Efficient and Sustainable Strategy for the Hierarchical Separation of

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Efficient and Sustainable Strategy for the Hierarchical Separation of Lignin-Based Compounds Using Ionic Liquid/Compressed CO2 Xiaofu Sun,† Chengyi Huang,† Zhimin Xue,‡ Chuanyu Yan,† and Tiancheng Mu*,† †

Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Lignin has received increasing attention as a potential starting material for providing valuable low-molecularweight aromatic compounds. Herein, a strategy for lignin degradation and low-molecular-weight compound hierarchical separation is presented. Ionic liquid and compressed CO2 were used in the two-step protocol. 1-Butyl-3-methylimidazolium acetate was used to induce the cleavage of C−O and C−C linkage as well as dehydration reaction of lignin at high temperatures, especially combined with sonication assistance. The low-molecular-weight aromatic compounds from lignin can be followed to separate efficiently and hierarchically by adjusting the pressure of compressed CO2. The size distribution and chemical structure of lignin samples were investigated to explain the mechanism, indicating that our method is easy and efficient.

1. INTRODUCTION The incentive from the shortage of fossil sources and sustainable development is motivating extensive research on new and alternative fuels or materials derived from biomass.1−3 Lignocellulosic biomass consists mainly of polymeric carbohydrates (cellulose and hemicellulose) and the aromatic polymer lignin, and each of them possesses unique structural and physicochemical properties, which offer specific opportunities to satisfy a multitude of applications. Among them, lignin occupies about 20−30% of wood and is mostly presented as an intractable byproduct in the pulp and paper industries.4 It has received increasing attention as a potential precursor or complex starting material for providing valuable low-molecular-weight aromatic compounds (see Scheme S1 of the Supporting Information).5−7 Some traditional methods, such as sulfite, kraft, and soda pulping, have been used in processing lignin in industry.6 However, these harsh and energyconsuming conditions have limits in producing quantities of value-added products. Toward this objective, some environmentally benign production processes have been investigated, including oxidative, reductive, and redox-neutral catalytic degradation reactions.6,8−13 Besides, the conversion of fine chemicals from lignin by microbial action could also provide additional benefits in terms of availability.14 However, these approaches also reveal significant shortcomings that have yet to be addressed. These degradation processes require relatively high temperatures. Most importantly, the products cannot be in situ obtained hierarchically. Therefore, it is necessary to develope more mild and efficient methods for obtaining value-added products from lignin. Composed entirely of ions, ionic liquids (ILs) are solventfree ionic conductors with outstanding solvation potential, excellent chemical and thermal stability, and negligible volatility and molecular tunability.1,15 Applications of ILs in diverse fields, such as catalytic reaction, functional materials, gas absorption, extraction and separation, and life science, have been extensively developed.16 During the past decade, ILs have © XXXX American Chemical Society

been investigated as solvents for biomass pretreatment, including cellulose, chitin, and lignin.17−23 It is believed that some chemical transformations of lignin would be raised and controlled during the IL treatment process. CO2 is abundant and nontoxic, and has moderate critical temperature and pressure (31.1 °C and 7.38 MPa).24 The properties of solvents can be changed by the addition of CO2 and be tuned continuously through controlling the temperature or pressure of CO2. Compressed CO2 can be easily recaptured and recycled, and thus, it can be used as an anti-solvent for solute separation selectively and circularly.25,26 As shown in Scheme 1, in this work, we report a strategy for lignin degradation and hierarchical separation that involves a two-step method. In the first step, 1-butyl-3-methylimidazolium acetate ([Bmim]OAc) was used to induce the cleavage of C−O and C−C linkages as well as dehydration reaction of lignin (30 wt % in IL) at high temperatures. It is followed by highly efficient and hierarchical separation of low-molecular-weight aromatic compounds by controlling the conditions of compressed CO2.

2. RESULTS AND DISCUSSION 2.1. Effect of Compressed CO2 on the Size Distribution of Different Lignin Samples. We began our investigation with exploring the size distribution and molecular weight of lignin samples. Consistent with screening of intramolecular electrostatic interactions by interactions between particles,27 the average particle size decreased with increasing concentration (3−15 mM) for the native lignin (LigN) in aqueous solutions (Table 1) according to the dynamic light scattering (DLS) experiments. Figure 1 presents the size distribution of seven lignin samples in aqueous solutions with the same concentration (12 mM). In solutions, lignin exists as Received: February 11, 2015 Revised: March 5, 2015

A

DOI: 10.1021/acs.energyfuels.5b00334 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Scheme 1. Schematic Illustration for Lignin Degradation and Separation

including using ethanol (Lig-E) and compressed CO2 (Lig-C), have a smaller average particle size than Lig-N. In the meantime, the molecular weights of the lignin samples were estimated by gel permeation chromatography (GPC) and shown in Table S1 of the Supporting Information. The decrease of Mw and Mn can provide important insights into lignin depolymerization and fragmentation reactions during the IL dissolution process. The use of compressed CO2, based on its special anti-solvent property, could be precipitated selectively and considerably different from ethanol. It indicated that some small molecular fractions may be lost during the dissolution and regeneration processes. Lignin could be precipitated incompletely using compressed CO2 as an antisolvent. Through the change of the CO2 pressure, Lig-C6 has a

Table 1. Effect of the Concentration of Lig-N on Its Average Particle Size in Aqueous Solution average particle size, d (nm) entry

concentration (mM)

Pk1

Pk2

Pk3

1 2 3 4 5

3 6 9 12 15

19.43 18.70 15.79 15.32 12.90

328.0 322.4 318.1 313.2 312.5

5026 4982 4937 4857 4834

both a single molecule and aggregates. It can be seen clearly that all of the recovered [Bmim]OAc-treated lignin samples,

Figure 1. Size distribution of different lignin samples in aqueous solutions, including (A) Lig-N, (B) Lig-E (using ethanol) (black) and Lig-SE (using ethanol under assistance of sonication) (red), (C) Lig-C6 (6 MPa CO2) (black) and Lig-SC6 (using 6 MPa CO2 under assistance of sonication) (red), and (D) Lig-C18 (18 MPa CO2) (black) and Lig-SC18 (using 18 MPa CO2 under assistance of sonication) (red). B

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Energy & Fuels larger size with a narrower size distribution than Lig-C18, thus resulting in the reduced polydispersity of lignin. Hydrogen bonds between neighboring hydroxyl groups, ether groups, and carboxylic groups and non-bonded orbital interaction (π−π aromatic ring interactions) among aromatic moieties have been proposed to be responsible for lignin macromolecular association.28 After dissolving in [Bmim]OAc, both of the interactions may be changed. Figure 2A shows the

Table 2. Thermal Response Properties of Lignin Samples Obtained from Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC)

a

entry

sample

Td (°C)

Tg (°C)

RW800a (%)

1 2 3 4 5 6 7

Lig-N Lig-E Lig-SE Lig-C6 Lig-SC6 Lig-C18 Lig-SC18

201 163 150 171 162 168 155

155 128 117 131 124 130 118

32 22 17 28 15 24 18

Residual weight of the lignin sample at 800 °C.

our method, which indicates that the molecular weight plays a key role on them. It is very important in lignin applications, such as carbon fiber precursors.33 In the case, more residual weight could be obtained using compressed CO2 than ethanol, which is highly beneficial for cost saving and sustainable development. 2.3. Effect of Compressed CO2 on the Chemical Structure of Different Lignin Samples. Lignin possesses polydispersity in both molecular weight and loading density of the functional groups. The X-ray photoelectron spectroscopy (XPS) survey spectra of the seven samples were very similar (see Figure S2 of the Supporting Information). Their elemental compositions were overall the same, with an impurity S content of less than 1%. The C 1s and O 1s core-level spectra were fitted to obtain more information about the functional groups. (see Figures S3−S9 of the Supporting Information). According to the binding energies of component peaks (see Table S2 of the Supporting Information),34 the relative content of each component peak of C 1s and O 1s spectra from XPS was summarized in Table 3. The chemical structure of lignin was changed significantly. Generally, the decrease of the Csp2 relative content and the increase of the Csp3 relative content in the regenerated lignin correlate to the lower Tg values and char yield. Among them, we found that, in comparison to traditional anti-solvent, compounds of larger particle size and lower polarity should be precipitated from [Bmim]OAc using compressed CO2. Simultaneously, we can selectively obtain the hierarchical small lignin-based molecules via controlling the pressure of CO2. As shown in the present work, smaller lignin fragments and more sp3 carbon atoms can be found in Lig-C18 than Lig-C6. Nuclear magnetic resonance (NMR) was also performed to characterize the information of the functional groups in each lignin sample in further detail. Figure S10 of the Supporting Information shows the change of the linkages of lignin samples during the dissolution and regeneration processes. It can be observed that some signals were changed in their chemical shifts or intensities. These changes of the chemical groups agreed well with the majority of the fitting of the core-level XPS spectra. The complex signals appearing around 100−110 ppm in the regenerated lignin spectra can be attributed to the newly formed S and G units. It implied that some cleavage of aliphatic C−O linkage may occur in [Bmim]OAc at high temperatures. The signals of aromatic C−O (140−162 ppm) and aromatic C−C (123−140 ppm) had no obvious differences in these samples, indicating that [Bmim]OAc and CO2 had a slight influence on the aromatic ring structure of lignin. The aliphatic C−C signal (0−50 ppm) can be presented in the regenerated lignin. It can be explained by the generation of some straight-

Figure 2. (A) Fluorescence spectra of Lig-N aqueous solutions with different concentrations, excited at a wavelength of 450 nm. From the bottom to the top are 3, 6, 9, 12, and 15 mM, respectively. (B) Changes of wavelength reached half of the fluorescence intensity at 450 nm in the normalized excitation spectra of seven lignin aqueous solutions, which were monitored at 530 nm.

emission spectra of Lig-N in aqueous solutions with different concentrations. The maximum emission band can be found at 531 nm. The excitation spectra of Lig-N monitored at an emission wavelength of 531 nm are shown in Figure 2B. According to the molecular exciton coupling theory and model (see Figure S1 of the Supporting Information),29,30 the red shift observed in the fluorescence spectra indicated that the aromatic rings of Lig-N may form a head to tail arrangement, corresponding to the J-aggregation. Similarly, the regenerated lignin samples exist as J-aggregation in aqueous solutions, which indicated that π−π arrangement type remains unchanged in the process of dissolution and regeneration. Studies have shown that a carboxylation reaction may exist in the system of [Bmim]OAc and CO2, leading to the formation of the carboxylate zwitterions imidazolium-2-carboxylate ([Bmim+− COO−]).31,32 As a result, the hydrogen bonds between lignin and IL may be disrupted. In the meantime, the volume expansion and change of solvatochromic parameters caused by CO2 may be dependent upon the pressure and temperature. Therefore, lignin fragments with different sizes could be precipitated hierarchically. In addition, we emphasize that the smaller lignin average size could be observed because of the assistance of sonication (LigSE and Lig-SC), which can be attributed to the further dissociation of Lig-N. In combination with the data of GPC, our study indicated that the polydispersity of lignin reduced at least 20%. 2.2. Effect of Compressed CO2 on the Thermal Properties of Different Lignin Samples. The thermal response properties have been found to be altered in the process of lignin dissolution and regeneration. As shown in Table 2, the decomposition temperature (Td) and glass transition temperature (Tg) decrease in the order: Lig-N > Lig-C6 > Lig-C18 > Lig-E > Lig-SC > Lig-SE. Lignin fragments can be tuned to achieve the desired thermal properties using C

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Energy & Fuels Table 3. Relative Content of Each Component Peak of C 1s and O 1s Spectra from XPS relative content of each component peak of C 1s (%) sample Lig-N Lig-E Lig-SE Lig-C6 Lig-SC6 Lig-C18 Lig-SC18

a

Csp2 36 30 12 35 23 28 26

Csp3 23 35 53 42 30 46 40

C−COOR

C−OR

CO

COOR

a

18 20 16 15 8 14 3 15 18 23 13 12 18 9 relative content of each component peak of O 1s (%)

NF NF NF NF NF NF NF

3 4 13 5 6 1 7

sample

PhO, Ph−CO, O−O

CO, COOH

R−OH, C−O−C

Ph−OH, C−O

chemisorbed H2O

Lig-N Lig-E Lig-SE Lig-C6 Lig-SC6 Lig-C18 Lig-SC18

20 25 24 12 36 33 23

48 21 22 49 34 37 38

14 24 28 11 13 25 11

11 22 10 23 9 3 27

7 8 16 5 8 2 1

NF = not found.

chain and small molecular compounds generated in the process. As expected, the degradation of C−C or C−O linkage and the formation of small aliphatic molecules were even more acute under the assistance of sonication. The altered chemical composition can be matched well with the thermal properties of lignin samples as well as the size of lignin discussed above. In comparison to aromatic groups, aliphatic groups have more pliability, which allows for a free volume expansion and volatilization under high temperatures.34 Obviously, the regenerated lignin from [Bmim]OAc with more aliphatic carbon atoms has lower thermal stability and residual weight. To quantify the major hydroxyl groups in different lignin, 31P NMR spectroscopy (Figure 3) was performed to follow the Figure 4. Changes of the hydroxyl group content of different lignin samples according to 31P NMR spectra, including (a) carboxylic acid −OH, (b) H unit −OH, (c) C/G unit −OH, (d) S unit −OH, and (e) aliphatic −OH. The relative content is the ratio of each lignin and LigN.

be increased in the regenerated lignin samples, implying the cleavage of C−O linkages in the lignin degradation processes. It corresponds well with the XPS spectra and cross-polarization/ magic angle spinning (CP/MAS) 13C NMR spectra. The contents of carboxylic acid −OH are almost the same because of the dehydration reaction rather than the oxidation reaction during the IL dissolution and anti-solvent regeneration. In addition, it can be found that the lignin-based compounds with more C/G unit and S unit −OH and less aliphatic −OH were prior to precipitated from [Bmim]OAc when using compressed CO2. The ratio is closely sensitive to the pressure of compressed CO2. Considering the fact of incomplete lignin precipitation using CO2, many small molecules, mainly the aliphatic compounds, were still in [Bmim]OAc.35 These compounds can also be obtained by the addition of ethanol. The range of formed products was then investigated by highperformance liquid chromatography (HPLC) measurements, allowing for the separation and analysis of small molecular lignin-based compounds, which are more valuable. The results in Table S4 of the Supporting Information confirmed that more

Figure 3. Quantitative 31P NMR spectra of different lignin samples (from the bottom to the top is Lig-N, Lig-E, Lig-SE, Lig-C6, Lig-SC6, Lig-C18, and Lig-SC18, respectively).

changes. Here, we used cyclohexanol as an internal standard and calculated the integrated peak areas of lignin derivatized with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholate (TMDP). The results for different hydroxyl group contents are listed in Table S3 of the Supporting Information. It can be found that aliphatic −OH is the most in the lignin, which mainly comes from the side chain of lignin. As shown in Figure 4, the content of aliphatic −OH is decreased after dissolving in IL, which can be attributed to the dehydration of lignin. In contrast, the C/G unit and S unit −OH contents are found to D

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4. EXPERIMENTAL SECTION

aromatics can be precipitated using compressed CO2 because of the selectivity of regeneration. More aliphatic alcohols and esters may still remain in IL. It also indicated that the disruption of the C−C and C−O bonds occurred in the dissolution process. Thanks to the different HPLC retention times of small molecular lignin-based compounds, we can obtain these highly value-added low-molecular-weight aromatic compounds from the native lignin directly without any additional catalyst. 2.4. Effect of Compressed CO2 on the Morphologies of Different Lignin Samples. The transmission electron microscopy (TEM) images of different lignin samples are shown in Figure 5 and Figure S11 of the Supporting

4.1. Samples and Reagents. Lignin (lignin, alkali: Aldrich 471003-100G) was purchased from Sigma-Aldrich (St. Louis, MO). The native lignin samples used were dried in the vacuum at 100 °C for about 24 h. [Bmim]OAc (>99% purity) was purchased from Lanzhou Greenchem ILs, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), China (Lanzhou, China). N2 (99.999%) and CO2 (99.995%) were purchased from Beijing Beiwen Gas Chemical Industry Co., Ltd. (Beijing, China). Other reagents mentioned in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. Unless otherwise specified, all reagents were used as received. 4.2. Lignin Dissolution and Hierarchical Precipitation Experiments. The dissolution of lignin was carried out in a flask, which is immersed in an oil bath with vigorous magnetic stirring under an open atmosphere. In a typical trial, 0.05 g of lignin powder was added to 5 g of dried [Bmim]OAc under the continuous and mild mechanic stirring at 120 °C. Then, additional lignin (0.1 wt % of the IL) was added until the solution became clear. By repeating this operation, 30 wt % lignin/[Bmim]OAc solution was obtained. For the precipitation of lignin, our experimental apparatus consisted of a high-pressure view cell, a high-pressure syringe pump, a gas cylinder, a water bath, a magnetic stirrer, and a pressure gauge. The specific experiment was in accordance with our previous method. Briefly, the lignin solution was loaded into the view cell, which was placed in a water bath. After air was removed, CO2 was charged into the cell until the thermal equilibrium was reached. The system was kept up under certain conditions for several hours and then removed under vacuum to exhaust CO2. The regenerated lignin using CO2 was obtained. The regeneration experiment was carried out at a constant temperature of 25 °C. 4.3. Dynamic Light Scattering (DLS) Measurements. According to the date of elemental analysis, the contents of elemental carbon, hydrogen, and oxygen of the present lignin were measured to be 47.34, 4.85, and 39.69 wt %, respectively. The average phenylpropanoid unit of lignin can be calculated to be C9H9.77O5.39(OCH3)0.73 based on the above date, with a monomer molecular weight of 227. The DLS experiments were performed using the Zetasizer Nano instrument (Malvern Instruments, Worcestershire, U.K.). A total of 2− 3 mL of lignin aqueous solution was centrifuged at 12 000 rpm for 10 min, and the supernatants were transferred to a fluorescence cuvette for the measurements. 4.4. Fluorescence Measurements. The ultraviolet−visible (UV− vis) absorption measurements were performed on a Cary 50 UV−vis spectrophotometer (Agilent, Santa Clara, CA). For the lignin samples with water as the solvent, water was scanned as a baseline. The fluorescence measurements were monitored using a F-4500 fluorescence spectrometer (Hitachi) with a programmable temperature controller (PolyScience). 4.5. Thermal Property Measurement. The TG curves were determined with a TA Instruments Q50-TG thermal analyzer using platinum crucibles. A total of 5−10 mg of lignin was loaded in the platinum crucible, and the measurements were carried out under flowing N2 at a heating rate of 10 °C min−1. DSC (Q-2000, TA Instruments) was used to determine Tg values at a heating rate of 10 °C/min under a N2 atmosphere. 4.6. XPS Analysis. XPS analysis of the lignin samples were performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically, the hydrocarbon C 1s line at 284.8 eV from adventitious carbon is used for energy referencing. 4.7. Quantitative 31P NMR and Solid-State CP/MAS 13C NMR. For the quantitative 31P NMR spectra, 10 mg of lignin was dissolved in 0.6 mL of pyridine and deuterated chloroform (1.6:1, v/v) solution. This was followed by the addition 0.1 mL of cyclohexanol as an internal standard and 3 mg of chromium(III) acetylacetonate as the relaxation reagent. Then, the mixture was reacted with 0.1 mL of phosphitylating reagent (TMDP), so that the hydroxyl groups of lignin

Figure 5. TEM images of lignin samples: (A) Lig-N, (B) Lig-SE, (C) Lig-C18, and (D) Lig-SC18.

Information. The morphologies of the native and regenerated lignin are significantly different. Lig-N has a relatively more homogeneous surface structure, while Lig-E shows a loose architecture. On the other hand, because of the change of the state of aggregation, Lig-C6 and Lig-C18 have a slight and crimped network with particular transparency. With the increase of the pressure, the flocculent morphology shows much higher density at 18 MPa. Additionally, it is worth noting that the lignin samples contain some small fragments with a cross-linked network structure under the assistance of sonication. Especially, the utilization of compressed CO2 may lead to the formation of many inerratic particles.

3. CONCLUSION In summary, we have hierarchically separated the lowmolecular-weight lignin-based compounds by degradation lignin in IL and precipitation using compressed CO2. The regenerated lignin samples with different chemical structures and size distribution have been proposed by controlling the pressure of compressed CO2 and the assistance of sonication. The cleavages of the C−O linkage, the degradation of the C−C linkage, and the dehydration reaction have been demonstrated to occur in these processes. We believe that the highly simple, efficient, and sustainable route has very wide applications in the production of quantities of value-added products from lignin directly. E

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Engineering plants and enzymes for biofuels production. Science 2007, 315 (5813), 804−807. (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 (37), 9620−9633. (4) Nanayakkara, S.; Patti, A. F.; Saito, K. Chemical depolymerization of lignin involving the redistribution mechanism with phenols and repolymerization of depolymerized products. Green Chem. 2014, 16 (4), 1897−1903. (5) Feghali, E.; Cantat, T. Unprecedented organocatalytic reduction of lignin model compounds to phenols and primary alcohols using hydrosilanes. Chem. Commun. 2014, 50 (7), 862−865. (6) Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. A photochemical strategy for lignin degradation at room temperature. J. Am. Chem. Soc. 2014, 136, 1218−1221. (7) Kim, K. H.; Brown, R. C.; Kieffer, M.; Bai, X. Hydrogen-donorassisted solvent liquefaction of lignin to short-chain alkylphenols using a micro reactor/gas chromatography system. Energy Fuels 2014, 28 (10), 6429−6437. (8) Hanson, S. K.; Wu, R.; Silks, L. A. C−C or C−O bond cleavage in a phenolic lignin model compound: Selectivity depends on vanadium catalyst. Angew. Chem., Int. Ed. 2012, 51 (14), 3410−3413. (9) Rahimi, A.; Azarpira, A.; Kim, H.; Ralph, J.; Stahl, S. S. Chemoselective metal-free aerobic alcohol oxidation in lignin. J. Am. Chem. Soc. 2013, 135 (17), 6415−6418. (10) Zakzeski, J.; Jongerius, A. L.; Weckhuysen, B. M. Transition metal catalyzed oxidation of Alcell lignin, soda lignin, and lignin model compounds in ionic liquids. Green Chem. 2010, 12 (7), 1225−1236. (11) Jiang, Z.; He, T.; Li, J.; Hu, C. Selective conversion of lignin in corncob residue to monophenols with high yield and selectivity. Green Chem. 2014, 16 (9), 4257−4265. (12) Onwudili, J. A.; Williams, P. T. Catalytic depolymerization of alkali lignin in subcritical water: Influence of formic acid and Pd/C catalyst on the yields of liquid monomeric aromatic products. Green Chem. 2014, 16 (11), 4740−4748. (13) Zhang, J.; Asakura, H.; van Rijn, J.; Yang, J.; Duchesne, P.; Zhang, B.; Chen, X.; Zhang, P.; Saeys, M.; Yan, N. Highly efficient, NiAu-catalyzed hydrogenolysis of lignin into phenolic chemicals. Green Chem. 2014, 16 (5), 2432−2437. (14) Kataeva, I.; Foston, M. B.; Yang, S.-J.; Pattathil, S.; Biswal, A. K.; Poole, F. L., II; Basen, M.; Rhaesa, A. M.; Thomas, T. P.; Azadi, P.; Olman, V.; Saffold, T. D.; Mohler, K. E.; Lewis, D. L.; Doeppke, C.; Zeng, Y.; Tschaplinski, T. J.; York, W. S.; Davis, M.; Mohnen, D.; Xu, Y.; Ragauskas, A. J.; Ding, S.-Y.; Kelly, R. M.; Hahn, M. G.; Adams, M. W. W. Carbohydrate and lignin are simultaneously solubilized from unpretreated switchgrass by microbial action at high temperature. Energy Environ. Sci. 2013, 6 (7), 2186−2195. (15) Cevasco, G.; Chiappe, C. Are ionic liquids a proper solution to current environmental challenges? Green Chem. 2014, 16 (5), 2375− 2385. (16) Petkovic, M.; Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S. Ionic liquids: A pathway to environmental acceptability. Chem. Soc. Rev. 2011, 40 (3), 1383−1403. (17) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc. 2002, 124 (18), 4974−4975. (18) Wu, Y.; Sasaki, T.; Irie, S.; Sakurai, K. A novel biomass-ionic liquid platform for the utilization of native chitin. Polymer 2008, 49 (9), 2321−2327. (19) George, A.; Tran, K.; Morgan, T. J.; Benke, P. I.; Berrueco, C.; Lorente, E.; Wu, B. C.; Keasling, J. D.; Simmons, B. A.; Holmes, B. M. The effect of ionic liquid cation and anion combinations on the macromolecular structure of lignins. Green Chem. 2011, 13 (12), 3375−3385. (20) Sun, X.; Tian, Q.; Xue, Z.; Zhang, Y.; Mu, T. The dissolution behaviour of chitosan in acetate-based ionic liquids and their interactions: From experimental evidence to density functional theory analysis. RSC Adv. 2014, 4 (57), 30282−30291.

were derivatized with TMDP. After that, the mixture was transferred into a 5 mm NMR tube and 31P NMR spectroscopy was performed on a Bruker DMX 300 spectrometer. For CP/MAS 13C NMR spectra, 100 mg of each lignin sample was performed in the experiment. The spectra were obtained at 100.4 MHz using a Bruker Avance III 400 M spectrometer. 4.8. HPLC Measurements. The liquid chromatography (LC) separations were analyzed by HPLC with a Shimadzu LC-15C pump, a Shimadzu UV−vis SPD-15C detector at 274.0 nm, and a Supelcosil LC-18 5 μm column at 35 °C. The mobile phase A was water + 1% acetic acid, and the mobile phase B was methanol + 1% acetic acid. The solvent gradient rate was used at 1 mL min−1. The mobile phase B was held at 20% for 1 min and raised to 90% in 15 min. It was kept for 5 min, and then the gradient was returned to 20% for re-equilibration. 4.9. Morphology by TEM. The morphologies of the different lignin samples were examined by TEM (JEOL JEM-2010).

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ASSOCIATED CONTENT

S Supporting Information *

Structures for lignin oligomer (L-1, L-2, and L-3) and some aromatic units degraded from lignin, including syringyl (S), guaiacyl (G), p-hydroxyphenyl (H), p-hydroxybenzoic acid (HA), and catechol (C) (Scheme S1), weight-average (Mw) and number-average (Mn) molecular weight of the different lignin samples (Table S1), schematic representation of the different types of π−π aggregation and their energy levels (Figure S1), XPS survey spectra of different lignin samples: (A) Lig-N, (B) Lig-E, (C) Lig-SE, (D) Lig-C6, (E) Lig-SC6, (F) Lig-C18, and (G) Lig-SC18 (Figure S2), binding energies of C 1s and O 1s component peaks used for the fitting of XPS core level spectra (Table S2), fitting of C 1s and O 1s core level XPS spectra for Lig-N (Figure S3), fitting of C 1s and O 1s core level XPS spectra for Lig-E (Figure S4), fitting of C 1s and O 1s core level XPS spectra for Lig-SE (Figure S5), fitting of C 1s and O 1s core level XPS spectra for Lig-C6 (Figure S6), fitting of C 1s and O 1s core level XPS spectra for Lig-SC6 (Figure S7), fitting of C 1s and O 1s core level XPS spectra for Lig-C18 (Figure S8), fitting of C 1s and O 1s core level XPS spectra for Lig-SC18 (Figure S9), 13C solid-state CP/MAS NMR spectra of different lignin samples, including (a) Lig-N, (b) Lig-C18, and (c) Lig-SC18 (Figure S10), quantification of the hydroxyl group content of different lignin samples according 31P NMR spectra (Table S3), mass yields of the products in different lignin samples according to HPLC measurement (Table S4), and TEM images of lignin samples: (A) Lig-E, (B) Lig-C6, and (C) Lig-SC6 (Figure S11). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +8610-62514925. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21473252) for financial support. REFERENCES

(1) Martinez-Palou, R.; Luque, R. Applications of ionic liquids in the removal of contaminants from refinery feedstocks: An industrial perspective. Energy Environ. Sci. 2014, 7 (8), 2414−2447. (2) Himmel, M. E.; Ding, S.-Y.; Johnson, D. K.; Adney, W. S.; Nimlos, M. R.; Brady, J. W.; Foust, T. D. Biomass recalcitrance: F

DOI: 10.1021/acs.energyfuels.5b00334 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b00334 Energy Fuels XXXX, XXX, XXX−XXX