Purification and characterization of kraft lignin

Holzforschung 2015; aop

Wenwen Fang, Marina Alekhina, Olga Ershova, Sami Heikkinen and Herbert Sixta*

Purification and characterization of kraft lignin Abstract: To upgrade the utilization of kraft lignin (KL) for high-performance lignin-based materials (e.g., carbon fiber), the purity, molecular mass distribution (MMD), and thermal properties need to be improved and adjusted to target values. Therefore, different methods, such as ultrasonic extraction (UE), solvent extraction, dialysis, and hot water treatment (HWT), were applied for the purification of KL. The chemical and thermal properties of purified lignin have been characterized by nuclear magnetic resonance, Fourier transform infrared, gel permeation chromatography, elemental analysis, differential scanning calorimetry, and thermogravimetric analysis. The lignin fractions obtained by UE with ethanol/acetone (E/A) mixture (9:1) revealed a very narrow MMD and were nearly free of inorganic compounds and carbohydrates. Further, the E/A-extracted lignin showed a lower glass transition temperature (Tg) and a clearly detectable melting temperature (Tm). Dialysis followed by HWT at 220°C is an efficient method for the removal of inorganics and carbohydrates; however, lignin was partly forming condensed structures during the treatment. Keywords: chemical analysis, hot water treatment, kraft lignin, thermal analysis, ultrasonic extraction DOI 10.1515/hf-2014-0200 Received July 6, 2014; accepted November 26, 2014; previously published online xx

Introduction Kraft lignin (KL) as a byproduct of sulfate (kraft) cooking process amounts to 45 × 106 t year-1, which is approximately 85% of the global lignin production (Tejado et al. 2007). However, only 1–2% of this lignin is isolated from black

*Corresponding author: Herbert Sixta, Department of Forest Products Technology, Aalto University, P.O. Box 16300, Vuorimiehentie 1, FI-00076 Espoo, Finland, e-mail: [email protected] Wenwen Fang, Marina Alekhina and Olga Ershova: Department of Forest Products Technology, Aalto University, Espoo, Finland Sami Heikkinen: Laboratory of Organic Chemistry, Helsinki University, Helsinki, Finland

liquor (BL) and used as material (Gosselink et al. 2004; Vishtal and Kraslawski 2011). Many efforts have been done to develop high value-added products from KL, such as carbon fibers (Kadkla et al. 2002; Baker and Rials 2013), composites (Kharade and Kale 1999; Thielemans et al. 2001), binders and resins (Cavdar et al. 2008), activated carbons (Carrott and Carrott 2007), and some low molecular weight chemicals (vanillin, hydroxylated aromatics, and quinones) (Borges da Silva et al. 2009). The problem, in this context, is the inhomogeneity of KL, such as its wide molecular weight distribution and high content of impurities. The inorganic contaminants of KL originate from the process chemicals (e.g., NaOH and Na2S) (Chakar and Ragauskas 2004). The content of inorganics in BL is approximately 30% (Mansouri and Salvadó 2006). However, the sodium content of the precipitated KL can be reduced to approximately 0.5% by acid washing, whereas the remaining sulfur content in KL is 1%–3% (Öhman 2006). Sulfur compounds are toxic and can cause odor problems during thermal treatment. This is because the majority of sulfur is chemically linked to KL (Svensson 2008). KL also contains a certain amount of carbohydrates originating from lignin-carbohydrate complexes (Iversen and Wännström 1986; Vishtal and Kraslawski 2011). KL purification is rewarding. A hardwood KL purified by organic solvent extraction showed excellent spinnability in contrast to the poor spinnability of unpurified lignin due to the presence of infusible inorganics (Baker et al. 2012). KL can be fractionated and purified by means of various organic solvent systems, such as methanol, acetone, and diethyl ether (Mörck et al. 1986; Thring et al. 1996; Ropponen et al. 2011; Saito et al. 2013). Brodin et al. (2009) improved the homogeneity and purity of lignin significantly by membrane fractionation of industrial BLs followed by acidification according to the LignoBoost process (Öhman et al. 2006) and further purification by ion exchange. Enzymatic hydrolysis followed by acid hydrolysis is able to remove almost all carbohydrates from lignin but introduces new protein contaminants (Argyropoulos et al. 2002). Sulfur removal is a priori difficult, as it is organically bonded with KL. Raney nickel reduction is efficient for this purpose (Svensson 2008) but is expensive for industrial realization, and sulfur may inactivate the catalyst.

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2 W. Fang et al.: Purification and characterization of kraft lignin

In this work, the various purification methods of KL, including dialysis, solvent extraction with acetone, ethanol, and their aqueous solutions, and hot water treatment (HWT), should be investigated. Purified lignin samples will be characterized in terms of chemical and thermal properties. The upgrading potential of laboratoryscale purification by HWT should also be included into the working schedule based on a commercial Indulin AT purified under optimal conditions in a 10 l digester.

Materials and methods Two KL samples, KL5 and KL10.5, were obtained from industrial softwood kraft BL supplied by Metsä Fiber (Rauma, Finland). KL10.5 was precipitated by the acidification of BL with CO2 to pH 10.5, and the precipitate was separated by centrifugation. The precipitated cake was redispersed in H2SO4 at pH 2.5 and subsequently washed until the wash water was neutral. The supernatant from the first stage was treated with 6 M H2SO4 to reduce the pH to 5 and then centrifuged and washed to obtain KL5. Deuterated chloroform (CDCl3) with 0.03% (v/v) tetramethylsilane (TMS), pyridine (99.8%), chromium(III) acetyl acetonate (Cr(acac)3) (99.99%), H2SO4 (95–97%), acetone (99.5%) were purchased from Sigma Aldrich (St. Louis, MO, USA). Acetic anhydride (VWR International, Fontenay-sous-Bois, France) and ethanol (96.1%, Altia Oyj, Rajamäki, Finland) were also used. The ash was removed by dialysis in Spectra/Por dialysis membrane tubing (Sigma Aldrich, St. Louis, MO, USA) with a molecular

weight cutoff of 1000 Da. The essential steps of the purification process are presented schematically in Figure 1. HWT was performed in a microwave reactor (Monowave 300, Anton Paar, Graz, Austria) under stirring at 120°C, 140°C, 160°C, 180°C, 200°C, and 220°C. A suspension of lignin-water (liquid-tosolid ratio of 10 ml g-1) was heated to the target temperature, kept at its set point for 15 min, and subsequently cooled down below 55°C. The heating up time for all experiments was relatively short and generally within 1 min. After treatment, the residue was recovered by centrifugation and the supernatant was discarded. Solvent extraction was performed by ultrasonic extraction (UE) and Soxhlet extraction (SE) with the solvents presented in Table 1. UE was performed at ambient temperature in an ultrasonic bath (Sonorex Digitec DT 52H) for 10 min with a liquid-to-solid ratio of 10 ml g-1 and then centrifuged. SE was performed for 2 or 3 h with a liquid-to-solid ratio of 30 ml g-1. The supernatant was vacuum evaporated and dried. A large-scale HWT of commercial Indulin AT was conducted in a 10 l digester equipped with a heat exchanger and temperature control. Approximately 600 g of lignin (oven-dried mass) were added to the digester together with 6 l water to reach a liquid-to-solid ratio of 10 ml g-1. The temperature in the digester was raised to 220°C, kept for 10 min, and then cooled down. The residue was recovered by centrifugation and the supernatant was discarded. The lignin and carbohydrates were analyzed after two-step hydrolysis according to the standard NREL/TP-510-42618. The ash content was gravimetrically determined after incineration at 575°C according to ISO 1762 (2001). Elemental analysis was performed in a FlashEA 1112 elemental analyzer series CHNS/O equipped with an Autosampler MAS200R (Thermo Fisher Scientific, Bremen, Germany). The amount of phenolic hydroxyl groups was determined according to Sousa et al. (2001). The thermogravimetric analysis (TGA) of lignin was performed by means of a Perkin-Elmer TGA7 instrument (Winter St. Waltham, MA, USA) with approximately 7 mg of sample. The sample was dried at 105°C for 20 min before it was heated to 1000°C at a heating rate of 15°C min-1. Nitrogen served as purge gas. The glass transition temperature (Tg) and the melting temperature (Tm) of lignin were determined by differential scanning calorimeter (DSC) in a TA Instruments Q200 apparatus (Mettler Toledo, Columbus, OH, USA). The samples were heated at 105°C for 1 h to expel any remaining moisture. Then, 8–10 mg lignin was accurately weighed and heated to 200°C with a heating rate 10°C min-1. Each sample was analyzed at least in triplicate. The molecular mass distribution (MMD) was determined by gel permeation chromatography (GPC) equipped with UV detection (UV-Vis Table 1 Different solvents as well as specific mixture ratios and extraction time used to evaluate the extraction. Solvents

Figure 1 Flow chart of the overall process concept.

Mixture ratios (by volume)

UE Acetone/water Acetone/ethanol SE Acetone Ethanol Acetone/water Ethanol/water Acetone/ethanol

10:1; 9:1 10:1; 9:1; 7:3; 3:7 – – 9:1; 8:2 9:1 7:3; 3:7; 1:9


Extraction time

10 min 10 min 2h 2 h; 3 h 2h 2h 2 h; 3 h

W. Fang et al.: Purification and characterization of kraft lignin 3

120

a

Carbohydrates Inorganics Acid soluble lignin Klason lignin

110 100 90 80 70 Percent (%, based on original lignin)

Detector 2487). The column was eluted with dimethyl sulfoxide with 0.1 M LiBr at a flow rate of 1 ml min-1. The GPC system consisted of two analytical columns (Suprema 1000 and Suprema 100, 20 μm, 8 mm I.D.×300 mm) and one pre-column (Suprema 20 μm). The columns, injector and UV detector were maintained at 80°C during the analysis. Fourier transform infrared (FTIR) spectra were recorded by a Thermo Scientific Nicolet Avatar 360 FTIR spectrometer (KBr pellet technique). Thirty-two scans were collected between 400 and 4000 cm-1 with a resolution of 4 cm-1. Nuclear magnetic resonance (NMR) spectra were recorded with Varian Unity Inova 600 NMR spectrometer and Varian Inova 500 NMR spectrometer. The NMR samples were prepared with CDCl3 as solvent with 0.03% TMS employed as internal standard. All lignin samples (300 mg) were acetylated with 6 ml pyridine-acetic anhydride (1:1) before analysis. A 5 mm broadband probe head was used. The experiment was performed at 27°C with a sample concentration of 105 mg ml-1. Quantitative 13C experiments were recorded with the addition of relaxation agent Cr(acac)3 at a concentration of 10 mM. Inverse gated 1 H decoupling technique was used to record quantitative 13C data. 1H decoupling during the acquisition period was achieved by means of the GARP-1 decoupling scheme.

60 5 0 120

KL5

Dialysis

SE-E/A 9/1

D+HWT220

KL10.5

SE-E/A 9/1

UE-E/A 9/1

HWT220

b

110 100 90 80

Results and discussions

70 60

Screening of purification methods

5

KL5 and KL10.5 were purified by HWT combined with dialysis and organic solvent extraction (Figure 1). The chemical composition of the original and purified KL is presented in Figure 2. Dialysis was efficient for the removal of inorganics. Carbohydrates were eliminated by HWT due to the cleavage of the chemical bonds between the carbohydrates and lignin. Nonpurified KL10.5 contained 3.5% carbohydrates and 0.26% inorganics, and both impurities were completely removed by HWT at 220°C (Figure 2b). However, KL5, which contained a large amount of inorganics (16.9%), required dialysis before HWT (Figure 2a). According to the lignin specifications for manufacturing carbon fibers developed by the Oak Ridge National Laboratory, the lignin content should be higher than 99% and the ash content should be lower than 0.1%. Both KL5 and KL10.5 purified by HWT at 220°C combined with dialysis could match these specifications. UE was a fast and efficient method for KL purification. As visible in Figure 2b, ultrasonic extracted KL10.5 was nearly free of carbohydrates and ash; the yield of purified lignin was also high (65%). In addition, UE treatment generates a higher amount of phenolic groups in the lignin structure and therefore increases the activity of purified lignin (Table 2). Ethanol/acetone (E/A) mixtures of different ratios were tested on KL5. E/A (9:1) was found to be the most efficient mixture in terms of purity and yield. Extractions with pure acetone, ethanol, and other E/A mixtures also improved

Treatment

Figure 2 Chemical composition of KL5 (a) and KL10.5 (b) before and after purification.

the purity significantly, but the yield was much lower (see Table S1 in Supplementary Data). The changes of yield, phenolic group, and content of impurities along with temperature during HWT are presented in Figure 3. The amount of carbohydrates decreased clearly with increasing temperature until their complete removal at 220°C. The carbohydrates Table 2 Elemental composition and phenolic group content of KLs before and after purification. Elemental analysis (%) Sample KL5 KL5-D+HWT220 KL5-SE-E/A (9:1) KL10.5 KL10.5-HWT220 KL10.5-UE-E/A (9:1) D, dialysis. a Based on lignin.


C

H

N

S

OHphen (mmol g-1)a

50.6 65.0 61.4

5.5 5.4 5.6

0.1 0.1 0.1

65.5 63.3

5.6 5.7

0.08 0.08

6.6 2.8 3.1 3.1 2.1 2.7

4.0 3.3 4.0 3.6 2.9 4.2


80 3.5 3 3.0 2 2.5

1 0 160 180 200 HWT Temperature (°C)

KL5 D+HWT220 SE-E/A 9/1

100

2.0 140

-0.7

a

220

Figure 3 Changes of the amount of impurities, yield, and phenolic group along with elevated temperature during HWT of KL10.5.

-0.6 -0.5

80

-0.4 60 -0.3 40

-0.2

20

-0.1

0 120

0.0 -0.7

b

KL10.5 UE-E/A 9/1 HWT220

100

-0.6 -0.5

80

Derivative wieght (%/°C)

4.0

120

Weight percent (%)

Weight percent (%)

90

4.5

OHph /Sample (mmol/g )

phOH Sugars Ash Yield

100

-0.4 60

are hydrolyzed through the acid that is formed from the acetyl side groups of the hemicelluloses at elevated temperature. During HWT, lignin depolymerization occurs predominantly through the cleavage of α-O-4 and β-O-4 bonds. Condensation reactions are succeeding depolymerization reactions, in which the carbonium ion intermediate, formed through the scission of the aryl ether bond, reacts with another electron-rich carbon by radical coupling and forms stable C-C linkages (Borrega et al. 2011). The decrease of phenolic groups indicates the occurrence of condensation reactions during HWT, and the condensation reactions are clearly favored at high temperatures as seen in Figure 3. Funaoka et al. (1990) performed an extensive research on the condensation of lignin during the heating of wood and found that the condensation of lignin started above 120°C and the reaction rate doubled by increasing the temperature from 180°C to 220°C. For both KL5 and KL10.5, the sulfur content decreased after purification. HWT was especially efficient; however, 2.1%–2.8% of sulfur could not be removed (Table 2). The carbon content of HWT lignin is slightly higher than that of organic solvent-extracted lignin possible due to condensation of lignin during HWT.

Thermal properties Both crude lignin and purified lignin (Figure 4) decomposed over a broad temperature range (200–550°C) due to its complex structure and various functional groups (Brebu and Vasile 2010). The degradation peaks in four major temperature regions could be observed in the differential thermogravimetry (DTG) curves. Weight loss between 110°C and 200°C could be attributed to the vaporization of

-0.3 40

-0.2

20 0 100

-0.1

200

300

400

500

600

700

800

0.0 1000

Temperature (°C)

Figure 4 TGA/DTG curves of KL5 (a) and KL10.5 (b) before and after purification.

residual water in the sample. Water cannot be completely removed at 105°C due to the interaction of water with the hydroxyl groups of lignin (Hatakeyama and Hatakeyama 2010). A second peak at approximately 200°C–300°C is due to the thermal decomposition of low molar mass (MM) phenols, such as guaiacol. The predominated mass losses appeared at 350°C–450°C, indicating the decomposition of the aromatic ring. A fourth degradation step after 800°C was observed for untreated KL5, which is probably due to the decomposition of ash in KL5 that has a very high ash content of 16.9%. For the organic solvent-extracted lignin, the weight losses at the first two peaks were increased. This could be explained by the depolymerization of lignin during extraction, which generates more phenolic groups and therefore low MM phenols. During HWT at 220°C, the condensation of lignin occurred and the low MM products probably degraded, so there was almost no weight loss below 250°C. The char residue of KL at 1000°C was about 31%–42% (Table 3). The Tg values of KL before and after purification are shown in Table 3. The Tg value of softwood KL was reported to be approximately 140°C with variations (Brodin et al. 2009; Hatakeyama and Hatakeyama 2010; Ropponen et al. 2011). The differences are attributed to the content of moisture, the nature of contaminants



Table 3 Thermal analysis of KLs before and after purification.

0

Major degradation temperature (°C)

Sample

Char at 1000°C First Second Third Fourth Tg (°C) (%)

KL5 KL5-D+HWT220 KL5-SE-E/A (9:1) KL10.5 KL10.5-HWT220 KL10.5-UE-E/A (9:1)

160 – 152 – – 170

360 435 346 418 437 441

> 800 – – – – –

140 126 128 153 131 128

-10

37.8 36.9 35.0 34.4 41.1 31.5

D, dialysis.

(e.g., hemicelluloses), and the MM deviation. Recently, Sevastyanova et al. (2014) found a correlation between Tg and Mn of KLs fractionated by ultrafiltration, which follows the Fox-Flory equation. However, our data do not fit the correlation between Tg and Mn. This can be explained by the different chemical composition of the KL samples observed in the present study. Besides, the Mn values of KL are in a too narrow range, which makes it difficult to find some reliable correlation with Tg. Only a few types of lignin have been reported to have Tm. For example, birch lignin from acetic acid pulping (Kubo et al. 1998) and recently a hardwood KL purified by organic solvent extraction showed good melting properties (Baker et al. 2012). As shown in Figure 5, all purified KL demonstrated discernible Tm approximately 180°C; however, the shapes of the melting peaks were different. The lignin purified by organic solvents displayed sharp peaks, especially ultrasonic extracted lignin, whereas HWT lignin showed broad peaks. This is probably related to the differences in their MMD (Figure 6). A homogeneous polymer displays a sharp melting peak in the DSC curve, whereas a heterogeneous polymer melts in a broad temperature range. This phenomenon was also observed for blended materials (Hameed et al. 2013).

Molecular mass distribution (MMD) According to the data in Table 4, UE resulted in more homogeneous lignin with lower Mw and polydispersity. Unlike UE, the molecular mass (MM) of SE lignin increased slightly, which is probably due to the long heating time and subsequent condensation. The MM and polydispersity of HWT samples were extremely high, especially for KL10.5. As illustrated in Figure 6, the HWT lignin had a broader MMD profile with a new fraction of high MM, which is supposed to be the condensed lignin.

-15

KL5 D+HWT

SE-E/a 9/1

-20

-25 Heat flow (mV)

– – 271 329 – 291

-5

a

-30 0

-5

-10

b KL10.5 UE-E/A 9/1 HWT

-15

-20

-25

-30 60

80

100

120

140

160

180

200

Temperature (°C)

Figure 5 DSC curves of KL5 (a) and KL10.5 (b) before and after purification.

FTIR characterization The FTIR spectra of KL10.5 before and after purification are quite similar, which reveals that the “core” of the lignin structure did not change significantly during purification (Figure 7). However, the relative intensity of the shoulder at 1499 cm-1 to that of the band at 1513 cm-1 increased after HWT at 220°C. This is probably an indication for condensed aromatic rings and the presence of β-5, 5-5 linkages (Faix and Beinhoff 1988), whereas it remained almost unchanged during UE. The band at 1650 cm-1 in KL10.5, assigned to conjugated carbonyl groups that are reactive sites with phenyl nuclei, disappeared after heating to 220°C. This phenomenon was also reported by Funaoka et al. (1990). One explanation for the disappearance of conjugated carbonyl groups is their condensation with adjacent phenyl nuclei. The absorption at 1030 cm-1 assigned to alcoholic hydroxyl group decreased after heating, suggesting the condensation of side chains with phenyl nuclei. As shown in Figure 6, a new fraction



1596 1513

1710

a

KL5 D+HWT220 SE-E/A 9/1

1268 1213

1452 1463

1124

1429 1365

1031

1079 854 816

HWT220

dw/d logM

UE-E/A 9/1 1650

b

KL10.5

KL10.5 HWT220 UE-E/A 9/1

1800

1600

1400

1200

1000

800

Wavenumber (cm-1)

Figure 7 FTIR spectra of KL10.5 before and after purification in the 1800 to 800 cm-1 region. Assignment of the labeled peaks: 1596 cm-1, aromatic skeletal vibrations plus C = O stretch; 1513 cm-1, aromatic skeletal vibrations; 1268 cm-1, aromatic ring breathing (G) (Faix 1992).

0

1

2

3

4

5

6

7

log M

Figure 6 MMD of KL10.5 (a) and KL5 (b) and their purified samples.

appeared during HWT at 220°C with an extremely high Mw, which supports the assumption of lignin condensation. Besides, the increased carbon content also proves this assumption (Table 2).

NMR analysis The untreated KL10.5 and the purified lignin were subjected to NMR analysis. The HWT lignin could not be completely dissolved in CDCl3 even after acetylation; therefore, only untreated KL10.5 and UE lignins were analyzed by quantitative 13C and 1H NMR (see Figure S1 in Supplementary Data). The assignment

of the resonances is based on 2D NMR spectroscopy of dissolved wood lignin (spectra not shown) and literature data (Lundquist 1992; Capanema et al. 2004). From the integrals of the aromatic regions (Table 5), the condensed and protonated aromatic carbon appeared almost unchanged before and after UE, which indicates

Table 5 Quantification results of some lignin moieties obtained by integration of 13C and 1H NMR spectra of the acetylated samples.

Structure

NMR

OMearom Carom, oxygenated Carom condensed Carom protonated O-Acphen O-Acaliph

13

C C 13 C 13 C 1 H 1 H 13

Integration limits (ppm)

KL10.5

UE-E/A (9:1)

58.0–53.7 155.0–140.5 140.5–124.3 124.3–106 2.4–2.2 2.2–1.6

0.84 1.43 2.46 2.10 0.52 0.97

0.79 1.41 2.51 2.08 0.62 0.85

The signal of benzyl ring was used as reference and as internal standard to relate the different spectra.

Table 4 Molecular mass data for KLs before and after purification. KL5 Mol mass Mw Mn Mw/Mn

KL10.5

KL5

D+HWT220

SE-E/A (9:1)

KL10.5

HWT220

UE-E/A (9:1)

3553 1132 3.1

15 570 1829 8.5

4048 1329 3.0

5174 1472 3.5

51 225 2020 25.4

2870 1230 2.3

D, dialysis.

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Table 6 Chemical composition and yield of Indulin AT before and after purification. KL (%)

ASL (%)

Ligtotal (%)

Sugars (%)

Ash (%)

Sulfur (%)

Yield (%)

91.4 96.5

2.6 3.2

94 99.7

1.4 0.09

2.8 0.7

1.44 0.96

76%

Indulin AT HWT220-10 l

KL, Klason lignin; ASL, acid-soluble lignin.

that UE treatment did not affect the overall structure of lignin and no condensation occurred during the treatment. The results of quantitative 13C are supported by FTIR and MMD data. The 1H spectrum was used for the quantification of hydroxyl groups. The content of phenolic hydroxyl groups increased significantly after UE, indicating the cleavage of aryl-ether bonds. The decrease of aliphatic hydroxyl groups in UE lignin may be attributed to the cleavage of the Cγ and Cβ bonds in aryl chain (Leschinsky et al. 2008).

Purification of an industrial KL sample To demonstrate the potential of HWT for the purification of KL in a large scale, commercial Indulin AT was purified by HWT under optimal conditions in a 10 l digester. The purification of Indulin AT in laboratory scale was performed with both UE and HWT (Table S1 in Supplementary Data). The carbohydrates were almost completely removed by UE with E/A mixtures, but 0.6%–0.8% ash remained without a dialysis step. The amount of carbohydrates decreased with increasing temperature during HWT, and only 0.14% carbohydrates remained at 220°C. Additionally, the yield of purified lignin can reach 78.6% at 220°C. The ash content was not changed much with increasing temperature during HWT and remained in the range of 0.6%–0.9%. Therefore, the large-scale HWT of Indulin AT was conducted in a 10 l digester at 220°C and the chemical composition of purified Indulin AT is listed in Table 6. The carbohydrate content of purified Indulin AT was 0.09% and the yield was 76%. It is obvious that the purification with HWT at 220°C performed in a large-scale digester is as effective as in a laboratory-scale microwave reactor. Besides, approximately 33% sulfur was removed by HWT.

Conclusions Purified KL was obtained in high yields by two rapid procedures: first by UE with E/A mixture (9:1) and second by dialysis followed by HWT at 220°C. The impurities could be

removed without changing the core structure of KL. Even for highly contaminated KL5, which contained 16.9% inorganics, 3.7% carbohydrates, and 6.6% sulfur, a high purity ( > 97%) could be obtained with a high yield (70%) by HWT combined with dialysis. A substantial improvement in homogeneity and decrease of molecular mass was found after UE. Additionally, UE treatment resulted in lignin with good melting properties and more hydroxyl groups. This demonstrates that purified KL is a potential feedstock for value-added products. The yield losses during UE are limitations for industrial use. The inorganic compounds in KL could be eliminated by dialysis. HWT was found to be an efficient method for carbohydrates removal; however, severe condensation reactions occurred due to the harsh conditions and the phenolic groups were eliminated to some extent, which decreased the reactivity of the treated lignin. The feasibility of the large-scale purification of KL by HWT could be demonstrated. Although the purification procedures developed in this work are very efficient for the removal of inorganics and carbohydrates, the removal efficiency for sulfur is low. The development of a commercially viable procedure to remove covalently bound sulfur remains a challenge for the valorization of KL. Acknowledgments: The financial support from WoodWisdom and FIBIC is gratefully acknowledged. The authors thank Fraunhofer IAP for the elemental analysis and MMD measurement. Nolvi Leena is thanked for the help with DSC measurement.

References Argyropoulos, D.S., Sun, Y., Paluš, E. (2002) Isolation of residual kraft lignin in high yield and purity. J. Pulp Paper Sci. 28:50–54. Baker, D.A., Rials, T.G. (2013) Recent advances in low-cost carbon fiber manufacture from lignin. J. Appl. Polym. Sci. 130:713–728. Baker, D.A., Gallego, N.C., Baker, F.S. (2012) On the characterization and spinning of an organic-purified lignin toward the manufacture of low-cost carbon fiber. J. Appl. Polym. Sci. 124:227–234. Borges da Silva, E.A., Zabkova, M., Araffljo, J.D., Cateto, C.A., Barreiro, M.F., Belgacem, M.N., Rodrigues, A.E. (2009) An integrated process to produce vanillin and lignin-based polyurethanes from kraft lignin. Chem. Eng. Res. Des. 87: 1276–1292.



Borrega, M., Nieminen, K., Sixta, H. (2011) Effects of hot water extraction in a batch reactor on the delignification of birch wood. Bioresources 6:1890–1903. Brebu, M., Vasile, C. (2010) Thermal degradation of lignin – a review. Cellulose Chem. Technol. 44:353–363. Brodin, I., Sjöholm, E., Gellerstedt, G. (2009) Kraft lignin as feedstock for chemical products: the effects of membrane filtration. Hozforschung 63:290–297. Capanema, E.A., Balakshin, M.Y., Kadla, J.F. (2004) A comprehensive approach for quantitative lignin characterization by NMR spectroscopy. J. Agric. Food Chem. 52:1850–1860. Carrott, S., Carrott, R. (2007) Lignin – from natural adsorbent to activated carbon: a review. Bioresour. Technol. 98:2301–2312. Cavdar, A.D., Kalaycioglu, H., Hiziroglu, S., Mater. J. (2008) Some of the properties of oriented strandboard manufactured using kraft lignin phenolic resin. Process. Technol. 202: 559–563. Chakar, F.S., Ragauskas, A.J. (2004) Review of current and future softwood kraft lignin process chemistry. Ind. Crop. Prod. 20:131–141. Faix, O. (1992) Fourier transform infrared spectroscopy. In: Methods in lignin chemistry. Eds. Lin, S.Y., Dence, C.W. Springer-Verlag, Berlin. pp. 83–109. Faix, O., Beinhoff, O. (1988) FTIR spectra of milled wood lignins and lignin polymer models (DHPs) with enhanced resolution obtained by deconvolution. J. Wood Chem. Technol. 8:502–522. Funaoka, M., Kako, T., Abe, I. (1990) Condensation of lignin during heating of wood. Wood Sci. Technol. 24:277–288. Gosselink, R.J.A., De Jong, E., Guran, B., Abächerli, A. (2004) Coordination network for lignin – standardisation, production and applications adapted to market requirements (EUROLIGNIN). Ind. Crops. Prod. 20:121–129. Hameed, N., Xiong, R.Y., Salim, N.V., Guo, Q.P. (2013) Fabrication and characterization of transparent and biodegradable cellulose/poly (vinyl alcohol) blend films using an ionic liquid. Cellulose 20:2517–2527. Hatakeyama, H., Hatakeyama, T. (2010) Thermal properties of isolated and in situ lignin. In: Lignin and Lignans. Eds. Heitner, C., Dimmel, D.R., Schmidt, J.A. CRC Press, Boca Raton. pp. 301–319. Iversen, T., Wännström, S. (1986) Lignin-carbohydrate bonds in a residual lignin isolated from pine kraft pulp. Holzforschung 40:19–22. Kadkla, F., Kubo, S., Gilbert, R.D., Venditti, R.A. (2002) Lignin-based carbon fibers. In: Chemical Modification, Properties, and Usage of Lignin. Eds. Hu, T.Q. Kluwer Academic/Plenum, New York. pp. 121–137. Kharade, A.Y., Kale, D.D. (1999) Lignin-filled polyolefins. J. Appl. Polym. Sci. 72:1321–1326. Kubo, S., Uraki, Y., Sano, Y. (1998) Preparation of carbon fibers from softwood lignin by atmospheric acetic acid pulping. Carbon 36:1119–1124. Leschinsky, M., Zuckerstätter, G., Weber, H.K., Patt, R., Sixta, H. (2008) Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 1: comparison of different lignin

fractions formed during water prehydrolysis. Holzforschung 62:645–652. Lundquist, K. (1992) Proton (1H) NMR spectroscopy. In: Methods in Lignin Chemistry. Eds. Lin, S.Y., Dence, C.W. Springer Verlag, Berlin. pp. 243–249. Mansouri, N.E.E., Salvadó, J. (2006) Structural characterization of technical lignins for the production of adhesives: application to lignosulfonate, kraft, soda-anthraquinone, organosolv and ethanol process lignins. Ind. Crops Prod. 24:8–16. Mörck, R., Yoshida, H., Kringstad, K.P., Hatakeyama, H. (1986) Fractionation of kraft lignin by successive extraction with organic solvents. 1. Functional groups 13C-NMR-spectra and molecular weight distributions. Holzforschung 40:51–60. Öhman, F. Precipitation and Separation of Lignin from Kraft Black Liquor. Ph.D. Thesis, Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden (2006). Öhman, F., Theliander, H., Tomani, P., Axegård, P. (2006) Method for separating lignin from black liquor. Patent WO2006031175. Ropponen, J., Räsänen, L., Rovio, S., Ohra-aho, T., Liitiä, T., Van de Pas, D., Tamminen, T. (2011) Solvent extraction as a means of preparing homogeneous lignin fractions. Holzforschung 65:543–549. Saito, T., Perkins, J.H., Vautard, F., Meyer, H.M., Messman, J.M., Tolnai, B., Naskar, A.K. (2014) Methanol fractionation of softwood kraft lignin: impact on the lignin properties. ChemSusChem. 7:221–228. Sevastyanova, O., Helander, M., Chowdhury, S., Lange, H., Wedin, H., Chang, L.M., Ek, M., Kadla, J.F., Crestini, C., Lindström, M.E. (2014) Tailoring the molecular and thermo-mechanical properties of kraft lignin by ultrafiltration. J. Appl. Polym. Sci. DOI: 10.1002/APP.40799. Sousa, F.D., Reimann, A., Björklund Jansson, M., Nilvebrant N.-O. (2001) Estimating the amount of phenolic hydroxyl groups in lignins. In: 11th International Symposium on Wood and Pulping Chemistry, Vol. III, Poster presentations. pp. 649–653. Svensson, S. (2008) Minimizing sulfur content in kraft lignin. Degree Project, Mälardalen University, Sweden. http://www. diva-portal.org/smash/get/diva2:1676/FULLTEXT01.pdf. Tejado, A., Pena, C., Labidi, J., Echeverria, J.M., Mondragon, I. (2007) Physico-chemical characterization of lignins from different sources for use in phenol-formaldehyde resin synthesis. Bioresour. Technol. 98:1655–1663. Thielemans, W., Can, E., Morye, S.S., Wool, R.P. (2001) Novel applications of lignin in composite materials. J. Appl. Polym. Sci. 83:323–331. Thring, R.W., Vanderlaan, M.N., Griffin, S.L. (1996) Fractionation of Alcell® lignin by sequential solvent extraction. J. Wood Chem. Technol.16: 139–154. Vishtal, A., Kraslawski, A. (2011) Challenges in industrial applications of technical lignins. Bioresources 6:3547–3568.

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