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Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3
Bioresource Technology 98 (2007) 145–149
Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3 Donni Adinata, Wan Mohd Ashri Wan Daud *, Mohd Kheireddine Aroua Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 28 June 2005; received in revised form 30 October 2005; accepted 5 November 2005 Available online 27 December 2005
Abstract Palm shell was used to prepare activated carbon using potassium carbonate (K2CO3) as activating agent. The influence of carbonization temperatures (600–1000 C) and impregnation ratios (0.5–2.0) of the prepared activated carbon on the pore development and yield were investigated. Results showed that in all cases, increasing the carbonization temperature and impregnation ratio, the yield decreased, while the adsorption of CO2 increased, progressively. Specific surface area of activated carbon was maximum about 1170 m2/g at 800 C with activation duration of 2 h and at an impregnation ratio of 1.0. 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Palm shell; Chemical activation; Pore development
1. Introduction Activated carbon is a well known as porous material, with large specific surface area, which is useful in adsorption of both gases and solutes from aqueous solution. Therefore, it has been widely used for separation of gases, recovery of solvents, removal of organic pollutants from drinking water and a catalyst support. As environmental pollution is becoming a more serious problem, the need for activated carbon is growing. It is a versatile adsorbent because of its good adsorption properties. Various materials are used to produce activated carbon and some of the most commonly used are agriculture wastes such as coconut shell (Kirubakaran et al., 1991), pistachio shell (Abe et al., 1990), saw dust (Xiongzun et al., 1986), walnut shell (Khan et al., 1985), tropical wood (Maniatis and Nurmala, 1992) and almond shell (Hayashi et al., 2000). Chemical activation has been shown as a efficient method to obtain carbons with high surface area and nar-
*
Corresponding author. Tel.: +60 379676897; fax: +60 379675371. E-mail address: [email protected] (W.M.A. Wan Daud).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.11.006
row micropore distribution. Although it is a frequently used to prepare activated carbons, the general mechanism of chemical activation is not well understood, and the various interpretations found in literature underline the process complexity. In general terms, chemical activation by alkalis consists in solid–solid or solid–liquid reaction involving the hydroxide reduction and carbon oxidation to generate porosity. McKee (1983) studied the gasification of graphite by alkali metal compounds and found that K2CO3 was reduced by carbon in an inert atmosphere (Hayashi et al., 2002b; Okada et al., 2003). Other chemicals, which frequently used are ZnCl2, H3PO4, ZnCl2, KOH and NaOH (Guo and Lua, 2003; Lua and Yang, 2004; Raymundo-Pinero et al., 2005). However, alkali hydroxides such as KOH and NaOH are hazardous, expensive and corrosive (Lillo-Rodenas et al., 2004) and ZnCl2 is unfriendly the environment and create waste disposal problem (Guo and Lua, 2002). Thus, a more benign chemical is desired; K2CO3 is not a hazardous chemical and not deleterious as it is frequently used for food additives. Palm shell (also known as endocarp) is a cheap and abundant agricultural by-product in tropical countries like
146
D. Adinata et al. / Bioresource Technology 98 (2007) 145–149
Malaysia and Indonesia. Palm shell has been successfully converted into a well-developed activated carbon by thermal activation (physical activation) and chemical activation using with carbon dioxide (CO2) and H2PO3 (Guo and Lua, 2000, 2002). However, activated carbon prepared from palm shell using K2CO3 activation for carbonaceous precursors has not been thoroughly investigated. The aim of this work was to use K2CO3 as activating agent and study the influences of carbonization temperatures and impregnation ratios on pore development and yield. 2. Methods
A typical carbonization run began by changing 100 g of impregnated sample in the reactor and heated up to the carbonization temperature in flowing stream of nitrogen (15 l/min) (Wan Daud and Wan Ali, 2004). The temperature of reactor was increased at the rate of 10 C/min, until it reached the final carbonization temperature. The carbonization temperature was varied from 600 to 1000 C with activation duration of 2 h. After carbonization, the sample was cooled down under nitrogen (N2) flow and was washed sequentially several times with hot water, and finally with cold distilled water to remove residual chemicals. Then the sample was dried at 110 C.
2.1. Material and chemical reagent
3. Results and discussion
Palm shell obtained from Malaysia oil palm shell (MOPS) were dried, crushed and sieved to a particle size fraction of 1–2 mm. Potassium carbonate (K2CO3) (purity 99.9%; Fisher Scientific) was dissolved in distilled water to prepare a saturated solution.
3.1. Yield of activated carbon and chemical recovery ratio of K2CO3
2.2. Raw material analysis An elemental analysis was carried out using a Perkin Elmer CHNO/S Analyzer 2400. Lignin, cellulose and halocellulose were determined using TAPPI method (T-13wd74, T-17wd-70 and T-9m-54, respectively). 2.3. Characterization of activated carbon The characterization of activated carbon samples was carried out using CO2 adsorption at 273 K using a Micromeritics ASAP 2010 surface area analyzer. The sample was placed inside the tube and the glass bulb was inserted inside the tube and stopper. The bulb was inserted by slanting the sample tube almost to a horizontal position and the bulb was allowed to gradually down the sample tube. Before the experiment began, the adsorbents were degassed (104 mmHg) at 393 K. The surface areas of the samples were measured based on Brunauer–Emmet–Teller (BET) method and Dubinin–Radushkevich (DR) method was applied to calculate the micropore volume. Pore size distribution (PSD) was obtained from Horvath–Kawazoe (HK) analysis and solid density of activated carbon was measured by helium displacement with an ultrapycnometer (AccuPyc 1330 pycnometer). 2.4. Preparation of activated carbon Palm shells were mixed with saturated solution K2CO3 and kneaded. This mixture was then dried in an oven at 110 C for 24 h to prepare the impregnated sample. In this work, 0.5, 0.75, 1.0, 1.5 and 2.0 impregnation ratios were used. The impregnation ratio is given by impregnation ratio ¼
ðweight of K2 CO3 in solutionÞ ðweight of palm shellÞ
The proximate analyses of palm shell were as follows (dry wt basis %): carbon 18.7, moisture 7.96, ash 1.1, volatile 72.47, C 50.01, H 6.9, N 1.9, S 0, O 41, cellulose 29, halocellulose 47.7 and lignin 53.4 more than 100%. The yield of activated carbon decreased as the activation temperature increased (Table 1). When chemical activation with K2CO3 impregnation was used, increasing the carbonization temperature decreased the yield progressively due to be release of volatile products as a result of intensifying dehydration and elimination reaction; it also indicated that char of the palm shell was gasified by K2CO3. McKee (1983) studied the gasification of graphite by alkali metal and found that K2CO3 was reduced in an inert atmosphere by carbon (Hayashi et al., 2002b; Okada et al., 2003). The presence of K2CO3 in the interior of the precursor restricted the formation of tar as well as other liquids such as acetic acid and methanol by formation of cross-links, and inhibited the shrinkage of the precursor particle by occupying certain substantial volumes (Guo and Lua, 2003). The results showed that the chemical recovery ratio of K2CO3 with selected conditions above was 0.99, 0.96, 0.86, 0.65 and 0.25 for carbonization temperature of 600, 700, 800, 900 and 1000 C, respectively. This results supported previous findings that K2CO3 was reduced by carbon in inert atmospheres, whereby char reacted with K2CO3 and then the later was removed during gasification. Above 800 C, carbon reduced the impregnated K2CO3 and was consumed through the formation of CO. Thus, the specific surface area and the pore volume increased (Hayashi et al., 2002b). As shown in Table 1, the yield was strongly affected by the impregnation ratio. Increasing the impregnation ratio decreased yield and increased ‘‘burn-off’’ of palm shell (Gomez-Serrano et al., 2005). Higher K2CO3 concentrations consistently yielded products with a much larger external surface area, as well as large micropore volumes (Evans et al., 1999). The chemical recovery ratio of K2CO3 was 0.15, 0.37, 0.86, 0.89, and 0.98 for the impregnation ratio of 0.5, 0.75, 1.0, 1.5, and 2.0, respectively, which showed high chemical recovery ratio of products
D. Adinata et al. / Bioresource Technology 98 (2007) 145–149
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Table 1 The pore characteristics and properties of activated carbons prepared from palm shell at different carbonization temperature and impregnation ratio Activation conditionsa
Yield (%)
Variation (%)
Solid density (g/cm3)
Variation (%)
BET (m2/g)
Variation (%)
600–2–1 700–2–1 800–2–1 900–2–1 1000–2–1 800–2–0.5 800–2–0.75 800–2–1.5 800–2–2.0
26.34 22.24 18.86 16.79 12.82 27.84 25.24 14.79 11.85
1.6 1.4 1.1 1.0 1.9 2.2 0.7 1.8 0.6
1.7659 2.0872 2.3567 2.5341 2.7341 2.7159 2.4561 1.9322 1.7113
0.4 0.5 0.1 1.6 1.1 1.0 1.2 1.1 0.5
319 425 1170 544 339 248 476 540 332
1.3 0.9 1.4 1.9 1.0 2.0 1.3 0.8 1.1
a
Note: a–b–c denote carbonization temperature (C)–during activation (h)–impregnation ratio of K2CO3.
subjected to high impregnation ratio. Hot water washing was, thus, very effective in reducing K2CO3, which has been found to be related to formation of atomic K. Thus, atomic K may intercalate and expand the inter layers of adjacent hexagonal network planes consisting of C atoms, enhancing pore formation though hexagonal planes that are not well developed as in graphite. K2CO3 was not detected on the surface, indicating that the washing of this sample was effective to remove K from the surface on the products (Okada et al., 2003). 3.2. Pore development and characteristic of activated carbon The results showed that the specific surface areas increased with the increase in carbonization temperature from 600 to 800 C and decreased slightly at 800– 1000 C. The maximum specific area obtained was 1170 m2/g. Therefore, it was deduced that K2CO3 was effective as activation reagent below 800 C. This progressive carbonization temperature increased the C–K2CO3 reaction rate, resulting in increasing carbon ‘‘burn-off’’. Concurrently, the volatiles from the samples continued to evolve with increasing carbonization temperature. The develotilization process further developed the rudimentary pore structure in the char, whereas the C–K2CO3 reaction enhanced the existing pores and created new porosities (Hayashi et al., 2002a). The surface area of activated carbon increases with increasing impregnation ratio from 0.5 to 1.0 and decreased slightly for ratios larger than 1. However, mesoporous surface area increased continually. The micropore surface areas were obtained by subtracting mesopore surface area from the corresponding BET surface area. Under the same condition when the impregnation ratio was 0.5, 0.75, 1.0, 1.5, and 2.0, the corresponding specific surface area of activated carbon was 248, 476, 1170, 540, and 332 m2/g, respectively. There was a pronounced decrease in micropore surface area when the ratio was higher than 1 due to enlargement of micropores to mesopores. These results suggested that to obtain the optimal value of surface area, the impregnation ratio should be around 1.0.
Adsorption capacity of activated carbon largely depends on the amount of micropores that are present in the activated carbon. Hence, its measurement can be directly related to quantity of micropore volume in the sample. The micropore volume was estimated from the adsorption of carbon dioxide at 273 K. The results showed that micropore volume of activated carbon prepared with selected conditions above was 0.23, 0.50, 0.57, 0.48, and 0.15 cm3/ g for the carbonization temperature of 600, 700, 800, 900, and 1000 C with activation duration of 2 h at an impregnation ratio of 1.0, respectively. The micropore volume increased with carbonization temperature from 600 to 800 C and then between 800 and 1000 C, the micropore volume of activated carbon slightly decreased, because of enlargement surface of the micropores to mesopores. When impregnation ratio was 0.5, 0.75, 1.0, 1.5, and 2.0, micropore volume of activated carbon was 0.13, 0.28, 0.57, 0.43, and 0.21 cm3/g, respectively. It showed that increasing the impregnation ratio from 0.5 to 1.0 progressively increased both the micropore volume. There was a pronounced decreased in micropore volume when the ratio was higher than 1.0 because rapid increase in mesopore volume increased and the total pore volume (micropore volume and mesopore volume) increased with increasing impregnation ratio. The results obtained showed that increasing the impregnation ratio decreased the density of activated carbon (Table 1). Higher impregnation ratio also tends to produce lower yield and density of activated carbon. When higher impregnation ratio was used, the weight losses were due to the increasing release of volatile products as result of intensifying dehydration and elimination reactions (Evans et al., 1999). By increasing the carbonization temperature the solid density increased progressively due to the release of low-molecular weight products (water, furan derivates and laevoglucose) due to of dehydration and elimination reactions (Guo and Lua, 2002; Lua and Yang, 2004). 3.3. CO2 adsorption isotherm and pore size distribution of activated carbon Figs. 1 and 2 show the CO2 adsorption isotherms on the prepared activated carbons at different carbonization
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Volume Adsorbed (cm3/g) STP
180
12
Average variation of each line is < 0.2%
160
Average variation of each line is < 0.2%
140 100 80 60 40 20 0 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
Pore Volume (cm3/g-nm)
10
120
4
0
Fig. 1. CO2 adsorption isotherm on the prepared activated carbon at different carbonization temperatures: (m) 600 C, (j) 700 C, (d) 800 C, (·) 900 C, (s) 1000 C.
Volume Adsorbed (cm3/g) STP
6
2
Relative Pressure (P/P0)
120
8
0.4
0.5
0.6
0.7
0.8
0.9
1
Pore Diameter (nm)
Fig. 3. The influence of carbonization temperatures on the pore size distribution of activated carbon: (·) 600 C (h) 700 C (d) 800 C, (D) 900 C, (s) 1000 C.
Average variation of each line is < 0.2%
100
Average variation of each line is < 0.2%
12
PoreVolume (cm3/g-nm)
80 60 40 20 0 0
0.005
0.01
0.015
0.02
0.025
0.03
10 8 6 4 2
0.035
Relative Pressure (P/P0) Fig. 2. Adsorption isotherm of CO2 on the prepared activated carbons at different impregnation ratios: (j) 0.5, (m) 0.75, (d) 1, (+) 1.5, (s) 2.
temperatures and impregnation ratios. The amount of CO2 adsorbed at P/P0 = 0.03 increased for activated carbons prepared at relatively higher carbonization temperature and impregnation ratio. The isotherm curves are of Type I isotherms, which generally exhibit the presence of micropores. Further increase in carbonization temperature and impregnation ratio through activation process widened the pores as well as increased the volume of micro pore and total of pore volume. The structural heterogeneity of porous material is generally characterized in term of the pore size distribution. The pore size distribution is closely related to both kinetic and equilibrium properties of porous materials used in industrial application. The pore size distribution of activated carbon produced from palm shell at different carbonization temperatures and impregnation ratios are given in Figs. 3 and 4. From these figures, it was obvious that the carbonization temperature and impregnation ratio had significant effect on the pore structure of activated carbon produced. At low temperature and impregnation ratio, the pore structure of mainly consisted of micropore; however, with the increase of carbonization temperature and impregnation ratio, the creation of micropore structure and widening of micropores to mesopores also increased and also
0 0.4
0.5
0.6
0.7
0.8
0.9
1
Pore Diameter (nm)
Fig. 4. The influence of impregnation ratios on the pore size distribution of activated carbon: (s) 0.5, (·) 0.75, (d) 1, (j) 1.5, (D) 2.
increased the total pore volume of activated carbon (Ismadji and Bhatia, 2000, 2001). 4. Conclusions The activated carbon prepared by chemical activation with K2CO3 attained a maximum value of 1170 m2/g at a carbonization temperature of 800 C with activation duration of 2 h and at an impregnation ratio of 1.0. The increase in the carbonization temperature and impregnation ratio decreased yield and increased the product adsorption capacity of CO2. From the results of yield of activated carbon and reagent recovery ratio, it was concluded that the carbon involved in the palm shell char was removed as CO by reduction of K2CO3 above 800 C. It was also found that for 2 h activation, the mesopore volume increased with the increase in impregnation ratio and carbonization temperature. Acknowledgement Financial support for this work was provided by IRPA Grant (08-02-03-0233-EA233), University Malaya.
D. Adinata et al. / Bioresource Technology 98 (2007) 145–149
References Abe, I., Tatsumoto, H., Ikuta, N., Kawafune, I., 1990. Preparation of activated carbon from pisthachio nut shell. Chem. Exp. 5, 177–180. Evans, M.J.B., Halliop, E., MacDonald, J.A.F., 1999. The production of chemically-activated carbon. Carbon 37, 269–274. Gomez-Serrano, V., Cuerda-Correa, E.M., Fernandez-Gonzalez, M.C., Alexandre-Franco, M.F., Macias-Garcia, 2005. Preparation of activated carbons from chestnut wood by phosphoric acid-chemical activation. Study of microporosity and fractal dimension. Mater. Lett. 59, 846–853. Guo, J., Lua, A.C., 2000. Characterization of adsorbent prepared from oil palm shell by CO2 activation for removal of gaseous pollutants. Mater. Lett. 55, 334–339. Guo, J., Lua, A.C., 2002. Textural and chemical characterization of adsorbent prepared from palm shell by potassium hydroxide impregnation at different stages. J. Colloid Interface Sci. 254, 227–233. Guo, J., Lua, A.C., 2003. Textural and chemical properties of adsorbent prepared from palm shell by phosphoric acid activation. Mater. Chem. Phys. 80, 114–119. Hayashi, J., Kazehaya, A., Muroyama, K., Watkinson, A.P., 2000. Preparation of activated carbon from lignin by chemical activation. Carbon 32, 1873–1878. Hayashi, J., Horikawa, T., Muroyama, K., Gomes, V.G., 2002a. Activated carbon from chickpea husk by chemical activation with K2CO3: preparation and characterization. Micropor. Mesopor. Mater. 55, 63–68. Hayashi, J., Horikawa, T., Takeda, I., Muroyama, K., Ani, F.N., 2002b. Preparing activated carbon from various nutshells by chemical activation with K2CO3. Carbon 40, 2381–2386. Ismadji, S., Bhatia, S.K., 2000. Investigation of network connectivity in activated carbons by liquid phase adsorption. Langmuir 16, 9303– 9313.
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Ismadji, S., Bhatia, S.K., 2001. A modified pore filling isotherm for liquid phase adsorption in activated carbon. Langmuir 17, 1488–1498. Khan, A., Singh, H., Bhatia, A.K., 1985. Activated carbon from walnut shells. Res. Ind. 30, 13–16. Kirubakaran, C.J., Krishnaiah, Seshadri, S.K., 1991. Experimental study of the production of activated carbon from coconut shells in fluidized bed reactor. Ind. Eng. Chem. Res 27, 2411–2416. Lillo-Rodenas, M.A., Juan-Juan, J., Cazorla-Amoros, D., LinaresSolano, A., 2004. About reactions occurring during chemical activation with hydroxides. Carbon 42, 1371–1375. Lua, A.C., Yang, T., 2004. Effect of activation temperature on the textural and chemical properties of potassium hydroxide activated carbon prepared from pistachio-nut shell. J. Colloid Interface Sci. 274, 594– 601. Maniatis, K., Nurmala, M., 1992. Activated carbon production from biomass. Biomass Energy Ind. Environ. 274, 1034–1308. McKee, D.W., 1983. Mechanisms of the alkali metal catalyzed gasification of carbon. Fuel 62, 170–175. Okada, K., Yamamoto, N., Kameshima, Y., Yasumori, A., 2003. Porous properties of activated carbons from waste newspaper prepared by chemical and physical activation. J. Colloid Interface Sci. 262, 179– 193. Raymundo-Pinero, E., Azais, P., Cacciaguerra, T., Cazorla-Amoros, D., Linares-Solano, A., Beguin, F., 2005. KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organization. Carbon 43, 786–795. Wan Daud, W.M.A., Wan Ali, W.S., 2004. Comparison on pore development of activated carbon produced from palm shell and coconut shell. Bioresour. Technol. 93, 63–69. Xiongzun, Z., Famnao, Z., Lie, L., Qingrong, L., 1986. A new technique to produce activated carbon (from saw dust of any humidity) by zinc chloride method. J. Nanjing Inst. Forest. 1, 19–30.
Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3 Donni Adinata, Wan Mohd Ashri Wan Daud *, Mohd Kheireddine Aroua Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 28 June 2005; received in revised form 30 October 2005; accepted 5 November 2005 Available online 27 December 2005
Abstract Palm shell was used to prepare activated carbon using potassium carbonate (K2CO3) as activating agent. The influence of carbonization temperatures (600–1000 C) and impregnation ratios (0.5–2.0) of the prepared activated carbon on the pore development and yield were investigated. Results showed that in all cases, increasing the carbonization temperature and impregnation ratio, the yield decreased, while the adsorption of CO2 increased, progressively. Specific surface area of activated carbon was maximum about 1170 m2/g at 800 C with activation duration of 2 h and at an impregnation ratio of 1.0. 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Palm shell; Chemical activation; Pore development
1. Introduction Activated carbon is a well known as porous material, with large specific surface area, which is useful in adsorption of both gases and solutes from aqueous solution. Therefore, it has been widely used for separation of gases, recovery of solvents, removal of organic pollutants from drinking water and a catalyst support. As environmental pollution is becoming a more serious problem, the need for activated carbon is growing. It is a versatile adsorbent because of its good adsorption properties. Various materials are used to produce activated carbon and some of the most commonly used are agriculture wastes such as coconut shell (Kirubakaran et al., 1991), pistachio shell (Abe et al., 1990), saw dust (Xiongzun et al., 1986), walnut shell (Khan et al., 1985), tropical wood (Maniatis and Nurmala, 1992) and almond shell (Hayashi et al., 2000). Chemical activation has been shown as a efficient method to obtain carbons with high surface area and nar-
*
Corresponding author. Tel.: +60 379676897; fax: +60 379675371. E-mail address: [email protected] (W.M.A. Wan Daud).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.11.006
row micropore distribution. Although it is a frequently used to prepare activated carbons, the general mechanism of chemical activation is not well understood, and the various interpretations found in literature underline the process complexity. In general terms, chemical activation by alkalis consists in solid–solid or solid–liquid reaction involving the hydroxide reduction and carbon oxidation to generate porosity. McKee (1983) studied the gasification of graphite by alkali metal compounds and found that K2CO3 was reduced by carbon in an inert atmosphere (Hayashi et al., 2002b; Okada et al., 2003). Other chemicals, which frequently used are ZnCl2, H3PO4, ZnCl2, KOH and NaOH (Guo and Lua, 2003; Lua and Yang, 2004; Raymundo-Pinero et al., 2005). However, alkali hydroxides such as KOH and NaOH are hazardous, expensive and corrosive (Lillo-Rodenas et al., 2004) and ZnCl2 is unfriendly the environment and create waste disposal problem (Guo and Lua, 2002). Thus, a more benign chemical is desired; K2CO3 is not a hazardous chemical and not deleterious as it is frequently used for food additives. Palm shell (also known as endocarp) is a cheap and abundant agricultural by-product in tropical countries like
146
D. Adinata et al. / Bioresource Technology 98 (2007) 145–149
Malaysia and Indonesia. Palm shell has been successfully converted into a well-developed activated carbon by thermal activation (physical activation) and chemical activation using with carbon dioxide (CO2) and H2PO3 (Guo and Lua, 2000, 2002). However, activated carbon prepared from palm shell using K2CO3 activation for carbonaceous precursors has not been thoroughly investigated. The aim of this work was to use K2CO3 as activating agent and study the influences of carbonization temperatures and impregnation ratios on pore development and yield. 2. Methods
A typical carbonization run began by changing 100 g of impregnated sample in the reactor and heated up to the carbonization temperature in flowing stream of nitrogen (15 l/min) (Wan Daud and Wan Ali, 2004). The temperature of reactor was increased at the rate of 10 C/min, until it reached the final carbonization temperature. The carbonization temperature was varied from 600 to 1000 C with activation duration of 2 h. After carbonization, the sample was cooled down under nitrogen (N2) flow and was washed sequentially several times with hot water, and finally with cold distilled water to remove residual chemicals. Then the sample was dried at 110 C.
2.1. Material and chemical reagent
3. Results and discussion
Palm shell obtained from Malaysia oil palm shell (MOPS) were dried, crushed and sieved to a particle size fraction of 1–2 mm. Potassium carbonate (K2CO3) (purity 99.9%; Fisher Scientific) was dissolved in distilled water to prepare a saturated solution.
3.1. Yield of activated carbon and chemical recovery ratio of K2CO3
2.2. Raw material analysis An elemental analysis was carried out using a Perkin Elmer CHNO/S Analyzer 2400. Lignin, cellulose and halocellulose were determined using TAPPI method (T-13wd74, T-17wd-70 and T-9m-54, respectively). 2.3. Characterization of activated carbon The characterization of activated carbon samples was carried out using CO2 adsorption at 273 K using a Micromeritics ASAP 2010 surface area analyzer. The sample was placed inside the tube and the glass bulb was inserted inside the tube and stopper. The bulb was inserted by slanting the sample tube almost to a horizontal position and the bulb was allowed to gradually down the sample tube. Before the experiment began, the adsorbents were degassed (104 mmHg) at 393 K. The surface areas of the samples were measured based on Brunauer–Emmet–Teller (BET) method and Dubinin–Radushkevich (DR) method was applied to calculate the micropore volume. Pore size distribution (PSD) was obtained from Horvath–Kawazoe (HK) analysis and solid density of activated carbon was measured by helium displacement with an ultrapycnometer (AccuPyc 1330 pycnometer). 2.4. Preparation of activated carbon Palm shells were mixed with saturated solution K2CO3 and kneaded. This mixture was then dried in an oven at 110 C for 24 h to prepare the impregnated sample. In this work, 0.5, 0.75, 1.0, 1.5 and 2.0 impregnation ratios were used. The impregnation ratio is given by impregnation ratio ¼
ðweight of K2 CO3 in solutionÞ ðweight of palm shellÞ
The proximate analyses of palm shell were as follows (dry wt basis %): carbon 18.7, moisture 7.96, ash 1.1, volatile 72.47, C 50.01, H 6.9, N 1.9, S 0, O 41, cellulose 29, halocellulose 47.7 and lignin 53.4 more than 100%. The yield of activated carbon decreased as the activation temperature increased (Table 1). When chemical activation with K2CO3 impregnation was used, increasing the carbonization temperature decreased the yield progressively due to be release of volatile products as a result of intensifying dehydration and elimination reaction; it also indicated that char of the palm shell was gasified by K2CO3. McKee (1983) studied the gasification of graphite by alkali metal and found that K2CO3 was reduced in an inert atmosphere by carbon (Hayashi et al., 2002b; Okada et al., 2003). The presence of K2CO3 in the interior of the precursor restricted the formation of tar as well as other liquids such as acetic acid and methanol by formation of cross-links, and inhibited the shrinkage of the precursor particle by occupying certain substantial volumes (Guo and Lua, 2003). The results showed that the chemical recovery ratio of K2CO3 with selected conditions above was 0.99, 0.96, 0.86, 0.65 and 0.25 for carbonization temperature of 600, 700, 800, 900 and 1000 C, respectively. This results supported previous findings that K2CO3 was reduced by carbon in inert atmospheres, whereby char reacted with K2CO3 and then the later was removed during gasification. Above 800 C, carbon reduced the impregnated K2CO3 and was consumed through the formation of CO. Thus, the specific surface area and the pore volume increased (Hayashi et al., 2002b). As shown in Table 1, the yield was strongly affected by the impregnation ratio. Increasing the impregnation ratio decreased yield and increased ‘‘burn-off’’ of palm shell (Gomez-Serrano et al., 2005). Higher K2CO3 concentrations consistently yielded products with a much larger external surface area, as well as large micropore volumes (Evans et al., 1999). The chemical recovery ratio of K2CO3 was 0.15, 0.37, 0.86, 0.89, and 0.98 for the impregnation ratio of 0.5, 0.75, 1.0, 1.5, and 2.0, respectively, which showed high chemical recovery ratio of products
D. Adinata et al. / Bioresource Technology 98 (2007) 145–149
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Table 1 The pore characteristics and properties of activated carbons prepared from palm shell at different carbonization temperature and impregnation ratio Activation conditionsa
Yield (%)
Variation (%)
Solid density (g/cm3)
Variation (%)
BET (m2/g)
Variation (%)
600–2–1 700–2–1 800–2–1 900–2–1 1000–2–1 800–2–0.5 800–2–0.75 800–2–1.5 800–2–2.0
26.34 22.24 18.86 16.79 12.82 27.84 25.24 14.79 11.85
1.6 1.4 1.1 1.0 1.9 2.2 0.7 1.8 0.6
1.7659 2.0872 2.3567 2.5341 2.7341 2.7159 2.4561 1.9322 1.7113
0.4 0.5 0.1 1.6 1.1 1.0 1.2 1.1 0.5
319 425 1170 544 339 248 476 540 332
1.3 0.9 1.4 1.9 1.0 2.0 1.3 0.8 1.1
a
Note: a–b–c denote carbonization temperature (C)–during activation (h)–impregnation ratio of K2CO3.
subjected to high impregnation ratio. Hot water washing was, thus, very effective in reducing K2CO3, which has been found to be related to formation of atomic K. Thus, atomic K may intercalate and expand the inter layers of adjacent hexagonal network planes consisting of C atoms, enhancing pore formation though hexagonal planes that are not well developed as in graphite. K2CO3 was not detected on the surface, indicating that the washing of this sample was effective to remove K from the surface on the products (Okada et al., 2003). 3.2. Pore development and characteristic of activated carbon The results showed that the specific surface areas increased with the increase in carbonization temperature from 600 to 800 C and decreased slightly at 800– 1000 C. The maximum specific area obtained was 1170 m2/g. Therefore, it was deduced that K2CO3 was effective as activation reagent below 800 C. This progressive carbonization temperature increased the C–K2CO3 reaction rate, resulting in increasing carbon ‘‘burn-off’’. Concurrently, the volatiles from the samples continued to evolve with increasing carbonization temperature. The develotilization process further developed the rudimentary pore structure in the char, whereas the C–K2CO3 reaction enhanced the existing pores and created new porosities (Hayashi et al., 2002a). The surface area of activated carbon increases with increasing impregnation ratio from 0.5 to 1.0 and decreased slightly for ratios larger than 1. However, mesoporous surface area increased continually. The micropore surface areas were obtained by subtracting mesopore surface area from the corresponding BET surface area. Under the same condition when the impregnation ratio was 0.5, 0.75, 1.0, 1.5, and 2.0, the corresponding specific surface area of activated carbon was 248, 476, 1170, 540, and 332 m2/g, respectively. There was a pronounced decrease in micropore surface area when the ratio was higher than 1 due to enlargement of micropores to mesopores. These results suggested that to obtain the optimal value of surface area, the impregnation ratio should be around 1.0.
Adsorption capacity of activated carbon largely depends on the amount of micropores that are present in the activated carbon. Hence, its measurement can be directly related to quantity of micropore volume in the sample. The micropore volume was estimated from the adsorption of carbon dioxide at 273 K. The results showed that micropore volume of activated carbon prepared with selected conditions above was 0.23, 0.50, 0.57, 0.48, and 0.15 cm3/ g for the carbonization temperature of 600, 700, 800, 900, and 1000 C with activation duration of 2 h at an impregnation ratio of 1.0, respectively. The micropore volume increased with carbonization temperature from 600 to 800 C and then between 800 and 1000 C, the micropore volume of activated carbon slightly decreased, because of enlargement surface of the micropores to mesopores. When impregnation ratio was 0.5, 0.75, 1.0, 1.5, and 2.0, micropore volume of activated carbon was 0.13, 0.28, 0.57, 0.43, and 0.21 cm3/g, respectively. It showed that increasing the impregnation ratio from 0.5 to 1.0 progressively increased both the micropore volume. There was a pronounced decreased in micropore volume when the ratio was higher than 1.0 because rapid increase in mesopore volume increased and the total pore volume (micropore volume and mesopore volume) increased with increasing impregnation ratio. The results obtained showed that increasing the impregnation ratio decreased the density of activated carbon (Table 1). Higher impregnation ratio also tends to produce lower yield and density of activated carbon. When higher impregnation ratio was used, the weight losses were due to the increasing release of volatile products as result of intensifying dehydration and elimination reactions (Evans et al., 1999). By increasing the carbonization temperature the solid density increased progressively due to the release of low-molecular weight products (water, furan derivates and laevoglucose) due to of dehydration and elimination reactions (Guo and Lua, 2002; Lua and Yang, 2004). 3.3. CO2 adsorption isotherm and pore size distribution of activated carbon Figs. 1 and 2 show the CO2 adsorption isotherms on the prepared activated carbons at different carbonization
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Volume Adsorbed (cm3/g) STP
180
12
Average variation of each line is < 0.2%
160
Average variation of each line is < 0.2%
140 100 80 60 40 20 0 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
Pore Volume (cm3/g-nm)
10
120
4
0
Fig. 1. CO2 adsorption isotherm on the prepared activated carbon at different carbonization temperatures: (m) 600 C, (j) 700 C, (d) 800 C, (·) 900 C, (s) 1000 C.
Volume Adsorbed (cm3/g) STP
6
2
Relative Pressure (P/P0)
120
8
0.4
0.5
0.6
0.7
0.8
0.9
1
Pore Diameter (nm)
Fig. 3. The influence of carbonization temperatures on the pore size distribution of activated carbon: (·) 600 C (h) 700 C (d) 800 C, (D) 900 C, (s) 1000 C.
Average variation of each line is < 0.2%
100
Average variation of each line is < 0.2%
12
PoreVolume (cm3/g-nm)
80 60 40 20 0 0
0.005
0.01
0.015
0.02
0.025
0.03
10 8 6 4 2
0.035
Relative Pressure (P/P0) Fig. 2. Adsorption isotherm of CO2 on the prepared activated carbons at different impregnation ratios: (j) 0.5, (m) 0.75, (d) 1, (+) 1.5, (s) 2.
temperatures and impregnation ratios. The amount of CO2 adsorbed at P/P0 = 0.03 increased for activated carbons prepared at relatively higher carbonization temperature and impregnation ratio. The isotherm curves are of Type I isotherms, which generally exhibit the presence of micropores. Further increase in carbonization temperature and impregnation ratio through activation process widened the pores as well as increased the volume of micro pore and total of pore volume. The structural heterogeneity of porous material is generally characterized in term of the pore size distribution. The pore size distribution is closely related to both kinetic and equilibrium properties of porous materials used in industrial application. The pore size distribution of activated carbon produced from palm shell at different carbonization temperatures and impregnation ratios are given in Figs. 3 and 4. From these figures, it was obvious that the carbonization temperature and impregnation ratio had significant effect on the pore structure of activated carbon produced. At low temperature and impregnation ratio, the pore structure of mainly consisted of micropore; however, with the increase of carbonization temperature and impregnation ratio, the creation of micropore structure and widening of micropores to mesopores also increased and also
0 0.4
0.5
0.6
0.7
0.8
0.9
1
Pore Diameter (nm)
Fig. 4. The influence of impregnation ratios on the pore size distribution of activated carbon: (s) 0.5, (·) 0.75, (d) 1, (j) 1.5, (D) 2.
increased the total pore volume of activated carbon (Ismadji and Bhatia, 2000, 2001). 4. Conclusions The activated carbon prepared by chemical activation with K2CO3 attained a maximum value of 1170 m2/g at a carbonization temperature of 800 C with activation duration of 2 h and at an impregnation ratio of 1.0. The increase in the carbonization temperature and impregnation ratio decreased yield and increased the product adsorption capacity of CO2. From the results of yield of activated carbon and reagent recovery ratio, it was concluded that the carbon involved in the palm shell char was removed as CO by reduction of K2CO3 above 800 C. It was also found that for 2 h activation, the mesopore volume increased with the increase in impregnation ratio and carbonization temperature. Acknowledgement Financial support for this work was provided by IRPA Grant (08-02-03-0233-EA233), University Malaya.
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