Characteristics of activated carbons prepared from pistachio-nut shells by potassium hydroxide activation

Microporous and Mesoporous Materials 63 (2003) 113–124 www.elsevier.com/locate/micromeso

Characteristics of activated carbons prepared from pistachio-nut shells by potassium hydroxide activation Ting Yang, Aik Chong Lua

*

School of Mechanical and Production Engineering, Division of Thermal and Fluids Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 7 October 2002; received in revised form 8 May 2003; accepted 17 June 2003

Abstract High-surface-area activated carbons in granular form were prepared by chemical activation of pistachio-nut shells with potassium hydroxide. The effects of the preparation variables on the carbon pore structure were studied in order to optimize these parameters. It was found that the chemical to shell impregnation ratio, the activation temperature and the activation hold time were the important parameters that affect the characteristics of the activated carbons produced. For the experimental parameters investigated, the optimum conditions for preparing activated carbon with a high surface area and pore volume were identified. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Activated carbon; Chemical activation; Characterization; Pistachio-nut shell

1. Introduction Activated carbon can be produced from a variety of carbonaceous source materials. There are many studies in the literature relating to the preparation of activated carbons from agricultural wastes [1–6]. However, there are very few research works that report on the preparation of effective adsorbents from pistachio-nut shells. Hence, this study was undertaken to find out whether pistachio-nut shell is a good precursor for activated carbons to be used especially for gas adsorption.

*

Corresponding author. Tel.: +65-6790-5535; fax: +65-67924062/6791-1859. E-mail address: [email protected] (A.C. Lua).

Pistachio trees originated from the Middle East where they grew wild for centuries. According to the Foreign Agricultural Service of the United States Department of Agriculture, the world pistachio production was 0.21 Mt in 1999, i.e., a huge amount of nut-shells are available. Normally, pistachio-nut shells are used as boiler fuels or disposed off in landfills. To utilize these abundant solid wastes, it is proposed to use them as starting materials for the preparation of activated carbons because of their high carbon and low-ash contents. Generally, effective adsorbents can be prepared via a physical or a chemical method. In a chemical activation process, the starting materials are first impregnated with a chemical agent (for example, zinc chloride ZnCl2 , phosphoric acid H3 PO4 or potassium hydroxide KOH). This chemical is a dehydrating agent that can influence the pyrolytic

1387-1811/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1387-1811(03)00456-6

114

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

Table 1 Properties of the pistachio-nut shells Solid density (g/cm3 )

Hardness (HRC)a

Proximate analysis (wt.%) Moisture

Volatile

Fixed carbon

Ash

1.43

26.3

4.04

73.37

21.60

0.99

a

Note: HRC denotes Rockwell Hardness C scale (dimensionless).

decomposition and retard the formation of tars during the carbonization process, thereby increasing the yield of activated carbon [7]. The chemical process normally takes place at a temperature lower than that used in the physical activation process, possibly reducing energy costs in the production of activated carbons, although it may pose certain environmental problems. Chemical activation using KOH has been reported by many researchers [8–20]. However, the mechanism of chemical activation by KOH is not very clear although some mechanisms have been proposed [11–13,15]. This paper describes the preparation of granular high-surface-area activated carbons from pistachio-nut shells by chemical activation. These carbons will be prepared for use in gas adsorption applications. The properties of the pistachio-nut shells are shown in Table 1. The effects of different operating parameters (viz., the chemical to precursor impregnation ratio, the activation temperature and hold time) on the yield, surface area and pore evolution of the activated carbons were studied.

2. Experimental The pistachio-nut shells were dried at 110 °C for 12 h to reduce the moisture content. They were then crushed with a coffee grinder and sieved to a size range of 2.0–2.8 mm. Chemical activation with KOH was performed as follows: 10 grams of shell were placed in a vertical stainless-steel reactor (length 550 mm, interior diameter 38 mm) which was inserted in an electrical heating furnace (818P, Lenton). Carbonization was carried out under N2 (99.9995% purity) gas flow at 150 cm3 /min (STP) at 500 °C for 2 h. The temperature was increased at a rate of 10 °C/min. The heating rate, temper-

ature and dwell time of the furnace could be programmed. KOH pellets were mixed with the chars from the carbonization process in a glass beaker at a ratio of KOH to original shell masses ranging from 1:4 to 1:1. 100 cm3 water was added into the beaker to dissolve the KOH pellets. All the mixtures were dried at 120 °C for overnight in an oven. The resulting samples were then placed in the same furnace which was used for the carbonization process, and heated at a rate of 10 °C/min to 300 °C and then held for 1 h. This was to ensure that all the moisture in the samples had been driven out so as to avoid possible carbon loss through steam activation when the temperature was further increased. Subsequently, the temperature was increased to the final temperature (500–900 °C) at the same rate and held for 1.5–4 h before cooling down. This whole activation process was carried out under a nitrogen flow of 150 cm3 /min. The resulting products after activation were thoroughly washed with water followed by 0.1 M hydrochloric acid and finally distilled water to remove the residual KOH until the pH value of the washed solution ranged from 6 to 7. In a comparative test, KOH pellets were mixed with the original nut-shells. The mixture was only subjected to the activation process as described above. The aim of this test was to study the effect of the sequence of the impregnation stage on the properties of the resulting activated carbons. In some other tests, CO2 gas (flow rate of 100 cm3 / min) was used, instead of N2 gas, during the holding period of the final activation temperature so as to study the combined effects of chemical and CO2 activation. The yield of activated carbon was calculated on a chemical-free basis and could be regarded as an indicator of the process efficiency in the chemical activation process. Proximate analysis was carried

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

qp ¼ 1=ðVt þ 1=qs Þ

ð1Þ

Hence, the particle porosity (ep ) can be computed from qp and qs : ep ¼ 1  ðqp =qs Þ

ð2Þ

3. Results and discussion 3.1. Effect of the ratio of KOH to nut-shell masses The overall yields of activated carbons prepared at various ratios of KOH to shell masses are shown in Fig. 1. Activated carbons used in this part of the study were prepared by impregnating chars with various amounts of KOH and then activated at 700 °C for 2 h. For the range of mass ratios studied, the yields of activated carbons when CO2 was used during the holding period of the activation process are always lower than in the experiments in which N2 was used during the holding period. This is expected because in the former case, the combined chemical and CO2 activation effects will result in a more severe reaction and a higher carbon consumption than in the latter case. Initially, increasing the ratio of KOH to shell

N2 gas during holding period 22

CO2 gas during holding period

20 18

Yield (%)

out using a thermogravimetric analyzer (TA-50, Shimadzu). Nitrogen adsorption at )196 °C was measured by means of a conventional porosimeter (ASAP 2010, Micromeritics). The Brunauer– Emmett–Teller (BET) surface area was calculated from the isotherms using the BET equation [21]. The cross-sectional area of a nitrogen molecule was assumed to be 0.162 nm2 . The Dubinin– Radushkevich (DR) equation was used to calculate the micropore volume. The total pore volume was estimated to be the liquid volume of adsorbate (N2 ) at a relative pressure of 0.985. The pore size distribution was determined using the Barrett, Joyner and Halenda (BJH) model [22]. The average pore diameter was calculated as 4 times the total pore volume divided by the BET surface area. The solid density (qs ) of the sample was measured by helium displacement with an ultra-pycnometer (UPY-1001, Quantachrome) [23]. The apparent density (qp ) was thus calculated from the total pore volume per unit sample mass (Vt ) and qs :

115

16 14 12

Activation temperature = 700 oC

10 8

Activation hold time = 2 h

6 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Ratio of KOH to shell masses Fig. 1. Effect of the ratio of KOH to shell masses on the yield of the chemical activation process.

masses (up to 33.3% for pure chemical activation and 50% for combined chemical and CO2 activation) reduces the weight loss of the activated carbon due to the inhibition of tar production by KOH, leading to increased carbon yield. This trend was also reported by Ahmadpour and Do [14] in their study on the preparation of activated carbon from macadamia nut-shell by chemical activation. Above these initial mass ratios, the oxidation reaction is predominant, and therefore the yield decreases with increasing mass ratio as a result of increasing burn-off of carbon and release of volatiles. However, in the N2 case, the yield levels off for mass ratios of 0.5 and greater due to the absence of an oxidizing gas. It is believed that the potassium hydroxide acts as an oxidizer for the oxidation reaction which will be discussed later. Fig. 2 illustrates the effects of the ratio of KOH to shell masses on the pore and surface characteristics of the chemically activated carbons. In the case of using N2 during the holding period, the BET surface area and micropore volume peak at ratios of 0.50 and 0.75, respectively, and thereafter decrease with increasing ratio. The mechanism of pore evolution with varying ratio is as follows: Upon increasing the ratio from 0.25 to 0.5, predominantly micropores but some mesopores or macropores are progressively formed and hence the BET surface area of the activated carbon continues to increase up to a maximum of 1578.7 m2 /g for a ratio of 0.5. The presence of the

116

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124 N2 gas during holding period

N2 gas during holding period

0.8

CO2 gas during holding period

CO2 gas during holding period

1800

Micropore volume (cm3/g)

BET surface area (m2/g)

1900

1700 1600 1500 1400 1300 1200

0.7

0.6

0.5

0.4

(b)

(a)

1100 0.2

0.4

0.6

0.8

0.3 0.2

1.0

Ratio of KOH to shell masses

0.8

1.0

N2 gas during holding period

CO2 gas during holding period

CO2 gas during holding period

0.25

0.20

0.15

0.10

90

80

70

60

(d)

(c) 0.05 0.2

0.6

N2 gas during holding period

Micropore volume percent (%)

Non-micropore volume (cm3/g)

0.30

0.4

Ratio of KOH to shell masses

0.4

0.6

0.8

1.0

Ratio of KOH to shell masses

50 0.2

0.4

0.6

0.8

1.0

Ratio of KOH to shell masses

Fig. 2. Effect of the ratio of KOH to shell masses on the (a) BET surface area, (b) micropore volume, (c) non-micropore volume, and (d) micropore volume of chemically activated carbons (other activation conditions: temperature ¼ 700 °C, hold time ¼ 2 h).

hydroxyl group acts as the oxidizer for the C– KOH reaction, resulting in the consumption of carbon and the formation of pores within the internal carbon structures. The continuous devolatilization of the char sample also contributes to the increase in the BET surface area, regardless of the different ratios. At a ratio of 0.5, the micropore volume has nearly peaked, whilst the non-micropore volume (calculated by subtracting the mi-

cropore volume from the total pore volume) has reached its maximum value. When the ratio is increased from 0.5 to 0.75, there are only a marginal increase and decrease in the micropore and nonmicropore volumes, respectively. The BET surface area decreases slightly, which is attributed to the pore widening effect as the pore walls are ‘‘burntoff’’ and become thinner. A further increase of the ratio to 1 results in significant reductions in the

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

BET surface area and micropore volume. These reductions are due to an enhanced C–KOH reaction, resulting in the widening of pores through the complete ‘‘burning-off’’ of some walls between neighboring pores and continuous pore wall thinning. The general widening of bigger pores results in a reduced non-micropore volume which is not compensated sufficiently by the conversion of micropores to mesopores or macropores. For the range of ratios investigated, the micropore volume to the total pore volume remains nearly constant, as shown in Fig. 2(d), indicating a proportional increase or decrease of these two components concurrently. Fig. 2 also shows the KOH chemical activation results in which CO2 was used during the holding period. Increasing the ratio of KOH to shell masses from 0.33 to 0.5 increases the BET surface area and micropore volume, predominantly through the development of micropores arising from the carbon consumption in the combined C– KOH and C–CO2 reactions. Regardless of the different ratios, the continuous evolution of volatiles has also contributed to increases in the BET surface area and micropore volume, which peak at a ratio of 0.5, yielding 1830.8 m2 /g and 0.712 cm3 / g, respectively. The increase in the non-micropore

117

volume in Fig. 2(c) is due to the widening of existing micropores to mesopores and macropores. Further increases of the ratio beyond 0.5 show the reversed trend of decreasing the BET surface area and micropore volume. These reductions are due to the excessive ‘‘burning’’ of pore walls; both complete burning-off of some walls between neighboring pores and the pore wall thinning effect arising from the combined effects of C–KOH and C–CO2 reactions. However, the non-micropore volume continues to increase for increasing ratio, especially when the ratio is increased from 0.75 to 1.0. The general widening of larger pores is overcompensated by the more dominating effect of widespread conversion of micropores to mesopores or even macropores. This is distinctly different from the previous case where N2 was used during the holding period. Therefore, in Fig. 2(d), there is a sharp decline in the micropore to total pore volume when the ratio is increased from 0.75 to 1.0. It is evident from these results that chemical activation in the presence of CO2 , at an optimum ratio of KOH to shell masses, produces a greater number of pores than N2 , resulting in a higher BET surface area for the activated carbon. The results of proximate analyses for chemically activated carbons by KOH are shown in Table 2.

Table 2 Results of proximate analyses (dry basis) for chemically activated carbons Activation conditions a-b-c-d

Volatile (%)

Fixed carbon (%)

Activation conditions a-b-c-d

Volatile (%)

Fixed carbon (%)

700-2-0.25-N2 700-2-0.33-N2 700-2-0.5-N2 700-2-0.75-N2 700-2-1.0-N2

8.2 13.7 11.6 10.1 8.9

88.3 83.0 87.7 88.7 90.6

700-1.5-0.5-N2 700-2.0-0.5-N2 700-2.5-0.5-N2 700-3.0-0.5-N2 700-3.5-0.5-N2 700-4.0-0.5-N2

8.2 11.6 8.6 10.9 11.1 11.4

91.5 87.7 91.1 87.9 87.4 87.2

700-2-0.33-CO2 700-2-0.5-CO2 700-2-0.75-CO2 700-2-1.0-CO2

8.8 10.5 8.4 8.6

90.2 87.6 90.2 90.7

500-2-0.5-N2 600-2-0.5-N2 700-2-0.5-N2 800-2-0.5-N2 900-2-0.5-N2

19.3 14.7 11.6 9.7 8.9

79.1 84.6 87.7 87.8 88.1

800-1.5-0.5-N2 800-2.0-0.5-N2 800-2.5-0.5-N2 800-3.0-0.5-N2 800-3.5-0.5-N2 800-4.0-0.5-N2

8.0 9.7 6.9 8.1 8.3 8.5

89.9 87.8 90.9 89.6 89.2 89.1

Note: a-b-c-d denotes activation temperature (°C)-activation hold time (h)-ratio of KOH to shell masses-gas type during activation holding period.

118

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

For the effects of varying mass proportions of KOH under N2 or CO2 conditions, these results reveal the following: For low ratios of KOH to shell masses, an increasing ratio (0.25–0.33 for N2 and 0.33–0.5 for CO2 ) decreases the fixed carbon content due to the ‘‘burn-off’’ of carbon from the C–KOH reaction under N2 atmosphere or combined C–KOH and C–CO2 reactions under CO2 atmosphere. However, further increases in the KOH content produce an increasing fixed carbon content with increasing ratio. At these relatively high ratios, excessive ablation of the external carbon particle occurs, resulting in the release of volatile matter becoming more dominant than the burn-off of carbon. Hence, this ablation promotes and aids the release of the volatiles, thereby enriching the fixed carbon content as shown in Table 2. Fig. 3 shows the isotherms of the chemically activated carbons for nitrogen adsorption at )196 °C for different ratios of KOH to shell masses. For the case of using nitrogen gas during the holding period in Fig. 3(a), all the isotherms exhibit very similar trends. The nitrogen uptake is significant only in the low-pressure range, i.e. relative pres-

600

sure less than 0.2. In the high-pressure range, no further adsorption is observed, and therefore the adsorption curve has reached equilibrium. These isotherms are of Type I, which represents microporous solids having a relatively small external surface area, according to the International Union of Pure and Applied Chemistry (IUPAC) classification. Increasing the ratio from 0.25 to 0.50 increases the volume of nitrogen adsorbed, in-line with the increasing BET surface area as given in Fig. 2(a). At the ratio of 0.75, its isotherm coincides with that for a ratio of 0.5; again these results are in fair agreement with the BET values in Fig. 2(a). Finally, at a ratio of 1.0, the nitrogen adsorption reduces sharply, in tandem to the fall in the BET surface area. When CO2 was used during activation, the isotherms as presented in Fig. 3(b) differ slightly from those in Fig. 3(a). The knee of the isotherm is more open with the plateau forming at a higher relative pressure, especially for the activated carbon prepared at a ratio of 1.0. The isotherm for the highest ratio of 1 indicates a significant development of meso- and macropores which are also reflected in Fig. 2(c). The rise and decline of the isotherms with increasing ratio are

Ratio = 1.0

Ratio = 1.0

Ratio = 0.75

Ratio = 0.75 Ratio =0. 50

Ratio = 0.50

Volume adsorbed (cm3/g)

Volume adsorbed (cm3/g)

Ratio = 0.25

500

400

300

500

400

300

(a) N2 gas during holding period

200

0.0

0.2

0.4

0.6

Relative pressure

Ratio = 0.33

600

Ratio = 0.33

0.8

(b) CO2 gas during holding period 1.0

200

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure

Fig. 3. Effect of the ratio of KOH to shell masses on the adsorption isotherms of chemically activated carbons (other activation conditions: temperature ¼ 700 °C, hold time ¼ 2 h).

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

consistent with the trends for the BET surface area as shown in Fig. 2(a). Based on the results shown in Fig. 2, the optimum ratio of KOH to shell masses is 0.5 for maximum BET surface area and micropore volume in the activated carbons for both N2 and CO2 cases, and this ratio was used for subsequent tests. 3.2. Effect of the chemical activation temperature The activated carbon prepared at 800 °C using CO2 during the holding period was found to disintegrate into powder, because of the severity of the combined effects of the C–KOH and C–CO2 reactions. Since a powder form is not suitable for gas-phase adsorption applications, only N2 activation during the holding period was studied in the subsequent tests. The yield and some other physical properties of the activated carbons prepared at different temperatures are shown in Table 3. Increasing the activation temperature reduces the yield of the activated carbons continuously. This is expected since an increasing amount of volatiles is released at increasing activation temperature from 500 to 900 °C. This decreasing trend in yield is paralleled by the increasing carbon consumption with increasing activation temperature due to the C– KOH reaction. These phenomena are also manifested in the decreasing volatile content and increasing fixed carbon for increasing activation temperature in Table 2. These two opposing trends in Table 2 are due to the predominance of the loss in volatiles over carbon consumption for increasing activation temperature. Fig. 4 shows the effect of activation temperature on the pore and surface properties of chemi-

119

cally activated carbons. Increasing the temperature from 500 to 800 °C progressively increases both the BET surface area and micropore volume. This progressive temperature rise increases the C–KOH reaction rate, resulting in increasing carbon burnoff. Concurrently, the continuous evolution of volatiles from the char samples also increases with increasing temperature. The devolatilization process further develops the rudimentary pore structure in the char, whereas the C–KOH reaction enhances the existing pores and creates new ones. These two reactions produce an increasing BET surface area and micropore volume with increasing activation temperature as shown in Fig. 4(a). However, for further temperature increase from 800 to 900 °C, both the BET surface area and micropore volume decrease due to excessive carbon burn-off, resulting in the widening of pores and even the loss of some walls between the pores. This pore widening effect is also reflected in Fig. 4(b) in which the non-micropore volume remains relatively low for activation temperatures of 500– 700 °C. At these temperatures, the pores are predominantly microporous, contributing up to 86% of the total pore volume. At 800 °C, the steep increase in the non-micropore indicates the commencement of the pore widening effect which gets more intensive at 900 °C. At the same time, the micropore volume decreases, showing the conversion of micropores to meso- and macropores. In conjunction with these pore structural changes, the average pore diameter is less than 2 nm for activation temperatures of 500–700 °C (Table 3), indicating that the resulting activated carbons are mainly microporous in nature. However, at 900 °C, the average pore diameter is about 2.2 nm which shows that the activated carbons have been

Table 3 Yield, densities, porosity and average pore diameter of chemically activated carbons at different activation temperatures under N2 atmosphere Activation conditions

Yield (%)

Solid density (g/cm3 )

Apparent density (g/cm3 )

Porosity

Average pore diameter (nm)

500-2-0.5 600-2-0.5 700-2-0.5 800-2-0.5 900-2-0.5

21.2 21.1 19.6 16.2 12.4

1.690 2.027 2.275 2.465 2.735

1.020 0.943 0.831 0.722 0.722

39.6 53.9 63.5 70.7 73.6

1.95 1.92 1.93 2.01 2.24

Note: a-b-c denotes activation temperature (°C)-activation hold time (h)-ratio of KOH to shell masses.

120

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124 Non-micropore volume

BET surface area

Micropore volume percentage

Micropore volume

0.7 0.7

1600 0.5

1400 1200

0.4 1000

0.6 80

0.5 0.4

70

0.3

60

0.2

50

0.1

800 600

Micropore volume percentage(%)

0.6

1800

90

Non-micropore volume (cm3/g)

2000

Micropore volume (cm3/g)

BET surface area (m2/g)

2200

(a) 500

600

700

800

(b)

0.3

900

0.0

500

Activation temperature (oC)

600

700

800

40

900

Activation temperature (oC)

Fig. 4. Effect of activation temperature on the (a) BET surface area and micropore volume, and (b) non-micropore volume and micropore volume of chemically activated carbons under N2 environment (other activation conditions: ratio of KOH to shell masses ¼ 0.5, hold time ¼ 2 h).

0.20 Ratio of KOH to shell masses = 0.5

Pore volume (cm3/g)

converted to mainly mesoporous materials. Table 3 also shows that, with increasing continuous devolatilization and carbon burn-off with increasing activation temperature, the porosity of the carbonaceous structure continues to increase. These effects result in an increased solid density and hence reduce the apparent densities of the activated carbons. The pore size distributions of the chemically activated carbons at various activation temperatures are shown in Fig. 5. For carbons activated at relatively low temperature, namely 500–700 °C, the pores are predominantly microporous. At 800 °C, some new mesopores begin to form in the carbon structure. At a high-activation temperature of 900 °C, mesopores and even macropores are formed, besides the prevalent micropores. These phenomena are consistent with the characteristics of the activated carbons as discussed earlier in Fig. 4 and Table 3. As the intended application for the activated carbons developed in this work is the adsorption of gases, the presence of micropores in the carbon structure is important. Based on the results shown in Fig. 4(a), activation temperatures of 700 and 800 °C produce comparable quantities of micro-

0.16

Hold time = 2 h 500oC

0.12

600oC 700oC

0.08

800oC 900oC

0.04 0.00 0

10

2

3

4

5 6 7

1

10

2

3

4

5 6 7

2

10

Average pore diameter (nm) Fig. 5. Effect of activation temperature on the pore size distributions of chemically activated carbons under N2 environment.

pore volume. Thus, these two activation temperatures will be used to activate the chars for subsequent studies. 3.3. Effect of the hold time during chemical activation Fig. 6 shows the pore development and surface area characteristics of the activated carbons pre-

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124 BETsurface area Micropore volume

BETsurface area Micropore volume

0.70

0.75

1800 0.67

1700 1600

0.63

1500 0.59 1400 o

(a) 700 C

0.67

2200 2100

0.64 2000 0.61

1900 1800

0.58

Micropore volume (cm3/g)

0.71

BET surface area (m2/g)

1900

2300

Micropore volume (cm 3/g)

BET surface area (m2/g)

2000

1300

121

o 1700 (b) 800 C

0.55

0.55 1

2

3

4

1

Activation hold time (h)

2

3

4

Activation hold time (h)

Non-micropore volume

Non-micropore volume

Micropore percentage

Micropore percentage

0.22

0.6

84

0.14

82 0.10 80 (c ) 7 0 0

0.06 1

2

o

3

Non-micropore volume (cm /g)

86

Micropore percentage (%)

3

Non-micropore volume (cm /g)

0.18

0.5

71

66 0.4 61 0.3 56

C

3

(d ) 8 0 0

4

78

Activation hold time (h)

0.2

o

Micropore percentage (%)

76

88

C

51 1

2

3

4

Activation hold time (h)

Fig. 6. Effect of activation hold time on the pore volume and surface area characteristics of chemically activated carbons under N2 environment (other activation condition: ratio of KOH to shell masses ¼ 0.5).

pared at different hold times. For activation at 700 °C, the trends for the BET surface area and micropore volume are both similar as the hold time is increased. Increasing the hold time from 1.5 to 2 h increases the BET surface area, the micropore and non-micropore volumes. These properties increase with the continuous devolatilization of the char; coupled with pore enhancement and the formation

of new pores which are both due to carbon burnoff as a result of the C–KOH reaction. The proximate analysis in Table 2 shows that the fixed carbon content decreases whilst the volatile content increases when the hold time is increased from 1.5 to 2 h, indicating that the carbon burn-off is more dominant than the devolatilization process. However, at a hold time of 2.5 h, the BET surface

122

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

area and micropore volume decrease due to the pore wall thinning effect and the subsequent substantial release of volatiles (Fig. 6(a)). The proximate analysis in Table 2 shows an increase in the fixed carbon content, but the volatile content decreases, verifying that devolatilization is the predominant process. When the hold time is increased from 2.5 to 3 h, the BET surface area, micropore volume and non-micropore volume increase dramatically as shown in Fig. 6(a) and (c). These increases are due to the formation of new micropores and the conversion of some micropores to meso- and macropores through the C–KOH reaction, besides the continuous release of volatiles from the sample. Due to the presence of these meso- and macropores, the micropore percentage inevitably decreases. When the hold time is increased beyond 3 h, the BET surface area, the micropore volume and the non-micropore volume of the activated carbons all decrease because of the widening of pores through the complete burningoff of some walls between neighboring pores and the pore wall thinning effect as a result of prolonged hold time. The proximate analyses in Table 2 show that for hold times of 3 h and above, carbon burn-off due to the C–KOH reaction is the predominant effect and therefore the fixed carbon content continuously decreases with increasing hold time. For the activation at 800 °C, pore development and surface area characteristics of the activated carbons are quite similar to those for 700 °C, except for some minor differences. At 700 °C, the activated carbon subjected to 3 h hold time yielded the maximum BET surface area and micropore volume, whilst for 800 °C, the activated carbons that produced the maximum micropore volume and BET surface area were at two different hold times, viz., 1.5 and 3 h, respectively. Summarizing, the activated carbon prepared at 700 °C and a hold time of 3 h produced good micropore characteristics (micropore volume ¼ 0.744 cm3 /g, BET surface area ¼ 1931.8 m2 /g and total pore volume ¼ 0.933 cm3 /g) whilst the activated carbon prepared at 800 °C and a hold time of 3 h produced good BET surface area characteristics (BET surface area ¼ 2251.4 m2 /g, total pore volume ¼ 1.10 cm3 /g and micropore volume ¼ 0.5985 cm3 /g). These high-micropore vol-

ume and high BET surface areas obtained in this study as compared to BET values of 1408 m2 /g from oil-palm shell [18], 2451 m2 /g from coconut shell [17] and 1600 m2 /g from macadamia shell [14] for similar chemical impregnation technique, will render the pistachio-nut-shell activated carbons to be suitable for gas adsorption applications. 3.4. Variation of the preparation method The direct impregnation of KOH onto raw pistachio-nut shells was also carried out. However, the activated carbons were obtained in the powder form, and it was difficult to separate the powders from the volatile matters. This problem was also reported by Hu and Srinivasan [17]. The structure of the raw material is less organized as compared to that of the char. When impregnated with KOH, there is a non-homogeneous distribution of potassium ions, leading to large variations in expansion within the raw material structure, and therefore this can lead to high stresses and eventually crack formation. However, the char structure is less mosaic and can hold far more potassium, and therefore the break-up phenomenon does not occur. 3.5. Activation mechanism Much work has been carried out in the area of chemical activation using KOH. Marsh et al. [24] studied the effect of KOH on different cokes and stated that the presence of oxygen in the alkali resulted in the removal of cross-linking and stabilizing of carbon atoms in the crystallites. Potassium metal liberated at the reaction temperatures may intercalate and force apart the separate lamellae of the crystallite. Removal of these potassium salts (by washing) and carbon atoms (by activation reaction) from the internal volume of the carbon creates the micropores in the structure. Otowa et al. [11,12] produced high-surface-area activated carbons from petroleum coke by KOH and studied the activation mechanisms. They found that considerable amounts of K2 CO3 and hydrogen were formed and only a small amount of CO2 was contained in the effluent gas. Also, they concluded that high temperature would cause

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

several atomic layers of carbon being widened and hence, forming large pores. Some possible reactions were proposed:

123

ð7Þ

the preparation conditions, such as the chemical impregnation ratio, the activation temperature and the activation hold time. Under the experimental conditions studied, the best conditions for preparing activated carbons with high-surface area and pore volume by char impregnation are: an impregnation ratio of 0.5 (KOH to raw material on mass basis), an activation hold time of 3 h, and an activation temperature of 800 °C. With these experimental conditions, an activated carbon with a BET surface area of 2259.4 m2 /g and a total pore volume of 1.10 cm3 /g was obtained. Impregnating KOH with char results in activated carbon in granular form whilst direct impregnation with raw material will produce activated carbon in powder form. Too high an activation temperature and impregnation ratio will result in the burn-off of carbon structures and widening of the micropores to meso- and macropores.

ð8Þ

References

2KOH ! K2 O þ H2 O ðdehydrationÞ

ð3Þ

C þ H2 O ! H2 þ CO ðwater-gas reactionÞ

ð4Þ

CO þ H2 O ! H2 þ CO2 ðwater-gas shift reactionÞ ð5Þ K2 O þ CO2 ! K2 CO3 ðcarbonate formationÞ ð6Þ When the activation temperature exceeds 700 °C, a considerable amount of metallic potassium is formed due to the following possible reactions: K2 O þ H2 ! 2K þ H2 O ðreduction by hydrogenÞ

K2 O þ C ! 2K þ CO ðreduction by carbonÞ Ahmadpour and Do [13] studied the preparation of activated carbon from coal by chemical activation using KOH and ZnCl2 and summarized some activation mechanisms for these two chemicals based on a literature survey. They found that increasing the ratio of KOH to coal masses would mainly be responsible for pore creation whilst the pore width and mesopore volume remained almost constant. However, in this study, it was found that the percentage of micropore volume was nearly constant with increasing ratio of KOH to shell masses as seen in Fig. 2(d) when N2 gas was used. This means that KOH activation will produce a constant proportion of total pore volume and micropore volume.

4. Conclusions High-surface-area activated carbons were prepared from chemical activation of pistachio-nut shells with KOH. The pistachio-nut shell was found to be a good raw material for developing activated carbons with good pore characteristics. The porosity of the carbons is highly dependent on

[1] S. Balci, T. Dogu, H. Yucel, J. Chem. Tech. Biotech. 60 (1994) 419. [2] J. Laine, S. Yunes, Carbon 30 (1992) 601. [3] Z.H. Hu, E.F. Vansant, Carbon 33 (1995) 561. [4] F. Rodriguez-Reinoso, J.M. Martin-Martinaz, M. MolinaSabio, I. Petez-Lledo, C. Prado-Burguete, Carbon 23 (1985) 19. [5] K. Gergova, N. Petrov, S. Eser, Carbon 32 (1994) 693. [6] J. Rodriguez-Mirasol, T. Cordero, J.J. Rodriguez, Carbon 31 (1993) 87. [7] R. Kandiyoti, J.I. Lazaridis, B. Dyrvold, C. Ravindra, Fuel 63 (1984) 1583. [8] H. Marsh, D.C. Crawford, T.M. OÕGrady, A.N. Wennerberg, Carbon 20 (1982) 419. [9] A.N. Wennerberg, T.M. OÕGrady, Active carbon process and composition, US Patent 4,082,694, assigned to Standard Oil Company (Indiana), 1978. [10] T.M. OÕGrady, A.N. Wennerberg, in: J.D. Bacha, J.W. Newman, J.L. White (Eds.), Petroleum-derived Carbons, High-surface-area Active Carbon, ACS Symp. Ser. 303, American Chemical Society, Washighton, 1986, p. 302. [11] T. Otowa, Y. Nojima, T. Miyazaki, Carbon 35 (1997) 1315. [12] T. Otowa, R. Tanibata, M. Itoh, Gas Sep. Purif. 7 (1993) 241. [13] A. Ahmadpour, D.D. Do, Carbon 34 (1996) 471. [14] A. Ahmadpour, D.D. Do, Carbon 35 (1997) 1723. [15] Z.H. Hu, E.F. Vansant, Carbon 33 (1995) 1293. [16] J. Laine, A. Calafat, Carbon 29 (1991) 949.

124

T. Yang, A.C. Lua / Microporous and Mesoporous Materials 63 (2003) 113–124

[17] Z.H. Hu, M.P. Srinivasan, Micropor. Mesopor. Mater. 27 (1999) 11. [18] J. Guo, A.C. Lua, Micropor. Mesopor. Mater. 32 (1999) 111. [19] D. Lozano-Castello, M.A. Lillo-Rodenas, D. CazorlaAmoros, A. Linares-Solano, Carbon 39 (2001) 741. [20] C. Moreno-Castilla, F. Carrasco-Marin, M.V. LopezRamon, M.A. Alvarez-Merino, Carbon 39 (2001) 1415.

[21] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 1982, p. 41. [22] E.P. Barrett, L.C. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [23] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 1997, p. 195. [24] H. Marsh, D.S. Yan, T.M. OÕGrady, A. Wennerberg, Carbon 22 (1984) 603.