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Chemical activation of Quercus agrifolia char using KOH: Evidence of cyanide presence
Microporous and Mesoporous Materials 85 (2005) 331–339 www.elsevier.com/locate/micromeso
Chemical activation of Quercus agrifolia char using KOH: Evidence of cyanide presence Alejandro Robau-Sa´nchez
a,*
, Alfredo Aguilar-Elgue´zabal a, Julia Aguilar-Pliego
b
a
b
Laboratorio de Cata´lisis, Departamento de Quı´mica de los Materiales, Centro de Investigacio´n en Materiales Avanzados, CIMAV, Miguel de Cervantes 120, Complejo Industrial Chihuahua, CP 31109, Chihuahua, Mexico Universidad Auto´noma Metropolitana-Azcapotzalco, Area de Quı´mica Aplicada edif. G-bis, San Pablo 180, Col. Reynosa-Tamaulipas, CP 02200, Del. Azcapotzalco, Me´xico DF, Mexico Received 24 May 2005; received in revised form 29 June 2005; accepted 1 July 2005 Available online 22 August 2005
Abstract Activation of Quercus agrifolia char with KOH using a rotary batch reactor is presented in this work. Several samples of activated carbon showing a very high degree of activation (up to 3081 m2/g) with predominance of microporosity were obtained. Nitrogen and argon were used as gaseous media in the activation procedure. Samples were evaluated using nitrogen adsorption applying BET and DR equation for porosity assessment. The existence of CN in activated samples indicates the occurrence of not previously reported chemical reactions in this process. According to this result, both structural and gaseous nitrogen would play an active role in the chemical reaction. Following these results, a reaction mechanism for KOH–C–N2 system is proposed. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Activated carbon; Chemical activation; Porosity; Reaction mechanism
1. Introduction Chemical activation is a well-known process as a method for activated carbon preparation. This is a complex process because of the numerous process variables and its multiple combinations. The chemical process of activation with KOH is particularly interesting because activated carbons with extremely high degrees of activation can be obtained, with porous structure in which micropores are predominant. Raw materials of both mineral and organic sources have been processed with KOH activation procedure. Among materials of mineral origin are found bitumi-
* Corresponding author. Tel.: +52 614 4391118; fax: +52 614 4391112. E-mail address: [email protected] (A. Robau-Sa´nchez).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.003
nous coal [1,2], petroleum coke [3,4] and anthracite [5–7]. Highly porous activated carbons obtained from organic and lignocellulosic materials are reported too. Some examples of these materials are walnut shells [8], sucrose char [9], oil palm stone [10], corncob [11], olive mill waste water [12], and peanut hulls [13]. The complexity of the reaction process and the variety of raw materials that can be used make the activation with KOH a challenging and interesting subject of research. As a result of the efforts dedicated to elucidate the reaction mechanism of KOH–C system, several reaction products have been identified. During the activation process, a considerable amount of K2CO3 is formed [4]. As well, hydrogen and CO2 are contained in the effluent gas. Metallic potassium is a reaction product too and above 760 °C vaporized potassium is removed out the reaction chamber by the effluent gases. The metal vapors can be observed as condenses in colder parts of the activation equipment.
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Otowa et al. [4] proposed a reaction mechanism aimed to explain the formation of above mentioned reaction products. According to the authors, the first step of the proposed mechanism would be the thermal dehydration of KOH ð1Þ
2KOHðs;lÞ = K2 OðsÞ + H2 OðgÞ
The formation of CO2 and H2 would be then the consequence of the combined effect of the following reactions: CðsÞ + H2 OðgÞ = H2ðgÞ + COðgÞ
ð2Þ
COðgÞ + H2 OðgÞ = H2ðgÞ + CO2ðgÞ
ð3Þ
The potassium carbonate would appear then as the previously formed K2O and CO2 react ð4Þ
K2 OðsÞ + CO2ðgÞ = K2 CO3ðs;lÞ
Metallic potassium would be formed by the reduction of K2O by carbon or hydrogen at high temperatures K2 OðsÞ + H2ðgÞ = 2KðlÞ + H2 OðgÞ
ð5Þ
K2 OðsÞ + CðsÞ = 2KðlÞ + CO2ðgÞ
ð6Þ
In Fig. 1 are Gibbs free energies corresponding to reactions (1), (5) and (6) which show up as the main reactions of above exposed reaction mechanism. The authors concluded that the creation of the porosity is caused by the intercalation of metallic potassium into the carbon matrix. As observed, the essential hypothesis for this reaction mechanism is the formation of K2O. However, the values of Gibbs free energy corresponding to reaction (1) suggest that K2O should be produced by another chemical reaction, not included in that proposal. Recent published papers [5,7] contain a detailed study of KOH–C system. Authors evaluated the importance of
several process variables and pointed out the role of the purging gases during the activation process. The authors report both the influence of nitrogen flow on the attained activation degree and the consequences of using other gases different than N2 (CO2 and steam). They remark that the use of CO2 should be avoided since it induces an inhibition of the activation process because of the carbonation of KOH. On the other hand, steam produces better porosity than CO2 but less than N2. The authors [7] studied the evolution of formation of K2CO3, H2, CO2 and CO, using for this purpose FTIR and TPD techniques. These experimental results, supported by theoretical thermodynamic calculations, were used to propose the following reaction mechanism. Again, aiming to explain the formation of K2CO3, H2 and metallic potassium, reaction (7) was formulated 6KOHðlÞ + CðsÞ = 2Kðg;lÞ + 2K2 CO3ðs;lÞ + 3H2ðgÞ
This reaction would explain the formation of metallic potassium, hydrogen and K2CO3. Carbonate could be also formed according to the alternative way proposed in reaction (8) 4KOHðlÞ + 2CO2ðgÞ = 2K2 CO3ðs;lÞ + 2H2 O(g)
70
deltaG, kcal
60 50
K2O + H2(g) = 2K(l) + H2O(g)
(5)
40
K2O + C = 2K(l) + CO(g)
(6)
2KOH(l) = K2O + H 2O(g)
(1)
20 10
o
T, C
0 350 -10
450
550
650
ð8Þ
Reaction (8) implies of course the availability of CO2 in the studied system. In Fig. 2 the evolution of Gibbs free energies of these reactions can be observed. The increasing of the nitrogen flow enhances the porosity, being this effect more perceptible when lower KOH-char ratios are used [5]. This phenomenon is attributed by the authors to the removal of the reaction products by the effluent gases from the reaction chamber and consequently to the displacement of the activation reaction toward the products. This effect could strongly depend on the type of furnace used and on
80
30
ð7Þ
750
850
950
-20
Fig. 1. Gibbs free energies for reactions proposed in Ref. [4].
1050
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333
5 o
T, C 0 400
450
500
550
600
650
700
750
800
850
900
950
1000 1050
deltaG, kcal
-5
-10
-15
4KOH(l) + 2CO2(g) = 2K2CO3 + 2H2O(g)
(7)
6KOH(l) + C = 2K(g,l) + 2K2CO3 + 3H2(g)
(8)
-20
-25
-30
Fig. 2. Gibbs energies for reactions proposed in Ref. [7].
the geometrical distribution of the carbon with respect to the N2 flow. In our work, the possible participation of N2 in chemical reactions that take place during the activation of char by KOH is considered. The results of the activation of Quercus agrifolia char with KOH using nitrogen as a purging gas are reported in this paper and are used to propose a new reaction mechanism.
2. Experimental Quercus agrifolia char, obtained by pyrolysis at 500 °C, was used as the precursor in this work. The char was crushed and sieved to a particle range of 0.6–1.0 m. The fixed carbon content was obtained from a TGA analysis, showing a value of 75 wt%. KOH from Baker was ground to powder and mixed physically with 20 g of char at given proportion (4/1 and 2/1). The activation was made using a stainless steel rotary batch reactor heated in a horizontal tube furnace Thermolyne F79430-70 equipped with a programmable controller. The temperature profile at the working zone was measured before experiments, using a load similar to those intended to be used during experimentation, in order to establish the temperature variation all along the zone. The temperature profile was found to be stable and the differences were less than 3 °C. The rotation speed of the reactor was 0.79 rpm. The samples were heated at 5 °C/ min from ambient temperature to the final temperature, 760 °C. At this temperature were kept for 60 min and then cooled down under the same gas (nitrogen) and flow used during the carbonization. Activated samples were washed with distillated water, 5 M HCl and distillated water again several times until a pH higher than 6 was measured. After washing, samples were dried at 110 °C overnight. In the sample nomencla-
ture are included the KOH/char ratio, the corresponding gas flow and the activation temperature. Cyanide was determined by titrimetric method using silver nitrate and potassium iodide as indicator. For the porosity characterization of the samples, physical adsorption of N2 at 77 K was carried out in an automatic system (Quantachrome Autosorb 1). Micropore volumes (VMI) have been calculated from adsorption isotherms applying the Dubinin–Radushkevich (DR) equation. Despite the unrealistic values of BET internal surface areas that characterize the obtained materials, this parameter is included in the characterization since permits a quick evaluation of activation degree. Total pore volume (VT) was determined at 0.95 p/p0 and mesopore volume (VME) was calculated as the difference between VT and VMI.
3. Results and discussion 3.1. Porosity The experimental plan related to this work included two levels of KOH/C ratio (2 and 4) and three levels of gas flow rate (150, 250 and 500 cc/min). Obtained samples and its corresponding characterization are listed in Table 1. As a rule, all the produced samples show high adsorption parameters (above 1300 m2/g), showing a very high proportion of micropores (higher than 94% of total pore volume). The influence of the gas flow rate is quite different for each KOH/C ratio. At low KOH/C ratio (2/1), the increasing of the flow rate does not produce activated carbons with much higher adsorption capacities (less than 2000 m2/g). This result differs from those reported in Ref. [5] where the influence of gas flow rate was found to be more marked for low KOH/C ratio.
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334
Table 1 Experimental plan and characterization results of obtained samples
K-4-N-500 K-4-N-250 K-4-N-150 K-2-N-500 K-2-N-250 K-2-N-150
KOH/C ratio
Atmosphere N2 (cc/min)
BET area (m2/g)
VMI DR (cc/g)
VME (cc/g)
VT (cc/g)
VMI/VT
Cyanide content (KCN) (g)
4:1 4:1 4:1 2:1 2:1 2:1
500 250 150 500 250 150
2376 3081 1785 1762 1393 1547
1.13 1.43 0.85 0.85 0.67 0.75
0.052 0.091 0.022 0.013 0.008 0.015
1.18 1.52 0.88 0.86 0.68 0.76
0.96 0.94 0.97 0.99 0.99 0.98
0.37 0.40 0.17 0.52 0.16 0.29
When the KOH/C ratio is increased, the gas flow rate becomes an important variable. As can be seen in Table 1 and Fig. 3, there is a maximum in adsorption capacity for 250 cc/min flow. For smaller flows, the adsorption capacity abruptly decreases. The mesoporosity, a parameter related to pore widening, seems to be more influenced by the KOH/C ratio regardless the used gas flow rate (see Fig. 4). For higher KOH/C ratio (4/1) a widening of pores is taking place and a higher volume of mesopores is formed (0.022 cc/g and higher), a phenomenon previously reported [1] in literature. However, in Table 1 can be easily observed that the VME still is very low when compared to VMI (less than 6% of total pore volume).
VME, cc/g
0.10 0.08 0.06 0.04 N2 250 cc/min
0.02 N2 500 cc/min
0.00 4:1
2:1 KOH/C ratio
Fig. 4. Comparison of mesopore volume.
3.2. The reaction mechanism
purging gas used for experimentation and became the required source for cyanide forming. According to this, we consider that nitrogen takes part in chemical reactions. Under such consideration, the initial composition of the active chemicals of the studied activation system can be extended to potassium hydroxide–carbon–nitrogen (KOH–C–N2). According to thermodynamic considerations, several reactions were identified and proposed as part of the KOH–C–N2 reaction system, among them those that
As described previously, the activated samples were washed with distillated water in the first step of the washing stage. The obtained solution was analyzed and the presence of cyanide was identified. As shown in Table 1 and Fig. 5, there was detected cyanide in all samples. The essential condition for cyanide formation is the existence of carbon and nitrogen. Nitrogen was the
3500
KOH/C 2/1 Surface area, m2/g
3000
KOH/C 4/1 2500
2000
1500
1000 100
150
200
250
300
350
400
450
Gas flow rate, cc/min Fig. 3. Influence of gas flow rate on the activation degree.
500
550
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335
Grams of KCN
0.50 0.40 0.30 0.20 0.10
KO /C 1 2:
Flow rate, cc/min
1 4:
500
/C
375
H
250
KO
150
H
0.00
Fig. 5. Cyanide in activated samples.
explain the formation of cyanide. Depending on the kind of reactants, the proposed chemical reactions were grouped in two categories. Under the first category were included the primary reactions, which reactants are KOH and carbon and eventually, but not necessarily, nitrogen. The second category consists of secondary chemical reactions,
20
which do not necessarily include among its reactants the basic chemicals of the system (KOH, carbon, nitrogen). 3.2.1. Primary reactions Fig. 6 collects the evolution of Gibbs energies as a function of temperature of primary reactions. Reaction
deltaG, kcal o
T, C 0 500 550 600 650 700 750 800 850 900 950 1000 1050 -20 -40 -60 -80 -100 -120 -140 -160 -180 3KOH
+
C
=
6KOH
+
4C
+ N 2(g)
= 2KCN (l)
6KOH
+
4C
+ N 2(g)
=
6KOH
+ 5.5C
+ N 2(g)
6KOH
+ 5.5C
2KOH
+
2KOH
+
K (l)
+ K2 CO3
+
+ 2K 2 CO3
+
3H 2(g)
(9)
+ 2K 2 CO3
+
3H 2(g)
(10)
= 2KCN (l)
+ 2K 2 CO3
+
1.5CH
4(g)
(11)
+ N 2(g)
=
+ 2K 2 CO3
+
1.5CH
4(g)
(12)
3C
+ N 2(g)
= 2KCN (l)
+
CO2(g)
+
H 2(g)
(13)
3C
+ N 2(g)
=
+
CO2(g)
+
H 2(g)
(14)
K 2(CN)
2(g)
K 2(CN) 2(g)
K 2(CN)
2(g)
1.5H
2(g)
Fig. 6. Gibbs energies of primary reactions of KOH–C–N2 system.
(7)
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(7), proposed by Lillo-Ro´denas et al. in Ref. [7], is included in the proposed reaction mechanism. The chemical reaction of KOH, carbon and nitrogen can take a few paths. In the following reaction potassium cyanide, potassium carbonate and hydrogen are among reaction products. 6KOHðlÞ + 4C + N2ðgÞ = 2KCNðlÞ + 2K2 CO3 + 3H2ðgÞ ð9Þ Reaction (9) shows a good thermodynamic premise, which is a common factor to all cyanide-forming reactions. The high consumption of both KOH (6 mol) and carbon (4 mol) is a characteristic feature of this reaction too. An alternative cyanide compound that would be formed from the same reactants that participate in reaction (9) is the K2(CN)2, as described by the following reaction: 6KOHðlÞ + 4C + N2ðgÞ = K2 (CN)2ðgÞ + 2K2 CO3 + 3H2ðgÞ ð10Þ The amounts of reactants are similar to those involved in reaction (9) and, again, K2CO3 and hydrogen are among reaction products. Thermodynamically, this reaction is more favored than reaction (9). Methane is a compound that could be formed as an alternative to the hydrogen. Methane-forming reactions would produce as well carbonate, cyanide and dicyanide. The following reactions depict these processes: 6KOHðlÞ + 5.5C + N2ðgÞ = 2KCNðlÞ + 2K2 CO3 + 1.5CH4ðgÞ ð11Þ 6KOHðlÞ +5.5C+N2ðgÞ =K2 (CN)2ðgÞ +2K2 CO3 +1.5CH4ðgÞ ð12Þ
These two reactions show an increased consumption of carbon. So far in all analyzed reactions the amount of formed potassium carbonate is the same. Other reaction paths that would produce cyanide and hydrogen, substituting the carbonate by carbon dioxide, would take place according to following reactions: 2KOHðlÞ + 3C + N2ðgÞ = 2KCNðlÞ + CO2ðgÞ + H2ðgÞ
ð13Þ
2KOHðlÞ + 3C + N2ðgÞ = K2 (CN)2ðgÞ + CO2ðgÞ + H2ðgÞ ð14Þ All the six proposed reactions support the existence of cyanide in the produced activated carbons. According to this, cyanide would be formed during the activation process and would be one of the products of the KOH–C–N2 reaction system. 3.2.2. Secondary reactions The secondary reactions describe processes in which both primary reactants and substances previously
formed in other reactions participate to form new products. One of these reactions, proposed in Ref. [7], describes the carbonation of KOH and was presented in Section 1 as reaction (8). This reaction is included in this category of secondary reactions too. The variation of Gibbs energy as a function of the temperature for this group of reactions is plotted in Fig. 7. Some peculiar reactions, which incidentally show the most favorable values of Gibbs energy in the proposed reaction mechanism, are those in which metallic potassium is involved. 10Kðl;gÞ + 6CO2ðgÞ + N2ðgÞ = 2KCNðlÞ + 4K2 CO3
ð15Þ
10Kðl;gÞ + 6CO2ðgÞ + N2ðgÞ = K2 (CN)2ðgÞ + 4K2 CO3
ð16Þ
Cyanide and dicyanide are formed along with potassium carbonate. The presence of CO2 among reactants is a particularity of these reactions and, for temperatures higher than 760 °C, all the reactants of reactions (15) and (16) would be in the gaseous phase. Another characteristic of these reactions is the large amounts of CO2 (6 mol) and metallic potassium (10 mol) that would be consumed to produce only one mole of cyanide or dicyanide but four moles of carbonate. These reactions are important since they would compete advantageously with reaction (8) in CO2 removal, avoiding the carbonation of KOH. Eter-like structures reported to be part of the chars [14] could react with KOH to form an oxide, according to the following reaction: 26KOH + C7 H16 O = 7K2 CO3 + 6K2 O + 21H2ðgÞ
ð17Þ
Two aspects characterize this chemical reaction. First, thermodynamically the intensity of the reaction (17) would grow very fast as the temperature ascends, according to the slope in Fig. 7. The second question concerning this reaction is that the consumption of KOH would be huge (26 mol). On the other side, it would produce large quantities of carbonate, hydrogen and of a new product, K2O. The extent of such reaction depends of course of the availability of the mentioned eter structure. The presence of K2O in the system constitutes another way of metallic potassium formation as shown in reaction (18). 3K2 O + C = 4KðlÞ + K2 CO3
ð18Þ
Some reaction products, as far as we know not previously reported, has been presented so far. Some of this compounds can be quiet difficult to detect because of a number of reasons. The interaction between the chemical species that are part of this system seems to be complex and intense. For example, methane can be formed as explained in reactions (12) and (13) and one could expect this gas to be easily detected in the effluent gases. However, above 650 °C, methane can react with CO2
A. Robau-Sa´nchez et al. / Microporous and Mesoporous Materials 85 (2005) 331–339
337
o
T, C
deltaG, kcal 0 500
550
600
650
700
750
800
850
900
950 1000 1050
-50 -100 -150 -200 -250 -300 -350 -400 2KOH
+
CO 2(g)
= K2CO3
+ H2O(g)
(8)
10K (l)
+
6CO 2(g)
+ N2(g)
=
+ 2KCN (l)
(15)
10K (l)
+
6CO 2(g)
+ N2(g)
= 4K2CO3
+ K 2(CN)2(g)
(16)
26KOH
+
C7H16O
= 7K2CO3
+ 6K2O
3K2O
+
C
= 4K(l)
+ K 2CO3
(18)
CH4
+
CO 2
= 2CO
+ 2H2
(19)
2KCN (l)
4K2CO3
+ 21H 2(g)
= K2(CN) 2(g)
(17)
(20)
Fig. 7. Gibbs energies of secondary reactions of KOH–C–N2 system.
to produce hydrogen and carbon monoxide, as shown in reaction (19). CH4ðgÞ + CO2ðgÞ = 2H2ðgÞ + COðgÞ
ð19Þ
This reaction might consume the methane previously formed. At the same time, the carbon dioxide is an active reactant in several reactions such as (8), (15) and (16). Moreover, the well-known Boudouard reaction (C + CO2 = 2CO) begins around 700 °C, consuming part of the available CO2 and modifying the carbon matrix. The later is a not very desirable event since such reaction would probably attack and reduce the porosity created by the potassium intercalation. The proposed reaction mechanism explains the presence of cyanide but suggest the formation of other new substances such as methane. Further work is in progress in order to demonstrate the presence of methane experimentally, as well as to characterize the evolution of CO2, CO and H2. However, the results of the analysis of effluent gases should be interpreted very carefully. In the char structure remain non pyrolized substances that, as temperature is increased, decompose and produce CO2,
CO, H2 and methane, among others. The presence of these compounds in the gases, evolved while heating Quercus agrifolia char up to 900 °C, was corroborated as part of our progressing study of the subject. The description of this characterization is beyond the scope of this work. Therefore, in order to avoid a misleading experimentation and characterization of the gaseous effluents, an additional pyrolysis of the char should be done prior to experimentation. This procedure should make sure that the detected gases are exclusively product of the chemical reactions of the activation process. 3.2.3. The cyanide presence The essential condition for cyanide formation is the existence of carbon and nitrogen. Two possible nitrogen sources would take part in cyanide-forming reactions. The first source would be the structural nitrogen, which is a component of the original char, as showed in Table 2, and the second possible source is the nitrogen of the gas flow. In order to discern the contribution of each nitrogen source to the cyanide formation, some additional experiments were done. In these experiments, the nitrogen
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338
Table 2 Proximate and elemental analysis of char
Char
Fixed carbon (%)
Volatile matter (dry basis) (%)
Ash (%)
C (%)
H (%)
O (%)
N (%)
S (%)
73.5
19.5
7.02
65.8
1.95
14.3
0.37
0.09
Table 3 Experimental conditions and characterization of argon and nitrogen series, KOH activation KOH/C ratio
K-4-Ar-500 K-4-Ar-250 K-2-Ar-500 K-2-Ar-250 K-4-N-500 K-4-N-250 K-2-N-500 K-2-N-250
4:1 4:1 2:1 2:1 4:1 4:1 2:1 2:1
Atmosphere (cc/min) N2
Ar
0 0 0 0 500 250 500 250
500 250 500 250 0 0 0 0
Vmi DR (cc/g)
Vme (cc/g)
Vt (cc/g)
BET area (m2/g)
Cyanide content (g) CN
1.13 1.36 0.78 0.74 1.13 1.43 0.85 0.67
0.066 0.060 0.011 0.011 0.052 0.091 0.013 0.008
1.19 1.42 0.79 0.75 1.18 1.52 0.86 0.68
2383 2874 1611 1533 2376 3081 1762 1393
0.08 0.04 0.08 0.05 0.37 0.40 0.52 0.16
flow was replaced by argon. The results are listed in Table 3 where, for comparison purposes, some of the results previously listed in Table 1 were included too. As can be observed, when argon is the purging gas, cyanide is formed as well. However, there is more cyanide detected when N2 is used than in the case when argon is the purging gas. These results suggest that both structural and gaseous nitrogen would participate in cyanide-forming reactions. The transformation of the liquefied KCN into the gaseous K2(CN)2 is possible in accordance with the reaction (20). 2KCNðlÞ = K2 (CN)2ðgÞ
ð20Þ
Potassium dicyanide would be then removed out of the reaction chamber by the effluent gases. This event could be one of the factors that would make the content of cyanide in activated samples to be low and random (Table 1). The values of potassium cyanide detected in activated samples are low (less than 0.5 g) despite the numerous reactions that produce this substance proposed in this work. Moreover, the cyanide content does not keep any relation with the activation parameter or developed porosity. Considering these observations, one could think that cyanide is not remaining in the samples and that the evaporation could be the main factor affecting the cyanide presence in the system. Consequently, an evaluation of the evaporation process was made applying the following equation [15]: A P 0 a fm V E ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p M R T
area of the melt in cm2); P0 is the equilibrium partial pressure of K2(CN)2 over KCN(l), in atmospheres; a is the activity coefficient of KCN(l) (assumed to be unity); fm is the molar fraction of KCN(l) in the melt; M is the molecular weight of K2(CN)2, in g/mol; R is the universal gas constant (82.06 cc atm/mol K); T is the temperature, K. From this equation, the values of the evaporation rate for a given temperature can be obtained. The plot of the evaporation rate as a function of temperature is represented in Fig. 8. The molar fraction of KCN would be difficult to establish, so this parameter was taken equal to 0.05 in the shown plot. The evaporation rate was recalculated as the amount of KCN evaporated in 60 min, the dwelling time of the experiments. The value
grams of KCN evaporated in 60 min 8 7 6 5 4
3.228
3 2 1
ðIÞ
where VE is the evaporation rate, moles per second; A is the area over which evaporation takes place (surface
0 450
500
550
600
650
700
750
800
850
900
950 1000 1050 o Temperature, C
Fig. 8. Evaluation of KCN evaporation.
A. Robau-Sa´nchez et al. / Microporous and Mesoporous Materials 85 (2005) 331–339
of evaporated KCN shown over the curve corresponds to the temperature of 760 °C. This calculation suggests that the evaporation of cyanide could be the ruling factor that keeps the content of this chemical low (less than 0.5 g) in activated samples.
339
in which each reaction contributes to the proposed reaction mechanism. The characterization of the evolved gases could be the approach that would permit the achievement of this goal.
References 4. Conclusion From the results presented in this study is possible to conclude that Quercus agrifolia char is a good starting material for the preparation of activated carbons using KOH as activating agent. Obtained activated carbons showed a very high micropore volume (up to 1.43 cc/ g) and correspondingly high surface areas (up to 3081 m2/g) with very low mesoporosity. Cyanide was detected in activated samples and it was shown that both structural and gaseous nitrogen play an active role in the activation reactions. Several reactions proposed as part of a reaction system explain the formation of cyanide and predict the existence of other reaction products (CH4) not previously reported. Further work is in progress in order to demonstrate the presence of this gas in the activation atmosphere. The KOH–C–N2 activation system is complex and the reactions proposed as part of the reaction mechanism could be controlled by different factors. The limiting-reagent regime and the diffusion processes seem to be the element that modulates the studied chemical system and consequently the extent of the porosity in produced materials. Finally, it is a challenging and interesting task for those interested in this topic to establish the proportion
[1] A. Ahmadpour, D.D. Do, Carbon 34 (1996) 471. [2] H. Li-Yeh, T. Hsisheng, Fuel Processing Technology 64 (2000) 155. [3] T. Otowa, Y. Nojima, T. Miyazaki, Carbon 35 (1997) 1315. [4] T. Otowa, R. Tanibata, M. Itoh, Gas Separation & Purification 7 (1993) 241. [5] D. Lozano-Castello, M.A. Lillo-Rodenas, D. Cazorla-Amoros, A. Linares-Solano, Carbon 39 (2001) 741. [6] D. Lozano-Castello, D. Cazorla-Amoros, A. Linares-Solano, Fuel Processing Technology 77–78 (2002) 325. [7] M.A. Lillo-Rodenas, D. Cazorla-Amoros, A. Linares-Solano, Carbon 41 (2003) 267. [8] H. Zhonghua, E.F. Vansant, Carbon 33 (1995) 561. [9] M.J.B. Evans, E. Halliop, J.A.F. MacDonald, Carbon 37 (1999) 269. [10] J. Guo, A. Chong Lua, Microporous and Mesoporous Materials 32 (1999) 111. [11] W.T. Tsai, C.Y. Chang, S.Y. Wang, C.F. Chang, S.F. Chien, H.F. Sun, Bioresource Technology 78 (2001) 203. [12] C. Moreno-Castilla, F. Carrasco-Mary´n, M.V. Lo´pez-Ramo´n, M.A. Alvarez-Merino, Carbon 39 (2001) 1415. [13] B.S. Girgis, S.S. Yunis, A.M. Soliman, Materials Letters 57 (2002) 164. [14] J. Pastor-Villegas, V. Go´mez-Serrano, C.J. Dura´n-Valle, F.J. Higes-Rolando, Journal of Analytical and Applied Pyrolysis 50 (1999) 1. [15] S. Kamath, Thesis: Cyanide–carbonate–oxygen system: kinetics and mechanisms, New Mexico Institute of Mining and Technology, Master of Science in Materials Engineering and Science, Socorro, NM, 1997.
Chemical activation of Quercus agrifolia char using KOH: Evidence of cyanide presence Alejandro Robau-Sa´nchez
a,*
, Alfredo Aguilar-Elgue´zabal a, Julia Aguilar-Pliego
b
a
b
Laboratorio de Cata´lisis, Departamento de Quı´mica de los Materiales, Centro de Investigacio´n en Materiales Avanzados, CIMAV, Miguel de Cervantes 120, Complejo Industrial Chihuahua, CP 31109, Chihuahua, Mexico Universidad Auto´noma Metropolitana-Azcapotzalco, Area de Quı´mica Aplicada edif. G-bis, San Pablo 180, Col. Reynosa-Tamaulipas, CP 02200, Del. Azcapotzalco, Me´xico DF, Mexico Received 24 May 2005; received in revised form 29 June 2005; accepted 1 July 2005 Available online 22 August 2005
Abstract Activation of Quercus agrifolia char with KOH using a rotary batch reactor is presented in this work. Several samples of activated carbon showing a very high degree of activation (up to 3081 m2/g) with predominance of microporosity were obtained. Nitrogen and argon were used as gaseous media in the activation procedure. Samples were evaluated using nitrogen adsorption applying BET and DR equation for porosity assessment. The existence of CN in activated samples indicates the occurrence of not previously reported chemical reactions in this process. According to this result, both structural and gaseous nitrogen would play an active role in the chemical reaction. Following these results, a reaction mechanism for KOH–C–N2 system is proposed. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Activated carbon; Chemical activation; Porosity; Reaction mechanism
1. Introduction Chemical activation is a well-known process as a method for activated carbon preparation. This is a complex process because of the numerous process variables and its multiple combinations. The chemical process of activation with KOH is particularly interesting because activated carbons with extremely high degrees of activation can be obtained, with porous structure in which micropores are predominant. Raw materials of both mineral and organic sources have been processed with KOH activation procedure. Among materials of mineral origin are found bitumi-
* Corresponding author. Tel.: +52 614 4391118; fax: +52 614 4391112. E-mail address: [email protected] (A. Robau-Sa´nchez).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.003
nous coal [1,2], petroleum coke [3,4] and anthracite [5–7]. Highly porous activated carbons obtained from organic and lignocellulosic materials are reported too. Some examples of these materials are walnut shells [8], sucrose char [9], oil palm stone [10], corncob [11], olive mill waste water [12], and peanut hulls [13]. The complexity of the reaction process and the variety of raw materials that can be used make the activation with KOH a challenging and interesting subject of research. As a result of the efforts dedicated to elucidate the reaction mechanism of KOH–C system, several reaction products have been identified. During the activation process, a considerable amount of K2CO3 is formed [4]. As well, hydrogen and CO2 are contained in the effluent gas. Metallic potassium is a reaction product too and above 760 °C vaporized potassium is removed out the reaction chamber by the effluent gases. The metal vapors can be observed as condenses in colder parts of the activation equipment.
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Otowa et al. [4] proposed a reaction mechanism aimed to explain the formation of above mentioned reaction products. According to the authors, the first step of the proposed mechanism would be the thermal dehydration of KOH ð1Þ
2KOHðs;lÞ = K2 OðsÞ + H2 OðgÞ
The formation of CO2 and H2 would be then the consequence of the combined effect of the following reactions: CðsÞ + H2 OðgÞ = H2ðgÞ + COðgÞ
ð2Þ
COðgÞ + H2 OðgÞ = H2ðgÞ + CO2ðgÞ
ð3Þ
The potassium carbonate would appear then as the previously formed K2O and CO2 react ð4Þ
K2 OðsÞ + CO2ðgÞ = K2 CO3ðs;lÞ
Metallic potassium would be formed by the reduction of K2O by carbon or hydrogen at high temperatures K2 OðsÞ + H2ðgÞ = 2KðlÞ + H2 OðgÞ
ð5Þ
K2 OðsÞ + CðsÞ = 2KðlÞ + CO2ðgÞ
ð6Þ
In Fig. 1 are Gibbs free energies corresponding to reactions (1), (5) and (6) which show up as the main reactions of above exposed reaction mechanism. The authors concluded that the creation of the porosity is caused by the intercalation of metallic potassium into the carbon matrix. As observed, the essential hypothesis for this reaction mechanism is the formation of K2O. However, the values of Gibbs free energy corresponding to reaction (1) suggest that K2O should be produced by another chemical reaction, not included in that proposal. Recent published papers [5,7] contain a detailed study of KOH–C system. Authors evaluated the importance of
several process variables and pointed out the role of the purging gases during the activation process. The authors report both the influence of nitrogen flow on the attained activation degree and the consequences of using other gases different than N2 (CO2 and steam). They remark that the use of CO2 should be avoided since it induces an inhibition of the activation process because of the carbonation of KOH. On the other hand, steam produces better porosity than CO2 but less than N2. The authors [7] studied the evolution of formation of K2CO3, H2, CO2 and CO, using for this purpose FTIR and TPD techniques. These experimental results, supported by theoretical thermodynamic calculations, were used to propose the following reaction mechanism. Again, aiming to explain the formation of K2CO3, H2 and metallic potassium, reaction (7) was formulated 6KOHðlÞ + CðsÞ = 2Kðg;lÞ + 2K2 CO3ðs;lÞ + 3H2ðgÞ
This reaction would explain the formation of metallic potassium, hydrogen and K2CO3. Carbonate could be also formed according to the alternative way proposed in reaction (8) 4KOHðlÞ + 2CO2ðgÞ = 2K2 CO3ðs;lÞ + 2H2 O(g)
70
deltaG, kcal
60 50
K2O + H2(g) = 2K(l) + H2O(g)
(5)
40
K2O + C = 2K(l) + CO(g)
(6)
2KOH(l) = K2O + H 2O(g)
(1)
20 10
o
T, C
0 350 -10
450
550
650
ð8Þ
Reaction (8) implies of course the availability of CO2 in the studied system. In Fig. 2 the evolution of Gibbs free energies of these reactions can be observed. The increasing of the nitrogen flow enhances the porosity, being this effect more perceptible when lower KOH-char ratios are used [5]. This phenomenon is attributed by the authors to the removal of the reaction products by the effluent gases from the reaction chamber and consequently to the displacement of the activation reaction toward the products. This effect could strongly depend on the type of furnace used and on
80
30
ð7Þ
750
850
950
-20
Fig. 1. Gibbs free energies for reactions proposed in Ref. [4].
1050
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333
5 o
T, C 0 400
450
500
550
600
650
700
750
800
850
900
950
1000 1050
deltaG, kcal
-5
-10
-15
4KOH(l) + 2CO2(g) = 2K2CO3 + 2H2O(g)
(7)
6KOH(l) + C = 2K(g,l) + 2K2CO3 + 3H2(g)
(8)
-20
-25
-30
Fig. 2. Gibbs energies for reactions proposed in Ref. [7].
the geometrical distribution of the carbon with respect to the N2 flow. In our work, the possible participation of N2 in chemical reactions that take place during the activation of char by KOH is considered. The results of the activation of Quercus agrifolia char with KOH using nitrogen as a purging gas are reported in this paper and are used to propose a new reaction mechanism.
2. Experimental Quercus agrifolia char, obtained by pyrolysis at 500 °C, was used as the precursor in this work. The char was crushed and sieved to a particle range of 0.6–1.0 m. The fixed carbon content was obtained from a TGA analysis, showing a value of 75 wt%. KOH from Baker was ground to powder and mixed physically with 20 g of char at given proportion (4/1 and 2/1). The activation was made using a stainless steel rotary batch reactor heated in a horizontal tube furnace Thermolyne F79430-70 equipped with a programmable controller. The temperature profile at the working zone was measured before experiments, using a load similar to those intended to be used during experimentation, in order to establish the temperature variation all along the zone. The temperature profile was found to be stable and the differences were less than 3 °C. The rotation speed of the reactor was 0.79 rpm. The samples were heated at 5 °C/ min from ambient temperature to the final temperature, 760 °C. At this temperature were kept for 60 min and then cooled down under the same gas (nitrogen) and flow used during the carbonization. Activated samples were washed with distillated water, 5 M HCl and distillated water again several times until a pH higher than 6 was measured. After washing, samples were dried at 110 °C overnight. In the sample nomencla-
ture are included the KOH/char ratio, the corresponding gas flow and the activation temperature. Cyanide was determined by titrimetric method using silver nitrate and potassium iodide as indicator. For the porosity characterization of the samples, physical adsorption of N2 at 77 K was carried out in an automatic system (Quantachrome Autosorb 1). Micropore volumes (VMI) have been calculated from adsorption isotherms applying the Dubinin–Radushkevich (DR) equation. Despite the unrealistic values of BET internal surface areas that characterize the obtained materials, this parameter is included in the characterization since permits a quick evaluation of activation degree. Total pore volume (VT) was determined at 0.95 p/p0 and mesopore volume (VME) was calculated as the difference between VT and VMI.
3. Results and discussion 3.1. Porosity The experimental plan related to this work included two levels of KOH/C ratio (2 and 4) and three levels of gas flow rate (150, 250 and 500 cc/min). Obtained samples and its corresponding characterization are listed in Table 1. As a rule, all the produced samples show high adsorption parameters (above 1300 m2/g), showing a very high proportion of micropores (higher than 94% of total pore volume). The influence of the gas flow rate is quite different for each KOH/C ratio. At low KOH/C ratio (2/1), the increasing of the flow rate does not produce activated carbons with much higher adsorption capacities (less than 2000 m2/g). This result differs from those reported in Ref. [5] where the influence of gas flow rate was found to be more marked for low KOH/C ratio.
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334
Table 1 Experimental plan and characterization results of obtained samples
K-4-N-500 K-4-N-250 K-4-N-150 K-2-N-500 K-2-N-250 K-2-N-150
KOH/C ratio
Atmosphere N2 (cc/min)
BET area (m2/g)
VMI DR (cc/g)
VME (cc/g)
VT (cc/g)
VMI/VT
Cyanide content (KCN) (g)
4:1 4:1 4:1 2:1 2:1 2:1
500 250 150 500 250 150
2376 3081 1785 1762 1393 1547
1.13 1.43 0.85 0.85 0.67 0.75
0.052 0.091 0.022 0.013 0.008 0.015
1.18 1.52 0.88 0.86 0.68 0.76
0.96 0.94 0.97 0.99 0.99 0.98
0.37 0.40 0.17 0.52 0.16 0.29
When the KOH/C ratio is increased, the gas flow rate becomes an important variable. As can be seen in Table 1 and Fig. 3, there is a maximum in adsorption capacity for 250 cc/min flow. For smaller flows, the adsorption capacity abruptly decreases. The mesoporosity, a parameter related to pore widening, seems to be more influenced by the KOH/C ratio regardless the used gas flow rate (see Fig. 4). For higher KOH/C ratio (4/1) a widening of pores is taking place and a higher volume of mesopores is formed (0.022 cc/g and higher), a phenomenon previously reported [1] in literature. However, in Table 1 can be easily observed that the VME still is very low when compared to VMI (less than 6% of total pore volume).
VME, cc/g
0.10 0.08 0.06 0.04 N2 250 cc/min
0.02 N2 500 cc/min
0.00 4:1
2:1 KOH/C ratio
Fig. 4. Comparison of mesopore volume.
3.2. The reaction mechanism
purging gas used for experimentation and became the required source for cyanide forming. According to this, we consider that nitrogen takes part in chemical reactions. Under such consideration, the initial composition of the active chemicals of the studied activation system can be extended to potassium hydroxide–carbon–nitrogen (KOH–C–N2). According to thermodynamic considerations, several reactions were identified and proposed as part of the KOH–C–N2 reaction system, among them those that
As described previously, the activated samples were washed with distillated water in the first step of the washing stage. The obtained solution was analyzed and the presence of cyanide was identified. As shown in Table 1 and Fig. 5, there was detected cyanide in all samples. The essential condition for cyanide formation is the existence of carbon and nitrogen. Nitrogen was the
3500
KOH/C 2/1 Surface area, m2/g
3000
KOH/C 4/1 2500
2000
1500
1000 100
150
200
250
300
350
400
450
Gas flow rate, cc/min Fig. 3. Influence of gas flow rate on the activation degree.
500
550
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335
Grams of KCN
0.50 0.40 0.30 0.20 0.10
KO /C 1 2:
Flow rate, cc/min
1 4:
500
/C
375
H
250
KO
150
H
0.00
Fig. 5. Cyanide in activated samples.
explain the formation of cyanide. Depending on the kind of reactants, the proposed chemical reactions were grouped in two categories. Under the first category were included the primary reactions, which reactants are KOH and carbon and eventually, but not necessarily, nitrogen. The second category consists of secondary chemical reactions,
20
which do not necessarily include among its reactants the basic chemicals of the system (KOH, carbon, nitrogen). 3.2.1. Primary reactions Fig. 6 collects the evolution of Gibbs energies as a function of temperature of primary reactions. Reaction
deltaG, kcal o
T, C 0 500 550 600 650 700 750 800 850 900 950 1000 1050 -20 -40 -60 -80 -100 -120 -140 -160 -180 3KOH
+
C
=
6KOH
+
4C
+ N 2(g)
= 2KCN (l)
6KOH
+
4C
+ N 2(g)
=
6KOH
+ 5.5C
+ N 2(g)
6KOH
+ 5.5C
2KOH
+
2KOH
+
K (l)
+ K2 CO3
+
+ 2K 2 CO3
+
3H 2(g)
(9)
+ 2K 2 CO3
+
3H 2(g)
(10)
= 2KCN (l)
+ 2K 2 CO3
+
1.5CH
4(g)
(11)
+ N 2(g)
=
+ 2K 2 CO3
+
1.5CH
4(g)
(12)
3C
+ N 2(g)
= 2KCN (l)
+
CO2(g)
+
H 2(g)
(13)
3C
+ N 2(g)
=
+
CO2(g)
+
H 2(g)
(14)
K 2(CN)
2(g)
K 2(CN) 2(g)
K 2(CN)
2(g)
1.5H
2(g)
Fig. 6. Gibbs energies of primary reactions of KOH–C–N2 system.
(7)
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(7), proposed by Lillo-Ro´denas et al. in Ref. [7], is included in the proposed reaction mechanism. The chemical reaction of KOH, carbon and nitrogen can take a few paths. In the following reaction potassium cyanide, potassium carbonate and hydrogen are among reaction products. 6KOHðlÞ + 4C + N2ðgÞ = 2KCNðlÞ + 2K2 CO3 + 3H2ðgÞ ð9Þ Reaction (9) shows a good thermodynamic premise, which is a common factor to all cyanide-forming reactions. The high consumption of both KOH (6 mol) and carbon (4 mol) is a characteristic feature of this reaction too. An alternative cyanide compound that would be formed from the same reactants that participate in reaction (9) is the K2(CN)2, as described by the following reaction: 6KOHðlÞ + 4C + N2ðgÞ = K2 (CN)2ðgÞ + 2K2 CO3 + 3H2ðgÞ ð10Þ The amounts of reactants are similar to those involved in reaction (9) and, again, K2CO3 and hydrogen are among reaction products. Thermodynamically, this reaction is more favored than reaction (9). Methane is a compound that could be formed as an alternative to the hydrogen. Methane-forming reactions would produce as well carbonate, cyanide and dicyanide. The following reactions depict these processes: 6KOHðlÞ + 5.5C + N2ðgÞ = 2KCNðlÞ + 2K2 CO3 + 1.5CH4ðgÞ ð11Þ 6KOHðlÞ +5.5C+N2ðgÞ =K2 (CN)2ðgÞ +2K2 CO3 +1.5CH4ðgÞ ð12Þ
These two reactions show an increased consumption of carbon. So far in all analyzed reactions the amount of formed potassium carbonate is the same. Other reaction paths that would produce cyanide and hydrogen, substituting the carbonate by carbon dioxide, would take place according to following reactions: 2KOHðlÞ + 3C + N2ðgÞ = 2KCNðlÞ + CO2ðgÞ + H2ðgÞ
ð13Þ
2KOHðlÞ + 3C + N2ðgÞ = K2 (CN)2ðgÞ + CO2ðgÞ + H2ðgÞ ð14Þ All the six proposed reactions support the existence of cyanide in the produced activated carbons. According to this, cyanide would be formed during the activation process and would be one of the products of the KOH–C–N2 reaction system. 3.2.2. Secondary reactions The secondary reactions describe processes in which both primary reactants and substances previously
formed in other reactions participate to form new products. One of these reactions, proposed in Ref. [7], describes the carbonation of KOH and was presented in Section 1 as reaction (8). This reaction is included in this category of secondary reactions too. The variation of Gibbs energy as a function of the temperature for this group of reactions is plotted in Fig. 7. Some peculiar reactions, which incidentally show the most favorable values of Gibbs energy in the proposed reaction mechanism, are those in which metallic potassium is involved. 10Kðl;gÞ + 6CO2ðgÞ + N2ðgÞ = 2KCNðlÞ + 4K2 CO3
ð15Þ
10Kðl;gÞ + 6CO2ðgÞ + N2ðgÞ = K2 (CN)2ðgÞ + 4K2 CO3
ð16Þ
Cyanide and dicyanide are formed along with potassium carbonate. The presence of CO2 among reactants is a particularity of these reactions and, for temperatures higher than 760 °C, all the reactants of reactions (15) and (16) would be in the gaseous phase. Another characteristic of these reactions is the large amounts of CO2 (6 mol) and metallic potassium (10 mol) that would be consumed to produce only one mole of cyanide or dicyanide but four moles of carbonate. These reactions are important since they would compete advantageously with reaction (8) in CO2 removal, avoiding the carbonation of KOH. Eter-like structures reported to be part of the chars [14] could react with KOH to form an oxide, according to the following reaction: 26KOH + C7 H16 O = 7K2 CO3 + 6K2 O + 21H2ðgÞ
ð17Þ
Two aspects characterize this chemical reaction. First, thermodynamically the intensity of the reaction (17) would grow very fast as the temperature ascends, according to the slope in Fig. 7. The second question concerning this reaction is that the consumption of KOH would be huge (26 mol). On the other side, it would produce large quantities of carbonate, hydrogen and of a new product, K2O. The extent of such reaction depends of course of the availability of the mentioned eter structure. The presence of K2O in the system constitutes another way of metallic potassium formation as shown in reaction (18). 3K2 O + C = 4KðlÞ + K2 CO3
ð18Þ
Some reaction products, as far as we know not previously reported, has been presented so far. Some of this compounds can be quiet difficult to detect because of a number of reasons. The interaction between the chemical species that are part of this system seems to be complex and intense. For example, methane can be formed as explained in reactions (12) and (13) and one could expect this gas to be easily detected in the effluent gases. However, above 650 °C, methane can react with CO2
A. Robau-Sa´nchez et al. / Microporous and Mesoporous Materials 85 (2005) 331–339
337
o
T, C
deltaG, kcal 0 500
550
600
650
700
750
800
850
900
950 1000 1050
-50 -100 -150 -200 -250 -300 -350 -400 2KOH
+
CO 2(g)
= K2CO3
+ H2O(g)
(8)
10K (l)
+
6CO 2(g)
+ N2(g)
=
+ 2KCN (l)
(15)
10K (l)
+
6CO 2(g)
+ N2(g)
= 4K2CO3
+ K 2(CN)2(g)
(16)
26KOH
+
C7H16O
= 7K2CO3
+ 6K2O
3K2O
+
C
= 4K(l)
+ K 2CO3
(18)
CH4
+
CO 2
= 2CO
+ 2H2
(19)
2KCN (l)
4K2CO3
+ 21H 2(g)
= K2(CN) 2(g)
(17)
(20)
Fig. 7. Gibbs energies of secondary reactions of KOH–C–N2 system.
to produce hydrogen and carbon monoxide, as shown in reaction (19). CH4ðgÞ + CO2ðgÞ = 2H2ðgÞ + COðgÞ
ð19Þ
This reaction might consume the methane previously formed. At the same time, the carbon dioxide is an active reactant in several reactions such as (8), (15) and (16). Moreover, the well-known Boudouard reaction (C + CO2 = 2CO) begins around 700 °C, consuming part of the available CO2 and modifying the carbon matrix. The later is a not very desirable event since such reaction would probably attack and reduce the porosity created by the potassium intercalation. The proposed reaction mechanism explains the presence of cyanide but suggest the formation of other new substances such as methane. Further work is in progress in order to demonstrate the presence of methane experimentally, as well as to characterize the evolution of CO2, CO and H2. However, the results of the analysis of effluent gases should be interpreted very carefully. In the char structure remain non pyrolized substances that, as temperature is increased, decompose and produce CO2,
CO, H2 and methane, among others. The presence of these compounds in the gases, evolved while heating Quercus agrifolia char up to 900 °C, was corroborated as part of our progressing study of the subject. The description of this characterization is beyond the scope of this work. Therefore, in order to avoid a misleading experimentation and characterization of the gaseous effluents, an additional pyrolysis of the char should be done prior to experimentation. This procedure should make sure that the detected gases are exclusively product of the chemical reactions of the activation process. 3.2.3. The cyanide presence The essential condition for cyanide formation is the existence of carbon and nitrogen. Two possible nitrogen sources would take part in cyanide-forming reactions. The first source would be the structural nitrogen, which is a component of the original char, as showed in Table 2, and the second possible source is the nitrogen of the gas flow. In order to discern the contribution of each nitrogen source to the cyanide formation, some additional experiments were done. In these experiments, the nitrogen
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338
Table 2 Proximate and elemental analysis of char
Char
Fixed carbon (%)
Volatile matter (dry basis) (%)
Ash (%)
C (%)
H (%)
O (%)
N (%)
S (%)
73.5
19.5
7.02
65.8
1.95
14.3
0.37
0.09
Table 3 Experimental conditions and characterization of argon and nitrogen series, KOH activation KOH/C ratio
K-4-Ar-500 K-4-Ar-250 K-2-Ar-500 K-2-Ar-250 K-4-N-500 K-4-N-250 K-2-N-500 K-2-N-250
4:1 4:1 2:1 2:1 4:1 4:1 2:1 2:1
Atmosphere (cc/min) N2
Ar
0 0 0 0 500 250 500 250
500 250 500 250 0 0 0 0
Vmi DR (cc/g)
Vme (cc/g)
Vt (cc/g)
BET area (m2/g)
Cyanide content (g) CN
1.13 1.36 0.78 0.74 1.13 1.43 0.85 0.67
0.066 0.060 0.011 0.011 0.052 0.091 0.013 0.008
1.19 1.42 0.79 0.75 1.18 1.52 0.86 0.68
2383 2874 1611 1533 2376 3081 1762 1393
0.08 0.04 0.08 0.05 0.37 0.40 0.52 0.16
flow was replaced by argon. The results are listed in Table 3 where, for comparison purposes, some of the results previously listed in Table 1 were included too. As can be observed, when argon is the purging gas, cyanide is formed as well. However, there is more cyanide detected when N2 is used than in the case when argon is the purging gas. These results suggest that both structural and gaseous nitrogen would participate in cyanide-forming reactions. The transformation of the liquefied KCN into the gaseous K2(CN)2 is possible in accordance with the reaction (20). 2KCNðlÞ = K2 (CN)2ðgÞ
ð20Þ
Potassium dicyanide would be then removed out of the reaction chamber by the effluent gases. This event could be one of the factors that would make the content of cyanide in activated samples to be low and random (Table 1). The values of potassium cyanide detected in activated samples are low (less than 0.5 g) despite the numerous reactions that produce this substance proposed in this work. Moreover, the cyanide content does not keep any relation with the activation parameter or developed porosity. Considering these observations, one could think that cyanide is not remaining in the samples and that the evaporation could be the main factor affecting the cyanide presence in the system. Consequently, an evaluation of the evaporation process was made applying the following equation [15]: A P 0 a fm V E ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2p M R T
area of the melt in cm2); P0 is the equilibrium partial pressure of K2(CN)2 over KCN(l), in atmospheres; a is the activity coefficient of KCN(l) (assumed to be unity); fm is the molar fraction of KCN(l) in the melt; M is the molecular weight of K2(CN)2, in g/mol; R is the universal gas constant (82.06 cc atm/mol K); T is the temperature, K. From this equation, the values of the evaporation rate for a given temperature can be obtained. The plot of the evaporation rate as a function of temperature is represented in Fig. 8. The molar fraction of KCN would be difficult to establish, so this parameter was taken equal to 0.05 in the shown plot. The evaporation rate was recalculated as the amount of KCN evaporated in 60 min, the dwelling time of the experiments. The value
grams of KCN evaporated in 60 min 8 7 6 5 4
3.228
3 2 1
ðIÞ
where VE is the evaporation rate, moles per second; A is the area over which evaporation takes place (surface
0 450
500
550
600
650
700
750
800
850
900
950 1000 1050 o Temperature, C
Fig. 8. Evaluation of KCN evaporation.
A. Robau-Sa´nchez et al. / Microporous and Mesoporous Materials 85 (2005) 331–339
of evaporated KCN shown over the curve corresponds to the temperature of 760 °C. This calculation suggests that the evaporation of cyanide could be the ruling factor that keeps the content of this chemical low (less than 0.5 g) in activated samples.
339
in which each reaction contributes to the proposed reaction mechanism. The characterization of the evolved gases could be the approach that would permit the achievement of this goal.
References 4. Conclusion From the results presented in this study is possible to conclude that Quercus agrifolia char is a good starting material for the preparation of activated carbons using KOH as activating agent. Obtained activated carbons showed a very high micropore volume (up to 1.43 cc/ g) and correspondingly high surface areas (up to 3081 m2/g) with very low mesoporosity. Cyanide was detected in activated samples and it was shown that both structural and gaseous nitrogen play an active role in the activation reactions. Several reactions proposed as part of a reaction system explain the formation of cyanide and predict the existence of other reaction products (CH4) not previously reported. Further work is in progress in order to demonstrate the presence of this gas in the activation atmosphere. The KOH–C–N2 activation system is complex and the reactions proposed as part of the reaction mechanism could be controlled by different factors. The limiting-reagent regime and the diffusion processes seem to be the element that modulates the studied chemical system and consequently the extent of the porosity in produced materials. Finally, it is a challenging and interesting task for those interested in this topic to establish the proportion
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