Computing Adsorption Isotherms for Benzene, Toluene, and p-Xylene

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Computing Adsorption Isotherms for Benzene, Toluene, and p-Xylene in Heulandite Zeolite Michelle M. Laboy and Ivonne Santiago Department of Civil Engineering, University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680

Gustavo E. Lo´ pez* Department of Chemistry, University of Puerto Rico at Mayaguez, P.O. Box 9019, Mayaguez, Puerto Rico 00681-9019

When computer simulations were performed in the grand canonical ensemble, adsorption isotherms for benzene, toluene, and p-xylene in Heulandite zeolite were constructed. Nitrogen adsorption was simulated to test a feasible computational strategy. Simulations were performed at three temperatures (200, 298, and 473 K), at pressures ranging from 0 to 200 kPa, and at water contents ranging from 0% to 4%. It was found that the adsorption of the organic species was not significantly affected by increasing the pressure over 10 kPa. Also, increasing the water content of the zeolite reduced the adsorption of these aromatics significantly. On the other hand, as the temperature was increased the amount of adsorbed material was only slightly affected. To access adsorption selectivity information, various mixtures of the aromatics were studied. Results from the simulations show that adsorption of benzene was higher than that of toluene, and toluene adsorption was higher than that of p-xylene. A relation between the electronic environment of the molecular species and the amount of adsorbed material was established. The results obtained are compared with experimental data available on other synthetic and natural zeolites. 1. Introduction During the past few years the global concern for environmental problems has increased dramatically. The increasing industrialization and the development of transportation technology require great use of derived forms of petroleum, which give rise to byproducts of forms of petroleum that are major causes of pollution. Among the constituents of these byproducts, there are volatile organic compounds (VOCs), such as benzene, toluene, and xylene that are potential carcinogens. Furthermore, there are thousands of leaking underground storage tanks that house petroleum. It is estimated that the number of leaking gasoline tanks varies from 75 000 to 100 000 in the United States.1 Gasoline contains less than 3% benzene, less than 11% toluene, and less than 12% xylene.1 For the removal of these VOCs, adsorbents such as activated carbon are very effective but very expensive. Development of effective and economically feasible technologies is needed. Zeolites are arising as a possible developing new and inexpensive technology to treat air contaminated with these products. Zeolites are a group of microporous, crystalline aluminosilicate minerals containing alkali and alkaline-earth cations, as well as water, in their naturally occurring structures.2 The three-dimensional channels within the structure of the zeolite are frequently interconnected and are used as molecular sieves. Molecules that are small enough can pass through the channel systems, while those whose size and shape are not compatible with the apertures of the zeolite are trapped.3 Because of the capacity of * To whom correspondence should be addressed. E-mail: [email protected].

the zeolites to adsorb and desorb water, these molecular sieves have been used as desiccants and in water purification systems. Other major uses of zeolites include catalysis, ion exchange, and water softening units. An example of a naturally occurring zeolite is Heulandite (HEU), which is the most abundant and commonly used type.4,5 Even though zeolites have a great potential for the removal of contaminants, the molecular description of the adsorption process is very limited. It is of foremost importance to study the mechanisms of adsorption and how sorbate polarity and size, temperature, pressure, and moisture content affect them. Studies of the separation processes of aromatic compounds using zeolites had been performed, both experimentally6-10 and theoretically.11-14 Zeolite types that have been used for these purposes are the silica-rich MFI type (silicalite and ZSM-5) and the dealuminated Faujasite type.5,6,10 It has been found5 that the adsorption of organic compounds in zeolites depends on the Si/Al ratio, the nature of the adsorbed molecule, the zeolite structure type, pore size, and temperature. However, no conclusive evidence has shown how the electronic environment, polarity, and size of these species affect their adsorption onto Heulandite zeolite. To improve the adsorption capability of organic compounds, dealumination processes are used.5 The properties of dealuminated HEU are seldom known from a microscopic point of view. When computer simulations were performed in the grand canonical ensemble, adsorption isotherms were constructed for various molecular species adsorbed on pure Heulandite zeolite. Initially, the adsorption of molecular nitrogen was studied at various conditions in order to test a feasible computational strategy. The effects of temperature and humidity content were

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considered. After careful examination of computational parameters, adsorption isotherms were constructed for the adsorption of toluene, benzene, and p-xylene in pure form. Various mixtures were also studied in order to access adsorption selectivity information. A relation between the electronic environment of the molecular species and the amount of adsorbed material was established. Ideal conditions to obtain the maximum amount of adsorbed material were presented. The simulations were performed using the sorption module of Cerius2_3.0 software from Molecular Simulations Inc. (MSI).14 The HEU zeolite was simulated as a rigid framework with various moisture contents ranging from 0% to 4.12%. Hence, adsorption isotherms for each sorbate were computed at various humidity conditions. Adsorption simulations were performed at 200, 298, and 473 K, at a pressure range of 0-200 kPa. Selectivity simulations on benzene, toluene, and p-xylene were performed for a mixture of components. 2. Method As stated earlier, the main objective of this work was to construct adsorption isotherms for simple molecules adsorbed in HEU zeolite using statistical thermodynamic models.16-18 Specifically, the simulations were performed using the grand canonical Monte Carlo (GCMC) ensemble, which creates, destroys, translates, and rotates molecules in order to obtain thermodynamic equilibrium in an open system.15,16,18,19 The adsorption isotherms were computed by calculating the mean loading of the sorbate in the zeolite at a specific vapor pressure. Only adsorption in the internal channels of the zeolite was considered. 2.1. Interparticle Potential. Two types of interactions were considered in this study: sorbate-sorbate and sorbate-zeolite framework interactions. For the simulation of the adsorption of N2 and water, the two types of interactions were modeled using the so-called Universal Force Field.20 This force field describes bond stretches with a harmonic term, the angle bending with a Fourier cosine expansion, and torsion and inversion with cosine Fourier expansion terms. The van der Waals interactions are described using Lennard-Jones potentials, and the multipolar interactions are modeled using the point charge model. For the simulation of benzene, toluene, and p-xylene, the Sorption Demontis Force Field21 was used. Specifically, the sorbate-sorbate interactions are described using atom-atom Buckingham potentials. The electrostatic interactions are modeled by placing partial charges on the hydrogen and carbon atoms forming the aromatic compound.13,21 The sorbate-zeolite interaction takes into account the electrostatic interaction of all atoms and the short-range atom-atom interaction between the aromatic compound and the oxygen atoms of the zeolite. The short-range interactions with the silicon atoms are neglected considering that the oxygen atoms of the tetrahedral SiO4 structure shield them.21 Initially, high-energy configurations were rejected to save computational time during the GCMC simulation. These configurations are those in which the sorbate molecules and the framework are very close together; i.e., the distance between the atoms of the sorbate and the framework is less than half their van der Waals radii. van der Waals energies between the sorbate and the framework were calculated by summing all pair interactions within a specific volume, in which the

radius is determined by a cutoff distance. The van der Waals energy term within the zeolite framework is restricted to the minimum image convention, in which an atom is considered to interact with its closest neighbor atoms in a periodic box around it. In the case of sorbate-sorbate energy, the interactions are not limited to the atoms within the minimum image border but to the molecules whose centers of mass are within it. The Coulomb and the electrostatic energies between the sorbate and the framework were evaluated using Ewald summation. Aromatic molecules and the zeolite framework were treated as rigid units. 2.2. Simulation Procedure. The simulations were performed in a superlattice of 12 unit cells of HEU zeolite. Minimization with respect to energy was performed in order to obtain the most stable structure. The total number of atoms in this superlattice was 1296. The HEU zeolite was constructed with a net neutral charge; i.e., no aluminum substitution was induced. Thus, the zeolite had no cation-exchange capacity. Four other frameworks were constructed by simulating the adsorption of water at room temperature and four different pressures. These simulations resulted in frameworks, which represented four different moisture contents of natural zeolite containing water adsorbed from the environment. The model for the adsorption of water was the TIP3P potential, which has been successfully used in the description of pure water.22 The various Monte Carlo step sizes for the simulations were adjusted in order to obtain a 50% acceptance probability. The interaction cutoff distance was fixed at 8.0 Å, which is larger than the diameter of the cavities in the zeolite, so it accounts for all of the necessary interactions. The length of the simulation depended on the sorbate. Initially, several test runs at different lengths were performed for each sorbate until the loading and energy converged. For N2, the simulations consisted of 500 000 steps. For benzene, 4 million steps were necessary, whereas toluene required 6 million steps. For p-xylene, simulations were performed at 9 million steps (simulations of 200 K required 10 million steps). For the selectivity simulations of the aromatics, runs were performed at 14 million steps. The equilibration of the system was monitored by measuring the configuration energy and the number of adsorbed molecules as a function of Monte Carlo steps. Both quantities exhibit a small fluctuation around a central value after sufficiently long runs. Also, snapshots of the sorbates inside the zeolite structure were analyzed, and the amount of sorbate in the different adsorption channels was monitored. The effect of humidity in the adsorption isotherms was considered in two different ways. The first group of simulations considered the adsorption of water in the zeolite at room temperature for a period of time long enough so that the system reached equilibrium. The final configuration from this run is saved and used as an initial framework for the adsorption of the compounds mentioned above. No water molecules are created nor destroyed during the construction of these isotherms. The main focus of this approach is to determine the amount of adsorbed material at constant humidity. From the number of water molecules adsorbed and the size of the zeolite, the humidity content is calculated in terms of weight percent. This approach will be termed as model A. The second approach was based on simulating a mixture of water and the ad-

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Figure 1. Adsorption isotherms for N2 in HEU dry zeolite at three different temperatures.

sorbed material. Both sorbates were simulated at pressures and temperatures that correspond to the ones used in model A. For example, when the zeolite is prepared at 2.30% humidity in the first approach, the partial pressure of water was fixed to 100 kPa. When the second approach is used, the partial pressure of water in the mixture is set to 100 kPa and the partial pressure of benzene to the desired value. This method will be referred to as model B. The pressure and temperature ranges used were chosen in order to simulate experimental conditions. 3. Results and Discussion Figure 1 shows three adsorption isotherms for N2 at 0% humidity. All of the curves have the usual type I shape9,23 observed for adsorption isotherms, namely, an exponential increment in the adsorption as the partial pressure increases, followed by a saturated behavior. At 200 K, the saturation value is observed at approximately 20 kPa, and at 298 K, the saturation value is observed after 100 kPa (1 atm). As expected, as the temperature decreases, the amount of adsorbed material increases. At 473 K the amount of N2 adsorbed is almost negligible because of possible molecular desorption. Figure 2 shows adsorption isotherms at 298 K and five different humidity contents for N2. These isotherms were constructed using model A as the initial structure. It can be observed that, at all values of partial pressure examined, as the amount of moisture content increases, the adsorbed material decreases. This is due to the fact that water molecules will occupy a large amount of adsorption sites in the zeolite and hence less nitrogen will be adsorbed. Moreover, the zeolite will have a stronger preference for water than for nitrogen due to the strong dipole moment of the H2O molecule. This behavior is also observed at the other two temperatures, but obviously the amount of adsorbed material decreases at 473 K and increases at 200 K. When both effects, temperature and moisture content, are compared, the temperature will have a more pronounced effect in the amount of adsorbed material. The same behavior was observed at the other two temperatures considered, namely, a decrease in adsorption with an increase in moisture content. When model B was used as the initial configuration in the simulations, the

Figure 2. Adsorption isotherms for N2 in HEU zeolite at T ) 298 K and five different humidity contents.

results at ambient temperature were basically the same as the ones obtained from model A. However, at 200 and 473 K a small variation in the adsorption isotherms was found. This variation was similar in all of the systems considered in this study and will be discussed later. Figures 3 and 4 show the results for the adsorption of benzene, toluene, and p-xylene in the HEU zeolite using model A as the initial configuration. Some basic trends can be clearly identified for all of the organic molecules studied. First, it can be observed that the amount of adsorbed material is not considerably affected by increments in temperature (Figure 3). Two main reasons, which are based on energetic and steric arguments, could be postulated in order to explain this phenomenon. First, it is well-known that the hydrogen atoms in the aromatic compounds have a strong interaction with the oxygen atoms of the zeolite.20 This interaction causes the desorption process to be less efficient, and hence as the temperature is increased, the amount of adsorbed material does not vary much. The second reason for this behavior is that the pore sizes of the HEU zeolite system are large enough as to allow the adsorbed molecules to be packed in a very efficient way. This packing causes a very strong interaction between the adsorbed material, hence forming energetically stable systems that will not desorb efficiently. To explain the above arguments, various snapshots are presented in Figure 5 where the different adsorbed molecules show various preferential spatial orientations; i.e., a large fraction of molecules are adsorbed in the larger channels of the zeolite and exhibit different orientations. Figure 5a shows a unit cell of HEU zeolite with a labeled axis that helps in the identification of the various regions in the channel. The channel axis labeled by letter C goes through the center of the larger opening of the channel and goes parallel to the channel direction. The Cartesian coordinates X, Y, and Z are also depicted in this figure. Figure 5b shows a snapshot of benzene molecules, which are basically accommodated in two main conformations in the larger channels of the zeolite. In the channel openings, the benzene molecules are oriented with the ring perpendicular to the C axis, and in the intersections of the channels, the molecules are oriented parallel to the C axis. On the other hand, in the smaller channels only one main conformation is observed, that is, perpendicular to the channel axis with the benzene molecules very close together and the rings

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Figure 4. (a) Adsorption isotherms for benzene in HEU zeolite at T ) 298 K and different humidity contents, (b) same as part a but for toluene, and (c) same as part a but for p-xylene. Figure 3. (a) Same as Figure 1 but for benzene, (b) same as part a but for toluene, and (c) same as part a but for p-xylene.

almost aligned with each other, in a face to face configuration. This conformation increases the sorbatesorbate interaction, as can be seen in Figure 5b, where the molecules are tightly packed. In the case of p-xylene, which is depicted in Figure 5d, the molecules are farther away from each other and tightly fit in the channel. In the opening of the larger channels, p-xylene is mostly adsorbed with the ring parallel to the C axis and the methyl groups aligned with this axis. In the smaller channels, the molecules are away from the intersection of the channels and completely perpendicular to the channel axis, with the methyl groups always parallel to the Y direction. In the case of toluene molecules, which are asymmetric species, the orientation is parallel to the channel axis and the methyl groups are as far as possible, minimizing their interaction with respect to each other (Figure 5c). When an internal comparison is made between the three organic species, it can be seen that the amount of benzene adsorbed is approximately 2 times the amount of adsorbed toluene and approximately 4 times the amount of p-xylene. This phenomenon can be explained in terms of the delocalization of the π orbital in the compounds considered. Namely, the methyl groups in toluene and p-xylene cause a dispersion of the π

electronic cloud, which is concentrated on the center of the ring. If the electronic cloud is more delocalized, then the interaction with the zeolite framework will be weaker. Therefore, as the delocalization of the π electron decreases (p-xylene > toluene > benzene), the amount of adsorbed material increases. Another important argument for this behavior is the size of the molecule. As the size of the molecules increases, the diffusion inside the zeolite is more difficult, and hence the number of stable conformations that could be accepted decreases. Therefore, the packing of the molecules plays an important role in the amount of adsorbed material, as explained before. An interesting feature that can be noticed in the isotherms for p-xylene and toluene (Figure 3c) is a variation in the shape of the curves when compared to the other systems. This curve shape corresponds to a type IV isotherm,8 where it has been suggested that the sorbate-sorbate interaction might play a significant role in the adsorption process. In the case of the adsorption of p-xylene, an initial saturation value is observed at approximately 10 kPa. However, after this saturation value is reached, another increment on adsorbed material is observed, followed by a new saturation regime. This behavior in the isotherms is more pronounced at 473 K than at the other temperatures. Clearly, this phenomenon can be expected at higher temperatures where the zeolite has not reached saturation at low

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Figure 5. (a) HEU zeolite unit cell with various orientational axes labeled, (b) final configuration snapshot of benzene molecules adsorbed inside the 12 HEU unit cells, (c) same as part b but for toluene, and (d) same as part b but for p-xylene.

pressures. As the amount of adsorbed material is larger and the sorbate-sorbate interaction becomes stronger, reorganization and reorientation of the molecules is more efficient. Figure 4 shows the adsorption isotherms at various humidity contents for the organic compounds considered. It can be observed that, as the humidity content increases, the effect in the adsorption of toluene and p-xylene is decreased in a larger amount when compared to benzene. This could be explained by the fact that benzene has more conformational stability than the bigger compounds in the sense that it can assume more stable orientations around the water molecules or by forming benzene clusters. The functional groups in toluene and p-xylene cause difficulties in the possible formation of molecular clusters, and only a few stable orientations will be permitted and a smaller amount of material will be adsorbed. As stated, the results obtained using model B as the initial configuration are somewhat different from the previously discussed results at temperatures of 200 and 473 K. At 200 K, water adsorption increases signifi-

cantly at the same pressures as those used in model A. Hence, the amount of aromatic compound adsorbed decreases because of the large amount of water adsorbed. To achieve lower water content, it is necessary to lower the pressure used. In this case, the adsorption results are similar to model A data. When the temperature is increased to 473 K, the adsorption of water is significantly reduced, resulting in a higher adsorption of the aromatics. It is important to recall that the aromatics are considerably less affected by temperature increments. This implies that water adsorption, which is strongly dependent on temperature, is overcome by the adsorption of the aromatics at high temperatures. Again, to get a similar water content in model B at 473 K, the water pressure has to be increased. These results indicate that the temperature is an important factor in determining the selectivity of these systems because it determines which component will be desorbed more efficiently. To consider adsorption selectivity of the HEU zeolite, isotherms were constructed for various binary mixtures composed of the organic compounds previously men-

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Figure 6. (a) Adsorption isotherms for a mixture of benzene and toluene in HEU dry zeolite, (b) same as part a but for a benzene/pxylene mixture, and (c) same as part a but for a toluene/p-xylene mixture. Each two points at a given pressure correspond to the mean loading of each component in the mixture in a single run.

tioned. Specifically, mixtures of benzene/toluene, benzene/ p-xylene, and toluene/p-xylene were considered. For all mixtures the vapor pressure of both components was set equal. In all cases, the zeolite was simulated with no water content and the isotherms were constructed at 298 K. Figure 6a shows the adsorption isotherms for a mixture of benzene/toluene, with the upper curve

corresponding to benzene and the lower curve to toluene. It can be clearly observed that at all partial pressures considered the amount of benzene adsorbed is approximately 6 times larger than the amount of toluene. Figure 6b shows adsorption isotherms for the benzene/p-xylene mixture. It can be seen that the adsorption of benzene is approximately 10 times higher

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than p-xylene adsorption at most pressures. However, the shape of the curves indicates that the sorbatesorbate interaction starts playing a significant role in the adsorption process of benzene at high pressures. This behavior is due to the fact that benzene adsorption is initially controlled by the sorbate-framework interaction, and at higher pressures, the effect of sorbatesorbate interactions between benzene molecules overcomes that of p-xylene molecules. After approximately 15 kPa the adsorption of benzene increases, indicating the possible formation of benzene clusters, i.e., the increase in sorbate-sorbate interactions. Consequently, the adsorption of p-xylene decreases. Figure 6c shows the case of the toluene/p-xylene mixture. The p-xylene isotherm has clearly the type IV form, which indicates that when the sorbate-sorbate effects in p-xylene and toluene are compared, it is more pronounced for pxylene. This repeated behavior in p-xylene adsorption, on both pure and two-component systems, is caused by the poor adsorption of this species based only on zeolitesorbate interaction. Hence, the adsorption of p-xylene will rely mostly on the sorbate-sorbate interaction rather than the sorbate-framework interaction. In the case of benzene, both types of interaction have a significant contribution to the adsorption process. Preliminary, tertiary mixtures for the aromatics were simulated at 298 K. The behavior observed is very similar to the binary mixtures. Benzene is more preferably adsorbed than the other two compounds. Toluene and p-xylene adsorptions do not vary much when compared to the previous results. When the results of this study are compared to experimental data of adsorption in other zeolite frameworks,9,23 our results indicate that the dealuminated HEU zeolite is a very competitive alternative for the separation process of aromatic compounds from air. The most effective saturation value for benzene in our model is around 1.4 mol/kg, which is near the optimum adsorption found in the silicalite system: 1.3 mol/kg.23 When the pore sizes are compared, the HEU zeolite has large openings with dimensions similar to the FAU system and larger than ZSM-5. The largest opening of the HEU zeolite has dimensions of up to 7.2 Å. The FAU system has dimensions of 7.4 Å, and the ZSM-5 (silicalite) system has a maximum dimension of 5.6 Å. The benzene molecule, for example, has a kinetic diameter of 5.85 Å,9,23 a width of 5 Å calculated from bond lengths and angles, and a length of 6.6 Å.9,23 It has been approximated in simulations on previous works as a sphere of 5.270 Å.13 It was observed that the aromatic molecules were inserted inside the channels of the zeolite; there was not an observed preference for the intersections of the channels as stated in other works in silicalite,13 thus maximizing the fill of the zeolite inner volume. This shows that, in terms of space and steric effects, the HEU is a competitive natural resource. Future considerations will be made in terms of the Si/ Al ratio and the modification of the HEU framework, to maximize adsorption efficiency. 4. Conclusion The use of molecular sieves for the removal of organic contaminants is a very promising alternative. In particular, because of its considerable abundance, zeolites are the most widely recognized molecular sieves used in separation processes. In this study, a comprehensive analysis of the interaction of simple molecules with pure

zeolite has been presented. By initially considering molecular nitrogen, various important features of the process of adsorption in zeolite were uncovered. Namely, as the temperature of the system decreases, the amount of adsorbed material increases and at lower temperatures a saturation value is obtained at relatively low pressures. Also, as the humidity content is increased, the amount of adsorbed material decreases because the zeolites adsorb in a more selective fashion the strongly polar water molecule. The adsorption of three pure organic compounds was also considered. It was found that benzene was adsorbed in larger amounts than toluene and toluene more than p-xylene. The differences in the electronic environment as well as the steric effects of the methyl groups are responsible for the adsorption differences. As the molecules have the electronic charge more localized, the interaction with the zeolite framework is stronger because of electrostatic interactions, and as a result, the adsorption is higher. Also, molecules with fewer functional groups will pack more efficiently in the channels of the zeolite and hence increase the amount of adsorbed material. The size of the molecule decreases the amount of stable conformations that could be accepted inside the HEU zeolite channels, which are not relatively higher than the size of the molecules being considered. When binary mixtures of the organic compounds were considered, benzene was adsorbed 5 times more than toluene and toluene 5 times more than p-xylene. However, some of the adsorption isotherms presented an unusual type IV form characteristic of a considerable influence in the adsorption due to sorbate-sorbate interaction. The results of this study suggest several important features that should be taken into account when using HEU to remove VOC from air: 1. HEU must be as dry as possible in order to achieve maximum adsorption. Hence, the use of hot air streams, which will cause low adsorption of small polar compounds and possible desorption of water, is highly recommended. Then the adsorption of the aromatics will occur at the air stream temperature. 2. At ambient temperature, adsorption of VOC onto HEU is possible. Decreasing the temperature of the system to temperatures down to 200 K (-73 °C) will not enhance adsorption of the VOC onto HEU. 3. As the structure of the VOC is more compact, and the electronic environment is more localized, adsorption will be more efficient. 4. The same structural arguments used to explain the adsorption of pure compounds could be applied when binary and ternary mixtures are considered. The results of this study show that HEU can be effectively used for removing VOC from air. However, the framework used in this study has no silicon substituted for aluminum. The incorporation of aluminum atoms on the framework can decrease the adsorption of material. Preliminary simulations with an aluminummodified framework show that as the content of aluminum increases the amount of adsorbed VOC can be reduced considerably. Also, in the present study we have only considered the adsorption inside of the zeolite. However, a considerable amount of material can be adsorbed on the surface of the zeolite. In particular, large organic compounds that will not fit inside the cavities of the zeolite can be adsorbed on the surface. Work related to these issues is in progress. Further-

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more, other simulations are being performed to study how modification of the external surface of the zeolite affects the adsorption of these VOCs. Comparison between the adsorption of aromatics in HEU zeolite and the experimental and theoretical data obtained from other works shows that the HEU system is a competitive resource for the removal of organic compounds from air, especially at high temperatures. Acknowledgment This work was supported by the NSF/EPSCoR Infrastructure Thrust Area and the NSF/EPSCoR Material Science Thrust Area. M.M.L. acknowledges support from the Alliance Minority Program (AMP) from NSF. G.E.L. is a Camille and Henry Dreyfus teacher scholar. Literature Cited (1) Cadena, F.; Vigil, F.; Peters, R. W. Tailoring of Zeolites for Adsorption of Benzene and Toluene from Solution. Proceedings of the 47th Industrial Wastes Conference, Purdue University, Lafayette, IN, 1992. (2) Bohn, H. L.; McNeil, B. L.; O’Connor, G. A. Soil Chemistry; John Wiley & Sons: New York, 1985. (3) Ca´zares-Rivera, E. Removal of single ring aromatics by tailored zeolites. Doctoral dissertation, Civil Engineering, New Mexico State University, Las Cruces, New Mexico. (4) Galli, E.; Gottardi, G. The Structure of Potassium-Exchanged Heulandite at 293, 373 and 593 K. Acta Crystallogr. 1983, B39, 189. (5) Newsam, J. M. The Zeolite Cage Structure. Science 1986, 231, 1093. (6) Fajula, F.; Plee, D. Application of molecular sieves in view of cleaner technology. Gas and liquid phase separations. Adv. Zeolite Sci. Appl. 1994, 633. (7) Pope, C. G. Sorption of Benzene, Toluene, and p-Xylene on Silicalite and H-ZSM-5. J. Phys. Chem. 1986, 90, 835. (8) Pope, C. G. Sorption of Benzene, Toluene, and p-Xylene on ZSM-5. J. Phys. Chem. 1984, 88, 6312. (9) Talu, O.; Guo, C.; Hayhurst, D. Phase Transition and Structural Heterogeneity; Benzene Adsorption on Silicalite. AIChE J. 1989, 35 (4), 573. (10) Talu, O.; Li, J. Adsorption Equilibrium of Benzene-pXylene Vapor Mixture on Silicalite. Chem. Eng. Sci. 1994, 49 (2), 189.

(11) Henson, N. J.; Cheetham, A. K.; Redondo, A.; Levine, S. M.; Newsam, J. M. Computer Simulations of benzene in Faujasitetype zeolites. Zeolites Relat. Microporous Mater. 1994, 2059. (12) Snurr, Q.; Bell, A.; Theodoru, D. A Hierarchical Atomistic/ Lattice Simulation Approach for the Prediction of Adsorption Thermodynamics of Benzene in Silicalite. J. Phys. Chem. 1994, 98, 5111. (13) Snurr, Q.; Bell, A.; Theodoru, D. Prediction of Adsorption of Aromatic Hydrocarbons in Silicalite from Grand Canonical Monte Carlo Simulations with Biased Insertions. J. Phys. Chem. 1993, 97, 13742. (14) Grauert, B.; Fiedler, K.; Stach, H.; Janchen, J. Modeling of the Adsorption of Aromatics on Silicalite on Molecular-Statistical Basis. Zeolites: Facts, Figures, Future; Elsevier: New York, Vol. 49B, p 701. (15) . Molecular Simulations Inc. Cerius2 Property Prediction; San Diego, 1997. (16) Lo´pez, G.; Matos, N. Classical Monte Carlo study of phase transitions in rare-gas clusters adsorbed on model surfaces. J. Chem. Phys. 1998, 109 (3), 1141. (17) Lo´pez, G.; Quintana, I.; Ortiz, W. Determination of the structure and stability of water clusters using temperaturedependent techniques. Chem. Phys. Lett. 1998, 287, 429. (18) Allen, M. P.; Tildesley, D. J. Computer Simulations of Liquids; Clarendon Press: Oxford U.K., 1987. (19) Reference 19. (20) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF, A full periodic table force field for molecular mechanics and molecular dynamics simulations. J. Am. Chem. Soc. 1992, 114, 10024. (21) Demontis, P.; Yashonath, S.; Klein, M. Localization and Mobility of Benzene in Sodium-Y Zeolite by Molecular Dynamics Calculations. J. Phys. Chem. 1989, 93, 5016. (22) Jorgensen, W.; Chandrasekhar, J.; Madura, J. Comparison of simple potential functions for simulating liquid water. J. Chem. Phys. 1983, 79 (2), 926. (23) Talu, O.; Guo, C.; Hayhurst, D. Heterogeneous Adsorption Equilibria with Comparable Molecule and Pore Sizes. J. Phys. Chem. 1989, 93, 7294.

Received for review November 19, 1998 Revised manuscript received September 7, 1999 Accepted September 12, 1999 IE980732O