Polyacrylonitrile Carbon Fibers: the Effect of Fractionation and

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Lignin/Polyacrylonitrile Carbon Fibers: the Effect of Fractionation and Purification on Properties of Derived Carbon Fibers Huan Liu, Zhong Dai, QIping Cao, Xiaojuan Shi, Xing Wang, Haiming Li, Ying Han, Yao Li, and Jinghui Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00868 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Sustainable Chemistry & Engineering

A simple acid precipitation approach to fractionate and purify lignin from corn stalk refining residues for the preparation of high-quality lignin-based carbon fibers 1657x622mm (96 x 96 DPI)

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Lignin/Polyacrylonitrile Carbon Fibers: the Effect of Fractionation and Purification on Properties of Derived Carbon Fibers

Huan Liua, Zhong Daia, Qiping Caoa, Xiaojuan Shia, Xing Wanga, Haiming Lia, Ying Hana, Yao Lia,*, Jinghui Zhoua,*

a

Liaoning Province Key Laboratory of Pulp and Papermaking Engineering, Dalian

Polytechnic University, Dalian, Liaoning Province, China. * E-mail: [email protected], * E-mail: [email protected].

Keyword: Lignin, Polyacrylonitrile, Carbon fibers, Fractionation, Acid precipitation,


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Abstract: The effects of lignin chemical structures on the quality of lignin-based carbon fibers are still not clear. For this reason, we address the challenge by using a simple acid precipitation method to separate and purify lignin and study the effects of physicochemical characteristics of fractionated lignin on the properties of lignin-based CFs. The precipitation carried out by sequential acidification at different pH levels (10, 8, 6, 4, and 2) is indeed effective in obtaining fractionated lignin samples with different chemical structures, including molecular weights, units’ composition ratios, and polydispersity indexes (PDIs). All the fractionated lignin samples are respectively blended with polyacrylonitrile at the ratio of 1:1 (w/w) and electro-spun into fibers. Results suggest that fractionated lignin sample with large molecular weight, low PDI, and strong thermal stability can produce the carbon fibers with excellent performance, such as good spinnability, high crystallization and mechanical strength.



Introduction With the exhaustion of mineral fuel resources and the increase in environmental pollution, conventional non-renewable resources are being replaced by renewable resources.1,

2

Lignin has received an increasing attention due to its abundant

renewable resource, which is only next to cellulose on global scale. Annually, the pulp and paper industry can produce approximately 50 million tons of lignin. However, only about 2% of the lignin is utilized effectively,

3, 4

and most of the lignin is

discharged into rivers or directly burned as fuel, thereby causing serious environmental pollution and grievous waste of renewable resources. 5 Lignin is a unique bio-macromolecule with an exciting chemical structure, including large molecular weight (Mw), high carbon content, and aromatic monomers. Its unique chemical characteristics make it a potential precursor to produce carbon fibers (CFs).6-9 CFs are industrially important materials because of their excellent performances, such as low density, high creep resistance, good tensile strength and corrosion resistance.10, 11 Currently, almost 90% of commercial CFs are mostly made of the polyacrylonitrile (PAN), which is non-renewable and expensive.12 The utilization of lignin not only reduces the cost of CFs production but also enhances the sustainability and cost-effectiveness of pulp and paper industries.8, 13 Many kinds of lignin-based CFs have been developed, especially sub-micron CFs. A recent method to produce sub-micron diameter lignin-based CFs is electrospinning. However, compared with the pure petroleum-based CFs, the quality of current lignin-based CFs is consistently unsatisfactory, because of the inherent high


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heterogeneity of lignin.14-17 Lignin contains highly branched chain structures and a large number of benzene rings (large intermolecular π-π interaction), 18-23 which may affect its solvent, spinnability, and miscibility with PAN, thereby affecting the fiber morphology and performance.24-26 Meanwhile, lignin is a bio-macromolecule with different monomer units, diverse chemical connection modes. Therefore, the fractionation and purification of lignin is especially necessary to reveal the effect of lignin with different structural characteristics on the lignin-based CFs.27-32 In the present work, we develop a simple approach of fractionating and purifying lignin (obtained from corn stalk refining residues) by sequential acidification at five different pH levels (10, 8, 6, 4, and 2). Many differences of chemical structure are noted for the fractionated lignin, including the units’ composition ratio, Mw, and molecular geometries. The lignin-based CFs prepared using the fractionated lignin samples blended with PAN exhibit different fiber morphologies and mechanical properties. These results reveal the effects of the physicochemical characteristics of pH fractionated lignin on the manufacture and properties of lignin-based CFs, especially for gramineae lignin. Materials and methods Materials Corn stalk refining residues of cellulosic ethanol were provided by COFCO Corporation. PAN with a weight-average Mw of 150,000 was provided by Sigma–Aldrich. N, N-dimethylformamide (DMF), hydrochloric acid (HCl), sodium and hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent



Corporation. Fractionation and purification of lignin The fractionated strategy of the lignin was illustrated Fig. 1a. The corn stalk refining residues were used as the starting material, and they were fractionated under various pH values (10, 8, 6, 4, and 2). Corn stalk refining residues (30 g) and aq. NaOH (12 g, in 300 mL water) were added into a 500 mL three-neck boiling flask. The mixture was quickly stirred for 12 h at 35 °C and the dissolved solids were filtered from the mixture. After that, HCl (37%) was added dropwise to the supernatant until pH=10. The mixture was stirred for 12 h at 35 °C, and a black solid formed. The black solid was obtained by centrifugation (3,000 rpm, 10 min). The acidification process of the centrifugal supernatant was continued until pH=8. The same method was used to sequentially obtain the lignin samples at pH=6, 4, and 2. Each obtained black solid was freeze-dried and milled in a mortar. The lignin samples were named lignin-10, lignin-8, lignin-6, lignin-4, and lignin-2. For instance, lignin-10 represents the lignin sample precipitated from the pH=10 solution. Preparation of lignin/PAN carbon fibers The PAN (3 g), lignin (3 g), and DMF (24 g) were blended in a conical flask (50 mL). And then the mixture was stirred for 12 h at 35 °C. After being naturally cooled ( room temperature), the mixture was transferred to a 5 mL syringe with a pump as the electrospinning fluid. The lignin-based precursor fibers were obtained by electrospinning that the operating distance was 20 cm, the injection rate was 0.8 mL/h, and the potential difference was 25 kV (+10 and −15 kV). After being dried to remove


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the solvent (a vacuum at 60 °C), the lignin-based precursor fibers were heated from 20 °C to 220 °C in a muffle furnace at a heating rate of 0.5 °C/min. The final temperature (220 °C) was maintained for 12 h to obtain the lignin-based stabilized fibers. Finally, the obtained fibers were carbonized in a tube furnace at 1,000 °C for 240 min (heating rate of 4.0 °C/min, under N2 ) to obtain the lignin-based CFs. Measurements Fourier transform infrared spectroscopy (FT-IR) was measured on a Perkin Elmer spectrometer by transmittance mode (KBr discs method). The lignin samples were firstly acetylated for the analysis of gel permeation chromatography (GPC), rendering them soluble in tetrahydrofuran completely.33 100 mg lignin was mixed with pyridine: aceticanhydride (1:2 v/v, 5 mL) under a nitrogen atmosphere for 72 h, and the solvent was is removed from the mixture by rotary evaporation. And then, the resulting acetylated lignin was rinsed by ether. NMR spectra (2D-HSQC) were determined by a Bruker AVANCE 400 MHz spectrometer, and the data were acquired by 60 mg of the lignin was dissolved in 0.6 mL of DMSO-d6. Total running time was 20 h. Thermal analysis was measured by using Q500 TGA and Q250 DSC from TA instruments. TGA results of lignin samples were obtained by being heated from 25 °C to 700 °C under nitrogen atmosphere at a heating rate of 10 °C/min. DSC tests were run from 0 to 300 °C under nitrogen atmosphere at a heating rate of 10 °C/min. In this case, for each thermal properties measurement, dried lignin samples were 5~10 mg. The morphologies of fibers were conducted on a JSM 7800F electron microscope (JEOL, Tokyo, Japan) with the primary electron energy of 15 kV. The carbon structure



of the resulted fibers was determined by an In Via Raman spectrometer (Ranishaw Co., UK). The spectra were recorded with back-scattered light from a 480 nm laser. The scan was done from 750 to 2000 cm-1. The crystal properties of carbon fibers were determined on a Shimadzu XRD 7000S diffractometer with a scanning step rate of 2 °C/min (50 kV, 200 mA, Cu Kα radiation, λ=0.154 nm). The tensile properties of carbon fibers were measured according to uniaxial tensile testing, which was carried out by using a Model 5569 Instron tension tester (Norwood, USA). Sample width was 4 cm and thickness was 1 cm, and measuring rate was 2 mm/min. The reported data was the average of the five separate measurements. Results and discussion Structural characterization of the fractionated lignin Lignin is mainly composed of three structural monomers: syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H). To define the linking mode between units, we numbered the carbon atoms in the benzene ring from 1 to 6 and labeled the carbon atoms in the aliphatic chains of the benzene ring as α, β, and γ (Fig. 1b). The major linking modes of the units were β–β, β–5, and β–O–4. Other linking modes include β–1, 4–O–5, 5–5, α–O–γ, and α–O–4. For instance, a β–O–4 linking mode indicated ether bonds were formed between the β-carbon of aliphatic side chain and the 4-carbon position of benzene ring. The NMR spectra (2D-HSQC) of lignin samples were recorded in Fig. 2a, and the main chemical structures and linking modes of lignin were shown in Fig. 2b. The characteristic peaks of the lignin samples were listed in Table 1. Differences in the


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chemical structures of lignin samples were clearly observed in the aliphatic side-chain region (δC/δH 50–90/2.5–5.5 ppm). Prominent correlation signals (A) of β–O–4 structure (Cα-Hα, Cβ-Cβ and Cγ-Cγ) were found at δC/δH 71.8/4.85, 60.8/3.28–4.01 and 85.4/4.12 ppm, respectively. The signals for resinol (β–β′/α–O–γ′, B) were identified for α–C, and β–C positions at δC/δH 82.3/4.89, and 55.4/3.27 ppm, respectively. At the aromatic region (δC/δH 100–135/6.0–8.0 ppm), the signals of S, G, and H units for fractionated lignin were observed. The signal of S units showed the C2,6–H2,6 units’ correlations at δC/δH 104.6/6.72 ppm. The G units showed different correlations for C2–H2, C5–H5, and C6–H6 at δC/δH 111.6/6.97, 115.6/6.66, and 119.7/6.81 ppm, respectively. Large quantities of H units were detected at δC/δH 127.8/7.16 ppm.

34

The S/G ratios of lignin-2, lignin-4, lignin-6, lignin-8, and

lignin-10 were 1.83, 1.30, 1.27, 1.08, and 1.05, respectively. The high Mw lignin fractions has higher content of G monomer units, because that C5 bonds (aryl–aryl or aryl-aliphatic can increase the Mw) are easy to be formed.35, the lignin units were estimated by 2D-HSQC NMR spectra

36

37

The main linkages of

, As shown in Table 2,

the fractionated lignin samples had different proportions of linkages. The amounts of β–O–4 structures of lignin-2, lignin-4, lignin-6, lignin-8, and lignin-10 were 31.52%, 39.44%, 40.86%, 46.57%, and 48.23%, respectively. The fractionated lignin samples with a large proportion of β–O–4 had more linear structures.38-40 Overall, the pH fractionation and purification method was indeed effective in fractionating lignin with different chemical structures, such as units’ composition ratio and molecular



geometries. According to the GPC analyses, it was found that the lignin precipitation under different pH values had different Mw and polydispersity index (PDI) (Table 3 and Fig. 3). The Mw of the lignin-2, lignin-4, lignin-6, lignin-8, and lignin-10 were 4667, 5722, 6901, 7717, and 8137 g/mol, respectively. The Mw of the lignin samples increased significantly with increasing precipitation pH value. PDI indicated the uniformity level of the lignin molecules. The PDI of the lignin-2, lignin-4, lignin-6, lignin-8, and lignin-10 were 2.85, 2.32, 2.16, 1.90, and 2.13, respectively. The fractionated lignin samples were determined by FT–IR spectra were shown in Fig. 4. The fractionated lignin samples had rather similar FT-IR spectra, showing typical gramineous lignin.41 The stretching vibration peak of hydroxyl groups was centred at 3450 cm–1 and the peaks around 2931 and 2842 cm–1 were attributed to C–H stretching vibration for CH3 and CH2 groups. The peak at 1711 cm–1 was owing to carbonyl stretching vibration. The typical peaks of the aromatic ring vibrations were at 1614, 1513, and 1460 cm–1. The bands at 1326 and 1117 cm–1 were associated with the S units and the bands at 1218, 1155 and 1033 cm–1 were associated with the G units in the lignin molecules. It was found that the intensity of S-bands was higher than the G-bands for all lignin fractions. The thermal properties of the lignin samples were illustrated in Fig. 5a. There were three stages for the thermal decomposition process of the lignin fractions. In the first stage, the mass loss of the lignin samples, which occurred at 100 °C, was due to the moisture from sample. The second stage (from 200 to 400 °C) was induced by the


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mass loss of side chain lignin and remaining hemicellulose.35 It was according with the NMR spectrum (2D-HSQC) of lignin-8. The third stage (beyond 400 °C) was because of the decomposition of the lignin units. The thermal degradation process of the fractionated lignin samples was slightly different. In particular, lignin-10 had better thermal stability compared with the other lignin samples because of its high yield (lignin-2: 49.54%, lignin-4: 49.01%, lignin-6: 44.09%, lignin-8: 49.65%, and lignin-10: 51.49%). As an important thermodynamic property of lignin, melting enthalpy was convenient and reliable to explain the phase transition behaviour of the materials. The DSC results of the fractionated lignin samples (Fig. 5b) indicated that melting enthalpy (at 130 °C) of lignin samples significantly increased with increasing precipitation pH value. Lignin-10 required the most energy to melt compared with other lignin samples. The excellent thermal stability was conducive to maintain the morphology of the lignin-based CFs during the thermo-stabilization process. Properties of lignin-based carbon fibers The morphologies of lignin-based stabilized fibers (SFs) are crucial to enhance the mechanical performances of the resulting CFs.42 The collapse and defect of the fibers morphology led to a sharp decline of mechanical strength.43 As shown in Fig. 6, the SFs morphologies were clearly observed. SFs showed the different fibrous morphology. Lignin-2, lignin-4, and lignin-6 SFs had wrinkled surfaces, and the lignin-8 and lignin-10 SFs showed smooth surfaces and smaller average diameters. This phenomenon was mainly attributed to the higher Mw and better thermal stability of lignin-8 and lignin-10. SFs were further carbonized, and the SEM photos of the



resulting CFs were shown in Fig. 7. There were many folds and defects on the surface of lignin-2, lignin-4, and lignin-6 CFs. Such surface morphologies were disadvantageous for the CFs’ mechanical strength. By contrast, lignin-8 and lignin-10 CFs maintained their complete filamentous morphology without any defects. To further compare the differences of the CFs prepared by different lignin samples, we examined the morphologies of the cross sections of the lignin-based CFs by SEM (Fig. 8). There was not the regular “cylinder” for the CFs prepared by the low Mw lignin samples (lignin-2, lignin-4, and lignin-6). Furthermore, an irregular fracture appeared on the cross-sections of lignin-2, lignin-4, lignin-6, and lignin-8 CFs. Only lignin-10 CFs maintained the regular “cylinder” and the cross-section was very neat. To characterize crystalline structures, the XRD patterns of the lignin-based CFs (Fig. 9a) were measured. One broad diffraction peak at 2θ = 24° was due to the crystallographic planes of (002), while another peak at 2θ = 43° corresponded to planes (100) in the carbonaceous structure. After removing the lignin of low Mw, the more uniform lignin molecules accelerated the formation of crystallite structures in the lignin-based CFs, thereby improving the mechanical performances. The broad peak at 2θ = 24° illustrated that the graphite crystallites had the extremely small sizes.44, 45 The distances between the interfacial crystallite layers (002) of lignin-2, lignin-4, lignin-6, lignin-8, and lignin-10 CFs were calculated by Bragg’s law to be 0.3702, 0.3627, 0.3602, 0.3565, and 0.3562 nm, respectively. The average interplanar spacing of lignin-10 CFs was the shortest. As shown in Raman spectroscopy (Fig. 9b), the two major broad peaks were


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attributed to the D band (1345 cm−1) and the G band (1600 cm−1) respectively. The D band originated from a hybrid vibration corresponded to the edges of graphene layer, hinting the graphitic structure contained the amount of defects. The G band was attributed to the sp2 carbon bonds. The D/G ratios of lignin-2, lignin-4, lignin-6, lignin-8, and lignin-10 CFs were 0.98, 0.94, 0.90, 0.88 and 0.87, respectively. The D/G ratios of the lignin sample CFs decreased with the increasing of Mw. For the practical application as reinforcing materials, the mechanical properties of CFs were investigated by using tensile tests. The CFs samples were cut into small pieces of 4 cm length and 1 cm width. For the tests, the effective length of CFs was 3 cm and the stress of CFs could be calculated through the following formula (1):

The surface density was the weight (g) of the CFs samples divided by its area (m2). The stress (N tex−1) was translated into the real tensile force (GPa) by multiplying it with the density of the CFs samples, which was confirmed by using Sartorius Secura balance. The change in the CFs samples’ width was negligible during tensile testing because that the transverse strain was extremely small for the CFs samples. Young’s modulus and the tensile stress of the lignin-based CFs were shown in Fig. 10, and the detailed tensile results were listed in Table 4. The standard deviation of sample was obtained by five repeated measurements. It was observed that there were significant differences for the lignin-based CFs prepared by different lignin samples in tensile strength and Young's modulus. The maximum stress of the CFs increased from 13.19 MPa (lignin-2) to 21.21 MPa (lignin-10) and Young's modulus significantly increased.



Conclusions We developed a simple fractionation and purification approach at different pH values, which was indeed effective in obtaining fractionated lignin samples with different chemical structures, including units’ composition ratios, molecular geometries, Mw, and PDI. Five kinds of lignin-based CFs were fabricated by the five fractionated lignin samples. Among them, the CFs prepared by fractionated lignin obtained at pH=10, had the smoothest surface, the highest crystallinity and the best mechanical properties. We conclude that a large Mw, low PDI, highly linear molecule geometries, and good thermal stability are necessary factors for lignin to obtain lignin-based CFs. The information can guide the development of bio-renewable lignin-based CFs, which can serve as a cost-effective alternative to the nonrenewable petroleum-based CFs. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 31170554, No. 31770635 and No. 31470604), Fundamental Research Funds of Dalian Polytechnic University (No. 2016J001). Supporting Information

Section 1: Spectrum of 2D-HSQC NMR for original lignin. Section 2: FT-IR spectrum of the original lignin. Section 3: TG curve of the original lignin. Section

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Content graphic: Fig. 1 Scheme of sequential lignin precipitation by acidification (a) and the main chemical units of lignin (b) Fig. 2 Spectra of 2D-HSQC NMR for lignin samples (a) and the main chemical structures and linking modes of lignin (b) Fig. 3 The molecular weight of fractionated lignin samples were measured by GPC Fig. 4 FT-IR spectra of the lignin samples Fig. 5 TGA (a) and DSC (b) of the lignin samples Fig. 6 SEM images of the morphology of the lignin-based SFs prepared by the different lignin samples Fig. 7 SEM imaging of the morphology of the lignin-based CFs prepared by the different lignin samples Fig. 8 SEM imaging of the morphology of the CFs cross-sections prepared by different lignin samples Fig. 9 XRD (a) and Raman (b) spectra of the lignin-based CFs prepared by the different lignin samples Fig. 10 Tensile stress (a) and Young’s moludus (b) of the lignin-based CFs prepared by the different lignin samples Table 1 Assignment of main lignin signals in the 2D-HSQC NMR spectra of lignin samples Table 2 Quantitative informations of the substructures on fractionated lignin (expressed as per 100 Ar, %) samples by 2D-HSQC NMR



Table 3 The molecular weight of fractionated lignin by sequential acid precipitation Table 4 Properties of the lignin-based CFs prepared by the lignin-2, lignin-4, lignin-6, lignin-8 and lignin-10


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Figure 7



Figure 8


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Figure 9



Figure 10


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Table 1 Lignin δC/δH

Assignment

structure Cβ-Hβ in phenylcoumaran substructures Cβ

53.1/3.50 (C)

Bβ

55.4/3.27

Cβ-Hβ in resinol substructures (B)

OCH3(OMe)

55.9/3.73

C-H in methoxyls

Aγ

60.8/4.12

Cγ-Hγ in β-O-4 substructures(A)

Aα

71.8/4.85

Cα-Hα in β-O-4 substructures(A)

Aβ(s)

87.2/4.08

Aβ(G/H)

84.9/4.34

Bα

82.3/4.89

Cβ-Hβ in β-O-4 substructures linked to a S unit (A) Cβ-Hβ in β-O-4 substructures linked to a G/H unit (A) Cα-Hα in resinol substructures (B) Cγ-Hγ in phenylcoumaran substructures

Cγ

62.2/3.76

S2,6

104.6/6.72

S'2,6

106.9/7.29

G2

111.6/6.97

C2-H2 in guaiacyl units (G)

G5

115.6/6.66


(C) C2,6-H2,6 in etherified syringyl units (S) C2,6-H2,6 in syringyl units with Cα=O groups (S')

G6

119.7/6.81


H2,6

127.8/7.16

C2,6-H2,6 in p-hydroxyphenyl units (H)

pCA3,5

145.0/7.52

C3,5-H3,5 in p-coumarate (PCA)

pCA8

115.4/6.32

C8–H8 in ferulate (PCA)

pCA2,6

130.3/7.52

C2,6-H2,6 in p-coumarate (PCA)

FA2

112.0/7.25

C2-H2 in ferulate (FA)

FA6

122.7/7.14

C6-H6 in ferulate (FA)



Table 2 pH2

pH4

pH6

pH8

pH10

β-O-4

31.52

39.44

40.86

46.57

48.23

β-β

7.83

9.46

9.08

15.07

15.92

β-5

5.12

5.58

5.18

8.18

9.24

S

49.80

42.17

45.98

37.58

28.69

G

27.21

32.27

36.15

34.72

27.22

H

22.99

25.56

17.87

27.70

44.09

S/G

1.83

1.30

1.27

1.08

1.05

a: Expressed as per 100 Ar b: Molar percentages S+G+H=100%


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Table 3 Lignin sample

Mw (g/mol)

Mn (g/mol)

PDI(Mw/ Mn)

pH2

4467

1639

2.85

pH4

5722

2464

2.32

pH6

6901

3191

2.16

pH8

7717

4064

1.90

pH10

8170

3814

2.13



Page 36 of 37

Table 4 Simple

Density (g/cm3)

lignin-2

1.53

13.19±2

lignin-4

1.58

lignin-6

1.65

lignin-8 lignin-10

Tensile strength (MPa) Young’s modulus (GPa)

Strain at break (%)

Yield (%)

2.64±0.4

1.88±0.04

40.85

14.94±2

3.05±0.2

1.74±0.02

38.98

15.62±3

3.75±0.4

0.94±0.04

42.87

1.67

17.32±2

3.79±0.1

0.82±0.02

46.90

1.69

21.21±3

4.54±0.1

1.40±0.01

57.87


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TOC A simple acid precipitation approach to fractionate and purify lignin from corn stalk refining residues for the preparation of lignin-based carbon fibers.


Polyacrylonitrile Carbon Fibers: the Effect of Fractionation and

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