Langmuir 1997, 13, 1225-1228
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Characterization of Modified Active Carbons by Adsorption of Pure Water and Benzene Vapors and Ternary Liquid Mixture Benzene + Diethyl Ketone + n-Heptane† J. Goworek,*,‡ A. SÄ wia¸ tkowski,§ and S. Biniak| Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland, Institute of Chemistry, Military Technical Academy, 00-908 Warsaw, Poland, and Faculty of Chemistry, Nicolaus Copernicus University, 87-100 Torun´ , Poland Received November 22, 1995. In Final Form: July 2, 1996X Thermally treated and oxidized active carbon samples were investigated. The chemical character of active carbon surface was estimated by using water vapor adsorption, thermogravimetric measurements, and X-ray photoelectron spectroscopy. Porous structure was analyzed on the basis of benzene vapor isotherms. Oxidation as well as heating in an argon atmosphere changed markedly the concentration of the surface oxygen functional groups without substantial changes in the porous structure of the carbons. Excess adsorption isotherms from ternary solutions benzene (1) + diethyl ketone (2) + n-heptane (3) on chemically modified active carbon samples were measured. Analysis of the excess adsorption isotherms of benzene from equimolar solutions diethyl ketone + n-heptane and analysis of the changes of this binary solvent composition indicated a close correlation between the concentration of functional groups on the carbon surface and adsorption of liquid components.
1. Introduction The adsorption selectivity of active carbons depends on the chemical character of their surface. A significant role is here ascribed to the surface functional groups containing mainly oxygen. The variety of compounds (functional groups) present on the surface make a complete description of the chemical structure of the active carbons very complicated. Adsorption from the liquid mixtures containing polar and nonpolar components is strongly determined by the chemical composition of carbon surfaces.1-5 The range of preferential adsorption of these components is related to the number of the surface sites of appropriate polarity. Great difficulty in description of the adsorption process is introduced by the chemical and physical heterogeneity of the solid adsorbent. In the case of the liquid phase there are at least two components involved in competitive adsorption at the solid surface. In contrast to the extensive literature concerning adsorption from binary liquid mixtures,6-10 the ternary liquid mixtures were much less investigated. Adsorption from multicomponent mixtures was formulated in terms of classical † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. ‡ Maria Curie-Skłodowska University. § Military Technical Academy. | Nicolaus Copernicus University. X Abstract published in Advance ACS Abstracts, February 15, 1997.
(1) Bansal, R. C.; Dhami T. L. Carbon 1977, 15, 153. (2) Jankowska, H.; SÄ wia¸ tkowski, A.; Os´cik, J.; Kusak, R. Carbon 1983, 21, 117. (3) Jankowska, H.; SÄ wia¸ tkowski, A.; Goworek, J. Mater. Sci. Forum 1988, 25-26, 501. (4) Goworek, J.; Kaz´mierczak, J.; SÄ wia¸ tkowski, A. Carbon 1990, 28, 849. (5) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. R., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (6) Sircar, S.; Myers, A. L. J. Phys. Chem. 1970, 74, 2829. (7) Everett, D. H. Specialist Periodical Reports; Colloid Science; Royal Chemical Society: London, 1987; Vol. 3, Chapter 2. (8) Everett, D. H. Pure Appl. Chem. 1986, 58, 967. (9) Goncalves da Silva, A. M.; Soares, V. A. M.; Calado, J. G. G. J. Chem. Soc., Faraday Trans. 1971, 87, 755. (10) Everett, D. H. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Academic Press: London, 1983.
S0743-7463(95)01065-1 CCC: $14.00
thermodynamics more then 20 years ago, assuming surface homogeneity.11 Next, this theory was extended to adsorption onto heterogeneous solid surfaces.12,13 Most of these treatments concern prediction of the adsorption from multicomponent mixtures using parameters characterizing the binary mixtures of the same components.11,14 Recently, we proposed a simple approach to the testing of mixed and multilayer surface phases, consisting of the analysis of changes in mixed (equimolar) solvent composition during adsorption of component interacting specifically with the surface. The interpretation of adsorption data solution can be enreached when complemented with information from the adsorption of the pure components from the gas phase. For a better understanding of the interfacial phenomena on a microscopic scale, the adsorbent surface should be well-defined and the surface species estimated by using independent techniques. In the previous papers, we attempted to correlate the content of the surface species with some parameters describing adsorption from the liquid phase.2-4 In the present paper the adsorption from ternary liquid mixtures (benzene + diethyl ketone + n-heptane) on chemically modified active carbons was investigated. Analysis of the experimental data of adsorption from ternary mixtures, measured at a constant ratio of the mole fractions of polar (diethyl ketone) and nonpolar (n-heptane) components in the bulk phase gives information concerning the competition of these components for carbon surface at simultaneous preferential adsorption of the third component (benzene). The aim of the present work is to illustrate the influence of chemical modification of carbon surface (oxidation, thermal decomposition of the surface oxides) on adsorption of liquid components. 2. Experimental Section Active carbon Norit R4-ex was used in the experiments. The mineral matter (ash) was removed from commercially obtained (11) Minka, C.; Myers, A. L. AIChE J. 1973, 19, 453. (12) Boro´wko, M.; Jaroniec, M.; Os´cik, J.; Kusak, R. J. Colloid Interface Sci. 1979, 69, 311. (13) Deryło-Marczewska, A.; Jaroniec, M.; Os´cik, J.; Marczewski, A. W.; Kusak, R. Chem. Eng. Sci. 1987, 42, 2143. (14) Myers, A. L. In Fundamentals of Adsorption; Mersmann, A. B., Scholl, S. E., Eds.; Engineering Foundation: New York 1991; p 609.
© 1997 American Chemical Society
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Table 1. Information Concerning the Surface Species on the Chemically Modified Active Carbon Norit R4-ex Content of the surface functional groups (mmol/g) active carbon sample
modification method
-COOH
-COO-
-OH
dCO
basic
a0a (mmol/g)
N HT Ox
nonmodified heat treated in Ar at 980 °C oxidized with concd HNO3
0.19 0.01 0.63
0.18 0.09 0.28
0.15 0.10 0.41
0.18 0.07 0.46
0.20 0.52 0.09
0.72 0.18 2.78
a
a0 is the number of primary water adsorption centers.
Figure 2. Oxygen evolved by thermal decomposition of surface functional groups as CO and CO2 (a) and mass loss of modified active carbon samples (b) as a function of the degassing temperature: dotted line, HT; solid line, Ox. Figure 1. Water vapor adsorption isotherms for modified Norit R4-ex active carbon samples (1) N, (2) HT, and (3) Ox at 25 °C. carbon by treatment with concentrated hydrofluoric and hydrochloric acids, so that its content in the carbon did not exceed 0.2 wt %. One part of demineralized active carbon was oxidized by using concentrated nitric acid at 80 °C, another part was deoxidized by heating at 980 °C in an argon stream, and the remaining part was used without further modification. The obtained carbon samples are denoted as Ox, HT, and N, respectively. For estimation of the effects of used modification methods, the content of various types of the oxygen surface functional groups was determined by neutralization of bases of various strength15 and hydrochloric acid16 (see Table 1). For all carbon samples the adsorption isotherms of water vapor at 25 °C were also measured. From these data (Figure 1) within the range of low and medium relative pressures (p/ps, < 0.5) the numbers of primary adsorption centers a0 were calculated using the Dubinin-Serpinski equation17 (see Table 1). Additionally, for characterization of the chemical properties of tested carbon surfaces, thermogravimetric (TG) measurements with parallel recording of amount of gaseous products18 were performed. In these experiments, a universal thermoanalyzer (Model TA 1) (Mettler) and a quadrupole mass spectrometer (Model QMG 101) (Balzers) were used. The obtained results for samples HT and Ox are presented in Figure 2. The character of the surface species present on the carbon surface was also analyzed with the X-ray photoelectron spectroscopy (XPS) method. The XPS spectra were recorded using an ESCALAB 210 spectrometer from VG Scientific Ltd. (Figure 3). To explain how the chemical modification of active carbon influences its porous structure for all investigated samples, the adsorption/ desorption isotherms of benzene vapors were measured at 20 °C.19 On the basis of these data, the parameters of the Dubinin(15) Boehm, H. P. In Advances in Catalysis; Eley, E. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1966; Vol. 16, p 179. (16) Weller, S.; Young, T. F. J. Am. Chem. Soc. 1948, 70, 4155. (17) Dubinin, M. M.; Serpinski, V. V. Carbon 1981, 19, 402. (18) Kaz´mierczak, J.; Biniak, S.; SÄ wia¸ tkowski, A.; Radeke, K. H. J. Chem. Soc. Faraday Trans. 1991, 87, 3557.
Figure 3. O(1s) XPS spectra of active carbon samples: (a) N (I, 531.2 eV; II, 533.4 eV; III, 535.3 eV); (b) Ox (I, 530.6 eV; II, 532.8 eV; III, 535.5 eV). Raduskhevich equation W01, W02, B1, and B2 as well as the volume of mesopores Vme and specific surface area SBET were estimated20 (See Table 2). The surface excess adsorption isotherms from ternary liquid mixtures of benzene (1) + diethyl ketone (2) + n-heptane (3) on (19) Goworek, J.; Biniak, S.; SÄ wia¸ tkowski, A. In Process Technology Procceedings; Vansant, E. F., Ed.; Elsevier: Amsterdam, 1994; Vol. 11, p 349. (20) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 51.
Modified Active Carbon Adsorption
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Table 2. Parameters Characterizing the Porous Structure of the Chemically Modified Active Carbon Norit R4-exa active carbon W01 W02 W0 Vme SBET sample (cm3/g) 106B1 (cm3/g) 106B2 (cm3/g) (cm3/g) (m2/g) N HT Ox
0.352 0.338 0.336
0.787 0.632 0.666
0.186 0.175 0.170
2.18 2.53 2.18
0.538 0.513 0.506
0.148 0.131 0.121
1240 1170 1150
a W and W are the parts of limiting volume of the adsorption 01 02 space W0 from the DR equation referring to micropores and supermicropores, respectively; B1 and B2 are the parameters of the DR equation characterizing the micropore dimensions; Vme is the mesopore volume; SBET is the BET surface area.
active carbon samples were determined by using a static, immersional method. The ternary solutions were prepared gravimetrically by mixing of component 1 with binary solvent 2 + 3 with equimolar composition. The ratio of the mole fractions of the third and second component x30/x20 was constant and equal to 1. A detailed procedure of the liquid adsorption experiments was described earlier.21
3. Results and Discussion Surface Properties of Active Carbons. The obtained active carbon samples Ox, HT, and N are strongly differentiated with respect to chemical properties of their surface. As is seen from Table 1, the oxidation process increases the content of carboxyl groups 3.3 times, the lactone groups 1.6 times, and phenolic groups or carbonyl groups 2.7 times. However, thermal treatment in argon atmosphere reduces the concentration of carboxyl groups practically to zero, lactone groups by half, phenolic groups 1.5 times, and carbonyl groups 2.6 times. In the case of basic groups, the opposite tendency is observed. Oxidation leads to more than 2-fold decrease of content of these groups and thermal treatment increases the number of these groups to a similar extent. The number of primary water adsorption centers (Table 1) changes also during the modification of the carbon surface. The situation is similar as in the case of oxygen functional groups of acidic character; i.e., oxidation causes a 4-fold increase and thermal treatment a 4-fold decrease. The destruction of the surface functional groups during thermal treatment was investigated by using thermogravimetric analysis. The total loss of mass during heating process in vacuum at 900-1000 °C is 1.7% for sample HT and 13% for sample Ox. Further increase of temperature does not change substantially the mass of the sample (see Figure 2). The gases evolved during the thermal destruction of oxygen functional groups consist mainly of CO for which a maximal intensity occurs at 700 °C and sharply decreases within the temperature interval 800-900 °C. In the case of CO2, the maximal intensity of its evolving appears at 300 °C and is followed by further decrease to 900 °C. These observations are in accordance with thermogravimetric curves. Evolution of CO2 is related mainly to the thermal destruction of carboxyl groups and evolution of CO to the destruction of carbonyl groups. The above presented results concerning the changes of the chemical properties and composition of the carbon surface are confirmed by the results of investigations with X-ray photoelectron spectroscopy (XPS). For illustrative purposes, the O(1s) spectra for N and Ox carbon samples are presented in Figure 3. The measured O(1s) spectra for the N and Ox carbon samples (Figure 3) were fitted by three peaks:22,23 I, 531.0(21) Goworek, J.; Nieradka, A. Colloids Surf. 1995, 97, 27. (22) Yoshida, A.; Tanahashi, I.; Nishino, A. Carbon 1990, 28, 611.
531.5 eV (CdO and/or C-O-C groups); II, 532.7-533.3 eV (C-OH and/or ether groups); III, 535.4-535.8 eV (chemisorbed oxygen or adsorbed water). Oxidation of active carbon R4-ex leads to an increase of relative intensity in the O(1s) region spectra particularly in the case of the CdO bonds in -COO- or -COOH surface groups. The C(1s) region has been resolved into individual component peaks that represent graphitic carbon (284.6 eV) and carbon present in alcohol or ether groups (286.1286.3 eV), carbonyl groups (287.6-287.7 eV), carboxyl or ester groups (288.5-288.8 eV), and carbonate groups (290.6-290.7 eV). Similar results were observed for active carbons obtained for various raw materials,24 glassy carbon,25 and PAN-based carbon fibers26 oxidized with nitric acid. Comparing relative concentrations of the carbon-oxygen groups for samples N and Ox, one can state that oxidation with nitric acid leads to substantial increase of alcohol, ether, carbonyl, and carboxyl groups. The changes of the surface species concentration are accompanied by simultaneous changes of the porous structure of carbon samples (see Table 2). Oxidation reduces the volume of micropores W01 and supermicropores W02 as well as the volume of mesopores Vme and specific surface area SBET. This effect can be connected with blocking part of the pores by functional groups created during the oxidation process.27 The porous structure is changed also during thermal treatment, which can be explained by partial sintering and cracking of the carbon matrix at higher temperature. However, changes of porosity caused by chemical modification are relatively small. Thus, the obtained samples of modified active carbon may be assumed as a model material for studying the influence of the surface chemistry on the adsorption process. Adsorption from the Liquid Phase. The specific surface excess of component 1 (benzene) n1σ(n) for each concentration of ternary mixtures was calculated from the relation
n1σ(n) )
n0(x10 - x1l) m
(1)
where n0 is the total number of moles of liquid components in contact with m g of the adsorbent and x10 and x1l are the mole fractions of the component 1 in the initial and equilibrium solutions, respectively. In the investigated systems the components of the liquid mixture were so chosen that components 1 (benzene) and 2 (ketone) took part almost exclusively in the competition for the adsorbent surface. The surface excess of the ith component for ternary liquid mixture is given in terms of the surface and the bulk mole fraction of this component by:
niσ(n) ) ns(xis - xil)
(2)
where ns is the total number of moles in the surface phase (surface layer capacity), xis and xil are the mole fractions of ith component in the surface and the bulk phase, respectively. Our efforts to calculate the surface layer capacity using the general equation of adsorption isotherm given in ref 12 (eq 11) leads to unrealistic ns values for investigated (23) Zhong, S.; Padeste, C.; Kazacos, M.; Skyllas-Kazacos, M. J. Power Sources 1993, 45, 29. (24) Albers, D.; Deller, K.; Despeyroux, B. M.; Scha¨fer, A.; Seibolt, K. J. Catal. 1992, 133, 467. (25) Kelemen, S. R.; Freund, H. Energy Fuels 1988, 2, 111. (26) Gardner, S. D.; Singamsetty, C. S. K.; Booth, G. L.; He, GuoRen; Pittman, C. U., Jr. Carbon 1995, 33, 587. (27) Biniak, S.; Kaz´mierczak, J.; SÄ wia¸ tkowski, A. Adsorpt. Sci. Technol. 1989, 6, 182.
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Figure 4. (a) Excess adsorption isotherms for ternary mixture benzene (1) + diethyl ketone (2) + n-heptane (3) for x30/x20 ) 1 at 25 °C on active carbon samples: 1, N; 2, HT; 3, Ox. (b) x30/x20 as a function of x1l for the system mentioned above.
systems. This effect is probably connected with nonideality of the bulk phase and lack of constancy of the ratio of the mole fractions of components 2 (ketone) and 3 (nheptane). Figure 4a presents the experimental results for investigated systems in the form of specific excess adsorption isotherms niσ(n) ) f(xil). As is seen, benzene is the component preferentially adsorbed practically in the whole concentration range. The shape of the isotherm follows up type II or IV in the Schay-Nagy classification.28 It suggests that within the linear range of isotherm the surface is not completely covered by benzene molecules and ketone is also present in the surface layer. Competition of liquid components illustrates the dependence of the ratio of the mole fractions of n-heptane and ketone x3l/x2l against x1l. Figure 4b shows the dependencies of the ratio of the mole fractions of n-heptane to ketone x3l/ x2l against x1l in the equilibrium solution for ternary mixture benzene (1) + diethyl ketone (2) + n-heptane (3). At the adsorption equilibrium, due to adsorption, the ratio x3l/x2l is different that in the initial solutions x30/x20. It appears that for investigated systems the values of x3l/x2l are higher than x30/x20 ) 1 in the whole concentration range. The small tendency of x3l/x2l to decrease at lower x1l concentration illustrates the displacement process of ketone molecules by benzene from the interface. Because the observed effect is insignificant, one can conclude that (28) Schay, G.; Nagy, L. G.; Szekrenyesy, T. Period. Polytech., Chem. Eng. 1960, 4, 95.
Goworek et al.
the competition of both components for silica surface is similar. Within this range of concentrations the carbon surface is only partially covered by benzene molecules. When the concentration of benzene in the bulk solution increases, two remaining components are displaced from the surface phase. Moreover, the value x3l/x2l > 1 for all investigated samples indicates that at preferential adsorption of benzene, ketone is also present in the surface phase. The binary solvent contains ketone and n-heptane, i.e., components very different in polarity. Taking into account high values of x3l/x2l one can assume that n-heptane is totally excluded from the interface. Degree of displacement of diethyl ketone depends on the chemical character of the surface and followed the order carbon HT > carbon N > carbon Ox. As may be expected, ketone competes strongly with benzene for polar surface species of carbon surface. Its concentration is especially high on the Ox carbon sample. On hydrophobic parts of the surface, the preferential adsorption of ketone is questionable. The resulting excess adsorption of benzene above x1l = 0.8 is negative, and at certain composition of the liquid mixture x1a, ∆x1l equals zero and consequently n1σ(n) ) 0. The point of the excess adsorption isotherm for which x1l ) x1a ) x1s corresponds to the adsorption azeotropic composition when the concentration of benzene in the surface phase is the same as that of the bulk. The displacement data represented by the ratio of x3l/ x2l give information about the solid-liquid components interactions. Relatively high values of x3l/x2l for the Ox sample indicate the permanent presence of ketone molecules in the surface phase even at high concentration of benzene in the bulk solution and positive excess adsorption of this component. For all investigated systems the mixed character of the surface phase should be assumed. The range of preferential adsorption of benzene correlates with concentration of the surface functional groups. 4. Summary Summing up, one can state that heat treatment and oxidation with nitric acid change substantially the concentration of the surface species on the commercial active carbon. The sample heated in argon atmosphere contains a small amount of the oxygen functional groups. The opposite is true in the case of oxidized sample. The presence of these groups influences substantially the adsorption from gas and liquid phase. The hydrophilic adsorption takes place mainly on oxygen functional groups. The analysis of the changes of mixed solvent composition allows the estimation of the competition of liquid components for the carbon surface. Thus, the investigation of adsorption from ternary mixtures of suitably selected components may be of supplementary character for the studies on the adsorption properties of activated carbons. Acknowledgment. This work was partially supported by the Committee for Scientific Research (Poland), Project 3 P40500105. We thank Mr. J. Sobczak from the Department of Applied Surface Science, Institute of Physical Chemistry (PAN), Warsaw (Poland), for XPS measurements. LA951065W
