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Effects of microstructure of precursors on characteristics of pitch based activated carbons
Microporous and Mesoporous Materials 102 (2007) 337–340 www.elsevier.com/locate/micromeso
Effects of microstructure of precursors on characteristics of pitch based activated carbons Guo Chun-yu 1, Wang Cheng-yang
*
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Received 16 August 2006; received in revised form 6 November 2006; accepted 6 November 2006 Available online 16 January 2007
Abstract In this paper, isotropic pitch and anisotropic pitch with similar physical properties were chose to be precursors for activated carbons. Chemical activations with same conditions were carried out and the characteristic difference of activated carbons from different pitch precursors was discussed. As a result, isotropic pitch with more noncrystallite carbon atoms or edge carbon atoms on the microstructural defects had more reactive ability and more pores, 1.36 cm3/g, were manufactured through sufficient chemical activation. However, the crystallites of anisotropic pitch had larger size and good tropism so that the activations mostly reacted on the interspaces of carbonaceous layers. Therefore, isotropic pitch based activated carbon with sufficient chemical activation had higher specific surface area (2913) m2/g and better iodine adsorption value (2630 mg/g). Ó 2006 Elsevier Inc. All rights reserved. Keywords: Precursors; Isotropic; Anisotropic; Chemical activation
Activated carbons with lower cost, simple technological process and multiplicity have occupied the adsorbent market for many years. There existed so many kinds of precursors to manufacture activated carbons, such as plant, petroleum coke, resin and so on. As usual, commercial activated carbons with lower specific surface area (500– 1500 m2/g) and pore volume under 1 cm3/g were prepared from woody precursors like coconut shell with physical activation using vapor as activation agent [1]. This sort of activated carbon products could be used as ordinary absorbent for purification [2]. Activated carbons with better porosities were usually prepared with chemical activation using KOH as activation agent [3]. Catalytic activation was another kind of manufacture method to pre-
*
Corresponding author. E-mail addresses: [email protected] (C.-y. Guo), [email protected] (C.-y. Wang). 1 Ph.D. student, engaged in the study of preparation of porous carbonaceous materials. 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.11.022
pare mesoporous activated carbons involved in some particular fields, like electric double-layer capacitors and catalytic supporters [4,5]. The characteristic difference could be from not only different preparation methods, but also different precursors. As for the precursor itself, the abilities of chemical reaction were different from different positions on the precursor. The crystallite structures of carbon materials were made up of many aromatic layers and the carbon atoms on the layers were hard to be activated because of the stronger covalence bonds, however, the edge carbon atoms (noncrystallite carbon atoms) with unsaturated bonds (C@C) had more reactive abilities. Pitches with low cost and various microstructures were investigated for several years and there were many reports about pitch based activated carbons [6–9]. On the base of prior researches, isotropic pitch and anisotropic pitch with similar physical characteristics were chose to be precursors for activated carbons in this experiment. Chemical activations with same conditions were carried out and the effects of microstructure of precursors on characteristics of activated carbons were discussed.
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
In this paper, chemical activations were carried out, utilizing isotropic pitch and anisotropic pitch as the starting materials for activated carbons. The precursors were smashed and mixed with KOH aqueous solution at a certain weight ratio, respectively, keeping for 1 h. Before putting into the horizontal cylindrical furnace, the mixture should be vacuum-dried for 24 h. Then they were heated to 800 °C at a determinate heating speed, holding for 1 h. The activated products were washed by diluted hydrochloride acid to get rid of the alkaline compounds. Then pH value was adjusted to 6–7 by distilled water. The activated carbons named TP from isotropic pitch and YP from anisotropic pitch with the same treatment condition were finally gained after being dried for 24 h at 105 °C. X-ray diffraction (XRD) was utilized to analyze the microstructural difference between isotropic pitch and anisotropic pitch. The morphologies of the precursors after activation were observed by JSM-6700F type field emission scanning electron microscope (SEM). Iodine adsorption test (GB/T 7702.7-1997) and N2 adsorption test (Quantachrome NOVA Automated 2000 automatic adsorption system) were carried out. Specific surface area and pore size distribution of activated carbons were determined by BET and DFT method, respectively. As shown in Table 1, the mean diameter of pitch particles, softening point of these two kinds of pitch with a common origin were similar to each other. The difference of toluene insoluble content suggested the essential distinction between the samples, namely ‘‘isotropic’’ and ‘‘anisotropic’’. Furthermore, the microstructural difference between these two kinds of pitch was analyzed by XRD,
Table 1 Some properties of pitch Pitch
Md (lm)
SP (°C)
TI (%)
Origin
Isotropic Anisotropic
1–10 1–10
265 270
7.3 94.5
Coal tar pitch Coal tar pitch
Notes: Md, mean diameter; SP, softening point; TI, toluene insoluble content.
Isotropic pitch
Intensity
338
Anisotropic pitch
20
40
60
80
100
2Theta (Deg) Fig. 1. XRD analysis of isotropic pitch and anisotropic pitch.
shown in Fig. 1. Carbonaceous materials were determined by 2h around 26° and 43°. Narrow and keen-edged peak around d002 showed that this kind of material should have better periodic accumulation of graphite crystallites than the sample with broader peak d002. Peak d002 with narrow and sharp shape indicated that the crystallites of anisotropic pitch had larger size and good tropism. On the other hand, isotropic pitch with broader peak d002 had more microstructural defects and noncrystallite carbon atoms. As a result, the periodic accumulation of graphite crystallites of isotropic pitch was not as good as anisotropic pitch by means of XRD method, qualitatively. The morphologies of activated carbons from isotropic pitch and anisotropic pitch were observed by SEM, as shown in Fig. 2. A plenty of holes appeared on the surface of isotropic pitch in all directions after chemical activation, however, this kind of phenomena was not observed on TP activated from anisotropic pitch. The significant changing of surface morphology of isotropic pitch after activation must be the result of the microstructural characteristics: There existed a great deal microstructural defects on the
Fig. 2. SEM images of samples TP and YP (a) TP activated from isotropic pitch (b) YP activated from anisotropic pitch.
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
339
0.10
TP YP
0.06
3
dv/dw [cm /g.A]
0.08
aromatic layers of isotropic pitch with broader peak d002 so that this kind of precursor could be easily etched by activation agent in all directions. During the heat treatment, the more chemical reactions carried out, the more gaseous species were released and the more holes appeared on the surface of isotropic pitch after the gaseous species escaping from the sample TP. The crystallites of anisotropic pitch with narrow and sharp peak d002, however, had better tropism and the carbon atoms on the carbonaceous layers were hard to be activated because of the stronger covalence bonds. Therefore, anisotropic pitch with less active carbon atoms preformed less chemical reactions during heat treatment and the gaseous species run away mostly from the interlayers not the layer surface. As a result, the smooth surface of anisotropic pitch did not change much after chemical activation. (shown in Fig. 3). Some microstructural parameters of samples TP and YP activated from isotropic pitch and anisotropic pitch, respectively, were shown in Table 2. Activation was carried out more sufficiently for isotropic pitch because of the isotropic microstuctural characteristics and the sample TP had lower activation yield, higher specific surface area, higher pore volume and higher adsorption ability, comparing with sample YP. The pore size distributions of samples TP and YP were shown in Fig. 4. Obviously, TP owned more micropores and mesopores than YP so that the former sample belonged to mesoporous activated carbons. As result, a hysteresis loop appeared on the isotherm curve of TP, however, sample YP which belonged to microporous activated carbons reached adsorption saturation under lower P/P0. In addition, the adsorption ability of TP was higher than YP according to the higher position of isotherm curve, as shown in Fig. 5.
0.04
0.02
0.00 0
10
20
30
40
50
Pore diameter (A) Fig. 4. Pore size distribution of samples TP and YP.
1000
Pore volume [cm3 /g] STP
Fig. 3. Sketch map showing the presumed theory of chemical activation with different precursors (a) TP activated from isotropic pitch (b) YP activated from anisotropic pitch.
800
TP YP
600
400
200 0.0
0.2
0.4
0.6
0.8
1.0
P/Po Fig. 5. Isotherm of samples TP and YP.
In general, it was necessary to analyze the microstructure of precursors for activated carbons preparation. Microstructure of precursors was one of important factors to influence the final performance of activated carbons. Isotropic pitch with more noncrystallite carbon atoms or edge carbon atoms on the microstructural defects had more reactive ability and more pores were manufactured through sufficient chemical activation. However, the crystallites of
Table 2 Porosity characteristics of TP from isotropic pitch and YP from anisotropic pitch Sample
AY (%)
SBET (m/g)
Vtot (cm3/g)
<1 nm (cm3/g)
1–2 nm (cm3/g)
2–3 nm (cm3/g)
>3 nm (cm3/g)
IAV (mg/g)
L (nm)
TP YP
48.0 63.7
2913 1810
1.36 0.81
0.25 0.16
0.67 0.49
0.32 0.12
0.12 0.04
2630 1998
2.12 1.99
Notes: AY, activation yield; SBET, specific surface area from BET method; Vtot, total pore volume; IAV, iodine adsorption value; L, average pore diameter.
340
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
anisotropic pitch had larger size and good tropism so that the activations mostly reacted on the interspaces of carbonaceous layers. As a result, isotropic pitch based activated carbon with sufficient chemical activation had better characteristics. References [1] J. Laine, S. Yunes, Carbon 4 (30) (1992) 601–604. [2] L.R. Radovic, C. Moreno-Castilla, J. Rivera-Utrilla, Chemistry and Physics of Carbon 27 (2001) 27.
[3] Z. Hu, M.P. Sriniivasan, Y. Ni, Carbon 6 (39) (2001) 877–886. [4] Liu Hsin-Yu, Wang Kai-Ping, Teng Hsisheng, Carbon 43 (2005) 559– 566. [5] A. Erhan Aksoylu, M. Madalena, A. Freitas, et al., Carbon 39 (2001) 175–785. [6] Kap Seung Yang, Young Jo Yoon, Moo Sung Lee, et al., Carbon 40 (2002) 897–903. [7] Kyoichi Oshida, Carbon 40 (2002) 2699–2711. [8] C. Blanco, O. Fleurot, R. Menendez, et al., Carbon 37 (1999) 1059– 1064. [9] Y. Korai, S. Ishida, F. Watanabe, et al., Carbon 12 (35) (1997) 1733– 1737.
Effects of microstructure of precursors on characteristics of pitch based activated carbons Guo Chun-yu 1, Wang Cheng-yang
*
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Received 16 August 2006; received in revised form 6 November 2006; accepted 6 November 2006 Available online 16 January 2007
Abstract In this paper, isotropic pitch and anisotropic pitch with similar physical properties were chose to be precursors for activated carbons. Chemical activations with same conditions were carried out and the characteristic difference of activated carbons from different pitch precursors was discussed. As a result, isotropic pitch with more noncrystallite carbon atoms or edge carbon atoms on the microstructural defects had more reactive ability and more pores, 1.36 cm3/g, were manufactured through sufficient chemical activation. However, the crystallites of anisotropic pitch had larger size and good tropism so that the activations mostly reacted on the interspaces of carbonaceous layers. Therefore, isotropic pitch based activated carbon with sufficient chemical activation had higher specific surface area (2913) m2/g and better iodine adsorption value (2630 mg/g). Ó 2006 Elsevier Inc. All rights reserved. Keywords: Precursors; Isotropic; Anisotropic; Chemical activation
Activated carbons with lower cost, simple technological process and multiplicity have occupied the adsorbent market for many years. There existed so many kinds of precursors to manufacture activated carbons, such as plant, petroleum coke, resin and so on. As usual, commercial activated carbons with lower specific surface area (500– 1500 m2/g) and pore volume under 1 cm3/g were prepared from woody precursors like coconut shell with physical activation using vapor as activation agent [1]. This sort of activated carbon products could be used as ordinary absorbent for purification [2]. Activated carbons with better porosities were usually prepared with chemical activation using KOH as activation agent [3]. Catalytic activation was another kind of manufacture method to pre-
*
Corresponding author. E-mail addresses: [email protected] (C.-y. Guo), [email protected] (C.-y. Wang). 1 Ph.D. student, engaged in the study of preparation of porous carbonaceous materials. 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.11.022
pare mesoporous activated carbons involved in some particular fields, like electric double-layer capacitors and catalytic supporters [4,5]. The characteristic difference could be from not only different preparation methods, but also different precursors. As for the precursor itself, the abilities of chemical reaction were different from different positions on the precursor. The crystallite structures of carbon materials were made up of many aromatic layers and the carbon atoms on the layers were hard to be activated because of the stronger covalence bonds, however, the edge carbon atoms (noncrystallite carbon atoms) with unsaturated bonds (C@C) had more reactive abilities. Pitches with low cost and various microstructures were investigated for several years and there were many reports about pitch based activated carbons [6–9]. On the base of prior researches, isotropic pitch and anisotropic pitch with similar physical characteristics were chose to be precursors for activated carbons in this experiment. Chemical activations with same conditions were carried out and the effects of microstructure of precursors on characteristics of activated carbons were discussed.
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
In this paper, chemical activations were carried out, utilizing isotropic pitch and anisotropic pitch as the starting materials for activated carbons. The precursors were smashed and mixed with KOH aqueous solution at a certain weight ratio, respectively, keeping for 1 h. Before putting into the horizontal cylindrical furnace, the mixture should be vacuum-dried for 24 h. Then they were heated to 800 °C at a determinate heating speed, holding for 1 h. The activated products were washed by diluted hydrochloride acid to get rid of the alkaline compounds. Then pH value was adjusted to 6–7 by distilled water. The activated carbons named TP from isotropic pitch and YP from anisotropic pitch with the same treatment condition were finally gained after being dried for 24 h at 105 °C. X-ray diffraction (XRD) was utilized to analyze the microstructural difference between isotropic pitch and anisotropic pitch. The morphologies of the precursors after activation were observed by JSM-6700F type field emission scanning electron microscope (SEM). Iodine adsorption test (GB/T 7702.7-1997) and N2 adsorption test (Quantachrome NOVA Automated 2000 automatic adsorption system) were carried out. Specific surface area and pore size distribution of activated carbons were determined by BET and DFT method, respectively. As shown in Table 1, the mean diameter of pitch particles, softening point of these two kinds of pitch with a common origin were similar to each other. The difference of toluene insoluble content suggested the essential distinction between the samples, namely ‘‘isotropic’’ and ‘‘anisotropic’’. Furthermore, the microstructural difference between these two kinds of pitch was analyzed by XRD,
Table 1 Some properties of pitch Pitch
Md (lm)
SP (°C)
TI (%)
Origin
Isotropic Anisotropic
1–10 1–10
265 270
7.3 94.5
Coal tar pitch Coal tar pitch
Notes: Md, mean diameter; SP, softening point; TI, toluene insoluble content.
Isotropic pitch
Intensity
338
Anisotropic pitch
20
40
60
80
100
2Theta (Deg) Fig. 1. XRD analysis of isotropic pitch and anisotropic pitch.
shown in Fig. 1. Carbonaceous materials were determined by 2h around 26° and 43°. Narrow and keen-edged peak around d002 showed that this kind of material should have better periodic accumulation of graphite crystallites than the sample with broader peak d002. Peak d002 with narrow and sharp shape indicated that the crystallites of anisotropic pitch had larger size and good tropism. On the other hand, isotropic pitch with broader peak d002 had more microstructural defects and noncrystallite carbon atoms. As a result, the periodic accumulation of graphite crystallites of isotropic pitch was not as good as anisotropic pitch by means of XRD method, qualitatively. The morphologies of activated carbons from isotropic pitch and anisotropic pitch were observed by SEM, as shown in Fig. 2. A plenty of holes appeared on the surface of isotropic pitch in all directions after chemical activation, however, this kind of phenomena was not observed on TP activated from anisotropic pitch. The significant changing of surface morphology of isotropic pitch after activation must be the result of the microstructural characteristics: There existed a great deal microstructural defects on the
Fig. 2. SEM images of samples TP and YP (a) TP activated from isotropic pitch (b) YP activated from anisotropic pitch.
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
339
0.10
TP YP
0.06
3
dv/dw [cm /g.A]
0.08
aromatic layers of isotropic pitch with broader peak d002 so that this kind of precursor could be easily etched by activation agent in all directions. During the heat treatment, the more chemical reactions carried out, the more gaseous species were released and the more holes appeared on the surface of isotropic pitch after the gaseous species escaping from the sample TP. The crystallites of anisotropic pitch with narrow and sharp peak d002, however, had better tropism and the carbon atoms on the carbonaceous layers were hard to be activated because of the stronger covalence bonds. Therefore, anisotropic pitch with less active carbon atoms preformed less chemical reactions during heat treatment and the gaseous species run away mostly from the interlayers not the layer surface. As a result, the smooth surface of anisotropic pitch did not change much after chemical activation. (shown in Fig. 3). Some microstructural parameters of samples TP and YP activated from isotropic pitch and anisotropic pitch, respectively, were shown in Table 2. Activation was carried out more sufficiently for isotropic pitch because of the isotropic microstuctural characteristics and the sample TP had lower activation yield, higher specific surface area, higher pore volume and higher adsorption ability, comparing with sample YP. The pore size distributions of samples TP and YP were shown in Fig. 4. Obviously, TP owned more micropores and mesopores than YP so that the former sample belonged to mesoporous activated carbons. As result, a hysteresis loop appeared on the isotherm curve of TP, however, sample YP which belonged to microporous activated carbons reached adsorption saturation under lower P/P0. In addition, the adsorption ability of TP was higher than YP according to the higher position of isotherm curve, as shown in Fig. 5.
0.04
0.02
0.00 0
10
20
30
40
50
Pore diameter (A) Fig. 4. Pore size distribution of samples TP and YP.
1000
Pore volume [cm3 /g] STP
Fig. 3. Sketch map showing the presumed theory of chemical activation with different precursors (a) TP activated from isotropic pitch (b) YP activated from anisotropic pitch.
800
TP YP
600
400
200 0.0
0.2
0.4
0.6
0.8
1.0
P/Po Fig. 5. Isotherm of samples TP and YP.
In general, it was necessary to analyze the microstructure of precursors for activated carbons preparation. Microstructure of precursors was one of important factors to influence the final performance of activated carbons. Isotropic pitch with more noncrystallite carbon atoms or edge carbon atoms on the microstructural defects had more reactive ability and more pores were manufactured through sufficient chemical activation. However, the crystallites of
Table 2 Porosity characteristics of TP from isotropic pitch and YP from anisotropic pitch Sample
AY (%)
SBET (m/g)
Vtot (cm3/g)
<1 nm (cm3/g)
1–2 nm (cm3/g)
2–3 nm (cm3/g)
>3 nm (cm3/g)
IAV (mg/g)
L (nm)
TP YP
48.0 63.7
2913 1810
1.36 0.81
0.25 0.16
0.67 0.49
0.32 0.12
0.12 0.04
2630 1998
2.12 1.99
Notes: AY, activation yield; SBET, specific surface area from BET method; Vtot, total pore volume; IAV, iodine adsorption value; L, average pore diameter.
340
C.-y. Guo, C.-y. Wang / Microporous and Mesoporous Materials 102 (2007) 337–340
anisotropic pitch had larger size and good tropism so that the activations mostly reacted on the interspaces of carbonaceous layers. As a result, isotropic pitch based activated carbon with sufficient chemical activation had better characteristics. References [1] J. Laine, S. Yunes, Carbon 4 (30) (1992) 601–604. [2] L.R. Radovic, C. Moreno-Castilla, J. Rivera-Utrilla, Chemistry and Physics of Carbon 27 (2001) 27.
[3] Z. Hu, M.P. Sriniivasan, Y. Ni, Carbon 6 (39) (2001) 877–886. [4] Liu Hsin-Yu, Wang Kai-Ping, Teng Hsisheng, Carbon 43 (2005) 559– 566. [5] A. Erhan Aksoylu, M. Madalena, A. Freitas, et al., Carbon 39 (2001) 175–785. [6] Kap Seung Yang, Young Jo Yoon, Moo Sung Lee, et al., Carbon 40 (2002) 897–903. [7] Kyoichi Oshida, Carbon 40 (2002) 2699–2711. [8] C. Blanco, O. Fleurot, R. Menendez, et al., Carbon 37 (1999) 1059– 1064. [9] Y. Korai, S. Ishida, F. Watanabe, et al., Carbon 12 (35) (1997) 1733– 1737.