Fractionation and Characterization of Kraft Lignin by Sequential

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Fractionation and Characterization of Kraft Lignin by Sequential Precipitation with Various Organic Solvents Xiao Jiang, Dhanalekshmi Savithri, Xueyu Du, Siddhesh Nitin Pawar, Hasan Jameel, Hou-Min Chang, and Xiaofan Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02174 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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ACS Sustainable Chemistry & Engineering

Fractionation and Characterization of Kraft Lignin by Sequential Precipitation with Various Organic Solvents Xiao Jianga,b, Dhanalekshmi Savithrib, Xueyu Dub, Siddhesh Pawarb, Hasan Jameelb*, Hou-min Changb and Xiaofan Zhoua a: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, No. 159 Longpan Road, Nanjing 210037, Jiangsu, P.R. China b: Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Dr., Raleigh, NC 27695, USA *

Corresponding author E-mail: [email protected]

Abstract The value-added utilizations of technical lignin are restricted by its heterogeneous features, such as high polydispersity, complex functional group distribution, ununiformed reactivity, etc. Fractionation of lignin into more homogeneous parts represents a promising approach to overcome this challenge. In the present study, softwood kraft lignin was fractionated into four different portions (F1, F2, F3 and F4) by first dissolving it in methanol-acetone mixture followed by sequential precipitation with various organic solvents (ethyl acetate, 1:1 ethyl acetate/petroleum ether, petroleum ether) of decreasing solubility parameters. The yields of various fractions F1, F2, F3, and F4, were 48%, 39%, 10%, and 3%, respectively. The results from gel permeation chromatography indicated that the molecular weights of each fraction decreased from F1 to F4. The lowest molecular weight fraction F4 contained mainly monomeric and dimeric aromatic structures such as guaiacol and vanillin formed from lignin degradation.

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All fractions showed lower polydispersity than the starting kraft lignin. Chemical structures of F1, F2 and F3 were elucidated by elemental composition, methoxyl-content analysis, UV spectroscopy and various NMR techniques (31P, 13C and 2D HSQC). The results indicated that: (1) the sulfur content in each fraction was similar; (2) all residual polysaccharides only existed in fraction F1; (3) the contents of various structural linkages and thermal properties of each fraction varied as a function of its molecular weight. Overall, the solvent assisted fractionation method of kraft lignin yielded four fractions with varied molecular weights and polydispersities. As these fractions exhibited different chemical properties they can be used favorably in various applications.

Keywords: Pine kraft lignin, BioChoice lignin, Fractionation, Molecular weight distribution

Introduction Lignin is the second most abundant natural biopolymer with a highly branched and irregular chemical structure. It is composed of three different monomeric unis (C6-C3 units), i.e., coniferyl, sinapyl

and

p-coumaryl

alcohols,

linked

through

a

repetitive

enzyme-mediated

dehydrogenation/radical coupling process during its biosynthesis. Technical lignin is produced as a by-product from the pulp and paper industry and is mainly burnt to recover chemical and energy. Only a small fraction of lignin is used for other applications, such as dispersant in cement and gypsum blends 1-2, as an emulsifier 3-4, or as materials for the production of adhesives, resins and carbon fiber

5-6

, etc. Due to the growing concern about the importance of utilizing


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sustainable resources, lignin has drawn extensive attention because of its abundance and potential to produce high value chemicals as an alternative feedstock to fossil oil. However, the valorization of lignin is highly restricted by its heterogeneous features in terms of structural diversity, high polydispersity, and ununiformed reactivity, etc. The structure of lignin could change considerably based on the sources of feedstock, process conditions and the isolation method that would lead to different physicochemical properties. For instance, in the dominant industrial kraft pulping process, the lignin structure was altered through complex degradation and condensation reactions, such as the cleavage of β-O-4’ linkage, hydrolysis of αethers, demethylation, formation of vinyl ether and stilbene, and condensation at C-5 position of the aromatic ring. The diversity of various functional groups present in kraft lignin makes its utilization quite challenging. It has been confirmed that the chemical structure as well as the contents of certain functional groups vary to some degree with molecular weight

7-8

. Thus,

fractionation has become a convenient way to obtain specific lignin samples with definite molecular weight. Several methods have been proposed to fractionate lignin, such as selective precipitation based on the pH

9-10

, organic solvent extractions

11-12

, ultrafiltration

13-15

, etc. But

each method has its own disadvantages, such as low purity, high cost associated with solvents and ultrafiltration. Therefore, an efficient and economical method for lignin fractionation needs to be developed. In the present study, an economical and industrially feasible fractionation process was successfully developed by using inexpensive solvents. In this approach, softwood kraft lignin was fractionated into four portions according to their molecular weight by sequential precipitation with various organic solvents of decreasing solubility parameters 16. Thereafter, the



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fractionated lignin samples were fully characterized by various analytical methods owing their importance for the potential of value-added application.

Experimental Materials BioChoiceTM lignin (BCL) was kindly provided by Domtar Inc. (USA), which is a pine kraft lignin produced at its Plymouth mill by the LignoBoost process. The BCL was thoroughly washed with deionized water and dried prior to use. All chemicals used were of certified ACS reagent grade and purchased from Fisher Scientific Inc. (USA). The solvents, methanol, acetone, ethyl acetate and petroleum ether were used as received.

Sequential precipitation of BCL Fractionation process for lignin was developed based on the correlation of lignin solubility with hydrogen-bonding capacities and solvent solubility parameter 16. To begin with about 5g of BCL was first dissolved in 10 ml methanol-acetone mixture (7:3, v/v) and was then sequentially precipitated with various organic solvents of decreasing solubility parameters (i.e., ethyl acetate, 9.1; 1:1 (v/v) ethyl acetate/petroleum ether, 8.05 ~ 8.17; and petroleum ether, 7 ~ 7.24) as shown in Figure 1. Lignin fractions thus obtained were designated as F1, F2, F3 and F4. Due to insufficient yield and low molecular weight, fraction F4 was not fully characterized.


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Figure 1. Scheme for fractionation of BCL

Gel permeation chromatographic (GPC) analysis The GPC was used to determine the molecular weights and molecular weight distributions of lignin samples. All lignin samples were acetylated before analysis17. The analysis was carried out using a Waters GPC instrument equipped with a UV detector (set at 280 nm) using tetrahydrofuran (THF) as the eluent at a flow rate of 0.7 ml/min at 35 °C. A sample concentration of 1 mg/ml and an injection volume of 50 µL were used. Two ultra styragel linear columns linked in series (Styragel HR 1 7.8 x 300mm and Styragel HR 5E 7.8 x 300mm) were used. A series of monodispersed polystyrene standards were used as calibration standards.

Elemental and methoxyl content analyses Elemental analysis for carbon, hydrogen, nitrogen and sulfur elements were conducted at the Environmental and Agricultural Testing Service laboratory in the Department of Crop and Soil Science at North Carolina State University. The oxygen content was calculated from the



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sum of C, H, N and S contents by difference. The literature procedure [18] was used for the methoxyl content analysis. An oven dried lignin sample (about 20 mg, exact weight was recorded) was treated with 5 ml of 57% hydroiodic acid (HI) at 130 °C in a vial sealed with Supelco mininert valve for 30 min, and the vial was shaken every 5 min. After the reaction, the vial was cooled in an ice bath and about 100 µl of internal standard solution (97 mg/ml ethyl iodide in pentane) was added using a syringe. The mixture was then extracted with 10 ml of pentane and the pentane extract was analyzed by GC. The GC analysis was carried out using an Agilent 6890N GC instrument equipped with a CP-Sil 13 CB column (25 m × 0.32 mm i.d., thickness: 1.2 µm). The helium carrier gas flow rate was kept at 1.1 mL/min. The column temperature was held at 40 °C for 2 min, and then increased to 80 °C at 8 °C/min. The inlet temperature was set at 110 °C. A flame ionization detector was employed with hydrogen and air with flow rates of 40 and 300 mL/min, respectively. The detector temperature used was 150 °C.

Functional groups analysis The UV-VIS spectroscopy was used to determine the α-carbonyl, stilbene and catechol contents 18-19. The detailed procedure was exact same as described in literature 20. The α-carbonyl content was calculated based on the difference in absorbance at 305 nm between neutral and sodium borohydride reduced lignin samples. The stilbene content was calculated using ionization difference spectra at 378 nm. The catechol amount was estimated based on the difference in absorbance at 560 nm between equivalent concentration of lignin solution and catechol, pyrogallol-1-methyl ether, p-methylcatechol and 3,5-dimethylcatechol solutions. The UV spectra were recorded on a HP8453E UV-VIS spectrophotometer (Hewlett Packard Company, Palo Alto City, CA, USA).


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NMR analysis 31

P NMR analysis was conducted to determine the hydroxyl content following a reported

method 21-24. The 31P NMR spectrum was acquired using a Bruker 300 MHz spectrometer with a Quad probe. The

13

spectrometer. The

13

C and 2D HSQC NMR spectra were recorded using a Bruker 500 MHz C NMR experiment was conducted on the spectrometer equipped with a 5

mm BBO probe with Z-Axis Gradient. The spectra were acquired with inverse gated proton decoupling pulse sequence including 90° pulse width, a relaxation delay of 1.7 s, and acquisition time of 1.2 s, and a total of 20 K scans at 300.15 K. The 2D HSQC NMR spectra were acquired on the spectrometer equipped with a 5 mm double resonance broadband BBI inverse probe using a coupling constant J1 C-H of 147 Hz. The experimental parameters used were 160 transients (scans per block) acquired using 1K data points in F2 (1H) dimension for an acquisition time of 151 ms and 256 data points in F1 (13C) for an acquisition time of 7.68 ms for a total of 16.5 h.

Thermal analyses The thermal properties of lignin samples were determined using both thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) analysis. The TGA was performed using a TA Instruments Q500 TGA. The lignin sample was heated from 30 °C to 600 °C at a heating rate of 10 °C/min under a dry nitrogen atmosphere. Differential scanning calorimetric analysis was carried out using a TA Instruments Q100 DSC with an aluminum hermetic pan. The measurement was made at a scan rate of 10 °C/min over a temperature range from 40 °C to 180 °C under a nitrogen atmosphere. Since the glass transition temperature (Tg) of lignin is affected by many factors (molecular weight, low molecular weight contaminants including water and solvents, crosslinking and thermal history) 25, totally 3 heating scans were used in the present



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study. The first scan was from 40 °C to 105 °C to remove the moisture, the second scan was from 40 °C to 180 °C to eliminate the thermal history and enthalpy relaxation, and the third scan was recorded from 40 °C to 180 °C. The Tg was defined as the mid-point of the temperature at which the heat capacity occurred.

Results and Discussion Determination of yields and molecular weight distributions in lignin fractions BCL was first fractionated into four parts (F1, F2, F3 and F4) using the protocol described in Figure 1. The yields of each fraction obtained are shown in Table 1. The yield of insoluble fraction in ethyl acetate (F1) was 48%, whereas that from 1:1 ethyl acetate/petroleum ether mixture (F2) and petroleum ether (F3) solvents were 39% and 10%, respectively. Only 3% remained dissolved in petroleum ether (F4), which contained mainly monomeric and dimeric lignin degradation products. Table 1. Yields and molecular weight distributions of lignin fractions from BCL Lignin sample

Yield, %

Mn (g/mol)

Mw (g/mol)

PDI (Mw/Mn)

BCL

---

1562

5202

3.33

F1

48

3395

10244

3.02

F2

39

1647

2468

1.50

F3

10

626

770

1.23

F4

3

364

407

1.12

The molecular weight distributions of the four fractions are shown in Figure 2 and Table 1 along with the GPC curve of the original BCL sample. The GPC curve of each fraction was normalized with respect to the yield of that fraction. As can be seen in Figure 2, the fractionation results in the separation of fractions according to their molecular weight, and the average


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molecular weight of each fraction decreases from F1 to F4. While the ethyl acetate insoluble fraction (F1) showed bimodal distribution, all other fractions indicated unimodal distribution and relatively lower polydispersity as shown in Table 1. The number- and weight average molecular weights of each fraction as estimated using polystyrene standard were also displayed in Table 1.

Normalized UV absorbance (a.u.)

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1.0

BCL F1 F2 F3 F4

0.8

0.6

0.4

0.2

0.0 15

20

25

30

35

Retention Time (min)

Figure 2. Normalized GPC chromatograms of BCL and lignin fractions Elemental and methoxyl-content analyses The elemental and methoxyl-content analyses were performed in order to determine the molecular formula information of each lignin fraction. As can be seen in Table 2, the sulfur content increased from fractions F1 to F3 (weight percent), but it remained same in the C9 formula (0.08 per C9 unit). It has been reported that the sulfur content in kraft lignin was approximately 29% inorganic, 1% elemental and 70% organically bonded 20. Since the BCL has been well-washed prior to fractionation, the sulfur remained in each fraction was attributed mostly to the organically (covalently) bonded sulfur. The methoxyl contents of F1 and F2 were very close and were lower than BCL, whereas the fraction F3 had higher methoxyl content of 13.5%. The total methoxyl contents’ balance from F1 to F3 (sum as a function of fraction yield)



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was 12.0%, this only accounted for 97% of BCL (F4 was not characterized). The fraction F4 may contain much higher methoxyl content compared to other fractions since it was composed of mainly monomeric and dimeric lignin degradation products (The theoretical methoxyl contents of guaiacol and vanillin are 25.0% and 20.4%, respectively). Surprisingly, the methoxyl contents were higher in the low-molecular weight fractions considering the higher catechol content observed in the lower molecular weight fraction (as discussed later). It is quiet surprising that the same fraction had both higher methoxyl and catechol contents compared to others. This is because the catechol was originally formed by demethylation reaction of the methoxyl group. However, the relatively higher oxygen content observed in fraction F1 was attributed to the carbohydrates (as shown by 2D HSQC) present. It is possible that the presence of these carbohydrates caused the low methoxyl content in F1. Other possibility would be that the H-units existing in the pine lignin can undergo condensation during kraft pulping, thus retained in the F1 and F2 fractions. This also could cause the low methoxyl content in high molecular weight fraction. The proportion of catechol to methoxyl-content increased from F1 to F3 which indicated that the low molecular weight fractions underwent more demethylation reaction during kraft pulping.

Table 2. Elemental and methoxyl contents of BCL and lignin fractions Lignin C, % H, % N, % S, % O, % OMe, %

C9 formula

Unit weight

BCL

65.0

6.0

0.1

1.4

27.5

12.9

C9H8.5O2.3(OCH3)0.75S0.08

180

F1

64.2

5.9

0.2

1.3

28.5

12.1

C9H8.5O2.5(OCH3)0.71S0.08

179

F2

65.9

5.9

0.1

1.4

26.7

12.3

C9H8.3O2.2(OCH3)0.70S0.08

174

F3

66.9

6.2

0.1

1.5

25.4

13.5

C9H8.5O2.0(OCH3)0.76S0.08

172


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Functional groups analysis by UV spectroscopy The contents of various functional groups such as α-carbonyl, stilbene and catechol were estimated by UV spectroscopy. The results from the analysis are shown in Table 3. There was no significant difference in α-carbonyl contents among the various fractions when compared with those of BCL. However, there were noticeable differences in stilbene and catechol contents, and both of them increased with decreasing molecular weight as expected. It was because stilbene and catechol structures were formed by the degradation of lignin during kraft pulping.

Table 3. α-Carbonyl, stilbene and catechol contents of lignin fractions Samples

α-Carbonyl

Stilbene

Catechol

BCL

6.9

4.2

10.8

F1

7.4

2.9

4.4

F2

5.3

5.1

15.0

F3

6.4

7.0

20.3

% per 100 aromatic rings;

31

P NMR analysis Various hydroxyl and carboxylic groups present in the lignin fractions were also determined

by 31P NMR analysis and the results are shown in Table 4. The aliphatic hydroxyl group content of each fraction decreased with decreasing molecular weight whereas the phenolic hydroxyl content exhibited a reverse trend. The phenolic hydroxyl content of 77% for BCL found in this study was identical to the value in literature

26

. By comparing the phenolic hydroxyl group

content between different molecular weight fractions, it was postulated that there was more



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catechol in the lower molecular weight fraction. The carboxylic group content also showed the same trend as that of the phenolic hydroxyl groups. Table 4. Aliphatic, phenolic hydroxyl and carboxylic contents of lignin factions Samples

Aliphatic-OH

5-Substituted phenolic-OH

Non-condensed G-phenolic-OH

Total phenolic-OH

R-COOH

BCL

36.7

36.5

40.5

77.0

6.7

F1

50.0

32.9

30.4

63.4

4.7

F2

28.9

40.0

45.0

84.9

7.3

F3

17.5

33.9

69.7

103.4

10.5

% per 100 aromatic rings

13

C and 2D HSQC NMR analyses Both the original BCL and its fractionated samples were characterized by 13C NMR and 2D

HSQC techniques. The comparison of the

13

C and 2D HSQC spectra of BCL with those of the

fractionated samples F1, F2 and F3 are shown in Figure 3 and Figure 4. It is clear from Figure 3 that the main differences observed were in the side-chain regions of lignin structures. The main cross-signals detected in the spectra were assigned by comparison with the method from Capanema et al 27-30, and the main substructures identified are listed in Table 5. The intensity of signals for β-O-4’ (A), β-5’ (B) and β-β’ (C) structures were decreased with decreasing molecular weight, i.e., from F1 to F3. While the decrease in the intensity of β-O-4’ structure with decreasing molecular weight was expected, the decrease in the β-5’ structure with decreasing molecular weight was found interesting. In view of the fact that the stilbene structure, which is derived mainly from the β-5’ structure, was increased with decreasing molecular weight (Table 3). The increase in the stilbene (L) content with decreasing molecular weight, as determined by UV spectroscopy, was also confirmed by the 2D HSQC NMR spectra in the aromatic region. In


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addition, all signals related to the carbohydrates were present only in the F1 fraction, the high molecular weight fraction. This suggested that the residual carbohydrates in kraft lignin may be present in the form of lignin-carbohydrate complexes.

Table 5. Chemical shift assignment of main substructures in kraft lignin Label Assignment δC/ δH Lignin structure 33.8/2.54 Eα Cα-Hα in secoisolariciresinol substructures (E) 42.4/1.91 Eβ Cβ-Hβ in secoisolariciresinol substructures (E) 53.3/3.48 Bβ Cβ-Hβ in β-5’ substructures (B) 53.5/3.08 Cβ Cβ-Hβ in pinoresinol substructures (C) 60.0/3.43 Aγ Cγ-Hγ in β-O-4’ substructures (A) 61.3/4.13 Iγ Cγ-Hγ in coniferyl alcohol end groups (I) 62.7/3.70 Bγ Cγ-Hγ in β-5’ substructures (B) 70.7/3.76 and 4.17 Cγ Cγ-Hγ in pinoresinol substructures (C) 71.0/4.79 Aα Cα-Hα in β-O-4’ substructures (A) 73.8/4.45 Fα Cα-Hα in G-CH(OH)-COOH substructures (F) 81.1/4.77 Dβ Cβ-Hβ in dibenzodioxocin substructures (D) 84.0/4.31 Aβ Cβ-Hβ in β-O-4’ substructures (A) 84.8/4.63 Cα Cα-Hα in pinoresinol substructures (C) 86.5/5.52 Bα Cα-Hα in β-5’ substructures (B) 109.0/5.55 and111.9/6.15 Kα Cα-Hα in enol ether substructures (K) 128.2/7.0-7.3 Lα/β Cα-Hα and Cβ-Hβ in stilbene substructures (L) 142.7/7.29 Kβ Cβ-Hβ in enol ether substructures (K) Associated carbohydrate Ara5 61.9/3.52 C5-H5 in α-L-(1→4) linked arabinosyl units (Ara) 63.3/3.32 and 3.92 X5 C5-H5 in β-D-(1→4) linked xylosyl units (Xyl) X2 72.4/3.10 C2-H2 in β-D-(1→4) linked xylosyl units (Xyl) X3 73.8/3.30 C3-H3 in β-D-(1→4) linked xylosyl units (Xyl) X4 75.1/3.55 C4-H4 in β-D-(1→4) linked xylosyl units (Xyl) Ara3 77.1/3.71 C3-H3 in α-L-(1→4) linked arabinosyl units (Ara) Ara2 81.6/3.89 C2-H2 in α-L-(1→4) linked arabinosyl units (Ara) Ara4 86.6/4.34 C4-H4 in α-L-(1→4) linked arabinosyl units (Ara) X1 101.5/4.31 C1-H1 in β-D-(1→4) linked xylosyl units (Xyl) Ara1 107.8/4.82 C1-H1 in α-L-(1→4) linked arabinosyl units (Ara)



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BCL

F1

F2

F3

Figure 3. 2D HSQC NMR spectra of the different lignin fractions

The quantification of lignin structures were accomplished by a combination of

13

C NMR

and 2D HSQC NMR data according to literature 20. The specific structure per methoxyl unit was calculated as follows: X = (2DX / 2Dcluster) × 13Ccluster / 13Cmethoxyl


(1)

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where X was the substructure of interest; 2DX was the integration of the signal of substructure X in 2D HSQC spectra; 2Dcluster was the integration of the whole region of the signal of substructure X in 2D HSQC spectra; 13Ccluster was the integration of the corresponding cluster in 13

C spectra; 13Cmethoxyl was the integration of methoxyl signal in 13C spectra. Since the methoxyl

content of each fraction was already determined (Table 2, C9 formula), the value from above equation was converted to the result per C9 unit. The contents of various lignin sub-structures were estimated and are shown in Table 6. Compared with pine MWL in literature

31

,

substructures content changes (decreasing contents of β-O-4’, β-5’, dibenzodioxocin and coniferyl alcohol; increasing content of phenolic hydroxyl from 31P NMR and formation of enol ether and stilbene) were observed indicating significant structural changes in lignin during kraft pulping. In addition, it was quite clear that the contents of different substructures in each fraction exhibited either increasing or decreasing trend with the molecular weight gradient. -OMe

DMSO

Aromatic C & C=C bond

Carboxylic C

Lignin side chain & Polysaccharide signals

BCL

F1

F2

F3

Figure 4. 13C spectra of BCL and fractionated lignins



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Table 6. Estimation of the contents of various lignin substructures by 13C NMR and 2D HSQC NMR BCL

F1

F2

F3

Total balance*

A: β-O-4’

2.2

3.3

1.1

0.5

2.1

B: β-5’

0.9

1.2

0.5

0.4

0.8

C: β-β’ pinoresinol

2.4

3.3

1.4

1.3

2.2

D: Dibenzodioxocin

0.5

0.3

0.5

0.7

0.4

E: β-β’ secoisolariciresinol

3.5

2.7

3.0

5.3

3.1

F: G-CH(OH)-COOH

0.7

0.3

0.7

1.6

0.6

I: Coniferyl alcohol

0.7

1.3

0.7

0.3

0.9

K: Enol ether

2.1

1.5

2.2

3.7

1.9

L: Stilbene

4.5

3.4

4.0

11.9

4.5

Substructures

% per 100 aromatic rings; * sum as a function of fraction yield.

Thermal analyses The thermal properties of BCL and fractionated lignin samples were investigated by thermogravimetric analysis and differential scanning calorimetric analysis as shown in Figure 5 and Table 7. The thermal decomposition behavior of fractionated lignins exhibited a good correlation with their molecular weight. The 10% weight loss of F1, F2 and F3 observed at 251 ºC, 231 ºC and 209 ºC (defined as decomposition temperatures, Td) reveal that the thermal degradation occurred quickly as the molecular weight of the fractions decreased. All fractionated lignins degraded faster than BCL (10% weight loss at 277 ºC), even though the molecular weight of F1 was higher than BCL. This phenomenon indicated that the thermal stability of lignin was affected not only by its molecular weight but also by its chemical composition. Similar result was also reported by Tomobori et al

32

. The residue char of BCL, F1, F2 and F3 were 45.2%,

46.6%, 37.5% and 23.3%, respectively. If lignin is used as a carbon precursor to produce activated carbon or carbon fibers, fraction F1 with highest residue char yield could be more


Page 17 of 24

valuable than other fractions. The glass transition temperature (Tg) of BCL was found at 147 ºC, while that of fractions F2 and F3 were at 146 ºC and 86 ºC, respectively. There was no obvious Tg observed for F1 indicating that this fraction did not soften sufficiently and might require further modifications for thermal processing. Overall, the Tg of lignin was related to its molecular weight, as the molecular weight increased, the rigidity of lignin molecule increased, resulting in the increase of Tg.

BCL F1 F2 F3

Weight (%)

100 80 60 40

Deriv. Weight (%/°C)

20 0

100

200

300

400

500

600

0

100

200

300

400

500

600

0.5 0.4 0.3 0.2 0.1 0.0 -0.2

Heat Flow (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.3

-0.4 -0.5 20

40

60

80

100

120

140

160

180

Temperature (°C)

Figure 5. TG, DTG and DSC curves of BCL and fractionated lignins

Table 7. Td, Tg and residue char of lignin fractions from BCL Lignin sample

Td, °C

Tg, °C

Residue char, %

BCL

277

147

45.2

F1

251

---

46.6

F2

231

146

37.5

F3

209

86

23.3



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Outlook of fractionation towards valorization Lignin fractionation has been studied with the aim of producing relatively homogeneous lignin samples for valorizing applications. In spite of the progress made with fractionation processes, the practical value-added utilization of fractionated lignin samples has been barely investigated or reported. It is logical to integrate the characteristics of fractionated lignins with potential practical applications and designate a commercial feasible direction for sustainable lignin valorization. In view of the elaborate characterization of fractionated lignins in this study, the proposed applications of each fraction are described below. The fraction F1 possesses highest molecular weight and polydispersity, is suitable for producing lignin-based activated carbons. A pyrolysis process also seems to be a reasonable approach to utilize this fraction. The TGA of F1 have showed a weight loss of about 48% at 500 °C, hence the volatiles produced can be condensed and recovered as bio-oil and syngas. The residue after pyrolysis can be thermally treated at temperatures from 600-900 °C for carbonization resulting in the formation of char. Then, the char can be activated by oxidizing gases (normally CO2) or other activating agents (such as ZnCl2, H3PO4) to form porous activated carbons

33

. Besides pyrolysis and producing

activated carbons, the catalytic hydrogenation and phenolation reaction are also promising methods to utilize fraction F1. The catalytic hydrogenation can generate low-molecular-weight chemicals with high value 34, while phenolation will decrease the molecular weight and introduce more reactive sites into lignin 35 strucutre. The fractions F2 and F3 have relatively low molecular weight and polydispersity with high free phenolic hydroxyl groups. Hence these two fractions might be good for melt spinning to produce carbon fiber because of their relatively low Tg (146 °C and 86 °C) and high purity (no lignin-carbohydrate complexes contamination as


Page 19 of 24

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revealed by 2D HSQC) 36. High free phenolic hydroxyl content also indicate that they have good reactivity towards formaldehyde and should be useful as phenol substituent in phenolformaldehyde production

35

. Moreover, the fractions F2 and F3 also possess high methoxyl

content and can be used in DMSO production wood adhesive synthesis

38

37

. The demethylated lignin can also be used for

. The fraction F4, was obtained in low yield and had insufficient

characterization. However, the fraction F4 might have potential to be used as fuel additives since it can dissolve in petroleum ether, but definitely needs further investigation. Overall, the above interpretation of valorization fractionated lignins are on account of their unique features and provided a reasonable guide to achieve sustainable valorization of lignin.

Conclusions Kraft lignin can be fractionated into various portions of different molecular weights by sequential precipitation in solvents of decreasing solubility parameter (ethyl acetate, 1:1 ethyl acetate/petroleum ether, petroleum ether). The resulting fractions were more homogeneous in terms of molecular weight and polydispersity. However, some structural differences were found in these fractions, which can be summarized as: (1) the sulfur content in each fraction was similar; (2) the residual polysaccharides existed only in high molecular weight fraction F1 (ethyl acetate insoluble); (3) the contents of different structural linkages in each fraction varied as a function of molecular weight. The linkages existed in original wood sample have decreased with decreasing molecular weight whereas linkages formed during kraft pulping followed the reverse order; (4) the decomposition temperature decreased with the decreasing molecular weight, no Tg was observed for F1, whereas F2 (1:1 ethyl acetate/petroleum ether insoluble) and F3 (petroleum ether insoluble) had a Tg of 146 °C and 86 °C. The fractionation process developed in present



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study is efficient and economical, and it provides a promising way to obtain more homogeneous lignin faction. The valorization of fractionated lignin should depend on its unique feature, namely high molecular weight fraction might be suitable for pyrolysis, producing activated carbons or fragmentation modification. The low molecular weight fraction with good reactivity and thermal property may be used in carbon fiber and adhesive application.

Author information Corresponding Author *E-mail: [email protected]

Acknowledgements This research project was supported financially by a USDA grant through Domtar. The authors are also grateful to the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD).

References (1)

Yang, D.; Qiu, X.; Zhou, M.; Lou, H., Properties of sodium lignosulfonate as dispersant of coal

water slurry. Energy Convers. Manage. 2007, 48, 2433-2438. (2)

Matsushita, Y.; Imai, M.; Iwatsuki, A.; Fukushima, K., The relationship between surface tension

and the industrial performance of water-soluble polymers prepared from acid hydrolysis lignin, a saccharification by-product from woody materials. Bioresour. Technol. 2008, 99, 3024-3028. (3)

Boeriu, C. G.; Bravo, D.; Gosselink, R. J.; van Dam, J. E., Characterisation of structure-

dependent functional properties of lignin with infrared spectroscopy. Ind. Crops Prod. 2004, 20, 205-218. (4)

Sena-Martins, G.; Almeida-Vara, E.; Duarte, J., Eco-friendly new products from enzymatically

modified industrial lignins. Ind. Crops Prod. 2008, 27, 189-195. (5)

Zakzeski, J.; Bruijnincx, P. C.; Jongerius, A. L.; Weckhuysen, B. M., The catalytic valorization of

lignin for the production of renewable chemicals. Chem. Rev. 2010, 110, 3552-3599.


Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6)

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, 8-16. (7)

Mörck, R.; Yoshida, H.; Kringstad, K. P.; Hatakeyama, H., Fractionation of kraft lignin by

successive extraction with organic solvents. 1. Functional groups (13) C-NMR-spectra and molecular weight distributions. Holzforschung 1986. (8)

Sun, R.; Tomkinson, J.; Lloyd Jones, G., Fractional characterization of ash-AQ lignin by

successive extraction with organic solvents from oil palm EFB fibre. Polym. Degrad. Stab. 2000, 68, 111119. (9)

Wang, G.; Chen, H., Fractionation of alkali-extracted lignin from steam-exploded stalk by

gradient acid precipitation. Sep. Purif. Technol. 2013, 105, 98-105. (10)

Santos, P. S. B. d.; Erdocia, X.; Gatto, D. A.; Labidi, J., Characterisation of Kraft lignin separated

by gradient acid precipitation. Ind. Crops Prod. 2014, 55, 149-154. (11)

Cui, C.; Sun, R.; Argyropoulos, D. S., Fractional Precipitation of Softwood Kraft Lignin:

Isolation of Narrow Fractions Common to a Variety of Lignins. ACS Sustainable Chem. Eng. 2014, 2, 959-968. (12)

Li, M.-F.; Sun, S.-N.; Xu, F.; Sun, R.-C., Sequential solvent fractionation of heterogeneous

bamboo organosolv lignin for value-added application. Sep. Purif. Technol. 2012, 101, 18-25. (13)

Toledano, A.; Serrano, L.; Garcia, A.; Mondragon, I.; Labidi, J., Comparative study of lignin

fractionation by ultrafiltration and selective precipitation. Chem. Eng. J. 2010, 157, 93-99. (14)

Toledano, A.; García, A.; Mondragon, I.; Labidi, J., Lignin separation and fractionation by

ultrafiltration. Sep. Purif. Technol. 2010, 71, 38-43. (15)

Jönsson, A.-S.; Nordin, A.-K.; Wallberg, O., Concentration and purification of lignin in

hardwood kraft pulping liquor by ultrafiltration and nanofiltration. Chem. Eng. Res. Des. 2008, 86, 12711280. (16)

Schuerch, C., The solvent properties of liquids and their relation to the solubility, swelling,

isolation and fractionation of lignin. J. Am. Chem. Soc. 1952, 74, 5061-5067. (17)

Glasser, W. G.; Davé, V.; Frazier, C. E., Molecular weight distribution of (semi-) commercial

lignin derivatives. J. Wood Chem. Technol. 1993, 13, 545-559. (18)

Dence, C. W., The determination of lignin. In Methods in lignin chemistry, Springer: 1992, pp 33-

61. (19)

S. Ingemar, F.; Joseph, M.; Erich, A., Chromophores in Kraft Lignin. In Lignin Structure and

Reactions, AMERICAN CHEMICAL SOCIETY: 1966; Chapter 7, pp 75-89.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(20)

Page 22 of 24

Hu, Z.; Du, X.; Liu, J.; Chang, H.-m.; Jameel, H., Structural Characterization of Pine Kraft

Lignin: BioChoice Lignin vs Indulin AT. J. Wood Chem. Technol. 2016, 36, 432-446. (21)

Argyropoulos, D. S., Quantitative phosphorus-31 NMR analysis of lignins, a new tool for the

lignin chemist. J. Wood Chem. Technol. 1994, 14, 45-63. (22)

Granata, A.; Argyropoulos, D. S., 2-Chloro-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaphospholane, a

reagent for the accurate determination of the uncondensed and condensed phenolic moieties in lignins. J. Agric. Food Chem. 1995, 43, 1538-1544. (23)

Argyropoulos, D., 31P NMR in wood chemistry: A review of recent progress. Res. Chem.

Intermed. 1995, 21, 373-395. (24)

Balakshin, M.; Capanema, E., On the quantification of lignin hydroxyl groups with 31P and 13C

NMR spectroscopy. J. Wood Chem. Technol. 2015, 35, 220-237. (25)

Sevastyanova, O.; Helander, M.; Chowdhury, S.; Lange, H.; Wedin, H.; Zhang, L. M.; Ek, M.;

Kadla, J. F.; Crestini, C.; Lindstrom, M. E., Tailoring the Molecular and Thermo-Mechanical Properties of Kraft Lignin by Ultrafiltration. J. Appl. Polym. Sci. 2014, 131. (26)

Marton, J., Lignins: reactions in alkaline pulping, in: Lignins: Occurrence, Formation, Structure

and Reactions. Wiley- Interscience: New York, 1971. (27)

Liitiä, T. M.; Maunu, S. L.; Hortling, B.; Toikka, M.; Kilpeläinen, I., Analysis of technical lignins

by two-and three-dimensional NMR spectroscopy. J. Agric. Food Chem. 2003, 51, 2136-2143. (28)

Balakshin, M. Y.; Capanema, E. A.; Chen, C.-L.; 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, 6116-6127. (29)

Capanema, E. A.; Balakshin, M. Y.; Chen, C.-L.; Gratzl, J. S.; Gracz, H., Structural analysis of

residual and technical lignins by 1H-13C correlation 2D NMR-spectroscopy. Holzforschung 2001, 55, 302-308. (30)

Kim, H.; Ralph, J.; Akiyama, T., Solution-state 2D NMR of ball-milled plant cell wall gels in

DMSO-d 6. BioEnergy Res. 2008, 1, 56-66. (31)

Holtman, K. M.; Chang, H.-m.; Kadla, J. F., Solution-state nuclear magnetic resonance study of

the similarities between milled wood lignin and cellulolytic enzyme lignin. J. Agric. Food Chem. 2004, 52, 720-726. (32)

Saito, T.; Perkins, J. H.; Vautard, F.; Meyer, H. M.; Messman, J. M.; Tolnai, B.; Naskar, A. K.,

Methanol Fractionation of Softwood Kraft Lignin: Impact on the Lignin Properties. Chemsuschem 2014, 7, 221-228. (33)

Carrott, P.; Carrott, M. R., Lignin–from natural adsorbent to activated carbon: a review.

Bioresour. Technol. 2007, 98, 2301-2312.


Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34)

Pandey, M. P.; Kim, C. S., Lignin depolymerization and conversion: a review of thermochemical

methods. Chem. Eng. Technol. 2011, 34, 29-41. (35)

Podschun, J.; Saake, B.; Lehnen, R., Reactivity enhancement of organosolv lignin by phenolation

for improved bio-based thermosets. Eur. Polym. J. 2015, 67, 1-11. (36)

Baker, D. A.; Rials, T. G., Recent advances in low‐cost carbon fiber manufacture from lignin. J.

Appl. Polym. Sci. 2013, 130, 713-728. (37)

Laurichesse, S.; Avérous, L., Chemical modification of lignins: Towards biobased polymers.

Prog. Polym. Sci. 2014, 39, 1266-1290. (38)

Liu, Y.; Li, K., Preparation and characterization of demethylated lignin-polyethylenimine

adhesives. J. Adhes. 2006, 82, 593-605.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fractionation and Characterization of Kraft Lignin by Sequential Precipitation with Various Organic Solvents Xiao Jianga,b, Dhanalekshmi Savithrib, Xueyu Dub, Siddhesh Pawarb, Hasan Jameelb*, Hou-min Changb and Xiaofan Zhoua

For table of content use only

Synopsis The value-added utilizations of technical lignin are restricted by its heterogeneous features, fractionation can homogenize lignin to some extent. This study offers a commercial feasible fractionation method to achieve sustainable lignin valorization.


Fractionation and Characterization of Kraft Lignin by Sequential

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