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Application of inverse gas chromatography at infinite dilution to study the effects of oxidation of activated carbons
0008-6223192 $5.00+ .oO Copyright 0 1991Pergamon Pressplc
CarbonVol. 30, No. 1, pp. 63-69, 1992 Printed in GreatBritain.
APPLICATION OF INVERSE GAS CHROMATOGRAPHY AT INFINITE DILUTION TO STUDY THE EFFECTS OF OXIDATION OF ACTIVATED CARBONS JACEK JAGIEEEO*,
Department
TERESA
J. BANDOSZ
and
JAMES A.
SCHWARZ?
of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244-1190, U.S.A. (Received 22 April 1991; accepted in revised version 20 May 1991)
Abstract-Effects of the oxidation with nitric acid of active carbons from different origins have been studied by inverse gas chromatography. It is shown that the oxidation of activated carbons with nitric acid increases their acidity considerably, and at the same time decreases their adsorption affinity for n-alkanes, which is manifested by the decrease of the dispersive component of surface energy, $, and adsorption enthalpies of alkanes. A correlation has been established between the acidity of carbons measured by base titration and the energy of the specific interactions with TI bonds of unsaturated hydrocarbons. Key Words-Activated carbons, inverse gas chromatography, acidity, surface energy.
alkane and alkene adsorption,
surface
acidic oxides on activated carbons play an important role in catalytic processes[6]; they also affect adsorption, especially of polar substances. Furthermore, it was recently shown[7] that oxidation of activated carbons may increase adsorption of hydrogen, which could be very important for hydrogen storage applications. The degree of surface oxidation, the nature of the resulting acidic centers, and the adsorption properties of oxidized carbon depend on several conditions, which include the chemical reagent used as oxidant, temperature of reaction, time of reaction, type of carbon, its ash content, and microstructure. Puri, et al.[8] reported that the products obtained after treatment of outgassed charcoals with HNO, was highly acidic and contained over 20% oxygen. About 90% of this oxygen was removed in the form of carbon dioxide under vacuum at elevated temperatures. The final product had a larger surface area, and the pore size distribution was changed when compared to the untreated charcoal. On the other hand, Matsura[9] demonstrated that the surface area after oxidation was reduced, while there was an increase in the number and type of surface acidic groups. This result was explained by breaking of micropore walls by oxidation to produce oxygenated terminal groups, and by mechanical destruction of the pores by the surface tension of the oxidizing solutions. Thus, the surface area was reduced by both of these actions, especially the surface area of the carbons having small micropores. The oxidation of activated carbon may change both its physical structure and the chemical character of its surface. Structural characteristics, such as pore size distribution or BET area can be obtained using nitrogen adsorption isotherms. Surface chemical
1. INTRODUCTION
Activated carbons are widely used in a large number of industrial processes. As the number of applications increases, so does the need for the preparation and controlled modification of active carbon to obtain optimum specific properties according to the particular application. Oxidation is one of the processes that can effectively change the properties of a carbon surface. The mechanism of oxidation reactions was investigated by several authors[l-31. Among the methods leading to the formation of surface oxygen complexes, two main categories can be distinguished: utilizing the oxidizing gases like oxygen, ozone, nitrous oxide, nitric oxide, etc., and reactions in oxidizing solutions such as nitric acid, alkaline permanganate, hydrogen peroxide, acidic permanganate, and acidic dichromate. Chemisorption of oxygen on carbons causes formation of surface oxides. Their structures have not yet been investigated completely because of the great number of possible surface groups, as evident from the fact that activated carbons are chemically heterogeneous. The most common oxygen groups on the surface are carboxyl, lactonic, carbonyl, and phenolic. These groups have acidic character and can be relatively easily determined by classical chemical methods[l,4,5]. The remainder of the oxygen inventory bound to the carbon surface is considered as etheric oxygen, which is difficult to evaluate quantitatively because of its chemical inactivity. Surface
~__~~ .*Permanent address: Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakbw, Poland. fAuthor to whom correspondence should be addressed. 63
64
J.
JAGIETW
changes due to oxidation modification of acidic functional groups can be investigated by well-known methods[4,5] that are based on titration using bases of different strengths. However, the impact of the acid properties of activated carbons on their adsorption properties remains an open issue. The objectives of this paper are to study the effect of oxidation of activated carbons on their dispersive and specific interaction capacities by inverse gas chromatography at infinite dilution, and to demonstrate that surface acidity can be correlated with the specific adsorption energy of unsaturated hydrocarbons. This correlation gives a reliable and convenient method for the evaluation of acidic properties of activated carbons using the IGC technique. 2. INVERSE GAS CHROMATOGRAPHY
AT
INFINITE DILUTION
Gas solid chromatography, when applied to the investigation of solid surface properties, is usually called inverse gas chromatography. This method is based on the study of physical adsorption of appropriate molecular probes by means of chromatographic (dynamic) experiments. It is assumed, in the case of infinite dilution chromatography, that when very small amounts of solutes are injected, the adsorption is described by Henry’s law. This assumption is fulfilled when measured retention volume, V,, is independent of amount injected, and this can be easily verified experimentally. V, is the fundamental quantity measured in this method; it is directly related to the standard free energy of adsorption by the formula: AGo
=
-RTlnE
sm
(1)
where R and Tare the gas constant and temperature, B is a constant related to the standard states of gas and adsorbed phases, m and S are mass and specific surface of adsorbent. From the temperature dependence of V, the enthalpy of adsorption, AHO, can be easily calculated
AC;0 and AH0 obtained under the conditions of infinite dilution are dependent on the interaction of probe molecuies with the surface only, whereas interaction between adsorbed molecules are neglected. Analysis of these values obtained for appropriate molecular probes can provide information about certain surface properties. In general, it can be assumed that adsorbate-adsorbent interactions can be classified as dispersive (nonspecific) or polar (specific). Among probes which can undergo only dispersive interactions, nalkanes are of great practical importance. It is well known~IO] that AC” for n-alkanes varies linearly with their number of carbon atoms, n. The difference
et al.
in the AC’ of two subsequent n-alkanes, AGcH2, is then independent of n and on any reference state, which makes this quantity a very useful parameter to represent the dispersive interaction capacity of solid surface. Dorris and Gray[ll] proposed the method whereby AG,,,, is used to evaluate the dispersive component of the surface energy, $‘, by the formula:
where N is Avogadro’s number, aCHzis the surface area of a CH, group and -yCH2 is the surface energy of a hypothetical surface made of CH2 groups only (for example, polyethylene). Several empirical methods were proposed to obtain specific interaction parameters. The generally accepted way of separating specific (polar) effects from dispersive interactions is to assume their additivity and to make use of some dispersive interacting probe, treated as a reference. Practically, however, it is not trivial to find a suitable reference for a given specific probe which would have the same dispersive interactions with the surface. To solve this problem, empirical linear relationships of AG” versus different physicochemical variables, Y, for nalkanes have been used[ 12-141. AGalknne(Y) = c, + c2Y,
(4)
where c, and cZ are parameters. In the method proposed by Saint-Flour and Papier[l2], the empirical linear variation of AG” of nalkanes against the logarithm of the vapor pressure (P,) provides the reference. The difference between AG of the polar probe and ACafkaneinterpolated from the alkane straight line at POof the polar probe is taken as a specific interaction parameter, AGEp -AGsp
= AG(Y)
- AGatk”““(Y)
(5)
Here variable Y denotes log (P,,). A similar method has been proposed by Schultz, et a1.[13], who utilized the linear relationship of AGalkaneas a function of Y’ = a(g,)“2, where a is the surface area of the solute molecule and g,, its surface energy in the liquid state. It was shown[l4], however, that both of these methods fail, giving negative values of AGsp for polar probes when applied to carbonaceous materials like carbon fibers or graphite powders. It was also suggested that dispersive interactions of adsorbed molecules with a carbon surface are related to the deformation polarization parameter, PD. It was demonstrated[l4] that AGalkaneis a linear function of Y,, = Pa and, with this definition of Y”, eqn (5) gives correctly positive values for AGsf’. All the above methods assume that the dispersive component of adsorption free energy, AGD, of a solute is a linear function of a different physicochemical variable, Y, and that AGD is defined by this variable. Physical adsorption of gases on activated carbons is of great practical importance. Therefore, the use of adsorbate probes whose elec-
6.5 Oxidation of activated carbons NaOH neutralizes carboxyl, phenol& and lactonic tronic properties are sensitive to the chemical nature groups; Na,CO,-carboxyl and lactonic; and Naof the adsorption sites on activated carbon, while HCO, only carboxyl groups. The number of surface still preserving a physical adsorption process, are basic sites was calculated from the amount of hyof more significance than using variables “Y” that drochloric acid that reacted with the carbon. reflect only the physicochemical properties of the The chromatographic experiments were peradsorbate. formed with an Antek 3000 gas chromatograph Recently it was proposed[l5] to apply n-alkenes (from Antek Instruments Inc.) equipped with a in comparison with n-alkanes to study the effects of flame ionization detector. The stainless steel cola double bond placed in the hydrocarbon chain. This umns (20 cm long, 2.17 mm in diameter) were filied double bond (n electrons) interacts in a specific way with carbon particles of size ranging from 0.2 to 0.4 with the surface, especially with electron acceptor mm. Helium was used as a carrier gas with a flow sites, and this introduces an additional contribution rate of about 25 cm”/min. Injector and detector temto the total energy of adsorption of alkenes. The peratures were set at 250°C. The samples were conspecific interaction parameter e, is defined by the ditioned at 380°C in the chromatographic column following eqn: under helium gas flow for 12 hours prior to the mea= ,Qlkr~(~) _ A@lkanr(n), surement. The hydrocarbons used in this work were - E, ('4 HPLC grade (Aldrich Chemical Co.). Very small volumes of gaseous solutes were injected using a SOwhich is the special case of eqn (5) where variable p.1Hamilton syringe. The range of experimental temY is simply the number of carbon atoms in the hydrocarbon chain. The advantage of this method is peratures was 2SO-370°C. and under these conditions all chromatographic peaks were symmetrical that the quantity ln is clearly defined (the de~nition and retention times did not depend on amount inis based on well-known electronic structure of aljected, indicating that the Henry’s law region was kanes and alkenes) and easily measured. This parameter can be taken as a measure of the specific attained. Retention volumes were corrected for the gas compressibility. The precision of the mea(electron acceptor) interaction capacity of the surface. surement of retention times was 5%. The temperature of the column was stabilized with accuracy at +-O.l”C. 3. EXPERIMENTAL
Four activated carbons from different sources (Westvaco, Norit, Calgon, North American), whose characteristics are given in Table 1, were oxidized with 15 N (73%) HNO, solution at 25°C for 2.5 hours with continuous stirring. The vigorous oxidation reaction was indicated by an exothermic effect and release of brown fumes (nitric oxides). The temperature rise was not measured. After treatment, the samples were washed with distilled water to zero acid removal and oven dried at 100°C. The oxygenated surface groups were determined according to the method of Boehm]4]. One gram of carbon sample was placed in 25 ml of the following 0.05N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 hours and then 5 ml of the each filtrate was pipetted and the excess of base and acid titrated with HCI and NaOH, respectively. The numbers of acidic sites of various types were calculated under the assumption that
4. RESULTS
AND
DISCUSSION
Nitric acid is very often used as an oxidizing agent[7,16-181. The nature of surface groups formed was investigated by Zawadzki[l9]. At the initial stage of carbon oxidation, the oxidizing agent is a nitric acid solution (HNO,, (OH)*NO ’ , NO,+, NO,-, H,O ‘, f&O) which, during reduction, forms other nitrogen oxides. These oxides can oxidize carbons and then are reduced to N2, The results of oxidation are structures containing nitrogen-oxygen bonds, iono-radical structures C-O, carbonyl groups, nitrogroups that transform to carboxyls, quinones, and lactonic groups as a result of intermolecular arrangements. Hadzi and Novak[20] suggested that the fixation of a nitrogroup on a benzene ring has an inductive effect, attracting the eiectrons, stabilizing the anion of the corresponding acid, and thus increasing the acidity. Table 2 collects the results of the Boehm titration
Table 1. General characteristics of carbon samples
J. JAGIEJ+O et al.
66
Table 2. Results of Boehm titration
of the carbon samples. As expected[21,22], the oxidation treatment increases significantly the acidic sites. The result of oxidation with nitric acid is to increase the number of carboxyl groups about three times in the case of Westvaco and Calgon, and about ten times in the case of North American and Norit, while the effect of oxidation on phenolic and lactonic groups is smaller (the number of these groups decrease for Westvaco). The greatest changes observed for Norit and North American carbons may be attributed to their larger content of metallic elements (Table l), which can have a catalytic influence on the oxidation reaction[23]. On the other hand, the increment of the total number of acidic groups is similar for each carbon, about 50 meq/lOOg. In general, the final acidity will depend on the initial acidity of the carbon. However, the increment in acidity due to nitric acid treatment should be controlled by the aggressive action of nitric acid, which is determined by its concentration and contact time with the carbon. These remain constant in our experiments and, indeed, we find that the increment in acidity is also approximately constant. Inverse gas chromatography results are reported in Table 3. The dispersive component of surface energy, 9, was calculated from eqn (3) using AG,,, values obtained from the linear relationship of the
adsorption free energy, AGo, versus number of carbon atoms for alkanes from propane to heptane. An example of this dependence is presented in Fig. 1; similar linearity was observed for all samples of carbons. It was possible to describe very accurately the dependence of ln(V,) as a function of the number of carbon atoms and temperature using one simple eqn which follows for alkanes from relation (2), ln(VN)
Westvaco Westvaco ox. Norit Norit ox. Calgon Calnon ox. North Am. North Am.
+na+y+c
-*
b
results
AG CH2 kJ/mol
& mJ/m2
AHCH2 kJ/mol
AH hexane kJ/mol
kJ/mol
5.19 f0.15 5.01 f0.12
313 fl8 292 *14
9.7 r1.1 9.3 f2.4
72.6 f1.9 65.2 f3.6
-0.35 f0.15 0.55 f0.15
7.03 6.4 7.07 6.87 6.65 6.39
575 475 580 549 513 474
11.5 11.3 10.6 11.5 11.8 10.2
77;3 73.5 77.2 75.9 73.8 71.5
-1.84 -0.28 -1.70 -0.25 -1.00 -0.25
50.08 M.12 *0.12 f0.12 kO.08 f0.12
f13 f18 f21 f19 f12 f17
(7)
where n is a number of carbon atoms, AH,,, is the increment of AH per one carbon atom and a, b, and c are empirical constants. This regular linear dependence on rz enables us to exclude molecular sieving effects, which were studied using IGC by DomingoGarcia, et al. [24], who observed a dramatic drop in AGo and AH0 of hydrocarbons having diameters exceeding the size of pores for the molecular sieving carbon. The first four columns in Table 3 report values of the different thermodynamic quantities describing adsorption of hydrocarbons at infinite dilution and, as such, characterize the dispersive interactions with the carbon surface. However, from our error analysis it follows that AG,,,, which is obtained from isothermal data, is calculated more precisely than
Table 3. Inverse gas chromatography Sample
=
f0.8 f2.6 f1.2 f2.6 f0.7 f2.4
f3.0 f3.6 f2.7 f3.6 f2.7 f3.1
e11
f0.15 f0.15 f0.15 f0.15 f0.15 &O.15
Oxidation of activated carbons
1.0 3
4
7
6
5
number of carbon atoms Fig. 1. Variation of the retention volume VN of alkancs measured tor Norit carbon at the following temperatures: 523 K (m), 573 K (O), 603 K (*), 623 K (+I), 643 K (0).
AHCHZ or AH” obtained
from the temperature dependence of the retention volume. Thus, it seems that AG,,, values are more reliable than enthalpies, especially when small changes are observed. We report A G,,,, and rf; however, they are related, the former is measured experimentally and the latter is derived. The results consistently show that oxidation
3
4
5
6
of all carbons decreases the dispersive interactions with alkanes. The E, values reported in Table 3 were obtained from the difference between AGo of hexene and hexane, this parameter is generally negative for our carbon samples. Figure 2 illustrates the concept of E, using as examples chromatographic results for silica
7
8
9
10
number of carbon atoms Fig. 2. Dependence of AGo of alkane (-) and alkene (---) adsorption versus number of carbon atoms measured for Norit carbon at 423 K and Davison silica[27] at 623 K.
68
J. JAGIE#O et&.
0
20
40
60
80
100
120
140
number of acidic groups (meq/lOOg) Fig. 3. Correlation between specific interaction energy of hexene, en, and total acidity of carbon surface; arrows indicate the oxidation effect.
(Davison) and activated carbon (Norit). A positive E, value for the silica is related to the high acidity of its surface, and to the strong specific interactions with alkenes. Smaller interaction energy with alkenes than alkanes is probably a general property of the carbon surface._ Results of extensive studies of graphitized carbon blacks by Kiselev, et al. [10,25,26] also report lower values of -AGO and - AN0 for alkenes than alkanes; in particular, taking their results for graphitized carbon black at 175“Cf26] we obtain E, = - l.lkJ/mol. In the absence of specific interactions, molecular geometry will play an important role during adsorbate/adsorbent interactions. The higher interaction energy for saturated hydrocarbons is consistent with the idea that the alkane configuration on the carbon surface is more favorable for interaction than that of the alkene. Our results show that for oxidized carbon surfaces, E, increases, which can be explained by the contribution of specific interactions with electron acceptor surface sites induced by the oxidation. If this assertion is valid, there should be a relationship between the acidity measured by the Boehm titration and the values of E,. Such a relationship would be equivalent to a correlation between the total number of acidic groups, a surface chemical property, and the adsorption thermodynamics of alkene probes. We tested this hypothesis, and the results are shown in Fig. 3. A near-linear dependence of E, on the number of acidic groups is found. This result demonstrates that such a correlation between surface chemical and adsorption thermodynamic properties of alkenes does exist and, surprisingly, that microstructural effects between different carbons play a minor role in assessing surface properties. These results are presented to demonstrate a general trend; we cannot expect a unique dependence
because each quantity is related to somewhat different properties of the carbon surface. For example, based on previous results[22] our treatment by nitric acid probably did not alter structural characteristics, such as surface area or pore-size distribution, appreciably. Had these been affected, the chemical effects revealed by E, might have been “swamped.” Whatever the case, under the conditions of this study, it is clearly seen that the acidic character of the carbon surface increases the adsorption energy of alkenes. 5. CONCLUSION The oxidation of activated carbons with nitric acid increases their acidity considerably and changes their adsorption properties. Inverse gas chromatography studies show the existence of a correlation between the acidity of carbons and the energy of the specific interactions with 7~ bonds of unsaturated hydrocarbons. This suggests that IGC is a useful method for the evaluation of acidic properties of activated carbons. The dispersive component of surface energy, r:, and enthalpies of alkane adsorption decrease with the oxidation of carbons, which indicates that the oxidized carbon has a lower affinity for hydrocarbons than that of the untreated carbon surface. Acknowledgment-The
work was supported by the New York State Energy Research and Development Authority under Contract 139-ERER-POP-90. REFERENCES
1. B. R. Puri, in Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, pp. 191-241. Marcel Dekker, New York (1970). 2. J. Zawadzki, Carbon 16,491 (1978). 3. S. S. Barton and B. H. Harrison, Carbon 13, 283 (1975).
Oxidation of activated carbons 4. H. P. Boehm, Advan. Catalysti 16, 179 (1966). 5. H. P. Boehm, and M. Voll, Carbon 8, 227 (1970). 6. R. C. Bansal and T. L. Dhamii, Carbon 18, 137 (1980). 7. R. K. Agarwal, J. S. Noh, J. A. Schwarz, and P. Davini, Carbon 25, 219 (1987). 8. B. R. Puri, S. Singh, and 0. P. Makajan, J. Indian Chem. Sot. 42, 427 (1965). 9. Y. Matsura, J. Appl. Chem. Biotechnof. 25,39 (1975). 10. A. V. Kiselev and Y. I. Yashin, Gas Adsorption Chromatography, Plenum Press, New York (1969). 11. G. M. Dorris and D. G. Gray, J. Colloid Interface Sci. 71, 93 (1979).
12. C. Saint-Flour and E. Papirer, J. Colloid Interface Sci. 91, 69 (1983).
13. J. Schultz, L. LavieIIe, and C. Martin, J. Adhesion 23, 45 (1987).
14. S. Dong, M. Brendle, and J. B. Donnet, Chromatographia 28, 469 (1989).
15. M. Sidqi, G. Ligner, J. JagieJlo, Papirer, Chromatographia 28, 588 16. S. Hagiwara, K. Tsutsumi, and H. 16, 89 (1978). 17. Y. Matsura. S. Hagiwara, and H. 14, 163 (1976).
H. Balard, and E. (1989).
Takahaski, Carbon
69
18. E. Papirer, J. Dentzer, S. Li, and J. B. Donnet, Carbon 29, 69 (1991). 19. J. Zawadzki, In Chemistry and Physics of Carbon (Edited by P. A. Thrower), Vol. 21, pp. 187-217. Marcel __ Dekker, New York (1989). 20. D. Hadzi and A. Novak. Trans. Faradavi Sot. 51.1614 (1955). 21. H. P. Boehm, E Diekl, W. Heck, and R. Sappok, Angew. Chem. 76, 742 (1964). 22. J. S. Noh and J. A. Schwarz, Carbon 28, 675 (1990). 23. P. L. Walker, M. Shelf, and R. A. Anderson, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 4, pp. 36-95. Marcel Dekker, New York (1968). 24. M. Domingo-Garcia, I. Fernandez-Morales, F. J. Lopez-Garzon, C. Moreno-Castilla, and M. J. PradosRamirez. J. Colloid Interface Sci. 136. 160 (1990). 25. N. N. Avgul and A. V.’ Kiselev, In Chenkstry’and Physics of Curbon (Edited by P. L. Walker, Jr.), Vol. 6, pp. 1-122. Marcel Dekker, New York (1970). 26. 0. G. Eisen, A. V. Kiselev, A. E. Pilt, S. A. Rang, and K. D. Shcherbakova, Chromatographia 4, 448 (1971).
Takahaski, Carbon
27. C. Contescu, J. Jagie4lo, and J. A. Schwarz. J. Catal. 131, 443 (1991).
CarbonVol. 30, No. 1, pp. 63-69, 1992 Printed in GreatBritain.
APPLICATION OF INVERSE GAS CHROMATOGRAPHY AT INFINITE DILUTION TO STUDY THE EFFECTS OF OXIDATION OF ACTIVATED CARBONS JACEK JAGIEEEO*,
Department
TERESA
J. BANDOSZ
and
JAMES A.
SCHWARZ?
of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244-1190, U.S.A. (Received 22 April 1991; accepted in revised version 20 May 1991)
Abstract-Effects of the oxidation with nitric acid of active carbons from different origins have been studied by inverse gas chromatography. It is shown that the oxidation of activated carbons with nitric acid increases their acidity considerably, and at the same time decreases their adsorption affinity for n-alkanes, which is manifested by the decrease of the dispersive component of surface energy, $, and adsorption enthalpies of alkanes. A correlation has been established between the acidity of carbons measured by base titration and the energy of the specific interactions with TI bonds of unsaturated hydrocarbons. Key Words-Activated carbons, inverse gas chromatography, acidity, surface energy.
alkane and alkene adsorption,
surface
acidic oxides on activated carbons play an important role in catalytic processes[6]; they also affect adsorption, especially of polar substances. Furthermore, it was recently shown[7] that oxidation of activated carbons may increase adsorption of hydrogen, which could be very important for hydrogen storage applications. The degree of surface oxidation, the nature of the resulting acidic centers, and the adsorption properties of oxidized carbon depend on several conditions, which include the chemical reagent used as oxidant, temperature of reaction, time of reaction, type of carbon, its ash content, and microstructure. Puri, et al.[8] reported that the products obtained after treatment of outgassed charcoals with HNO, was highly acidic and contained over 20% oxygen. About 90% of this oxygen was removed in the form of carbon dioxide under vacuum at elevated temperatures. The final product had a larger surface area, and the pore size distribution was changed when compared to the untreated charcoal. On the other hand, Matsura[9] demonstrated that the surface area after oxidation was reduced, while there was an increase in the number and type of surface acidic groups. This result was explained by breaking of micropore walls by oxidation to produce oxygenated terminal groups, and by mechanical destruction of the pores by the surface tension of the oxidizing solutions. Thus, the surface area was reduced by both of these actions, especially the surface area of the carbons having small micropores. The oxidation of activated carbon may change both its physical structure and the chemical character of its surface. Structural characteristics, such as pore size distribution or BET area can be obtained using nitrogen adsorption isotherms. Surface chemical
1. INTRODUCTION
Activated carbons are widely used in a large number of industrial processes. As the number of applications increases, so does the need for the preparation and controlled modification of active carbon to obtain optimum specific properties according to the particular application. Oxidation is one of the processes that can effectively change the properties of a carbon surface. The mechanism of oxidation reactions was investigated by several authors[l-31. Among the methods leading to the formation of surface oxygen complexes, two main categories can be distinguished: utilizing the oxidizing gases like oxygen, ozone, nitrous oxide, nitric oxide, etc., and reactions in oxidizing solutions such as nitric acid, alkaline permanganate, hydrogen peroxide, acidic permanganate, and acidic dichromate. Chemisorption of oxygen on carbons causes formation of surface oxides. Their structures have not yet been investigated completely because of the great number of possible surface groups, as evident from the fact that activated carbons are chemically heterogeneous. The most common oxygen groups on the surface are carboxyl, lactonic, carbonyl, and phenolic. These groups have acidic character and can be relatively easily determined by classical chemical methods[l,4,5]. The remainder of the oxygen inventory bound to the carbon surface is considered as etheric oxygen, which is difficult to evaluate quantitatively because of its chemical inactivity. Surface
~__~~ .*Permanent address: Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakbw, Poland. fAuthor to whom correspondence should be addressed. 63
64
J.
JAGIETW
changes due to oxidation modification of acidic functional groups can be investigated by well-known methods[4,5] that are based on titration using bases of different strengths. However, the impact of the acid properties of activated carbons on their adsorption properties remains an open issue. The objectives of this paper are to study the effect of oxidation of activated carbons on their dispersive and specific interaction capacities by inverse gas chromatography at infinite dilution, and to demonstrate that surface acidity can be correlated with the specific adsorption energy of unsaturated hydrocarbons. This correlation gives a reliable and convenient method for the evaluation of acidic properties of activated carbons using the IGC technique. 2. INVERSE GAS CHROMATOGRAPHY
AT
INFINITE DILUTION
Gas solid chromatography, when applied to the investigation of solid surface properties, is usually called inverse gas chromatography. This method is based on the study of physical adsorption of appropriate molecular probes by means of chromatographic (dynamic) experiments. It is assumed, in the case of infinite dilution chromatography, that when very small amounts of solutes are injected, the adsorption is described by Henry’s law. This assumption is fulfilled when measured retention volume, V,, is independent of amount injected, and this can be easily verified experimentally. V, is the fundamental quantity measured in this method; it is directly related to the standard free energy of adsorption by the formula: AGo
=
-RTlnE
sm
(1)
where R and Tare the gas constant and temperature, B is a constant related to the standard states of gas and adsorbed phases, m and S are mass and specific surface of adsorbent. From the temperature dependence of V, the enthalpy of adsorption, AHO, can be easily calculated
AC;0 and AH0 obtained under the conditions of infinite dilution are dependent on the interaction of probe molecuies with the surface only, whereas interaction between adsorbed molecules are neglected. Analysis of these values obtained for appropriate molecular probes can provide information about certain surface properties. In general, it can be assumed that adsorbate-adsorbent interactions can be classified as dispersive (nonspecific) or polar (specific). Among probes which can undergo only dispersive interactions, nalkanes are of great practical importance. It is well known~IO] that AC” for n-alkanes varies linearly with their number of carbon atoms, n. The difference
et al.
in the AC’ of two subsequent n-alkanes, AGcH2, is then independent of n and on any reference state, which makes this quantity a very useful parameter to represent the dispersive interaction capacity of solid surface. Dorris and Gray[ll] proposed the method whereby AG,,,, is used to evaluate the dispersive component of the surface energy, $‘, by the formula:
where N is Avogadro’s number, aCHzis the surface area of a CH, group and -yCH2 is the surface energy of a hypothetical surface made of CH2 groups only (for example, polyethylene). Several empirical methods were proposed to obtain specific interaction parameters. The generally accepted way of separating specific (polar) effects from dispersive interactions is to assume their additivity and to make use of some dispersive interacting probe, treated as a reference. Practically, however, it is not trivial to find a suitable reference for a given specific probe which would have the same dispersive interactions with the surface. To solve this problem, empirical linear relationships of AG” versus different physicochemical variables, Y, for nalkanes have been used[ 12-141. AGalknne(Y) = c, + c2Y,
(4)
where c, and cZ are parameters. In the method proposed by Saint-Flour and Papier[l2], the empirical linear variation of AG” of nalkanes against the logarithm of the vapor pressure (P,) provides the reference. The difference between AG of the polar probe and ACafkaneinterpolated from the alkane straight line at POof the polar probe is taken as a specific interaction parameter, AGEp -AGsp
= AG(Y)
- AGatk”““(Y)
(5)
Here variable Y denotes log (P,,). A similar method has been proposed by Schultz, et a1.[13], who utilized the linear relationship of AGalkaneas a function of Y’ = a(g,)“2, where a is the surface area of the solute molecule and g,, its surface energy in the liquid state. It was shown[l4], however, that both of these methods fail, giving negative values of AGsp for polar probes when applied to carbonaceous materials like carbon fibers or graphite powders. It was also suggested that dispersive interactions of adsorbed molecules with a carbon surface are related to the deformation polarization parameter, PD. It was demonstrated[l4] that AGalkaneis a linear function of Y,, = Pa and, with this definition of Y”, eqn (5) gives correctly positive values for AGsf’. All the above methods assume that the dispersive component of adsorption free energy, AGD, of a solute is a linear function of a different physicochemical variable, Y, and that AGD is defined by this variable. Physical adsorption of gases on activated carbons is of great practical importance. Therefore, the use of adsorbate probes whose elec-
6.5 Oxidation of activated carbons NaOH neutralizes carboxyl, phenol& and lactonic tronic properties are sensitive to the chemical nature groups; Na,CO,-carboxyl and lactonic; and Naof the adsorption sites on activated carbon, while HCO, only carboxyl groups. The number of surface still preserving a physical adsorption process, are basic sites was calculated from the amount of hyof more significance than using variables “Y” that drochloric acid that reacted with the carbon. reflect only the physicochemical properties of the The chromatographic experiments were peradsorbate. formed with an Antek 3000 gas chromatograph Recently it was proposed[l5] to apply n-alkenes (from Antek Instruments Inc.) equipped with a in comparison with n-alkanes to study the effects of flame ionization detector. The stainless steel cola double bond placed in the hydrocarbon chain. This umns (20 cm long, 2.17 mm in diameter) were filied double bond (n electrons) interacts in a specific way with carbon particles of size ranging from 0.2 to 0.4 with the surface, especially with electron acceptor mm. Helium was used as a carrier gas with a flow sites, and this introduces an additional contribution rate of about 25 cm”/min. Injector and detector temto the total energy of adsorption of alkenes. The peratures were set at 250°C. The samples were conspecific interaction parameter e, is defined by the ditioned at 380°C in the chromatographic column following eqn: under helium gas flow for 12 hours prior to the mea= ,Qlkr~(~) _ A@lkanr(n), surement. The hydrocarbons used in this work were - E, ('4 HPLC grade (Aldrich Chemical Co.). Very small volumes of gaseous solutes were injected using a SOwhich is the special case of eqn (5) where variable p.1Hamilton syringe. The range of experimental temY is simply the number of carbon atoms in the hydrocarbon chain. The advantage of this method is peratures was 2SO-370°C. and under these conditions all chromatographic peaks were symmetrical that the quantity ln is clearly defined (the de~nition and retention times did not depend on amount inis based on well-known electronic structure of aljected, indicating that the Henry’s law region was kanes and alkenes) and easily measured. This parameter can be taken as a measure of the specific attained. Retention volumes were corrected for the gas compressibility. The precision of the mea(electron acceptor) interaction capacity of the surface. surement of retention times was 5%. The temperature of the column was stabilized with accuracy at +-O.l”C. 3. EXPERIMENTAL
Four activated carbons from different sources (Westvaco, Norit, Calgon, North American), whose characteristics are given in Table 1, were oxidized with 15 N (73%) HNO, solution at 25°C for 2.5 hours with continuous stirring. The vigorous oxidation reaction was indicated by an exothermic effect and release of brown fumes (nitric oxides). The temperature rise was not measured. After treatment, the samples were washed with distilled water to zero acid removal and oven dried at 100°C. The oxygenated surface groups were determined according to the method of Boehm]4]. One gram of carbon sample was placed in 25 ml of the following 0.05N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 hours and then 5 ml of the each filtrate was pipetted and the excess of base and acid titrated with HCI and NaOH, respectively. The numbers of acidic sites of various types were calculated under the assumption that
4. RESULTS
AND
DISCUSSION
Nitric acid is very often used as an oxidizing agent[7,16-181. The nature of surface groups formed was investigated by Zawadzki[l9]. At the initial stage of carbon oxidation, the oxidizing agent is a nitric acid solution (HNO,, (OH)*NO ’ , NO,+, NO,-, H,O ‘, f&O) which, during reduction, forms other nitrogen oxides. These oxides can oxidize carbons and then are reduced to N2, The results of oxidation are structures containing nitrogen-oxygen bonds, iono-radical structures C-O, carbonyl groups, nitrogroups that transform to carboxyls, quinones, and lactonic groups as a result of intermolecular arrangements. Hadzi and Novak[20] suggested that the fixation of a nitrogroup on a benzene ring has an inductive effect, attracting the eiectrons, stabilizing the anion of the corresponding acid, and thus increasing the acidity. Table 2 collects the results of the Boehm titration
Table 1. General characteristics of carbon samples
J. JAGIEJ+O et al.
66
Table 2. Results of Boehm titration
of the carbon samples. As expected[21,22], the oxidation treatment increases significantly the acidic sites. The result of oxidation with nitric acid is to increase the number of carboxyl groups about three times in the case of Westvaco and Calgon, and about ten times in the case of North American and Norit, while the effect of oxidation on phenolic and lactonic groups is smaller (the number of these groups decrease for Westvaco). The greatest changes observed for Norit and North American carbons may be attributed to their larger content of metallic elements (Table l), which can have a catalytic influence on the oxidation reaction[23]. On the other hand, the increment of the total number of acidic groups is similar for each carbon, about 50 meq/lOOg. In general, the final acidity will depend on the initial acidity of the carbon. However, the increment in acidity due to nitric acid treatment should be controlled by the aggressive action of nitric acid, which is determined by its concentration and contact time with the carbon. These remain constant in our experiments and, indeed, we find that the increment in acidity is also approximately constant. Inverse gas chromatography results are reported in Table 3. The dispersive component of surface energy, 9, was calculated from eqn (3) using AG,,, values obtained from the linear relationship of the
adsorption free energy, AGo, versus number of carbon atoms for alkanes from propane to heptane. An example of this dependence is presented in Fig. 1; similar linearity was observed for all samples of carbons. It was possible to describe very accurately the dependence of ln(V,) as a function of the number of carbon atoms and temperature using one simple eqn which follows for alkanes from relation (2), ln(VN)
Westvaco Westvaco ox. Norit Norit ox. Calgon Calnon ox. North Am. North Am.
+na+y+c
-*
b
results
AG CH2 kJ/mol
& mJ/m2
AHCH2 kJ/mol
AH hexane kJ/mol
kJ/mol
5.19 f0.15 5.01 f0.12
313 fl8 292 *14
9.7 r1.1 9.3 f2.4
72.6 f1.9 65.2 f3.6
-0.35 f0.15 0.55 f0.15
7.03 6.4 7.07 6.87 6.65 6.39
575 475 580 549 513 474
11.5 11.3 10.6 11.5 11.8 10.2
77;3 73.5 77.2 75.9 73.8 71.5
-1.84 -0.28 -1.70 -0.25 -1.00 -0.25
50.08 M.12 *0.12 f0.12 kO.08 f0.12
f13 f18 f21 f19 f12 f17
(7)
where n is a number of carbon atoms, AH,,, is the increment of AH per one carbon atom and a, b, and c are empirical constants. This regular linear dependence on rz enables us to exclude molecular sieving effects, which were studied using IGC by DomingoGarcia, et al. [24], who observed a dramatic drop in AGo and AH0 of hydrocarbons having diameters exceeding the size of pores for the molecular sieving carbon. The first four columns in Table 3 report values of the different thermodynamic quantities describing adsorption of hydrocarbons at infinite dilution and, as such, characterize the dispersive interactions with the carbon surface. However, from our error analysis it follows that AG,,,, which is obtained from isothermal data, is calculated more precisely than
Table 3. Inverse gas chromatography Sample
=
f0.8 f2.6 f1.2 f2.6 f0.7 f2.4
f3.0 f3.6 f2.7 f3.6 f2.7 f3.1
e11
f0.15 f0.15 f0.15 f0.15 f0.15 &O.15
Oxidation of activated carbons
1.0 3
4
7
6
5
number of carbon atoms Fig. 1. Variation of the retention volume VN of alkancs measured tor Norit carbon at the following temperatures: 523 K (m), 573 K (O), 603 K (*), 623 K (+I), 643 K (0).
AHCHZ or AH” obtained
from the temperature dependence of the retention volume. Thus, it seems that AG,,, values are more reliable than enthalpies, especially when small changes are observed. We report A G,,,, and rf; however, they are related, the former is measured experimentally and the latter is derived. The results consistently show that oxidation
3
4
5
6
of all carbons decreases the dispersive interactions with alkanes. The E, values reported in Table 3 were obtained from the difference between AGo of hexene and hexane, this parameter is generally negative for our carbon samples. Figure 2 illustrates the concept of E, using as examples chromatographic results for silica
7
8
9
10
number of carbon atoms Fig. 2. Dependence of AGo of alkane (-) and alkene (---) adsorption versus number of carbon atoms measured for Norit carbon at 423 K and Davison silica[27] at 623 K.
68
J. JAGIE#O et&.
0
20
40
60
80
100
120
140
number of acidic groups (meq/lOOg) Fig. 3. Correlation between specific interaction energy of hexene, en, and total acidity of carbon surface; arrows indicate the oxidation effect.
(Davison) and activated carbon (Norit). A positive E, value for the silica is related to the high acidity of its surface, and to the strong specific interactions with alkenes. Smaller interaction energy with alkenes than alkanes is probably a general property of the carbon surface._ Results of extensive studies of graphitized carbon blacks by Kiselev, et al. [10,25,26] also report lower values of -AGO and - AN0 for alkenes than alkanes; in particular, taking their results for graphitized carbon black at 175“Cf26] we obtain E, = - l.lkJ/mol. In the absence of specific interactions, molecular geometry will play an important role during adsorbate/adsorbent interactions. The higher interaction energy for saturated hydrocarbons is consistent with the idea that the alkane configuration on the carbon surface is more favorable for interaction than that of the alkene. Our results show that for oxidized carbon surfaces, E, increases, which can be explained by the contribution of specific interactions with electron acceptor surface sites induced by the oxidation. If this assertion is valid, there should be a relationship between the acidity measured by the Boehm titration and the values of E,. Such a relationship would be equivalent to a correlation between the total number of acidic groups, a surface chemical property, and the adsorption thermodynamics of alkene probes. We tested this hypothesis, and the results are shown in Fig. 3. A near-linear dependence of E, on the number of acidic groups is found. This result demonstrates that such a correlation between surface chemical and adsorption thermodynamic properties of alkenes does exist and, surprisingly, that microstructural effects between different carbons play a minor role in assessing surface properties. These results are presented to demonstrate a general trend; we cannot expect a unique dependence
because each quantity is related to somewhat different properties of the carbon surface. For example, based on previous results[22] our treatment by nitric acid probably did not alter structural characteristics, such as surface area or pore-size distribution, appreciably. Had these been affected, the chemical effects revealed by E, might have been “swamped.” Whatever the case, under the conditions of this study, it is clearly seen that the acidic character of the carbon surface increases the adsorption energy of alkenes. 5. CONCLUSION The oxidation of activated carbons with nitric acid increases their acidity considerably and changes their adsorption properties. Inverse gas chromatography studies show the existence of a correlation between the acidity of carbons and the energy of the specific interactions with 7~ bonds of unsaturated hydrocarbons. This suggests that IGC is a useful method for the evaluation of acidic properties of activated carbons. The dispersive component of surface energy, r:, and enthalpies of alkane adsorption decrease with the oxidation of carbons, which indicates that the oxidized carbon has a lower affinity for hydrocarbons than that of the untreated carbon surface. Acknowledgment-The
work was supported by the New York State Energy Research and Development Authority under Contract 139-ERER-POP-90. REFERENCES
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