ARTICLE pubs.acs.org/Biomac
Lignocellulose Aerogel from Wood-Ionic Liquid Solution (1-Allyl-3-methylimidazolium Chloride) under Freezing and Thawing Conditions Jian Li,† Yun Lu,† Dongjiang Yang,‡ Qingfeng Sun,† Yixing Liu,*,† and Huijun Zhao*,‡ †
Key Laboratory of Bio-Based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, China ‡ Environmental Futures Centre and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia
bS Supporting Information ABSTRACT: This article reports a facile preparation of a lignocellulose aerogel from a solution of wood in an ionic liquid by cyclic freezethaw (FT) process. Trema orientalis wood flour was dissolved in 1-allyl-3-methylimidazolium chloride (AMImCl), an effective ionic liquid, and then repeatedly frozen at 20 °C and thawed at 20 °C for several times, and then finally regenerated in water. The hydrogel obtained was solventexchanged to acetone, washed with liquid carbon dioxide, and finally dried by releasing the carbon dioxide at critical temperature to obtain the lignocellulose aerogel. The aerogel had an open 3D fibrillar network and could be transformed from nanofibrillar to sheet-like skeletons with hierarchical micro- and nanoscale morphology and porosity by adjusting the FT treatment cycles. The frequency of FT cycles influenced the intensity, specific surface, crystallinity, and thermostability of the aerogel. This research highlights new opportunities for the development of porous and flexible aerogel scaffolds.
’ INTRODUCTION With the rapid diminishment of oil resources and serious environmental pollution caused by the refining of petrochemical products, alternative and sustainable resources have attracted unprecedented interest among researchers.1 Because of its unique reproducibility and carbon neutrality, lignocellulose, as a representative of the biomass resources, has been one of the popular topics in current research.26 It mainly consists of cellulose, hemicellulose, and lignin in various proportions.7 Among them, cellulose is the most extensively utilized, followed by the hemicellulose and lignin. Lignin is an irregular polymer with several monoaromates. It forms a 3D network in which cellulose and hemicellulose fibers are embedded.7 The complex structure makes it hard to be developed for practical application.8 For instance, wood biomass is hard to be utilized because of its high lignin contents.3 Recently, a solvent system to dissolve lignocellulose has been used to investigate the chemical modification, structural identification, and efficient utilization of lignocellulose. In general, the lignocellulose is dissolved by the solvent system through the disruption of the intra- and intermolecular hydrogen bonds of the components. The selected solvent systems are mainly classified into the organic solvents systems and ionic liquid (IL) solvent systems. The organic solvent system primarily utilizes dimethyl sulfoxide/tetrabutyl ammonium fluoride,9,10 dimethyl sulfoxide/N-methylimidazole,9 or dimethyl sulfoxide/ r 2011 American Chemical Society
lithium chloride.11 However, there are drawbacks during the dissolving procedure. For instance, the double solvent systems seriously degrade the lignocellulose components and thus generate many side products during the dissolving process; extreme pretreatment methods, such as ball milling and acetylating, have to be carried out to overcome recalcitrance of the lignocellulose biomass. The solvents are hard to be recycled, which increases the fabrication cost. Apparently, more efficient, fully recyclable solvents that require low-energy, simplified processes are needed.12 Compared with the organic solvents systems, the ILs provide a broader scope for the development and application of lignocellulose biomass3,1217 because of their unique properties such as low melting point, high stability, low vapor pressure, and tunable solubility. Up to now, ILs have been widely applied in the dissolution and regeneration of lignin and lignocellulose as well as in applications of developing new materials.1824 Among the suitable ILs, 1-allyl-3-methylimidazolium chloride (AMImCl) is the most effective for the dissolution of wood chips.18,21,25 Recently, cellulosic aerogels in previous literature are mostly based on dissolving cellulose in solvent systems2629 and drying using supercritical CO230 or freeze-drying using liquid Received: February 12, 2011 Revised: March 17, 2011 Published: March 22, 2011 1860
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Biomacromolecules nitrogen.31 However, few such porous aerogels by dissolving lignocellulose biomass in ILs have been reported up to date because the precipitate obtained from a solution/dispersion of spruce sawdust in the IL is not always coagulated. The aerogel structure is brittle and thus results in a uncontrollable gel process.22 Additionally, attempts to dissolve and regenerate biopolymer mixtures (cellulose, spruce wood, and mixtures of cellulose, lignin, and xylan) have been performed. After critical drying, these attempts were expected to prepare lignocellulosic aerogels toward a nanofibrillar aerogel structure and higher internal surface area. However, such wood aerogels are too fragile to coagulate because it is very difficult to control the dissolving time and pH value of the solution.22 In this work, a simple route is introduced to prepare lignocellulose aerogels from the aerogel skeletons formed by entangled nanoscopic fiber of cellulose I and undissolved lignocellulose fragments. It renders sufficient strength to prevent the aerogels from collapsing during the solvent extraction, thus allowing the suppression of brittleness and enhancement of the applicability of this material. Furthermore, additional crosslinkers are not added in the process because the preserved intramolecular hydrogen bonds and the presence of crystalline regions act as cross-linkers in the freezing-thawing (FT) process. Thereby, the biomass gel can be converted to an aerogel by critical CO2 drying without collapse. The effectively physical method of gelation of lignocellulose is cooling wood/IL solutions to 20 °C and thawing back to room-temperature several times. This article examines the effect of the conditions of varying FT cycles to form the lignocellulose aerogel. It further reports the morphological, structural, crystallinity, and thermal properties of the aerogel. The properties of the product have been characterized by scanning electron microscopy (SEM), nitrogen adsorption (BET), X-ray diffraction (XRD), and thermogravimetric analysis (TGA).
’ MATERIALS AND METHODS Materials. Nalita wood (Trema orientalis) flour passed through a 60mesh sieve was used as the native source of lignocellulose, and then dried in vacuum at 105 °C for 12 h before use. The AMImCl (Shanghai Boyle Chemical) was directly used without further purification. The CO2 (Harbin Liming Gas) has a purity of 99.9%. Other chemicals, including acetone, and deionized water were of laboratory grade and used as received. Preparation of Lignocellulose Aqueous Gels. The preparation procedure of lignocellulose aerogels is shown in Figure 1. Wood flour (10 g) was mixed with 120 g of AMImCl in a 500 mL beaker. The beaker was immersed in an oil bath at 80 °C under constant stirring for 4 h to form a brownish homogeneous mixture. The homogeneous viscous solution was transferred to 11 appropriate molds and sealed individually. The molds were labeled in sequence, from LFT0 to LFT10. All samples, except LFT0, underwent cyclic freezethaw (FT) treatment. The typical FT process is described as follows: LFT1 was frozen at 20 °C for 10 h; then, it was vacuum-thawed for 6 h to room temperature. The required cryogenic temperature in this process was 17 °C. After undergoing several FT cycles, all samples were immersed in the first coagulation bath with deionized water. The bath was replenished at least thrice until no Cl was detected using AgNO3 solution. The immersion time in each bath was longer than 3 h. The same cyclic FT process was also applied to the other samples. Drying of the Lignocellulose Aqueous Gels to Prepare Aerogels. Acetone is miscible with water and liquid CO2 to act as an
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Figure 1. Schematic presentation of lignocellulose aerogel preparation. “intermediate” liquid to avoid the presence of any water during critical drying. The obtained lignocellulose hydrogels were immersed in aqueous acetone solutions with 10, 30, 60, and 90 wt % acetone before the precipitate was finally immersed in pure acetone. Each immersion time was 30 min. The washed solid cake was completely immersed in pure acetone before storage. The solvent-exchange step was repeated thrice until the complete removal of water. The wet (acetone), regenerated lignocellulose was placed in a viewautoclave, which was filled with acetone to prevent uncontrolled drying. Liquid CO2 was pumped into the cell at 5 °C and ∼500 psi pressure to fill the cell completely. After ∼30 min, the mixed liquid was exhausted from the cell as fresh CO2 was injected in it to keep the biomass completely submerged in liquid. The entire process was observed through the cell window. This acetone-to-CO2 solventexchange cycle was repeated twice until the cell was filled with fresh liquid CO2. Finally, the content of the cell was heated to 35 °C, and the pressure reached 1250 psi. The liquid phase in the pores of the aerogel precursor was exchanged with critical CO2 through a dynamic washing step, and no liquidvapor interface could be formed during the pressure reduction. The cell was slowly depressurized to avoid the cracking of the lignocellulose 3D network to obtain the dry biomass aerogel. Characterization. The changes in the macroscopic appearance of the lignocellulose aerogel before and after FT treatment were observed through visual examination. The 3D cellulosic network of the obtained aerogels and associated nanomaterials were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were taken with an FEI Quanta 200 scanning electron microscope. Drops of dilute cellulosic network suspensions were deposited onto glow-discharged carbon-coated TEM grids. The excess liquid was absorbed by a piece of filter paper. After the specimen has been completely dried, it was observed under an FEI Tecnai G2 electron microscope operated at 80 kV. The XRD patterns were measured for the raw wood flour and the regenerated biomass specimens with an X-ray diffractometer (D/max 2200, Rigaku) using Ni-filtered Cu KR radiation (λ = 1.5406 Å) at 40 kV and 30 mA. Scattered radiation was detected in the range of 2θ = 540° at a scan rate of 4°/min. The crystallinity index (CI) was calculated from the heights of the (200) peak (I200, 2θ = 22.2°) and the minimum intensity between the (200) and (110) peaks (Iam, 2θ = 16.5°) using the Segal method (eq 1). The I200 represents both crystalline and amorphous material, whereas 1861
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Figure 2. Photographs and SEM of final products cross section. (a) SEM of LFT0 without any FT action. The inset shows the photograph of LFT0. (b) SEM of LFT5 undergoes 5 FT cycles. The inset shows the photograph of LFT5. Iam represents the amorphous material. CI ð%Þ ¼
Iam 100 1 I200
ð1Þ
The thermogravimetric analysis (TGA) was performed to compare the degradation characteristics of the raw wood flour, the directly regenerated lignocellulosic material without FT action, and the treated lignocellulose aerogel under different FT cycles obtained with critical dying. The thermal stability of each sample was determined using a thermogravimetric analyzer (Pyris 6, PerkinElmer) with a heating rate of 10 °C/min in a nitrogen environment. The measurements of the porosity of the lignocellulose aerogel, including the pore size and their distribution and specific surface area, were performed by measuring the density and BrunauerEmmetTeller (BET) surface area. The BETsurface area and BarretJoynerHalenda (BJH) pore dimensions were obtained with a 3H-2000PS2 unit (Beishide Instrument S&T). The unit measures and calculates nitrogen absorption and desorption isotherms at the temperature of liquid nitrogen. It uses the isotherms to calculate the surface area by the BET method and determines the mesopore diameter distribution by the BJH method (assuming open cylindrical pores). Prior to the measurement, the samples were dried at 120 °C until the required vacuum was reached.
Figure 3. Density distribution of aerogel specimens experienced FT treatment.
’ RESULTS AND DISCUSSION Visual Examination, Whole Density and Porosity Measurement. To compare the change in the structure of the
lignocellulose aerogel before and after FT treatment, the photographs of LFT0 (without FT treatment) and LFT5 (undergoing quintuplicate FT treatment) are shown in Figure 2. Apparently, LFT5 gel maintained a well-defined form induced from the molding process (inset of Figure 2b), indicating the superior molding ability of the FT treated samples. However, LFT0 gel collapsed completely during the coagulation bath without FT treatment. Additionally, the obtained aerogels had well-defined shapes without a major collapse or shrinkage. The bulk density of LFT1 to LFT10 was calculated by weighing the sample and dividing the weight by the sample volume, measured with a micrometer (Figure 3). Among the FT treated samples, LFT6, a sample after six cycles of FT treatment, exhibited the lowest density because the 3D network becomes
Figure 4. N2 adsorptiondesorption isotherms of as-prepared LFT4 aerogel.
more compact. A slight shrinkage was still observed during critical drying due to the density of the fibrillar network, which kept gels’ initial morphology even under aqueous conditions. Because the starting solution had the same concentration, the 1862
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Figure 5. SEM of cross section of obtained aerogels. (a) LFT1, (b) LFT4, (c) LFT7, and (d) LFT10.
whole densities of samples are nearly proportional to the extent of shrinkage after drying. Figure 2b shows the porous structural network of lignocellulose aerogel. We measured pore size and pore volume of specimens and raw material to determine the potential mesoporous structure. Analysis of nitrogen adsorption at liquid nitrogen temperature gives information on the structure of porous materials. The N2 absorptiondesorption isotherms of LFT4 are shown in Figure 4. The absorption isotherm was inverse-S shaped and fit the type II adsorption curve, which means the diameter of the pore was >10 nm.33 BJH method was used to determine the pore size distribution. According to the data listed in Table S1 of the Supporting Information, the FT cycles led to the formation of interconnected fibrillar network structure of aerogels, which slightly increases the specific surface area and pore volume, and the porosity achieved as high as 97%. Microscopic Morphology of Lignocellulose Aerogel by SEM and TEM. In general, the wood flour mostly dissolved in IL within 4 h. The undissolved fractions precipitated in the coagulation bath. The dissolved substances formed a net-shaped structure during the FT cycles and thus regenerated 3D microfibrillar networks. The compactness degree of the cross-linking network can be adjusted by different cycles of FT treatment. The morphology of the lignocellulose aerogel before and after FT treatment according
to SEM images is shown in Figure 2. The microstructure of the untreated specimen (Figure 2a) shows no evidence of cellulosic network structure. In contrast, after only one FT treatment, the uncollapsed columnar aerogel LFT1 (Figure 5a) showed a porous structure. Additionally, the distribution of the pores became denser with the increase in the FT treatment cycles (Figure 5b). However, the microporous structure disappeared, and a dense film-like structure, which is associated with common open-pore web structure, was observed from the samples after quintuplicate FT treatment. As presented in Figure 5c,d, the film-like structure rather than a network structure was observed from the samples after septuplicate and decuplicate FT treatment, respectively. The construction of cellulose fibril networks should be ascribed to the formation of the IL crystal during the freezing process. Despite the influence of the wood/IL solution concentration on the density of the network, the frequency of crystallization is considered as the vital parameters to form the mesh structure. Obviously, the morphology of the lignocellulose aerogels can be influenced to a certain degree by selecting different FT cycles. As shown in Figure 6a, the ionic network is depicted as a chain in which the anion and cation are represented by a spherical particle and an ellipsoid with a polar headgroup (the imidazolium ring),34 respectively. The anions and cations can form H-bonds with linear polymer molecules, which made the ionic cluster as a single-stranded chain of ions. Figure 6a represents the proposed 1863
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Figure 6. Schematic illustration of nanostructural organization in a wood/ionic liquid solution showing how cellulosic network can form under FT action.
dissolved state of the starting wood/IL solution. Both anions and cations are involved in the dissolution process to destroy the H-bond network between the microfibrils and other polysaccharides. In this process, anions and cations of the IL also form electron donorelectron acceptor (EDA) with the oxygen and hydrogen atoms of the C-6 and C-3 hydroxyl groups of neighbored cellulose chains. These interactions result in the separation of different molecules of wood and make them dissolved in the IL. The freeze process was shown in Figure 6b. First, the cellulose chains are squeezed out from the cellulose/IL due to the crystallization of AMImCl resulted from the low-temperature treatment. Second, the long period of low-temperature treatment results in the formation of strong hydrogen bond between squeezed molecules, which immobilize the cellulose chains. Besides the hydrogen bonds, the crystallization of the cellulose molecules also happens in the frozen state to form physical crosslink network. The cross-link points make the network stable, and the network only breaks down under some special harsh conditions. Therefore, the polymer chains retain the 3D network structure while the chilled gel thaws at appropriate temperature ranges. As the temperature slowly rises to 20 °C, the hydrogen bonding within linear molecules and ions (clusters) reduces gradually, leading to the release of ions (clusters) from the fixed positions around cellulose molecules. As shown in Figure 6c, the domains of the ionic chains structure are scarcely affected during the gradual thermal expansion process, and the activity of
cellulose chains improved in this process; the 3D gel structure was formed because of the further entanglement of polymer chains. These disengaged anions and cations (clusters) will form new hydrogen bonds with the active molecular chains. In the next crystallization process, these cellulose molecules not only penetrate through but also entangle with exiting cross-linked network, forming new entangled nets with higher compactness degree. Therefore, the continuous FT cycles do not destroy the formed cross-link point while forming new physical cross-linking, which efficiently enhances the cross-link density. After coagulation, solvent exchange, and critical drying, all ions of IL can be removed completely, and the 3D structure of lignocellulose aerogel forms (Figure 6d). The cross-link points in the gel are probably composed of the microcrystalline structure of cellulose and the intermolecular hydrogen bonding among lignin, hemicellulose, and cellulose. Given that the number of freezing and thawing cycles impacts the number of crystalline regions of lignocellulose gels, an increase in FT cycles results in the enhancement of stability of the network. The TEM images of the cellulose fibril networks of LFT1 and LFT4 are compared in Figure 7. Figure 7a shows that the netshaped structure of LFT1 contained fewer layers, and Figure 7b presents compacter networks with multiple layers after quadruplicate FT treatment. Figure 7 with high magnification strongly supports the schematic of mechanism (Figure 6) and is in good agreement with the morphology shown in Figure 5a,b. This confirms that FT cycles play an important role in the formation of physical cross-linking network. 1864
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Figure 7. (a) TEM image of cellulose fibrils network structure in LFT1. (b) TEM images of denser cellulose fibrils network structure in LFT4.
Figure 8. (a) X-ray diffraction patterns and (b) crystallinity distribution of as-prepared biomass specimens at different FT cycles.
X-ray Diffraction in Qualitative Measurements of Lignocellulose Aerogels. As mentioned above, the possible mechan-
ism of cross-link point formation can be ascribed to the formation of intermolecule hydrogen bonding and crystallization of the soft segment regions of cellulose. This implies that the crystallinity of specimens varies with the cyclic FT process. XRD was employed to investigate the crystallinity of all specimens. The XRD patterns and distribution of crystallinity (calculated as eq 1) are illustrated in Figure 8. The observed XRD patterns of the specimens in Figure 8a revealed that all diffractograms represent typical cellulose I formation, which exhibited peaks at around 2θ = 14.8, 16.5, and 22.2°. This also indicates that the crystal form of cellulose was not changed during the dissolution, FT treatment, and regeneration processes, but the crystallinity of the specimen evidently decreased after FT treatment, as shown in Figure 8b. This suggests that the FT treatment had a significant effect on the crystallinity to prepare amorphous cellulose. Therefore, there was a steady increase in the degree of crystallinity as the FT cycles were increased from 1 to 6. The increase was more significant, however, between 1 and 4 than between 4 and 6. Therefore, as the cycle was further raised, the crystallinity leveled off or started decreasing. Under such conditions, entanglements among the polymer chains may act to inhibit the folding of polymer chains and thus the formation of crystals,
and the microcrystalline gain rate decreases and the crystalline region in the polymer chain consistently becomes an amorphous region. The crystallinity initially increased with the increasing frequency of FT treatment; then, it subsequently decreased after six FT cycles. The variation in crystallinity indicates that the crystallization of the IL leads to the decrystallization of the original high molecular material. It is known that the freezing process of wood/IL solution results in the formation of numerous crosslinking points. The number of freezing and thawing cycles impact the size and number of crystalline regions of lignocellulose gels. Rather, an increase in FT cycles serves to add stability to existing crystals. Crystallinity is the key factor in determining the thermal properties and stability of a material. According to the measured crystallinity, the higher degree of crystallinity of the specimens appears after five to seven FT cycles. Thermostability Analysis of Lignocellulose Aerogels. In view of the importance of thermal stability in many applications of organic porous materials, thermal decomposition of lignocellulose aerogels and raw material were examined by thermogravimetry (TGA) in a nitrogen environment. In the present work, several well-defined points were selected for the characterization of a thermogravimetric experiment of biomass samples.35 These quantities are shown in Figure 9 and listed in Table S2 of the Supporting Information. 1865
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Figure 9. (a) TG curves for the thermal decomposition of wood flour, LFT0, LFT1, LFT4, LFT7, and LFT10 during pyrolysis at heating rate of 10 °C min1 and (b) DTG evolution profile for wood flour, LFT0, LFT1, LFT4, LFT7, and LFT10 during pyrolysis at heating rate of 10 °C min1.
In all TGA curves, the small weight losses below 150 °C apparently resulted from evaporation of adsorbed water. The decomposition behavior of the aerogels was nearly the same as that of raw material below 150 °C. The start of the hemicellulose decomposition is ∼248 °C.36 The FT treatment could fix some hemicellulose on the gel matrix, making these fixed low-molecularweight polysaccharides insoluble in water during the coagulation and solvent exchange process. The remaining various types and amount of hemicellulose of the specimens make the difference of the Thc.onset. There are significant differences of Tpeak and Toffset due to cellulose and ligin decomposition, respectively, between the raw material and regenerated specimens by 10 °C/min heating. For the cellulose component, the decomposition of wood cellulose is slightly faster than the corresponding aerogel counterpart (temperatures Toffset of ca. 10 °C lower), although lignin may also play an important role in the final part of the DTG curves. However, the Tpeak values did not exhibit a significant change of regenerated samples (LFT0, LFT1, LFT4, LFT7, and LFT10), indicating FT treatment did not improve cellulose thermal stability to expected level, although the degree of crystallinity contribute to the improvement of thermal stability of the cellulose.37 Furthermore, there was a ca. 5% decrease in the aerogel char yields at 500 °C, indicating that the regenerated samples yield less char owing to the removal of the partial lignin in the solventexchange processes.22 The char yields increased progressively with FT cycles, suggesting that the FT treatments fixed more lignin on the formed network. Analyzed results indicated that the thermal stability of the product was not decreased in dissolution and regeneration process.
’ CONCLUSIONS Highly porous lignocellulose aerogels were prepared from wood via dissolving wood in AMImCl and sequent FT treatment, regeneration, and critical CO2 drying process. It was observed that the FT treatment, used as a physical cross-linking technique, can form continuous 3D network structures. In this project, we examined the low density and high porosity of the aerogels. The shrinkage of the volume was correlated with the FT process, which is considered to be an effective method to prepare lignocellulose aerogels and retain the thermal stability. In addition, the crystallinity could be influenced by the frequency of FT cycles. This study also proposed a possible mechanism of crosslink point formation. Therefore, it seems that four to seven FT cycles could produce porous biomass aerogels with high crystallinity.
’ ASSOCIATED CONTENT
bS
Supporting Information. The specific surface area, pore size, pore volume, and porosity parameters of the raw material and aerogels (S1) and several defined points for the characterization of the thermogravimetric curves of raw material and aerogels (S2). This material is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected], h.zhao@griffith.edu.au.
’ ACKNOWLEDGMENT This work was financially supported by the State Key Program of The National Natural Science Foundation of China (grant no. 30630052). ’ REFERENCES (1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science 2006, 311, 484–489. (2) Huber, G. W.; Dumesic, J. A. Catal. Today 2006, 111, 119–132. (3) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044–4098. (4) Huber, G. W.; Corma, A. Angew. Chem., Int. Ed. 2007, 46, 7184–7201. (5) Vispute, T. P.; Zhang, H.-Y.; Sanna, A.; Xiao, Rui.; Huber, G. W. Science 2010, 330, 1222–1227. (6) Valenzuela, M. B.; Jones, C. W.; Agrawal, P. K. Energy Fuels 2006, 20, 1744–1752. (7) Kim, H.; Ralph, J.; Akiyama, T. Bioenerg. Res. 2008, 1, 56–66. (8) Lynd, L. R.; Weimer, P. J.; van Zyl, W. H.; Pretorius, I. S. Microbiol. Mol. Biol. Rev. 2002, 66, 506–577. (9) Lu, F.-C.; Ralph, J. Plant J. 2003, 35, 535–544. (10) Heinze, T.; Dicke, R.; Koschella, A.; Kull, A. H.; Klohr, E. A.; Koch, W. Macromol. Chem. Phys. 2000, 201, 627–631. (11) Wang, Z.-G.; Yokoyama, T.; Chang, H.-M.; Matsumoto, Y. J. Agric. Food Chem. 2009, 57, 6167–6170. (12) Li, C.-Z.; Wang, Q.; Zhao, Z.-K. Green Chem. 2008, 10, 177–182. (13) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974–4975. (14) Fukaya, Y.; Sugimoto, A.; Ohno, H. Biomacromolecules 2006, 7, 3295–3297. (15) Xie, H.-L.; Shi, T.-J. Holzforschung 2006, 60, 509–512. 1866
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