Article pubs.acs.org/JAFC

Characterization of Lignin Extracted from Birch Wood by a Modified Hydrotropic Process Konstantin Gabov,† Richard J. A. Gosselink,§ Annika I. Smeds,‡ and Pedro Fardim*,† †

Laboratory of Fiber and Cellulose Technology and ‡Laboratory of Wood and Paper Chemistry, Åbo Akademi University, Porthansgatan 3, FI-20500 Åbo, Finland § Wageningen UR Food and Biobased Research, Bornse Weilanden 9, NL-6708 WG Wageningen, The Netherlands S Supporting Information *

ABSTRACT: In this work an environmentally friendly hydrotropic process was used to extract lignin from industrial birch wood chips. Two hydrotropic treatments were performed, a conventional and a modified process. The lignins were characterized using FTIR, pyrolysis−gas chromatography−mass spectrometry (pyrolysis-GC-MS), 31P and 1H−13C HSQC NMR, and size exclusion chromatography (SEC). The chemical (carbohydrates, extractives, etc.) and elemental compositions of the lignins were also determined. The yields of both lignins were 16.1% (dry wood basis), and the obtained lignins had very low contents of non-lignin compounds. The treatments resulted in significant changes of the structure of the lignins, a decrease in aliphatic hydroxyls and an increase in phenolic ones. The lignin isolated by the modified treatment underwent more substantial change than the reference one. It is believed that the data presented will facilitate utilization of hydrotropic lignin and promote the adoption of the hydrotropic process in the pulp and biorefinery industry. KEYWORDS: hydrotropic lignin, modified hydrotropic treatment, lignin characterization, NMR, pyrolysis-GC-MS, FTIR, SEC



INTRODUCTION Nowadays, lignin attracts great attention as the second most abundant renewable polymer on Earth and as a source of aromatic compounds, which can be used as a replacement for petroleum-based counterparts. Lignin is a complex cross-linked aromatic polymer that consists of phenylpropane units (syringylpropane, guaiacylpropane, and hydroxyphenylpropane) linked through C−C and C−O−C bonds in different combinations such as aryl−alkyl, alkyl−alkyl, and aryl−aryl.1 The composition of lignin varies significantly depending on plant species and methods and conditions of isolation and purification. It can differ in the ratio of the phenylpropane units and type and amount of linkages and functional groups. Lignins can also contain various impurities, the nature and amount of which will depend on the above-mentioned factors. In this study, technical lignin was obtained from birch wood industrial chips using an environmentally friendly hydrotropic process. Hydrotropic treatment of biomass is a process that is carried out with concentrated aqueous solutions of hydrotropic agents, or hydrotropes, at elevated temperatures.2 Hydrotropes are amphiphilic salts that have aromatic anions and, at high concentrations (30−40%), considerably improve the aqueous solubility of poorly soluble substances.3 Figure 1 shows some examples of hydrotropes. Hydrotropic treatment has several attractive features that make it an interesting alternative for biomass refineries.2 Among them are a simple recovery of a hydrotropic solution, which comprises few steps, the possibility of using one solution for about six or seven successive treatments, and the environmental friendliness of the process. Furthermore, recent studies showed that hydrotropic treatment has great potential as a process for the fractionation of hardwood biomass into lignin and cellulose.4,5 © XXXX American Chemical Society

Figure 1. Hydrotropic salts: sodium salicylate, 1; sodium cumenesulfonate, 2; sodium phenolsulfonate, 3; sodium xylenesulfonate, 4.

Hydrotropic lignin has not been as extensively studied as hydrotropic pulp. However, several papers can be found dealing with the subject. It was claimed that the structure of hydrotropic lignin was not altered to any great extent, and the lignin was very close to the native lignin of wood.6 On the other hand, some researchers stated the opposite.7,8 On the basis of the yield of aromatic aldehydes from alkaline nitrobenzene oxidation and the amount of phenol reacted with lignins, Zoldners and Surna8 concluded that hydrotropic lignin was more condensed than dioxane lignin, which served in the study as a representative of the native wood lignin. However, the authors also showed that the reactivity and degree of modification of the hydrotropic lignin significantly depended on the treatment conditions.8 A number of papers were devoted to utilization of hydrotropic lignin. Kalninsh et al.9 showed that hydrotropic Received: March 27, 2014 Revised: October 6, 2014 Accepted: October 7, 2014

A

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treatments were disintegrated and washed as described elsewhere5 and screened in a Valmet TAP03 rotary screen using a 0.1 mm slit basket. The accepted fraction was used for the determination of residual lignin content. Hydrotropic lignins from the collected spent solutions were precipitated by dilution with 10-fold (v/v) of hot distilled water. After addition of the hot water, the temperature of the diluted solution was around 75 °C. The precipitated lignin was separated by filtration through a glass filter funnel no. 2. At the beginning of the filtration, some amount of the filtrate was recirculated back onto the filter until the filtrate became free of lignin particles. The precipitate was further washed with around 8-fold (v/v, based on the volume of spent solution taken for precipitation) of hot distilled water. The temperature of the suspension during the washing was maintained at 75−80 °C. The washed lignins were freeze-dried and stored in a cold room at a temperature of 10 °C. The amounts of dissolved, precipitated, and nonprecipitated lignins were calculated according to eqs 1−4.

lignin could replace 55% of phenol in phenol−formaldehyde resins. Gromov and Pormale10 performed hydrogenation of lignin directly during hydrotropic treatment with a nickel catalyst to obtain phenols. Kreicberg and Grabovskij7 carried out nitrobenzene oxidation of aspen hydrotropic lignin in alkaline media and obtained aromatic aldehydes with a yield of 8.8% based on Klason lignin of the hydrotropic lignin. Another paper11 describes oxidative degradation of aspen hydrotropic lignin with oxygen in an alkaline media in the presence of copper oxide. The yield of low molecular weight products was 39.3% based on Klason lignin of the hydrotropic lignin. The products were further fractionated into aldehyde, acidic, phenolic, and neutral fractions.11 All of these studies have shown that hydrotropic lignin is a valuable biobased polymer that can be used to produce chemicals and as a substituent of phenol in phenol− formaldehyde resins. Despite all of the research, there is no detailed information about the composition and chemical structure of this, rather novel, type of technical lignin. However, characterization of lignin has a great importance for the evaluation of its potential for various applications. In this study, hydrotropic lignin was isolated from industrial birch wood chips by conventional (reference) and modified hydrotropic treatments.5 The difference between these two processes is that in the latter the hydrotropic solution was modified by the addition of formic acid and hydrogen peroxide. Such modification led to a better delignification, and the obtained pulp had higher cellulose content than the reference one; however, the modified process was more severe, which resulted in a lower yield of pulp with a lower viscosity.5 Therefore, it was interesting to investigate how the modification of hydrotropic solution with hydrogen peroxide and formic acid would affect the chemical composition of lignin. The lignins were characterized with Fourier transform infrared spectroscopy (FTIR), pyrolysis−gas chromatography−mass spectrometry (pyrolysis-GC-MS), semiquantitative 1H−13C HSQC NMR, 31P NMR spectroscopy techniques, and size exclusion chromatography (SEC) to elucidate their structures and functional groups. The chemical and elemental compositions of the lignins were studied as well. We believe that the characterization of hydrotropic lignins presented will facilitate their utilization and promote the hydrotropic process in the pulp and biorefinery industry.



dissolved lignin (or degree of delignification) based on lignin in wood, %

Ldis = (Lwood − Lpulp × m pulp /m wood)/Lwood × 100

(1)

precipitated lignin (lignin yield) based on wood, %

Lprecip/wood = Lprecip ss × mss /m wood × 100

(2)

precipitated lignin (lignin yield) based on the original lignin in wood, %

Lprecip/lignin = Lprecip ss × mss /mlignin wood × 100

(3)

nonprecipitated lignin based on the original lignin in wood, %

Lnonprecip = Ldis − Lprecip/lignin

(4)

Lwood and Lpulp are lignin contents in birch wood and in pulp, %; mpulp and mwood are the amounts of pulp and wood, g; Lprecip ss is the amount of lignin precipitated from spent solution, g/g; mss is the amount of spent solution calculated as a sum of hydrotropic solution, water in wood, and dissolved wood, g; mlignin wood is the amount of lignin in birch wood, g. Chemical Composition of Lignins. Residual carbohydrates in the lignin samples were determined using acid methanolysis and gas chromatography (GC) as described elsewhere.5 The analysis was carried out using an AutosystemXL (PerklinElmer, Norwalk, CT, USA) gas chromatograph. The columns used were 25 m × 0.200 mm i.d., 0.11 μm, HP-1 (Agilent Technologies, Santa, Clara, CA, USA) and 25 m × 0.199 mm i.d., 0.11 μm, HP-5 (Agilent Technologies). The operating parameters of the GC were the following: injection amount, 1 μL; split ratio, 1:25; injector temperature, 250 °C; temperature program of the oven, 100 °C, held for 1 min, increased at 4 °C/min to 170 °C and then at 12 °C/min to 300 °C, and, finally, held for 7 min. The temperature of the detector (flame ionization detector, FID) was 310 °C. Hydrogen was used as a carrier gas. Extractives in lignins were analyzed by GC after silylation. The amount of samples weighed for the analysis was about 8 mg with the accuracy of 0.1 mg. Two milliliters of the internal standard containing 0.02 mg/mL cholesterol, heneicosanoic acid, 1,3-dipalmitoyl-2oleoylglycerol, and cholesteryl heptadecanoate in methyl tert-butyl ether was added to the samples. The samples were mixed well with the internal standard and evaporated under a nitrogen flow at 50 °C. To ensure complete removal of the solvent, the samples were kept in a vacuum oven for 20 min at 40 °C. The dried samples were dissolved in 40 μL of pyridine and silylated with 80 μL of N,O-bis(trimethylsilyl)trifluoroacetamide and 40 μL of trimethylchlorosilane at 70 °C for 45 min. After cooling at room temperature, the samples were transferred

MATERIALS AND METHODS

Materials. Birch wood chips collected from a Finnish pulp and paper mill were screened in the laboratory, and a 4−6 mm fraction was used for the hydrotropic treatments. Technical grade sodium xylenesulfonate having purity >90% (Sigma-Aldrich) was used as a hydrotropic agent. Other chemicals were obtained from commercial sources and were used without further purification. Isolation of Hydrotropic Lignins. Hydrotropic lignins were extracted in a 10 L rocking digester using a 36% aqueous hydrotropic solution. Two treatments were performed, namely, a conventional reference method and one with the addition of formic acid and hydrogen peroxide. Lignins obtained with these treatments were designated R and M lignins, respectively. A dosage of hydrogen peroxide for the modified treatment was 2.5% based on wood, and the amount of formic acid was added to lower the pH to 3.5. The conditions of the extractions were the following: liquor-to-wood ratio, 4 (w/w); heating rate, 1.5 °C/min; dwell temperature, 170 °C; and dwell time, 120 min. The pH values of the prepared hydrotropic solutions were 9.2 and 3.5 for the reference and modified solutions, respectively. Prior to the extractions, the chips were vacuumimpregnated with the hydrotropic solutions at room temperature for 30 min using a water pump. The solid residues (or pulps) after the B

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Other Analysis. Contents of lignin in the birch wood and in the wood residues (pulps) after the treatments were determined as described elsewhere.5

into GC vials and analyzed by Clarius 500 (PerkinElmer) and AutosystemXL (PerkinElmer) gas chromatographs equipped with 6−7 m × 0.530 mm i.d., 0.15 μm, HP-1 (Agilent Technologies) and 25 m × 0.200 mm i.d., 0.11 μm, HP-1 (Agilent Technologies) columns, respectively. The injection parameters and temperature programs are described elsewhere.12 The short column was used to analyze groups of components, whereas the long column was used for the analysis of individual substances (except for steryl esters and triglycerides). The identification and assignment of the peaks were done with the help of GC-MS.13 Elemental Analysis. Carbon, hydrogen, nitrogen, and sulfur contents of the lignins were measured with a Thermo Scientific Flash 2000 series elemental analyzer. The description of the analysis can be found elsewhere.14 Oxygen content was calculated by subtracting the sum of C, H, N, and S from 100%. The elemental compositions of lignins obtained by the reference and modified treatments were as follows: carbon, 65.51 and 64.50%; hydrogen, 5.87 and 5.54%; nitrogen, 0.23 and 0.22%; sulfur, 0.16 and 0.39%; oxygen, 28.23 and 29.35%, respectively. Pyrolysis-GC-MS. The mass ratios of the p-hydroxyphenyl (H), guaiacyl (G), and syringyl units (S) in the hydrotropic lignins as well as in the lignin in the wood were determined using pyrolysis-GC-MS. Before the analysis, the wood was ground using a cutting mill with a sieve cassette with 1 mm openings. After that, the wood meal was sieved to obtain a fraction S) CH asymmetric deformations in methyl and methylene groups aromatic skeletal vibrations aliphatic CH symmetric deformation in methyl (not methoxyl) + OH deformation in phenols S ring breathing + G ring substituted in position 5 G ring breathing and CO stretching CO stretching in phenols and ethers aromatic CH in-plane deformations (S units) aromatic CH in-plane deformations in G units + CO deformations in primary alcohols CH out-of-plane deformation of trans-double bond (conjugated with an aromatic ring, α,β-double bond) aromatic CH out-of-plane deformation (only in GS lignin type) aromatic CH out-of-plane deformation (only in GS and H lignin types)

The spectra showed bands typical of hardwood lignin. A relatively high peak at 1325 cm−1 and a strong band at 1116 cm−1 are characteristic for an IR spectrum of hardwood lignins.29,30 In the case of softwood lignin, the former is usually very small, and the latter appears at 1140 cm−1. Such shifts have been explained by Faix,29 who found that the band at 1140 cm−1 is shifted toward lower wavenumbers as the content of S units in lignin increases. Bands at 833 and 913 cm−1 are also specific to hardwood lignins, and they are absent in spectra of softwood lignins.29,30 The spectra of both hydrotropic lignins were almost identical, implying that the lignins have similar structures. Compared to other lignins,31 the spectra look very similar to the spectrum of Alcell lignin. This could suggest that E

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Figure 4. HSQC spectra of hydrotropic lignins: (A) R and (B) M. The assigned lignin structures are shown in Figure 5.

Both lignins were quite similar in the amount of bonds. The only difference was the amount of β-O-4′ linkage. With respect to this, the M lignin seems to be more degraded than the reference one. The reason for this is more severe conditions during the modified treatment due to a higher acidity and the presence of an oxidant. This fact can be also supported by the lower yield of cellulosic pulp obtained with the modified process and lower content of residual lignin in the pulp. In addition, it has been shown previously5 that the pulp from the modified process has lower viscosity and a smaller amount of residual hemicelluloses in comparison with the reference one, which also indicates the severity of the modified treatment. 31 P NMR. The data from other analyses were extended by quantitative 31P NMR spectrometry. Before 31P NMR measurements, the sample is derivatized with 2-chloro-4,4,5,5-

followed by radical coupling and radical exchange reactions.22,32,36 The former path leads to the formation of β-1′, β-β′, and β-5′ structures.32 β-1′ and β-5′ can be converted to stilbenes (as was discussed under FTIR) with the loss of a Cγ− OH group as formaldehyde.32,37 Besides these reactions, lignin also undergoes counterproductive condensation reactions. In the case of the modified process, additional various reactions could take place due to added hydrogen peroxide.5 The ratio of the units (H/G/S) calculated from the NMR data differs from the values obtained by pyrolysis-GC-MS, especially the results for the reference lignin (Table 2). However, the results of NMR can be considered more accurate than those of pyrolysis-GC-MS. According to the NMR, both lignins showed difference in the ratio of the units. F

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the fifth position (condensed structures).38 Therefore, it is quite likely that such a high amount of 5-substituted phenolic hydroxyls was affected by a high content of S units in the lignins. The contents of phenolic and aliphatic hydroxyl groups in birch (Betula pendula) MWL determined by 31P NMR are 1.2 and 5.4 mmol/g, respectively.39 By comparison of these numbers with the contents of aliphatic and phenolic hydroxyl groups of the hydrotropic lignins, it is clearly seen that the content of phenolic hydroxyl groups in the lignins increased after the treatment, whereas the amount of aliphatic hydroxyls decreased (Table 3). Both the increase of phenolic hydroxyl groups and the decrease of aliphatic OH are consequences of the reactions discussed above. Aliphatic hydroxyls could also be converted into carboxyl groups as a result of oxidation reactions.36 The amounts of carboxyl groups were 0.30 and 0.32 mmol/g for R and M lignins (Table 3). The contributions of the extractives to the total content of carboxyl groups were not significant and were 6.0 and 4.9% in the case of R and M lignins, respectively. Generally, the lignins did not differ from each other in the amount of functional groups, except the contents of aliphatic hydroxyl groups. Again, as was discussed previously, the reason for this was the different composition of the hydrotropic solutions employed in the treatments. SEC. The molecular weights of lignins determined by alkaline SEC are shown in Table 3. The values do not differ considerably; however, the M lignin has higher Mw than the reference one. Because the M lignin underwent greater changes due to the severity of the process, the difference between the molecular weights can be explained by the higher degree of condensation in the case of lignin obtained according to a modified process. Comparison with Other Technical Lignins. Table 3 displays some characteristics of two commercial hardwood technical lignins and the hydrotropic lignins. The data are presented only for hardwoods, because lignins from nonwood and softwood raw materials are different by nature. The lignin obtained from the alkaline process has a higher content of carboxyl groups in comparison to the lignins isolated in acidic conditions. This can be explained by the higher oxidation rate of phenolic units by air in alkaline media in comparison to an acidic one.40 A similar trend as with the carboxylic groups can be observed for carbohydrates. Lower contents of carbohydrates of Alcell and hydrotropic lignins can be attributed to the extensive hydrolytic processes taking place in the acidic media, as was discussed earlier in this paper. Among the listed lignins, hydrotropic lignins are expected to be closer to Alcell lignin due to the similarity of the nature of the processes. Indeed, the contents of carbohydrates and carboxyl groups are quite similar (Table 3). In addition, the FTIR spectra of ththate hydrotropic lignins were similar to the spectrum of the Alcell lignin. However, due to the difference in the wood species and treatment agents as well as the process conditions (temperature, duration, liquor to solid ratio, etc.), the lignins also showed differences. For example, the Alcell lignin has a lower molecular weight and a lower content of phenolic hydroxyl groups than the hydrotropic lignins (Table 3) Also, the Alcell lignin is sulfur-free, and the hydrotropic lignins contain small amounts of sulfur, which, however, might be due to contamination in the hydrotropic agent. In conclusion, the present study showed that 67% of the original birch wood lignin could be recovered by conventional

Figure 5. Lignin structures detected by 2D NMR: (A) β-O-4′; (B) β5′ (phenylcoumaran structure); (C) β-β′ (resinol structure); (H) phydroxyphenylpropane unit; (G) guaiacylpropane unit; (S) syringylpropane unit; (S′) syringylpropane unit with a carbonyl group at Cα.

Table 2. Content of the Main Linkages in the Hydrotropic Lignins, Lignin Units, and Their Ratio Determined by 1 H−13C HSQC NMR linkages, per 100 units

units, %

lignin

β-O-4′ (A)

β-5′ (B)

β-β′ (C)

H, %

G, %

S, %

S′, %

S/G

R M

15.0 11.8

3.3 3.2

3.2 3.0

1 2

19 22

78 73

2 3

4.2 3.4

tetramethyl-1,3,2-dioxaphospholane, which reacts with OH moieties of functional groups. Therefore, this technique is used for estimation of aromatic and aliphatic hydroxyl and carboxyl groups in lignin samples. The amount of functional groups calculated from the 31P NMR is summarized in Table 3. Both lignins had a relatively high content of phenolic hydroxyls attached to 5-substituted units. The signal of these units consisted of two poorly resolved signals of S phenolic units and G phenolic units substituted at Table 3. Some Characteristics of the Hydrotropic and Two Hardwood Technical Lignins lignins hydrotropic

carbohydrates, % sulfur, % aliphatic OH, mmol/g 5-substituted (condensed + S) phenolic OH, mmol/g G−OH, mmol/g H−OH, mmol/g total phenolic, mmol/g COOH, mmol/g Mw, g/mol Mn, g/mol polydispersity a

R

M

Alcella

HW sodaa

0.32 0.16 1.38 2.43

0.14 0.39 1.13 2.39

0.30

8.04

1.08 1.81

1.34 1.62

0.64 0.15 3.22 0.30 5306 1035 5.1

0.60 0.17 3.16 0.32 5910 1143 5.2

0.70 0.20 2.71 0.30 3470 850 4.1

0.51 0.34 2.47 1.06 4570 1105 4.0

Data for Alcell and HW soda are from the literature.19 G

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(11) Telysheva, G. M.; Sergeeva, V. N.; Mozheiko, L. N. Alkaline oxidative decomposition of hydrotropic and sulfuric acid lignins. Latv. PSR Zinat. Akad. Vestis, Kim. Ser. 1966, 6, 720−728. (12) Strand, A.; Sundberg, A.; Vähäsalo, L.; Holmbom, B. Influence of pitch composition and wood substances on the phase distribution of resin and fatty acids at different pH levels. J. Dispersion Sci. Technol. 2011, 32, 702−709. (13) Smeds, A. I.; Eklund, P. C.; Monogioudi, E.; Willför, S. M. Chemical characterization of polymerized products formed in the reactions between matairesinol and pinoresinol with the stable radical 2,2-diphenyl-1-picrylhydrazyl. Holzforschung 2012, 66, 283−294. (14) Vega, B.; Wondraczek, H.; Zarth, C. S. P.; Heikkilä, E.; Fardim, P.; Heinze, T. Charge-directed fiber surface modification by molecular assemblies of functional polysaccharides. Langmuir 2013, 29, 13388− 13395. (15) Ralph, J.; Marita, J. M.; Ralph, S. A.; Hatfield, R. D.; Lu, F.; Ede, R. M.; Peng, J.; Quideau, S.; Helm, R. F.; Grabber, J. H.; Kim, H.; Jimenez-Monteon, G.; Zhang, Y.; Jung, H.-J. G.; Landucci, L. L.; MacKay, J. J.; Sederoff, R. R.; Chapple, C.; Boudet, A. M. Solutionstate NMR of lignins. In Advances in Lignocellulosics Characterization; Argyropoulos, D. S., Ed.; TAPPI Press: Atlanta, GA, USA, 1999; pp 55−108. (16) Ralph, S. A.; Ralph, J.; Landucci, L. NMR database of lignin and cell wall model compounds. U.S. Forest Products Laboratory, Madison, WI, USA, http://ars.usda.gov/Services/docs.htm?docid= 10491, 2004 (accessed January 2009). (17) Martínez, A. T.; Rencoret, J.; Marques, G.; Gutiérrez, A.; Ibarra, D.; Jiménez-Barbero, J.; del Río, J. C. Monolignol acylation and lignin structure in some nonwoody plants: a 2D NMR study. Phytochemistry 2008, 69, 2831−2843. (18) Ralph, J.; Landucci, L. L. NMR of lignins. In Lignin and Lignans: Advances in Chemistry; Heitner, C., Dimmel, D. R., Schmidt, J. A., Eds.; CRC Press (Taylor & Francis Group): Boca Raton, FL, USA, 2010; pp 137−234. (19) Gosselink, R. J. A.; van Dam, J. E. G.; De Jong, E.; Scott, E. L.; Sanders, J. P. M.; Li, J.; Gellerstedt, G. Fractionation, analysis, and PCA modeling of properties of four technical lignins for prediction of their application potential in binders. Holzforschung 2010, 64, 193− 200. (20) Gromov, V. S.; Odincov, P. N. The hydrotropic pulping of aspen wood and the influence of various factors on it. Tr. Inst. Lesokhoz. Probl. Akad. Nauk Latv. SSR, Vopr. Lesokhim. Khim. Drev. 1957, 12, 79−90. (21) Gromov, V. S.; Odincov, P. N. Composition and properties of the lignin hydrotropically isolated from hardwood. Tr. Inst. Lesokhoz. Probl. Akad. Nauk Latv. SSR, Vopr. Lesokhim. Khim. Drev. 1957, 12, 91−100. (22) Leschinsky, M.; Zuckerstätter, G.; Weber, H. K.; Patt, R.; Sixta, H. Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: comparison of different lignin fractions formed during water prehydrolysis. Holzforschung 2008, 62, 645−652. (23) Goundalkar, M. J.; Bujanovic, B.; Amidon, T. E. Analysis of noncarbohydrate based low-molecular weight organic compounds dissolved during hot-water extraction of sugar maple. Cellul. Chem. Technol. 2010, 44, 27−33. (24) Hergert, H. L.; Goyal, G. C.; Lora, J. H. Limiting molecular weight of lignin from autocatalysed organosolv pulping of hardwood. In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W. G., Northey, R. A., Schultz, T. P., Eds.; ACS Symposium Series 742; American Chemical Society: Washington, DC, USA, 2000; pp 265− 277. (25) Fengel, D.; Wegener, G. Wood: Chemistry, Ultrastructure, Reactions; de Gruyter: Berlin, Germany, 1984; p 613. (26) Gosselink, R. J. A.; Abächerli, A.; Semke, H.; Malherbe, R.; Käuper, P.; Nadif, A.; van Dam, J. E. G. Analytical protocols for characterization of sulfur-free lignin. Ind. Crops Prod. 2004, 19, 271− 281.

and modified hydrotropic treatments. The structure of lignins was altered during the hydrotropic treatments; the content of phenolic hydroxyl groups increased and that of aliphatic hydroxyls decreased. The lignin from the modified treatment underwent more considerable changes than the reference one, although generally both lignins were quite similar to each other. Also, the lignins showed some similarities with technical organosolv Alcell lignin. Both hydrotropic lignins have very little non-lignin contaminants and potentially present valuable raw materials for further chemical and thermal conversion or for making shaped biobased materials.



ASSOCIATED CONTENT

* Supporting Information S

Material balance of lignin for the reference (Figure S1A) and modified (Figure S1B) hydrotropic treatments and 31P NMR spectra of the reference lignin (Figure S2A) and lignin from the modified hydrotropic process (Figure S2B). This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*(P.F.) Phone: +358 504096424. E-mail: pfardim@abo.fi. Funding

The Graduate School for Biomass Refining (BIOREGS) is acknowledged for its financial support. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Jacinta van der Putten and Guus Frissen are kindly acknowledged for the SEC analysis and NMR analysis, respectively.



REFERENCES

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dx.doi.org/10.1021/jf5037728 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Characterization of lignin extracted from birch wood by a modified hydrotropic process.

In this work an environmentally friendly hydrotropic process was used to extract lignin from industrial birch wood chips. Two hydrotropic treatments w...
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