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J. Phys. Chem. B 2006, 110, 250-257
On the Difficulties of Predicting the Adsorption of Volatile Organic Compounds at Low Pressures in Microporous Solid: The Example of Ethyl Benzene Moise´ s L. Pinto,* Joa˜ o Pires, Ana P. Carvalho, and Manuela B. de Carvalho Departamento de Quı´mica e Bioquı´mica and CQB, Faculdade de Cieˆ ncias, UniVersidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal ReceiVed: June 24, 2005; In Final Form: NoVember 2, 2005
Adsorption isotherms of toluene and ethyl benzene, at 25 °C and 40 °C, were determined in two microporous activated carbons and one zeolite. Significant differences were found in the adsorption behavior, at low pressures, between the two vapors on the same adsorbent material. The quantities of adsorbed ethyl benzene at 25 °C, in the low-pressure region, were lower than what was observed at 40 °C in all the studied adsorbents, contrary to what was found for toluene. This fact was not related to kinetic effects at the two temperatures nor to vapor swelling of the adsorbents structure. Also, there was no molecular sieving since at high pressures the toluene and ethyl benzene occupied the same adsorption space. The differences found in the ethyl benzene adsorption at the two temperatures pose difficulties in the analysis of the adsorption data and, therefore, in the prediction of results. This is discussed in the analysis of the results with the application of the DubininAstakhov equation and in the estimation of the isosteric heats of adsorption. The adsorption potentials of two possible ethyl benzene conformations were estimated for the adsorption in the pores of activated carbon from the Horvath and Kawazoe model, and the values compared with those found experimentally. The results were interpreted in terms of the ethyl benzene conformation effects when the molecule is confined in pores that are about the same size of one of the conformations.
Introduction The ability of solid adsorbents to retain organic molecules has attracted considerable attention to these materials, especially in applications related to environmental protection. In applications to the adsorption of volatile organic compounds (VOCs), the microporous materials (pore widths less than 2 nm1) are normally preferred since they can adsorb and be saturated at very low vapor pressures. The adsorbents with narrow pores are usually chosen since they can adsorb high amounts at low pressures but, in this case, for the application to relatively large molecules, sieving can also occur. However, the possibility that a more complex organic molecule can adopt different conformations may confound any prediction of the right adsorbent based only on the dimensions of the pores of the adsorbent and in the application of relatively simple adsorption models. It is therefore important to study the influence of the conformation of the adsorbed molecules, from both the theoretical and the practical views. As a general rule, the entropic effects of the adsorbed phase cannot be easily accessed and frequently cannot be completely distinguished from the surface heterogeneity, especially in highly heterogeneous surfaces such as those found in many adsorbent materials.2 On the other hand, the molecular conformations in the adsorbed phases, investigated by molecular simulation techniques, were found to be important for understanding the adsorption of organic molecules.3-6 These conformational effects have more importance when the molecules are confined in narrow spaces and/or when strong adsorption potentials are present. Thus, these effects are expected to be found experimentally in the low-pressure region. It should be emphasized * Corresponding author. E-mail:
[email protected]; Telephone: (+351) 217500898; Fax: (+351) 217500088.
that, in the context of the VOCs adsorption, these facts can have important practical implications, since a large number of these molecules are noxious, even in concentrations that correspond to very low partial pressures, and consequently need to be adsorbed in this range of pressures. In the present work, the toluene and ethyl benzene vapors were chosen since these are two similar VOCs but, in the ethyl benzene case, if the ethyl group is perpendicular to the aromatic ring the entrance in the smallest micropores can be hindered. The toluene and ethyl benzene adsorption was determined in two microporous activated carbons and in one zeolite at two temperatures. The differences found between the isotherms at different temperatures and between toluene and ethyl benzene can be interpreted in terms of differences in the molecular structure of the two adsorbates and their relation to the micropore structure of the adsorbents, since their chemical nature is very similar and small changes in the temperature do not change the adsorbents’ structure. Even in the case of relatively simple molecules, the adsorption in microporous adsorbents seems to be highly dependent on the various possible conformations of the molecules in the pores. For example, some authors have concluded that the ethyl group rotation in the ethyl benzene molecule could explain the abnormal diffusivity of this molecule in some zeolites.7-9 When the pore dimensions of the solid are close to the size of the adsorbed molecules, the conformation effects become more important and, in the limit, molecular sieving can occur. To put forward these effects in the ethyl benzene molecule, the selected adsorbents have pore dimensions close to the dimensions of that molecule. The molecular conformations are expected to influence the equilibrium distance of the adsorbed molecule from the pore walls and, in this way, to influence the adsorption potential of the molecule in the pores. Two models, based on the adsorption
10.1021/jp0534380 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
On the Difficulties of Predicting the Adsorption of VOCs
J. Phys. Chem. B, Vol. 110, No. 1, 2006 251
potential, were chosen to analyze the vapor adsorption results on the different adsorbents. Horva´th and Kawazoe10 have proposed a method for calculating the micropore size distribution based on the slit potential model of Everett and Powl.11 This model was extended by Saito and Foley for the cylinder pore geometry12 and by Cheng and Yang for the spherical geometry.13 All these models are based on the intermolecular potential concept that was used by many authors not only to calculate pore size distributions but also to obtain information on other important properties of the adsorbate/adsorbent systems.14 Briefly, according to the Horvath and Kawazoe (HK) model, the adsorption potential can be related to the width of the slit pores (l) by10
RT ln(p/p°) ) N aA a + N A A A σ10 σ4 σ10 σ4 + (1) L 4 3 9 3 σ (l - 2d0) 3(l - d0) 9(l - d0) 3d0 9d09
[
]
where L is the Avogadro number, Na and NA are the numeric surface densities of adsorbent and adsorbate respectively, and the dispersion constants Aa and AA are expressed according to the Kirkwood-Mu¨ller formalism:
Aa )
6mec2RaRA Ra/χa + RA/χA
(2)
3mec2RAχA 2
(3)
AA )
where me is the mass of the electron, c is the velocity of light, Ra and χa are the polarizability and diamagnetic susceptibility of the adsorbent surface atom, and RA and χA are the polarizability and diamagnetic susceptibility of the adsorbate molecule. The d0 value is the arithmetic mean of the diameters of the adsorbent atom and adsorbate molecule, and σ ) 0.858d0. Recently some authors have proposed some improvements in the HK model for the calculation of the micropore size distributions, and the application of this model to the analysis of adsorption isotherms as well as their limitations, are still the object of discussion.15-23 An equation proposed by Dubinin and Astakhov (DA) for the analysis of adsorption isotherms of vapors in microporous solids is also based in the adsorption potential.24-26 The wellknown DA equation has the form
w ) w0 exp[-(A/E)n]
(4)
where w is the adsorbed volume, at temperature T and relative pressure p/p°, w0 is the adsorption space in the micropores, A is the adsorption potential (A ) -RTln(p/p°)) and n and E are temperature invariant parameters. E is the characteristic adsorption energy and can be related with the energy of the adsorbate/ adsorbent system,24,27,28 and molecular properties of the adsorptive.29 This equation has been applied to the adsorption of a large variety of organic molecules in different adsorbents.30,31 In this work, the parameters w0, E, and n for each adsorbent were obtained by fitting this equation to the toluene and ethyl benzene adsorption data in a given adsorbent. Particular attention was given to testing the validity of the temperature independence of these parameters, and also to predictions of the adsorption of ethyl benzene from the data of the toluene adsorption.32,33 The predictions of the ethyl benzene adsorption, made by eqs 1 and 4, were compared with the experimental results, and the deviations found were interpreted in terms of the possible
conformational effects of the adsorbed phase, particularly for the ethyl benzene molecule. In the case of the HK equation, the adsorption potential of two extreme conformations of the ethyl benzene molecule in slit pores are compared with the experimentally obtained values in the two activated carbons used. The HK equation was useful to evaluate the effect of the conformation change of ethyl benzene in the adsorption potential. Experimental Section Adsorbent Materials. Two activated carbons and one zeolite were used, in the form of pellets of about 3 mm in diameter. The zeolite was a sodium form of X zeolite (BDH, “Molsiv” molecular sieve, type 13X). The two activated carbons were RB3 (Norit) and CarbonTech (CarboTech). The zeolite and the activated carbons RB3 and CarbonTech will be designated as 13X, RB3, and CT, respectively. Before the adsorption experiments, all samples (about 1 g) were crushed in a mortar to particles smaller than 1 mm, to ensure that the masses used in the adsorption experiments were representative of the respective material. Low-Temperature Nitrogen Adsorption. The nitrogen adsorption isotherms at -196 °C were obtained in an automatic apparatus (ASAP 2010; Micromeritics). Before the adsorption experiments, the samples, about 50 mg in weight, were outgassed under vacuum better than 10-2 Pa for 2.5 h at 300 °C. An analytic balance (AE 240, Mettler) was used to determine the mass of the clean solid, and the adsorbed amounts are expressed per mass of clean solid. The adsorption temperature was controlled with a cryogenic bath of liquid nitrogen. The adsorbed amounts were converted to liquid volume using the molar volume of nitrogen at -196 °C.34 Room-Temperature Vapor Adsorption. The toluene and ethyl benzene adsorption isotherms, at 25 °C and 40 °C, were obtained by the gravimetric method in a balance suited for vacuum (MK2-G5 and Disbal, C. I. Electronics). Samples of about 50 to 65 mg were outgassed with a heating program of 10 °C min-1 from room temperature to 300 °C and kept at this temperature for 4 h, under a final vacuum better than 10-2 Pa. The vapor was then introduced in the system, and allowed to equilibrate with the sample. Pressure readings were made with a capacitance transducer (CMR 262, Pfeiffer Vacuum). The temperature was controlled using a water bath (GD120, Grant), with 0.05 °C precision. The toluene (Aldrich,
