatile platform for cellulose processing and derivatization. Chem. Eng. J. 147, 13–21. Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P., Moyna, G., Rogers, R.D., 2007. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methylimidazolium chloride. Green Chem. 9, 63– 69. Gilardi, G., Abis, L., Cass, A.E.G., 1995. C-13 CP/MAS solid-state NMR and FT-IR spectroscopy of wood cell-wall biodegradation. Enzyme Microb. Tech. 17, 268– 275. Heinze, T., Liebert, T., 2001. Unconventional methods in cellulose functionalization. Prog. Polym. Sci. 26, 1689–1762. Istek, A., Gonteki, E., 2009. Utilization of sodium borohydride (NaBH4) in kraft pulping process. J. Environ. Biol. 30, 951–953. Jebrane, M., Sebe, G., 2008. A new process for the esterification of wood by reaction with vinyl esters. Carbohyd. Polym. 72, 657–663. Kilpelainen, I., Xie, H., King, A., Granstrom, M., Heikkinen, S., Argyropoulos, D.S., 2007. Dissolution of wood in ionic liquids. J. Agric. Food Chem. 55, 9142–9148. Klemm, D., Heublein, B., Fink, H.P., Bohn, A., 2005. Cellulose: fascinating biopolymer and sustainable raw material. Angewandte Chemie-International Edition 44, 3358–3393. Lucas, M., Macdonald, B.A., Wagner, G.L., Joyce, S.A., Rector, K.D., 2010. Ionic liquid pretreatment of poplar wood at room temperature: swelling and incorporation of nanoparticles. Appl. Mater. Interf. 2, 2198–2205. Mansikkamaki, P., Lahtinen, M., Rissanen, K., 2005. Structural changes of cellulose crystallites induced by mercerisation in different solvent systems; determined by powder X-ray diffraction method. Cellulose 12, 233–242. Myllymaeki, V., Aksela, R., Myllymaki, V., 2005. Method for dissolving lignocellulosic material, such as wood, involves mixing lignocellulosic material with ionic liquid solvent under microwave irradiation and/or pressure in absence of water for complete dissolution. pp. 1654307A1654301, Kemira Oyj (Kemh). Rogers, R.D., Seddon, K.R., 2003. Ionic liquids-solvents of the future? Science 302, 792–793. Satge, C., Verneuil, B., Branland, P., Granet, R., Krausz, P., Rozier, J., Petit, C., 2002. Rapid homogeneous esterification of cellulose induced by microwave irradiation. Carbohydr. Polym. 49, 373–376. Stirling, R., Bicho, P., Daniels, B., Morris, P.I., 2010. Kraft pulping of wood treated with carbon-based preservatives. Holzforschung 64, 285–288. Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H., Rogers, R.D., 2009. Complete dissolution and partial delignification of wood in the ionic liquid 1ethyl-3-methylimidazolium acetate. Green Chem. 11, 646–655. Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogers, R.D., 2002. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc. 124, 4974–4975.
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