Langmuir 2004, 20, 5277-5284
5277
Adsorption of Butane Isomers and SF6 on Kureha Activated Carbon: 1. Equilibrium Weidong Zhu,*,† Johan C. Groen,‡ Freek Kapteijn,† and Jacob A. Moulijn† Reactor & Catalysis Engineering, Applied Catalyst Characterization, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received June 30, 2003. In Final Form: February 24, 2004 Adsorption equilibria of butane isomers and SF6 on Kureha activated carbon were investigated using the volumetric method and the tapered element oscillating microbalance (TEOM) technique. The isotherm data of the butane isomers measured by the TEOM technique are in good agreement with those determined by the volumetric method. Single-component adsorption isotherms are reported at temperatures in the range from 298 to 393 K and at pressures up to 120 kPa. SF6 molecules are mainly adsorbed in the larger micropores, resulting in a lower adsorption capacity. The amount adsorbed for n-butane is slightly higher than that for isobutane in the whole range investigated. This is attributed to the fact that the linear n-butane molecule can adsorb in the smaller micropores. The To´th model appropriately describes the equilibrium data of the butane isomers, while the isotherm data of SF6 can be fitted by the Langmuir model. The isosteric heats associated with adsorption for these three adsorptives show different loading dependences. The present study indicates that the activated carbon can be well characterized by the probe molecules having different molecular sizes.
Introduction The gas or vapor adsorption phenomenon is widely used in the field of gas separation and purification as well as in the characterization of porous solids. Each of these applications requires basic adsorption data in a particular range of experimental conditions (temperature, pressure, and gas or vapor composition). These data consist of adsorption isotherms, selectivity curves, heats of adsorption, and kinetics. One of the most useful adsorbents is activated carbon, but there are many different brands to choose from and their properties depend on the starting material and the manufacturing process. In general, microporous activated carbon has a much higher surface area compared to that of zeolites, resulting in a higher adsorption capacity for nonpolar adsorptives. There are a lot of adsorption isotherm data for different adsorptives on activated carbon available in the open literature. The accumulation of a data bank can be found in a handbook by Valenzuela and Myers.1 This paper presents experimental results for the equilibrium adsorption of the butane isomers and SF6 on the activated carbon commercialized by Kureha Chemical Industry, Japan. The aim of the present study is twofold. In our previous work, a novel technique, the so-called tapered element oscillating microbalance (TEOM) technique, was used to measure adsorption isotherms on zeolites.2-5 Although the isotherms of light alkanes on * To whom correspondence should be addressed. Fax: +31-152785006. E-mail:
[email protected]. † Reactor & Catalysis Engineering. ‡ Applied Catalyst Characterization. (1) Valenzuela, D. P.; Myers, A. L. Adsorption Equilibrium Data Handbook; Prentice Hall: Englewood Cliffs, New Jersey, 1989. (2) Zhu, W.; van de Graaf, J. M.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1998, 37, 1934-1942. (3) Zhu, W.; Kapteijn, F.; Moulijn, J. A.; Den Exter, M. C.; Jansen, J. C. Langmuir 2000, 16, 3322-3329. (4) Zhu, W.; Kapteijn, F.; Moulijn, J. A.; Jansen, J. C. Phys. Chem. Chem. Phys. 2000, 2, 1773-1779. (5) Zhu, W.; Kapteijn, F.; van der Linden, B.; Moulijn, J. A. Phys. Chem. Chem. Phys. 2001, 3, 1755-1761.
silicalite-1 determined by the TEOM technique are in remarkable agreement with those obtained by employing either the volumetric or the gravimetric method in the literature,6 a comparison study of in-house measurements for the same system has not been reported. In the present study, the isotherms of the butane isomers on the activated carbon are measured by both the volumetric and TEOM techniques. The second objective is to characterize the activated carbon by means of the probe molecules n-butane, isobutane, and SF6 having different molecular sizes. The bulky, symmetrical SF6 molecule has been used as an ideal probe to characterize interactions between adsorptive and carbonaceous materials.7-13 The butane isomers were chosen because they have very similar physical properties but a significant difference in their molecular diameters. This paper gives an interpretation of the difference in the adsorption behaviors for these three adsorptives. In addition, thermodynamic properties such as isosteric heat associated with adsorption are presented to characterize interactions between the adsorptive and the adsorbent. Experimental Section Adsorbent. The commercial sample, spherical bead activated carbon, was supplied by Kureha Chemical Industry. This activated carbon is referred to as Kureha carbon. Kureha carbon particles in this study were of spherical shape and had an average diameter of 0.34 mm, as determined by scanning electron microscopy (SEM) and shown in Figure 1. (6) Zhu, W.; Kapteijn, F.; Moulijn, J. A. Adsorption 2000, 6, 159167. (7) Muris, M.; Dupont-Pavlovsky, N.; Bienfait, M.; Zeppenfeld, P. Surf. Sci. 2001, 492, 67-74. (8) Jagiełło, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. J. Chem. Soc., Faraday Trans. 1995, 91, 2929-2933. (9) Kiselev, S. B.; Ely, J. F.; Belyakov, M. Y. J. Chem. Phys. 2000, 112, 3370-3383. (10) Jagiełło, J.; Bandosz, T. J.; Schwarz, J. A. Langmuir 1996, 12, 2837-2842. (11) Lo´pez-Ramo´n, M. V.; Jagiełło, J.; Bandosz, T. J.; Schwarz, J. A. Langmuir 1997, 13, 4435-4445. (12) Pribylov, A. A.; Kalinnikova, I. A.; Shekovtsova, L. G.; Kalashnikov, S. M. Russ. Chem. Bull. 2000, 49, 1993-1999. (13) Cao, D. V.; Sircar, S. Adsorption 2001, 7, 73-80.
10.1021/la030264+ CCC: $27.50 © 2004 American Chemical Society Published on Web 05/27/2004
5278
Langmuir, Vol. 20, No. 13, 2004
Zhu et al.
Figure 1. Scanning electron micrographs of the Kureha carbon particles used in this study. Table 1. Textural Properties of Kureha Carbon property
value
total micropore volume, cm3 g-1 BET surface area, m2 g-1 He grain density, kg m-3 particle density, kg m-3
0.56 (0.22, pore width < 0.9 nm) 1300 1860 1098a
a Calculated from the grain density and the total micropore volume.
Figure 2. Pore size distribution by the DFT simulation of Kureha carbon from nitrogen adsorption. Porous texture analysis of Kureha carbon was carried out by the adsorption of N2 at 77 K. The grain density of the adsorbent particles was measured by He pycnometry. The textural properties of Kureha carbon are summarized in Table 1. The pore size distribution was evaluated in terms of the density functional theory (DFT) simulation14-16 using the isotherm data of nitrogen adsorption at 77 K and relative pressures up to 0.2. Only micropores contribute to the total pore volume and the surface area. This was further confirmed by mercury intrusion porosimetry; no significant additional porosity was observed in the pore size range from 2 nm to 100 µm. Figure 2 presents a plot of differential pore volume as a function of pore width in which two maxima at 0.6 and 1.1 nm, respectively, can be seen. Volumetric Method. A Micromeritics ASAP 2010 gas adsorption analyzer (stainless steel version) was used to measure the adsorption isotherms of the butane isomers on Kureha carbon in the pressure range from 0.002 to 120 kPa. The instrument was equipped with a turbomolecular vacuum pump and three (14) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786-4796. (15) Lastoskie, C.; Gubbins, K. E.; Quirke, N. Langmuir 1993, 9, 2693-2702. (16) Olivier, J. P.; Conklin, W. B.; Szombathely, M. V. In Characterization of Porous Solids III; Rouquerol, J., Rodrigues-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, The Netherlands, 1994; pp 81-89.
different pressure transducers (0.13, 1.33, and 133 kPa) to enhance the sensitivity in different pressure ranges. The staticvolumetric technique is used to determine the volume of the gas adsorbed at different partial pressures: upon adsorption, a pressure decrease is observed in the gas phase, which is a direct measure for the amount adsorbed. The sample cell was loaded with 155.7 mg of Kureha carbon particles. Prior to the adsorption measurements, the adsorbent particles were outgassed in situ in a vacuum at 623 K for 16 h to remove any adsorbed impurities. The obtained dry sample weight was used in the calculation of isotherm data. Adsorption measurements were subsequently done at different temperatures from 298 to 393 K. The temperature was controlled within (0.3 K by means of a circulating oil bath. Five different temperatures (298, 338, 353, 373, and 393 K) for the adsorption of the butane isomers were used to reduce the uncertainty in the derived adsorption parameters. TEOM Technique. A Rupprecht & Patashnick TEOM 1500 mass analyzer (100 mg sample volume) was used in an experimental setup designed for measurements of equilibrium, transient adsorption, and desorption in porous materials. This novel technique has been used by several research groups.17-23 A detailed description of the TEOM operating principles is given elsewhere.2 On the basis of the operating principles, the TEOM technique yields information about mass changes rather than absolute sample masses. The total mass change measured consists of the amount adsorbed and the mass change caused by the change of the gas density in the tapered sample tube. The change in the gas density depends on the type of gas and the operating conditions. To correct for the mass change caused by the change in the density of the gas phase, reference experiments have to be performed. As mentioned in our previous papers,2-5 the relationship between the mass change in the reference runs and the partial pressure of an adsorbing gas is almost linear. If equilibrium adsorption for strong adsorptives is operated at very low partial pressures, this mass change caused by the density change in the gas phase will become negligible. A sample of 13.5 mg of the adsorbent particles was used for the adsorption experiments. Quartz wool was used at the top and bottom of the sample bed to keep the adsorbent particles firmly packed, which is essential for a stable measurement. The (17) Hershkowitz, F.; Madiera, P. D. Ind. Eng. Chem. Res. 1993, 32, 2969-2974. (18) Chen, D.; Rebo, H. P.; Moljord, K.; Holmen, A. Chem. Eng. Sci. 1996, 51, 2687-2692. (19) Liu, K.; Fung, S. C.; Ho, T. C.; Rumschitzki, D. S. J. Catal. 1997, 169, 455-468. (20) Rekoske, J. E.; Barteau, M. A. J. Phys. Chem. B 1997, 101, 1113-1124. (21) Petkovic, L. M.; Larsen, G. J. Catal. 2000, 191, 1-11. (22) Alpay, E.; Chadwick, D.; Kershenbaum, L. S.; Barrie, P. J.; Sivadinarayana, C.; Gladden, L. F. Chem. Eng. Sci. 2003, 58, 27772784. (23) Giaya, A.; Thompson, R. W. Microporous Mesoporous Mater. 2002, 55, 265-274.
Adsorption of Butane Isomers and SF6
Langmuir, Vol. 20, No. 13, 2004 5279
Table 2. Summary of the Physical and Molecular Properties of the Adsorptives Investigateda adsorptive
MW, g mol-1
R, Å3
µ,b D
σk,c nm
Tb, K
T c, K
pc, kPa
∆H 0v, kJ mol-1
MVLd, cm3 mol-1
n-butane isobutane SF6
58.12 58.12 146.05
8.20 8.14 6.54
99.999%), and the sample gas was used to establish the partial pressure. The temperature in the sample bed was controlled with an accuracy of (0.5 K, and the deviation of the component concentration in the inlet of the sample bed was within 0.2%. The TEOM measurements were accurate to 1 µg. Prior to the experiments, the adsorbent particles were outgassed in the following way. After a temperature rise with a rate of 10 K min-1 in situ in a helium flow rate of 200 cm3 (NTP) min-1 (NTP: 298 K and 101.325 kPa), the sample was heated at 573 K for 24 h in order to remove adsorbed impurities. Adsorptives. The gaseous adsorptives such as the butane isomers and SF6 were 3.5 grade (>99.95%). The physical and molecular properties of the adsorptives investigated are listed in Table 2.
d
Molar volume,
Figure 4. Adsorption isotherms of isobutane on Kureha carbon measured by the volumetric method. The lines are the To´th model fits: (+) 298 K; (2) 338 K; (b) 353 K; (1) 373 K; ([) 393 K.
Figure 5. Adsorption isotherms of SF6 on Kureha carbon measured by the TEOM technique. The lines are the To´th model fits: (+) 298 K; (2) 338 K; (b) 373 K; (1) 393 K.
Results and Discussion Isotherms. The isotherms of n-butane, isobutane, and SF6 on Kureha carbon are given in Figures 3-5. Experimental isotherm data for the butane isomers on Kureha carbon, shown in Figures 3 and 4, were measured by the volumetric method, while the isotherms of SF6 presented in Figure 5 were determined by the TEOM technique. All the isotherms exhibit a type-I adsorption isotherm (Brunauer classification) over the temperature and pressure range studied. Figure 6 shows a comparison of the isotherm data of the butane isomers at 373 K measured by the volumetric method and the TEOM technique. The experimental data from the TEOM measurements are in good agreement with those from the volumetric measurements. Under the same conditions, the amount adsorbed for n-butane is slightly higher than that for isobutane. These in-house observations verify for the first time the agreement between the TEOM and volumetric techniques for the same system. This also implies the quality of the measured isotherm data.
Figure 6. Comparison of the isotherm data of n-butane and isobutane on Kureha carbon at 373 K measured by the TEOM (closed symbols) and volumetric (open symbols) techniques: (2, 4) n-butane; (b, O) isobutane.
The isotherms of the butane isomers were reversible over the complete pressure and temperature range investigated. For the sake of clarity, the desorption data are omitted. The complete desorption of SF6 in Kureha carbon was a very slow process. Figure 7 presents some
5280
Langmuir, Vol. 20, No. 13, 2004
Zhu et al.
Table 3. Extracted Values of Adsorption Isotherm Parameters and Standard Deviations adsorptive n-butane
isobutane
SF6
a
T, K
qsat, mol kg-1
298 338 353 373 393 298 338 353 373 393 298 338 373 393 298d 338d 373d 393d
6.19 ( 6.45 ( 0.06 6.34 ( 0.07 6.56 ( 0.11 6.70 ( 0.13 5.78 ( 0.03 5.76 ( 0.08 5.76 ( 0.10 5.76 ( 0.13 5.83 ( 0.15 3.57 ( 0.00 2.83 ( 0.23 1.58 ( 0.03 1.26 ( 0.00 3.57 ( 0.00 2.15 ( 0.04 1.58 ( 0.01 1.26 ( 0.00
0.03c
Vpa, cm3 g-1
K, kPa-1
m
0.504 0.525 0.516 0.534 0.545 0.474 0.472 0.472 0.472 0.478 0.275 0.218 0.122 0.097 0.275 0.166 0.122 0.097
56.6 ( 7.27 ( 0.45 3.03 ( 0.17 1.34 ( 0.09 0.576 ( 0.030 23.8 ( 1.4 4.00 ( 0.18 2.07 ( 0.16 0.912 ( 0.074 0.386 ( 0.025 (7.17 ( 0.02) × 10-2 (6.57 ( 0.74) × 10-2 (4.33 ( 0.01) × 10-2 (3.82 ( 0.00) × 10-2 (7.17 ( 0.02) × 10-2 (5.27 ( 0.30) × 10-2 (4.23 ( 0.08) × 10-2 (3.82 ( 0.00) × 10-2
0.333 ( 0.320 ( 0.005 0.332 ( 0.005 0.328 ( 0.006 0.332 ( 0.006 0.362 ( 0.005 0.350 ( 0.002 0.349 ( 0.008 0.354 ( 0.009 0.359 ( 0.009 1.000 ( 0.001 0.715 ( 0.094 0.996 ( 0.045 1.000 ( 0.000
3.2c
σmodelb, mol kg-1 0.004c
0.02 0.02 0.02 0.02 0.01 0.03 0.02 0.02 0.02 0.02 0.00 0.04 0.01 0.00 0.00 0.05 0.01 0.00
Corresponding pore volume estimated from the fitted saturation amount adsorbed and the molar volume of the adsorptive (MVL shown
in Table 2). b Standard deviation model: model.
x∑n (qcal-qexp)2/(n-j), for j fitting parameters. c 95% confidence limit. d Fitted by the Langmuir
Figure 7. Uptakes and desorption curves of SF6 on Kureha carbon at 338 K and different partial pressures: (a) uptake from 0 to 0.480 kPa and desorption from 0.480 to 0 kPa; (b) uptake from 0 to 1.553 kPa and desorption from 1.553 to 0 kPa; (c) uptake from 0 to 0.775 kPa and desorption from 0.775 to 0 kPa.
adsorption and desorption cycles of SF6 in Kureha carbon at 338 K and different partial pressures. In cycle a, when a stepwise increase of a SF6 partial pressure from 0 to 0.480 kPa was set up for the system, first, SF6 instantaneously adsorbed in the outgassed sample, which was followed by a very slow uptake process to reach equilibrium. The total mass change in the slow uptake region was ∼0.1 mg, corresponding to an amount adsorbed of 0.05 mol kg-1. During the desorption in pure helium, first, the total mass sharply decreased to 0.1 mg and then it maintained almost constant, although there was a trend to further desorb. Therefore, it can be proposed that globally there are two types of adsorption locations for SF6 in Kureha carbon, one type contributed by the large pores, in which SF6 molecules easily adsorb and desorb, and another type formed by the “small” pores, in which SF6 molecules can just penetrate, resulting in very slow adsorption and desorption processes. It is also pointed out that the capacity of these small pores for SF6 adsorption is limited. In addition, it was found that this strong adsorption of SF6 on Kureha carbon was independent of the operating conditions. Figure 7 also shows two other adsorption and desorption cycles for this observation. By neglecting this small amount (0.05 mol kg-1), the isotherms of SF6 were reversible in the pressure and temperature range investigated.
Isotherm Model. For practical utility, the experimental data should be correlated with an analytical expression that includes adjustable parameters as a function of temperature. Various empirical methods have been proposed to fit isotherm data in adsorption science. In many cases, however, it may not be easy to determine a set of isotherm parameters that can represent all the experimental data at multiple temperatures with good accuracy. Although Kureha carbon is a pure microporous material, the pore size distribution is wide and its range covers from 0.4 to 1.9 nm, as shown in Figure 2. Therefore, Kureha carbon is considered to be a heterogeneous adsorbent. For the adsorption on heterogeneous adsorbents such as activated carbon, the To´th model is often used to correlate isotherm data1,28
Kp q ) qsat [1 + (Kp)m]1/m
(1)
where q is the amount adsorbed, qsat is the saturation amount adsorbed, K is the equilibrium constant, p is the pressure, and m is the parameter that characterizes the system heterogeneity.28 Being a three-parameter model, the To´th equation can appropriately describe the isotherm data, also shown in Figures 3-5. The extracted values of the adsorption parameters in the To´th model are listed in Table 3. The estimated saturation capacities are almost constant at the five temperatures for isobutane, while they increase slightly with temperature for n-butane, but these changes are not so significant and the maximal difference in the temperature range investigated is ∼0.5 mol kg-1. The extracted saturation capacities for n-butane are higher than those for isobutane. The kinetic diameter of n-butane is smaller than that of isobutane; see Table 2. In Kureha carbon, part of the micropores are smaller than 0.5 nm and n-butane molecules can access these micropores, while isobutane cannot. This probably gives (24) Lide, D. R. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2001. (25) McClellan, A. L. Tables of Experimental Dipole Moments; W. H. Freeman & Co.: San Francisco, CA, 1963. (26) Breck, D. W. Zeolite Molecular Sieves-Structure, Chemistry, and Use; Wiley: New York, 1974. (27) Webster, C. E.; Cottone, A.; Drago, R. S. J. Am. Chem. Soc. 1999, 121, 12127-12139. (28) Do, D. D. Adsorption Analysis: Equilibrium and Kinetics; Imperial College Press: London, 1998.
Adsorption of Butane Isomers and SF6
an explanation for the difference in the saturation capacity between n-butane and isobutane. The extracted saturation capacity for SF6 adsorption on Kureha carbon significantly decreases with increasing temperature. It is well-known that, in the case of supercritical adsorption, the adsorbed phase does not occupy the whole pore volume, even at very high pressures.29 This is different from subcritical adsorption, where, in principle, all pores are filled by adsorptive molecules when the pressure approaches a critical pore-size-dependent value called the pore filling pressure, which is smaller than the vapor pressure.29 The equilibrium temperature range for the adsorption of SF6 covers from the subcritical temperature 298 K to the supercritical temperature domain 338-393 K, while all the operating temperatures for the butane isomers are below their critical temperatures. The observed strong temperature dependence of the extracted saturation amount adsorbed for SF6 may be attributed to the different operating temperature domains (sub- and supercritical temperatures). From the values of qsat and the molar volume of the adsorptive, MVL, presented in Table 2, the pore filling volume, Vp, can be estimated, and its values are also included in Table 3. For the linear n-butane molecule, the estimated Vp is reasonably consistent with that determined from the isotherm of nitrogen at 77 K. This indicates that n-butane has a high degree of occupancy in Kureha carbon. Compared to n-butane, the single-branched isobutane has a lower degree of occupancy. The values of Vp for SF6 are much lower than those for the butane isomers. SF6 molecules of spherical shape with a kinetic diameter of 0.55 nm, reported by Breck,26 are mainly adsorbed in the larger micropores. The results in Figure 7 clearly support this consideration. SF6 molecules instantaneously adsorb in Kureha carbon, and only a limited number of the molecules can tightly adsorb in the micropores, of which the size should be slightly larger than the molecular diameter. From the pore size distribution simulated by the DFT method (Figure 2), the pore volume of Kureha carbon with a pore size above 0.55 nm is ∼0.5 cm3 g-1, which, in principle, can accommodate SF6 molecules. However, both equilibrium and kinetic adsorption results are not consistent with this picture. The density functional theory method is the current widely used model for analyzing micropore distribution in activated carbon, which assumes an array of semi-infinite, rigid slits of distributed width whose walls are modeled as energetically uniform graphite.30 However, these local isotherms can cause some deviations from the actual course of adsorption for experimentally studied porous solids, especially in the case of their high surface heterogeneity.31 In addition, slit-shaped pores are an extremely simplified representation of the void spaces within the real solid; slit-shaped pores do not include “corners” that would be observed in the real solid when two or more planes of carbon meet.32 The value of Vp for SF6 (0.275 cm3 g-1), extracted from the isotherm at 298 K, is much lower than 0.5 cm3 g-1 in the pore size range 0.55-1.9 nm accessible to SF6, as determined by the DFT method. This discrepancy could be caused by the errors inherent to the DFT simulation. A comparison study of the HorvathKawazoe (HK) and DFT methods shows that the DFT method seems to shift the pore size distribution to larger sizes,31,33-34 evidencing the cause of the discrepancy. (29) Nguyen, C.; Do, D. D. J. Phys. Chem. B 2001, 105, 1823-1828. (30) Olivier, J. P. Carbon 1998, 36, 1469-1472. (31) Kruk, M.; Jaroniec, M.; Choma, J. Carbon 1998, 36, 1447-1458. (32) Davies, G. M.; Seaton, N. A. Carbon 1998, 36, 1473-1490. (33) Korili, S. A.; Gil, A. Adsorption 2001, 7, 249-264.
Langmuir, Vol. 20, No. 13, 2004 5281
For the equilibrium constant K in the To´th model, the value for all three adsorptives decreases with an increase in the operating temperature. The magnitude of the equilibrium constant depends on the properties both of the adsorptive and of the adsorbent and corresponds to the affinity of the adsorbent to the adsorptive. At the same temperature, the K value for n-butane is larger than that for isobutane; see Table 3. This indicates that the adsorbent has a higher adsorption affinity to n-butane than isobutane. Compared to the butane isomers, SF6 has a much lower K value, and this indicates SF6 is a weaker adsorptive on Kureha carbon. The parameter m in the To´th model reflects a degree of the system heterogeneity.28 The larger the deviation from unity, the system is said to be more heterogeneous. The extracted values of m for either n-butane or isobutane are almost constant in the whole temperature range studied, and the average values are 0.33 and 0.35 for n-butane and isobutane, respectively. This implies a strong degree of heterogeneity for the adsorption of the butane isomers on Kureha carbon. In addition, for n-butane, the adsorbent appears to be slightly more heterogeneous than it is for isobutane in terms of the m value. It is interesting to note that the m values of SF6 are equal to unity, except for the data at 338 K. When m ) 1, the To´th isotherm reduces to the famous Langmuir model
Kp q ) qsat 1 + Kp
(2)
The isotherm data of SF6 are well correlated with the Langmuir equation, and the values of the parameters are also presented in Table 3. Except for the data at 338 K, the estimated values of qsat and K are almost the same in the two models. The estimated qsat values for both isotherm models show strong temperature dependences. This behavior emphasizes a difficulty in applying both To´th and Langmuir isotherms in a strict way for SF6 on Kureha carbon. Other isotherm models, such as the Unilan model1,28 and those based on the theory of micropore volume filling introduced and developed by Dubinin,35 have also been considered, but from the fitting results, the To´th model describes the present case best over the full range. For engineering purposes, the To´th isotherm is quite attractive to be applied due to its simplicity. In addition, because of its correct behavior at low and high pressures, the To´th model is usually recommended as the first choice of an isotherm equation for correlating adsorption data.28 Adsorption Thermodynamics. From the isotherm data at 298, 338, 353, 373, and 393 K for the butane isomers and at 298, 338, 373, and 393 K for SF6, a number of thermodynamic properties can be derived. The Henry law constant, KH, quantifies the extent of the adsorption of a given adsorptive by a solid. The magnitude of KH depends on the properties of both the adsorptive and the solid. The Henry law constant is defined by the following equation:1
KH ) lim pf0
dq (pq) ) lim(dp ) pf0
(3)
For both To´th and Langmuir isotherms, the Henry law constant can be expressed by the following formula: (34) Vallandares, D. L.; Reinoso, F. R.; Zgrablich, G. Carbon 1998, 36, 1491-1499. (35) Dubinin, M. M. Chem. Rev. 1960, 60, 235-241.
5282
Langmuir, Vol. 20, No. 13, 2004
Zhu et al.
KH ) qsatK
(4)
The calculated values of KH are listed in Table 4. From KH values at multiple temperatures, the isosteric heat at zero coverage can be derived from its definition1
) [
(
]
d ln KH d ln KH ) Rg dT d(1/T)
2 Qst 0 ) -RgT
Table 4. Derived Values of the Henry Law Constant and the Isosteric Heat at Zero Coverage adsorptive
T, K
n-butane
298 338 353 373 393 298 338 353 373 393
(5) isobutane
Qst 0
where is the isosteric heat of adsorption at zero coverage and Rg is the universal gas constant. The observed linearity of a plot of ln KH versus 1/T shown in Figure 8 leads to the determination of the isosteric heat at zero coverage. The derived values of the isosteric heat associated with adsorption are presented in Table 4. Linders et al.36 used a novel low pressure pulse-response technique (similar to a temporal analysis of products technique) to measure the heat of adsorption (-∆Hads) for n-butane on Kureha carbon. The reported value of -∆Hads is 45.2 kJ mol-1, which is in good agreement with the current result, 46.3 kJ mol-1, of Qst 0 for n-butane. The extracted values for all three adsorptives are larger than their of Qst 0 corresponding enthalpies of vaporization shown in Table 2, as expected. By assuming the nonspecific interactions involved, the adsorption potential is almost entirely the product of dispersion forces,37 where the interaction between the adsorptive and the adsorbent is proportional to the polarizability of the adsorptive.38 As presented in Table 4, the derived Qst 0 value shows the following order: n-butane > isobutane > SF6. The molecular polarizability of the adsorptives also follows the same order; see Table 2. From the derived isotherms, the isosteric heat of adsorption as a function of loading can be calculated by the following equation1
(
Qst ) RgT2
)
∂ ln p ∂T
q
[
) -Rg
]
∂ ln p ∂(1/T)
SF6a
298 338 373 393
KH, mol kg-1 kPa-1 350 46.9 19.2 8.79 3.86 138 23.0 11.9 5.25 2.24 0.256 0.113 0.0668 0.0481
-1 Qst 0 , kJ mol
46.3
41.8
16.9
a Values derived from the adsorption isotherm parameters fitted by the Langmuir model.
Figure 8. ln KH vs 1/T plots for n-butane, isobutane, and SF6 on Kureha carbon: (+) n-butane; (2) isobutane; (b) SF6.
(6)
q
where Qst is the isosteric heat of adsorption. It is evident that ln p versus 1/T at the same loading should approach linearity, leading to the determination of Qst as a function of loading. These results are summarized in Figure 9. The curves of Qst versus q are generally used to measure whether an adsorption system is energetically heterogeneous. Sircar and Rao39 classified the pure gas isosteric heat of adsorption as a function of loading into three cases. (1) An adsorbent is said to be energetically homogeneous for adsorption if all the sites of adsorption have the same energy of adsorption. Consequently, Qst is independent of q. However, even for an energetically homogeneous adsorbent, Qst can vary with q if (a) the adsorptiveadsorptive lateral interactions in the adsorbed phase are significant, (b) the adsorptive orientation on the adsorbent surface changes with coverage, (c) there is multilayer adsorption or condensation of the adsorptive in the mesopores, and (d) there is a change in the physical structure of the adsorbed phase. (2) An adsorbent is said to be energetically heterogeneous for adsorption if it consists of a distribution of sites of different adsorption energies. Even though all sites are populated by the (36) Linders, M. J. G.; van den Broeke, L. J. P.; Nijhuis, T. A.; Kapteijn, F.; Moulijn, J. A. Carbon 2001, 39, 2113-2130. (37) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; John Wiley & Sons: New York, 1978. (38) Richards, R. E.; Rees, L. V. C. Langmuir 1987, 3, 335-340. (39) Sircar, S.; Rao, M. B. In Surfaces of Nanoparticles in Porous Materials; Schwarz, J. A., Contescu, C. I., Eds.; Marcel Dekker: New York, 1999; pp 501-528.
Figure 9. Isosteric heat of adsorption as a function of loading: (a) n-butane; (b) isobutane; (c) SF6.
adsorptive molecules at all gas pressures, the higher energy sites are predominantly occupied by the adsorptive molecules at the lower gas pressures. Lower energy sites are progressively occupied by the adsorptive molecules as the gas pressure increases. Consequently, Qst decreases with increasing loading. (3) In a special case, an adsorbent may not be homogeneous even if Qst remains constant as a function of q. This may be caused by mutual cancelation of two effects: an increase of Qst with an increase of q due to lateral interactions and a decrease of Qst with an increase of q due to adsorbent heterogeneity. The isosteric heat for n-butane monotonically decreases with increasing loading. Kureha carbon consists of micropores with different widths relative to the molecular diameter of the adsorptive. Physically, molecules prefer
Adsorption of Butane Isomers and SF6
to adsorb onto sites of high energy, and hence, as adsorption progresses, molecules then adsorb onto sites of decreasing energy.28 On the basis of this concept, the linear n-butane molecules first penetrate into narrower pores, resulting in a stronger interaction between the adsorptive and the adsorbent. This implies a higher value of Qst at lower loadings. After completely filling smaller pores, n-butane molecules will gradually be accommodated in larger pores, in which the adsorption affinity becomes weaker. Therefore, a monotonic decrease in the isosteric heat as a function of loading is due to the heterogeneity. It is surprising to see that the isosteric heat of adsorption for isobutane shows a different loading dependence from that for n-butane, and in the range of the amount adsorbed from zero coverage up to 3.1 mol kg-1, the value of Qst almost maintains constant. Three reasons may be given for this loading independence of Qst for isobutane. The energy of adsorption is made up of two terms: adsorptivesolid interaction energy and adsorptive-adsorptive interaction energy. The first of these will decrease with increasing loading on a heterogeneous surface, and the second will increase with increasing loading on any surface. The observed isosteric heat is the sum of these two, so that a constant isosteric heat of adsorption with increasing loading means that the changes due to the two contributions are canceling each other out. Second, the pore size distribution of Kureha carbon may be centered in the range where the micropores are similar in size to the molecular diameter of isobutane, resulting in an apparent homogeneous adsorbent for this adsorptive. Additionally, some functional groups on the carbon surface can lead to specific interactions with isobutane molecules. Although, in general, activated carbon is hydrophobic in nature, some carbon-oxygen surface groups can be formed during manufacturing.40 Linders et al.40,41 investigated the effects of the presence of water on the adsorption of hexafluoropropylene (HFP) on activated carbon. The experimental results showed that the amount adsorbed of HFP could be dramatically reduced by the presence of water. This gives the evidence that there is some specific potential available on the surface of activated carbon for polar adsorptives. For the specific interactions involved, the dipole moment of an adsorptive plays an important role in the adsorption potential. Some investigators42-44 studied the correlation of the dipole moment and the adsorption energies on carbonaceous materials, and indeed, they found some pronounced effects of the dipole moment on the interactions between the adsorptive and the adsorbent. Isobutane is a representative weak polar molecule and has a small dipole moment; see Table 2. The loading independence of Qst for isobutane on Kureha carbon may be due partially to the presence of such a constant specific interaction. The formation of an ordered structure in the micropores leads to the interactions between the adsorptive and the carbon surface where the pore width is less relevant. The isosteric heat significantly increases with an increase in loading for SF6 on Kureha carbon. This can partially be attributed to the adsorptive-adsorptive (40) Linders, M. J. G.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A.; van Bokhoven, J. J. G. M. AIChE J. 2001, 47, 1885-1892. (41) Linders, M. J. G.; van der Weijst, M. B. L.; van Bokhoven, J. J. G. M.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 2001, 40, 31713180. (42) Wang, Z. M.; Kaneko, K. J. Phys. Chem. B 1998, 102, 28632868. (43) Meeks, O. R.; Rybolt, T. R. J. Colloid Interface Sci. 1997, 196, 103-109. (44) Okamura, J. P.; Sawyer, D. T. Anal. Chem. 1971, 43, 17301733.
Langmuir, Vol. 20, No. 13, 2004 5283
lateral interactions in the adsorbed phase. Bulky SF6 molecules might be adsorbed in parallel inside the same large pore, or they can be closely packed with increasing loading. This could lead to a development of interactions between adsorptives as well as to an increase in the interactions between the adsorptive and the adsorbent, resulting in increasing Qst with loading. However, these interactions would not give rise to a steeply rising curve, as shown in Figure 9 for SF6. This strong increase in the isosteric heat with loading may be associated with the used isotherms determined from the subcritical temperature 298 K to the supercritical temperatures 338, 373, and 393 K. When the isotherm temperature range passes over the critical temperature of the adsorptive, some kind of underlying phase change, for example, from liquidlike to supercritical fluid, in the adsorbed layer may take place. Indeed, the derived adsorption capacity steeply decreases with increasing temperature, as shown in Table 3, indicating the occurrence of changes in the adsorbed layer. This not only reflects a difficulty in applying both To´th and Langmuir isotherms but also indicates an uncertainty in the derived isosteric heat of adsorption for SF6 on Kureha carbon. Thus, the steep increase in the isosteric heat with loading could be mainly caused by the determination from the isotherms at multiple temperatures, in which a phase change in the adsorbed layer may occur. The most commonly used characterization of the internal structure of microporous carbons is the pore size distribution and the specific surface area as well as the surface chemistry (functional groups and their distribution). Adsorptives having different molecular shapes and sizes on the same adsorbent may exhibit substantially different adsorption properties that cannot be predicted a priori. The approach followed here shows that Kureha carbon is tailored to adsorb the specific molecules n-butane, isobutane, and SF6. When the adsorptive molecules are larger in size than the pores, the adsorbing species are rejected. The information, extracted from the comprehensive analysis of the equilibrium data, provides an estimate of the limiting adsorption capacities qsat and Vp. These quantities should be considered as micropore volumes seen by the adsorptive molecules. A difference in the extracted value of Vp for these three adsorptives is fully attributed to their different molecular sizes that discriminate between pores. Because the adsorptives have different molecular sizes and strengths of interaction with the adsorbent, they probe different ranges of pore size. This gives a more complete picture of the pore size distribution than what could be obtained with the commonly used adsorptive N2 in combination with the DFT simulation. The further analysis of adsorption thermodynamic properties provides further insight into the interactions between the adsorptive and the adsorbent. The present study also follows a general way to characterize microporous activated carbon.8,10-11 Conclusions The adsorption isotherms of n-butane, isobutane, and SF6 on Kureha carbon have been accurately measured over a temperature between 298 and 393 K and pressures up to 120 kPa. The isotherms of the butane isomers measured by both the TEOM and volumetric methods show good agreement. This further verifies the TEOM technique as applied to adsorption on porous materials. The To´th model gives a good description of the adsorption isotherms of the butane isomers. For SF6, the fitted m parameter in the To´th model is almost equal to unity, and the isotherm data can be well described by the Langmuir model. Both equilibrium and kinetic adsorption results
5284
Langmuir, Vol. 20, No. 13, 2004
indicate that SF6 molecules are mainly adsorbed in the larger pores. Linear n-butane molecules have a higher adsorption capacity on Kureha carbon compared to that of single-branched isobutane molecules. The difference in the limiting adsorption capacity for these three adsorptives can be explained by the fact that these molecules see different ranges of pore sizes, and hence, the present study also gives complementary information on the pore size distribution of Kureha carbon. Nomenclature K KH m MVL MW p p0 pc q qsat Qst Qst 0 Rg
equilibrium constant, kPa-1 Henry law constant, mol kg-1 kPa-1 parameter in the To´th isotherm molar volume, cm3 mol-1 molecular weight, g mol-1 pressure, kPa standard pressure at 298.15 K and 101.315 kPa critical pressure, kPa amount adsorbed, mol kg-1 saturation amount adsorbed, mol kg-1 isosteric heat, kJ mol-1 isosteric heat at zero coverage, kJ mol-1 universal gas constant, 8.314 J mol-1 K-1
Zhu et al. t T Tb Tc Vp
time, s temperature, K boiling point at 101.325 kPa, K critical temperature, K pore volume estimated from the extracted saturation amount adsorbed and the molar volume of the adsorptive, cm3 g-1
Greek Letters R µ σk σmodel ∆Hads ∆H 0v ∆m
molecular polarizability, Å3 molecular dipole moment, D kinetic diameter, nm standard deviation model: x∑n (qcal-qexp)2/(n-j), for j fitting parameters, mol kg-1 molar enthalpy of adsorption, kJ mol-1 molar enthalpy of vaporization at 298.15 K and 101.325 kPa, kJ mol-1 mass change measured with the TEOM technique, mg
Acknowledgment. The authors are grateful to an anonymous reviewer for fruitful discussions on the isosteric heat curves. LA030264+
