Article pubs.acs.org/JAFC
Selective Cleavage of the Aryl Ether Bonds in Lignin for Depolymerization by Acidic Lithium Bromide Molten Salt Hydrate under Mild Conditions Xiaohui Yang,†,‡ Ning Li,‡ Xuliang Lin,‡ Xuejun Pan,*,‡ and Yonghong Zhou† †
Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042, China Department of Biological Systems Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States
‡
S Supporting Information *
ABSTRACT: The present study demonstrates that the concentrated lithium bromide (LiBr) solution with acid as catalyst was able to selectively cleave the β-O-4 aryl ether bond and lead to lignin depolymerization under mild conditions (e.g., in 60% LiBr with 0.3 M HCl at 110 °C for 2 h). Four industrial lignins from different pulping and biorefining processes, including softwood kraft lignin (SKL), hardwood kraft lignin (HKL), softwood ethanol organosolv lignin (EOL), and acid corncob lignin (ACL), were treated in the LiBr solution. The molecular weight, functional group, and interunit linkages of the lignins were characterized using GPC, FTIR, and NMR. The results indicated that the β-O-4 aryl ether bonds of the lignins were selectively cleaved, and both LiBr and HCl played crucial roles in catalyzing the cleavage of the ether bonds. KEYWORDS: lignin, depolymerization, lithium bromide, inorganic molten salt hydrate, aryl ether bond
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INTRODUCTION Driven by the inevitable depletion of fossil fuels and increasing environmental concerns, lignocellulosic biomass has been identified as a potential feedstock and sustainable resource for the production of liquid fuels and chemicals. Significant progress has been made in the conversion of the carbohydrates (cellulose and hemicelluloses) in the lignocellulosic biomass into biofuels (such as cellulose bioethanol, butanol, and biohydrocarbon fuels) and biobased chemicals (such as lactic acid, succinic acid, levulinic acid, and their derivatives).1 However, lignin, the most abundant natural aromatic polymer on earth, retains the least utilized biomass component because of its complex and heterogeneous macromolecular structure and noncompetitiveness of lignin-derived products with petroleum-based ones in properties, performance, appearance, and cost.2,3 At present, the majority of the lignin generated in paper industry is utilized as a low-value boiler fuel in the plant operation, which is also the most common proposal for the lignin from biorefineries.4,5 Although burning lignin for heat and power production is still a valuable contribution to saving fossil sources, it is a low-value and an energy-inefficient application of lignin because of the need for dewatering (concentration) prior to combustion. In addition, a large volume of lignin is produced annually by the paper industry, and the volume is expected to surge with the introduction of future biorefineries.6,7 Considering these factors, it is highly desirable to develop efficient methods to utilize lignin.8,9 Furthermore, converting lignin into value-added products is also critical to increase revenue and establish economically feasible biorefining processes.5 Many strategies have been developed to utilize lignin or convert lignin into materials, chemicals, and biofuels, such as the application of lignin in polymeric materials and composites,10,11 gasification and pyrolysis of lignin to syngas © XXXX American Chemical Society
and bio-oil, respectively, followed by synthesis and upgrading to chemicals, fuels, and materials,12−14 hydrogenolysis of lignin to aromatic compounds15−17 and oxidative cracking of lignin into aromatics and organic acids.18,19 Selective depolymerization of lignin is a promising way to produce low molecular weight lignin or aromatics as the precursors of hydrocarbon fuels and platform chemicals.20,21 In addition, partial depolymerization could improve the reactivity of the lignin when preparing adhesives and the comparability of the lignin with other components in composites.22,23 For example, acidolysis, thioacidolysis, and derivatization followed by reductive cleavage (DFRC) methods were able to selectively cleave the β-O-4 ether linkages and depolymerize lignin.24 Molten salt hydrate is a concentrated inorganic salt solution having a molar ratio of water to salt equal to or less than the coordination number of the cation (i.e., all water molecules are in the inner coordination sphere of the cation).25 Many molten salt hydrates have been found to be able to swell, dissolve, and decompose cellulose.26−31 Lithium bromide (LiBr) molten salt hydrate was recently reported to be capable of dissolving and hydrolyzing pure cellulose.25,32 Further, it was observed that the concentrated LiBr solution could not only efficiently hydrolyze cellulose and hemicelluloses of lignocellulosic biomass into monomeric sugars under mild conditions without the need of pretreatment and enzymes but also depolymerize lignin while the carbohydrates were hydrolyzed.33,34 For example, the weight-average molecular weights of the lignins left over when Aspen and Douglas fir were saccharified in the LiBr solution were approximately 2500 and 3000 g/mol, respectively,35 which Received: August 25, 2016 Revised: October 14, 2016 Accepted: October 15, 2016
A
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
pressure glass vial with a magnetic stirrer. Lignin (0.5 g) was suspended in 5 mL of 0−60% (w/w) LiBr aqueous solution with 0.04−0.64 M HCl and treated at 90−150 °C for 0.5−4 h. When the reaction finished, the vial was immediately removed from the oil bath and cooled down in icy water to stop the reaction. The solid in the reaction mixture (depolymerized lignin) was collected by filtration, washed with water, and dried under vacuum. Acetylation of Lignin. Lignin (0.1 g) was dispersed/dissolved in 2 mL of pyridine−acetic anhydride (1:1, v/v) in a vial and kept in a dark cabinet at room temperature for 72 h. The solution was added dropwise into 120 mL of ice-cold water containing 1 mL of concentrated HCl with constant stirring. The precipitated lignin acetate was collected on a 10 μm nylon membrane by filtration, washed with water, and dried in air and then under vacuum. Fourier Transformation Infrared Spectroscopy (FTIR). The FT-IR spectra of the lignins before and after treatment in the LiBr solution were recorded on a Spectrum 100 FTIR Spectrometer (PerkinElmer, Waltham, MA). Each spectrum was scanned 32 times in the range from 4000 to 400 cm−1 with a resolution 2 cm−1 in the transmission mode. NMR Spectroscopy. The nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV III 500 MHz spectrometer (Billerica, MA). Lignin sample (50 mg) was dissolved in 0.5 mL of dimethyl sulfoxide-d6 (DMSO-d6). Heteronuclear single-quantum coherence (HSQC) spectra were recorded at 25 °C using the hsqcetgpsisp 2.2 pulse program. The number of collected complex points was 1024 for the 1H dimension with a relaxation delay of 1 s. The number of transients was 512 in 13C dimension. The 1JC−H used was 146 Hz. Prior to Fourier transformation, the data matrixes were zero filled up to 1024 points in the 13C dimension. Data processing was performed using standard Bruker Topspin 3.2-NMR software. Estimation of Lignin Molecular Weight. The number-average and weight-average molecular weights (Mn and Mw, respectively) of the acetylated lignin samples were estimated using GPC on an ICS3000 system (Dionex, Sunnyvale, CA) with three 300 mm × 7.8 mm i.d. Phenogel 5U columns (10 000, 500, and 50 Å) and a 50 mm × 7.8 mm i.d. Phenogel 5U guard column (Phenomenex, Torrance, CA). Lignin acetate (5 mg) was dissolved in 1 mL of HPLC-grade tetrahydrofuran (THF) without stabilizer, and 50 μL of the solution was injected onto the GPC columns. THF was used as eluent at a flow rate of 1 mL/min. The column temperature was 30 °C. Polystyrene standards were used for calibration. Lignin samples and polystyrene standards were detected using a variable wavelength detector (VWD) at 280 and 254 nm, respectively. Quantitation of Aryl Hydroxyl Group (ArOH). Aryl or phenolic hydroxyl (ArOH) of the lignins was estimated using 1H NMR according to the method reported previously.37,38 In brief, lignin acetate (50 mg) and p-nitrobenzaldehyde (5 mg as internal standard) were dissolved in 0.5 mL of CDCl3. The 1H NMR spectrum was recorded at 500 MHz with a total of 128 scans. The content of ArOH was calculated from the integration ratio of the protons of the functional group to the protons of the internal standard.
were much lower than those of the corresponding native lignins. However, to our knowledge, no research has been done on how lignin is depolymerized in a molten salt hydrate. Therefore, the main objective of the present study was to investigate whether and how lignin is depolymerized in the LiBr molten salt hydrate. Four representative industrial lignins from different sources including softwood kraft lignin (SKL), hardwood kraft lignin (HKL), softwood ethanol organosolv lignin (EOL), and acid corncob lignin (ACL) were treated in acidic LiBr solution. The effect of reaction conditions (temperature, acid loading, LiBr concentration, and reaction time) and lignin sources on the lignin depolymerization was examined. The structural changes of the lignins during the depolymerization were studied using Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and nuclear magnetic resonance (NMR) spectroscopy techniques. The mechanism of lignin depolymerization in LiBr solution is discussed.
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MATERIALS AND METHODS
Lignins and Chemicals. SKL and HKL were generously provided by Westvaco (New York, NY). They contained 86.8% and 84.6% acidinsoluble (Klason) lignin, 1.7% and 1.5% sugars, 2.6% and 2.4% ash, and 2.8% and 2.4% moisture, respectively. EOL was prepared from lodgepole pine using the organosolv ethanol process, as described previously,36 which contained 92.2% acid-insoluble lignin, 1.9% sugars, 1.4% ash, and 2.3% moisture. Crude ACL was provided by Vland Biotech (Qingdao, China), which was generated in furfural production from corncob using an acid method. The crude ACL contained 63.7% acid-insoluble lignin, 17.5% sugars, 4.3% ash, and 4.2% moisture. The compositions of the lignins listed above were analyzed according to the NREL protocols. The molecular weight and functional group of the lignins above are summarized in Table 1, which were estimated with GPC and 1H NMR, as described below. LiBr was purchased from VWR (Radnor, PA).
Table 1. Characterization of the Lignin Samples before and after Treatment in LiBr Solutiona lignin
Mn
Mw
DPI
ArOH mmol/g lignin
SKL TSKL HKL THKL EOL TEOL ACL TACL
1200 900 1100 1200 1300 1000 700 1100
3600 1800 1800 2300 3500 2200 2600 3100
3.80 2.00 1.63 1.92 2.69 2.20 3.71 2.82
3.0 4.7 4.3 6.0 2.8 3.6 2.7 3.2
lignin yield (%) 91 84 88
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93
a
Note: SKL, softwood kraft lignin; TSKL, treated softwood kraft lignin; HKL, hardwood kraft lignin; THKL, treated hardwood kraft lignin; EOL, ethanol organosolv lignin; TEOL, treated ethanol organosolv lignin; ACL, acid corncob lignin; TACL treated acid corncob lignin.
RESULTS AND DISCUSSION Selection of the Industrial Lignins. Four industrial lignins were selected for this study, including SKL and HKL from paper industry, EOL from organosolv ethanol pretreatment of lodgepole pine for cellulose ethanol production, and ACL from furfural production. Kraft pulping is the most dominant chemical pulping method in paper industry, in which wood lignin is depolymerized by Na2S and NaOH in the pulping liquor and dissolved into the spent pulping liquor (black liquor). When the black liquor is acidified to about pH 2, the precipitated lignin is called the kraft lignin. Ethanol organosolv pretreatment is one of the most effective and widely investigated biomass pretreatment methods for cellulose ethanol production, in which aqueous ethanol with a catalyst (usually an acid) is used to depolymerize and dissolve lignin
Purification of ACL. Crude ACL (50 g) was dissolved in 500 mL of 0.5 M NaOH solution and then heated at 80 °C for 1 h. The pH of the solution was then brought to 2 with 2 M HCl to precipitate the lignin. The lignin sediment was filtered, washed to pH 6−7 with deionized water, and dried in air and then under vacuum. The yield of purified ACL was 85% (based on the Klason lignin of the crude ACL). The purified ACL contained 93.6% acid-insoluble lignin, 1.4% sugars, 2.1% ash, and 3.4% moisture. In the following experiments, if not indicated, the purified ACL was used. Depolymerization of Lignin in LiBr Solution. The depolymerization of lignin in acidic LiBr solution was carried out in a 50 mL B
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 1. Effect of temperature (A, at 60% LiBr, 0.16 M HCl, and 1 h), concentration of LiBr (B, at 0.16 M HCl, 110 °C, and 1 h), reaction time (C, at 60% LiBr, 0.16 M HCl, and 110 °C), and concentration of HCl (D, at 60% LiBr, 110 °C, and 2 h) on lignin (SKL) depolymerization.
110 °C was a favorable temperature in the range investigated for the depolymerization of SKL in LiBr solution. In order to understand the effect of LiBr concentration on the lignin depolymerization, SKL was treated in the LiBr solution with varying concentrations from 0 to 60% (w/w). As shown in Figure 1B, without addition of LiBr (0% concentration), SKL was not depolymerized in the acidic water but Mw slightly increased from 3600 (Table 1) to 3900 g/ mol, implying that condensation probably occurred due to the acidic condition and high temperature. When 10% LiBr was used, Mw dropped sharply. However, when LiBr concentration was up to 20%, Mw reversely increased slightly and then kept decreasing when the LiBr concentration was further increased. The PDI of the lignin followed the same pattern as Mw when LiBr concentration increased. It was unexpected that 10% LiBr resulted in lower Mw than 20%, 30%, and 40% LiBr. The point at 10% LiBr was thought to be probably an outlier, because this phenomenon was not observed when EOL was treated at different LiBr concentrations (0−60%), in which EOL Mw kept decreasing with the LiBr concentration and reached the lowest Mw at 60% LiBr. To double check this point, the treatment of SKL in 10% LiBr solution was repeated two more times, but similar results were observed. The cause of the result is not clear at present. The results above indicated that both lignins reached the lowest Mw at 60% LiBr. This is consistent with the unique nature of the molten salt hydrate. First, at 60% concentration, which is very close to the theoretical concentration (61.7%) of LiBr forming a perfect molten salt hydrate, almost all Li+ is coordinated with water molecules and Br− is naked and free in the solution with high concentration.34 Second, the Hammett acidity (H+ activity) of Brønsted acid is substantially enhanced in a molten salt hydrate system.39 This
from lignocellulosic biomass. The EOL used in this study was prepared from lodgepole pine using acidic ethanol following the method described in a previous study.36 ACL used in the present study was from a biorefinery producing furfural from corncob, in which corncob hemicellulose (xylan) was converted into furfural using an acid process and the cellulose and lignin in the corncob were left over as a solid residue. ACL was separated after the cellulose in the solid residue was hydrolyzed by cellulases. Due to incomplete hydrolysis of the cellulose, ACL contained a significant amount of carbohydrates. The lignins above were from different biomass species (hardwood, softwood, and herbage) and experienced varied chemical environments (alkali, acid, and organic solvent), and they are representative industrial lignins and also available in large quantity. Optimization of Reaction Conditions for Lignin Depolymerization in Acidic LiBr Solution. Using SKL, the effects of reaction conditions on the depolymerization of the lignin in acidic LiBr solution were investigated. It was found that temperature was a crucial parameter in the lignin depolymerization. As shown in Figure 1A, Mw of SKL decreased with temperature and reached the minimum at 110 °C, and then Mw reversely increased when temperature was further elevated to 130 °C, suggesting that elevating temperature not only enhanced the lignin depolymerization but also promoted lignin condensation. When temperature was further elevated to 150 °C, Mw dropped again but was still higher than that at 110 °C, suggesting that depolymerization and condensation occurred simultaneously but the former was more significant than the latter at 150 °C. The polydispersity index (PDI = Mw/Mn) of the lignin showed almost the same trend as Mw with temperature. The results above suggest that C
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 2. Weight-average molecular weight (Mw) and polydispersity index of the lignins before and after LiBr treatment: (A) SKL and TSKL; (B) HKL and THKL; (C) EOL and TEOL; (D) ACL and TACL.
g/mol to 1800 and 900 g/mol of TSKL, respectively, indicating that the SKL was significantly depolymerized. Similarly, the Mw and Mn of EOL were reduced from 3500 and 1300 g/mol to 2200 and 1000 g/mol, respectively, suggesting that the ethanol organosolv lignin was also effectively depolymerized. However, HKL and ACL were not depolymerized in the LiBr solution, and their Mw values (2300 and 3100 g/mol, respectively) after LiBr treatment were even higher than those before treatment (1800 and 2600 g/mol, respectively). Why HKL and ACL were not depolymerized might be related to the structure and origin of the lignins. First, the very low Mw (1800 g/mol) of HKL implied that the lignin had experienced extensive depolymerization during the kraft pulping, and most of the β-O-4 ether linkages between lignin structural units had been cleaved, which was verified by the NMR result that only a trace amount of βO-4 linkages was detected in HKL. Since the LiBr treatment was only able to cleave the ether linkages but not C−C bonds, it could not further depolymerize HKL. Instead, condensation occurred to HKL during the LiBr treatment due to the acidity and temperature, leading to the slightly increased molecular weight. Second, as mentioned above, ACL was from a biorefinery that produces furfural from corncob using an acid process. In other words, ACL had experienced a high-acidity and high-temperature environment, and thereby it was expected to be more resistant to acidic depolymerization. The results in Table 1 show that ACL Mw increased slightly from 2600 to 3100 g/mol after LiBr treatment. The increased Mw was not necessarily caused by condensation and could result from the removal of low molecular weight fractions from the lignin during the LiBr treatment, which was consistent with the
was probably why 60% LiBr solution was able to depolymerize the lignin more than the dilute LiBr solutions, because both H+ and Br− were involved in the depolymerization, leading to cleavage of aryl ether bonds, as discussed below. The effect of treatment time on the lignin depolymerization is shown in Figure 1C. The Mw and PDI of SKL decreased during the first 2 h and then increased, which was probably caused by the condensation of the lignin when the reaction was extended. The effect of HCl as the catalyst on the lignin depolymerization is shown in Figure 1D. The Mw and PDI decreased sharply with the increase of HCl concentration, but it seemed that HCl concentration above 0.3 M was not necessary, because that did not further improve the depolymerization of the lignin. On the basis of the results in Figure 1, the suggested reaction conditions for the depolymerization of SKL were 60% LiBr with 0.3 M HCl at 110 °C for 2 h. Molecular Weight of the Lignins before and after the LiBr Treatment. To investigate whether the LiBr treatment could effectively depolymerize different lignins, SKL, HKL, EOL, and ACL were treated in acidic LiBr solution under the suggested reaction conditions above. The lignins treated with LiBr solution were named as treated softwood kraft lignin (TSKL), treated hardwood kraft lignin (THKL), treated ethanol organosolv lignin (TEOL), and treated acid corncob lignin (TACL), respectively. The yields of TSKL, THKL, TEOL, and TACL varied between 84% and 93%, as summarized in Table 1. Mw, Mn, and PDI of all the lignin samples before and after the treatment in LiBr solution are presented in Table 1. The treatment lowered the Mw and Mn of SKL from 3600 and 1200 D
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 3. FTIR spectra of the lignins before and after LiBr treatment: (A) SKL and TSKL; (B) HKL and THKL; (C) EOL and TEOL; and (D) ACL and TACL.
TACL during LiBr treatment. However, the sharp peaks at the low molecular weight end were significantly reduced, suggesting that the low molecular weight fractions were removed, which was probably the main reason why the Mw of TACL estimated by GPC was slightly higher than that of ACL. Change of Aryl Hydroxyl Groups. It is known that the content of aryl hydroxyl group (ArOH) could be used as an indicator of whether aryl ether bonds were cleaved, because cleavage of the ether bonds resulted in the formation of new ArOH. As shown in Table 1, the content of ArOH of all the lignins increased after LiBr treatment, which provided another evidence of the cleavage of aryl ether bonds (such as β-O-4 and α-O-4 ether bonds) of the lignins during LiBr treatment. FTIR Spectra of the Lignins before and after the LiBr Treatment. The FTIR spectra of the four lignins before and after LiBr treatment are presented in Figure 3, which provided additional evidence of lignin depolymerization. The peak at 1030 cm−1, which is attributed to symmetric stretching vibration of C−O−C of ethers, was observed in all lignins before and after treatment, but the intensity of the peak decreased after treatment. In particular, the peaks of TSKL and TEOL were significantly weaker than those of SKL and EOL, suggesting that a portion of the ether bonds were cleaved, which was consistent with the observations above that SKL and EOL were depolymerized more extensively than other lignins in the LiBr treatment. Weaker absorption at 1030 cm−1 in HKL and ACL than SKL and EOL verified again that the former had fewer ether bonds than the latter, which was the major reason why the former were unable to be depolymerized as efficiently as the latter by LiBr treatment. The bands at 1200 cm−1
changes of molecular weight distribution before and after LiBr treatment. The molecular weight distributions of the four lignins before and after LiBr treatment are presented in Figure 2. TSKL and TEOL (Figure 2A and 2C, respectively) had a lower proportion of high molecular weight fractions compared with SKL and EOL, suggesting that the high molecular weight fractions of the lignins were partially depolymerized during the LiBr treatment. In addition, their Mw distribution curves shifted slightly toward the low molecular weight end after the LiBr treatment. These observations support that the low Mw of TSKL and TEOL resulted from the depolymerization of high molecular weight portions of SKL and EOL. Moreover, the lignins became more homogeneous in molecular weight after LiBr treatment, because TSKL and TEOL had narrower molecular weight distributions (PDIs 2.0 and 2.26, respectively) than SKL and EOL (2.89 and 2.62, respectively). In contrast, as shown in Figure 2B, the peaks of the low molecular weight fractions of HKL disappeared after LiBr treatment. In addition, the peak of the high molecular weight portion in THKL was significantly higher than that in HKL, suggesting that condensation occurred during LiBr treatment. Furthermore, the Mw distribution curve of THKL shifted slightly toward the high end of molecular weight. All these observations and evidence provided explanations why the molecular weight of HKL increased after the LiBr treatment. As shown in Figure 2D, ACL behaved differently from other lignins during LiBr treatment. The Mw distribution curve changed little, and no significant change was observed at the high molecular weight end of the TACL Mw curve, indicating that no further condensation occurred to E
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 4. 2D HSQC NMR spectra of the lignins in DMSO-d6 before and after LiBr treatment: (A) SKL; (B) TSKL; (C) HKL; (D) THKL; (E) EOL; (F) TEOL; (G) ACL; and (H) TACL. F
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 5. Proposed mechanism of β-O-4 ether bond cleavage in lignin in acidic LiBr solution.
attributed to C−O vibrations were also observed. The peaks of the treated lignins at 1200 (C−O of phenols) and 1390 cm−1 (in-plane bending vibration of O−H) became stronger compared with those of the untreated lignins, implying the formation of new phenolic OH groups, which was consistent with the analysis of ArOH above. 2D HSQC NMR Spectra. NMR analysis further verified the cleavage of the ether bonds in lignin during LiBr treatment. There existed β-O-4 (δC/δH, 71.6/4.75 (α), 84.6/4.27 (β), and 60.5/(3.60, 3.42) (γ) ppm), β-5 (δC/δH, 87.6/5.47 (4), 53.7/ 3.46 (β), 63.4/(3.65, 3.36) (γ) ppm), and β−β (δC/δH, 85.7/ 4.62 (α), 54.2/3.06 (β) and 71.4/(4.143.60, 3.74) (γ) ppm) structures in SKL and EOL, as shown in 2D HSQC NMR spectra of Figure 4A and 4E, respectively. SKL also contained X1 unit. It was apparent that the LiBr treatment effectively cleaved the aryl ether bonds β-O-4 and α-O-4 (in β-5 unit) (Figure 4B and 4F). However, HKL only contained β−β and other structures with no or little β-O-4 and β-5 (Figure 4C), which explained why HKL could not be further depolymerized in the LiBr treatment. It seemed that cleavage of intramolecular ether bonds of β−β units occurred as well, as shown in Figure 4D. ACL had β-O-4 and β-5 units (Figure 4G), and the aryl ethers (β-O-4 and α-O-4) were cleaved during LiBr treatment (Figure 4H). However, this did not lead to the depolymerization of ACL, and the Mw of TACL did not decease but increased after LiBr treatment, as discussed above in Table 1 and Figure 2D. This was presumably attributed to the condensed nature of ACL caused by its previous high-acidity and high-temperature experience in furfural production. During LiBr treatment, even if the ether bonds were cleaved, the lignin
skeleton networked through C−C linkages that originally existed in the lignin or were newly formed through condensation was stable and could not be broken by LiBr treatment. As shown in Figure 4B, 4D, 4F, and 4H, a Hibbert’s ketone structure, 1-hydroxy-3-(4-hydroxy-3-methoxyphenyl)propan-2-one (HK) (δCγ/δHγ = 67.6/4.16 ppm), was observed in all LiBr-treated lignins, which was one of the products from cleavage of β-O-4 structures. On the Depolymerization Mechanism of Lignin in the LiBr Treatment. The evidence and results above clearly suggested that cleavage of β-O-4 and α-O-4 ether bonds and condensation (repolymerization) occurred to the lignins during treatment in the acidic LiBr solution and cleavage of β-O-4 ether bond led to depolymerization of the lignins. The question remains as to how the β-O-4 ether bond was cleaved. The reaction medium, the acidic concentrated LiBr solution, used in this study contained H+, Br−, and Li+. It is reasonable to assume these ions played roles in the lignin depolymerization. It is known that H+ is able to catalyze cleavage of the β-O-4 bond. The results in Figure 1D also demonstrated that sufficient HCl was essential to efficiently depolymerize the lignin in the LiBr system. Lundquist and Lundgren40 pioneered the study on the acidolysis of lignin. Matsumoto and coworkers41−46 systematically revisited the mechanism of lignin acidolysis and clarified the details of the cleavage of the β-O-4 bond in acidic aqueous dioxane. The acknowledged mechanism of β-O-4 bond cleavage induced by acid is summarized in Figure 5. The reaction started from the proton attack on α-C of an β-O-4 substructure I, which led to the formation of a benzyl cation intermediate II. In the next step, the β-H abstracted from G
DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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charge on the ACS Publications website at DOI: 10.1021/ acs.jafc.6b03807. 1 H NMR spectra of SKL, TSKL, HKL, THKL, EOL, TEOL, ACL, and TACL acetates in CDCl3 with pnitrobenzaldehyde as internal standard; 2D HSQC NMR spectra (aromatic region) of the lignins in DMSO-d6 before and after LiBr treatment (PDF)
II produced an enol ether substructure III. The enol ether III was then hydrolyzed and consequently led to cleavage of the βO-4 bond through intermediates IV and V. The products included a Hibbert’s ketone-type (HK) substructure VI, which was detected in the lignin depolymerization products (Figures 4B, 4D, 4F, and 4H), and a new phenolic unit VII. Alternatively, the cation intermediate II could be directly transferred to the intermediate IV through a β to α H transfer, but this path was found to be minor42 compared to the path via the enol ether III. It was confirmed that the acid-catalyzed cleavage of β-O-4 bonds was initiated by the formation of the benzyl cation intermediate II, but the rate-determining step was the formation of the enol ether III through the β-H elimination from the intermediate II. In addition to the β-O-4 bond cleavage, condensation could occur between the cation intermediate II and the electron-rich carbon (usually C-6 or C-2) of another aromatic ring, resulting in the condensed structure X, which is supported by the changes in the aromatic region of 2D HSQC NMR spectra of the lignins before and after LiBr treatment. The contours of C2/6 of all the lignins are reduced after LiBr treatment, suggesting that condensation occurred at these sites. The essential role of LiBr in the lignin depolymerization was confirmed, as shown in Figure 1B, that the lignin was not depolymerized but condensed without the addition of LiBr. Considering the nature of Br−, it could promote cleavage of the β-O-4 bond in two ways, as proposed in Figure 5. First, as a nucleophile, Br− could substitute α-OR and lead to the formation of the benzyl cation intermediate II via the intermediate VIII. Second, as a base, Br− could abstract the hydrogen from β-C of the intermediate II and thereby promote the formation of the enol ether III. In summary, as discussed above, both H+ and Br− were hypothesized to play crucial roles in catalyzing cleavage of the β-O-4 bond. The unique natures of the LiBr molten salt hydrate (concentrated free Br− and enhanced H+ activity) promoted the H+- and Br−-induced β-O-4 bond cleavage. It is unclear at present whether Li+ played a role in the lignin depolymerization. The confirmation of the proposed mechanisms is under investigation using lignin model compounds. In conclusion, this study demonstrated that acidic lithium bromide molten salt hydrate was able to selectively cleave the β-O-4 ether bond and thereby lead to the depolymerization of lignin under mild conditions (e.g., in 60% LiBr with 0.3 M HCl at 110 °C for 2 h). It was confirmed that acid (HCl) and LiBr were both critical in inducing the ether bond cleavage. The results suggested that the system was effective only to the lignin containing β-O-4 ether bonds but ineffective to the lignin having few β-O-4 ether bonds and condensed structure. The method provides a new approach for the depolymerization of lignin under mild reaction conditions (low temperature and acidity). The method has potential application in producing low molecular weight lignin fragments and aromatics as the precursors of hydrocarbon fuels and platform chemicals. In addition, the depolymerization leads to new aromatic hydroxyl groups, which improves the reactivity of the lignin such as in lignin-based adhesives.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1-608-2624951; Fax: +1-608-2621228; E-mail: xpan@ wisc.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the China Scholarship Council (CSC) for supporting X.Y. to conduct this research at the University of WisconsinMadison. This work was partially supported by grants from the NSF (CBET 1159561 and CBET 1236562) and USDA McIntire Stennis (WIS01597) to X.P.
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REFERENCES
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DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jafc.6b03807 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
