Research Article pubs.acs.org/journal/ascecg
Biomass-Derived γ‑Valerolactone-Based Solvent Systems for Highly Efficient Dissolution of Various Lignins: Dissolution Behavior and Mechanism Study Zhimin Xue,*,† Xinhui Zhao,‡ Run-cang Sun,‡ and Tiancheng Mu*,† †
Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, No. 17 Qinghua Donglu, Beijing 100083, China ‡ Department of Chemistry, Renmin University of China, No. 59 Zhongguancun Street, Beijing 100872, China S Supporting Information *
ABSTRACT: Binary solvent systems consisting of biomassderived γ-valerolactone (GVL) and one cosolvent (e.g., water, ionic liquids, DMSO, and DMF) were developed as highly efficient systems for dissolution of various types of lignin. It was found that the content of cosolvent in GVL significantly affected the solubility of lignins. More importantly, we first concluded that the relationship between the solubility of lignin and hydrogen bond basicity parameter β value of solvents depends both on the solvent and on the lignin, which clarifies the existing dispute on this topic. Additionally, the dissolved lignin can be easily recovered by the addition of ethanol without its structure noticeably changing. The as-proposed systems are not only mild and highly efficient but also versatile and flexible (with different components and concentrations), thus adapting to the highly diversity of lignin. KEYWORDS: Lignin, Solubility, Solvatochromic parameters, Correlation, γ-Valerolactone, Water, DMSO, Ionic liquids
■
INTRODUCTION With the depletion of fossil resources and the increasing demand for fuels, chemicals, and energy, the utilization of renewable and abundant biomass to take the place of traditional fossil resources has become a hot research field in chemistry.1−3 As one of three major components of lignocellulosic biomass, lignin, consisting of phenyl propanoid units, has attracted much interest as the only aromatic native biopolymer on the earth, which can be utilized as raw materials for materials and valueadded chemicals, such as resins,4,5 surfactants,6,7 and various aromatic chemicals,8−12 etc. However, poor solubility in most commonly used solvents is one of the main challenges for the efficient utilization of lignin. Therefore, many efforts have been devoted for improving the lignin dissolution. Until now, several solvent systems have been developed for lignin dissolution, including ionic liquids (ILs),13−15 ILs−water mixtures,16 and liquid ammonia.17 Very recently, a novel solvent system composed of N-methyl-2-pyrrolidone and C1− C4 carboxylic acid has also demonstrated excellent lignin solubility.18 However, these solvent systems still suffered from various defects. For example, the ILs are usually expensive, very viscous, and not so stable at the high temperature needed for the lignin dissolution, hindering the ILs for large-scale industrial applications.19 For the systems of liquid ammonia, high pressure and low temperature are necessary during the handling of lignin for long time periods.17 More importantly, the © XXXX American Chemical Society
structures and properties of lignin obtained from different plants and preparation methods are not quite similar,20−22 which results in the observation that one good solvent for one kind of lignin might be a poor solvent for other lignins. Hence, finding green, robust, and cost-effective means to dissolve various lignins is still highly desirable. Generally, the desired solvents for lignin should satisfy the following criteria, including being cheap, nontoxic, not easily volatile, less viscous, highly efficient, and easily tuned to fit different types of lignin. γ-Valerolactone (GVL), a naturally occurring chemical in fruits, can be derived from carbohydrate-based biomass and widely used as an intermediate for chemicals and a food or fuel additive. Due to the inertness toward oxygen and water, high boiling and flash points, low melting point, and vapor pressure,23,24 GVL-based solvents have been proposed by Horváth et al. as sustainable solvents for diverse purposes.25,26 However, using GVL as a sustainable solvent is only the beginning stage.27,28 Continued and extensive efforts need to be focused on developing more applications for GVL. Since aprotic solvent dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) and ionic liquids are known good solvents for lignin,29−31 and water is also used to mix with other Received: March 30, 2016 Revised: May 18, 2016
A
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Lignin Solvent in GVL/Water, GVL/DMSO, GVL/DMF, GVL/[Bmim]OAc, and GVL/[Amim]Cl
Figure 1. Solubilities (g/100 g solvent) of DAL in different solvent systems at different temperatures.
solvents for lignin dissolution,16,32 thus, in this work, we designed novel binary solvent systems consisting of GVL and another appropriate solvent, including water, DMSO, DMF, 1butyl-3-methylimidazolium acetate ([Bmim]OAc), and 1-allyl3-methylimidazolium chloride ([Amim]Cl), for efficient dissolution of lignin at near room temperature (Scheme 1). We found that these proposed binary solvent mixtures have excellent performance on the dissolution of various types of lignin, including dealkaline lignin (DAL), enzymatic hydrolysis lignin (EHL), kraft lignin (KL), and organosolv lignin (OL).
■
Chemical Industry (TCI, Shanghai). The enzymatically hydrolyzed lignin (EHL) was isolation from cellulolytic enzyme hydrolysis of corncob and supplied by Shandong Longlive Bio-Technology Co., Ltd. (Shandong, China).32 Formic acid organosolv lignin (OL) was prepared from dewaxed bamboo (Yunnan, China) according to ref 33. Kraft lignin was obtained from Sigma-Aldrich Chemical Co. The Reichardt’s dye (30), 4-nitroaniline, Nile red, and N,N-dimethylformamide (DMF) were obtained from J&K Scientific. The dimethyl sulfoxide (DMSO) was bought from Beijing Yili Fine Chemicals. The N,N-diethyl-4-nitroaniline was obtained from Fluorochem (Old Glossop, U.K.). All the above chemicals and reagents were used directly without further purification. The ionic liquids 1-butyl-3methylimidazolium acetate ([Bmim]OAc) and 1-allyl-3-methylimidazolium chloride ([Amim]Cl) were purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China), with a purity over
EXPERIMENTAL SECTION
Materials. Lignin (dealkaline, from North American soft wood), γvalerolactone (GVL), and 4-nitroanisole were purchased from Tokyo B
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering 99.9%. [Bmim]OAc and [Amim]Cl was dried at 60 °C under vacuum for 96 h before use; water content of the ionic liquids is less than 1000 ppm after the drying process. Solubility of Lignin. In a typical experiment, 1.0 g of GVL-based solvent was added to a 25 mL glass flask. The flask was then immersed in an oil bath (DF-101S, Henan Yuhua Instrument Factory), and the temperature instability of the oil bath was estimated to be ±1.0 K. After the temperature reaches the desired value, ca. 0.01 g lignin sample was added into the solution. The mixture was kept at an appropriate temperature and stirred under N2 gas. Additional lignin (another 0.1 wt % of the solvent) was added after the added lignin was totally solubilized, and the above procedure was repeated until lignin could not be dissolved under a certain temperature within 2 h. For each situation, the temperature was increased by 10 K, from 313 to 343 K. Solvatochromic Parameters Measurement. The dyes used for solvatochromic parameters measurement were first dissolved in methanol with a concentration of 1.0 × 10−3 mol/L. Each time, 50 μL of dye solution was added into a centrifuge tube. The residual methanol was then carefully removed by vacuum drying at 40 °C for 30 min. After the methanol was removed, the dye was mixed with 2 g solution for examination. The dye/solutions were put into quartz cells with 1.0 cm light path length. The absorption spectra were recorded with a Varian Cary 50 UV−vis spectrophotometer at room temperature (25 °C). The dyes used in the measurement, detailed procedures, as well as equations used for calculating the solvatochromic parameters were given in the Supporting Information.
Table 1. Maximum Solubility of DAL, EHL, KL, and OL in Different GVL-Based Binary Solvent Systems solubility (g/100 g solvent) solvent syst
T (K)
DAL
EHL
KL
OL
GVL/water
313 343 313 343 313 343 313 343 313 343
38.1 53.9 20.5 44.1 11.1 17.6 6.6 13.9 6.5 28.2
16.3 44.9 21.1 23.5 12.6 20.5 7.2 13.8 16.1 20.5
23.5 35.1 22.9 31.9 23.5 33.3 20.9 28.0 13.4 26.6
12.6 23.8 13.1 20.5 6.6 17.9 11.2 20.8 9.1 15.6
GVL/DMSO GVL/DMF GVL/[Bmim]OAc GVL/[Amim]Cl
solvents at the same temperature (Figure 1 and Figures S1− S3). For temperature effect, the solubility increases on average 88% from 313 to 343 K. For DAL in GVL/[Amim]Cl the solubility even increases from 6.5 (at 313 K) to 28.2 (at 343 K), which indicates that temperature has a significant effect on the solubilities of lignin in various solvents. Among the investigated solvent systems, the GVL/water system has relatively good solvation ability for different types of lignins (Figure 2). For example, the solubility of DAL in GVL/water is much higher than that in GVL/ILs and DMF, while for EHL, the difference still exists but is not so significant. However, we still could say the dissolution behavior of EHL, KL, and OL in the proposed GVL-based systems was generally consistent with the results for DAL. Most of the solubility data of lignin that have been reported are based on mixed solvents. As far as we know, no solubility data of DAL in pure DMSO, water, DMF, and the ILs used in this study have been reported. However, the solubility data of KL in DMSO and some kinds of ionic liquids have been reported.29−31 The reported solubility of KL (pine) in DMSO at 90 °C was larger than 20 wt %,29 which is consistent with our result of solubility (ca. 17 g/100 g solvent) of KL (from SigmaAldrich Chemical Co.) at 70 °C. The solubility data of lignin in different ionic liquids are very different and can vary from insoluble for [Bmim]BF6 to 344 g/L for [Mmim][MeSO4],30 which is not easy to explain. This supports the conclusion that the dissolution of lignin is very complicated. The difference among the solubility values of various lignins in the same solvent is understandable because it is well-known that the structure and property of lignin prepared from different methods are very different.22 Lignin is a highly functionalized biomacromolecule, which contains a variety of active functional groups, such as phenolic, hydroxyls, methoxyl, carboxylic, and benzyl hydrogen.34,35 The solubility of lignin is mainly influenced by the structure, such as the structural fragment, the interunit linkages (especially β-O-4), and the degree of lignin grafted after treating raw lignin with different methods. Besides that, molecular weight and the carbohydrate impurities of lignin also have a significant influence on its solubility.32,33 For example, DAL and KL have different structural units.36 In the meantime, they have different residue solution pH values in the treated process. The β-O-4 of the DAL was more seriously broken than the other lignins. These factors all influence the lignin solubility in a certain solvent. Mechanism of Dissolution. As discussed above, good solubility of lignins could be achieved in GVL-based binary solvent systems, and the content of cosolvent affected the
■
RESULTS AND DISCUSSION Solubility of Lignin. The solubility of lignin in GVL-based solvent systems was determined initially using commercially available dealkaline lignin (DAL). Figure 1 shows the solubilities of DAL in different solvent systems at different temperatures. Notably, GVL/water is the most efficient system for DAL dissolution in our present work (Figure 1A−E). As shown in Figure 1A, the water content has a key effect on the solubility of DAL. Our experiment indicated that the DAL did not dissolve in pure GVL or water (Figure 1F). However, with increasing water content from 0 to 50 wt %, the solubility of DAL increased dramatically, and then reached the maximum value at a water content of 50 wt %. However, when the water content further increased from 50 wt % to 100 wt %, the DAL solubility slowly decreased. The temperature could also affect the solubility of DAL in GVL/water system. The solubility increased with the temperature from 313 to 343 K, and the value for the DAL solubility was 53.9 g lignin/100 g solvent at 343 K with a water content of 50 wt %. To our surprise, the solubility of DAL in GVL/water could reach 38.1 g lignin/100 g solvent even at 313 K. Cosolvents also have significant effects on the solubility of lignin which can be seen from Figure 1, and the solubility at a given temperature generally decreases in the following order: GVL/water > GVL/DMSO > GVL/[Amim] Cl > GVL/[Bmim]OAc > GVL/DMF. Besides, the solubility of DAL in pure GVL or other cosolvent was much lower than the results in binary solvent systems (Figure 1F). These results suggested that the designed GVL-based binary solvent systems were superior for the dissolution of DAL. Delighted by the excellent performance of GVL-based binary solvent systems for DAL dissolution (Figure 1), we further examined the solubilities of EHL, KL, and OL in the abovementioned solvent systems at different temperatures, and the results were presented in Table 1 and Figures S1−S3. The asproposed solvent systems also had good solubility for EHL, KL, and OL. However, the solubilities of these three lignins are commonly lower than that of DAL in the corresponding C
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Solubilities (g/100 g solvent) of different lignins in GVL/H2O solvent systems at different temperatures and water contents.
water acts as an antisolvent. We also showed the 1H NMR measurements of the GVL/DMSO-d6 mixtures (Figure S4). It was shown that each of H chemical shifts of GVL was not obviously changed after addition of different concentrations of DMSO-d6. Comparing this with the systems of GVL/D2O, we can conclude that no hydrogen bond exists in GVL/DMSO-d6. GVL/D2O can offer more hydrogen bond acceptors and donors to interact with lignin, so more lignin solubility can be obtained in the GVL/D2O system. The Kamlet−Taft hydrogen bond basicity parameter (β value) describes the ability of the solvent system to donate electrons to form a hydrogen bond with protons from solute. The β value (Tables S1−S5) shows that the maximum value corresponds to the maximum lignin solubility. In this way the mechanism of dissolution can be explained by the formation of strong hydrogen bond interactions between the solute and the solvent system. The solubility of biomass polymers including chitosan, cellulose, and lignin might have a relationship with the Kamlet−Taft empirical parameters (e.g., α, β, π*) of the solvents,39−41 which are used to describe the polarity of solvent systems.42,43 Studies have been carried out to verify this hypothesis. Unfortunately, no certain and convincing conclusions have been obtained.15,44 To give a thorough and deep understanding of the present solvent systems and further our understanding of the relationship between the solubility of lignin and the β value, the Kamlet−Taft parameters were determined (Tables S1−S5), and the solubilities of lignin versus β values of solvents were plotted in Figure 4. The adjusted R2 values of all systems are given in Table S6. For the same lignin (let us take DAL as an example, other lignins have similar results) in different solvents, Figure 4, Figure S5, and Table S6 show that there is no correlation between solubility of lignin and the β value of the solvent (for GVL/water and GVL/[Amim]Cl), but there are some relationships (for GVL/[Amim]Cl and GVL/DMSO) and closer relationships (for GVL/DMF) between the solubility and β value. Also, for different lignins in the same solvent (here we take GVL/water as an example because generally lignins have relatively higher solubility in GVL/water), Table S6 indicates that there is no correlation between solubility of lignin
performance significantly. When water/DMSO/DMF/ILs was added into GVL as a cosolvent, the solvent property may be changed and varied with the change of the cosolvent content. We examined the intrinsic interaction between GVL and water by 1H nuclear magnetic resonance (NMR) spectra (Figure 3).
Figure 3. 1H NMR spectra of GVL/D2O solvent systems at various concentrations. From bottom to top: 10, 30, 50, and 70 wt % D2O, respectively.
It was found that the signal of each H in GVL shifted downfield, indicating the existence of a strong hydrogen bond between GVL and water and the property change of the solvent system compared with the pure GVL and water. The change of the solvent property for GVL-based binary solvent systems was beneficial for the break of the strong hydrogen bonding in lignin and the combination of the aromatic nucleus and aliphatic chain regions, and thus resulted in much higher lignin solubility. In addition, solvent viscosity is another significant factor that affects the lignin solubility in the present solvent systems. Higher viscosity may slow down the lignin transportation, leading to the poor solubility of lignin. This may be the reason that GVL/[Bmim]OAc and GVL/[Amim]Cl had lower solubility for lignins.37,38 The 1H NMR shifts cannot explain the observation that the solubility decreases when a water content of 50% is exceeded. It can be explained that when the water content is above 50%, GVL can interact with a water molecule strongly, and no extra hydrogen bond acceptor and donor can be offered to interact with lignin. In this situation, D
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. Relationship of lignin (DAL, EHL, KL, and OL) solubility vs β value for GVL-based solvent systems.
Figure 5. SEM images of the (A) initial lignin (DAL) and (B) after dissolution in GVL/water (10 wt %) solvent system and regeneration into ethanol.
and the β value of the solvent (for DAL, −0.032), but there are some relationships (for EHL, 0.224 and OL, 0.489) and closer relationships (for KL, 0.846) between the solubility and β value. Since there is the complication of the dissolution mechanism of lignins induced by the diversity of structure of lignins, the correlation between two other solvatochromic parameters α and π* of GVL-based solvents with the solubility of various lignins was carried out to better clarify the relationships between the solubility of various lignins and solvatochromic parameters (Figures S5−S7 and Tables S6−S7). Overall, β values follow the trend in lignin solubility in some extent; however, the linear relationship is very weak (Figures S5 and Tables S1−S6). The relationship between α and π* of the selected solvents with the solubility of lignins is even poorer than that of β (Figures S6 and S7 and Table S7). Thus, we could conclude that whether the solubility of lignin has a close relationship with the value of solvatochromic parameters or not depends both on the solvent nature and on the lignin type. It is noteworthy that in most cases the R2 values are too low to derive meaningful relationships between the β value and the solubility of lignin. This conclusion might be extrapolated to other biomass polymers such as cellulose and chitosan, and related investigations are carried out in our group.
Finally, the recovery of the dissolved lignin (DAL) from GVL/water solvent system was examined, and we found that the lignin could be easily precipitated from GVL/water solvent system by the addition of ethanol. The regenerated lignin materials were characterized by scanning electron microscopy (SEM) and two-dimensional heteronuclear single quantum coherence (2D-HSQC) NMR spectra. From the SEM images in Figure 5, we can observe the bulk structure of the initial lignin and the regenerated one by ethanol after the dissolution in GVL/water (10 wt %) solvent system. The initial lignin has a relatively more homogeneous surface structure, while the regenerated lignin shows a little looser architecture. On the other hand, it was worth noting that the regenerated lignin samples contain some small fragments with a cross-linked network structure, which may be caused by the interaction between the lignin and GVL/water solvent system. From the 2D-HSQC NMR spectra the complete structural information on lignin could be obtained. The side chain regions (δC/δH) lie in about 50−90/2.8−6.2, while aromatic regions (δC/δH) locate at about 100−150/6.2−8.0. The insets of the Figure 5 indicate that β-O-4 structures (Aγ) have decreased slightly and the methoxy groups (OMe) have increased a little after the dissolution and regeneration. However, the difference E
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
ACS Sustainable Chemistry & Engineering is not so significant.45 The full 2D-HSQC NMR spectra (Figure S8) confirmed that only slight changes in lignin structure could be found after regeneration. The Fourier-transform infrared (FT-IR) spectra (Figure S9), elemental analyses (EA) (Table S8), and X-ray photoelectron spectroscopy (XPS) (Figure S10 and Table S9) indicated that the backbone of the lignin structure (detailed assignment of peaks was given in Figure S5), the elemental composition of the lignin, and the surface electronic states of lignin do not change noticeably, respectively, which is consistent with the 2D-HSQC NMR spectra results. We can also conclude from the above characterization of lignins that GVL-based solvent systems are mild for lignin dissolution. Please note that the precipitation of lignin from GVL/water solution by the addition of a third solvent is technically not suitable for a commercial scale. Since the solubilities of various lignins in the mixture of GVL and water decrease significantly with the water content, it is easy to precipitate the lignins by the addition of more water, which would act as an antisolvent.
ACKNOWLEDGMENTS
■
REFERENCES
(1) Besson, M.; Gallezot, P.; Pinel, C. Conversion of Biomass into Chemicals over Metal Catalysts. Chem. Rev. 2014, 114, 1827−1870. (2) Straathof, A. J. J. Transformation of Biomass into Commodity Chemicals Using Enzymes or Cells. Chem. Rev. 2014, 114, 1871−1908. (3) He, M.; Sun, Y.; Han, B. Green Carbon Science: Scientific Basis for Integrating Carbon Resource Processing, Utilization, and Recycling. Angew. Chem., Int. Ed. 2013, 52, 9620−9633. (4) Karumuri, S.; Hiziroglu, S.; Kalkan, A. K. Thermoset-CrossLinked Lignocellulose: A Moldable Plant Biomass. ACS Appl. Mater. Interfaces 2015, 7, 6596−6604. (5) Liu, W.; Zhou, R.; Goh, H. L. S.; Huang, S.; Lu, X. From Waste to Functional Additive: Toughening Epoxy Resin with Lignin. ACS Appl. Mater. Interfaces 2014, 6, 5810−5817. (6) Qian, Y.; Zhang, Q.; Qiu, X.; Zhu, S. CO2-responsive diethylaminoethyl-modified lignin nanoparticles and their application as surfactants for CO2/N2-switchable Pickering emulsions. Green Chem. 2014, 16, 4963−4968. (7) Gupta, C.; Washburn, N. R. Polymer-Grafted Lignin Surfactants Prepared via Reversible Addition−Fragmentation Chain-Transfer Polymerization. Langmuir 2014, 30, 9303−9312. (8) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344, 1246843. (9) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 2015, 115, 11559−11624. (10) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110, 3552−3559. (11) Fan, H.; Yang, Y.; Song, J.; Meng, Q.; Jiang, T.; Yang, G.; Han, B. Free-radical conversion of a lignin model compound catalyzed by Pd/C. Green Chem. 2015, 17, 4452−4458. (12) Yang, Y.; Fan, H.; Song, J.; Meng, Q.; Zhou, H.; Wu, L.; Yang, G.; Han, B. Free radical reaction promoted by ionic liquid: a route for metal-free oxidation depolymerization of lignin model compound and lignin. Chem. Commun. 2015, 51, 4028−4031. (13) Tan, S. S. Y.; MacFarlane, D. R.; Upfal, J.; Edye, L. A.; Doherty, W. O. S.; Patti, A. F.; Pringle, J. M.; Scott, J. L. Extraction of lignin from lignocellulose at atmospheric pressure using alkylbenzenesulfonate ionic liquid. Green Chem. 2009, 11, 339−345. (14) Mora-Pale, M.; Meli, L.; Doherty, T. V.; Linhardt, R. J.; Dordick, J. S. Room temperature ionic liquids as emerging solvents for the pretreatment of lignocellulosic biomass. Biotechnol. Bioeng. 2011, 108, 1229−1245. (15) Hart, W. E. S.; Harper, J. B.; Aldous, L. The effect of changing the components of an ionic liquid upon the solubility of lignin. Green Chem. 2015, 17, 214−218. (16) Wang, Y.; Wei, L.; Li, K.; Ma, Y.; Ma, N.; Ding, S.; Wang, L.; Zhao, D.; Yan, B.; Wan, W.; Zhang, Q.; Wang, X.; Wang, J.; Li, H. Lignin dissolution in dialkylimidazolium-based ionic liquid−water mixtures. Bioresour. Technol. 2014, 170, 499. (17) Strassberger, Z.; Prinsen, P.; van der Klis, F.; van Es, D. S.; Tanase, S.; Rothenberg, G. Lignin solubilisation and gentle fractionation in liquid ammonia. Green Chem. 2015, 17, 325−334. (18) Mu, L.; Shi, Y.; Chen, L.; Ji, T.; Yuan, R.; Wang, H.; Zhu, J. [NMethyl-2-pyrrolidone] [C1−C4 carboxylic acid]: a novel solvent system with exceptional lignin solubility. Chem. Commun. 2015, 51, 13554−13557. (19) Cao, Y.; Mu, T. Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis. Ind. Eng. Chem. Res. 2014, 53, 8651−8664.
CONCLUSIONS In conclusion, GVL-based binary solvent systems, including GVL/water, GVL/DMSO, GVL/DMF, GVL/[Bmim]OAc, and GVL/[Amim]Cl, have been developed for the efficient dissolution of lignins at low temperatures. It was found that the cosolvent content in GVL-based systems strongly affected the lignin solubility. Among the five examined solvent systems, GVL/water was the most efficient one. The solubility of lignin could reach 38.1 g/100 g even at 313 K with the content of water being 50 wt %. The higher performance can be attributed to the hydrogen bond interaction between lignin and GVL/ water solvent systems as well as the low viscosity. More importantly, we find that whether there is a relationship between the solubility of lignin and β value of solvent depends both on the solvent and the lignin. Besides that, the dissolved lignin can be easily recovered by the addition of ethanol or more water, and the structure of the lignin stays almost unchanged after the dissolution and regeneration. The asdeveloped solvent systems have advantages of renewability, easy preparation, and low cost, and are mild and very efficient for lignin dissolution compared with ILs and other solvents. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00639. Details for procudures; solubilities in as-proposed solvent systems; 1H NMR spectra; relationship of lignin solubility vs β, α, π* value for GVL-based solvent systems; 2D-HSQC NMR spectra; FT-IR spectra; XPS spectra; and CH elemental analyses of initial DAL and regenerated DAL (PDF)
■
■
The authors acknowledge National Natural Science Foundation of China (21503016, 21473252) for financial support.
■
■
Research Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: 00861062337090. *E-mail:
[email protected]. Phone: 00861062514925. Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
the polarity of ionic liquids. Phys. Chem. Chem. Phys. 2011, 13, 16831− 16840. (41) Taft, R. W.; Kamlet, M. J. The solvatochromic comparison method. 2. The alpha-scale of solvent hydrogen-bond donor (HBD) acidities. J. Am. Chem. Soc. 1976, 98, 2886−2894. (42) Xu, A.; Wang, J.; Wang, H. Effects of anionic structure and lithium salts addition on the dissolution of cellulose in 1-butyl-3methylimidazolium-based ionic liquid solvent systems. Green Chem. 2010, 12, 268−275. (43) Sun, X.; Xue, Z.; Mu, T. Precipitation of chitosan from ionic liquid solution by the compressed CO2 anti-solvent method. Green Chem. 2014, 16, 2102−2106. (44) Doherty, T. V.; Mora-Pale, M.; Foley, S. E.; Linhardt, R. J.; Dordick, J. S. Ionic liquid solvent properties as predictors of lignocellulose pretreatment efficacy. Green Chem. 2010, 12, 1967− 1975. (45) Wen, J.-L.; Yuan, T.-Q.; Sun, S.-L.; Xu, F.; Sun, R.-C. Understanding the chemical transformations of lignin during ionic liquid pretreatment. Green Chem. 2014, 16, 181−190.
(20) Garcia Calvo-Flores, F.; Dobado, J. A. Lignin as Renewable Raw Material. ChemSusChem 2010, 3, 1227−1235. (21) Shu, R.; Long, J.; Xu, Y.; Ma, L.; Zhang, Q.; Wang, T.; Wang, C.; Yuan, Z.; Wu, Q. Investigation on the structural effect of lignin during the hydrogenolysis process. Bioresour. Technol. 2016, 200, 14− 22. (22) Constant, S.; Wienk, H. L. J.; Frissen, A. E.; de Peinder, P.; Boelens, R.; van Es, D. S.; Grisel, R. J. H.; Weckhuysen, B. M.; Huijgen, W. J. J.; Gosselink, R. J. A.; Bruijnincx, P. C. A. New insights into the structure and composition of technical lignins: a comparative characterisation study. Green Chem. 2016, 18, 2651. (23) Luo, W.; Sankar, M.; Beale, A. M.; He, Q.; Kiely, C. J.; Bruijnincx, P. C. A.; Weckhuysen, B. M. High performing and stable supported nano-alloys for the catalytic hydrogenation of levulinic acid to γ-valerolactone. Nat. Commun. 2015, 6, 6540. (24) Song, J.; Zhou, B.; Zhou, H.; Wu, L.; Meng, Q.; Liu, Z.; Han, B. Porous zirconium−phytic acid hybrid: a highly efficient catalyst for Meerwein−Ponndorf−Verley reductions. Angew. Chem., Int. Ed. 2015, 54, 9399−9403. (25) Horvath, I. T. Solvents from nature. Green Chem. 2008, 10, 1024−1028. (26) Fegyverneki, D.; Orha, L.; Lang, G.; Horvath, I. T. Gammavalerolactone-based solvents. Tetrahedron 2010, 66, 1078−1081. (27) Luterbacher, J. S.; Rand, J. M.; Alonso, D. M.; Han, J.; Youngquist, J. T.; Maravelias, C. T.; Pfleger, B. F.; Dumesic, J. A. Nonenzymatic Sugar Production from Biomass Using BiomassDerived γ-Valerolactone. Science 2014, 343, 277−280. (28) Liguori, F.; Moreno-Marrodan, C.; Barbaro, P. Environmentally Friendly Synthesis of γ-Valerolactone by Direct Catalytic Conversion of Renewable Sources. ACS Catal. 2015, 5, 1882−1894. (29) Sun, N.; Rodríguez, H.; Rahman, M.; Rogers, R. D. Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass? Chem. Commun. 2011, 47, 1405− 1421. (30) Pu, Y.; Jiang, N.; Ragauskas, A. J. Ionic liquid as a green solvent for lignin. J. Wood Chem. Technol. 2007, 27 (1), 23−33. (31) Yan, P.; Xu, Z.; Zhang, C.; Liu, X.; Xu, W.; Zhang, Z. C. Fractionation of lignin from eucalyptus bark using amine-sulfonate functionalized ionic liquids. Green Chem. 2015, 17, 4913−4920. (32) Yan, B.; Li, K.; Wei, L.; Ma, Y.; Shao, G.; Zhao, D.; Wan, W.; Song, L. Understanding lignin treatment in dialkylimidazolium-based ionic liquid−water mixtures. Bioresour. Technol. 2015, 196, 509. (33) Li, M.-F.; Sun, S.-N.; Xu, F.; Sun, R.-C. Sequential solvent fractionation of heterogeneous bamboo organosolv lignin for valueadded application. Sep. Purif. Technol. 2012, 101, 18−25. (34) Kai, D.; Tan, M. J.; Chee, P. L.; Chua, Y. K.; Yap, Y. L.; Loh, X. J. Towards lignin-based functional materials in a sustainable world. Green Chem. 2016, 18, 1175−1200. (35) Sun, X.; Huang, C.; Xue, Z.; Yan, C.; Mu, T. Efficient and Sustainable Strategy for the Hierarchical Separation of Lignin-Based Compounds Using Ionic Liquid/Compressed CO2. Energy Fuels 2015, 29, 2564−2570. (36) Li, H.; Zhang, Q.; Gao, P.; Wang, L. Preparation and characterization of graft copolymer from dealkaline lignin and styrene. J. Appl. Polym. Sci. 2015, 132, DOI: 10.1002/app.41900. (37) Andanson, J.-M.; Bordes, E.; Devemy, J.; Leroux, F.; Padua, A. A. H.; Gomes, M. F. C. Understanding the role of co-solvents in the dissolution of cellulose in ionic liquids. Green Chem. 2014, 16, 2528− 2538. (38) Sescousse, R.; Le, K. A.; Ries, M. E.; Budtova, T. Viscosity of Cellulose− Imidazolium- Based Ionic Liquid Solutions. J. Phys. Chem. B 2010, 114, 7222−7228. (39) Jessop, P. G.; Jessop, D. A.; Fu, D.; Phan, L. Solvatochromic parameters for solvents of interest in green chemistry. Green Chem. 2012, 14, 1245−1259. (40) Ab Rani, M. A.; Brant, A.; Crowhurst, L.; Dolan, A.; Lui, M.; Hassan, N. H.; Hallett, J. P.; Hunt, P. A.; Niedermeyer, H.; PerezArlandis, J. M.; Schrems, M.; Welton, T.; Wilding, R. Understanding G
DOI: 10.1021/acssuschemeng.6b00639 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
