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Fast Fractionation of Technical Lignins by Organic Co-solvents Yun-Yan Wang, Mi Li, Charles Cai, Charles Wyman, and Arthur Jonas Ragauskas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04546 • Publication Date (Web): 18 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Sustainable Chemistry & Engineering

1

Fast Fractionation of Technical Lignins by Organic

2

Co-solvents

3

Yun-Yan Wang†, Mi Li‡, Charles M. Cai∥ Charles E. Wyman∥ and Arthur J. Ragauskas*†, ‡, §

4

†

5

Tennessee Institute of Agriculture, 2506 Jacob Drive, Knoxville, Tennessee 37996, USA

6

‡

7

Bethel Valley Road, Oak Ridge, Tennessee 37831, USA

8

∥

9

University of California, 1084 Columbia Avenue, Riverside, California 92521, USA

Department of Forestry, Wildlife, and Fisheries, Center for Renewable Carbon, University of

Joint Institute for Biological Science, Biosciences Division, Oak Ridge National Laboratory, 1

Department of Chemical and Environmental Engineering, Bourns College of Engineering,

10

§

11

Drive, Knoxville, Tennessee 37996, USA

Department of Chemical and Biomolecular Engineering, University of Tennessee, 1512 Middle

12

13

Email of the corresponding author: [email protected]

14

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ABSTRACT: A simple and fast fractionation method was developed to obtain carbohydrate-

16

free lignin with well-defined characteristics, such as narrowly distributed molecular weights and

17

tunable chemical structure for specific applications. In this study, an industrial softwood kraft

18

lignin and a hardwood CELF lignin (co-product lignin obtained from Co-solvent Enhanced

19

Lignocellulosic Fractionation

20

tetrahydrofuran-methanol co-solvents, respectively. Hexane was applied as the anti-solvent for

pretreatment)

were dissolved

1 ACS Paragon Plus Environment

in

acetone-methanol

and

ACS Sustainable Chemistry & Engineering 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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sequential precipitation of both lignin preparations. A thorough characterization including

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various NMR techniques (31P, quantitative

23

conducted to correlate the molecular weight of obtained lignin fractions with their structural

24

features including distributions of aliphatic and phenolic hydroxyl groups and relative abundance

25

of interunit linkages. It was found that approximately 10% of the softwood kraft lignin was

26

lignin carbohydrate complexes, and the latter one could be removed efficiently by decreasing

27

polarity of the co-solvent.

13

C and 2D-HSQC), GPC, DSC and TGA, was

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KEYWORDS: Softwood kraft lignin, CELF poplar lignin, Sequential Fractionation, Structural

30

analyses, Molecular weight, Lignin carbohydrate complexes

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INTRODUCTION

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Lignin, the most abundant aromatic biopolymer, constitutes 15~30% of terrestrial plant

34

lignocellulosic materials. It can be composed of up to three primary subunits: guaiacyl (G),

35

syringyl (S) and p-hydroxyphenyl (H). Softwood lignin is primarily composed of guaiacyl

36

subunits and hardwood lignin is formed by a mixture of guaiacyl and syringyl subunits. Recently,

37

the increasing interest in lignocellulosic biorefinering has brought the utilization of lignin back

38

into focus. The annual production of lignin is predicted to be 62 million tons in the United State

39

by 2022.1 Numerous efforts have been devoted to convert lignin into a wide range of value-

40

added products. One approach involves deconstruction of lignin to small molecular weight

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phenolics or related aromatics through pyrolysis and/or catalytic reforming approaches.2 More

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frequently, owing to its multifunctional properties, lignin is crosslinked or blended with other 2 ACS Paragon Plus Environment

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polymers or monomers to produce materials such as thermoset resins 3-6, foams 7-8, adhesives 9-13

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and thermoplastics14-15.

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However, the utilization of technical lignins remains underdeveloped and is most often

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combusted as a low-cost fuel, and approximately 1~2% of commercial lignins are used as low-

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value industrial additives.16 Depending on the reaction conditions, technical lignins differ in

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structural composition, molecular weight, polydispersity, and functional group distribution.

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Medium molecular weight Alcell® lignin obtained by sequential organic solvent extraction with

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increasing hydrogen bonding capacity, generated polyurethane films exhibiting better tensile

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behavior than those of high and low molecular weight fractions with the same lignin content.17 In

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addition to their structural complexity, carbohydrate impurities are another obstacle that prevents

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technical lignins from effective conversion into many commercial polymeric materials.18-20 For

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example, in the production of lignin-based carbon fiber, ideal technical lignins for melt-spinning

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should possess moderate molecular weights with a narrow distribution, meanwhile the impurities

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including residual carbohydrates and ash were preferred to be less than 1%.21 The solvent-based fractionation of lignin has been widely investigated in the past decades.22-

57 58

29

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technical lignins, and then more water was added to the system as anti-solvents to precipitate

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lignin. The properties of co-solvent used in the lignin fractionation have been shown to have

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significant effects on the characteristics of lignin obtained. An accumulation of carbohydrates

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was found in low molecular weight kraft lignin obtained from aqueous organic solvent

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fractionation.27 However carbohydrates tend to be enriched in high molecular weight fractions,

64

when technical lignin fractions were separated by pure organic solvent extraction

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methodologies.30-31 In this study, methanol was chosen to replace water as the co-solvent that

Among the proposed methods, water has been frequently used as a co-solvent to dissolve



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provides sufficient hydrogen bonding capacity and polarity. Two technical lignin samples,

67

softwood kraft lignin and hardwood CELF lignin, prepared under different reaction conditions

68

were dissolved in low-boiling-point organic co-solvents, and then fractionally precipitated with

69

hexane. The structural evolution across fractions of different molecular weight were thoroughly

70

characterized.

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MATERIALS AND METHODS

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Materials. The CELF poplar lignin is a co-product of Co-solvent Enhance Lignocellulosic

74

Fractionation (CELF) process. During the CELF pretreatment, the biomass (7.5 wt.% loading)

75

underwent one-step delignification and hemicellulose hydrolysis in tetrahydrofuran (THF)-water

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(1:1, v/v) co-solvent containing 0.5 wt.% H2SO4 at 160 °C with 15-mincontact time and was

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isolated according to literature procedures.32-33 Commercial softwood kraft black liquor lignin

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was purified according to a published method.2 In detail, 100 g of dry kraft lignin was suspended

79

in 1000 mL 0.1 M NaOH with EDTA-2Na+ (0.5 g /100 mL). After stirring for 1 h, the mixture

80

was filtered through a filter paper (Whatman 1). The filtrate was acidified to pH 6.0 by adding

81

2.0 M H2SO4 dropwise. After stirring for 1 h, the filtrate was gradually acidified to pH 3.0 using

82

2.0 M H2SO4, and the resulting viscous mixture was kept at -20 °C overnight. After thawing, the

83

precipitates were collected by centrifugation and washed thoroughly with distilled water. The

84

air-dried powder was Soxhlet extracted with pentane for 24 h, designated as kraft lignin. The

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chemicals and solvents used in this work were purchased from Sigma-Aldrich and Fisher

86

Scientific and used as received.


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Lignin Fractionation. The kraft lignin and CELF lignin were fractionated according to the

88

scheme presented in Figure 1. Two co-solvent systems were employed in this study: acetone-

89

methanol (7:3, v/v) for kraft lignin and THF-methanol (5:5, v/v) for CELF lignin.

90

CELF poplar lignin (2.00 g) was suspended in 20 mL THF-methanol (TM) co-solvent. The

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suspension was vigorously stirred at RT for 30 min, the undissolved materials (designed as TM-

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ins) was removed by centrifugation (4000 × g, -4 °C). Hexane (2.00 ml) was added as the anti-

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solvent to the supernatant, and the resulting precipitated fraction (designated as TM-2H) was

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collected by centrifugation (4000 × g, -4 °C). This procedure was repeated in the foregoing step

95

as demonstrated in Figure 1. Five fractions of CELF lignin were obtained as precipitates after

96

adding hexane sequentially, and they were labeled as TM-2H, TM-6H, TM-10H, TM-14H and

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TM-20H. The insoluble fractions were air-dried at 40 ºC. After adding 20 mL hexane, the final

98

soluble fraction (designated as TM-20H sol) was dried in a vacuum oven under reduced pressure

99

at 35 ºC. After removing the solvent, the isolated fractions were dried at 40 ºC for 24 h. The

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softwood kraft lignin sample was fractionated by acetone-methanol (AM) using hexane as the

101

antisolvent in the same manner as described above. Each fraction was denoted based upon the

102

corresponding co-solvent and the total amount of anti-solvent added.

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104

105 106

Figure 1. Flow chart of lignin fractionation.

107

108

Nuclear Magnetic Resonance (NMR) Analysis. The quantitative

31

P NMR spectra

109

were acquired on a 500 MHz Varian VMNRS spectrometer using a 5-mm triple resonance probe.

110

A 90º pulse width, 1.2 s acquisition time, 25 s pulse delay were used in collecting 64 scans.

111

20~30 mg (accurately weighed) lignin sample was phosphitylated with 60 µL 2-chloro-4,4,5,5

112

tetramethyl-1,3,2-dixoaphospholane (TMDP) in 700 µL pyridine/CDCl3 (1.6:1, v/v) containing

113

chromium(III) acetylacetonate (1 mg/mL) as the relaxation agent and N-hydroxy-5-norbornene-

114

2,3-dicarboximide (2.5 mg/mL) as the internal standard.34-35 The TMDP-water phosphitylation

115

product (δ132.2 ppm) was used as the internal reference. The quantitative analysis was carried

116

out using MestReNova software according to a published method.36


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117


Heteronuclear single quantum coherence (HSQC) and quantitative

13

C NMR spectra were

118

acquired on a Bruker Avance III HD 500-MHz spectrometer. Lignin samples (~50 mg) were

119

dissolved in 0.5 mL DMSO-d6 in 5mm-NMR tube. HSQC experiments were carried out using a

120

N2 cryoprobe (BBO 1H &

121

following acquisition parameters: spectra width 12 ppm in F2 (1H) dimension with 1024 data

122

points (acquisition time 85.2 ms), 166 ppm in F1 (13C) dimension with 256 increments

123

(acquisition time 6.1 ms), a 1.0 s delay, a JC–H of 145 Hz, and 128 scans. The central DMSO-d6

124

solvent peak (δC/δH at 39.5/2.49) was used for chemical shifts calibration. Relative abundance

125

of lignin compositional subunits and interunit linkage were estimated by using the Bruker

126

TopSpin software to integrate the volume of contours in HSQC spectra, and cross-peaks were

127

assigned according to literatures.11, 28, 37

128

with an inverse-gated decoupling pulse sequence, a 2.0-s pulse delay, and 30,000 scans at 30°C.

129

For the quantitative 13C NMR analysis, 0.01 mg/mL chromium(III) acetylacetonate was added to

130

decrease the relaxation time.

19

F-5mm) and a Bruker pulse sequence (hsqcetgpspsi2.2) with the

13

C NMR acquisition was performed using a 90° pulse

131

Gel Permeation Chromatography (GPC). About 2 mg dried lignin sample was

132

accurately measured and dissolved in 1 mL anhydrous pyridine. Acetic anhydride (1.00 mL)

133

was added to the mixture soon after complete dissolution of lignin. The resulting stirred solution

134

was kept dark for 24 h, whereupon 2 mL ethanol was added, and the solvents were removed

135

using rotary evaporator under vacuum at 40 ºC. Repeat ethanol addition, followed by evaporation

136

was preformed to remove the last trace of pyridine and remaining acetic anhydride. The dried

137

acetylated lignin was dissolved in 2 mL THF. The solution was filtered through a 0.45 µm PTFE

138

membrane before injecting to an Agilent GPC SECurity 1200 system equipped with several

139

Waters Styragel columns (Water Corporation, Milford, MA), an Agilent UV detector and a 7 ACS Paragon Plus Environment


Page 8 of 32

140

refractive index detector. HPLC-grade THF was used as the eluent at flow rate of 1.0 mL/min at

141

30 ºC. Standard calibration was performed by using polystyrene standards with weight-average

142

molecular weight ranging from 580 ∼ 13,000 g/mol.38 The UV detector signal was collected at λ

143

= 280 nm.

144

Thermal Gravimetric Analysis (TGA). Lignin samples (5 mg) were loaded to ceramic

145

crucibles (PerkinElmer, American Fork, UT). The TGA of lignin was operated on a PerkinElmer

146

Pyris 1 TGA heating in nitrogen and air atmosphere, respectively. The sample was heated from

147

25 to 105 ºC at 10 ºC/min. After incubating at 105 ºC for 15 min, it was heated further from 105

148

to 900 ºC at a heating rate of 10 ºC/min.

149

Differential Scanning Calorimetry (DSC). About 5 mg

of powdery sample was

150

encapsulated in an standard aluminum pan and lid (TA Instruments. The DSC measurements

151

were performed in heat-cool-heat mode on a TA Q2000 differential scanning calorimeter (TA

152

Instruments) with heating/cooling rate of 20 °C/min.

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RESULTS AND DISCUSSION

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The studies39-40 of lignin solubility in a single solvent started with the Hildebrand solubility

156

parameter (δ), which is defined as the square root of a substance’s cohesive energy density:

157

= (/) /

(1)

158

Where E is the total energy of vaporization at zero pressure and V is the molar volume of the

159

pure solvent. As E is composed of three parts which are contributed by nonpolar/dispersion


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160

forces, dipole forces and hydrogen bonding forces, δ can be divided into three corresponding

161

parts as following:

162

= + +

(2)

163

δD, δP and δH are the Hansen solubility parameters (HSP) for the dispersion, polar and hydrogen

164

bonding interactions, respectively. Ra is the difference between the HSP for a given solvent and

165

polymer. For a good solvent, Ra/Ro 10%, e.g. 19.8 % yield

200

for the AM-4H (4 mL hexane in total solvent mixture) kraft lignin fraction and 16.5% yield for

201

the AM-6H (6 mL hexane in total solvent mixture) kraft lignin fraction (Table 2). On the other

202

hand, CELF lignin that is less hydrophilic required more hexane for fractionation. In both cases,

203

fractionation stopped at 20 mL hexane (50% of the total solvent mixture) in the co-solvent

204

systems due to the limited yield.

205

Table 2. The yields of AM-kraft lignin and TM-CELF lignin fractionated by hexane. Kraft lignin Fraction AM-ins AM-1H AM-2H AM-4H AM-6H AM-8H AM-10H AM-13H AM-20H AM-20H sol

hexane, vol. % 0 4.8 9.1 16.7 23.1 28.6 33.3 39.4 50.0 50.0

CELF lignin yield, % 3.9 3.4 2.3 19.8 16.5 10.6 8.0 8.6 10.3 16.6

Fraction TM-ins TM-2H TM-6H TM-10H TM-14H TM-20H TM-20H sol

hexane, vol. % 0 9.1 23.1 33.3 41.2 50.0 50.0

yield, % 0.5 0.7 29.5 17.5 9.7 9.8 31.1

206

207

The changes of weight-average molecular weight (Mw) and polydispersity index (PDI =

208

Mw/Mn) as a function of hexane volume was depicted in Figure 2. AM-ins, AM-1H, AM-2H,

209

TM-ins and TM-2H fractions were only partially soluble in pyridine, therefore their molecular

210

weight characterization results were not representative for the whole sample. The Mw of TM-



211

CELF lignin fractions decreased from 14,500 to 2,300 g/mol with polydispersity decreased from

212

5.5 to 1.6. The final soluble fraction in THF-methanol containing 50% hexane account for 31%

213

of the total CELF lignin was mainly composed of oligomers with Mw = 1,200 g/mol and PDI

214

=1.5. The unfractionated kraft lignin exhibited a close Mw (4,300 g/mol) but with a narrower

215

distribution (PDI = 2.6), compared with the unfractionated CELF lignin (Mw = 4,400 g/mol, PDI

216

= 3.4). Similar trends were observed for AM-kraft lignin.

2.8

A

weight-average molecular weight polydispersity

Mw, g/mol

8000

2.6

7000

2.4

6000

2.2

5000

2.0

4000

1.8 unfractionated kraft lignin

3000

PDI

9000

1.6

2000

1.4 0

5

10

15

20

hexane, mL 6.0 14000

B

weight-average molecular weight polydispersity

5.5 5.0

12000

4.5

10000

4.0

8000

3.5

PDI

Mw, g/mol

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

Page 12 of 32

3.0

6000

2.5 4000

2.0

unfractionated CELF lignin

2000 0

5

10

1.5 15

20

hexane, mL

217

218 219 220

Figure 2. Changes in weight-average molecular weight (Mw) and polydispersity (PDI) of (A) kraft lignin and (B) CELF lignin fractions as a function of hexane volume present in in the acetone-methanol and THF-methanol, respectively. 12 ACS Paragon Plus Environment

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221

222

223

Functional Group Analysis by

31

P NMR. As shown in the 31P NMR spectra (Figure 3),

224

distinct differences between softwood kraft lignin and hardwood CELF lignin were observed in

225

the phenolic region (δ145~136 ppm) in relate to the compositional lignin subunits of different

226

wood species.34 In addition, phenolic OH (60% of total OH) was predominant in kraft lignin as a

227

result of aryl ether bond cleavage.44 A reversed order was observed for unfractionated CELF

228

lignin; aliphatic OH content (52% of total OH) was slightly higher than phenolic OH (46% of

229

total OH). Given the fact that the abundant phenolic OH contributes significantly to δH and δP

230

and enhances hydrophilicity of kraft lignin,43 fractions with higher phenolic OH content but

231

lower molecular weights were separated in the later stage, i.e. they were more tolerant to

232

increasing hexane percentage in the acetone-methanol co-solvent. These results implied that

233

lignin fractionation by sequential precipitation in pure organic co-solvent was driven by Mw

234

considerations rather than hydrophilicity (Figure 4). In fact, increasing phenolic OH and

235

simultaneous decreasing aliphatic OH with decreasing lignin molecular weight has been reported

236

in different studies of kraft lignin fractionation regardless of the solvent systems applied.26-28, 45

237

Actually, the contents of these hydroxyl groups changed linearly with the molecular weights of

238

kraft lignin fractions as depicted in Figure 4 A and B. For CELF lignin, the changes of aliphatic

239

and phenolic (guaiacyl and C5-substituted) OH contents with molecular weight reached plateau,

240

when Mw decreased from 14,500 to 6,300 g/mol (Figure 4 C and D).

241

242



Page 14 of 32

243

244

internal standard

aliphatic OH

phenolic OH

COOH

syringyl p-hydroxylphenyl guaiacyl

CELF lignin C5 substituted

kraft lignin 156

245 246

154

152

150

148

146

144

142

140

138

136

134 PPM

Figure 3. Quantitative 31P NMR spectra of CELF lignin and kraft lignin. The hydroxyl groups were phosphitylated with TMDP.

247


OH, mmol/g lignin

6

total

5

acetone− −methanol− −hexane fractionated kraft lignin

phenolic

4 3 2 aliphatic 1

carboxylic

guaiacyl


2

C-5 substituted 1

p-hydroxyphenyl 0

0 2000

4000

6000

2000

8000

4000

6000

8000

Mw, g/mol

Mw, g/mol THF− −methanol− −hexane fractionated CELF lignin

5 4 aliphatic

3 2

phenolic

1

carboxylic

phenolic OH, mmol/g lignin

4 total

6

OH, mmol/g lignin

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


phenolic OH, mmol/g lignin

Page 15 of 32

guaiacyl 3

2 C-5 substituted 1

p-hydroxyphenyl

0

0 0

4000

8000

12000

0

4000

Mw, g/mol

248 249 250

THF− −methanol− −hexane fractionated CELF lignin

8000

12000

Mw, g/mol

Figure 4. Changes of hydroxyl group contents as a function of Mw for kraft lignin (A & B) and CELF lignin (C & D).

13

251

Structural Analyses by HSQC and Quantitative

252

evolution of softwood kraft lignin, CELF poplar lignin fractionations were investigated by

253

semi-quantitative HSQC and quantitative

254

(xyl1), δC/δH 72.61/3.09 (xyl2), δC/δH 73.83/3.30 (xyl3), δC/δH 75.25/3.56 (xyl4), δC/δH

255

63.0/3.22; 3.93 (xyl5), δC/δH 107.98/4.82 (ara1), δC/δH 81.61/3.88 (ara2), δC/δH 77.27/3.70

256

(ara3) and δC/δH 86.1/4.03 (ara4) were observed in the first three fractions (AM ins, AM-1H

257

and AM-2H in Figure 5) which were initially insoluble in acetone-methanol (7:3, v/v) or then

258

precipitated with small amounts of hexane (1 mL and 2 mL). These cross-peaks belong to α-

259

L-(1→4)

13

C NMR. The detailed structure

C NMR. Intensive signals at δC/δH 101.63/4.32

arabinosyl units (ara) and β-D-(1→4) xylosyl units (xyl) that covalent attach to 15 ACS Paragon Plus Environment


Page 16 of 32

260

lignin to form xylan-enriched lignin carbohydrate complexes.46 The typical interunit linkages

261

of softwood lignin were found in the HSQC spectra of AM-ins, AM-1H, AM-2H kraft

262

fractions (Figure 5), including β−O−4′ alkyl aryl ether (A) and β−β′ resinol (B) and

263

β−5′phenylcoumaran (C) substructures (at noise level), and their signal intensities were weak

264

compared with those of hemicelluloses. The hemicellulose cross-peaks disappeared as

265

fractionation continued: starting from AM-4H, the HSQC spectra of kraft lignin fractions

266

showed clear aliphatic features of lignin without the interruption from hemicellulose, and no

267

distinguishable hemicelluloses cross-peaks were found in the HSQC spectrum of AM-20 sol

268

fraction. It was reported that xylan-rich lignin carbohydrate complex fraction was slightly

269

soluble in 90% aqueous dioxane.46 In this study, xylan might contribute to the incomplete

270

dissolution of the AM-ins, AM-1H and AM-2H in pyridine. The quantitative

271

results confirmed the presence of xylan with the prominent peak around δ101.8 ppm, and the

272

content of β-D-(1→4) xylosyl units was 14.9, 14.5 and 11.2 per 100 aromatic rings for the

273

AM-ins, AM-1H and AM-2H kraft lignin fractions, respectively. The peak centered at δ105.4

274

ppm observed in both

275

represents galactan which was found enriched in high molecular weight residual softwood

276

kraft lignin carbohydrate complexes,47 and its content was 18.6, 16.9 and 9.2 per 100

277

aromatic rings for the three fractions, respectively. Neither δ101.8 ppm or δ105.4 ppm was

278

observed in the 13C NMR spectra of AM-4H and AM-6H.

13

C NMR

13

C and HSQC NMR spectra of AM-ins, AM-1H and AM-2H could


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279 280 281


Figure 5. Sidechain in the 2D-HSQC spectra of the AM ins, AM-1H, AM-2H and AM-4H kraft lignin fractions.

282

283

284



OMe 3'

6' HO

5' 4' O

HO

2' 3' OMe

1 6

2

5

3

2'

1'

(A)

5' 6'

4'

2

O

O

2

5

3 4 O

1

1

6

OMe

2

6

(G)

5 MeO

(D)

3 OMe

4

HO 1

(C)

1

5 OMe

OMe

O

6

2

3'

HO

O

3 O

2'

5'

(B)

' 1

4 O

HO

285 286 287 288 289 290

1'

6 5

1' 6'

'

OMe

4 O

'

O

4' O

Page 18 of 32

3 4 O

(S) MeO

OMe

6

2

5

3 4 O

(S′)

(PB)

OMe

Figure 6. HSQC spectrum of CELF lignin: (A) aliphatic region and (B) aromatic region. Structure (A) β−O−4′ linked alkyl aryl ether substructure; (B) β−β′ linked resinol substructure; (C) β−5′ and α−O−4′ linked phenylcoumaran substructure; (D) cinnamaldehyde end group (G) guaiacyl unit; (S) syringyl unit; (S′) α−oxidized syringyl unit; (PB) p-hydroxybenzoate substructure.

291

292

Representative HSQC spectra of CELF poplar lignin are shown in Figure 6. Under acidic

293

conditions, lignin was able to repolymerize at C5, C6 and Cα positions after acidolysis of β−O−4′ 18 ACS Paragon Plus Environment

Page 19 of 32 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


294

linkages.48 The cross-peak centered at δC/δH

295

guaiacyl units containing C5 condensed with other lignin side chains.

296

condensation of syringyl units with C2,6−H2,6 correlations were observed at δC/δH 106.7/4.69;

297

105.6/6.43; 104.9/6.33 (Scond) probably occurred at the Cα due to the steric hindrance arisen

298

from the C5-substitued methoxyl group.48-50 More insight of the structural evolution in

299

relationship to lignin molecular weight were provided by semi-quantitative HSQC NMR (Figure

300

7). In general, high molecular weight lignin fractions tend to possess more β−O−4′ linkages,

301

while the subunits of oligomeric lignin molecules prefer to link through C−C bonds. When the

302

molecular weight dropped below 2000 g/mol, the relative abundance of β−O−4′ linkages of kraft

303

lignin dropped to ~ 12% (Figure 7 A), however CELF lignin preserved more than 50% β−O−4′

304

as the interunit linkages (Figure 7 B). Moreover, the S/G ratio of CELF lignin increased

305

dramatically from 1.1 to 4.0 as Mw decreased from 14,500 to 1,200 g/mol. Interestingly, in

306

Figure 7 B, the relative abundance of β−O−4′ and β−β′ and β−5′ changed little when the

307

molecular weight of CELF lignin fraction increased from 6,300 to 14,500 g/mol, which was

308

consistent with the findings in the

309

changes occurred on the macromolecules of medium and high molecular weight CELF lignin

310

during CELF pretreatment.

31

112.7/6.76 (Gcond) was C2−H2 correlation of 49

On the other hand,

P NMR analysis. These results imply that little structural



relative abundance, %

50

A

β−O− β− −4'

β−β' β−β

40


30

β−5' β−

20

10

2000

4000

6000

8000

Mw, g/mol 75

B

relative abundance, %

70

β−O− β− −4'

65 60

THF− −methanol− −hexane fractionated CELF lignin

55 50 20

β−β' β−β 15

10

β−5' β− 5 4

S/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

Relationship between S/G and Mw

3 2 1 0

2000

4000

6000

8000

10000

12000

14000

Mw, g/mol

311 312 313 314

Figure 7. The relationship between the relative abundance of interunit linkages, the S/G ratio and Mw of (A) acetone-methanol-hexane fractionated kraft lignin and (B) THF-methanol-hexane fractionated CELF lignin.


Page 20 of 32

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315

Thermal Analysis. The thermal properties of AM-kraft lignin and TM-CELF lignin

316

fractions were investigated by TGA and DSC. Two-stage thermal degradation were observed in

317

the TGA thermograms of both lignin species (Figure 8). In nitrogen atomsphere, heterolysis and

318

hemolytic cleavage of β−O−4′ bonds and dehydration of aliphatic hydroxyl groups occur around

319

200 °C, and then C−C interunit linkages are cleaved accompanying with demethoxylation of

320

aromatic rings between 350 and 400 °C.51-52 When the molecular weight of the lignin fractions

321

decreased, the degradation temperature of first stage tended to decrease and the second one

322

moved slightly to the opposite direction. This is consistent with the changes in the relative

323

abundance of β−O−4′ and C−C interunit linkages estimated by semi-quantitative HSQC NMR.

324

In Figure 8 A, the additional peak (~290 °C) appeared on the thermal curves of AM-ins, AM-1H

325

and AM-2H reflected the degradation of xylan;53 such peak was not found on the curves of

326

CELF lignin fractions (Figure 8 C). When the TGA measurements were performed in air (Figure

327

8 B and D), thermal degradation of second stage was remarkably detained owing to the

328

condensation reaction between aliphatic side-chains and phenolic hydroxyl groups in the

329

presence of oxygen.51 Especially, in the case of CELF lignin (Figure 8 D), its fractions were

330

subject to rearrangements as Mw decreased and ratio between phenolic and aliphatic hydroxyl

331

groups approached to 1. The DSC results listed in Table 3 showed that the molecular weight of

332

lignin fraction played an important role on its glass transition behavior, regardless of the

333

variation in chemical structure of different lignin fraction. The glass transition temperatures (Tg)

334

of parent kraft lignin and CELF lignin were significantly lower than their corresponding high and

335

medium-molecular weight fractions which account for ~70% of the parent lignin. In the studies

336

of polymeric materials containing very high lignin contents, the presence of low molecular-

337

weight lignin was able to reduce the rigidity of lignin-based thermoplastics.15 Therefore, lignin 21 ACS Paragon Plus Environment


338

fractionation by organic co-solvents could provide a source of low-cost plasticizers for lignin-

339

based polymeric materials.

340

341 342 343 344 345

Figure 8. TGA thermograms of the AM-kraft lignin (A&B) and TM-CELF lignin (C&D)fractions.


Page 22 of 32

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346


Table 3. Glass transition temperatures of kraft lignin and CELF lignin fractions. kraft lignin parent KL AM-2H AM-4H AM-6H AM-8H AM-10H AM-13H AM-20H

Tg, °C 174 215 213 209 201 194 180 160

CELF lignin parent CELF TM-6H TM-10H TM-14H TM-20H

Tg, °C 121 158 154 141 133

347

348

349

CONCLUSION

350

Acetone-methanol-hexane and THF-methanol-hexane were employed for fractionating softwood

351

kraft lignin and hardwood CELF lignin, respectively. The volume proportions of acetone-

352

methanol and THF-methanol were carefully chosen based upon the solvent HSPs. The sequential

353

precipitation with hexane as antisolvent works well with both kraft lignin and CELF lignin

354

fractionation. The sequence of lignin precipitation as a result of gradually adding hexane into the

355

co-solvents was determined by several factors in order: (1) the content of hemicellulose bonding

356

with lignin macromolecules; (2) the molecular weight of lignin itself; (3) the type and content of

357

OH groups. The

358

higher frequency of aliphatic OH groups on the macromolecular chains, while low molecular

359

weight one contained more phenolic OH groups due to the cleavage of alkyl aryl ether bonds.

360

Generally, β−O−4′ alkyl ether linkages tended to be replaced by C−C bonds to some extent in

361

low molecular weight lignin fractions. The chemical structure of CELF lignin macromolecules

362

underwent little modifications when its molecular weight was above 6,000 g/mol.

31

P NMR analyses demonstrated that high molecular weight lignin possessed



363

ACKNOWLEGEMENTS

364

We acknowledge the support through U.S.D.A. National Institute of Food and Agriculture Grant

365

no. 9008-004957 titled “Integrated Biorefinery to Produce Ethanol, High Value Polymers, and

366

Chemicals from Lignocellulosic Biomass”.

367

368

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DOI 10.1039/C1PY00086A

525 526

Table of Content

527 528

Synopsis

529 530

A simple and fast method to fractionate technical lignins with low-boiling point organic cosolvents.

531


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Fast Fractionation of Technical Lignins by Organic ... - ACS Publications

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