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Langmuir 1998, 14, 6728-6733
Halocarbon Adsorption in Nanoporous Materials: A Combined Calorimetric and Monte Carlo Study of Trichloroethylene (TCE) in Faujasite-Type Zeolites Caroline F. Mellot,*,† Anthony K. Cheetham,*,† Shani Harms,* Scott Savitz,‡ Raymond J. Gorte,‡ and Alan L. Myers‡ Materials Research Laboratory, University of California, Santa Barbara, California 93106, Institut Lavoisier, Universite´ de Versailles Saint-Quentin, Versailles Cedex 78035, France, and Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received March 20, 1998. In Final Form: September 9, 1998 Isosteric heats of adsorption of trichloroethylene (TCE) in a series of faujasite-type zeolites, siliceous faujasite, NaY (Si:Al ) 2.6), and NaX (Si:Al ) 1.2), have been studied by the combination of calorimetry and (N,V,T) Monte Carlo simulations, varying the sorbate loading up to ∼35 molecules per unit-cell. Excellent agreement is obtained between observed and calculated heats, confirming the applicability of our force field to the realm of unsaturated halocarbons for a large range of Si:Al ratio, cation content, and sorbate loading. The relative contributions of short-range and long-range interactions to the heat of adsorption are discussed, and the host/guest pair distribution functions (PDFs) from the MC simulations are analyzed in detail. At fixed loading, TCE heats of adsorption increase in the sequence of host basicity and cation content: siliceous faujasite: ∼40 kJ/mol < NaY: ∼55 kJ/mol < NaX: ∼80 kJ/mol (extrapolated to “zero” loading). Such a correlation is further elucidated from the host/guest PDFs by the enhancement of HTCE‚‚‚OZEO hydrogen bonding and ClTCE‚‚‚NaZEO electrostatic interactions from the siliceous faujasite to NaY and NaX. An increase in TCE loading gives rise to a systematic increase in adsorption heats (>10 kJ/mol); this is identified as of a predominantly dispersive nature arising from ClTCE‚‚‚ClTCE and HTCE‚‚‚ClTCE intermolecular interactions.
Introduction During the past decade, the study of normal and aromatic hydrocarbons in nanoporous materials has attracted remarkable interest in the context of separations and catalysis. In this context, the exploration of the structure, the thermodynamics and the diffusive properties of hydrocarbons in zeolites has benefited considerably from the accelerating development of computing facilities and simulation tools.1 The latter range from force field methods such as energy minimizations,2 Monte Carlo simulations (including conventional MC,3 kinetic MC,4 and configurational biaised MC5), and molecular dynamics calculations6 to first principle methods,7 including Hartree-Fock and DFT techniques. Ongoing work in this area is of prime importance for understanding the microscopic basis for host/guest interactions involved in adsorption and catalytic processes. Considering the growing range of existing structures, one of the challenges for simulations is to provide a means of predicting the behavior of materials in order to facilitate the selection of appropriate systems for a given application. * University of California. † Universite ´ de Versailles Saint-Quentin. ‡ University of Pennsylvania. (1) Catlow, C. R. A., Ed. Modeling of Structure and Reactivity in Zeolites; Academic Press: London, 1992. (2) Freeman, C. M.; Catlow, C. R. A.; Thomas, J. M.; Brode, S. Chem. Phys. Lett. 1991, 186, 136. (3) June, R. L.; Bell, A. L.; Theodorou, D. N. J. Phys. Chem. 1990, 94, 1508. (4) Auerbach, S. M.; Henson, N. J.; Cheetham, A. K.; Metiu, H. I. J. Phys. Chem. 1995, 99, 10600. (5) Smit, B.; Siepmann, J. I. J. Phys. Chem. 1994, 19, 8442. (6) Demontis, P.; Suffritti G. B. Chem. Rev. 1997, 97, 2870 and references therein. (7) Sauer, J. Chem. Rev. 1989, 89, 199 and references therein.
We have recently extended force field simulations into the realm of halocarbon adsorption by developing a new force field for chlorocarbon/zeolite systems.8,9 One of the driving forces for our work is the environmental need for developing new separation and catalytic conversion processes involving halocarbons.10 Zeolites and related nanoporous structures have been recently recognized as interesting alternatives over other media for the sequestration and conversion of halogenated molecules, such as perchloroethylene 11 and a variety of hydrofluorocarbons.12 In our initial work, we used our force field for predicting heats of adsorption for a simple saturated chlorocarbon, chloroform, in a series of sodium-exchanged faujasite-type zeolites. Energy minimizations8 and canonical Monte Carlo simulations9 were performed, in close conjunction with spectroscopic and calorimetric measurements, respectively. Our Monte Carlo simulations provided compelling validation of our force field for saturated chlorocarbons in a wide range of zeolite compositions in terms of Si:Al ratio (1.2-∞), cation content, and sorbate concentration. In the present work, we explore the adsorption of an unsaturated chlorocarbon, trichloroethylene (TCE), in faujasite-type zeolites. Indeed, together with chloroform and tetrachloroethylene, TCE is one of the most ubiquitous (8) Mellot, C. F.; Davidson; A. D.; Eckert, J.; Cheetham, A. K. J. Phys. Chem. B 1998, 102, 2530. (9) Mellot, C. F.; Cheetham, A. K.; Harms, S.; Savitz, S.; Gorte, R. J.; Myers, A. L. J. Am. Chem. Soc. 1998, 120, 5788. (10) Manzer, L. E. Science 1990, 249, 31. Hutchings, G. J.; Heneghan, C. S.; Hudson, I. D.; Taylor, S. H. Nature 1996, 384, 341. (11) Weber, G.; Bertrand, O.; Fromont, E.; Bourg, S.; Bouvier, F.; Bissinger, D.; Simonot-Grange, M. H. J. Chim. Phys. 1996, 93, 1412. (12) Corbin, D. R.; Mahler, B. A. World Patent, W.O. 94/02440, 1994. Grey, C. P.; Corbin, D. R. J. Chem. Phys. 1995, 99, 16821. Grey, C. P.; Poshni, F. I.; Gualtieri, A. F.; Norby, P.; Hanson, J. C.; Corbin, D. R. J. Am. Chem. Soc. 1997, 119, 1981.
10.1021/la980318t CCC: $15.00 © 1998 American Chemical Society Published on Web 10/20/1998
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soil and groundwater contaminants. A limited number of previous studies have examined the adsorption of trichloroethylene in zeolites, mainly relating to isotherm measurements13 and FTIR spectroscopy studies14 of TCE in ZSM-5 and Y-type zeolites. However, the underlying adsorption mechanism of TCE in zeolites is not yet understood at the molecular level. Our aim is to identify the driving forces for TCE adsorption by probing the influence of key parameters: the Si:Al ratio, the density of extraframework cations, and the sorbate loading. To this end, calorimetric measurements of TCE heats of adsorption were performed with the three faujasite-type zeolites: siliceous faujasite (Si:Al ) ∞), NaY (Si:Al ) 2.6), and NaX (Si:Al ) 1.2). In parallel with the calorimetric results, we carried out canonical (N, V, T) Monte Carlo simulations of TCE adsorption in the same systems. Taken together, the experimental and computational results yield new insight into the energetics and structures of zeolite/halocarbons systems and their impact on the potential applications of zeolites for separation purposes. Experimental and Simulation Methods Calorimetric Measurements. Room-temperature calorimetric measurements were performed on the following zeolites: siliceous faujasite (Si192O384), NaY (Na53Si139Al53O384), and NaX (Na88Si104Al88O384). The zeolite samples were the same as those in ref 9. The microcalorimeter for these measurements was the same Tian-Calvet instrument used in our earlier study of adsorbates on silicalite.15 The sample and reference cells were Pyrex cubes, 2.5 cm on a side, which were surrounded by thermopiles on the bottom and four sides. Each sample (∼1 g) was pressed into wafers and placed into the sample cell. The corresponding dry weights of the samples, used in the calculations of loading, were determined from TGA measurements. The samples were then degassed by ramping the temperature of the sample cell from 20 to 110 °C, in a vacuum, over the course of 12 h, after which the temperature was further ramped to 350 °C over the next 12 h. Finally, the sample was held in a vacuum at 350 °C for an additional 12 h. After cooling to room temperature, adsorbates were dosed onto the samples by injection of vapor from a 50-cm3 dosing loop on a six-way valve. The calibration constant was determined from the heat of adsorption calculated from the Clapeyron equation applied to high-precision adsorption isotherms for ethane in silicalite. Interaction Model and Force Field. In the Monte Carlo simulations, the host/guest and guest/guest interactions were modeled by the sum of a short-range term using a LennardJones potential and a Coulombic term accounting for the interaction between the dipole moment of the guest and the electrostatic field generated by the zeolite:
ELennard-Jones )
∑ (A /r ij
ij
12
- Bij/r6) )
∑ [(r* /r ) ij
ij
ij
12
- 2(r*ij/rij)6] (1)
ij
(2)
ij
ECoulombic )
∑ q q /r i j
ij
where Aij is the repulsive constant and, Bij, the dispersive constant, with ij ) Bij2/4Aij, r*ij ) (2Aij/Bij)1/6. The induction energy was not taken into account. The short-range parameters used here are identical to those of refs 8 and 9. We wish to underline that no fitting to experimental data on halocarbons was used in the derivation of the short-range parameters. (13) Kobayashi, S.; Mizuno, K.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Ohuchi, H. Ind. Eng. Chem. Res. 1991, 30, 2340. (14) Chintawar, P. S.; Greene, H. L. Appl. Catal. B Environ. 1997, 14, 37. Chintawar, P. S.; Greene, H. L. Appl. Catal. B Environ. 1997, 13, 81. Chintawar, P. S.; Greene, H. L. J. Catalysis 1997, 165, 12. (15) Dunne J. A.; Marivals, R.; Rao, M.; Sircar, S. Langmuir 1996, 12, 5888.
Table 1. Short-Range Parameters for Halocarbon-Zeolite Systems ij (K)
ij (K)
r*ij (Å)
r*ij (Å)
Zeolite-Halocarbon Short-Range Lennard-Jones Parametersa O‚‚‚C 87.06 3.25 Na‚‚‚C 13.24 3.69 O‚‚‚H 90.53 2.70 Na‚‚‚H 11.41 3.10 O‚‚‚Clb 181.93 3.43 Na‚‚‚Cl 212.48 2.90 C‚‚‚C C‚‚‚H C‚‚‚Cl
Halocarbon-Halocarbon Short-Range Parametersc 25.86 3.75 Cl‚‚‚Cl 119.8 3.82 26.73 3.36 Cl‚‚‚H 57.53 3.39 55.65 3.79 H‚‚‚H 27.63 2.96
a Short-range parameters between (C, H) TCE atoms and (O, Na)ZEO atoms were taken from previous Monte Carlo studies on hydrocarbons.22 Short-range parameters between ClTCE and (O, Na)ZEO were derived from Lennard-Jones parameters of argon in various zeolites.9,23 For more details on the derivation of these parameters, see ref 8. b In the case of the siliceous faujasite, the O-Cl parameter was lowered by a factor of 1.1 (O-Cl ) 165.4 K), accounting for the difference in framework oxygen polarizabilities between siliceous and cationic zeolites.21 c The intermolecular shortrange parameters were derived from previous studies on hydrocarbons concerning the (C, H) atoms24 and from data on liquid argon.25
Table 2. Partial Charges for the Zeolite-Halocarbon Potentiala siliceous Y NaY NaX
Si192O384 Na48Al48Si144O384 Na88Al88Si104O384
trichloroethylene C(1) C(2) H(1) Cl(1) Cl(21) Cl(22)
T(Si, Al)
O
Na
+2.40 +2.15 +1.95
-1.2 -1.2 -1.2
+1 +1
-0.064 +0.036 +0.172 -0.068 -0.054 -0.022
with
a The charge of the average T-atom (Al, Si) was chosen to satisfy the electroneutrality of each structure according to its Si:Al ratio, fixing the charges on the Na cations to +1 and that of framework oxygens to -1.2. Charges on the TCE molecule were derived from first principle calculations (see text).
An Ewald summation was used in the estimation of the electrostatic energy. Partial charges were adopted for the host structure, consistent with the half-ionicity reported in a recent high-resolution X-ray diffraction study.16 The increase of basicity from the siliceous faujasite to the NaX was accounted for through the decrease in the charge of the average T-atom (Si, Al) while fixing the charge on the Na cations to +1 and that of the oxygens to -1.2 in all structures. Trichloroethylene was described by a six point-charge model. Its partial charges were first estimated from a Hartree-Fock calculation using the 6-31G** basis set, fitting the electrostatic surface potential, and then scaled down so as to give a dipole moment of ∼1.1 D. All short-range and long-range parameters are summarized in Tables 1 and 2. Structure Models and Monte Carlo Simulations. While being topologically identical (Figure 1), siliceous faujasite, NaY, and NaX differ from one another by their in-framework Al content and therefore in their cationic composition and distribution. The three zeolite hosts were modeled as follows: (i) siliceous faujasite (Si:Al ) ∞), Si192O384; (ii) NaY (Si:Al ) 3), Na48Al48Si144O384, placing 16 Na cations in site SI (SI is at the center of an hexagonal prism) and 32 in site SII (SII is at the center of a 6-membered ring in the supercage); (iii) NaX (Si:Al ) 1.2), Na88Al88Si104O384, with 32 Na cations in site SI′ (SI′ is in the sodalite cage facing the 6-membered ring of a hexagonal prism), 32 in site SII, and 24 in site SIII′ (SIII′ is at the edge of a 4-membered ring, in the 12-membered ring window). For building the NaX (Na88Al88Si104O384) model, we started with our recent simulation of the NaLSX (Si:Al ) 1) structure17 and selected 24 Na cations (16) Ghermani, N. E.; Lecomte, C.; Dusausoy, Y. Phys. Rev. B 1996, 53, 5231.
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Figure 1. Representation of the faujasite-type framework and of extraframework cations in positions SI, SI′, and SII. among the 32 Na cations in site SIII′ of the optimized NaLSX (Si:Al ) 1) structure. This NaX (Si:Al ) 1.2) structure model was then energy minimized with the appropriate partial charges, using the cvff•aug force field and the Discover software (MSI).18 The Na cations in site SIII′ were allowed to relax, keeping the framework and the Na cations in the sites SI′ and SII fixed. In the minimized structure, these extra Na cations in site SIII′ have low coordination to the framework at a distance of 2.3-2.4 Å from two oxygens, O(1) and O(4). Regarding the adsorption of TCE, only the supercages are accessible to guest molecules. Dummy atoms were assigned with high repulsive Lennard-Jones parameters at the centers of the sodalite cages, so that insertions of guest molecules happened exclusively in the supercages. The Monte Carlo simulations (300 K) used the Metropolis scheme in the canonical (N,V,T) ensemble. In each system, the number of guest molecules, N, was varied from a single molecule per unit-cell up to ∼30-40 molecules per unit-cell, so as to reproduce the variation of loading in a fashion similar to the calorimetric measurements. All simulations were run by keeping the host rigid. Average energies were obtained over (1-2) × 106 iterations, after an equilibration period of (100200) × 103 steps, with a short-range summation taken up to a cutoff radius of 12 Å and an Ewald summation for the electrostatic term. The Sorption module of the MSI suite of software was used for these simulations.18
Results and Discussion I. Phenomenologic Findings. The calorimetric data for TCE adsorption in the three faujasites zeolites are plotted in Figure 2 (open symbols) as a function of loading (up to 35 molecules per unit-cell, i.e., ∼4 molecules per supercage on average). Two dominant features are observed. (i) The affinity of TCE for the host increases with increasing polarity/basicity of the zeolite: siliceous faujasite < NaY < NaX. The adsorption heats of TCE extrapolated to “zero” loading in siliceous faujasite (∼40 kJ/mol), NaY (∼55 kJ/mol), and NaX (∼80 kJ/mol) are particularly illustrative of this point. This sequence underlines the role of the dipolar nature of the halocarbon molecule and of its interaction with the zeolite electrostatic field (the slight increase at low loading in the siliceous zeolite is believed to be due to a low concentration of silanol defects). (ii) The adsorption heats increase with loading in a similar fashion in all three hosts, i.e., ∼10 kJ/mol from low loading to high loading, indicating the importance of sorbate-sorbate cooperative interactions in these systems. (17) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. J. Phys. Chem. 1997, 101, 4559. (18) Catalysis and Sorption Software Suite, Version 3.2; MSI: San Diego, CA.
Figure 2. Calorimetric heats of adsorption of trichloethylene at room temperature in the siliceous faujasite, NaY, and NaX zeolites as a function of loading. For comparison, the heats of adsorption of chloroform9 are also reported.
Features (i) and (ii) are qualitatively similar to those found for the adsorption of chloroform in the same hosts.9 However, the comparison of TCE and chloroform adsorption heats shows interesting differences. The two unsaturated and saturated halocarbons have almost identical adsorption heats at “zero” loading, whatever the zeolite host. For example, in the most polar zeolite, NaX, the heats of adsorption of TCE and chloroform are both of ∼80 kJ/mol at “zero” loading. This observation suggests that TCE and chloroform adsorption are driven by similar physicochemical features, i.e., the number of chlorine atoms per molecule and the dipole moment, indifferent to their unsaturated or saturated nature. These results are in contrast with the heats of adsorption of hydrocarbons in zeolites. For example, in NaX, the heat of adsorption at “zero” loading increases from methane (19.2 kJ/mol)19 to ethane (27 kJ/mol)19 to ethene (∼37 kJ/mol),20 showing the effect of the number of carbon atoms and the cation/ π-bond interactions. As far as loading is concerned, the heats of adsorption increase more rapidly with TCE than with chloroform. This would suggest that the sorbatesorbate interactions are stronger in the case of TCE. This is probably due to the planer nature of TCE, thus allowing better packing of the sorbate molecules. The aim of the following section is to elucidate and discuss the microscopic basis of TCE adsorption in faujasite-type zeolites. II. Simulation Findings. Figure 3 compares the Monte Carlo heats of adsorption of TCE in the three zeolites (filled symbols) with our calorimetric data (open symbols). Our MC simulations predict the experimental heats of adsorption remarkably well, especially as they capture both the trends observed as a function of the zeolite (19) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5896. (20) Stach, H.; Lohse, U.; Thamm, H.; Schirmer, W. Zeolite 1986, 6, 75. (21) Pellenq, J. M.; Nicholson, D. J. Chem. Soc., Faraday Trans. 1993, 89, 2499. (22) Yashonath, S.; Thomas, J. M.; Nowak, A. K.; Cheetham, A. K Nature 1988, 331, 601. Smit, B.; den Ouden, C. J. J. J. Phys. Chem. 1988, 92, 7169. (23) Mellot, C.; Lignie`res, J. Mol. Simul. 1997, 18, 349. (24) Nicholas, J. B.; Trouw, F. R.; Mertz, J. E.; Iton, L. E. J. Phys. Chem. 1993, 97, 4149. (25) Razmus D. M.; Hall, C. K. AIChE J. 1991, 37, 771.
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Figure 3. Experimental (open symbols) and simulated (filled symbols) isosteric heats of adsorption of trichloroethylene at room temperature in the three zeolites hosts: siliceous faujasite, NaY, and NaX.
polarity and those as a function of loading. To get further insight into the host/guest interactions, the total interaction energies of TCE with the host were decomposed into their short-range and long-range components (Figure 4ac). In the following, we discuss the trends of the isosteric heats of adsorption and of their short-range and long-range components. We also show how further understanding of the adsorption process and correlations with the host structure and chemical composition can be garnered from the analysis of the pair-distribution functions (PDFs) obtained from the MC simulations. In each TCE/zeolite system, all host/guest and guest/guest PDFs plots were obtained at each coverage over all accepted configurations of the (N,V,T) simulation run and were normalized to 1 molecule per unit-cell (see Figure 5a-c). Trichloroethylene in Siliceous Y. At “zero” loading, the adsorption energy of TCE reflects ∼90% short-range interactions and ∼10% electrostatic interactions. It is apparent from Figure 4a that the increase in adsorption heats with sorbate loading stems entirely from the increase in the short-range term (more than 10 kJ/mol increase within the range of 0-30 molecules per unit-cell (molec/ uc)), while the electrostatic contribution remains constant at ∼4.2 kJ/mol over the whole range. Figure 5a shows the host/guest pair distribution functions as collected during the Monte Carlo simulations and their evolution with increasing loading. At “zero” loading (Θ ) 1 molec/uc), it is apparent that host/guest interactions are dominated by the short-range interaction energy arising from ClTCE‚‚‚OZEO and HTCE‚‚‚OZEO contacts. Interestingly, the HTCE‚‚‚OZEO PDF reveals some hydrogen bonding, since a typical van der Waals contact would correspond to an H‚‚‚O distance of 2.6 Å. When the loading is increased (Θ ) 8 molec/uc and more), additional intermolecular interactions, ClTCE‚‚‚ClTCE and HTCE‚‚‚ClTCE, are observed. Although they show a wide distribution around the typical van der Waals distances of Cl‚‚‚Cl (3.6 Å) and H‚‚‚Cl (2.9 Å), they reveal that the loading increase is strongly dominated by mutual attraction of guest molecules, resulting in enhanced adsorption heats. These interactions are largely of a dispersive nature, arising because of the high polarizability of chlorine atoms. They
Figure 4. Simulated isosteric heats of adsorption for (a) siliceous faujasite, (b) NaY, and (c) NaX, decomposed into the short-range and long-range contributions to the internal energy.
are estimated by our simulations to reach 27% of the total adsorption energy in siliceous faujasite at high loading (∼30 molec/uc).
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Figure 5. Pair distribution functions for trichloroethylene in zeolites (a) siliceous faujasite, (b) NaY, and (c) NaX, taken from the Monte Carlo simulations at room temperature for increasing loadings.
Trichloroethylene in NaY. The striking feature concerning the NaY in comparison with the siliceous faujasite is that the bigger affinity of TCE for NaY stems mainly from the enhancement of electrostatic interactions (Figure 4b). The electrostatic contribution consists of 14.2 kJ/mol in NaY compared with 4.2 kJ/mol in the siliceous faujasite and is invariant with loading (25% of the total isosteric heat of adsorption at zero loading). Such an enhancement of the electrostatic term is a consequence of the additional interaction between the TCE dipole
moment and the electrostatic field generated by the aluminosilicate framework and the Na cations. The specific role of Na cations in site II (located at the center of the 6-ring windows of the supercages) is clearly seen in the ClTCE‚‚‚Na(II)ZEO PDF (Figure 5b) since a distinctive peak appears at a typical Cl‚‚‚Na distance of 2.7 Å. In a fashion similar to the siliceous faujasite, the increase of loading gives rise to an increase in the isosteric adsorption heats that can be attributed to additional intermolecular interactions. Both this increase in the
Halocarbon Adsorption in Nanoporous Materials
short-range term (Figure 4b) and the appearance of additional ClTCE‚‚‚ClTCE and HTCE‚‚‚ClTCE interactions (Figure 5b) as loading increases illustrate these dispersive intermolecular interactions. The PDFs of HTCE‚‚‚OZEO and ClTCE‚‚‚OZEO are similar to those observed in siliceous faujasite and are maintained when the loading increases, as expected. Trichloroethylene in NaX. Figure 4c shows the decomposition of the total adsorption heat of TCE in NaX into its short-range and long-range components. Interestingly, the further enhancement of the electrostatic term is the dominant cause of the higher affinity of TCE for NaX when compared to NaY. At “zero” loading, the total adsorption energy of TCE reflects ∼50% short-range interactions and ∼50% electrostatic interactions. The electrostatic contribution consists of 41.3 kJ/mol in NaX, instead of 14.2 kJ/mol in NaY and 4.2 kJ/mol in the siliceous faujasite. The host/guest PDFs, shown in Figure 5c, nicely illustrate the specific interactions involved in TCE adsorption in NaX. The key features are the appearance of a distinctive peak in the HTCE‚‚‚OZEO PDF at 2.4 Å, showing the promotion of hydrogen bonding in this zeolite, and the enhancement of the ClTCE‚‚‚Na(II,III′)ZEO interactions with a more pronounced peak around 2.8 Å in the PDF. The latter provides clear evidence for the crucial role of the additional Na cations in sites III′ of the NaX structure (located in the 12-ring windows) when compared with the corresponding ClTCE‚‚‚Na(II)ZEO PDF in NaY. In light of the above PDFs, the enhancement of the electrostatic interactions is the direct consequence of the greater ionicity of the NaX structure, and especially the greater number of Na cations accessible to the sorbate molecules in the supercages. Concerning the influence of loading, similar intermolecular interactions appear upon increasing loadings, as is shown by both the increase of the short-range term and the ClTCE‚‚‚ClTCE and HTCE‚‚‚ClTCE PDFs. However, the behavior with loading is slightly different from those observed in the other two zeolites. The short-range term increase is partially compensated by a decrease in electrostatic interactions. This is similar in fashion to our findings concerning the adsorption of chloroform in NaX,9 although it is less accentuated than in the case of chloroform. Our findings show the high heterogeneity of the potential surface in NaX toward TCE adsorption: the diminution of the electrostatic field term reveals the limited availability of highly favorable sorbate locations, leading to a cooperative decrease in electrostatic interactions. Conclusion We have investigated by calorimetry the heats of adsorption of TCE in three faujasite-type zeolites: siliceous
Langmuir, Vol. 14, No. 23, 1998 6733
faujasite, NaY and NaX. The heats of adsorption are enhanced according to the basicity and cation content of the host and to the sorbate loading. Our Monte Carlo simulations are in excellent agreement with our calorimetric findings, successfully capturing both the host composition dependence and the coverage dependence of TCE adsorption heats in all three zeolites. Our results underline the transferrability of the force field from chloroform to the realm of unsaturated halocarbons and to a large range of host chemical composition and sorbate loading. The Monte Carlo simulations yielded a further insight into the relative contribution of short-range and longrange interactions to the total interaction energy from one system to another, especially regarding its behavior with loading. Key structural details on TCE adsorption sites were obtained from the analysis of host/guest pair distribution functions, and revealed features similar to those of chloroform in the same systems:9 (i) Cl‚‚‚O van der Waals interactions, (ii) Cl‚‚‚Na (II, III) electrostatic interactions, and (iii) H‚‚‚O hydrogen bonding. Such structural information is of special interest when considering that such systems are prone to be partially disordered, since adsorption is driven by the framework oxygens and statistically disordered extraframework cations with reduced relationship with the symmetry of the host. Further diffraction analysis will be of importance in validating the structural features of TCE adsorption in these systems. Our results at “zero” loading suggest, unlike hydrocarbons, an analogy between the adsorption processes of saturated and unsaturated halocarbons. However, from the differences between TCE and chloroform adsorption heats, especially at high loadings, we anticipate potential separation applications for cation-exchanged faujasitetype zeolites. Future studies on selectivities and mixtures will be of special interest. Acknowledgment. Supported by the U.S. Department of Energy under grant No. DE-FG03-96ER14672. The work made use of computing facilities supported by the MRL Program for the National Science Foundation under Award No. DMR96-32716. A.L.M. and R.J.G. were funded by NFS under CTS 96-10030. C.F.M. acknowledges the French Ministe`re des Affaires Etrange`res for a Lavoisier fellowship. A.K.C. acknowledges the Fondation de l’Ecole Normale Supe´rieure for a Chaire Internationale de Recherche, Blaise Pascal. LA980318T
