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Adsorption of aromatic amines and o-substituted derivatives of phenol from organic solutions by activated carbons-effect of surface acidity
C&m Vol.21,No. 5,pp 485489.1983 Printed I"GreatBritain
ooOg&6223/83 Q 1983 Pergamon
$3.00 + Ml Press Ltd.
ADSORPTION OF AROMATIC AMINES AND o-SUBSTITUTED DERIVATIVES OF PHENOL FROM ORGANIC SOLUTIONS BY ACTIVATED CARBONSEFFECT OF SURFACE ACIDITY HIROKAZU
Department
ODA and CHIKAOYOKOKAWA
of Chemical Engineering, Faculty of Engineering, Kansai University, Senri-yama, Suita, Osaka, 564, Japan (Received 15 April 1982)
Abstract-In
order to study the effect of surface acidity of activated carbons on their adsorption characteristics, three kinds of aromatic amines and four kinds of o-substituted phenols were adsorbed from organic solutions by twelve kinds of activated carbons of different acidity and of known pore-structure. It was demonstrated that the surface acidity had a marked effect on the adsorption characteristics and that the effect was linked with the polarity of the solvents. 1. INTRODUCTION
the adsorption of aromatic amines and o-substituted phenols from organic solutions by various kinds of carbons.
In
the study of adsorption by activated carbons from solutions, attention has been paid to the relationship between the phenomena and such physico-chemical characteristics of the adsorbates as molecular weight, size and their affinity to the solvents[l], and the characteristics of the adsorbents such as specific surface area and pore-size distribution[2-61. Thus little attention has been paid to the problem of the surface acidity of activated carbons, although several articles have treated the subject[7-91. The problem, however, was studied by Oda et al.[lO] in aqueous solutions of phenol and benzoic acid and they have demonstrated that the surface acidity of activated carbons has a marked effect on the adsorption characteristics. A discussion of the relationships among the physico-chemical properties of adsorbates, adsorbents and solvents seems to have started. To extend the discussion, the present article deals with
2. EXPERIMENTAL 2.1 Materials used The activated carbons used were the same as those employed in the previous study[lO]. Some characteristics are presented in Table 1, and others are outlined in Section 2.3. Three kinds of amines, aniline, Nmethylaniline and N,N-dimethylaniline, four kinds of derivatives of phenol, salicylaldehyde, pyrocatechol, o-chlorophenol, o-cresol and phenol and two kinds of solvents, ethylalcohol and benzene, all of G.R. grade, were used without pre-treatment. 2.2 Adsorption experiments
Carbon samples of 0.5 g were taken precisely in Erlenmyer flasks of 100 ml capacity, and 50ml of
Table 1. Characteristics of the activated carbons used Activated
carbon
Specific area
surface
Pore
m2/g)
volume
Total
(ml/g)
acidity
(meq/g)
bneq/m2)x10-3
1
1160
0.22
0.44
0.38
2
1100
c.21
0.25
0.22
3
1040
0.20
0.32
u.31
4
980
0.19
0.35
0.36
5
780
0.15
0.29
u.37
6
680
0.13
0.41
0.60
HT-*l
1070
0.25
0.27
0.24
HT-2
1160
0.22
0.18
0.16
HT-3
1040
0.20
0.15
0.14
HT-4
1000
0.19
0.17
0.17
HT-5
790
0.15
0.13
0.16
HT-6
800
0.15
0.16
0.20
*HT means for
1 hour
that
each
under
activated
a nitogen
carbon
atmosphere.
485
was
heat-treated
at 800°c
486
H. ODA and C. YOKOKAWA
solutions of each substrate in different concentrations (O.Hl.05 mol/l.) were added. After vigorous agitation the flask was kept in the dark at room temperature until adsorption equilibrium was established. Analysis was done by gas chromatography. Equilibrium was reached in most cases after standing for a week. 2.3 Churucferistics of fhe activated cczrhons used Table 1 shows several characteristics of the carbons. The specific surface area is in a range of 680-l 160 m2/g and the pore-volume 0.25-O. 13 ml/g, Thus the carbons are classified into two groups, one of large surface area and pore-volume (Nos. l-4), and the other of small surface area and pore-volume (Nos. 5 and 6). The carbons, however, have been classified into two other groups according to their pore-size distribution[lO]. The heat-treatment seemed to cause no marked change in the pore-structure of the carbons (with the exception of No. 6 carbon, whose specific surface area increased) since the other including pore-size distribution[lO], indications, showed no distinctive change due to the treatment. The carbons can also be classified in terms of surface acidity[lO]. Group 1 is of high acidity (Nos. 1, 4 and 6) and the other is of relatively low acidity (Nos. 2, 3 and 5). In the table only the amount of total acidity is given although the acidic groups were classified into four groups according to a procedure based on the principles introduced by Boehm et a/. [I 11in the previous paper [IO]. The acidic groups of carbons were mostly weakly acidic (titrated by alcoholic sodium ethylate, aqueous sodium hydroxide and sodium carbonate). The amount of the strongly acidic groups, which were titrated with aqueous sodium bicarbonate, was about 15% for Nos. 1, 4, 5 and 6, about 3% for No. 3 and 0% for No. 2. Heat-treatment markedly reduces the acidity of carbons, making them comparable to each other. 3. RESULTS AND DISCUSSION
3.1 Adsorpfion of aromatic amines The adsorption isotherms of aniline, Nmethylaniline and N,N-dimethylaniline from ethanol solutions by Nos. l-3 are illustrated in Figs. 1-3, respectively. The results for the heat-treated carbons are also shown. The three carbons are comparable to each other in physical structure but are different in total acidity. According to the figures, the amount adsorbed at ~~lib~urn conditions for each carbon is in the order of N,N-dimethylaniline (DMA), Nmethylaniline (MA) and aniline (A). This agrees with the general understanding that the larger the molecular weight of the adsorbate, the larger the amount adsorbed when the substrates concerned are similar in nature. The order also agrees with the suggestion tThe dielectric constants of DMA, MA and A are 5.1,&O and 7.0, ~s~tiv~ly[~ 31. $The pKas of DMA, MA and A in ethanol solution are 4.4, 4.9 and 5.7, respectiveIy[l4].
1 Fig.
1
0
Fig.
4.l
Carbon No.1
1
I
No.2
Carbon
1
0
I
1
1
Carbon NO. 3
Fig. 3
8
1
I
2
8 3.0
2.0
1.0
L
0
,
0.1
0.2
L
0.3
I
0.4
Equilibriumconcentration (mol/l) Figs. l-3. Adsorption isotherms of aromatic amines from EtOH solution by original and HT-activated carbons. Originaf carbon
HT-carbon
:
:
A
A
N,N-dimethylanitine N-methylaniline Aniline.
of K.irkwood[ 121 that the smaller the dielectric constant the larger the amount adsorbed is.7 Moreover, the order also agrees with the order of pKa of the substrates in the solution.2 It is noticeable that the discrepancies observed among the isotherms of the three adsorbates are obvious for the carbons of Nos. 1 and 2 but are obscure for No. 2. The reason for the obvious discrepancies observed for Nos. 1 and 3 seems to be attributable to the fact that the surface acidity, especially acidity per unit surface area, is large for the carbons. Heat-treatment of the carbons resulted in a decrease in the amount adsorbed, and the decrease is larger for carbons of high acidity (Nos. 1 and 3). For No. 2 carbon, the effect of heat treatment is less marked except for aniline. Thus, it is clear that the
Effect of surface acidity
adsorption of amines by activated carbons is controlled by the surface acidity of the carbons, but the phenomena do not result from a simple relationship of the acid-base equivalent, because there is no stoichiometric balance between the decrease in the acidity and in the amount adsorbed. That is, according to Table 1, the decrease in the acidity is 0.18, 0.07 and 0.17 meq/g for the carbons of Nos. 1, 2 and 3, respectively while the decreases in the amount adsorbed are several times larger in the sense of acidbase stoichiometry, as Figs. l-3 show. The adsorption isotherms of the three amines from solution in benzene by carbons l-3 and their heattreated products are illustrated in Figs. 4-6. It is obvious that the amounts adsorbed from the benzene solutions are without exception, markedly smaller than those from the ethanol solutions. The tendency is particularly notable in the case of the heat-treated carbons. This seems to be due to the facts that ethanol is polar while benzene is non-polar in nature and that the surface acidic groups of the carbon retard adsorption of benzene. It is also noteworthy that the adsorption isotherms of the three amines from benzene solution by carbon 2 differ significantly from each other as is seen in Fig. 6, while there is no marked difference between them when the amines are
Fig.
Carbon
4
adsorbed from ethanol solution, as shown in Fig. 2. Moreover, the order of the amounts adsorbed changes for DMA and MA when the solvents are changed in the case of No. 2. Thus, the nature of the solvent has a marked effect on the characteristics of adsorption and the effect is stronger in the case of adsorption by carbons of low acidity. That is, the substitution of ethanol with benzene results in a marked decrease in the amount adsorbed but no change in the selectivity for the adsorbates when the carbons are highly acidic (Nos. 1 and 3), while the replacement of the solvent causes a reverse in the order of selective adsorption for carbons of low acidity. 3.2 Adsorption of o-rubstituted derivatives of phenol The adsorption isotherms of salicylaldehyde (SA) and o-chlorophenol (CP) from their solutions in ethanol by the six kinds of activated carbons (Nos. l-6) are illustrated in Figs. 7 and 8, respectively. The
No.1
01 0
Fia.
Carbon
5
487
I
0.1
Equllibrrum
NO.2
I
1
0.Z
concentration
0.3
0.4
(mol/l)
Fig. 7. Adsorption isotherms of salicylaldehyde from EtOH solution by original activated carbons. 0, No. 1; V, No. 2; A, No. 3; 0, No. 4; 0, No. 5; AL, No. 6.
Fig.
0
Carbon
6
0.1
0.2
No.3
0.3
0.4
Equilibrium cOncentratiOn ( mol/l) Figs. 4-6 Adsorption isotherms of aromatic amines from benzene solution by original and HT-activated carbons,
Original carbon
HT-carbon
; .n
: A
I
0.1 Equllibrlum
N,N-dimethylaniline N-methylaniline Aniline.
1
1
0.2 concentration
0.3
/
0.
(ml/l)
Fig. 8. Adsorption isotherms of o-chlorophenol from EtOH solution by original activated carbons. 0, No. 1; V’, No. 2; A, No. 3; 0, No. 4; 0, No. 5; A, No. 6.
H. ODAand C.
YOKOKAWA
Table 2. Amounts adsorbed* on original and HT activated carbon ( x lo- 3mol/m2-carbon) Activated Remarks
Adsornate salicyl-
Original HT
aldehyde (SA) Pyrocatechol (PC) o-chlorophenol (CP) o-Cresol (0-C)
Increments(%)
carbon No.
1
2
3
4
5
6
1.82
1.83
2.00
2.24
1.74
1.61
1.77
1.08
2.14
1.70
1.90
2.14
17.58
-7.58
-5.00
-4.46
1.72 -32.92
Original
1.55
1.09
1.52
1.44
1.12
HT
1.82
1.10
1.79
1.63
1.86
1.63
17.42
9.17
17.76
13.19
66.07
27.34
Increments(%)
1.28
Original
1.21
1.04
1.25
1.12
0.97
0.96
HT
1.92
1.25
1.34
1.58
1.44
1.25
58.68
20.19
7.20
41.07
48.45
30.20
Original
Increments(%)
0.69
0.80
0.83
0.76
0.62
0.62
HT
1.28
0.84
1.06
1.26
0.78
0.85
85.51
5.00
27.71
65.79
25.80
37.10
Increments(%)
* Equilibrium
concentration
(0.2lnOl/l)
isotherms for the carbons can be classified into two groups according to the amounts adsorbed, for a group of Nos. l-4 and for the other of Nos. 5 and 6. It is known that the specific surface area is large for Nos. 14 and is relatively small for Nos. 5 and 6. Similar results were also found for pyrocatechol (PC) and o-cresol (o-C), although the isotherms were not shown in the text. On the other hand, the amounts of the five kinds of adsorbates adsorbed by No. 3 were arranged in the order of SA, PC, CP, o-C and phenol (P), as shown in Fig. 9. This order was also valid for the other carbons employed. The molecular weights of the five adsorbates are comparable to each other. However, it should be noted that the amount of CP adsorbed, whose molecular weight is the largest, is about 60% that of SA. This was also the case for the other carbons. These findings do not agree with Traube’s concept (1). The order of the amounts adsorbed, however, agrees with the order of dipole moment of the substrates.? On the other hand, it is seen that the amounts adsorbed are larger for the adsorbates of small pKa.1 The effect of heat-treatment of carbons on the adsorption capacity of the carbons is shown in Table 2. The capacity increases with heat-treatment, except in the case of SA, and the degree of the increase (%) seems larger for carbbns of high acidity and with large surface areas (Nos. 1 and 4). Since the reasons for the irregularity seen in the table are quite obscure, it is difficult to discuss the relationship between surface acidity and adsorption capacity quantitatively. However, it is clear that the surface acidity of activated carbons has a marked effect on the adsorption characteristics, such as capacity and selectivity, because the heat-treatment of the carbons caused no clear change in the physical structure of the carbons. tDipole moments (D) are as follows; P: 1.45, o-C: 1.45, CP: 1.43, PC: 2.62 and SA: 2.99[15]. fThe values of pKa in ethanol solution are as follows; P: 12.8, o-C: 11.3, CP: 9.9, PC: 9.4 and SA: 8.61161.
3’o3
0
I
I
0.1
0.2
Equilibrium
0.3
concentration
0.4
ml/l)
Fig. 9. Adsorption isotherms of o-substituted derivatives of phenol by activated carbon No. 3. 0, Salicylaldehyde; A, pyrocatechol; 0, o-chlorophenol; V, otresol; 0, phenol.
4. SUMMARY The effect of surface acidity of activated carbons on the adsorption characteristics was studied from the stand-points of surface acidity of the carbons, polarity of the solvent and dipole moment and pKa of the adsorbate. It was demonstrated that the surface acidity of activated carbons had a marked effect on the adsorption characteristics, such as capacity and selectivity of the adsorption, and the effect was considered to also be linked with the physicochemical properties of the systems. REFERENCES 1. A. W. Adamson, Physical Chemistry of Surface, 2nd Edn. Academic Press, New York (1967). 2. R. S. Hansen and R. P. Craig, J. Phys. Chem. 58, 211 (1954). 3. V. L. Snoeyink and W. J. Weber, Jr., Environ. Sci. Tech. 1, 288 (1967). 4. P. Gupta and P. De, J. Indian Chem. Sot. 23, 353 (1946). 5. M. M. Dubinin, J. Colloid Interface Sci. 23, 487 (1967). 6. A. Gutsze and B. Jarecka, Carbon 5, 314 (1967). 7. D. Graham, J. Phys. Chem. 59, 896 (1955).
Effect of surface acidity 8. R. W. Coughlin and F. S. Ezara, Environ. Sci. Tech. 2, 291 (1968). 9. S. P. Nandi and P. L. Walker, Jr., Fuel 50, 345 (1971). 10. H. Gda, M. Kishida and C. Yokokawa, Carbon 19,243 (1981). 1I. H. P. Boehm and E. Diehl, Angew. Chem. Int. Edn. 3, 10 (1964). 12. K. G. Kirkwood,
J. Chem. Phy.r. 2, 351 (1934).
489
13. J. Timmermans, A. M. Piette and R. Philippe, Bull. Sot. Chem. Beiges. 64, 5 (1955). 14. B. Gutbemhl and E. Grunwald, J. Am. Chem. Sot. 75, 559 (1953).
15. M. Kotake, Constant of Organic Compound, p. 500. Asakura, Tokyo, Japan (1963). 16. G. Schwarzenbach and E. Rudin, H&. Chim. Acta. 22, 360 (1939).
ooOg&6223/83 Q 1983 Pergamon
$3.00 + Ml Press Ltd.
ADSORPTION OF AROMATIC AMINES AND o-SUBSTITUTED DERIVATIVES OF PHENOL FROM ORGANIC SOLUTIONS BY ACTIVATED CARBONSEFFECT OF SURFACE ACIDITY HIROKAZU
Department
ODA and CHIKAOYOKOKAWA
of Chemical Engineering, Faculty of Engineering, Kansai University, Senri-yama, Suita, Osaka, 564, Japan (Received 15 April 1982)
Abstract-In
order to study the effect of surface acidity of activated carbons on their adsorption characteristics, three kinds of aromatic amines and four kinds of o-substituted phenols were adsorbed from organic solutions by twelve kinds of activated carbons of different acidity and of known pore-structure. It was demonstrated that the surface acidity had a marked effect on the adsorption characteristics and that the effect was linked with the polarity of the solvents. 1. INTRODUCTION
the adsorption of aromatic amines and o-substituted phenols from organic solutions by various kinds of carbons.
In
the study of adsorption by activated carbons from solutions, attention has been paid to the relationship between the phenomena and such physico-chemical characteristics of the adsorbates as molecular weight, size and their affinity to the solvents[l], and the characteristics of the adsorbents such as specific surface area and pore-size distribution[2-61. Thus little attention has been paid to the problem of the surface acidity of activated carbons, although several articles have treated the subject[7-91. The problem, however, was studied by Oda et al.[lO] in aqueous solutions of phenol and benzoic acid and they have demonstrated that the surface acidity of activated carbons has a marked effect on the adsorption characteristics. A discussion of the relationships among the physico-chemical properties of adsorbates, adsorbents and solvents seems to have started. To extend the discussion, the present article deals with
2. EXPERIMENTAL 2.1 Materials used The activated carbons used were the same as those employed in the previous study[lO]. Some characteristics are presented in Table 1, and others are outlined in Section 2.3. Three kinds of amines, aniline, Nmethylaniline and N,N-dimethylaniline, four kinds of derivatives of phenol, salicylaldehyde, pyrocatechol, o-chlorophenol, o-cresol and phenol and two kinds of solvents, ethylalcohol and benzene, all of G.R. grade, were used without pre-treatment. 2.2 Adsorption experiments
Carbon samples of 0.5 g were taken precisely in Erlenmyer flasks of 100 ml capacity, and 50ml of
Table 1. Characteristics of the activated carbons used Activated
carbon
Specific area
surface
Pore
m2/g)
volume
Total
(ml/g)
acidity
(meq/g)
bneq/m2)x10-3
1
1160
0.22
0.44
0.38
2
1100
c.21
0.25
0.22
3
1040
0.20
0.32
u.31
4
980
0.19
0.35
0.36
5
780
0.15
0.29
u.37
6
680
0.13
0.41
0.60
HT-*l
1070
0.25
0.27
0.24
HT-2
1160
0.22
0.18
0.16
HT-3
1040
0.20
0.15
0.14
HT-4
1000
0.19
0.17
0.17
HT-5
790
0.15
0.13
0.16
HT-6
800
0.15
0.16
0.20
*HT means for
1 hour
that
each
under
activated
a nitogen
carbon
atmosphere.
485
was
heat-treated
at 800°c
486
H. ODA and C. YOKOKAWA
solutions of each substrate in different concentrations (O.Hl.05 mol/l.) were added. After vigorous agitation the flask was kept in the dark at room temperature until adsorption equilibrium was established. Analysis was done by gas chromatography. Equilibrium was reached in most cases after standing for a week. 2.3 Churucferistics of fhe activated cczrhons used Table 1 shows several characteristics of the carbons. The specific surface area is in a range of 680-l 160 m2/g and the pore-volume 0.25-O. 13 ml/g, Thus the carbons are classified into two groups, one of large surface area and pore-volume (Nos. l-4), and the other of small surface area and pore-volume (Nos. 5 and 6). The carbons, however, have been classified into two other groups according to their pore-size distribution[lO]. The heat-treatment seemed to cause no marked change in the pore-structure of the carbons (with the exception of No. 6 carbon, whose specific surface area increased) since the other including pore-size distribution[lO], indications, showed no distinctive change due to the treatment. The carbons can also be classified in terms of surface acidity[lO]. Group 1 is of high acidity (Nos. 1, 4 and 6) and the other is of relatively low acidity (Nos. 2, 3 and 5). In the table only the amount of total acidity is given although the acidic groups were classified into four groups according to a procedure based on the principles introduced by Boehm et a/. [I 11in the previous paper [IO]. The acidic groups of carbons were mostly weakly acidic (titrated by alcoholic sodium ethylate, aqueous sodium hydroxide and sodium carbonate). The amount of the strongly acidic groups, which were titrated with aqueous sodium bicarbonate, was about 15% for Nos. 1, 4, 5 and 6, about 3% for No. 3 and 0% for No. 2. Heat-treatment markedly reduces the acidity of carbons, making them comparable to each other. 3. RESULTS AND DISCUSSION
3.1 Adsorpfion of aromatic amines The adsorption isotherms of aniline, Nmethylaniline and N,N-dimethylaniline from ethanol solutions by Nos. l-3 are illustrated in Figs. 1-3, respectively. The results for the heat-treated carbons are also shown. The three carbons are comparable to each other in physical structure but are different in total acidity. According to the figures, the amount adsorbed at ~~lib~urn conditions for each carbon is in the order of N,N-dimethylaniline (DMA), Nmethylaniline (MA) and aniline (A). This agrees with the general understanding that the larger the molecular weight of the adsorbate, the larger the amount adsorbed when the substrates concerned are similar in nature. The order also agrees with the suggestion tThe dielectric constants of DMA, MA and A are 5.1,&O and 7.0, ~s~tiv~ly[~ 31. $The pKas of DMA, MA and A in ethanol solution are 4.4, 4.9 and 5.7, respectiveIy[l4].
1 Fig.
1
0
Fig.
4.l
Carbon No.1
1
I
No.2
Carbon
1
0
I
1
1
Carbon NO. 3
Fig. 3
8
1
I
2
8 3.0
2.0
1.0
L
0
,
0.1
0.2
L
0.3
I
0.4
Equilibriumconcentration (mol/l) Figs. l-3. Adsorption isotherms of aromatic amines from EtOH solution by original and HT-activated carbons. Originaf carbon
HT-carbon
:
:
A
A
N,N-dimethylanitine N-methylaniline Aniline.
of K.irkwood[ 121 that the smaller the dielectric constant the larger the amount adsorbed is.7 Moreover, the order also agrees with the order of pKa of the substrates in the solution.2 It is noticeable that the discrepancies observed among the isotherms of the three adsorbates are obvious for the carbons of Nos. 1 and 2 but are obscure for No. 2. The reason for the obvious discrepancies observed for Nos. 1 and 3 seems to be attributable to the fact that the surface acidity, especially acidity per unit surface area, is large for the carbons. Heat-treatment of the carbons resulted in a decrease in the amount adsorbed, and the decrease is larger for carbons of high acidity (Nos. 1 and 3). For No. 2 carbon, the effect of heat treatment is less marked except for aniline. Thus, it is clear that the
Effect of surface acidity
adsorption of amines by activated carbons is controlled by the surface acidity of the carbons, but the phenomena do not result from a simple relationship of the acid-base equivalent, because there is no stoichiometric balance between the decrease in the acidity and in the amount adsorbed. That is, according to Table 1, the decrease in the acidity is 0.18, 0.07 and 0.17 meq/g for the carbons of Nos. 1, 2 and 3, respectively while the decreases in the amount adsorbed are several times larger in the sense of acidbase stoichiometry, as Figs. l-3 show. The adsorption isotherms of the three amines from solution in benzene by carbons l-3 and their heattreated products are illustrated in Figs. 4-6. It is obvious that the amounts adsorbed from the benzene solutions are without exception, markedly smaller than those from the ethanol solutions. The tendency is particularly notable in the case of the heat-treated carbons. This seems to be due to the facts that ethanol is polar while benzene is non-polar in nature and that the surface acidic groups of the carbon retard adsorption of benzene. It is also noteworthy that the adsorption isotherms of the three amines from benzene solution by carbon 2 differ significantly from each other as is seen in Fig. 6, while there is no marked difference between them when the amines are
Fig.
Carbon
4
adsorbed from ethanol solution, as shown in Fig. 2. Moreover, the order of the amounts adsorbed changes for DMA and MA when the solvents are changed in the case of No. 2. Thus, the nature of the solvent has a marked effect on the characteristics of adsorption and the effect is stronger in the case of adsorption by carbons of low acidity. That is, the substitution of ethanol with benzene results in a marked decrease in the amount adsorbed but no change in the selectivity for the adsorbates when the carbons are highly acidic (Nos. 1 and 3), while the replacement of the solvent causes a reverse in the order of selective adsorption for carbons of low acidity. 3.2 Adsorption of o-rubstituted derivatives of phenol The adsorption isotherms of salicylaldehyde (SA) and o-chlorophenol (CP) from their solutions in ethanol by the six kinds of activated carbons (Nos. l-6) are illustrated in Figs. 7 and 8, respectively. The
No.1
01 0
Fia.
Carbon
5
487
I
0.1
Equllibrrum
NO.2
I
1
0.Z
concentration
0.3
0.4
(mol/l)
Fig. 7. Adsorption isotherms of salicylaldehyde from EtOH solution by original activated carbons. 0, No. 1; V, No. 2; A, No. 3; 0, No. 4; 0, No. 5; AL, No. 6.
Fig.
0
Carbon
6
0.1
0.2
No.3
0.3
0.4
Equilibrium cOncentratiOn ( mol/l) Figs. 4-6 Adsorption isotherms of aromatic amines from benzene solution by original and HT-activated carbons,
Original carbon
HT-carbon
; .n
: A
I
0.1 Equllibrlum
N,N-dimethylaniline N-methylaniline Aniline.
1
1
0.2 concentration
0.3
/
0.
(ml/l)
Fig. 8. Adsorption isotherms of o-chlorophenol from EtOH solution by original activated carbons. 0, No. 1; V’, No. 2; A, No. 3; 0, No. 4; 0, No. 5; A, No. 6.
H. ODAand C.
YOKOKAWA
Table 2. Amounts adsorbed* on original and HT activated carbon ( x lo- 3mol/m2-carbon) Activated Remarks
Adsornate salicyl-
Original HT
aldehyde (SA) Pyrocatechol (PC) o-chlorophenol (CP) o-Cresol (0-C)
Increments(%)
carbon No.
1
2
3
4
5
6
1.82
1.83
2.00
2.24
1.74
1.61
1.77
1.08
2.14
1.70
1.90
2.14
17.58
-7.58
-5.00
-4.46
1.72 -32.92
Original
1.55
1.09
1.52
1.44
1.12
HT
1.82
1.10
1.79
1.63
1.86
1.63
17.42
9.17
17.76
13.19
66.07
27.34
Increments(%)
1.28
Original
1.21
1.04
1.25
1.12
0.97
0.96
HT
1.92
1.25
1.34
1.58
1.44
1.25
58.68
20.19
7.20
41.07
48.45
30.20
Original
Increments(%)
0.69
0.80
0.83
0.76
0.62
0.62
HT
1.28
0.84
1.06
1.26
0.78
0.85
85.51
5.00
27.71
65.79
25.80
37.10
Increments(%)
* Equilibrium
concentration
(0.2lnOl/l)
isotherms for the carbons can be classified into two groups according to the amounts adsorbed, for a group of Nos. l-4 and for the other of Nos. 5 and 6. It is known that the specific surface area is large for Nos. 14 and is relatively small for Nos. 5 and 6. Similar results were also found for pyrocatechol (PC) and o-cresol (o-C), although the isotherms were not shown in the text. On the other hand, the amounts of the five kinds of adsorbates adsorbed by No. 3 were arranged in the order of SA, PC, CP, o-C and phenol (P), as shown in Fig. 9. This order was also valid for the other carbons employed. The molecular weights of the five adsorbates are comparable to each other. However, it should be noted that the amount of CP adsorbed, whose molecular weight is the largest, is about 60% that of SA. This was also the case for the other carbons. These findings do not agree with Traube’s concept (1). The order of the amounts adsorbed, however, agrees with the order of dipole moment of the substrates.? On the other hand, it is seen that the amounts adsorbed are larger for the adsorbates of small pKa.1 The effect of heat-treatment of carbons on the adsorption capacity of the carbons is shown in Table 2. The capacity increases with heat-treatment, except in the case of SA, and the degree of the increase (%) seems larger for carbbns of high acidity and with large surface areas (Nos. 1 and 4). Since the reasons for the irregularity seen in the table are quite obscure, it is difficult to discuss the relationship between surface acidity and adsorption capacity quantitatively. However, it is clear that the surface acidity of activated carbons has a marked effect on the adsorption characteristics, such as capacity and selectivity, because the heat-treatment of the carbons caused no clear change in the physical structure of the carbons. tDipole moments (D) are as follows; P: 1.45, o-C: 1.45, CP: 1.43, PC: 2.62 and SA: 2.99[15]. fThe values of pKa in ethanol solution are as follows; P: 12.8, o-C: 11.3, CP: 9.9, PC: 9.4 and SA: 8.61161.
3’o3
0
I
I
0.1
0.2
Equilibrium
0.3
concentration
0.4
ml/l)
Fig. 9. Adsorption isotherms of o-substituted derivatives of phenol by activated carbon No. 3. 0, Salicylaldehyde; A, pyrocatechol; 0, o-chlorophenol; V, otresol; 0, phenol.
4. SUMMARY The effect of surface acidity of activated carbons on the adsorption characteristics was studied from the stand-points of surface acidity of the carbons, polarity of the solvent and dipole moment and pKa of the adsorbate. It was demonstrated that the surface acidity of activated carbons had a marked effect on the adsorption characteristics, such as capacity and selectivity of the adsorption, and the effect was considered to also be linked with the physicochemical properties of the systems. REFERENCES 1. A. W. Adamson, Physical Chemistry of Surface, 2nd Edn. Academic Press, New York (1967). 2. R. S. Hansen and R. P. Craig, J. Phys. Chem. 58, 211 (1954). 3. V. L. Snoeyink and W. J. Weber, Jr., Environ. Sci. Tech. 1, 288 (1967). 4. P. Gupta and P. De, J. Indian Chem. Sot. 23, 353 (1946). 5. M. M. Dubinin, J. Colloid Interface Sci. 23, 487 (1967). 6. A. Gutsze and B. Jarecka, Carbon 5, 314 (1967). 7. D. Graham, J. Phys. Chem. 59, 896 (1955).
Effect of surface acidity 8. R. W. Coughlin and F. S. Ezara, Environ. Sci. Tech. 2, 291 (1968). 9. S. P. Nandi and P. L. Walker, Jr., Fuel 50, 345 (1971). 10. H. Gda, M. Kishida and C. Yokokawa, Carbon 19,243 (1981). 1I. H. P. Boehm and E. Diehl, Angew. Chem. Int. Edn. 3, 10 (1964). 12. K. G. Kirkwood,
J. Chem. Phy.r. 2, 351 (1934).
489
13. J. Timmermans, A. M. Piette and R. Philippe, Bull. Sot. Chem. Beiges. 64, 5 (1955). 14. B. Gutbemhl and E. Grunwald, J. Am. Chem. Sot. 75, 559 (1953).
15. M. Kotake, Constant of Organic Compound, p. 500. Asakura, Tokyo, Japan (1963). 16. G. Schwarzenbach and E. Rudin, H&. Chim. Acta. 22, 360 (1939).