Changes in the Structure and the Thermal Properties of Kraft Lignin

Research Article pubs.acs.org/journal/ascecg

Changes in the Structure and the Thermal Properties of Kraft Lignin during Its Dissolution in Cholinium Ionic Liquids Yan-Xia An, Ning Li,* Hong Wu, Wen-Yong Lou, and Min-Hua Zong* State Key Laboratory of Pulp and Paper Engineering, College of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Lignin valorization using ionic liquids (ILs) has attracted growing interest recently. Cholinium ILs, a novel kind of bio-ILs, are good lignin solvents. In this work, the changes in the structure and the thermal properties of kraft lignin during its dissolution in this type of bio-ILs were examined in detail by nuclear magnetic resonance spectroscopy, gel permeation chromatography, elemental analysis, thermogravimetric analysis, and differential scanning calorimetry. It was found that the β-O-4′ linkage was broken during kraft lignin dissolution, while the β-β′ and β-5′ linkages were formed. In addition, the dehydration and demethoxylation reactions occurred. Besides, the dissolution process resulted in a higher maximal decomposition temperature (Tm) and a higher glass transition temperature (Tg) of kraft lignin as well as the low residual char and the reduced molecular weights. Hence, these chemical transformations should be taken into account during lignin dissolution in ILs when the mechanism of lignin valorization using IL processes is proposed in the presence of catalysts. KEYWORDS: 2D HSQC NMR, Dissolution, Ionic liquids, Lignin valorization, Structural characterization

■

INTRODUCTION Lignin is one of the major components of lignocellulosic biomass (15−30% by weight). Although lignin is the second most abundant terrestrial polymer on Earth after cellulose, it is currently underutilized relative to cellulose. For example, this byproduct in the pulp and paper industry is mostly burned to generate energy, and only a small amount (1−2%) is commercially used for the production of materials and chemicals.1 In fact, lignin is the richest renewable resource of aromatics in nature.2,3 Therefore, the conversion of lignin to value-added products, especially aromatic compounds, has significant potential. Recently, lignin valorization has attracted increasing interest in the scientific community.4−6 However, the commercial high-value applications of lignin remain challenging, because of structural variability of lignin in different plant species as well as its structural heterogeneity. Lignin dissolution is critically important for its efficient valorization. Ionic liquids (ILs), types of salts with melting points of [Ch][Gly] ≈ [Ch][Glc] > [Ch][Arg] > [Ch][AcO]. It is contrary to the recent results using [Emim][AcO] as the solvent,25 demonstrating that the different reactions would happen in the different types of ILs. In addition, the relative ratio of the β-5′ linkage also increased slightly in kraft lignins after the IL treatment. The results indicate that [Lys]-based IL prefers the formation of the β-β′ linkage, while the β-5′ linkage is more readily produced in [Ch][Glc]. Besides, the 13C NMR semiquantitative results show that the contents of the methoxyl group are reduced after regeneration of lignin from [Ch][Gly], [Ch][Lys], and [Ch][Arg] (Table S4), suggesting the occurrence of the demethoxylation during lignin dissolution in amino acid-based ILs. According to the results of 2D HSQC NMR combined with 13C NMR data, the breakage of the β-O-

Table 1. Semiquantitative Results of the Interunit Linkages in Kraft Lignins by 2D HSQC Spectra signal integration relative to the aromatic region at position 2 (%) lignin untreated [Ch][AcO] [Ch][Glc] [Ch][Gly] [Ch][Lys] [Ch][Arg] a

yield (%) 100 85.9 91.2 82.3 83.5 85.5

± ± ± ± ±

a

0.86 0.99 0.70 0.58 1.05

β-O-4′

lignin purity (%) 93.3 95.0 97.3 95.7 94.8 96.4

7.03 5.94 6.05 6.36 6.32 6.44

± ± ± ± ± ±

0.44 0.21 0.47 0.02 0.10 0.01

β-β′ 3.79 9.48 18.16 18.49 25.35 12.57

± ± ± ± ± ±

β-5′ 0.04 0.52 0.21 0.06 0.40 0.06

1.93 2.63 12.70 4.16 2.68 3.16

± ± ± ± ± ±

0.02 0.14 0.12 0.01 0.04 0.02

The yield was calculated in terms of the mass of lignin after the regeneration. 2955

DOI: 10.1021/acssuschemeng.5b00915 ACS Sustainable Chem. Eng. 2015, 3, 2951−2958

Research Article

ACS Sustainable Chemistry & Engineering

the char residues at 700 °C decreased slightly after kraft lignin regeneration, and this is supported by the fact that the polymers with higher molecular weights gave more char residues.40 Generally, the increase in the molecular weights would result in a higher Tm as well as a higher Tg.28,41 Unexpectedly, both Tm and Tg of kraft lignins increased after regeneration, although the Mw decreased (Table 4). The reason for this may be that, in addition to the molecular weights, Tm and Tg are also influenced by the structure, functional groups, and morphology of the polymer.40,42 According to the NMR results described above, the more resistant β-β′ and β-5′ linkages were formed during kraft lignin dissolution, which may account for the shifts of Tm and Tg to a higher temperature region. The IL-treated lignins with higher Tm and lower polydispersity values may be promising feedstocks for the production of task-specific biobased polymers.43,44 Elemental Analysis. Table 5 shows the elemental compositions, the C900 formulas of kraft lignins, and the degrees of unsaturation. As expected, the untreated kraft lignin has a low sulfur content of 1.8%, which is in agreement with a recent report.28 After dissolution and regeneration, the sulfur content decreased to 1.1% in kraft lignins. Sevasyanova el al. found that the kraft lignin fractions with low molecular weights contained sulfur contents much higher than those with high molecular weights.28 Consequently, the decreased sulfur contents in the recovered kraft lignins might be ascribed to the loss of fractions with low molecular weights after lignin regeneration. In addition, the carbon contents increased, while the oxygen and hydrogen contents decreased after IL processing. Interestingly, the degree of unsaturation in kraft lignins increased significantly from 224 to 324−349, indicating the formation of more unsaturated bonds during dissolution, which is supported by the 31P NMR results described above. It is likely due to the occurrence of the dehydration reaction during the incubation of kraft lignin in ILs, which was also reported by other groups.25,45

reason remains unclear currently and is under investigation in our laboratory. Molecular Weights. The molecular weight of lignin exerted a significant effect on its reactivity and thermomechanical behaviors that are vital for its value-added applications.28 Therefore, kraft lignins were analyzed by GPC (Table 3). As Table 3. Weight-Average (Mw) and Number-Average (Mn) Molecular Weights (grams per mole) and Polydispersities (Mw/Mn) of Kraft Lignins lignin

Mw

Mn

Mw/Mn

untreated [Ch][AcO] [Ch][Glc] [Ch][Gly] [Ch][Lys] [Ch][Arg]

15390 10450 12465 14285 10460 12755

8624 7073 8417 8618 6008 7284

1.78 1.48 1.60 1.66 1.74 1.75

shown in Table 3, the Mw of the untreated kraft lignin was approximately 15390 g mol−1, which is much higher than the previously reported value;29 nonetheless, it is still within the Mw range (600−180000 g mol−1) of kraft lignin obtained from black liquor.28 After regeneration, the Mw of kraft lignins decreased to 10450−14285 g mol−1. Other groups also reported that the molecular weights of the technical lignins were reduced after IL treatment.21,25,39 In addition, the polydispersity (Mw/Mn) of kraft lignins decreased from 1.78 to 1.48−1.66 after regeneration from [AcO]-, [Glc]-, and [Gly]-based ILs, which suggests that these regenerated lignins have structures that are more homogeneous than those of the untreated kraft lignin. The reduced polydispersity may partially be attributed to the losses of low-molecular weight fractions during lignin regeneration. Thermal Analysis. The thermal properties of the untreated and IL-treated kraft lignins were investigated by TG and DSC; the residual char, the maximal decomposition temperature (Tm), and Tg are summarized in Table 4. It could be found that

■

CONCLUSIONS In conclusion, this study has demonstrated that significant changes in the structure and thermal properties of kraft lignin occur during its dissolution in cholinium ILs. In addition, the chemical reactions that occurred during kraft lignin dissolution in cholinium ILs appear to be different from those in imidazolium ILs. During its dissolution in cholinium ILs, kraft lignins underwent depolymerization as well as recondensation. The dehydration and demethoxylation reactions also occurred. As a result, the thermal properties of kraft lignin were altered, and its moelcular weights were reduced after regeneration from cholinium ILs. This study may provide new insight into the mechanistic proposal of kraft lignin depolymerization or valorization using IL processes in the presence of catalysts.

Table 4. Thermal Properties of the Untreated and ILTreated Kraft Lignins lignin

residual char (wt %)a

Tm (°C)b

Tg (°C)c


43.5 39.0 41.2 39.0 38.5 39.4

331 358 358 350 352 358

153 160 160 164 162 162

The residues at 700 °C, obtained from TG curves (Figure S5a). bThe temperature corresponding to the maximal decomposition rate, obtained from DTG curves (Figure S5b). cThe glass transition temperature, obtained from DSC curves (Figure S6). a

Table 5. Elemental Analysis and C900 Empirical Formulas of Kraft Lignins lignin

C (%)

H (%)

O (%)

N (%)

S (%)

C900 formula

degree of unsaturation


60.1 61.3 64.6 63.0 62.7 61.4

7.6 6.6 6.9 6.5 6.8 6.7

29.9 29.5 26.2 28.3 28.1 29.1

0.6 1.4 1.2 1.1 1.3 1.8

1.8 1.2 1.1 1.1 1.1 1.1

C900H1362O336N8S10 C900H1157O324N18S7 C900H1148O274N15S6 C900H1117O302N13S6 C900H1163O302N16S6 C900H1176O319N22S6

224 332 335 349 328 324

2956


Research Article

ACS Sustainable Chemistry & Engineering

■

(13) Liu, Q. P.; Hou, X. D.; Li, N.; Zong, M. H. Ionic liquids from renewable biomaterials: synthesis, characterization and application in the pretreatment of biomass. Green Chem. 2012, 14 (2), 304−307. (14) Hou, X. D.; Liu, Q. P.; Smith, T. J.; Li, N.; Zong, M. H. Evaluation of toxicity and biodegradability of cholinium amino acids ionic liquids. PLoS One 2013, 8 (3), e59145. (15) Hou, X. D.; Smith, T. J.; Li, N.; Zong, M. H. Novel renewable ionic liquids as highly effective solvents for pretreatment of rice straw biomass by selective removal of lignin. Biotechnol. Bioeng. 2012, 109 (10), 2484−2493. (16) Hou, X. D.; Li, N.; Zong, M. H. Renewable bio ionic liquidswater mixtures-mediated selective removal of lignin from rice straw: Visualization of changes in composition and cell wall structure. Biotechnol. Bioeng. 2013, 110 (7), 1895−1902. (17) Hou, X. D.; Xu, J.; Li, N.; Zong, M. H. Effect of anion structures on cholinium ionic liquids pretreatment of rice straw and the subsequent enzymatic hydrolysis. Biotechnol. Bioeng. 2015, 112 (1), 65−73. (18) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.; Davis, J. H. Novel brønsted acidic ionic liquids and their use as dual solvent−catalysts. J. Am. Chem. Soc. 2002, 124 (21), 5962− 5963. (19) Moreau, C.; Finiels, A.; Vanoye, L. Dehydration of fructose and sucrose into 5-hydroxymethylfurfural in the presence of 1-H-3-methyl imidazolium chloride acting both as solvent and catalyst. J. Mol. Catal. A: Chem. 2006, 253 (1−2), 165−169. (20) Zhao, G.; Jiang, T.; Gao, H.; Han, B.; Huang, J.; Sun, D. Mannich reaction using acidic ionic liquids as catalysts and solvents. Green Chem. 2004, 6 (2), 75−77. (21) Diop, A.; Jradi, K.; Daneault, C.; Montplaisir, D. Kraft lignin depolymerization in an ionic liquid without a catalyst. BioResources 2015, 10 (3), 4933−4946. (22) Ogawa, S.; Miyafuji, H. Reaction behavior of milled wood lignin in an ionic liquid, 1-ethyl-3-methylimidazolium chloride. J. Wood Sci. 2015, 61 (3), 285−291. (23) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of structural carbohydrates and lignin in biomass. NREL/TP-510-42618; National Renewable Energy Laboratory: Golden, CO, 2008. (24) An, Y.-X.; Zong, M.-H.; Wu, H.; Li, N. Pretreatment of lignocellulosic biomass with renewable cholinium ionic liquids: Biomass fractionation, enzymatic digestion and ionic liquid reuse. Bioresour. Technol. 2015, 192, 165−171. (25) Wen, J. L.; Yuan, T. Q.; Sun, S. L.; Xu, F.; Sun, R. C. Understanding the chemical transformations of lignin during ionic liquid pretreatment. Green Chem. 2014, 16 (1), 181−190. (26) del Río, J. C.; Rencoret, J.; Prinsen, P.; Martínez, Á . T.; Ralph, J.; Gutiérrez, A. Structural characterization of wheat straw lignin as revealed by analytical pyrolysis, 2D-NMR, and reductive cleavage methods. J. Agric. Food Chem. 2012, 60 (23), 5922−5935. (27) You, T.-T.; Mao, J.-Z.; Yuan, T.-Q.; Wen, J.-L.; Xu, F. Structural elucidation of the lignins from stems and foliage of Arundo donax Linn. J. Agric. Food Chem. 2013, 61 (22), 5361−5370. (28) Sevastyanova, O.; Helander, M.; Chowdhury, S.; Lange, H.; Wedin, H.; Zhang, L.; Ek, M.; Kadla, J. F.; Crestini, C.; Lindström, M. E. Tailoring the molecular and thermo−mechanical properties of kraft lignin by ultrafiltration. J. Appl. Polym. Sci. 2014, 131 (18), 40799. (29) Tejado, A.; Peña, C.; Labidi, J.; Echeverria, J. M.; Mondragon, I. Physico-chemical characterization of lignins from different sources for use in phenol−formaldehyde resin synthesis. Bioresour. Technol. 2007, 98 (8), 1655−1663. (30) Fernández-Costas, C.; Gouveia, S.; Sanromán, M. A.; Moldes, D. Structural characterization of Kraft lignins from different spent cooking liquors by 1D and 2D Nuclear Magnetic Resonance spectroscopy. Biomass Bioenergy 2014, 63 (0), 156−166. (31) Mansfield, S. D.; Kim, H.; Lu, F.; Ralph, J. Whole plant cell wall characterization using solution-state 2D NMR. Nat. Protoc. 2012, 7 (9), 1579−1589.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00915. Signal assignment, spectra of FTIR, 13C and 31P NMR, 13 C NMR quantitative data, TG, DTG, and DSC curves (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Telephone and fax: +86 20 2223 6669. E-mail: lining@scut. edu.cn. *Telephone: +86 20 8711 1452. Fax: +86 20 2223 6669. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was financially supported by the State Key Laboratory of Pulp and Paper Engineering (2015C03), the Natural Science Foundation of Guangdong Province (2014A030313263), and the Research Fund for Central Universities (2014ZG0045).

■

REFERENCES

(1) Mansouri, N.-E. E.; Salvadó, J. Structural characterization of technical lignins for the production of adhesives: Application to lignosulfonate, kraft, soda-anthraquinone, organosolv and ethanol process lignins. Ind. Crops Prod. 2006, 24 (1), 8−16. (2) Tuck, C. O.; Pérez, E.; Horváth, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of Biomass: Deriving More Value from Waste. Science 2012, 337 (6095), 695−699. (3) Clark, J. H. Green chemistry for the second generation biorefinerysustainable chemical manufacturing based on biomass. J. Chem. Technol. Biotechnol. 2007, 82 (7), 603−609. (4) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin valorization: Improving lignin processing in the biorefinery. Science 2014, 344 (6185), 1246843. (5) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The catalytic valorization of lignin for the production of renewable chemicals. Chem. Rev. 2010, 110 (6), 3552−3599. (6) Xu, C.; Arancon, R. A. D.; Labidi, J.; Luque, R. Lignin depolymerisation strategies: towards valuable chemicals and fuels. Chem. Soc. Rev. 2014, 43 (22), 7485−7500. (7) Chatel, G.; Rogers, R. D. Review: Oxidation of lignin using ionic liquidsAn innovative strategy to produce renewable chemicals. ACS Sustainable Chem. Eng. 2014, 2 (3), 322−339. (8) Hossain, M. M.; Aldous, L. Ionic liquids for lignin processing: Dissolution, isolation, and conversion. Aust. J. Chem. 2012, 65, 1465− 1477. (9) Zhang, Z. C. Catalytic transformation of carbohydrates and lignin in ionic liquids. WIREs Energy Environ. 2013, 2 (6), 655−672. (10) Pu, Y.; Jiang, N.; Ragauskas, A. J. Ionic liquid as a green solvent for lignin. J. Wood Chem. Technol. 2007, 27 (1), 23−33. (11) Stärk, K.; Taccardi, N.; Bösmann, A.; Wasserscheid, P. Oxidative depolymerization of lignin in ionic liquids. ChemSusChem 2010, 3 (6), 719−723. (12) Liu, S.; Shi, Z.; Li, L.; Yu, S.; Xie, C.; Song, Z. Process of lignin oxidation in an ionic liquid coupled with separation. RSC Adv. 2013, 3 (17), 5789−5793. 2957


Research Article

ACS Sustainable Chemistry & Engineering (32) Sette, M.; Wechselberger, R.; Crestini, C. Elucidation of lignin structure by quantitative 2D NMR. Chem. - Eur. J. 2011, 17 (34), 9529−9535. (33) Kim, H.; Ralph, J. Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d6/pyridine-d5. Org. Biomol. Chem. 2010, 8 (3), 576−591. (34) Balakshin, M. Y.; Capanema, E. A.; Chen; Gracz, H. S. Elucidation of the structures of residual and dissolved pine kraft lignins using an HMQC NMR technique. J. Agric. Food Chem. 2003, 51 (21), 6116−6127. (35) Sathitsuksanoh, N.; Holtman, K. M.; Yelle, D. J.; Morgan, T.; Stavila, V.; Pelton, J.; Blanch, H.; Simmons, B. A.; George, A. Lignin fate and characterization during ionic liquid biomass pretreatment for renewable chemicals and fuels production. Green Chem. 2014, 16, 1236−1247. (36) Tran, F.; Lancefield, C. S.; Kamer, P. C. J.; Lebl, T.; Westwood, N. J. Selective modification of the β-β linkage in DDQ-treated Kraft lignin analysed by 2D NMR spectroscopy. Green Chem. 2015, 17 (1), 244−249. (37) Zhang, L.; Gellerstedt, G. Quantitative 2D HSQC NMR determination of polymer structures by selecting suitable internal standard references. Magn. Reson. Chem. 2007, 45 (1), 37−45. (38) Granata, A.; Argyropoulos, D. S. 2-Chloro-4,4,5,5-tetramethyl1,3,2-dioxaphospholane, a reagent for the accurate determination of the uncondensed and condensed phenolic moieties in lignins. J. Agric. Food Chem. 1995, 43 (6), 1538−1544. (39) 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. (40) Wen, J.-L.; Sun, S.-L.; Xue, B.-L.; Sun, R.-C. Quantitative structures and thermal properties of birch lignins after ionic liquid pretreatment. J. Agric. Food Chem. 2013, 61 (3), 635−645. (41) Singh, S.; Varanasi, P.; Singh, P.; Adams, P. D.; Auer, M.; Simmons, B. A. Understanding the impact of ionic liquid pretreatment on cellulose and lignin via thermochemical analysis. Biomass Bioenergy 2013, 54, 276−283. (42) Yoshida, H.; Mörck, R.; Kringstad, K. P.; Hatakeyama, H. Kraft lignin in polyurethanes. II. Effects of the molecular weight of kraft lignin on the properties of polyurethanes from a kraft lignin−polyether triol−polymeric MDI system. J. Appl. Polym. Sci. 1990, 40 (11−12), 1819−1832. (43) Duval, A.; Lange, H.; Lawoko, M.; Crestini, C. Modification of kraft lignin to expose diazobenzene groups: Toward pH- and lightresponsive biobased polymers. Biomacromolecules 2015, 16 (9), 2979− 2989. (44) Gao, G.; Xu, W. Z.; Kadla, J. F. Reversible pH-responsive hydrogels of softwood kraft lignin and poly[(2-dimethylamino)ethyl methacrylate]-based polymers. J. Wood Chem. Technol. 2014, 35 (1), 73−90. (45) Kubo, S.; Hashida, K.; Yamada, T.; Hishiyama, S.; Magara, K.; Kishino, M.; Ohno, H.; Hosoya, S. A characteristic reaction of lignin in ionic liquids; glycelol type enol-ether as the primary decomposition product of β-O-4 model compound. J. Wood Chem. Technol. 2008, 28 (2), 84−96.

2958


Changes in the Structure and the Thermal Properties of Kraft Lignin

Recommend Documents