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The effect of chlorination on surface properties of activated carbon
Carbon Vol. 36, No. 11, pp. 1677–1682, 1998 © 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/98 $ — see front matter
PII: S0008-6223(98)00165-1
THE EFFECT OF CHLORINATION ON SURFACE PROPERTIES OF ACTIVATED CARBON M. J. B. E,a E. H,a S. Lb and J. A. F. MDa,* aDepartment of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ont., Canada K7K 7B4 bPhysical Protection Section, Defence Research Establishment Suffield, Medicine Hat, Alta., Canada T1A 8K6 (Received 10 February 1998; accepted in revised form 6 April 1998) Abstract—In an investigation of the reaction of chlorine with carbon surfaces and the relationship between chlorine fixation and other heteroatoms (such as oxygen and hydrogen) on the surface, various porous carbons have been treated with gaseous chlorine. The diversity of the carbons was depicted not only by precursor material (coal, coconut shell, sugar, polymer) and porosity, but also by pretreatment which altered the surface oxide concentration or ash content. After chlorination, all of the samples showed a reduced uptake of water vapour at saturation, but calorimetric studies showed that the molar enthalpy of adsorption of water is unaffected by the presence of chlorine on the surface and remains dependant on the concentration of surface oxide sites. However, the chlorinated samples were determined to be more acidic than the surface oxide concentration indicated, suggesting an inductive effect of chemisorbed chlorine. The fixation of chemisorbed chlorine on the carbon surface showed no correlation with the ash content of the original carbon. © 1998 Elsevier Science Ltd. All rights reserved. Key Words—A. Activated carbon, B. halogenation, B. chlorination, D. functional groups, D. surface properties.
this study. The ultimate aim was to reduce aging of the carbon surface by producing stable carbon–chlorine complexes without significantly reducing the adsorptive capacity of the given carbon.
1. INTRODUCTION
Stable carbon–chlorine surface complexes have been reported in the literature for many years [1] and although the optimum temperature for fixation of chlorine was determined to be 400°C it has been demonstrated that chlorination of a porous carbon at moderate temperatures, #180°C, results in a dramatic improvement in its capacity to resist aging (further addition of oxygen groups) in high humidity air [2–4]. The chlorine surface complex has been postulated to form partially by the addition of chlorine at the unsaturated sites created by eliminating the CO complex and partially by substitution of 2 combined hydrogen [1]. Dehydrohalogenation of the carbon to produce more olefinic bonds has also been suggested as a possible mechanism of chlorination [5]. Hall and Holmes proposed the possible replacement of the oxygen-containing functional groups by chlorine to be the reason for a reduction in the polar water adsorption sites [3] and Barton et al. [2] showed, using temperature programmed desorption, that the total concentration of surface oxide functionalities on chlorinated BPL carbon remained the same after chlorination but small reductions in the acidic oxide groups, those which desorb as CO , were 2 noticed. Since the condition of the surface prior to chlorination may alter the stability of the carbon–chlorine complexes created, a variety of carbons were used in
2. EXPERIMENTAL
2.1 Treatment of the carbon Chlorinated samples were produced from porous carbons with a variety of different characteristics. These included as-received Calgon BPL carbon (BPL-AR) and samples of BPL carbon which had been reduced in hydrogen, oxidised in nitric acid or hydrogen peroxide, or boiled in HCl. The latter treatment was an attempt to reduce the ash content of the carbon since there has been some speculation that the chlorine does not react with the carbon surface itself but with the ash. It was previously shown that treating the carbon with ethylene at 165°C reduced the surface concentration of oxygen groups [2]. An ethylene-treated sample of BPL carbon was chlorinated in this study. Carbons which had been produced from different precursor materials, and had different porosities, were also chlorinated. These carbons were a coconut-shell carbon (GMS-70 from the California Carbon Co.), a sugar carbon and a PVDC (polyvinylidene chloride homopolymer) carbon. The latter two carbons were produced in-house by a slow pyrolysis under an inert atmosphere to 750°C. The PVDC carbon was in the form of small beads and was used ‘‘as produced’’ but the sugar carbon was subjected to some air
*Corresponding author. Tel: +1 613 541 60006457; Fax: +1 613 542 9489; e-mail: [email protected] 1677
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activation at 500°C, resulting in approximately 16% burnoff, before use. The procedures for the preparation of the oxidised BPL carbon samples with either hydrogen peroxide [7] or nitric acid [8] have previously been described, as has the reduction in hydrogen [9]. A sample of BPL carbon was boiled in concentrated hydrochloric acid and then washed in water, in an attempt to remove some of the ash from this coal-based carbon. Samples to be chlorinated were degassed overnight at 110°C. The chlorine treatment took place in a sealed stainless steel reaction vessel maintained at a pressure of approximately one half an atmosphere of chlorine gas, at 165°C, for 30 minutes. In an attempt to eliminate irregularities in the treatment when direct comparisons were to be made between samples the carbons were in fact placed in the reaction vessel together. Hence they were subjected to exactly the same degassing conditions, temperature and reaction time. All chlorinated carbons were washed extensively with methanol at room temperature to remove physisorbed chlorine and dried at 110°C.
A 20% reduction of the ash was achieved, hence, the carbon still contained 5.0% ash. Other carbons chlorinated under the same conditions included an airactivated sugar carbon (<0.1% ash) and PVDC carbon beads which had no significant ash content. Table 1 shows the chlorine fixed on five carbons with varying ash contents. It is obvious that the ash content does not correlate with the amount of chlorine that was fixed on the surface. However, the amount of chlorine released from the carbons on heating to 900°C appears higher for the lower ash content carbons. The high ash content in the BPL carbons may be a factor in the amount of chlorine held on the carbon at 900°C, although the coconutshell carbon, with almost 4% ash, released most of its fixed chlorine on heating. The chemical nature of the ash may also be a contributing factor. The coalbased BPL carbon contains different metallic species than the coconut-shell-based carbon, and although the ash was not wholly analysed, neutron activation analyses detected trace amounts of aluminium, vanadium and titanium in the BPL carbon.
2.2 Characterisation of the carbon
3.2 Availability of hydrogen
All the carbon samples were characterised by temperature programmed desorption (TPD) to 900°C, nitrogen adsorption at 77 K, water adsorption at 298 K and enthalpy of immersion in water, using procedures that have been previously described [2]. Mass titration experiments, in 0.1M NaCl, and acid/base depletion, from HCl/NaCl and NaOH/NaCl solutions were carried out on some samples to determine the effect of chlorination on the acid/base properties of the carbons. Both methods have recently been described for a number of BPL carbons with varying surface oxide concentrations [10]. Wet chemical chlorine analyses were carried out by heating the powdered carbon in an excess of sodium carbonate to 550°C for 8 hours. The residue was then wetted with water, dissolved in 1 : 1 concentrated nitric acid and neutralised with 1M sodium hydroxide before being titrated with 0.100M silver nitrate. The results of these chemical analyses were consistent with those from previous neutron activation analyses [6 ]. The ash content of some of the carbons was measured by firing three, 1 g, dry carbon samples in a muffle furnace held at 850°C, to a constant weight (after at least 48 hours). All adsorption and analysis values are quoted per gram of sample with the understanding that each sample contains mainly carbon but also varying amounts of other heteroatoms, such as oxygen, hydrogen and chlorine, as well as ash. 3. RESULTS AND DISCUSSION
3.1 Effect of ash Boiling BPL carbon in HCl to reduce the ashforming mineral content was not entirely successful.
Since virtually all of the chlorine released in the TPD experiment was in the form of HCl, the availability of hydrogen on the surface of the carbon may be a contributing factor not only in the release of chlorine during temperature programmed desorption but also in the fixation of chlorine on the carbon surface if a dehydrochlorination reaction predominates. Figure 1 shows the total amount of desorbed hydrogen, [ H ], from TPD to 900°C, for the original carbons and the chlorinated equivalents. Although the chlorinated samples release hydrogen at lower temperatures as HCl, very little difference in the total amount of H desorbed is evident. Both the sugar carbon and the PVDC carbon released significantly larger quantities of hydrogen than the BPL carbons and the coconut carbon. The observation that the PVDC carbon fixed twice the amount of chlorine as the sugar carbon appears to lessen the possibility of the predominance of one of the proposed mechanisms of chlorine addition which involves the exchange of chlorine with chemisorbed hydrogen on the carbon surface. The chlorinated sugar carbon was the only chlorinated sample which retained a large quantity of hydrogen which could be desorbed from the carbon in the form of H . It cannot however be ruled 2 Table 1. Characteristics of various carbons
Sample
Ash content (% mass)
Chlorine held (% mass)
TPD/900°C, Cl removed (%)
BPL-AR BPL-deashed PVDC carbon Sugar carbon GMS-70
6.3±0.1 5.0±0.1 0 0.07±0.10 3.7±0.2
6.6 6.7 11.6 5.2 5.0
51 54 80 100 90
The effect of chlorination on surfaces properties of activated carbon
Fig. 1. Total hydrogen evolution, [ H ], from temperature programmed desorption to 900°C, from the original carbon samples and their chlorinated equivalents.
out that some sites are unavailable for chlorination simply due to pore size restrictions.
3.3 Controlled oxidation of carbon To study the effect of [O], a variety of BPL carbons with different surface oxide concentrations were selected for chlorination. The results of stepwise temperature programmed desorption of surface oxide groups from the various samples of BPL carbon before chlorination are shown in Fig. 2. Treating the carbon with ethylene at 165°C for 6 hours appeared to lower the surface oxygen concentration to the
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Fig. 3. Water adsorption isotherms at 298 K, for a number of carbons: 6, OXBPL65; #, BPL-red; (, BPL-AR; %, BPL–H O ; $, sugar carbon. Curves indicate modified DS 2 2 equation fits.
same level as reducing the carbon in a flowing H /N mixture at 500°C for 2 hours. 2 2 The corresponding water adsorption isotherms for some of these carbons and their respective modified Dubinin–Serpinskii equation fits [11] are shown in Fig. 3. These two plots illustrate how the surface oxide groups alter the initial low pressure portion of the water adsorption isotherm. As previously shown [12] for example, reducing the carbon surface shifts the onset of the region in which there is a large increase in water uptake to higher relative pressures and oxidising the surface facilitates water adsorption
Fig. 2. Desorption profiles at 300, 500, 750 and 900°C, of [O] from temperature programmed desorption for a number of BPL carbons: diagonal stripe, BPL reduced in hydrogen; solid bar, as-received BPL; horizontal stripe, hydrogen peroxide-treated BPL; cross hatched, nitric acid-treated BPL at 65°C (OXBPL65); and open bars, ethylene-treated BPL at 165°C.
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at low relative pressures. The hydrogen peroxide treatment does not alter the surface very much and none of the treatments were harsh enough to have severely modified the pore structure of the original carbon. Table 2 summarises the BET surface areas, number of primary water adsorption sites, a , from o the modified DS equation fit, and surface oxide concentrations for these BPL samples and the sugar carbon, before chlorination. It is obvious from the variance in the high relative pressure portions of the water isotherms and the differences in BET surface areas that the pore structures of the BPL carbon and the sugar carbon are dissimilar. Analysing the samples after chlorination showed that the hydrogen reduced BPL carbon (BPL-red ) picked up the most chlorine and the strongly nitric acid oxidised carbon, OXBPL65, the least. This may be due to oxygen-containing surface groups occupying a large number of sites making them unavailable to the chlorine. The highly oxidised carbon has much of its external surface covered with oxygen-containing functional groups [8]. It should be noted that this highly oxidised carbon and the air-activated sugar carbon, which also had a high concentration of surface oxides (6.4 mmol g−1), are the only two samples where [O] actually went down after chlorination and washing. The weight percent chlorine uptake for the two samples was similar at 5.2 and 4.6%. There seems to be no relationship between the BET surface area of the carbon and the fixation of chlorine on the surface.
3.4 Modification of acidic or basic sites The variation of the molar enthalpy of interaction, which was obtained by dividing the enthalpy of immersion in water by the water isotherm saturation value, a , with the [O] desorbed from the surface on s TPD for a number of chlorinated and non-chlorinated carbons is shown in Fig. 4. This relationship reinforces the notion that water binds to high energy primary sites which are essentially surface oxide groups and does not appear to be affected by the addition of chlorine to the surface. Included in this graph are data for carbons from different sources such as the sugar carbon, the PVDC carbon, the coconut-shell carbon (GMS-70) and a series of nitric acid oxidised BPL carbons previously reported [8]. This may account for some of the scatter. Unlike the as-received BPL carbon and the ethy-
Fig. 4. Net molar enthalpy of immersion in water, Dh , for w various carbons versus total oxygen desorbed, [O]: $, nonchlorinated; (, chlorinated carbons.
lene-treated BPL carbon, it has been established that the strongly oxidised BPL carbons are acidic in nature [10] and the chlorinated BPL carbons also display acidic characteristics. The mass titrations of a number of BPL carbons are shown in Fig. 5. The strongly nitric acid oxidised BPL carbon (OXBPL65) demonstrates the decrease in pzc (point of zero charge [13] or zpc [14]) with increased surface oxide concentration. The as-received BPL carbon and the ethylene-treated carbon are both basic carbons. The chlorinated equivalents are shown to have a much lower pzc although neither of these chlorinated carbons have significantly more surface oxides than the original BPL. It should be noted that the relationship between the total concentration of surface acidic groups, measured by the amount of base uptake, with [O] is
Table 2. Characteristics of various carbons Carbon
BET surface area (m2 g−1)
a* o (mmol g−1)
[O] (mmol g−1)
BPL-reduced BPL-AR BPL–H O 2 2 OXBPL65 Sugar carbon
1098 1088 1159 1099 530
0.32 0.71 1.7 3.1 3.1
0.73 1.2 1.3 6.4 5.0
*a =Primary site concentration. o
Fig. 5. Mass titrations of BPL carbons in 0.1M NaCl: &, BPL–C H ; %, chlorinated BPL–C H ; 6, BPL-AR; +, 2 4 2 4 BPL–Cl; $, OXBPL65.
The effect of chlorination on surfaces properties of activated carbon
also different for the chlorinated carbons when compared to oxidised carbons, reinforcing the observation that the chlorinated carbons are more acidic than the surface oxide concentration would indicate. A significant increase in base uptake from aqueous saline solution and decrease in acid uptake is observed when a chlorinated BPL sample is compared to the as-received BPL carbon. Figure 6Fig. 7 show typical acid and base uptakes for BPL-AR and a chlorinated BPL carbon along with the respective Langmuir fits of the reaction isotherms. Base uptakes, n , for the non-chlorinated and chlorinated B2 carbons were 0.20 and 0.46 meq g−1, respectively, while the acid uptakes, n , for the same samples A2 were 0.60 and 0.32 meq g−1. Interestingly, the sum of the acidic and basic sites changes little, suggesting that the chlorination causes an inductive effect, making the oxygen sites more acidic but not actually creating or destroying the majority of these groups on the surface of the carbon. The sum of these sites increases dramatically when more oxygen groups are
Fig. 6. Acid uptakes from 0.1M NaCl/HCl solutions for: &, BPL-AR; $, BPL–Cl. Curves represent Langmuir fits calculated from the linear plots.
Fig. 7. Base uptakes from 0.1M NaCl/NaOH solutions for: &, BPL-AR; $, BPL–Cl. Curves represent Langmuir fits calculated from the linear plots.
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added to the surface [10,15]. This observation may tentatively be rationalised by considering that as a result of the inductive effect of chemisorbed chlorine, very weak acidic oxide sites are made stronger so that they contribute to the observed acid site concentration and to the pzc (point of zero charge) observed in the mass titration experiments. This is not surprising given that chloroacetic acid (pK =2.85) is nearly a 100 times as strong as acetic acid (pK =4.75) [16 ]. a Chlorination of the surface acts then, in a manner similar to the introduction of chemisorbed oxygen except that new acid or potentially new acid sites are not produced. On this basis, chlorination can be looked upon as protecting easily oxidised areas on the carbon surface and thus inhibiting the aging process in humid air. These results are contrary to those of Puri and Bansal who observed no increase in base adsorption capacity and no change in the pH of the chlorinated samples in aqueous solution [17]. The water adsorption isotherms for the coconut and sugar carbons in this study also differ from those of Puri and Bansal in that although the higher relative pressure portions of the isotherms of chlorinated samples showed less water adsorption, the lower relative pressure portions did not show an increased uptake and the two isotherms (for chlorinated and unchlorinated ) do not intersect. Barton et al. [5] have previously attributed the increase in adsorption in the low pressure region of the water isotherm of chlorinated carbon cloth to the presence of strongly physisorbed chlorine. This physisorbed chlorine has been removed from the carbons in this study by extensive washing with methanol. Similar observations have been reported by Hall and Holmes [3]. Figure 8 shows the water adsorption isotherms on chlorinated and non-chlorinated PVDC carbon plotted on two different adsorption scales. The shape of the original water isotherm seems to be retained indicating that the main differences are due to pore blocking, probably from the
Fig. 8. Water adsorption isotherms at 298 K: %, PVDC carbon, right-hand scale; (, PVDC–Cl, left-hand scale.
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positioning of the large Cl groups on the surface. The 40% drop in surface area, from 1060 to 640 m2 g−1 after chlorination for PVDC carbon was not totally unexpected since this carbon, known to ˚ range [18], have fairly narrow micropores in the 6-A with little or no macropore and mesopore volume, held the most chlorine of any sample treated—11.6%. The proposed mechanism of the decomposition of the PVDC polymer to produce this carbon involves the formation of double bonds and a rigid aromatic structure [19]. The saturation of these bonds on chlorination may be deduced as the primary method of chlorine fixation. The intensity of the surface interaction with water appears not to be altered and the molar enthalpy of immersion increased only very slightly from 2.01 to 2.18 kJ mol−1. The act of treating a dry carbon with gaseous chlorine does not seem to alter the surface oxides on a mildly oxidised carbon such as the BPL-AR or GMS-70 carbons. Chlorination blocks or in some way reduces slightly some of the available porosity in these carbons but does not appear to remove primary water adsorption sites from the surface. These sites have often been considered to be associated with surface oxide groups.
4. CONCLUSIONS
The fixation of chlorine on carbon surfaces at moderate temperatures depends little on the surface oxygen concentration or on the ash content of the carbon. Carbons with similar concentrations of desorbable hydrogen from the surface fixed different amounts of chlorine. After chlorination the carbons show an increase in acidic properties which can be
attributed to the chlorine having an inductive influence on the surface oxygen functional groups. REFERENCES 1. Puri, B. R., in Chemistry and Physics of Carbon, Vol. 6, ed. P. L. Walker. Marcel Dekker, New York, 1970, p. 255. 2. Barton, S. S., Evans, M. J. B., Liang, S. and MacDonald, J. A. F., Carbon, 1996, 34, 975. 3. Hall, C. R. and Holmes, R. J., Carbon, 1992, 30, 173. 4. Hall, C. R. and Holmes, R. J., Carbon, 1993, 31, 881. 5. Barton, S. S., Evans, M. J. B., Koresh, J. E. and Tobias, H., Carbon, 1987, 25, 663. 6. Barton, S. S., Evans, M. B. J. and MacDonald, J. A. F., Pol. J. Chem., 1997, 71, 651. 7. Barton, S. S., Dacey, J. R. and Evans, M. J. B., Colloid Polym. Sci., 1982, 260, 726. 8. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Adsorption Sci. Technol., 1993, 10, 75. 9. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Carbon, 1991, 29, 1099. 10. Barton, S. S., Evans, M. J. B., Halliop, E. and MacDonald, J. A. F., Carbon, 1997, 35, 1361. 11. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Carbon, 1992, 30, 123. 12. McDermot, H. L and Arnell, J. C., J. Phys. Chem., 1954, 58, 492. 13. Noh, J. S. and Schwarz, J. A., J. Colloid Interface Sci., 1995, 130, 157. 14. Corapcioglu, M. O. and Huang, C. P., Carbon, 1987, 25, 569. 15. Bandosz, T. J., Jagiello, J. and Schwarz, J. A., Anal. Chem., 1992, 64, 891. 16. CRC Handbook of Chemistry and Physics, 62nd edn, ed. R. C. Weast and M. J. Astle. CRC Press, Boca Raton, FL, 1982, p. D142. 17. Puri, B. R. and Bansal, R. C., Indian J. Chem., 1967, 5, 556. 18. Barton, S. S., Evans, M. J. B. and Harrison, B. H., J. Colloid Interface Sci., 1974, 49, 462. 19. Dacey, J. R. and Cadenhead, D. A., Proceedings of the Fourth Conference on Carbon. Pergamon Press, Oxford, London, New York, Paris, 1960, p. 315.
PII: S0008-6223(98)00165-1
THE EFFECT OF CHLORINATION ON SURFACE PROPERTIES OF ACTIVATED CARBON M. J. B. E,a E. H,a S. Lb and J. A. F. MDa,* aDepartment of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ont., Canada K7K 7B4 bPhysical Protection Section, Defence Research Establishment Suffield, Medicine Hat, Alta., Canada T1A 8K6 (Received 10 February 1998; accepted in revised form 6 April 1998) Abstract—In an investigation of the reaction of chlorine with carbon surfaces and the relationship between chlorine fixation and other heteroatoms (such as oxygen and hydrogen) on the surface, various porous carbons have been treated with gaseous chlorine. The diversity of the carbons was depicted not only by precursor material (coal, coconut shell, sugar, polymer) and porosity, but also by pretreatment which altered the surface oxide concentration or ash content. After chlorination, all of the samples showed a reduced uptake of water vapour at saturation, but calorimetric studies showed that the molar enthalpy of adsorption of water is unaffected by the presence of chlorine on the surface and remains dependant on the concentration of surface oxide sites. However, the chlorinated samples were determined to be more acidic than the surface oxide concentration indicated, suggesting an inductive effect of chemisorbed chlorine. The fixation of chemisorbed chlorine on the carbon surface showed no correlation with the ash content of the original carbon. © 1998 Elsevier Science Ltd. All rights reserved. Key Words—A. Activated carbon, B. halogenation, B. chlorination, D. functional groups, D. surface properties.
this study. The ultimate aim was to reduce aging of the carbon surface by producing stable carbon–chlorine complexes without significantly reducing the adsorptive capacity of the given carbon.
1. INTRODUCTION
Stable carbon–chlorine surface complexes have been reported in the literature for many years [1] and although the optimum temperature for fixation of chlorine was determined to be 400°C it has been demonstrated that chlorination of a porous carbon at moderate temperatures, #180°C, results in a dramatic improvement in its capacity to resist aging (further addition of oxygen groups) in high humidity air [2–4]. The chlorine surface complex has been postulated to form partially by the addition of chlorine at the unsaturated sites created by eliminating the CO complex and partially by substitution of 2 combined hydrogen [1]. Dehydrohalogenation of the carbon to produce more olefinic bonds has also been suggested as a possible mechanism of chlorination [5]. Hall and Holmes proposed the possible replacement of the oxygen-containing functional groups by chlorine to be the reason for a reduction in the polar water adsorption sites [3] and Barton et al. [2] showed, using temperature programmed desorption, that the total concentration of surface oxide functionalities on chlorinated BPL carbon remained the same after chlorination but small reductions in the acidic oxide groups, those which desorb as CO , were 2 noticed. Since the condition of the surface prior to chlorination may alter the stability of the carbon–chlorine complexes created, a variety of carbons were used in
2. EXPERIMENTAL
2.1 Treatment of the carbon Chlorinated samples were produced from porous carbons with a variety of different characteristics. These included as-received Calgon BPL carbon (BPL-AR) and samples of BPL carbon which had been reduced in hydrogen, oxidised in nitric acid or hydrogen peroxide, or boiled in HCl. The latter treatment was an attempt to reduce the ash content of the carbon since there has been some speculation that the chlorine does not react with the carbon surface itself but with the ash. It was previously shown that treating the carbon with ethylene at 165°C reduced the surface concentration of oxygen groups [2]. An ethylene-treated sample of BPL carbon was chlorinated in this study. Carbons which had been produced from different precursor materials, and had different porosities, were also chlorinated. These carbons were a coconut-shell carbon (GMS-70 from the California Carbon Co.), a sugar carbon and a PVDC (polyvinylidene chloride homopolymer) carbon. The latter two carbons were produced in-house by a slow pyrolysis under an inert atmosphere to 750°C. The PVDC carbon was in the form of small beads and was used ‘‘as produced’’ but the sugar carbon was subjected to some air
*Corresponding author. Tel: +1 613 541 60006457; Fax: +1 613 542 9489; e-mail: [email protected] 1677
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activation at 500°C, resulting in approximately 16% burnoff, before use. The procedures for the preparation of the oxidised BPL carbon samples with either hydrogen peroxide [7] or nitric acid [8] have previously been described, as has the reduction in hydrogen [9]. A sample of BPL carbon was boiled in concentrated hydrochloric acid and then washed in water, in an attempt to remove some of the ash from this coal-based carbon. Samples to be chlorinated were degassed overnight at 110°C. The chlorine treatment took place in a sealed stainless steel reaction vessel maintained at a pressure of approximately one half an atmosphere of chlorine gas, at 165°C, for 30 minutes. In an attempt to eliminate irregularities in the treatment when direct comparisons were to be made between samples the carbons were in fact placed in the reaction vessel together. Hence they were subjected to exactly the same degassing conditions, temperature and reaction time. All chlorinated carbons were washed extensively with methanol at room temperature to remove physisorbed chlorine and dried at 110°C.
A 20% reduction of the ash was achieved, hence, the carbon still contained 5.0% ash. Other carbons chlorinated under the same conditions included an airactivated sugar carbon (<0.1% ash) and PVDC carbon beads which had no significant ash content. Table 1 shows the chlorine fixed on five carbons with varying ash contents. It is obvious that the ash content does not correlate with the amount of chlorine that was fixed on the surface. However, the amount of chlorine released from the carbons on heating to 900°C appears higher for the lower ash content carbons. The high ash content in the BPL carbons may be a factor in the amount of chlorine held on the carbon at 900°C, although the coconutshell carbon, with almost 4% ash, released most of its fixed chlorine on heating. The chemical nature of the ash may also be a contributing factor. The coalbased BPL carbon contains different metallic species than the coconut-shell-based carbon, and although the ash was not wholly analysed, neutron activation analyses detected trace amounts of aluminium, vanadium and titanium in the BPL carbon.
2.2 Characterisation of the carbon
3.2 Availability of hydrogen
All the carbon samples were characterised by temperature programmed desorption (TPD) to 900°C, nitrogen adsorption at 77 K, water adsorption at 298 K and enthalpy of immersion in water, using procedures that have been previously described [2]. Mass titration experiments, in 0.1M NaCl, and acid/base depletion, from HCl/NaCl and NaOH/NaCl solutions were carried out on some samples to determine the effect of chlorination on the acid/base properties of the carbons. Both methods have recently been described for a number of BPL carbons with varying surface oxide concentrations [10]. Wet chemical chlorine analyses were carried out by heating the powdered carbon in an excess of sodium carbonate to 550°C for 8 hours. The residue was then wetted with water, dissolved in 1 : 1 concentrated nitric acid and neutralised with 1M sodium hydroxide before being titrated with 0.100M silver nitrate. The results of these chemical analyses were consistent with those from previous neutron activation analyses [6 ]. The ash content of some of the carbons was measured by firing three, 1 g, dry carbon samples in a muffle furnace held at 850°C, to a constant weight (after at least 48 hours). All adsorption and analysis values are quoted per gram of sample with the understanding that each sample contains mainly carbon but also varying amounts of other heteroatoms, such as oxygen, hydrogen and chlorine, as well as ash. 3. RESULTS AND DISCUSSION
3.1 Effect of ash Boiling BPL carbon in HCl to reduce the ashforming mineral content was not entirely successful.
Since virtually all of the chlorine released in the TPD experiment was in the form of HCl, the availability of hydrogen on the surface of the carbon may be a contributing factor not only in the release of chlorine during temperature programmed desorption but also in the fixation of chlorine on the carbon surface if a dehydrochlorination reaction predominates. Figure 1 shows the total amount of desorbed hydrogen, [ H ], from TPD to 900°C, for the original carbons and the chlorinated equivalents. Although the chlorinated samples release hydrogen at lower temperatures as HCl, very little difference in the total amount of H desorbed is evident. Both the sugar carbon and the PVDC carbon released significantly larger quantities of hydrogen than the BPL carbons and the coconut carbon. The observation that the PVDC carbon fixed twice the amount of chlorine as the sugar carbon appears to lessen the possibility of the predominance of one of the proposed mechanisms of chlorine addition which involves the exchange of chlorine with chemisorbed hydrogen on the carbon surface. The chlorinated sugar carbon was the only chlorinated sample which retained a large quantity of hydrogen which could be desorbed from the carbon in the form of H . It cannot however be ruled 2 Table 1. Characteristics of various carbons
Sample
Ash content (% mass)
Chlorine held (% mass)
TPD/900°C, Cl removed (%)
BPL-AR BPL-deashed PVDC carbon Sugar carbon GMS-70
6.3±0.1 5.0±0.1 0 0.07±0.10 3.7±0.2
6.6 6.7 11.6 5.2 5.0
51 54 80 100 90
The effect of chlorination on surfaces properties of activated carbon
Fig. 1. Total hydrogen evolution, [ H ], from temperature programmed desorption to 900°C, from the original carbon samples and their chlorinated equivalents.
out that some sites are unavailable for chlorination simply due to pore size restrictions.
3.3 Controlled oxidation of carbon To study the effect of [O], a variety of BPL carbons with different surface oxide concentrations were selected for chlorination. The results of stepwise temperature programmed desorption of surface oxide groups from the various samples of BPL carbon before chlorination are shown in Fig. 2. Treating the carbon with ethylene at 165°C for 6 hours appeared to lower the surface oxygen concentration to the
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Fig. 3. Water adsorption isotherms at 298 K, for a number of carbons: 6, OXBPL65; #, BPL-red; (, BPL-AR; %, BPL–H O ; $, sugar carbon. Curves indicate modified DS 2 2 equation fits.
same level as reducing the carbon in a flowing H /N mixture at 500°C for 2 hours. 2 2 The corresponding water adsorption isotherms for some of these carbons and their respective modified Dubinin–Serpinskii equation fits [11] are shown in Fig. 3. These two plots illustrate how the surface oxide groups alter the initial low pressure portion of the water adsorption isotherm. As previously shown [12] for example, reducing the carbon surface shifts the onset of the region in which there is a large increase in water uptake to higher relative pressures and oxidising the surface facilitates water adsorption
Fig. 2. Desorption profiles at 300, 500, 750 and 900°C, of [O] from temperature programmed desorption for a number of BPL carbons: diagonal stripe, BPL reduced in hydrogen; solid bar, as-received BPL; horizontal stripe, hydrogen peroxide-treated BPL; cross hatched, nitric acid-treated BPL at 65°C (OXBPL65); and open bars, ethylene-treated BPL at 165°C.
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at low relative pressures. The hydrogen peroxide treatment does not alter the surface very much and none of the treatments were harsh enough to have severely modified the pore structure of the original carbon. Table 2 summarises the BET surface areas, number of primary water adsorption sites, a , from o the modified DS equation fit, and surface oxide concentrations for these BPL samples and the sugar carbon, before chlorination. It is obvious from the variance in the high relative pressure portions of the water isotherms and the differences in BET surface areas that the pore structures of the BPL carbon and the sugar carbon are dissimilar. Analysing the samples after chlorination showed that the hydrogen reduced BPL carbon (BPL-red ) picked up the most chlorine and the strongly nitric acid oxidised carbon, OXBPL65, the least. This may be due to oxygen-containing surface groups occupying a large number of sites making them unavailable to the chlorine. The highly oxidised carbon has much of its external surface covered with oxygen-containing functional groups [8]. It should be noted that this highly oxidised carbon and the air-activated sugar carbon, which also had a high concentration of surface oxides (6.4 mmol g−1), are the only two samples where [O] actually went down after chlorination and washing. The weight percent chlorine uptake for the two samples was similar at 5.2 and 4.6%. There seems to be no relationship between the BET surface area of the carbon and the fixation of chlorine on the surface.
3.4 Modification of acidic or basic sites The variation of the molar enthalpy of interaction, which was obtained by dividing the enthalpy of immersion in water by the water isotherm saturation value, a , with the [O] desorbed from the surface on s TPD for a number of chlorinated and non-chlorinated carbons is shown in Fig. 4. This relationship reinforces the notion that water binds to high energy primary sites which are essentially surface oxide groups and does not appear to be affected by the addition of chlorine to the surface. Included in this graph are data for carbons from different sources such as the sugar carbon, the PVDC carbon, the coconut-shell carbon (GMS-70) and a series of nitric acid oxidised BPL carbons previously reported [8]. This may account for some of the scatter. Unlike the as-received BPL carbon and the ethy-
Fig. 4. Net molar enthalpy of immersion in water, Dh , for w various carbons versus total oxygen desorbed, [O]: $, nonchlorinated; (, chlorinated carbons.
lene-treated BPL carbon, it has been established that the strongly oxidised BPL carbons are acidic in nature [10] and the chlorinated BPL carbons also display acidic characteristics. The mass titrations of a number of BPL carbons are shown in Fig. 5. The strongly nitric acid oxidised BPL carbon (OXBPL65) demonstrates the decrease in pzc (point of zero charge [13] or zpc [14]) with increased surface oxide concentration. The as-received BPL carbon and the ethylene-treated carbon are both basic carbons. The chlorinated equivalents are shown to have a much lower pzc although neither of these chlorinated carbons have significantly more surface oxides than the original BPL. It should be noted that the relationship between the total concentration of surface acidic groups, measured by the amount of base uptake, with [O] is
Table 2. Characteristics of various carbons Carbon
BET surface area (m2 g−1)
a* o (mmol g−1)
[O] (mmol g−1)
BPL-reduced BPL-AR BPL–H O 2 2 OXBPL65 Sugar carbon
1098 1088 1159 1099 530
0.32 0.71 1.7 3.1 3.1
0.73 1.2 1.3 6.4 5.0
*a =Primary site concentration. o
Fig. 5. Mass titrations of BPL carbons in 0.1M NaCl: &, BPL–C H ; %, chlorinated BPL–C H ; 6, BPL-AR; +, 2 4 2 4 BPL–Cl; $, OXBPL65.
The effect of chlorination on surfaces properties of activated carbon
also different for the chlorinated carbons when compared to oxidised carbons, reinforcing the observation that the chlorinated carbons are more acidic than the surface oxide concentration would indicate. A significant increase in base uptake from aqueous saline solution and decrease in acid uptake is observed when a chlorinated BPL sample is compared to the as-received BPL carbon. Figure 6Fig. 7 show typical acid and base uptakes for BPL-AR and a chlorinated BPL carbon along with the respective Langmuir fits of the reaction isotherms. Base uptakes, n , for the non-chlorinated and chlorinated B2 carbons were 0.20 and 0.46 meq g−1, respectively, while the acid uptakes, n , for the same samples A2 were 0.60 and 0.32 meq g−1. Interestingly, the sum of the acidic and basic sites changes little, suggesting that the chlorination causes an inductive effect, making the oxygen sites more acidic but not actually creating or destroying the majority of these groups on the surface of the carbon. The sum of these sites increases dramatically when more oxygen groups are
Fig. 6. Acid uptakes from 0.1M NaCl/HCl solutions for: &, BPL-AR; $, BPL–Cl. Curves represent Langmuir fits calculated from the linear plots.
Fig. 7. Base uptakes from 0.1M NaCl/NaOH solutions for: &, BPL-AR; $, BPL–Cl. Curves represent Langmuir fits calculated from the linear plots.
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added to the surface [10,15]. This observation may tentatively be rationalised by considering that as a result of the inductive effect of chemisorbed chlorine, very weak acidic oxide sites are made stronger so that they contribute to the observed acid site concentration and to the pzc (point of zero charge) observed in the mass titration experiments. This is not surprising given that chloroacetic acid (pK =2.85) is nearly a 100 times as strong as acetic acid (pK =4.75) [16 ]. a Chlorination of the surface acts then, in a manner similar to the introduction of chemisorbed oxygen except that new acid or potentially new acid sites are not produced. On this basis, chlorination can be looked upon as protecting easily oxidised areas on the carbon surface and thus inhibiting the aging process in humid air. These results are contrary to those of Puri and Bansal who observed no increase in base adsorption capacity and no change in the pH of the chlorinated samples in aqueous solution [17]. The water adsorption isotherms for the coconut and sugar carbons in this study also differ from those of Puri and Bansal in that although the higher relative pressure portions of the isotherms of chlorinated samples showed less water adsorption, the lower relative pressure portions did not show an increased uptake and the two isotherms (for chlorinated and unchlorinated ) do not intersect. Barton et al. [5] have previously attributed the increase in adsorption in the low pressure region of the water isotherm of chlorinated carbon cloth to the presence of strongly physisorbed chlorine. This physisorbed chlorine has been removed from the carbons in this study by extensive washing with methanol. Similar observations have been reported by Hall and Holmes [3]. Figure 8 shows the water adsorption isotherms on chlorinated and non-chlorinated PVDC carbon plotted on two different adsorption scales. The shape of the original water isotherm seems to be retained indicating that the main differences are due to pore blocking, probably from the
Fig. 8. Water adsorption isotherms at 298 K: %, PVDC carbon, right-hand scale; (, PVDC–Cl, left-hand scale.
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positioning of the large Cl groups on the surface. The 40% drop in surface area, from 1060 to 640 m2 g−1 after chlorination for PVDC carbon was not totally unexpected since this carbon, known to ˚ range [18], have fairly narrow micropores in the 6-A with little or no macropore and mesopore volume, held the most chlorine of any sample treated—11.6%. The proposed mechanism of the decomposition of the PVDC polymer to produce this carbon involves the formation of double bonds and a rigid aromatic structure [19]. The saturation of these bonds on chlorination may be deduced as the primary method of chlorine fixation. The intensity of the surface interaction with water appears not to be altered and the molar enthalpy of immersion increased only very slightly from 2.01 to 2.18 kJ mol−1. The act of treating a dry carbon with gaseous chlorine does not seem to alter the surface oxides on a mildly oxidised carbon such as the BPL-AR or GMS-70 carbons. Chlorination blocks or in some way reduces slightly some of the available porosity in these carbons but does not appear to remove primary water adsorption sites from the surface. These sites have often been considered to be associated with surface oxide groups.
4. CONCLUSIONS
The fixation of chlorine on carbon surfaces at moderate temperatures depends little on the surface oxygen concentration or on the ash content of the carbon. Carbons with similar concentrations of desorbable hydrogen from the surface fixed different amounts of chlorine. After chlorination the carbons show an increase in acidic properties which can be
attributed to the chlorine having an inductive influence on the surface oxygen functional groups. REFERENCES 1. Puri, B. R., in Chemistry and Physics of Carbon, Vol. 6, ed. P. L. Walker. Marcel Dekker, New York, 1970, p. 255. 2. Barton, S. S., Evans, M. J. B., Liang, S. and MacDonald, J. A. F., Carbon, 1996, 34, 975. 3. Hall, C. R. and Holmes, R. J., Carbon, 1992, 30, 173. 4. Hall, C. R. and Holmes, R. J., Carbon, 1993, 31, 881. 5. Barton, S. S., Evans, M. J. B., Koresh, J. E. and Tobias, H., Carbon, 1987, 25, 663. 6. Barton, S. S., Evans, M. B. J. and MacDonald, J. A. F., Pol. J. Chem., 1997, 71, 651. 7. Barton, S. S., Dacey, J. R. and Evans, M. J. B., Colloid Polym. Sci., 1982, 260, 726. 8. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Adsorption Sci. Technol., 1993, 10, 75. 9. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Carbon, 1991, 29, 1099. 10. Barton, S. S., Evans, M. J. B., Halliop, E. and MacDonald, J. A. F., Carbon, 1997, 35, 1361. 11. Barton, S. S., Evans, M. J. B. and MacDonald, J. A. F., Carbon, 1992, 30, 123. 12. McDermot, H. L and Arnell, J. C., J. Phys. Chem., 1954, 58, 492. 13. Noh, J. S. and Schwarz, J. A., J. Colloid Interface Sci., 1995, 130, 157. 14. Corapcioglu, M. O. and Huang, C. P., Carbon, 1987, 25, 569. 15. Bandosz, T. J., Jagiello, J. and Schwarz, J. A., Anal. Chem., 1992, 64, 891. 16. CRC Handbook of Chemistry and Physics, 62nd edn, ed. R. C. Weast and M. J. Astle. CRC Press, Boca Raton, FL, 1982, p. D142. 17. Puri, B. R. and Bansal, R. C., Indian J. Chem., 1967, 5, 556. 18. Barton, S. S., Evans, M. J. B. and Harrison, B. H., J. Colloid Interface Sci., 1974, 49, 462. 19. Dacey, J. R. and Cadenhead, D. A., Proceedings of the Fourth Conference on Carbon. Pergamon Press, Oxford, London, New York, Paris, 1960, p. 315.