Langmuir 2003, 19, 7941-7943
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Adsorption of Volatile Organic Compounds in Pillared Clays: Estimation of the Separation Factor by a Method Derived from the Dubinin-Radushkevich Equation Joa˜o Pires,* Moise´s L. Pinto, Ana Carvalho, and M. B. de Carvalho Departamento de Quı´mica e Bioquı´mica, Universidade de Lisboa, Faculdade de Cieˆ ncias, Campo Grande, 1749-016 Lisbon, Portugal Received March 3, 2003. In Final Form: June 16, 2003 In this work the results of adsorption of selected volatile organic compounds (VOCs), namely, methanol, acetone, methyl ethyl ketone, 1,1,1-trichloroethane, and trichloroethylene, in pillared clays are analyzed by a model recently proposed to estimate the separation factor of a selected pair of molecules in a given adsorbent. Although the precise design of the process of selective adsorption of VOCs would need more elaborated methodologies, the present model has the advantage of being developed for microporous adsorbents, the parameters and the calculations involved are accessible, and, in this way, it could provide useful information to material scientists during the work of optimization of the adsorption properties of a given adsorbent for a particular system that involves adsorption and/or recovering of noxious volatile organic compounds.
Introduction It is well recognized that the methodologies for analyzing mixed gas adsorption data are crucial for the design of separation processes by adsorption.1-3 Besides, the progress that was achieved in the description of singlegas adsorption, the extension of the various methodologies to multicomponent mixtures poses some difficulties.4 These questions are even more relevant when microporous adsorbents are considered, where models derived from certain assumptions have to be applied with caution. For instance the models based in the extended Langmuir equation, in the adsorbed solution theory, or in the vacancy solution theory1-3,5 all have premises that, in principle, are more adequate to a mechanism of adsorption that occurs more by the coverage of the surface area than by the filling of the volume of the micropores.1-3,5 It would be undoubtedly useful if a simple model, based in the filling of the micropores, could be used to obtain reliable information on the adsorption of mixtures from the pure component isotherms. Additionally, such a model should be extended to the adsorption of vapors, particularly due to the increased importance of the recovery, and possible reuse, of noxious volatile organic compounds (VOCs), by adsorption methods.6,7 Recently,4 a model was proposed for the adsorption of multicomponent mixtures in micropores which is based in the formerly empirical8 Dubinin-Radushkevich (D-R) expression of the theory of volume filling of micropores. Briefly, the proposed model4 is based in the D-R equation, θ ) exp{-(A/βE0)2}, which relates the degree of filling (θ) with the adsorption * To whom correspondence may be addressed. E-mail: jpiresil@ fc.ul.pt. (1) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Stoneham, 1987; Chapter 3. (2) Ruthven, D. M. Priciples of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984; Chapters 3 and 4. (3) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, 1998; Chapter 4. (4) Dobruskin, V. Kh. Langmuir 1996, 12, 987. (5) Suwanayuen, S.; Danner, R. P. AIChE J. 1980, 26, 76. (6) Ruhl, M. J. Ind. Eng. Prog. 1993, 37. (7) Pires, J.; Carvalho, A.; Carvalho, M. B. Microporous Mesoporous Mater. 2001, 43, 277. (8) Chen, S. G.; Yang, R. T. Langmuir 1994, 10, 224.
Figure 1. Adsorption isotherms (nitrogen at 77 K and other vapors at 298 K) in the aluminum oxide pillared clay (Al-PILC).
Figure 2. Adsorption isotherms (nitrogen at 77 K and other vapors at 298 K) in the zirconium oxide pillared clay (Zr-PILC).
potential, A ) RT ln(p0/p), being p the partial pressure and p0 the saturated pressure of the liquid adsorbate. E0 is an adsorbent-related constant, and β is the similarity coefficient which is defined in relation to a standard adsorbate and reflects the differences among the various adsorbates. The proposed methodology leads to an expression for the separation factor (S) for binary mixtures4
10.1021/la030086k CCC: $25.00 © 2003 American Chemical Society Published on Web 08/16/2003
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Figure 3. Separation factors versus coverage for the aluminum oxide pillared clay (Al-PILC) with the values of β estimated from the parachors (closed symbols) or the molar polarizations (open symbols) using benzene (squares) or carbon tetrachloride (triangles) as standard vapor.
S ) (β2/β1)0.5θ-1 exp(b2){exp{[(ln 1/θ)-0.5 - b]2} + π0.5b{1 - erf[(ln 1/θ)0.5 - b]}} (1) where b ) ∆βE0/2RT. In this way, eq 1 provides an accessible way to estimate the separation factor, which is the more important parameter for the selection of an appropriate adsorbent. This is particularly relevant in the cases where an optimization of the properties of a given adsorbent, for a given separation/recovery process, is under study. In fact, to obtain experimental adsorption data for mixtures, in parallel with the optimization of the properties of the adsorbent, would represent a difficult task. In this work, the validity of eq 1 is appraised in relation to the case of the adsorption in pillared clays (PILCs) of selected oxygenated and chlorinated VOCs molecules, namely, methanol, acetone, methyl ethyl ketone (MEK), 1,1,1-trichloroethane (TCA), and trichloroethylene (TCE).
Pires et al.
Figure 4. Separation factors versus coverage for the zirconium oxide pillared clay (Zr-PILC) with the values of β estimated from the parachors (closed symbols) or the molar polarizations (open symbols) using benzene (squares) or carbon tetrachloride (triangles) as standard vapor.
Experimental Section The pillared clays were obtained from Portuguese soils in Madeira archipelago by pillaring with oligomeric cations of aluminum, or of zirconium, by optimized procedures described elsewhere.9-11 Briefly, the oligomeric solutions were prepared from AlCl3 and NaOH, or from ZrOCl2, and added to the clay suspension. After centrifugation the intercalated materials were washed in a dialysis tube until a conductivity less than 1 mS m-1, freeze-dried and calcined at 623 K during 2 h after a ramp of 1 K min-1. The initial (nonintercalated) clays were montmrilonite type9,11 with the chemical formula (Si3.70Al0.30)IV(Al1.16Fe0.51Mg0.26)VI(Ca1/2, K, Na)0.39. After pillaring, the basal spacings (d001) increased from 12.7 in the initial clay to 18 and 16 Å for the Al-PILC and the Zr-PILC, respectively. The nitrogen adsorption isotherms at 77 K (Figures 1 and 2) were determined in a manual volumetric installation, made in Pyrex and equipped (9) Pires, J.; Carvalho, M. B.; Carvalho, A. Zeolites 1997, 19, 107. (10) Pereira, R. P.; Pires, J.; Carvalho, M. B. Langmuir 1998, 14, 4584. (11) Carvalho, M. B.; Pires, J.; Carvalho, A. P. Microporous Mater. 1996, 6, 65.
Adsorption of Organic Compounds in Clays with a transducer from model 600 (Datametrics, USA). Prior to the measures, the samples were outgassed for 2 h at 573 K, after a ramp of 10 K min-1, under dynamic vacuum better than 10-2 Pa. Specific surface areas (ABET) and microporous volumes (from D-R plots) were 366 and 269 m2 g-1 and 0.155 and 0.114 cm3 g-1, for the Al-PILC and the Zr-PILC, respectively. The adsorption isotherms of methyl ethyl ketone (BDH, 99.5%), methanol (Riedelde-Hae¨n, 99.8%) 1,1,1-trichloroehane (Aldrich, 99%), acetone (BDH, 99.5%), and trichloroethylene (Fluka, 99.5%) were determined by the gravimetric method using microbalances from C.I. Electronics (U.K.), which allowed a precision of 10 µg. The pressure readings were made with capacitance transducers from Shaevitz (U.K.). A combination of rotary/oil diffusion pumps, and heating the solids, in a similar way as described for the case of the nitrogen adsorption isotherms, accomplished the outgassing of the adsorbents. The temperature of adsorption was maintained at 298 ( 0.1 K with a water bath (VWR Scientific, USA). Prior to adsorption, the vapors were purified in situ by freeze-vacuumthaw cycles. The amounts adsorbed (including for the nitrogen adsorption) were reported in terms of liquid volume by gram of adsorbent material, that is, in cm3 g-1.
Results and Discussion In Figures 1 and 2 the adsorption isotherms of nitrogen at 77 K and of the studied VOC molecules at 298 K are shown for the Al- and Zr-PILCs, respectively. It can be seen that, in general, the isotherms have a shape that is compatible with the adsorption in microporous solids although, in the range of intermediate relative pressures, an inflection point is noticed in the curves, revealing that adsorption in the external area can also occur. In the calculation of S, some uncertainty can arise from the value of E0 to be used to obtain b in eq 1. In fact, although from the theory E0 is a constant related with the adsorbent, in practice the exact values obtained from isotherms of various vapors, usually present some variation. In the present case a mean value of E0, between those obtained from the isotherms of each studied pair of adsorbates was used. For the estimation of the β values, which are relevant to obtain the separation factor from eq 1, essentially two properties of the adsorbates, the molar polarization and the molar parachor, have been discussed in the literature.10,12 Benzene is usually chosen as standard adsorbate, but others, as for instance carbon tetrachloride, were also used.7,10 In Figures 3 and 4 the separation factors
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(from eq 1), using β estimated from the molar parachors or from the molar polarizations, and considering as standard adsorbates benzene or carbon tetrachloride, are plotted against the coverage, for the Al- and Zr-PILCs, respectively. Different situations can be disclosed from these figures. In the case of the methanol/acetone and TCA/TCE pairs, and considering each one of the physical properties (parachors or polarizations), the values of S are not significantly dependent on the standard adsorbate chosen, and parachors seem particularly adequate to be used in these cases. On the basis of the analysis of the separation factor, it could be accepted that both PILCs could be candidates for the process of separation/recovery of methanol/acetone vapors, particularly at low fillings, but not for the TCA/TCE pair since, in this situation, the separation factor is near 1 even for low θ values. The case of the acetone/MEK presents the larger discrepancies in the S values. One must bear in mind that the formalism that is related with the D-R equation, and consequently with eq 1, is expected to better approach the reality when the more relevant forces involved in the adsorption process are the dispersion forces. In the case of acetone and MEK, these molecules have the higher dipole moments of the studied VOCs (2.8 and 3.3 D12 for acetone and MEK, respectively) and in this situation other forces, besides dispersion forces, can also be involved. Although the final design of a given adsorption and/or recovery process would need the use of more elaborated methodologies, the model used4 seems to be adequate for the analysis of the data during a process of selection/ optimization of the adsorbent material, namely, due to the availability of data to estimate the variables involved, as the values of β, and the simplicity of the calculations. Acknowledgment. This work was partially made under the funding of Fundac¸ a˜o para a Cieˆncia e Tecnologia (FCT) to Centro de Cieˆncias Moleculares e Materiais. M. L. Pinto thanks FCT for a PhD grant. LA030086K (12) Reid, R. C.; Praunitz, J. M.; Poling, B. E. The properties of Gases and Liquids; McGraw-Hill: New York, 1988.
