Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10116-10127
pubs.acs.org/journal/ascecg
Survey of Lignin-Structure Changes and Depolymerization during Ionic Liquid Pretreatment Tanmoy Dutta,†,‡ Nancy G. Isern,§,∥ Jian Sun,†,‡ Eileen Wang,† Sarah Hull,† John R. Cort,§,∥ Blake A. Simmons,†,⊥ and Seema Singh*,†,‡ †
Deconstruction Division, Joint BioEnergy Institute, 5885 Hollis Street, Emeryville, California 94608, United States Biological and Engineering Sciences Center, Sandia National Laboratories, 7011 East Avenue, Livermore, California 94551, United States § Environmental Molecular Sciences Laboratory and Biological Sciences Division, and ∥Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥ Biological Systems and Engineering Division, 1 Cyclotron Road, Berkeley, California 94720, United States ‡
S Supporting Information *
ABSTRACT: A detailed study of chemical changes in lignin structure during the ionic liquid (IL) pretreatment process is not only pivotal for understanding and overcoming biomass recalcitrance during IL pretreatment but is also necessary for designing new routes for lignin valorization. Chemical changes in lignin were systematically studied as a function of pretreatment temperature, time, and type of IL used. Kraft lignin was used as the lignin source, and common pretreatment conditions were employed using three different ILs of varying chemical structure in terms of acidic or basic character. The chemical changes in the lignin structure due to IL pretreatment processes were monitored using 1H−13C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR), 31P NMR, elemental analysis, gel permeation chromatography (GPC), and Fourier transform infrared (FT-IR), and the depolymerized products were analyzed using gas chromatography mass spectrometry (GC-MS). Although, with pretreatment in acidic IL, triethylammonium hydrogensulfate ([TEA][HSO4]) results in the maximum decrease in the β-aryl ether bond, maximum dehydration and recondensation pathways were also evident, with the net process showing a minimum decrease in the molecular weight of regenerated lignin. However, 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) pretreatment yields a smaller decrease in the β-aryl ether content along with minimum evidence of recondensation, resulting in the maximum decrease in the molecular weight. Cholinium lysinate ([Ch][Lys]) pretreatment shows an intermediate result, with moderate depolymerization, dehydration, and recondensation observed. The depolymerization products after IL pretreatment are found to be a function of the pretreatment temperature and the specific chemical nature of the IL used. At higher pretreatment temperature, [Ch][Lys] pretreatment yields guaiacol, [TEA][HSO4] yields guaiacylacetone, and [C2C1Im][OAc] yields both guaiacol and guaiacylacetone as major products. These results clearly indicate that the changes in lignin structure as well as the depolymerized product profile depend on the pretreatment conditions and the nature of the ILs. The insight gained on lignin structure changes and possible depolymerized products during IL pretreatment process would help future lignin valorization efforts in a potential IL-based lignocellulosic biorefinery. KEYWORDS: Lignin, Lignin depolymerization, Ionic liquid, Pretreatment, Cholinium lysinate
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biorefinery configurations it is largely underutilized.2−4 In recent years, lignin valorization has attracted significant interest in the scientific community.3,5−7 Lignin is an amorphous,
INTRODUCTION
Lignocellulosic biomass is considered to be a sustainable and renewable resource for the production of fuels and chemicals.1 It is primarily composed of three major biopolymers, namely, cellulose, hemicellulose, and lignin. Although lignin constitutes 15−30 wt % of biomass and is the second most abundant biopolymer on Earth after cellulose, in terms of proposed © 2017 American Chemical Society
Received: June 28, 2017 Revised: August 9, 2017 Published: September 6, 2017 10116
DOI: 10.1021/acssuschemeng.7b02123 ACS Sustainable Chem. Eng. 2017, 5, 10116−10127
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ACS Sustainable Chemistry & Engineering
Figure 1. Common lignin interunit linkages: (A) β−O−4′, (B) β-5′, (C) β-β′, (D) dibenzodioxocin, (E) 4-O-5′, and (F) 5-5′ linkage and structures of three ILs.
study on the chemical transformation of alkaline lignin under a broad range of pretreatment conditions using the IL 1-ethyl-3methylimidazolium acetate ([C2C1Im][OAc]) and reported dissociation of the β−O−4′ linkage and degradation of β-β′ and β-5′ linkages, primarily at higher pretreatment temperatures.16 Sathitsuksanoh et al. characterized different lignin streams isolated during the pretreatment of wheat straw, miscanthus, and pine with [C2C1Im][OAc] using solution-state two-dimensional (2D) NMR and size exclusion chromatography (SEC). The results suggested that the lignin isolated from the pretreatment of three different feedstocks were depolymerized.13 Since the inception of IL-based pretreatment technology, various types of ILs such as acidic, protic, bionic IL, etc., have been studied and many of them have excellent pretreatment efficiencies.9 However, a comparative analysis of the transformations of lignin that occur during the ILpretreatment process as a function of IL is lacking. In this work, we aim to understand the chemical transformation of lignin during the IL pretreatment process as a function of the IL used. We have employed kraft lignin (KL) as a lignin source and used three different ILs: [C2C1Im][OAc], cholinium lysinate ([Ch][Lys]), and triethylammonium hydrogensulfate ([TEA][HSO4]). KL is one of the most common commercial lignin sources. Apart from KL’s easy availability, the exploration of chemical changes induced during the IL pretreatment process using a relatively inert lignin will provide a baseline for future studies with other lignin sources. [C2C1Im][OAc] is the most studied IL to date for the pretreatment of lignocellulosic biomass and is considered as a benchmark system. [Ch][Lys], often termed as a renewable IL or “bionic liquid”, both the cation and anion are derived from amino acids; it has been demonstrated to have excellent pretreatment efficiencies.25−30 Lastly, [TEA][HSO4] is a protic
heterogeneous aromatic polymer comprised of three phenylpropanoid monomers, namely p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) linked by C−C or C−O bonds. The common lignin interunit linkages include β−O−4′, 5−5′, β−5′, 4−O−5′, β−1, dibenzodioxocin, β−β′, and α−O−4′ (Figure 1). Although the relative proportions differ of these linkages depend on the origin of the lignin, the β−O−4′ bond is the major interunit linkage in lignin, irrespective of the source.8 For both the biofuel and pulp and paper industries, carbohydrate polymers are the main focus and lignin is traditionally separated from the biomass via different pretreatment methods. Ionic liquid (IL) pretreatment of lignocellulosic biomass has been demonstrated to be one of the most efficient pretreatment methods to reduce biomass recalcitrance and to effectively fractionate biomass components.9−12 Among the various factors affecting IL pretreatment efficiency, the ability of the IL to disrupt the lignin−carbohydrate interlinked structure and thereby enable lignin removal was found to be the most important factor for most of the efficient ILs.9 In order to effectively valorize lignin, it is important to understand lignin transformation during the pretreatment process. Previous literature reports confirm that changes in the chemical structure of lignin occur during the IL pretreatment process.13−16 However, few studies to date have focused on providing a detailed understanding of the chemical transformations of lignin during the IL pretreatment process; furthermore, most of them are focused on imidazolium-based ILs.16−24 George et al. reported a systematic study on changes in macromolecular structure of technical lignin during thermal treatment with imidazolium-based ILs having different anions. These studies show significant reductions of lignin molecular weight during the pretreatment process.19 Sun et al. carried out an extensive 10117
DOI: 10.1021/acssuschemeng.7b02123 ACS Sustainable Chem. Eng. 2017, 5, 10116−10127
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ACS Sustainable Chemistry & Engineering
Figure 2. Work flow diagram of different lignin streams in IL pretreatment of KL.
IL and attractive due to its low cost.31 Apart from the structural differences, the acidic or basic characteristics associated with these three ILs are also very different; [Ch][Lys] is strongly basic,11 [TEA][HSO4] is acidic,32 and [C2C1Im][OAc] is near neutral (Table S1). Apart from the variable chemical nature of ILs, we also seek to understand the effect of the pretreatment conditions on the lignin structure and therefore conducted the study using two common pretreatment temperatures and variable pretreatment times. The lignin streams (Figure 2) were characterized using two-dimensional (2D) 13C−1H heteronuclear single quantum coherence (HSQC) and 1D 31P nuclear magnetic resonance (NMR) methods to probe relative changes in the lignin interunit linkages, gel permeation chromatography (GPC) for determining the relative changes in the hydrodyanamic volume or molecular weight, Fourier transform infrared spectroscopy (FT-IR), elemental analysis for probing change in chemical structure, and gas chromatography mass spectrometry (GC-MS) for identifying the monomeric lignin depolymerization products formed during the pretreatment process. These results provide a deep insight about the chemical changes in lignin structure as a function of IL and pretreatment conditions.
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GPC analysis was performed using a Tosoh Ecosec HLC-8320 GPC equipped with a refractive index (RI) and diode array detector (DAD). Separation was achieved with an Agilent PLgel 5 μm Mixed-D column at 35 °C using a mobile phase of THF at a flow rate of 1.0 mL/min. The GPC standards, which contained polystyrene ranging from 162 to 29 150 Da, were purchased from Agilent and used for calibration. Absorbance of materials eluting from the column was detected at 280 nm (UV). NMR samples were prepared by dissolving 50 mg in 600 μL DMSO-d6. Two-dimensional 1H−13C HSQC (adiabatic HSQC, hsqcad.c) spectra were acquired at 298 K on a 600 MHz Varian Direct Drive (VNMRS) spectrometer equipped with a Varian triple resonance salt-tolerant cold probe. Spectra were collected in StatesTPPI mode using 6.1 and 14.5 μs nonselective 90° pulses and 12 and 166 ppm spectral widths for 1H and 13C respectively, 2048 complex points, 256 increments (Varian parameter ni) to give 81 Hz (0.65 ppm) digital resolution in the indirect dimension, 32 transients, and a recycle delay of 1.5 s. The KL and IL-pretreated lignin samples were analyzed by quantitative 31P NMR using published procedures.36 An accurately weighed amount of dried lignin sample (40−45 mg) was dissolved in 500 μL of anhydrous pyridine/CDCl3 mixture (1.6:1, v/ v). A total of 200 μL of an endo-N-hydroxy-5-norbornene-2,3dicarboximide solution (internal standard; 9.23 mg/mL) and 50 μL of a chromium(III) acetylacetonate solution (relaxation reagent; 5.6 mg/ mL) were added to the above pyridine/CDCl3 solution. Finally, 100 μL of phosphitylating reagent II (2-chloro-4,4,5,5-tetramethyl-1,2,3dioxaphospholane) was added and transferred into NMR tube for analysis. One-dimensional 31P spectra were acquired at 298 K on a 500 MHz Varian Inova spectrometer equipped with a Nalorac HCNP triple resonance probe, a 15.6 μs nonselective 90° pulse, 286 ppm spectral width, 64 000 complex points, 1024 transients, 25 s recycle delay, and WALTZ 1H decoupling during the acquisition period. Chemical shifts were referenced to the DMSO peak (δC/δH: 39.50/ 2.50 ppm). Spectra were processed using MestReNova 9.0 (Mestrelab Research, Spain), using cosine bell apodization and 2X zero filling, and referenced to residual protonated DMSO (for 1H and 13C) or indirectly to phosphoric acid (for 31P). Identification of chemical compounds in depolymerization products was carried out using an Agilent 6890N gas chromatography (GC) equipped with Agilent 5973N mass spectrometry (MS). The compounds were separated using an Agilent DB-5MS (30 m × 0.25 mm × 0.25 μm) capillary column. A 1.0 μL portion of the sample with a concentration of 1 g/L in EA was injected into the GC at an inlet temperature of 280 °C and was operated in a split mode (split flow of 30 mL/min, split ratio = 30). Helium was used as a carrier gas with a constant flow rate of 1 mL/min. The temperature of the GC was held at 50 °C for 5 min, was then increased at a rate 5 °C/min up to 300 °C, and was held at this temperature for 5 min. The MS was used until the end of the GC run with a solvent delay of 7.0 min. The ion source was maintained at a temperature of 280 °C, and the MS was operated in scan mode. The instrument control and data processing were carried out using the installed Agilent ChemStation software; to identify each peak, the NIST MS library was used. The monomeric lignin depolymerization products were quantified using calibration curves constructed from the standards.
MATERIAL AND METHODS
Chemicals and Materials. Kraft lignin (KL) was purchased from Sigma-Aldrich (batch number: 1001044421). All other chemicals were reagent grade and purchased from Sigma-Aldrich (St. Louis, MO), unless otherwise noted. [C2C1Im][OAc] was purchased from BASF with a 98% purity (lot no. 11-0005) and used as received. [Ch][Lys] and [TEA][HSO4] were synthesized following previously described literature procedures.33,34 IL Pretreatment. KL (0.5 g) was mixed with IL (4.50 g) in a glass pressure tube with a magnetic stirrer and pretreated in an oil bath at an array of times and temperatures under air. After pretreatment, the slurry was cooled to room temperature and diluted with 10 mL deionized (DI) water. The lignin was isolated (L1) by adjusting the pH of the solution to 2−3 using 6 N HCl and the solution was kept in a 4 °C refrigerator overnight to ensure total precipitation. The content was transferred to a 50 mL Falcon tube with an additional 20 mL DI water and centrifuged at high speed (8000 rpm) for 15 min to separate solids. The clear supernatant liquid was carefully aspirated and the resulting solid lignin was additionally washed with DI water and centrifuged (each 35 mL) five times. The regenerated solid lignin (L1) was then freeze-dried. To the first wash (supernatant liquid), 10 mL ethyl acetate (EA) was added and mixed vigorously. Then the organic layer was separated and the supernatant liquid was washed with additional 10 mL of EA. The combined organic layer was then dried over anhydrous magnesium sulfate and solvent was removed to isolate the monomeric lignin depolymerization products (L2). Characterization of Lignin. The molecular weight distribution of lignin was investigated using gel permeation chromatography (GPC). The lignin was acetylated with pyridine and acetic anhydride following a previously published procedure.35 The acetylated lignin was dissolved in tetrahydrofuran (THF) with a concentration of 1 g/L. 10118
DOI: 10.1021/acssuschemeng.7b02123 ACS Sustainable Chem. Eng. 2017, 5, 10116−10127
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ACS Sustainable Chemistry & Engineering Elemental analysis was performed on a CHNS 2400 PerkinElmer series II analyzer. The oxygen content was obtained by subtracting the contents of other elements. The C900 formula and degree of unsaturation (DU) were calculated, on the basis of the contents of various elements.
with a minor contribution of syringyl (S) units identified by the cross peaks at δC/δH: 110.9/6.98 (G2), 114.7/6.74 (G5), 119.0/ 6.80 (G6), and 104.0/6.72 (S2,6) respectively. The HSQC analysis also shows distinct β−O−4′ aryl ether (A), β−β′ (resinol, B), and β−5′ (phenylcoumaran, C) interunit linkages. The β−O−4′ aryl ether (A) was identified by the cross peaks at δC/δH: 71.8/4.88 (Aα), 83.6/4.32 (Aβ(G/H)), and 59.5−59.7/ 3.40−3.63 (Aγ(G/H)), whereas β−β′ (resinol, B), β−5′ (phenylcoumaran, C) interunit linkages were identified by cross peaks at δC/δH 84.8/4.69 (Bα), 86.7/5.49 (Cα), 81.0/5.10 (Dα), 53.5/ 3.10 (Bβ), and 53.4/3.49 (Cβ) respectively. The quantitative evaluation of interunit linkages was performed using an internal standard that represents aromatic C9 units in the lignin. The C2−H2 position of the G-unit and C2,6−H2,6 positions of S-unit are considered as stable. The total amount of C9 units present in the lignin can be estimated by the sum of half of C2,6−H2,6 signal of S-unit plus C2−H2 signal of G-unit. The relative amounts of interunit linkages are estimated from the volume integrals of the Aα, Bα, and Cα correlations and expressed as a fraction of 100 aromatic C9 units; B-unit integrals are divided by 2 as a resinol unit contains two C/H pairs per unit. In the untreated lignin (KL) among all the interunit linkages, the β− O−4′ linkage dominates with 12/100Ar; apart from that, other interunit linkages like β−β′ (3/100Ar) and β−5′ (5/100Ar) linkages were also identified. The HSQC analysis of the pretreated lignin clearly demonstrated the decrease in the Aα (β−O−4′) content, which could be a manifestation of depolymerization, dehydration or recondensation pathways involving β−O−4′ subunit of lignin during IL-pretreatment. The effect of pretreatment temperature on the chemical structure of lignin was first explored. KL was pretreated in two common pretreatment temperatures, 140 and 160 °C for 1 h.29,46 For all three ILs studied, higher pretreatment temperature favors a decrease in β−O−4′ content. Between these three ILs, the acidic IL ([TEA][HSO4]) shows the maximum temperature effect. A 25% increase in the β−O−4′ degradation is observed for [TEA][HSO4], as compared to 8% for [C2C1Im][OAc] and [Ch][Lys] by increasing the pretreatment temperature from 140 to 160 °C. At 140 °C, for all three IL pretreated lignin samples, no changes were observed for β−5′ linkages, but at 160 °C, a slight β−β′ degradation was observed. To determine the effect of time, pretreatment experiments were carried out at 160 °C for 1, 2, and 3 h, respectively, which are common biomass IL-pretreatment conditions. All three ILs show a high β−O−4′ bond degradation rate in the first hour which then levels off with an increase in pretreatment time. The effect of the type of IL on the chemical changes in the lignin structure during pretreatment shows some interesting trends. Among various lignin interunit linkages, a decrease in the β−O−4′ content was observed for all three ILs. [Ch][Lys] and [C2C1Im][OAc] show comparable levels of β−O−4′ bond degradation. The acidic IL ([TEA][HSO4]) was found to be most effective, with β−O−4′ bond degradation 2−3 fold higher than that for the other two ILs. The ILs show a different trend for the change in β−β′ interunit content after pretreatment. Both [Ch][Lys] and [C2C1Im][OAc] pretreatment were found to be more effective in decreasing the β−β′ content than the acidic IL. The dibenzodioxocin linkage was more or less unchanged during the pretreatment with all three ILs. From the relative changes in the different interunit linkages of lignin during the pretreatment in three different ILs, it is evident that
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RESULTS AND DISCUSSION Theoretical Survey of Lignin Interunit Linkages. The presence of various types of interunit linkages in lignin and the considerable variation in relative proportions of these linkages depending on the type or source of the lignin creates a complex heterogeneous structural network. The structural complexity of lignin poses a challenge for lignin depolymerization studies. Theoretical investigations of lignin linkages have been an area of research. These theoretical studies were conducted on lignin model compounds mimicking the interunit linkages present.37−41 The calculated bond dissociation energies (BDE) of different lignin bonds provide an idea about the lignin depolymerization chemistry. Lignin interunit linkages can be classified into two broad categories, namely, ether (C−O) and carbon−carbon (C−C) covalent linkages. In a particular substructure, the ether linkages typically have lower BDEs than carbon−carbon linkages. Parthasarathi et al. did a theoretical study involving 65 different lignin model compounds and reported their BDEs. The results show that α-O-4′ followed by β−O−4′ is the weakest among various ether linkages and β-1 followed by β-5′ are the strongest among various C−C linkages.37 Beckham et al., Younker et al., and Elder et al. did thorough theoretical studies of lignin linkages using different model compounds; a summary of BDEs from these studies of different covalent bonds in various lignin substructures (Figure 1) is listed in Table 1.37−42 Table 1. Bond Dissociation Energies of Common Lignin Interunit Linkages substructure
bond type
BDE (kcal/mol)
β−O−4′
Cβ−O−C4′ Cα−Cβ Cα−O−C4′ Cα−Cβ Cα−O/Cγ−O Cα−Cβ/Cβ−Cβ′ C4−O−C5′ C5−C5′
54−72 75−80 50−56 54−63 68/79 67/81 78−83 115−118
β−5′ β−β′ 4−O−5′ 5−5′
Although BDEs depend also on the substitutions present and can vary in an actual macromolecular lignin structure, this simple comparison gives a general idea about the relative bond strengths of major lignin interunit linkages. Characterization of Regenerated Lignin (L1). 2D-HSQC NMR Analysis. The changes in the chemical structure of the regenerated lignin (L1) in different IL pretreatment conditions were analyzed using 1H−13C HSQC NMR (Figures S1−4, Supporting Information). A comparison of untreated lignin (KL) with regenerated lignin provides an insight into the changes in the chemical structure in lignin as a function of pretreatment conditions. Table 2 tabulates different pretreatment conditions and the changes in the interunit linkages, and Figure 1 depicts the chemical structures of the interunit linkages. The peaks were assigned following literature protocols.43−45 The HSQC analysis of lignin samples indicates that initial lignin is primarily comprised of guaiacyl (G) units 10119
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Table 2. Relative Amounts of Interunit Linkages in L1 Determined Using 1H−13C HSQC NMR Spectra and Yields of L2 in Different Processing Conditions lignin interunit linkagec lignin KL E-140-1 E-160-1 E-160-2 E-160-3 C-140-1 C-160-1 C-160-2 C-160-3 T-140-1 T-160-1 T-160-2 T-160-3 a
IL [C2C1Im][OAc] [C2C1Im][OAc] [C2C1Im][OAc] [C2C1Im][OAc] [Ch][Lys] [Ch][Lys] [Ch][Lys] [Ch][Lys] [TEA][HSO4] [TEA][HSO4] [TEA][HSO4] [TEA][HSO4]
T (°C)
a
140 160 160 160 140 160 160 160 140 160 160 160
t (h)
b
L2 yield (%)
β−O−4′
β−5′
β−β′
8.4 8.4 8.0 6.7 2.6 3.5 3.9 4.8 2.1 2.0 2.0 1.8
12 12 11 10 10 12 11 11 10 10 7 5 5
5 5 5 5 5 5 5 5 5 5 5 5 5
3 3 2 2 2 3 2 2 2 3 2 2 2
1 1 2 3 1 1 2 3 1 1 2 3
Prossesing temperature. bProcessing time. cAbundances of different interunit linkages are expressed as per 100 aromatic units.
Table 3. Quantitative Estimation of Hydroxyl Groups in KL and Different Lignin Streams by Quantitative 31P NMR KL E-160-3 C-160-3 T-160-3
aliphatic−OH
condensed phenolic−OH
noncondensed phenolic OH
total phenolic−OH
carboxylic acid
3.44 3.31 2.58 1.29
2.37 2.24 1.63 2.73
1.32 1.39 1.08 1.61
3.69 3.63 2.71 4.34
0.38 0.44 0.26 0.17
[Ch][Lys] and [C2C1Im][OAc] behave similarly and that these two ILs differ significantly from [TEA][HSO4]. Quantitative 31P NMR. Quantitative 31P NMR analysis of phosphitylated lignin is a useful tool for estimating the amounts and distributions of different hydroxyl (alcohol) groups such as aliphatic hydroxyl, condensed and noncondensed phenolic hydroxyls and carboxylic acid groups in lignin.47 Figure S5 and Table 3 show the quantitative 31P spectra and relative amounts of different hydroxyl groups of KL and lignin isolated after IL pretreatment at 160 °C for 3 h in the three different ILs. The aliphatic hydroxyl content was found to be lower for all three IL pretreated lignin samples as compared to KL. The highest reduction was observed for [TEA][HSO4] pretreated lignin (∼62%), and the lowest observed for [C2C1Im][OAc] pretreatment (∼4%). This reduction in the aliphatic hydroxyl group content can be attributed to factors such as loss of the γmethylol group as formaldehyde, oxidation of Cα−OH groups to ketones, and loss of whole side chains to form β-1 linkages.48 This result corroborates the β−O−4′ degradation trend observed in HSQC NMR measurements presented above. Both condensed and noncondensed phenolic hydroxyl content was found to be higher for T-160-3 as compared to KL and lignins generated from [C2C1Im][OAc] and [Ch][Lys] pretreatment. The increase in the total phenolic hydroxyl groups in [TEA][HSO4] pretreated lignin as compared to the KL may suggest the acid-catalyzed cleavage of aryl ether bonds during IL pretreatment. As compared to KL, [Ch][Lys]pretreated lignin shows a reduction in aliphatic hydroxyl groups, and the total amount of phenolic hydroxyl groups was also found to be lower. This observation may suggest that, in contrast to [TEA][HSO4] pretreatment, [Ch][Lys] pretreatment leads to less aryl ether cleavage. The [C2C1Im][OAc] pretreated lignin demonstrates very little change in terms of the aliphatic and phenolic hydroxyl content as compared to KL, suggesting a minor chemical change during the pretreatment
process. The total phenolic to aliphatic hydroxyl content ratio in the case of [C2C1Im][OAc] pretreated lignin was found to be similar, and lower for [Ch][Lys] pretreated lignin, as compared to the KL. However, [TEA][HSO4] pretreatment shows an increase in the ratio of total phenolic- to aliphatichydroxyl content. This high aliphatic to total phenolic hydroxyl ratio and increase in the condensed phenolic hydroxyl content indicate high dissociation of phenolic ether linkages and subsequent higher recondensation via nonclassical linkages, ether bonds, and C−C coupling during the [TEA][HSO4]pretreatment process.49 The carboxyl content was found to be higher in the case of [C2C1Im][OAc]-pretreated lignin and lower in the cases of [TEA][HSO4] and [Ch][Lys] pretreated lignin as compared to KL. Molecular Weight Analysis. A depolymerization reaction via dissociation of β−O−4′ interunit linkage is expected to result in a decrease in the observed molecular weight, and any recondensation reactions would contribute to an increase in the molecular weight of lignin. At any point in the IL pretreatment process, depending on the conditions, competing depolymerization and recondensation reaction pathways could occur simultaneously and their net effect will be reflected in the lignin molecular weight. The regenerated lignin after pretreatment (L1) was characterized by GPC to obtain the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI). The results are tabulated in Table 4 and Figure S6. Because polystyrene is a random coil polymer, it is not an ideal reference for lignin; the molecular weight numbers give a rough estimation for comparisons between untreated and pretreated lignin.50 From the GPC elugram depicted in Figure S6, the main peak shows a gradual shift to the lower molecular weight region, with the largest shift observed for [C2C1Im][OAc] and lowest for [TEA][HSO4] pretreatment. Although all three IL pretreated lignins show 10120
DOI: 10.1021/acssuschemeng.7b02123 ACS Sustainable Chem. Eng. 2017, 5, 10116−10127
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corresponding to C−H stretching of methyl and methylene groups and the methyl group of methoxyl, respectively. Various peaks characteristic to aryl ring stretching, aromatic skeleton vibration, and aromatic C−H deformation were observed and listed in Table 5.
Table 4. Molecular Weight Results of KL and Regenerated Lignin (L1) after Pretreatment at 160 °C for 3 h L1
Mn (Da)
Mw (Da)
PDI
KL E-160-3 C-160-3 T-160-3
505 319 383 646
1347 830 979 1200
2.67 2.60 2.56 1.86
Table 5. FT-IR Band Assignments for Lignin Described in Figure 5
some lower molecular weight peaks, the main peak remains unchanged, indicating no major change in molecular weight. The starting KL shows Mw of 1347 Da and a PDI of 2.67. The molecular weights of L1 after pretreatment in three different ILs at 160 °C for 3 h gives an idea about the net change in the lignin molecular weight during pretreatment. The Mw of L1 from [C2C1Im][OAc] pretreatment shows a minimum value of 830 Da, whereas, [TEA][HSO4] pretreatment shows the highest Mw of 1200 Da and [Ch][Lys] shows an intermediate value of 979 Da. From the GPC results, it is evident that the hydrodynamic volume of the lignin after [TEA][HSO4] pretreatment is highest, and [C2C1Im][OAc] pretreatment is the lowest. The higher Mw of L1 in spite of having higher β−O−4′ dissociation, suggests possible recondensation pathways during the [TEA][HSO4] pretreatment process. FT-IR Characterization of Lignin. The KL and the acid precipitated lignin (L1) after pretreatment with three different ILs at the most severe pretreatment conditions (160 °C for 3 h) were characterized by FT-IR spectroscopy (Figure 3). Table 4 summarizes the band assignments of the lignin samples. The band assignments were carried out according to previously reported literature.51,52 As depicted in Figure 3, the FT-IR of all lignin show bands corresponding to predominately G-units. The three pretreated lignin samples shows similar spectral features to the initial KL, indicating that no major change in the main chemical structure of the lignin backbone occurs. However, changes in the relative intensity of the bands were observed, indicating relative changes in the functional groups during the IL pretreatment process. All lignin samples exhibit a broad band centered around 3430 cm−1 corresponding to the OH stretching, and peaks centered at 2938 and 2856 cm−1
number
wavenumber
assignment
1 2 3
3430 2938 1704
4 5 6 7 8
1680 1600 1513 1466 1428
9 10 11 12
1270 1226 1142 1085
13 14 15
1035 864 742
O−H stretching, H-bonded C−H stretching CH3 and CH2 groups CO stretching in unconjugated ketones, carbonyl, ester CO stretching in conjugated carbonyl, carboxyl aryl ring stretching, symmetric aryl ring stretching, asymmetric C−H deformation, asymmetric aromatic skeletal vibration combined with C−H in plane deformation guaiacyl ring and CO stretching C−C, C−O, and CO stretching aromatic C−H in plane deformation C−O deformation, secondary alcohol and aliphatic ether aromatic C−H in plane deformation guaiacyl ring C−H deformation out of plane guaiacyl ring skeletal deformation, substituent groups and side chains
During the IL pretreatment new bands near 1704 and 1680 cm−1, which were not distinct in KL, become prominent, The band centered at 1704 cm−1 can be assigned to carbonyl stretching in unconjugated ketones, carbonyls, and ester groups, whereas the 1680 cm−1 band is due to carbonyl stretching of conjugated carbonyl and carboxyl groups. The pretreated lignin also shows increased intensities for 1270 and 1226 cm−1 bands, which are associated with CO stretches. The increase in intensity of these peaks may arise from the oxidation of lignin (or formation of Hibbert ketones) during pretreatment. As evident in the Figure 3, these peaks are most
Figure 3. FT-IR spectrum of KL and regenerated lignin (L1) at 160 °C for 3 h. 10121
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Research Article
ACS Sustainable Chemistry & Engineering
accordingly. The degree of unsaturation (DU) in the KL was found to be 371, which was increased in all the lignin streams isolated after IL pretreatment, implicating an increase in the unsaturated bonds. Among the three pretreated lignin streams, lignin pretreated with [Ch][Lys] at 160 °C for 3 h (C-160-3, Table 6) demonstrated the lowest DU and lignin pretreated with [TEA][HSO4] at 160 °C for 3 h (T-160-3, Table 6) shows the highest DU. The HSQC NMR analysis of these lignin streams shows similar trends for the β−O−4′ degradation pattern, which can give rise to formation of unsaturated double bonds. Characterization of EA Soluble Depolymerized Lignin (L2). EA Soluble Depolymerized Lignin (L2). The lignindepolymerized products were extracted from the IL reaction mixture using EA, and the yields are listed in Table 1 and depicted in Figure 4. It is worth noting that some of the depolymerized lignin oligomers or monomeric lignin depolymerization products have different solubility in different ILs and can preferentially extracted by EA or remain in the IL solution and not be extracted. However, the EA extractable ligninderived compounds from these three different reaction mixtures give a relative measure of lignin depolymerization during the IL pretreatment process. As depicted in Figure 4, among three ILs, the highest yield of L1 was observed for [C2C1Im][OAc] pretreatment and lowest was observed for [TEA][HSO4] pretreatment. This data could be easily correlated with the GPC data obtained for L1, demonstrating the regenerated lignin size was highest for [TEA][HSO4] pretreatment and lowest for [C2C1Im][OAc] pretreatment. No distinct or convincing effect of pretreatment condition (either temperature or time) on the L2 yield was observed for [C2C1Im][OAc] and [TEA][HSO4] pretreatment. However, a steady increase in the L2 yield with increasing pretreatment severity was observed for [Ch][Lys] pretreatment. GC-MS Characterization of the Depolymerized Products. The oil obtained from the pretreatment process (L2) was characterized by GC-MS and depolymerized products were identified using the NIST mass spectral library. The yields of the major monomeric lignin depolymerization products are depicted in Figure 5. Apart from the calibrated monomeric products; the EA-extracted oil is composed of many other aromatic monomeric products, listed in Table S2. Nearly 18 aromatic compounds were identified and most of them are phenolic in nature. Some of the identified aromatic monomeric lignin depolymerization products also contain functional groups such as aldehyde, ketone, and carboxylic acids. Among the
prominent for the [TEA][HSO4] pretreatment and least for [Ch][Lys] pretreatment. The data suggests that [TEA][HSO4] pretreatment generates both unconjugated and conjugated carbonyl groups due to oxidation of lignin. Also a reduction in intensity of 1600 cm−1 band, assigned to aromatic skeletal vibration, was observed after IL pretreatment. This effect was more prominent for [TEA][HSO4] and [Ch][Lys] pretreatments. The intensity reduction of this band is believed to be a result of condensation reactions and/or splitting of aliphatic side chains of lignin.53,54 Elemental Analysis. To get more insight into the chemical changes of the lignin streams, elemental analysis on the lignin streams pretreated with three different ILs at the most severe pretreatment condition (160 °C for 3 h) was performed and compared with that of KL. The elemental composition and C900 formula along with the degree of unsaturation are listed in Table 6. The KL control shows 4.02% sulfur due to the nature Table 6. Elemental Analysis of KL and Regenerated Lignin (L1) at 160 °C for 3 h lignin
C (%)
H (%)
O (%)
S (%)
C900 formula
DU
KL E-160-3 C-160-3 T-160-3
48.79 53.78 49.79 52.47
4.82 4.97 4.67 4.71
42.22 24.72 26.99 24.00
4.02 2.63 2.55 1.02
C900H1059O585S28 C900H990O309S16 C900H1006O367S17 C900H962O309S7
371 406 398 420
of the lignin (Kraft process) and the %S decreases with IL pretreatment process. A similar decrease in sulfur content of KL after pretreatment was also observed in a few recent reports.55,56 It is also reported that the low molecular weight fractions of KL have higher sulfur content as compared to the high molecular weight fractions.56 Thus, removal of lower molecular weight fractions of lignin with high sulfur content during the pretreatment process could account for the observed reduction in the %S. The loss in the sulfur content could also arise from the loss of inorganic sulfur present in KL as contaminant. However, the %N in the starting KL was very negligible (
