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The influence of the impregnation method on yield of activated carbon produced by H3PO4 activation
Materials Letters 65 (2011) 1423–1426
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The influence of the impregnation method on yield of activated carbon produced by H3PO4 activation José Miguel González-Domínguez a,b, Carmen Fernández-González a, María Alexandre-Franco a, Alejandro Ansón-Casaos b, Vicente Gómez-Serrano a,⁎ a b
Departamento de Química Orgánica e Inorgánica, UEx, Badajoz 06071, Spain Grupo de Nanoestructuras de Carbono y Nanotecnología, Instituto de Carboquímica, CSIC, Zaragoza 50018, Spain
a r t i c l e
i n f o
Article history: Received 23 August 2010 Accepted 4 February 2011 Available online 17 February 2011 Keywords: Carbon materials Phosphoric acid activation Yields
a b s t r a c t The influence of the impregnation method on producing activated carbon (AC) from cherry stones (CS) using H3PO4 is studied. Mass changes associated with the impregnation, carbonization and washing processes were measured. With H3PO4 dilute solutions, the loading of substance on CS increases with concentration, in particular when the impregnation is effected in a single step instead of in successive steps and especially when using CS chars. The opposite applies when filtering the residual liquid and washing the resulting product. With the concentrated solution, regardless of whether it is previously oven-drying at 120 °C or not, most H3PO4 is loaded on CS. The concentration of the H3PO4 solution seems to control the processes of impregnation, carbonization and washing in the preparation of AC from CS by H3PO4 chemical activation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Activated carbon (AC) is widely used on an industrial scale as an adsorbent mainly in the purification/separation of liquids and gases and also as catalyst and catalyst support [1]. Furthermore, newer applications are ever emerging, particularly those concerning environmental protection and technological development. AC is commonly prepared by the well-known methods of physical activation and chemical activation. In the latter method, H3PO4 is the most widely used impregnation agent. Using this substance, most of the numerous studies carried out so far on the preparation of AC have focused on the influence of concentration of the impregnation solution and soaking temperature on the porous structure. Because of the highly polar character of H3PO4 and hence the control of the physical and chemical interactions occurring in the bulk of the solution and with the substratum during the impregnation treatment, the solution concentration is likely to be the primary factor of the activation process. Here, dilute and concentrated solutions of H3PO4 are used in the impregnation of an AC precursor (CS, 1–2 mm particle size; soaked in 5% v/v H2SO4), the influence of the impregnation method on the loading of H3PO4 is investigated. The tested methods include the previous modification of the composition of CS by heat treatment at low temperatures. The mass changes are analyzed and
⁎ Corresponding author. Fax: +34 924 271149. E-mail address: [email protected] (V. Gómez-Serrano). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.022
correlated with those produced in the carbonization and washing stages of the overall process of preparation of AC.
2. Experimental The impregnation of CS or ST (i.e. two chars prepared by heating CS at 200 and 300 °C for 2 h in N2, flow = 100 mL/min) was effected by mixing 25 g of substratum and 100 mL of H3PO4 solution, heating at 85 °C for 2 h and oven-drying at 120 °C for 24 h, instead of in a single heat treatment [2]. Here, three series of impregnated products (IP) were prepared using (see Table 1): • CS and a dilute H3PO4 solution (stock solution) and by carrying out the impregnation process once, twice or three times with more dilute H3PO4 solutions prepared from it, and by filtering the supernatant liquid, rinsing the resulting product with distilled water or with the filtrate (Series 1). In the successive impregnations, the resulting heterogeneous mixture after heating at 85 °C was always oven-drying prior to carrying out the subsequent impregnation. • CS and a concentrated H3PO4 solution and by effecting the impregnation not only with such a solution but also with the more concentrated solutions prepared previously from it by ovendrying at 120 °C for 2–24 h (Series 2). • ST and dilute and concentrated H3PO4 solutions (Series 3). The IP were heated at 500 °C for 2 h in N2 (100 mL/min) and the resulting carbonized products (CP) were thoroughly washed with
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Table 1 Preparation of AC from CS by H3PO4 chemical activation. Yields. H3PO4/g/100 mL
t/h
16 5.33 5.33 10.66 5.33 16 16 n-d 144.5
NI
Filtering
Washing
T/°C
IP codes
Series
Δm/g
Yi
Yc
Yw
1
200 300 200 300
D DL1 DL2 DI1 DL3 DF DFW DRL C0 C2 C6 C12 C24 S200D S300D S200C S300C
7.30 1.20a 1.85a 4.43 2.30a 1.34 0.48 n-d 135.54 143.20 126.29 127.45 134.96 9.09 15.04 132.80 n-d
45.6 22.5 34.7 41.6 43.1 8.4 3.0 n-d 93.8 99.1 87.4 88.2 93.4 56.8 94.0 91.9 n-d
62.7 50.3 60.4 56.7 64.6 51.1 28.6 54.2 27.9 29.5 25.1 30.3 33.5 61.6 66.9 30.5 65.3
71.0 92.8 67.5 79.0 57.3 94.1 97.8 57.1 27.4 26.9 31.8 25.6 23.9 64.3 78.7 19.4 66.1
2 3 Yes Yes
Yes
0 2 6 12 24
16 16 144.5 144.5
2
3
Abbreviations: n-d, not-determined; t, oven-drying time; NI, number of impregnations; and T, semi-carbonization temperature. IP codes: D, dilute solution; L, low concentration; I, intermediate concentration; F, filtration; FW, filtration and washing; RL, residual liquid; C, concentrated solution; and S, semicarbonized product. a Average values of duplicate impregnation treatments.
3. Calculations The yields of the processes of impregnation (Yi), carbonization (Yc), and washing (Yw) were calculated by expressions (1)–(4): Δm = mf −m0
ðmf Nm0 Þ
Yi = Δm = P0 × 100 Yc = Mf = M0 × 100 Yw = Cf = C0
ð1Þ ð2Þ
ðMf bM0 Þ
ðCf bC0 Þ
ð3Þ ð4Þ
where m0 stands for the initial mass of CS (or of previously H3PO4impregnated CS) or ST used in the impregnation with the H3PO4 solution, mf for the mass of IP, P0 for initial mass of H3PO4 present in the impregnation solution, M0 for the mass of IP carbonized at 500 °C, Mf for the mass of CP, C0 for the mass of CP washed with distilled water, and Cf for the mass of AC. Δm is the increase in the mass of CS or ST after its impregnation with H3PO4. Ideally, Δm should be equal to the amount of H3PO4 loaded on CS or ST and P0 − Δm to the amount of H3PO4 lost during the impregnation treatments carried out at 85 °C and 120 °C. Table 1 lists the calculated values of Yi, Yh, and Yw. 4. Results and discussion 4.1. Yield of the impregnation process (Yi) For Series 1, Yi greatly ranges between 45.6 wt.% and 3.0 wt.%. Furthermore, it varies by D N DI1 N DL1 and DL3 N DL2 N DL1 (Fig. 1). The increase in the loading of H3PO4 on CS with a greater presence of substance either in solution or in CS was likely connected with the high tendency exhibited by H3PO4 to association by molecular condensation. The much lower Yi for DF than for D indicates that the loading of H3PO4 largely occurred during the oven-drying treatment at 120 °C, instead of during the heat treatment at 85 °C. The substance to a large extent should bind weakly to CS and thereby it was leached out by simply washing with distilled water as Yi is as low as 3.0 wt.% for DFW. Then, owing to the presumably high H3PO4
content in the filtrate, the preparation of DRL was undertaken. For this sample, the calculated values of Yi and Δm are 41.6 wt.% and 3.62 g. The values of Yi and Δm for DL1, DI1, D, and C0 show a great influence of the H3PO4 concentration on the amount of H3PO4 loaded on CS. This is 18.6 times higher for C0 than for D, whereas the content of H3PO4 in the impregnation solution was only 9 times higher for C0. Thus, most of the large amount of H3PO4 present in the initial impregnation solution was loaded on CS. It also holds for C2–C24. From these results it is clear that with the concentrated H3PO4 solution a synergic effect occurred in the impregnation of CS which was likely connected with the strong dependence of the molecular association of H3PO4 on the concentration of its aqueous solutions. In the preparation of C2–C24 it was observed that, after oven-drying the H3PO4 solution for 2–24 h, the volume reduction was as low as ≈10% at most. Therefore, the vaporization of water greatly occurred only after the oven-heated H3PO4 solution was brought into contact with CS. Perhaps as a result of the loading of H3PO4 on CS, the supernatant
Δm, single impregnations 8
Cumulative Δm, successive impregnations Δm, successive impregnations
Mass increase (g)
distilled water in a Soxhlet extractor and oven-dried to obtain the activated carbons (AC).
D
6 DI1 DL3 4
DL2 2
DL3 DL2 DL1
0
Process variables
Fig. 1. Impregnation of CS with dilute H3PO4 solutions. Variation of the mass increase with process variables: H3PO4 content (g)/100 mL (squares), cumulative H3PO4 contents (g)/100 mL (diamonds), and number of impregnation (circles).
J.M. González-Domínguez et al. / Materials Letters 65 (2011) 1423–1426
100
DFW
DF DL1
80
DI1 DL2 DL3
C6 30
C0
S300D DL3 S200D
DI1 DRL
Yc (%)
DF 50
C2
C12
25
C24 20
15
S200C
0
10
20 120
130
140
150
Fig. 3. Variation of the water-washing yield with mass increase.
For C0–C24, Yc ranges between 27.9 and 33.5 wt.%. The great mass loss originated by the carbonization of C0–C24 must be largely associated with the removal of phosphorus species, which were thermally unstable because of the formation of weaker bonds with CS on account of a larger degree of association between the phosphorus species in the impregnated state of the IP. Nevertheless, it should be stated that the calculated content of phosphorus species in the C0derived CP is still as high as ≈84.4 wt.%. For S200D and S200C, the Yc values are of the same magnitude order as for D and C0 respectively. The significantly higher Yc for S300D than for D is in line with the markedly higher Δm for the former sample. For S300C, however, Yc is much higher than for C0–C24. 4.3. Yield of the washing process (Yw) As shown in Fig. 3, Yw decreases almost linearly with Δm. This was expected as the mass loss produced by washing of the IP-derived CP with distilled water is attributable to lixiviation of phosphorus species, which are very polar ions/molecules and therefore highly soluble in water (i.e. 548 g H3PO4/100 g H2O and P4O10 reacts with water). Perhaps, the solution process was favored as a result of a decrease in size of phosphorus species present in the PI because of the carbonization process. Nevertheless, it should be pointed out that the estimated content of phosphorus species remaining in the ACs is high. Thus, as a guide, it is 51 wt.% for D and 43 wt.% for C0.
70
D
DRL
Mass increase (g)
The much higher Yc for the IP prepared using the dilute H3PO4 solutions (i.e. usually N50 wt.%, Fig. 2) than for CS (i.e. 25.4–28.7 wt.% [3]) is indicative of a great thermal stabilization of the volatile fraction of CS as a result of the loading of H3PO4 [4]. Since Yc varies by D N DI1 N DL1 and DL3 N DL2 N DL1, the thermal stability increased as Δm increased. However, Yc is higher for DL2 than for DI1 and for DL3 than for D, and this proves that the method of impregnation also influenced the thermal stability, this being higher when the process was carried out in successive steps. The close values of Yc for DF and DL1 are in line with the values of Δm for this IP and reveal that the phosphorus species most strongly bound to CS, which were responsible for the higher thermal stability of the IP, were loaded on CS during the impregnation treatment performed at 85 °C. However, such species were very sensitive to the subsequent washing with distilled water as Yc was as low as 28.6 wt.% for DRL. The similar Yc for DRL and DI1 is consistent with calculated Δm and Yi values for DRL.
DL2
D S200D
60
4.2. Yield of the carbonization process (Yc)
60
S300D
Yw (%)
diluted and this facilitated the vaporization of water to dryness. For the dilute solutions, the solute-solvent interactions occurring in the bulk of the solution should be stronger than for the concentrated solution because of the smaller degree of H3PO4 association, and this should have an unfavorable effect on the loading of H3PO4 on CS. In this case, not only water but also H3PO4 should vaporize during the oven-drying treatment. The effect of the previous charring of CS at 200 or 300 °C on the degree of H3PO4 loading was dependent on the concentration of H3PO4 and on the charring temperature of CS. Notice that Δm and Yi increase for S200D and mostly for S300D as compared to D. These results denote that the removal of a fraction of volatile matter from CS resulted in the creation of active sites only for the loading of small size phosphorus species. Furthermore, it was enhanced with the rise of the charring temperature. For larger phosphorus species, however, such sites either were not accessible or the chemical affinity was low, in particular when S300 was used. With this char, the remaining liquid after the first impregnation at 85 °C was not completely removed by oven-drying at 120 °C and as a result it was necessary to separate it by filtration before oven-drying again.
1425
DL1
5. Conclusions
35 C24
S200C
C12
30
C2 DFW
C0 C6
25 0
10
20
130
140
Mass increase (g) Fig. 2. Variation of the carbonization yield with mass increase.
150
The method of impregnation has a marked influence on the yields of the process of preparation of AC from CS by H3PO4 chemical activation provided that a dilute H3PO4 solution is used and that CS chars are impregnated with H3PO4. However, the previous ovendrying at 120 °C of the concentrated acid solution for further concentration of the impregnation solution has less influence on yields. With the increase in the concentration of the impregnation solution, it seems that the H3PO4–H3PO4 interactions progressively predominate over the H3PO4–substrate interactions. It promotes the loading of H3PO4, decreases the thermal stability of H3PO4 in the impregnation state of the IP and favors the leaching of H3PO4 by washing with distilled water in a Soxhlet equipment.
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References [1] Marsh H, Rodríguez-Reinoso F. Activated carbon. Amsterdam: Elsevier; 2006. p. 454–508. [2] Molina-Sabio M, Rodríguez-Reinoso F, Caturla F, Sellés MJ. Carbon 1995;33(8): 1105–13.
[3] Gómez-Serrano V, Piriz-Almeida F, Durán-Valle CJ, Pastor-Villegas J. Carbon 1999;37(10):1517–28. [4] Olivares-Marín M, Fernández-González C, Macías-García A, Gómez-Serrano V. Carbon 2006;44:2347–50.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The influence of the impregnation method on yield of activated carbon produced by H3PO4 activation José Miguel González-Domínguez a,b, Carmen Fernández-González a, María Alexandre-Franco a, Alejandro Ansón-Casaos b, Vicente Gómez-Serrano a,⁎ a b
Departamento de Química Orgánica e Inorgánica, UEx, Badajoz 06071, Spain Grupo de Nanoestructuras de Carbono y Nanotecnología, Instituto de Carboquímica, CSIC, Zaragoza 50018, Spain
a r t i c l e
i n f o
Article history: Received 23 August 2010 Accepted 4 February 2011 Available online 17 February 2011 Keywords: Carbon materials Phosphoric acid activation Yields
a b s t r a c t The influence of the impregnation method on producing activated carbon (AC) from cherry stones (CS) using H3PO4 is studied. Mass changes associated with the impregnation, carbonization and washing processes were measured. With H3PO4 dilute solutions, the loading of substance on CS increases with concentration, in particular when the impregnation is effected in a single step instead of in successive steps and especially when using CS chars. The opposite applies when filtering the residual liquid and washing the resulting product. With the concentrated solution, regardless of whether it is previously oven-drying at 120 °C or not, most H3PO4 is loaded on CS. The concentration of the H3PO4 solution seems to control the processes of impregnation, carbonization and washing in the preparation of AC from CS by H3PO4 chemical activation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Activated carbon (AC) is widely used on an industrial scale as an adsorbent mainly in the purification/separation of liquids and gases and also as catalyst and catalyst support [1]. Furthermore, newer applications are ever emerging, particularly those concerning environmental protection and technological development. AC is commonly prepared by the well-known methods of physical activation and chemical activation. In the latter method, H3PO4 is the most widely used impregnation agent. Using this substance, most of the numerous studies carried out so far on the preparation of AC have focused on the influence of concentration of the impregnation solution and soaking temperature on the porous structure. Because of the highly polar character of H3PO4 and hence the control of the physical and chemical interactions occurring in the bulk of the solution and with the substratum during the impregnation treatment, the solution concentration is likely to be the primary factor of the activation process. Here, dilute and concentrated solutions of H3PO4 are used in the impregnation of an AC precursor (CS, 1–2 mm particle size; soaked in 5% v/v H2SO4), the influence of the impregnation method on the loading of H3PO4 is investigated. The tested methods include the previous modification of the composition of CS by heat treatment at low temperatures. The mass changes are analyzed and
⁎ Corresponding author. Fax: +34 924 271149. E-mail address: [email protected] (V. Gómez-Serrano). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.022
correlated with those produced in the carbonization and washing stages of the overall process of preparation of AC.
2. Experimental The impregnation of CS or ST (i.e. two chars prepared by heating CS at 200 and 300 °C for 2 h in N2, flow = 100 mL/min) was effected by mixing 25 g of substratum and 100 mL of H3PO4 solution, heating at 85 °C for 2 h and oven-drying at 120 °C for 24 h, instead of in a single heat treatment [2]. Here, three series of impregnated products (IP) were prepared using (see Table 1): • CS and a dilute H3PO4 solution (stock solution) and by carrying out the impregnation process once, twice or three times with more dilute H3PO4 solutions prepared from it, and by filtering the supernatant liquid, rinsing the resulting product with distilled water or with the filtrate (Series 1). In the successive impregnations, the resulting heterogeneous mixture after heating at 85 °C was always oven-drying prior to carrying out the subsequent impregnation. • CS and a concentrated H3PO4 solution and by effecting the impregnation not only with such a solution but also with the more concentrated solutions prepared previously from it by ovendrying at 120 °C for 2–24 h (Series 2). • ST and dilute and concentrated H3PO4 solutions (Series 3). The IP were heated at 500 °C for 2 h in N2 (100 mL/min) and the resulting carbonized products (CP) were thoroughly washed with
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J.M. González-Domínguez et al. / Materials Letters 65 (2011) 1423–1426
Table 1 Preparation of AC from CS by H3PO4 chemical activation. Yields. H3PO4/g/100 mL
t/h
16 5.33 5.33 10.66 5.33 16 16 n-d 144.5
NI
Filtering
Washing
T/°C
IP codes
Series
Δm/g
Yi
Yc
Yw
1
200 300 200 300
D DL1 DL2 DI1 DL3 DF DFW DRL C0 C2 C6 C12 C24 S200D S300D S200C S300C
7.30 1.20a 1.85a 4.43 2.30a 1.34 0.48 n-d 135.54 143.20 126.29 127.45 134.96 9.09 15.04 132.80 n-d
45.6 22.5 34.7 41.6 43.1 8.4 3.0 n-d 93.8 99.1 87.4 88.2 93.4 56.8 94.0 91.9 n-d
62.7 50.3 60.4 56.7 64.6 51.1 28.6 54.2 27.9 29.5 25.1 30.3 33.5 61.6 66.9 30.5 65.3
71.0 92.8 67.5 79.0 57.3 94.1 97.8 57.1 27.4 26.9 31.8 25.6 23.9 64.3 78.7 19.4 66.1
2 3 Yes Yes
Yes
0 2 6 12 24
16 16 144.5 144.5
2
3
Abbreviations: n-d, not-determined; t, oven-drying time; NI, number of impregnations; and T, semi-carbonization temperature. IP codes: D, dilute solution; L, low concentration; I, intermediate concentration; F, filtration; FW, filtration and washing; RL, residual liquid; C, concentrated solution; and S, semicarbonized product. a Average values of duplicate impregnation treatments.
3. Calculations The yields of the processes of impregnation (Yi), carbonization (Yc), and washing (Yw) were calculated by expressions (1)–(4): Δm = mf −m0
ðmf Nm0 Þ
Yi = Δm = P0 × 100 Yc = Mf = M0 × 100 Yw = Cf = C0
ð1Þ ð2Þ
ðMf bM0 Þ
ðCf bC0 Þ
ð3Þ ð4Þ
where m0 stands for the initial mass of CS (or of previously H3PO4impregnated CS) or ST used in the impregnation with the H3PO4 solution, mf for the mass of IP, P0 for initial mass of H3PO4 present in the impregnation solution, M0 for the mass of IP carbonized at 500 °C, Mf for the mass of CP, C0 for the mass of CP washed with distilled water, and Cf for the mass of AC. Δm is the increase in the mass of CS or ST after its impregnation with H3PO4. Ideally, Δm should be equal to the amount of H3PO4 loaded on CS or ST and P0 − Δm to the amount of H3PO4 lost during the impregnation treatments carried out at 85 °C and 120 °C. Table 1 lists the calculated values of Yi, Yh, and Yw. 4. Results and discussion 4.1. Yield of the impregnation process (Yi) For Series 1, Yi greatly ranges between 45.6 wt.% and 3.0 wt.%. Furthermore, it varies by D N DI1 N DL1 and DL3 N DL2 N DL1 (Fig. 1). The increase in the loading of H3PO4 on CS with a greater presence of substance either in solution or in CS was likely connected with the high tendency exhibited by H3PO4 to association by molecular condensation. The much lower Yi for DF than for D indicates that the loading of H3PO4 largely occurred during the oven-drying treatment at 120 °C, instead of during the heat treatment at 85 °C. The substance to a large extent should bind weakly to CS and thereby it was leached out by simply washing with distilled water as Yi is as low as 3.0 wt.% for DFW. Then, owing to the presumably high H3PO4
content in the filtrate, the preparation of DRL was undertaken. For this sample, the calculated values of Yi and Δm are 41.6 wt.% and 3.62 g. The values of Yi and Δm for DL1, DI1, D, and C0 show a great influence of the H3PO4 concentration on the amount of H3PO4 loaded on CS. This is 18.6 times higher for C0 than for D, whereas the content of H3PO4 in the impregnation solution was only 9 times higher for C0. Thus, most of the large amount of H3PO4 present in the initial impregnation solution was loaded on CS. It also holds for C2–C24. From these results it is clear that with the concentrated H3PO4 solution a synergic effect occurred in the impregnation of CS which was likely connected with the strong dependence of the molecular association of H3PO4 on the concentration of its aqueous solutions. In the preparation of C2–C24 it was observed that, after oven-drying the H3PO4 solution for 2–24 h, the volume reduction was as low as ≈10% at most. Therefore, the vaporization of water greatly occurred only after the oven-heated H3PO4 solution was brought into contact with CS. Perhaps as a result of the loading of H3PO4 on CS, the supernatant
Δm, single impregnations 8
Cumulative Δm, successive impregnations Δm, successive impregnations
Mass increase (g)
distilled water in a Soxhlet extractor and oven-dried to obtain the activated carbons (AC).
D
6 DI1 DL3 4
DL2 2
DL3 DL2 DL1
0
Process variables
Fig. 1. Impregnation of CS with dilute H3PO4 solutions. Variation of the mass increase with process variables: H3PO4 content (g)/100 mL (squares), cumulative H3PO4 contents (g)/100 mL (diamonds), and number of impregnation (circles).
J.M. González-Domínguez et al. / Materials Letters 65 (2011) 1423–1426
100
DFW
DF DL1
80
DI1 DL2 DL3
C6 30
C0
S300D DL3 S200D
DI1 DRL
Yc (%)
DF 50
C2
C12
25
C24 20
15
S200C
0
10
20 120
130
140
150
Fig. 3. Variation of the water-washing yield with mass increase.
For C0–C24, Yc ranges between 27.9 and 33.5 wt.%. The great mass loss originated by the carbonization of C0–C24 must be largely associated with the removal of phosphorus species, which were thermally unstable because of the formation of weaker bonds with CS on account of a larger degree of association between the phosphorus species in the impregnated state of the IP. Nevertheless, it should be stated that the calculated content of phosphorus species in the C0derived CP is still as high as ≈84.4 wt.%. For S200D and S200C, the Yc values are of the same magnitude order as for D and C0 respectively. The significantly higher Yc for S300D than for D is in line with the markedly higher Δm for the former sample. For S300C, however, Yc is much higher than for C0–C24. 4.3. Yield of the washing process (Yw) As shown in Fig. 3, Yw decreases almost linearly with Δm. This was expected as the mass loss produced by washing of the IP-derived CP with distilled water is attributable to lixiviation of phosphorus species, which are very polar ions/molecules and therefore highly soluble in water (i.e. 548 g H3PO4/100 g H2O and P4O10 reacts with water). Perhaps, the solution process was favored as a result of a decrease in size of phosphorus species present in the PI because of the carbonization process. Nevertheless, it should be pointed out that the estimated content of phosphorus species remaining in the ACs is high. Thus, as a guide, it is 51 wt.% for D and 43 wt.% for C0.
70
D
DRL
Mass increase (g)
The much higher Yc for the IP prepared using the dilute H3PO4 solutions (i.e. usually N50 wt.%, Fig. 2) than for CS (i.e. 25.4–28.7 wt.% [3]) is indicative of a great thermal stabilization of the volatile fraction of CS as a result of the loading of H3PO4 [4]. Since Yc varies by D N DI1 N DL1 and DL3 N DL2 N DL1, the thermal stability increased as Δm increased. However, Yc is higher for DL2 than for DI1 and for DL3 than for D, and this proves that the method of impregnation also influenced the thermal stability, this being higher when the process was carried out in successive steps. The close values of Yc for DF and DL1 are in line with the values of Δm for this IP and reveal that the phosphorus species most strongly bound to CS, which were responsible for the higher thermal stability of the IP, were loaded on CS during the impregnation treatment performed at 85 °C. However, such species were very sensitive to the subsequent washing with distilled water as Yc was as low as 28.6 wt.% for DRL. The similar Yc for DRL and DI1 is consistent with calculated Δm and Yi values for DRL.
DL2
D S200D
60
4.2. Yield of the carbonization process (Yc)
60
S300D
Yw (%)
diluted and this facilitated the vaporization of water to dryness. For the dilute solutions, the solute-solvent interactions occurring in the bulk of the solution should be stronger than for the concentrated solution because of the smaller degree of H3PO4 association, and this should have an unfavorable effect on the loading of H3PO4 on CS. In this case, not only water but also H3PO4 should vaporize during the oven-drying treatment. The effect of the previous charring of CS at 200 or 300 °C on the degree of H3PO4 loading was dependent on the concentration of H3PO4 and on the charring temperature of CS. Notice that Δm and Yi increase for S200D and mostly for S300D as compared to D. These results denote that the removal of a fraction of volatile matter from CS resulted in the creation of active sites only for the loading of small size phosphorus species. Furthermore, it was enhanced with the rise of the charring temperature. For larger phosphorus species, however, such sites either were not accessible or the chemical affinity was low, in particular when S300 was used. With this char, the remaining liquid after the first impregnation at 85 °C was not completely removed by oven-drying at 120 °C and as a result it was necessary to separate it by filtration before oven-drying again.
1425
DL1
5. Conclusions
35 C24
S200C
C12
30
C2 DFW
C0 C6
25 0
10
20
130
140
Mass increase (g) Fig. 2. Variation of the carbonization yield with mass increase.
150
The method of impregnation has a marked influence on the yields of the process of preparation of AC from CS by H3PO4 chemical activation provided that a dilute H3PO4 solution is used and that CS chars are impregnated with H3PO4. However, the previous ovendrying at 120 °C of the concentrated acid solution for further concentration of the impregnation solution has less influence on yields. With the increase in the concentration of the impregnation solution, it seems that the H3PO4–H3PO4 interactions progressively predominate over the H3PO4–substrate interactions. It promotes the loading of H3PO4, decreases the thermal stability of H3PO4 in the impregnation state of the IP and favors the leaching of H3PO4 by washing with distilled water in a Soxhlet equipment.
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