Heliyon 9 (2023) e14556
Available online 13 March 2023
2405-8440/© 2023 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Research article
Acetylated lignin from oil palm empty fruit bunches and its electrospun nanofibres with PVA: Potential carbon fibre precursor
Mahyuni Harahap
a,b, Yurika Almanda Perangin-Angin
b,c, Vivi Purwandari
a,b, Ronn Goei
d, Alfred ling Yoong Tok
d, Saharman Gea
b,c,*aDepartment of Chemistry, Universitas Sari Mutiara Indonesia, Jalan Kapten Muslim No. 79, Medan, 20124, Indonesia
bCellulosic and Functional Materials Research Centre, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Medan, 20155, Indonesia
cDepartement of Chemistry, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Padang Bulan, Medan, 20155, Indonesia
dSchool of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Block N4.1, 637335, Singapore
A R T I C L E I N F O Keywords:
Lignin Acetylation Electrospinning Carbon fibre
Oil palm empty fruit bunches
A B S T R A C T
The electrospinning of acetylated lignin/polyvinyl alcohol (PVA) nanofibres was carried out to expand the application of lignin materials obtained from oil palm empty fruit bunches (OPEFB).
Lignin was isolated by the steam explosion method and subsequently precipitated using H2SO4. Acetylated lignin was produced by mixing acetic anhydride and pyridine at a 2:1 v/v ratio.
Following the acetylation process, FTIR analysis showed the absorption of the C=O carbonyl group at wavenumber 1714.6 cm−1. The chemical structures of isolated and acetylated lignin were established using 1H NMR spectral analysis, and XRD examination demonstrated their amorphous character. The electrospinning process of acetylated lignin and PVA solution was then carried out at 15 kV voltage, 0.8 mL/h flow rate, and 12 cm distance between the needle and collector. The sample exhibited electrical conductivity of 443 μS/cm and viscosity of 2.8 ×10−3 Pa s. The morphology analysis showed that there were more beads on the surface of lignin/PVA nanofibres than acetylated lignin/PVA nanofibres. In addition, acetylated lignin/PVA nanofibre was more stable than lignin/PVA. The G-band of carbonized material increased with the presence of lignin. The works presented suggest the potential of using waste materials such as OPEFB as a suitable precursor for the preparation of carbon fibre.
1. Introduction
Carbon fibres are materials with a huge potential for various applications. Poly(acrylonitrile) (PAN) is the most used precursor for the synthesis of the carbon fibres. However, the precursor is derived from fossil fuels with some limitations, such as being high in cost and its non-sustainable nature for the long-term use [1]. The search for new biopolymer-based materials has gained significant interest, especially with the rising concern about the impact of petroleum used on the sustainability of the environment and economy.
Bio-sourced materials have been studied to replace petroleum-based materials.
Lignin, the second most abundant natural material resource after cellulose, is a by-product of the pulp/paper and cellulosic-ethanol industries [2]. More than 55–70 million tonnes of by-product was produced annually, 98% of which were directly combusted for
* Corresponding author. Cellulosic and Functional Materials Research Centre, Universitas Sumatera Utara, Jalan Bioteknologi No. 1, Medan, 20155, Indonesia.
E-mail address: [email protected] (S. Gea).
Contents lists available at ScienceDirect
Heliyon
journal homepage: www.cell.com/heliyon
https://doi.org/10.1016/j.heliyon.2023.e14556
Received 21 November 2022; Received in revised form 8 March 2023; Accepted 9 March 2023
energy production with only 2% of the by-product converted into value-added products such as a binder, filler additive, dispersant, adsorbent, and surfactant [3]. Lignin is the largest source of natural-based aromatic functional groups and is considered as a low-cost precursor for carbon fibre precursor [4]. Therefore, lignin has the potential for cutting-edge applications and improving the overall economy of the relevant industry, particularly those that are related to the carbon fibre materials [5]. Lignin has been extensively researched for its potential as an alternative precursor to PAN over the past 10 years [6–8]. Due to its wide availability, carbon-building chemical structure, low-level pollution, and high carbon content, lignin has been regarded as a potential low-cost alternative and the most promising sustainable precursor for the carbon fibre [9].
Electrospinning is a simple conventional technique to produce nanofibre and submicron fibre, which can be subsequently carbonized [10]. The technique has three main components, i.e. high-voltage electricity, a syringe, and a collector. During the elec- trospinning process, an electrically charged polymer solution is discharged from a syringe as a thin jet of emission that accumulates on the collector surface. While the thin jet is in the air, the polymer solution is collected on top of the collector as fibres [11]. The production of lignin-based nanofibre using the electrospinning method is still underdeveloped because the end-product needs to be functionalised and/or blended with other polymers or plasticizers [12]. Blending lignin with synthetic or natural polymers would improve its viscoelastic properties, which helps the electrospinning process [13]. Poursorkhabi et al., 2015 examined the concen- tration of lignin/polyethylene oxide (PEO) for electrospinning and reported that the electrospun fibre generated at the ratio of lig- nin/PEO: 95/5 wt/wt at a concentration of 7 wt%. The nanofibres produced have the potential to be further carbonized to produce lignin-based carbon fibres [14]. Another study reported that the immiscibility of polymers has placed restrictions on the suitable ratio between lignin and cellulose acetate for electrospinning [15]. Lignin-based electrospun fibres extracted from palm fronds and banana bunch blended with a plasticizer had been produced. Subsequently carbonized at 500 ◦C, lignin-based carbon fibres have a smooth appearance and met the criteria to be a promising adsorbent material [16].
This work focused into the feasibility of isolating lignin from oil palm empty fruit bunches (OPEFB) using the steam explosion method. Recent studies reported the conversion of biomass using a green solvent such as deep eutectic solvent [17,18], however, the production/selling price is still relatively high at 1.9 USD per kg [19]. Steam explosion pre-treatment improves the yield of lignin isolation by up to 15% with high-purity and high-quality lignin without changing its structure [20]. The extraction of cellulose from OPEFB has been studied extensively for several applications such as drug carrier [21], personal care and food industry [22], membrane [23], and filler [24], however, lignin derived from by-products is still less utilised. The biomass consists of 20–50% cellulose, 23–36%
hemicellulose, and 22–51% lignin (dry weight) [25].
In this research, different lignin loads were used throughout the electrospinning process. Electrospun nanofibres were produced by combining lignin with polyvinyl alcohol (PVA). PVA is a synthetic polymer that has been widely used as a matrix composite in the electrospinning technique due to its outstanding electrospinnability alongside its biocompatibility and biodegradability [26–30]. The modification of lignin using the acetylation process was carried out to enhance the miscibility of lignin into the PVA polymer matrix.
After that, the fibres were carbonized at 600 ◦C to produce carbon nanofibres. Raman analysis of the produced carbon fibres was carried out.
2. Materials and methods 2.1. Materials
OPEFB was obtained from PT Perkebunan Nusantara VI Adolina, Medan, North Sumatra, Indonesia. Materials used such as PVA(s)
(MW: 60,000 g/mol), NaOH, distilled water, CH3COOH glacial, NaOCl 2.5%, H2O2 37%, H2SO4(l) 97%, acetic anhydride, pyridine, and commercial lignin were purchased from Sigma Aldrich, USA. All chemical materials were used as it is without further purification.
2.2. Isolation of lignin from oil palm empty fruit bunches
Firstly, the OPEFB was rinsed using distilled water to remove any impurities. After that, it was dried at room temperature and chopped into small pieces which size is around 5 cm in length. The cut samples were put into a vacuum oven at 80 ◦C for 5 h to remove moisture and excess water. Next, 50 g of the dried samples were soaked in NaOH 6% solution (1:10w/v) for 24 h. After that, it was transferred to the reactor for steam explosion with 180 kPa pressure, where the reaction occurred at 130 ◦C for 2 h. This process produced a black liquor Kraft lignin, which was then acidified using a 5 N H2SO4 solution until pH 2 in order to precipitate the lignin.
The lignin produced was rinsed using distilled water several times until the pH was neutral. Finally, the lignin produced was dried in the vacuum oven at 60 ◦C for 5 h.
2.3. Acetylation of lignin
7 g lignin was mixed into 420 mL pyridine: acetic anhydride (1:2 v/v) solution mixture. The acetylation process was carried out in three-necked round bottom flask under a reflux condition in a nitrogen atmosphere at 60 ◦C for 6 h. After that, the product was precipitated using 5 N H2SO4 solution until pH 2. The mixture was filtered using a vacuum filter and the solid phase was collected and dried in the vacuum oven at 60 ◦C for 5 h.
2.4. Electrospinning of PVA/lignin
Three polymer solutions were prepared for the electrospinning process. The first one was a 10% (w/v) PVA in distilled water prepared as the control. The second solution was 10% lignin in PVA solution (w/w), and the last solution was 10% acetylated lignin in PVA solution (w/w). Each of the solutions was electrospun using an 18G syringe at 15 kV voltage and 0.8 mL/h flow rate with a 12 cm distance between the collector and tip of the needle. The collector drum was covered using aluminium foil and was spun at 110–120 rpm speed.
2.5. Carbonization
The synthesis of carbon fibre began by heating the spun fibres from 30 ◦C to 600 ◦C for a total time of 140 min in an inert state (using N2 gas, 5 ◦C/min). The first heating stage was from 30 ◦C to 200 ◦C and maintained isothermally for 30 min. The second stage was from 200 ◦C to 400 ◦C and maintained isothermally for 30 min. The last heating stage was from 400 ◦C to 600 ◦C and maintained isothermally for 1 h. The carbon fibres produced were characterised using a Raman spectrometer.
2.6. Characterization
The surface morphology of lignin and PVA/lignin spun fibres were observed using a scanning electron microscopy FESEM JEOL 7600F at 20 kV accelerating voltage equipped with Oxford Instruments energy dispersive X-rays (EDX) detector. The image was taken at a magnification of 5000×for lignin and 20000×for spun fibres. Before the analysis, the samples were coated using a thin layer of gold to improve the overall conductivity of the sample. The chemical functional groups of samples were analyzed using a Fourier transform Infrared (FTIR) Shimadzu IR-Prestige 21 spectrometer. The instrument was operated in a transmission mode at wave- numbers 4000-500 cm−1, 2 cm−1 resolutions, and 100 scans. A thermogravimetric analyser (TGA) STA TG/DTA 7300 was used for the thermal analysis. The sample was heated in a clean and closed platinum pan from 30 ◦C to 600 ◦C with a heating rate of 10 ◦C/s under nitrogen gas flow. The crystallinity of lignin before and after acetylation was measured using a Shimadzu XRD 2000. The instrument was performed over a theta range between 0◦and 80◦at a 0.02◦sec−1 scan rate at 35 kV and 25 mA emission current. The structural analysis of lignin before and after acetylation was done using a 1H NMR Agilent 500 MHz (Agilent NMR Technology) at room tem- perature. The conductivity of the electrospinning polymer solution was measured using an AC780 Isolv Merck conductometer. The instrument was calibrated using distilled water before placing it into the homogenous electrospinning solution. The viscosity of the electrospinning polymer solution was measured using a Redwood viscometer. Raman spectra were collected using a Raman spec- trometer (nano-Raman, INTEGRA) equipped with a synapse CCD detector and a confocal Olympus microscope.
3. Results
3.1. Lignin yield and morphological analysis
The lignin yield used for this study was calculated based on biomass weight from the ratio between the obtained mass of lignin extraction and the mass of lignin in the fibre before treatment. In this study, lignin yield extracted from OPEFB was 26%, a low lignin content compared to the previous work 36.0% and 36.7% [31,32] and other lignocellulosic materials such as corn cob (57.3%), and poplar (69.9%) [33,34]. However, the result was similar to other reported study 28.89% and 22% [35,36]. Steam explosion is a low-cost environmentally friendly method to extract lignin which involves the competition between lignin depolymerization and repolymerization reactions. When repolymerization reactions occur in a marked way in comparison to depolymerization, low lignin yield is obtained due to hindered lignin extraction. However, a high lignin yield would be obtained when the depolymerization re- action is more pronounced than the repolymerization reaction [31].
SEM images for commercial, isolated, and acetylated lignin samples are presented in Fig. 1, while morphological images for PVA nanofibre, lignin/PVA nanofibre, and acetylated lignin/PVA nanofibres are shown in Fig. 2. Commercial lignin (Fig. 1a) looked like blocks of irregular allotropes with a smooth surface, and large particle sizes with pores. Whereas, isolated lignin (Fig. 1b) had a flat and rough surface without pores. Acetylated lignin (Fig. 1c) had some small beads on the surface. Different processes were involved in the
Fig. 1.FESEM images of (a) commercial lignin, (b) isolated lignin, (c) acetylated lignin.
synthesis of lignin samples, which resulted in different surface morphologies [37]. The acetylation process eliminated hydrogen bonds resulting in the distinctive morphology.
Nanofibres produced in this study had a surface morphology of polymeric beads. PVA nanofibre as shown in Fig. 2a had beads on the surface. In addition, there were more beads on the surface of lignin/PVA nanofibres (Fig. 2b) than acetylated lignin/PVA nano- fibres (Fig. 2c). A previous study reported that the acetylation of lignin improves the compatibility with polymer matrix composite [38]. Gupta et al., 2015 also examined lignin/PEO polymers and reported that the polymers formed beads at 25% lignin concentration on the fibre surface [39].
The conductivity and viscosity of the electrospun polymer solution influenced the morphology. Table 1 summarized the electrical conductivity and viscosity of electrospun polymers. PVA 10% solution had the lowest conductivity of 201.8 μS/cm, while lignin/PVA and acetylated lignin/PVA had an electrical conductivity of 438.6 μS/cm and 443.3 μS/cm, respectively. Meanwhile, the viscosity decreased with the increase in the electrical conductivity, i.e. 5.1043 ×10−3 Pa s in PVA 10% solution, 3.3451 ×10−3 Pa s in lignin/
PVA solution, and 2.8196 ×10−3 Pa s in acetylated lignin/PVA solution. In general, a high electrical conductivity and low viscosity electrospinning solution would decrease the resulting fibre sizes [40]. However, it is difficult to produce fine fibres at low viscosity because the droplets in the solution require a lot of pressure during the fibre manufacturing process [11].
3.2. FTIR analysis
FTIR spectra of lignin, PVA, and electrospun nanofibre are presented in Fig. 3.
The functional groups between isolated lignin and acetylated lignin are shown in Fig. 3. Acetylated lignin shows wavenumbers of 1714.6 cm−1 and 1267.3 cm−1 indicating the presence of C=O and C–O compounds, respectively. The presence of the C=O carbonyl group in acetylated lignin/PVA nanofibre could be observed at 1705.1 cm−1 and the reduction in O–H absorption area in lignin was at 3371.6 cm−1. The O–H absorption of lignin/PVA nanofibre shifted to 3371.6 cm−1 due to the formation of relatively strong hydrogen bonds among the hydroxyl groups in PVA and lignin, which could form hydrogen bonds with the semi-crystalline polymers [3]. The characteristics of functional groups for all samples are summarized in Table 2.
3.3. XRD analysis of lignin
XRD patterns of lignin before and after acetylation are shown in Fig. 4.
Fig. 4 exhibits the diffraction peak for isolated lignin at 2θ =21.440◦and commercial lignin at 2θ =23.396◦, while acetylated lignin exhibited a peak at 2θ =24.178◦. Previous research has reported a peak at 2θ =22.37◦of acetylated lignin isolated from a softwood [41]. The diffraction pattern revealed that the as-prepared lignin was amorphous, which is in agreement with previously reported studies [37].
3.4. 1H NMR analysis
1H NMR spectroscopy was used to examine the chemical structure of isolated lignin and acetylated lignin. Fig. 5 shows the NMR proton signals of isolated lignin and acetylated lignin. The chemical shift in the signal spectrum from 2.3 ppm in lignin to 2.11 ppm in
Fig. 2. FESEM images of (a) PVA nanofibre, (b) lignin/PVA nanofibres, (c) acetylated lignin/PVA nanofibres.
Table 1
Conductivity and viscosity analysis result.
No Electrospinning Solution Conductivity (μs/cm) Viscosity (Pa.s)
1 PVA 10% 201.8 5.1043 ×10−3
2 Lignin/PVA 438.6 3.3451 ×10−3
3 Acetylated lignin/PVA 443.3 2.8196 ×10−3
acetylated lignin suggested the presence of phenolic and acetate aliphatic groups. Signals of 3.4 ppm and 2.5 ppm showed the presence of water and DMSO-d6 solvent. Protons related to methoxyl groups in lignin which are related to the G and S presented at around 3.82 ppm. The protons in –CH3 of the aromatic and aliphatic acetyl groups were assigned in the signals at 2.29 ppm and 2.01 ppm [42], but it was at 3.8 ppm in isolated lignin. The proton signal of the methoxyl group was about 3.6 ppm.
Fig. 3.FTIR spectra of isolated lignin, commercial lignin, acetylated lignin, acetylated lignin/PVA nanofibre, lignin/PVA nanofibre, and PVA nanofibre.
Table 2
Functional group analysis.
wavenumbers (cm−1) Description
Lignin Nanofibres
Commercial Isolated Acetylated PVA Lignin/PVA Acetylated lignin/PVA
3425.6 3425.6 3406.8 3387.0 3371.6 3371.6 O–H
2939.5 2924.1 2929.1 2939.5 2939.5 2939.5 C–H methyl
1604.8 1604.8 1595.5 – 1604.8 1620.2 Aromatic rings
1512.2 1512.2 1513.3 Aromatic rings
1342.5 1327.0 1371.7 1327.1 1327.0 Syringyl
1273.0 1219.0 1207.7 – 1273.0 1273.0 Guaiacyl
817.8 825.5 812.6 – – – C–H aromatic
1458.2 1465.9 1461.1 Ether bond
- – 1714.6 – 1705.1 1705.1 C=O carbonyl
- – 1267.3 1095.6 1095.6 1095.6 C–O acetyl
Fig. 4. XRD patterns of commercial lignin, isolated lignin, and acetylated lignin.
3.5. Thermal analysis
The TGA curves for lignin and electrospun nanofibre samples are presented in Fig. 6.
Fig. 6 illustrates three steps of degradation in the thermal analysis of all samples. First was the dehydration stage observed in isolated and commercial lignin samples that began at ambient temperature and progressed to 100 ◦C resulting in 4–7% weight loss.
Then, the weight was further decreased at the second decomposition stage at a temperature range between 225 ◦C and 350 ◦C observed in acetylated lignin/PVA nanofibre, lignin/PVA nanofibre, and PVA nanofibre with 31–54% weight loss. The weight loss was caused by the decomposition of PVA and lignin. Finally, the samples lost almost all of their weight at 600 ◦C and produced residual weight as follows: 1.9% in isolated lignin, 1.8% in commercial lignin, 48% in acetylated lignin, 20% in lignin/PVA nanofibre, 21% in acetylated lignin/PVA nanofibre, and 8% in PVA nanofibre. The number of residues indicated that acetylated lignin was more stable than isolated lignin and commercial lignin because it had a greater residue, while acetylated lignin/PVA nanofibre was more stable than lignin/PVA and PVA nanofibres. The results were in agreement with previous studies [43].
3.6. Raman analysis
Raman spectroscopy (Fig. 7) was used to evaluate the structural changes in the lignin samples after carbonization. There were two distinct peaks present in carbonized lignin/PVA nanofibre. The peaks correspond to D and G-bands, which were typical for lignin and PVA-based carbon fibres. The D-band appearing at ~ 1350 cm−1 was attributed to the breathing mode of carbon atoms in the aromatic rings. While the G-band appearing at ~1600 cm−1 was ascribed to the sp2 stretching plane of carbon hybridized bonds (C=C) in the aromatic rings [44]. In this study, the G-band of carbonized material increased with the presence of lignin. Lignin with higher mo- lecular weight may improve the graphitic structure and mechanical performance of the carbonized materials [9].
4. Conclusion
Lignin from OPEFB was successfully isolated by steam explosion method. Lignin isolated was subsequently acetylated using acetic anhydride and pyridine. The FTIR analysis showed the presence of C=O compounds in acetylated lignin. Thermal analysis (TGA) showed that acetylated lignin/PVA nanofibres were more stable than lignin/PVA and PVA nanofibres. Acetylated lignin/PVA nano- fibres had smoother morphology and were thermally more stable than non-acetylated lignin/PVA nanofibres. Acetylated lignin/PVA showed exceptional quality as a candidate material for carbon fibre precursor.
Fig. 5. NMR proton signals of isolated lignin (below) and acetylated lignin (above).
Author contribution statement
Mahyuni Harahap, Yurika Almanda Perangin Angin: Conceived and designed the experiments; Performed the experiments;
Analyzed and interpreted the data; Prepared the manuscript.
Vivi Purwandari: Contributed reagents, materials, analysis tools or data.
Ronn Goei: Performed the experiments; Contributed reagents, materials, analysis tools or data; Revised the manuscript.
Alfred ling Yoong Tok: Contributed reagents, materials, analysis tools or data; Prepared the manuscript.
Saharman Gea: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Prepared the manuscript.
Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Data availability statement
Data will be made available on request.
Fig. 6. TGA curves of lignin and electrospinning composite nanofibres.
Fig. 7.Raman spectra of PVA nanofibre, lignin/PVA nanofibre, and acetylated lignin/PVA nanofibre carbonized at 600 ◦C in nitrogen gas flow for 1 h.
Declaration of interest’s statement The authors declare no conflict of interest.
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