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Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production
Materials Today: Proceedings xxx (xxxx) xxx
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Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production Vijayaletchumy Karunakaran, Norfahana Abd-Talib, Tau-Len Kelly Yong ⇑ Universiti Kuala Lumpur Malaysian Institute of Chemical and Bioengineering Technology (UniKL MICET), Lot 1988 Kawasan Perindustrian Bandar Vendor, Taboh Naning, Alor Gajah, Melaka 78000, Malaysia
a r t i c l e
i n f o
Article history: Received 6 October 2019 Received in revised form 9 January 2020 Accepted 11 January 2020 Available online xxxx Keywords: Carbon fiber Lignin Subcritical phenol Oil palm biomass Empty fruit bunches
a b s t r a c t Sustainable approach is needed to find an alternative precursor for carbon fiber. The resources should be low in cost with properties comparable to the present precursor. Malaysia as the second largest producer of palm oil produced hundred million tons of waste especially oil palm empty fruit bunches (EFB) from its plantation. The extraction of its lignin from biomass is a challenge as it binds together with cellulose and hemicellulose to form a complex network. Hence, phenol under subcritical conditions has the potential to shorten the reaction time to dissolve relatively high molecular weight compounds without catalyst. This study aimed to determine the effect of temperature (260–300 °C), reaction time (1–10 min), and solid loading (6 and 10 g) towards ash, volatile and carbon content of the lignin obtained from EFB under subcritical phenol conditions. Highest carbon content in the lignin (43.7%) was achieved at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.5% and 0.3% respectively. The correlation of these properties is discussed in this paper to understand its suitability as a precursor for carbon fiber production. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 4th International Conference on Green Chemical Engineering and Technology: Materials Science.
1. Introduction The world production of biomass is estimated to be 1880 billion tons per year [1]. Malaysia as the second largest palm oil producer, significantly contributes 38% of global market and disposing almost 50 million tons of dry biomass every year [2]. Palm oil extracted from the fresh fruit bunch (FFB) accumulates only 10%, while the rest 90% are discarded [3]. Common disposal of oil palm empty fruit bunches (EFB) are through burning or dumping in plantation site for natural decomposition. However, such practices resulted in emissions of greenhouse gases and also creating breeding habitat for rats and snakes [4]. Existing manufacturing of carbon fibers mainly used polyacrylonitrile (PAN) as precursor while other viable precursors are derived from cellulosic fibers and mesophase pitch. The overall process including pretreatment, preparation, and purification for this precursor is rather complex and the carbon fiber produced generally has poor properties compared with the PAN-based carbon fiber. Thus, more than 96% of global carbon fibers are from ⇑ Corresponding author. E-mail address: [email protected] (T.-L. Kelly Yong).
PAN, although it is an expensive precursor [5]. Yet, previous studies had shown that lignin can be an alternative precursor for carbon fibers. It was reported that the cost of lignin-based carbon fiber is only USD 1.1 per kg, compared to PAN-based precursors which ranges from USD 6.0–10.0 per kg [6]. Lignin is an alternative precursor to produce carbon fiber as it is naturally occurring, readily available worldwide and cheaper. The structure of lignin is complex due its higher molecular weight, amorphous compound with highly branched and substituted polymer of phenyl propane units. Major phenyl propane units in lignin are p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) which differ in o-methyl substitution of the aromatic ring [7]. Researchers at Oak Ridge National Laboratory (ORNL) investigated lignin blends in low-cost production of carbon fiber and developed a set of specifications as shown in Table 1 [8]. This specification served as an excellent guideline in evaluating the suitability of the recovered lignin as precursor for carbon fiber production. There are various method on isolation of lignin from biomass. Mohtar et al. [9] investigated the recovery of lignin and other lignocellulosic component from EFB using ionic liquid. Several concerns were raised based on the effect of ionic liquid since it disrupts the stronger linkages between the lignocellulosic
https://doi.org/10.1016/j.matpr.2020.01.252 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 4th International Conference on Green Chemical Engineering and Technology: Materials Science.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 Specification for lignin as carbon fiber precursor by ORNL [8]. Criteria Value
Range (wt%)
Ash Content Volatile Content Carbon Content
<0.1 <5.0 60
components by the interaction of bonds. H’ng et al. [10] developed pretreatment of EFB by prehydrolysis soda-anthraquinone cooking followed by enzymatic hydrolysis using cellulolytic enzymes. The study highlighted the need to identify the characteristics of lignin in EFB pulp since the finding shows maximum carbon content of only 50.0% [10]. Li et al. [11] in their study of wood liquefaction of sweet gum in phenol with sulphuric and phosphoric acid catalysts produced lignin with high carbon content up to 85.0%. The research shown there was significant effect in the reaction time of 2.5–15.0 min. However, low phenol to wood (P/W) ratio caused carbonization of liquefied wood which resultant from the insufficient amount of phenol [11]. Phenol under subcritical conditions (6.13 MPa and 421.2 °C) has the potential to shorten the reaction time needed to dissolve the relatively high molecular weight compounds from cellulose, hemicellulose, and lignin without catalysts. Subcritical phenol behaved as non-polar organic solvent as it has low dielectric constant, thus an excellent solvent to convert biomass to value-added chemicals [12]. Realising the importance of carbon fiber applications especially in the composite and polymer industries, thus, the efforts to study the potential of lignin as an alternative precursor for carbon fiber production that are more sustainable, low cost, with milder manufacturing process to replace PAN and pitch. The fundamental properties of the lignin have to be determined in order to evaluate its feasibility. Hence, this research focused on the fundamental properties (ash, volatile and carbon content) of lignin obtained from EFB under subcritical phenol conditions as a suitable precursor for carbon fiber production. 2. Methodology 2.1. Material preparations
Table 2 Experimental conditions. Parameters
Range of parameters
Temperature (°C) Time (min) Solid loading (g)
260–300 1–10 6–10
dried in an oven at 70 °C for 12 h. Table 2 shows the experimental conditions. 2.3. Recovery and purification of lignin Lignin recovery and purification were carried out according to the NREL specification (Determination of Structural Carbohydrates and Lignin in Biomass) [15]. 300 mg of the solid residue were mixed 3 mL of 72% sulphuric acid and placed in a water batch shaker at 30 ± 3 °C and incubated for 60 ± 5 min. The flask was sealed with aluminum foil and autoclaved for another 60 ± 5 min at 121 °C. After completion, the residue was left to settle and subsequently filtered. The acid-insoluble residue was washed with hot water and dried in an oven at 105 °C for 12 h. 2.4. Analysis of recovered lignin 2.4.1. Ash content Ash content was determined according to TAPPI Standard Test Method T 413 om-93 [16]. The recovered lignin was ignited in muffle furnace at 900 °C for 4 h and repeated upon reaching a constant weight. The weight of ash residue is used to measure the ash content in the recovered lignin as shown in Eq. (1) below.
AshContentð%Þ ¼
Weight of ashðg Þ 100 Weight of recov ered lignin ðg Þ
ð1Þ
2.4.2. Volatile matter content The volatile matter content in the recovered lignin sample was determined following the standard by ORNL and Luo [17]. The weight loss was used to measure its volatile content as shown in Eq. (2) below.
Volatile Matter Contentð%Þ ¼
Weight of recov ered ligninðgÞ Weight loss ðg Þ 100% Weight of recov ered lignin ðgÞ
ð2Þ
The EFB was collected from Palm Oil Refinery Factory in Malim Sawit, Johor. Sample preparation followed standard procedures [13,14]. The samples were ground and sieved to obtain particles in the range of 210–500 mm. Subsequently, the samples were subjected to extraction with methanol to remove oil residues and extractives. After 6 h, the extracted samples were washed with water and dried in the oven for 48 h at 45 °C.
2.4.3. Carbon content The recovered lignin samples were analysed using CHNS/O to determine its percentage of carbon, hydrogen, nitrogen and sulphur.
2.2. Experimental conditions
3. Results and discussion
The reaction was conducted in an autoclave batch reactor with working volume of 100 mL. Weighed EFB (6 or 10 g) and phenol were mixed and loaded into the reactor. The reaction pressure was maintained under subcritical phenol conditions (6.13 MPa). The reaction time was recorded once the temperature inside the vessel reached the target temperature. When the reaction was completed, the reactor was immersed in a water bath to stop the reaction. The solid residue was separated from the liquid residue using vacuum filtration. Subsequently, the solid residue was washed with methanol several times to ensure the removal of phenol. The washing was stopped when the color of the methanol filtrate was almost colorless and the refractive index (RI) of the methanol approached 1.33141. The solid residue was subsequently
3.1. Elemental and compositional analysis Elemental analysis of EFB used in this study as comparison to others is shown in Table 3 below. The carbon content of the EFB in this study (48.6 wt%) is comparable to other study. Furthermore, the carbon content in EFB is considerably higher compared to other types of oil palm biomass except for oil palm trunk. High carbon content in the biomass is desirable as it is one of the vital requirements to produce carbon fiber. The compositional analysis in Table 4 shows that lignin content in the EFB is the highest (34.9 wt%) and comparable with the lignin content reported in EFB of other study. The lignin content in EFB is also considerably higher than other types of oil palm biomass. The
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 3 Elemental analysis of oil palm biomass. Types of Biomass
Elemental content (wt%) Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
Oil Oil Oil Oil
48.6 45.6 51.4 43.6
6.6 6.2 11.8 4.8
0.6 0.4 0.2 0.6
0.4 – – 0.5
43.8 47.8 51.2 50.6
palm palm palm palm
EFB EFB Trunk Frond
References
This study [18] [19] [20]
Table 4 Compositional analysis of oil palm biomass. Types of Biomass
Oil Oil Oil Oil
palm palm palm palm
EFB EFB Trunk Frond
Composition (wt%)
References
Cellulose
Hemicellulose
Lignin
Extractives
Ash
32.0 28.0 22.4 45.2
24.4 19.4 41.3 17.5
34.9 32.2 36.1 22.2
4.6 7.1 – 6.6
4.1 2.3 2.2 3.6
high lignin content in the biomass is important as lignin will be extracted as the precursor to produce carbon fiber. The low ash content in the biomass is preferable as this suppressed the formation of char especially at high temperature [21]. On the other hand, high content of extractives is unfavorable as this caused mechanical defects in the carbon fiber [22]. 3.2. Recovered lignin specification as carbon fiber precursor Ash is defined as the residue left after combustion at high temperature (900 °C). Minimum inorganic content matter in lignin is desirable as a precursor for carbon fiber production. The mechanical properties of the recovered lignin may reduce due to the presence of these inorganic minerals hence ORNL specification that the ash content must be less than 0.1% for it to be deemed as suitable as precursor for carbon fiber production [8]. The range of ash content obtained in this study are 14.1–46.7% which are higher than the specification. Lignin recovered from sulfite process has similar high ash content (25.0%) [26]. The high ash content can be attributed to sulfur from sulfuric acid during the purification stage. The presence of sulfur due to acid hydrolysis were discussed by Sameni et al. [27] where they obtained higher ash content of 4.3% in the lignin from pine species. The volatile matter content is determined by measuring the change in sample mass before and after heating at 250 °C for 6 h. Volatile content caused modifications in the carbon fiber structures due to evaporation during carbonization at high heating rate [28]. As specified by ORNL, the volatile content in the recovered lignin must be less than 5% as suitable precursor for carbon fiber production (Table 1). The range of volatile content obtained in this study is 0.1–0.3% which fulfilled the ORNL specifications. Thus, it can be concluded that the range of parameters used in this study are suitable to produce lignin with low volatile content. Carbon content in the recovered lignin is another important criterion to be considered as the precursor for carbon fiber production. As specified by ORNL, the carbon content has to be higher than 60%. In this study, the range of carbon content obtained are 29.4–43.7%. The low carbon content obtained in this study can be attributed to the high ash content in the recovered lignin. Lignin from hardwood source by Mainka et al. [29] has carbon content over 60.0%. Meanwhile, Faris et al. [30] determined the elemental properties of Kraft lignin from EFB and reported carbon content of 57.8%. The differences in carbon content in similar raw material can be attributed to the extraction method and reaction parameters.
This study [23] [24] [25]
3.3. Ash content Ash content of lignin originated from minerals such as sodium, potassium, calcium, aluminium, magnesium and silicone. Minerals may present higher in younger plants compared to matured plants due to uptake for growth and its fruit bearing potentials [31]. Figs. 1 and 2 below shows the ash content at different temperatures (260– 300 °C), reaction times (1–10 min) and solid loading (6 and 10 g). The lowest and highest ash content in the recovered lignin obtained in this study is 14.1% and 46.7% respectively. The ash content increased with temperature for all reaction times. The increase in ash content with temperature is due to the increase condensation rate of the ash with the intermediates produced during lignin decomposition under subcritical phenol conditions. This is consistent with Makela et al. [32] where they observed the increase in ash content from 48.0% to 67.0% for hydrothermal treatment of biomass at 180–260 °C. Shakya et al. [33] also observed significant increase of ash content from 6.1% to 43.3% when the temperature was increased from 250 °C to 350 °C at 60 min. As observed, the ash content increased with longer reaction time. This indicated that reaction time is an important parameter that affected the recovered lignin from EFB. Makela et al. [32] reported high content of sodium and chlorine in char composition of EFB. They reported that reaction time (0.08–14 h) increased the ash content up to 55.8%. Thus, shorter reaction time is essential for economical operation as well as producing highly purified ligninbased precursor [34].
Fig. 1. Ash content at different temperatures (260–300 °C), reaction times (1– 10 min) and 6 g solid loading.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 2. Ash content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
Fig. 4. Volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
Fig. 2 shows that the highest ash content (46.7%) in this study was obtained at 10 g, 10 min and 300 °C. Lower ash content of 36.9% was obtained at 6 g under the same reaction conditions (Fig. 1). Higher solid loading produced higher ash content in all temperature range used in this study. Kabir and Hameed [35] stated that reaction temperature and solid loading caused similar effect to the reaction due to the heat transfer within the particles. Makela et al. [32] similarly observed significant amount of potassium, magnesium, sulphur and silicone in ash composition of EFB. Furthermore, higher solid loading caused the increased amount of potassium present in lignin and subsequently in the ash
The volatile matter content generally decreased with longer reaction time as shown in Figs. 3 and 4. The significantly high ash content under the same conditions caused the lower volatile matter content in the lignin as similarly observed by Kabir and Hameed [35]. Higher reaction temperature showed prominent changes in volatility of lignin compared to reaction time. But prolonged reaction time produced lower volatile content. Volatile gases such as CO, CO2 and CH4 are produced by lignin degradation products. Yong and Matsumura [37] and Watkins et al. [38] concluded that biomass decomposed rapidly within few minutes. Carbon fiber production required low amount of volatiles in order to avoid foaming during spinning of the fibres [6]. In contrast to ash content, higher volatile content were obtained at lower solid loading. Acid treatment during lignin purification step created porosity due to breaking of the cell wall, that disrupted EFB structure which affected the volatile content [39]. Furthermore, the finding of this study are consistent with study by Yong et al. [40] which observed the decrease of volatile content with solid loading.
3.4. Volatile matter content Volatile matter content in the recovered lignin are important since it caused modifications in the carbon fiber structures due to evaporation especially during carbonization at high heating rate [28]. Figs. 3 and 4 shows the volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and solid loading (6 g and 10 g) towards volatile matter content. The volatile matter content decreased with reaction temperature. Nevertheless, the decrease in volatile content with temperature is considerably minimal. For example, it decreased from 0.3% (260 °C) to 0.2% (300 °C) at 1 min and 6 g. The decrease of volatile matter content with temperature could be attributed to the reduction of bond strengths in lignin at higher temperature producing macromolecules volatiles and subsequently volatilized. Thus, it can be concluded that lower volatile matter content can be obtained at higher reaction temperature under subcritical phenol conditions. The ash and volatile matter content are inverse of one another with temperature change. This is consistent with Mousa et al. [36] that reported the Kraft lignin produced ash content of 0.8% but volatile content as high as 59.6%.
Fig. 3. Volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and 6 g solid loading.
3.5. Carbon content Figs. 5 and 6 shows the effect of temperatures (260–300 °C), reaction times (1–10 min) and solid loading (6 g and 10 g) towards carbon content. The highest carbon content (43.7%) was obtained at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.4% and 0.3% respectively. In contrast, at 300 °C, the highest carbon content was (29.4%) with its ash and volatile content of 31.8% and 0.2% respectively. Subcritical conditions caused side reaction and yield secondary products especially at higher temperatures, which caused the decrease in carbon content in the solid residue [41]. The solubility of lignin caused carbon loss to liquid fraction. The rate of depolymerization are affected by temperature [42]. Lignin
Fig. 5. Carbon content at different temperatures (260–300 °C), reaction times (1–10 min) and 6 g solid loading.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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ing - review & editing. Tau-Len Kelly Yong: Conceptualization, Methodology, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Fig. 6. Carbon content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
are more soluble due to the increased cleavage of b-O-4 ether bonds into monomeric phenols at higher temperature. Bui et al. [43] deduced that liquefaction of biomass are enhanced at above 280 °C under sub- and supercritical conditions. Diffusion rate increased with temperature as the decrease in dielectric constant resulted in enhanced degradation of lignin [44]. This indicated that high carbon content is obtained at lower temperature with low ash and volatile content which are desirable. It is observed that the carbon content decreased minimally with prolonged reaction time. The highest carbon content of 43.7% was obtained at 260 °C, 1 min and 10 g. Thus, it can be concluded that reaction time does not affect the carbon content in this study. This is in agreement with Meilany et al. [45]. The overall trend shows that both fundamental properties (carbon content and volatile content) of the recovered lignin decreased with time. On the other hand, the ash content increased with time. Different recovery method and extraction produced lignin with different carbon content. Elemental study of enzymatically isolated residual lignin from EFB produced lignin with carbon content in the range of 44.2% to 50.4% which is comparable to this study [10]. The effect of solid loading towards both volatile content and carbon content are similar where lower solid loading produced higher carbon content. Higher solid loading created mechanical hindrance which may result in non-homogenous reaction especially the bonds breakage of the lignocellulosic components [45]. Presence of phenol with higher solid loading might favour formation of other products such as catechols [41]. Kabir and Hameed [35] also reported that higher solid loading affected the heat transfer between the solid particles thus affecting its carbon content. 4. Conclusion The aim of this study is to determine the effect of temperature (260–300 °C), reaction time (1–10 min), and solid loading (6 and 10 g) towards ash, volatile and carbon content of the recovered lignin EFB under subcritical phenol conditions as precursor material for carbon fiber production. The highest carbon content in the lignin (43.7%) was achieved at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.5% and 0.3% respectively. As shown in this study, temperature, reaction time and solid loading under subcritical phenol conditions has significant effect on the ash, volatile, and carbon content. Utilisation of EFB from oil palm biomass to produce advanced material as well as resolving the waste management issues faced by oil palm industries, could certainly catalysed the aim to achieve the National Biomass Strategy 2020. CRediT authorship contribution statement Vijayaletchumy Karunakaran: Visualization, Investigation, Writing - original draft, Data curation. Norfahana Abd-Talib: Writ-
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Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production Vijayaletchumy Karunakaran, Norfahana Abd-Talib, Tau-Len Kelly Yong ⇑ Universiti Kuala Lumpur Malaysian Institute of Chemical and Bioengineering Technology (UniKL MICET), Lot 1988 Kawasan Perindustrian Bandar Vendor, Taboh Naning, Alor Gajah, Melaka 78000, Malaysia
a r t i c l e
i n f o
Article history: Received 6 October 2019 Received in revised form 9 January 2020 Accepted 11 January 2020 Available online xxxx Keywords: Carbon fiber Lignin Subcritical phenol Oil palm biomass Empty fruit bunches
a b s t r a c t Sustainable approach is needed to find an alternative precursor for carbon fiber. The resources should be low in cost with properties comparable to the present precursor. Malaysia as the second largest producer of palm oil produced hundred million tons of waste especially oil palm empty fruit bunches (EFB) from its plantation. The extraction of its lignin from biomass is a challenge as it binds together with cellulose and hemicellulose to form a complex network. Hence, phenol under subcritical conditions has the potential to shorten the reaction time to dissolve relatively high molecular weight compounds without catalyst. This study aimed to determine the effect of temperature (260–300 °C), reaction time (1–10 min), and solid loading (6 and 10 g) towards ash, volatile and carbon content of the lignin obtained from EFB under subcritical phenol conditions. Highest carbon content in the lignin (43.7%) was achieved at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.5% and 0.3% respectively. The correlation of these properties is discussed in this paper to understand its suitability as a precursor for carbon fiber production. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 4th International Conference on Green Chemical Engineering and Technology: Materials Science.
1. Introduction The world production of biomass is estimated to be 1880 billion tons per year [1]. Malaysia as the second largest palm oil producer, significantly contributes 38% of global market and disposing almost 50 million tons of dry biomass every year [2]. Palm oil extracted from the fresh fruit bunch (FFB) accumulates only 10%, while the rest 90% are discarded [3]. Common disposal of oil palm empty fruit bunches (EFB) are through burning or dumping in plantation site for natural decomposition. However, such practices resulted in emissions of greenhouse gases and also creating breeding habitat for rats and snakes [4]. Existing manufacturing of carbon fibers mainly used polyacrylonitrile (PAN) as precursor while other viable precursors are derived from cellulosic fibers and mesophase pitch. The overall process including pretreatment, preparation, and purification for this precursor is rather complex and the carbon fiber produced generally has poor properties compared with the PAN-based carbon fiber. Thus, more than 96% of global carbon fibers are from ⇑ Corresponding author. E-mail address: [email protected] (T.-L. Kelly Yong).
PAN, although it is an expensive precursor [5]. Yet, previous studies had shown that lignin can be an alternative precursor for carbon fibers. It was reported that the cost of lignin-based carbon fiber is only USD 1.1 per kg, compared to PAN-based precursors which ranges from USD 6.0–10.0 per kg [6]. Lignin is an alternative precursor to produce carbon fiber as it is naturally occurring, readily available worldwide and cheaper. The structure of lignin is complex due its higher molecular weight, amorphous compound with highly branched and substituted polymer of phenyl propane units. Major phenyl propane units in lignin are p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) which differ in o-methyl substitution of the aromatic ring [7]. Researchers at Oak Ridge National Laboratory (ORNL) investigated lignin blends in low-cost production of carbon fiber and developed a set of specifications as shown in Table 1 [8]. This specification served as an excellent guideline in evaluating the suitability of the recovered lignin as precursor for carbon fiber production. There are various method on isolation of lignin from biomass. Mohtar et al. [9] investigated the recovery of lignin and other lignocellulosic component from EFB using ionic liquid. Several concerns were raised based on the effect of ionic liquid since it disrupts the stronger linkages between the lignocellulosic
https://doi.org/10.1016/j.matpr.2020.01.252 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the 4th International Conference on Green Chemical Engineering and Technology: Materials Science.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 1 Specification for lignin as carbon fiber precursor by ORNL [8]. Criteria Value
Range (wt%)
Ash Content Volatile Content Carbon Content
<0.1 <5.0 60
components by the interaction of bonds. H’ng et al. [10] developed pretreatment of EFB by prehydrolysis soda-anthraquinone cooking followed by enzymatic hydrolysis using cellulolytic enzymes. The study highlighted the need to identify the characteristics of lignin in EFB pulp since the finding shows maximum carbon content of only 50.0% [10]. Li et al. [11] in their study of wood liquefaction of sweet gum in phenol with sulphuric and phosphoric acid catalysts produced lignin with high carbon content up to 85.0%. The research shown there was significant effect in the reaction time of 2.5–15.0 min. However, low phenol to wood (P/W) ratio caused carbonization of liquefied wood which resultant from the insufficient amount of phenol [11]. Phenol under subcritical conditions (6.13 MPa and 421.2 °C) has the potential to shorten the reaction time needed to dissolve the relatively high molecular weight compounds from cellulose, hemicellulose, and lignin without catalysts. Subcritical phenol behaved as non-polar organic solvent as it has low dielectric constant, thus an excellent solvent to convert biomass to value-added chemicals [12]. Realising the importance of carbon fiber applications especially in the composite and polymer industries, thus, the efforts to study the potential of lignin as an alternative precursor for carbon fiber production that are more sustainable, low cost, with milder manufacturing process to replace PAN and pitch. The fundamental properties of the lignin have to be determined in order to evaluate its feasibility. Hence, this research focused on the fundamental properties (ash, volatile and carbon content) of lignin obtained from EFB under subcritical phenol conditions as a suitable precursor for carbon fiber production. 2. Methodology 2.1. Material preparations
Table 2 Experimental conditions. Parameters
Range of parameters
Temperature (°C) Time (min) Solid loading (g)
260–300 1–10 6–10
dried in an oven at 70 °C for 12 h. Table 2 shows the experimental conditions. 2.3. Recovery and purification of lignin Lignin recovery and purification were carried out according to the NREL specification (Determination of Structural Carbohydrates and Lignin in Biomass) [15]. 300 mg of the solid residue were mixed 3 mL of 72% sulphuric acid and placed in a water batch shaker at 30 ± 3 °C and incubated for 60 ± 5 min. The flask was sealed with aluminum foil and autoclaved for another 60 ± 5 min at 121 °C. After completion, the residue was left to settle and subsequently filtered. The acid-insoluble residue was washed with hot water and dried in an oven at 105 °C for 12 h. 2.4. Analysis of recovered lignin 2.4.1. Ash content Ash content was determined according to TAPPI Standard Test Method T 413 om-93 [16]. The recovered lignin was ignited in muffle furnace at 900 °C for 4 h and repeated upon reaching a constant weight. The weight of ash residue is used to measure the ash content in the recovered lignin as shown in Eq. (1) below.
AshContentð%Þ ¼
Weight of ashðg Þ 100 Weight of recov ered lignin ðg Þ
ð1Þ
2.4.2. Volatile matter content The volatile matter content in the recovered lignin sample was determined following the standard by ORNL and Luo [17]. The weight loss was used to measure its volatile content as shown in Eq. (2) below.
Volatile Matter Contentð%Þ ¼
Weight of recov ered ligninðgÞ Weight loss ðg Þ 100% Weight of recov ered lignin ðgÞ
ð2Þ
The EFB was collected from Palm Oil Refinery Factory in Malim Sawit, Johor. Sample preparation followed standard procedures [13,14]. The samples were ground and sieved to obtain particles in the range of 210–500 mm. Subsequently, the samples were subjected to extraction with methanol to remove oil residues and extractives. After 6 h, the extracted samples were washed with water and dried in the oven for 48 h at 45 °C.
2.4.3. Carbon content The recovered lignin samples were analysed using CHNS/O to determine its percentage of carbon, hydrogen, nitrogen and sulphur.
2.2. Experimental conditions
3. Results and discussion
The reaction was conducted in an autoclave batch reactor with working volume of 100 mL. Weighed EFB (6 or 10 g) and phenol were mixed and loaded into the reactor. The reaction pressure was maintained under subcritical phenol conditions (6.13 MPa). The reaction time was recorded once the temperature inside the vessel reached the target temperature. When the reaction was completed, the reactor was immersed in a water bath to stop the reaction. The solid residue was separated from the liquid residue using vacuum filtration. Subsequently, the solid residue was washed with methanol several times to ensure the removal of phenol. The washing was stopped when the color of the methanol filtrate was almost colorless and the refractive index (RI) of the methanol approached 1.33141. The solid residue was subsequently
3.1. Elemental and compositional analysis Elemental analysis of EFB used in this study as comparison to others is shown in Table 3 below. The carbon content of the EFB in this study (48.6 wt%) is comparable to other study. Furthermore, the carbon content in EFB is considerably higher compared to other types of oil palm biomass except for oil palm trunk. High carbon content in the biomass is desirable as it is one of the vital requirements to produce carbon fiber. The compositional analysis in Table 4 shows that lignin content in the EFB is the highest (34.9 wt%) and comparable with the lignin content reported in EFB of other study. The lignin content in EFB is also considerably higher than other types of oil palm biomass. The
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 3 Elemental analysis of oil palm biomass. Types of Biomass
Elemental content (wt%) Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
Oil Oil Oil Oil
48.6 45.6 51.4 43.6
6.6 6.2 11.8 4.8
0.6 0.4 0.2 0.6
0.4 – – 0.5
43.8 47.8 51.2 50.6
palm palm palm palm
EFB EFB Trunk Frond
References
This study [18] [19] [20]
Table 4 Compositional analysis of oil palm biomass. Types of Biomass
Oil Oil Oil Oil
palm palm palm palm
EFB EFB Trunk Frond
Composition (wt%)
References
Cellulose
Hemicellulose
Lignin
Extractives
Ash
32.0 28.0 22.4 45.2
24.4 19.4 41.3 17.5
34.9 32.2 36.1 22.2
4.6 7.1 – 6.6
4.1 2.3 2.2 3.6
high lignin content in the biomass is important as lignin will be extracted as the precursor to produce carbon fiber. The low ash content in the biomass is preferable as this suppressed the formation of char especially at high temperature [21]. On the other hand, high content of extractives is unfavorable as this caused mechanical defects in the carbon fiber [22]. 3.2. Recovered lignin specification as carbon fiber precursor Ash is defined as the residue left after combustion at high temperature (900 °C). Minimum inorganic content matter in lignin is desirable as a precursor for carbon fiber production. The mechanical properties of the recovered lignin may reduce due to the presence of these inorganic minerals hence ORNL specification that the ash content must be less than 0.1% for it to be deemed as suitable as precursor for carbon fiber production [8]. The range of ash content obtained in this study are 14.1–46.7% which are higher than the specification. Lignin recovered from sulfite process has similar high ash content (25.0%) [26]. The high ash content can be attributed to sulfur from sulfuric acid during the purification stage. The presence of sulfur due to acid hydrolysis were discussed by Sameni et al. [27] where they obtained higher ash content of 4.3% in the lignin from pine species. The volatile matter content is determined by measuring the change in sample mass before and after heating at 250 °C for 6 h. Volatile content caused modifications in the carbon fiber structures due to evaporation during carbonization at high heating rate [28]. As specified by ORNL, the volatile content in the recovered lignin must be less than 5% as suitable precursor for carbon fiber production (Table 1). The range of volatile content obtained in this study is 0.1–0.3% which fulfilled the ORNL specifications. Thus, it can be concluded that the range of parameters used in this study are suitable to produce lignin with low volatile content. Carbon content in the recovered lignin is another important criterion to be considered as the precursor for carbon fiber production. As specified by ORNL, the carbon content has to be higher than 60%. In this study, the range of carbon content obtained are 29.4–43.7%. The low carbon content obtained in this study can be attributed to the high ash content in the recovered lignin. Lignin from hardwood source by Mainka et al. [29] has carbon content over 60.0%. Meanwhile, Faris et al. [30] determined the elemental properties of Kraft lignin from EFB and reported carbon content of 57.8%. The differences in carbon content in similar raw material can be attributed to the extraction method and reaction parameters.
This study [23] [24] [25]
3.3. Ash content Ash content of lignin originated from minerals such as sodium, potassium, calcium, aluminium, magnesium and silicone. Minerals may present higher in younger plants compared to matured plants due to uptake for growth and its fruit bearing potentials [31]. Figs. 1 and 2 below shows the ash content at different temperatures (260– 300 °C), reaction times (1–10 min) and solid loading (6 and 10 g). The lowest and highest ash content in the recovered lignin obtained in this study is 14.1% and 46.7% respectively. The ash content increased with temperature for all reaction times. The increase in ash content with temperature is due to the increase condensation rate of the ash with the intermediates produced during lignin decomposition under subcritical phenol conditions. This is consistent with Makela et al. [32] where they observed the increase in ash content from 48.0% to 67.0% for hydrothermal treatment of biomass at 180–260 °C. Shakya et al. [33] also observed significant increase of ash content from 6.1% to 43.3% when the temperature was increased from 250 °C to 350 °C at 60 min. As observed, the ash content increased with longer reaction time. This indicated that reaction time is an important parameter that affected the recovered lignin from EFB. Makela et al. [32] reported high content of sodium and chlorine in char composition of EFB. They reported that reaction time (0.08–14 h) increased the ash content up to 55.8%. Thus, shorter reaction time is essential for economical operation as well as producing highly purified ligninbased precursor [34].
Fig. 1. Ash content at different temperatures (260–300 °C), reaction times (1– 10 min) and 6 g solid loading.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
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V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 2. Ash content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
Fig. 4. Volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
Fig. 2 shows that the highest ash content (46.7%) in this study was obtained at 10 g, 10 min and 300 °C. Lower ash content of 36.9% was obtained at 6 g under the same reaction conditions (Fig. 1). Higher solid loading produced higher ash content in all temperature range used in this study. Kabir and Hameed [35] stated that reaction temperature and solid loading caused similar effect to the reaction due to the heat transfer within the particles. Makela et al. [32] similarly observed significant amount of potassium, magnesium, sulphur and silicone in ash composition of EFB. Furthermore, higher solid loading caused the increased amount of potassium present in lignin and subsequently in the ash
The volatile matter content generally decreased with longer reaction time as shown in Figs. 3 and 4. The significantly high ash content under the same conditions caused the lower volatile matter content in the lignin as similarly observed by Kabir and Hameed [35]. Higher reaction temperature showed prominent changes in volatility of lignin compared to reaction time. But prolonged reaction time produced lower volatile content. Volatile gases such as CO, CO2 and CH4 are produced by lignin degradation products. Yong and Matsumura [37] and Watkins et al. [38] concluded that biomass decomposed rapidly within few minutes. Carbon fiber production required low amount of volatiles in order to avoid foaming during spinning of the fibres [6]. In contrast to ash content, higher volatile content were obtained at lower solid loading. Acid treatment during lignin purification step created porosity due to breaking of the cell wall, that disrupted EFB structure which affected the volatile content [39]. Furthermore, the finding of this study are consistent with study by Yong et al. [40] which observed the decrease of volatile content with solid loading.
3.4. Volatile matter content Volatile matter content in the recovered lignin are important since it caused modifications in the carbon fiber structures due to evaporation especially during carbonization at high heating rate [28]. Figs. 3 and 4 shows the volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and solid loading (6 g and 10 g) towards volatile matter content. The volatile matter content decreased with reaction temperature. Nevertheless, the decrease in volatile content with temperature is considerably minimal. For example, it decreased from 0.3% (260 °C) to 0.2% (300 °C) at 1 min and 6 g. The decrease of volatile matter content with temperature could be attributed to the reduction of bond strengths in lignin at higher temperature producing macromolecules volatiles and subsequently volatilized. Thus, it can be concluded that lower volatile matter content can be obtained at higher reaction temperature under subcritical phenol conditions. The ash and volatile matter content are inverse of one another with temperature change. This is consistent with Mousa et al. [36] that reported the Kraft lignin produced ash content of 0.8% but volatile content as high as 59.6%.
Fig. 3. Volatile matter content at different temperatures (260–300 °C), reaction times (1–10 min) and 6 g solid loading.
3.5. Carbon content Figs. 5 and 6 shows the effect of temperatures (260–300 °C), reaction times (1–10 min) and solid loading (6 g and 10 g) towards carbon content. The highest carbon content (43.7%) was obtained at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.4% and 0.3% respectively. In contrast, at 300 °C, the highest carbon content was (29.4%) with its ash and volatile content of 31.8% and 0.2% respectively. Subcritical conditions caused side reaction and yield secondary products especially at higher temperatures, which caused the decrease in carbon content in the solid residue [41]. The solubility of lignin caused carbon loss to liquid fraction. The rate of depolymerization are affected by temperature [42]. Lignin
Fig. 5. Carbon content at different temperatures (260–300 °C), reaction times (1–10 min) and 6 g solid loading.
Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252
V. Karunakaran et al. / Materials Today: Proceedings xxx (xxxx) xxx
5
ing - review & editing. Tau-Len Kelly Yong: Conceptualization, Methodology, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
Fig. 6. Carbon content at different temperatures (260–300 °C), reaction times (1–10 min) and 10 g solid loading.
are more soluble due to the increased cleavage of b-O-4 ether bonds into monomeric phenols at higher temperature. Bui et al. [43] deduced that liquefaction of biomass are enhanced at above 280 °C under sub- and supercritical conditions. Diffusion rate increased with temperature as the decrease in dielectric constant resulted in enhanced degradation of lignin [44]. This indicated that high carbon content is obtained at lower temperature with low ash and volatile content which are desirable. It is observed that the carbon content decreased minimally with prolonged reaction time. The highest carbon content of 43.7% was obtained at 260 °C, 1 min and 10 g. Thus, it can be concluded that reaction time does not affect the carbon content in this study. This is in agreement with Meilany et al. [45]. The overall trend shows that both fundamental properties (carbon content and volatile content) of the recovered lignin decreased with time. On the other hand, the ash content increased with time. Different recovery method and extraction produced lignin with different carbon content. Elemental study of enzymatically isolated residual lignin from EFB produced lignin with carbon content in the range of 44.2% to 50.4% which is comparable to this study [10]. The effect of solid loading towards both volatile content and carbon content are similar where lower solid loading produced higher carbon content. Higher solid loading created mechanical hindrance which may result in non-homogenous reaction especially the bonds breakage of the lignocellulosic components [45]. Presence of phenol with higher solid loading might favour formation of other products such as catechols [41]. Kabir and Hameed [35] also reported that higher solid loading affected the heat transfer between the solid particles thus affecting its carbon content. 4. Conclusion The aim of this study is to determine the effect of temperature (260–300 °C), reaction time (1–10 min), and solid loading (6 and 10 g) towards ash, volatile and carbon content of the recovered lignin EFB under subcritical phenol conditions as precursor material for carbon fiber production. The highest carbon content in the lignin (43.7%) was achieved at 260 °C, 1 min, and 10 g while its ash and volatile content were 23.5% and 0.3% respectively. As shown in this study, temperature, reaction time and solid loading under subcritical phenol conditions has significant effect on the ash, volatile, and carbon content. Utilisation of EFB from oil palm biomass to produce advanced material as well as resolving the waste management issues faced by oil palm industries, could certainly catalysed the aim to achieve the National Biomass Strategy 2020. CRediT authorship contribution statement Vijayaletchumy Karunakaran: Visualization, Investigation, Writing - original draft, Data curation. Norfahana Abd-Talib: Writ-
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Please cite this article as: V. Karunakaran, N. Abd-Talib and T. L. Kelly Yong, Lignin from oil palm empty fruit bunches (EFB) under subcritical phenol conditions as a precursor for carbon fiber production, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.252