Langmuir 2000, 16, 3823-3834
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Adsorption Studies of Methane, Ethane, and Argon in the Zeolite Mordenite: Molecular Simulations and Experiments Michael D. Macedonia, Darrin D. Moore, and Edward J. Maginn* Department of Chemical Engineering and The Center for Catalysis and Reaction Engineering, University of Notre Dame, Notre Dame, Indiana 46556
Michael M. Olken Corporate Catalysis Laboratory, The Dow Chemical Company, Midland, Michigan 48674 Received September 20, 1999. In Final Form: December 15, 1999 The adsorption of methane, ethane, and argon on sodium mordenite (Na-MOR) at ambient and cryogenic conditions is investigated experimentally and with grand canonical Monte Carlo (GCMC) simulations. Two different Na-MOR samples with silicon-to-aluminum (Si/Al) ratios of ∼5 and ∼9 are used in the experiments. Simulations are conducted on models with close to the experimentally observed Si/Al ratios and also on a purely siliceous model of MOR. In addition, the impact of varying the zeolite crystal symmetry from Cmcm to Pbcn is examined with GCMC. The agreement between the GCMC simulations and previous and current experimental measurements is quite good at ambient conditions. Differences between the ambient isotherms computed with the Cmcm and Pbcn structures are slight. However, the two structures exhibit qualitatively different argon adsorption behavior at cryogenic temperatures (87.3 K). The structure based on symmetry Pbcn shows a much better match with experimental isotherms than does the Cmcm structure. Cryogenic adsorption measurements are shown to be sensitive to subtle structural differences in the zeolite lattice. They also provide a rigorous test of force fields used in simulations. It is demonstrated that inclusion of cations and framework Al atoms with realistic charge distributions in the simulations is required to adequately match experimental results.
1. Introduction Zeolites are used for a wide variety of catalytic and separations applications. Mordenite (MOR) is a particularly useful zeolite for several catalytic applications, including cracking and isomerization of hydrocarbons, dewaxing of heavy petroleum fractions, and the disproportionation of alkyl benzenes. An understanding of the adsorption properties of this material can be helpful in determining the mechanisms of these processes, as well as in identifying further applications of these zeolites as catalysts and adsorbents. In this paper, we discuss the results of experimental and molecular modeling studies that have focused on examining the adsorption behavior of small molecules in MOR samples with different crystal structures and sodium ion (Na+) content. Argon, methane, and ethane were chosen as adsorbates because they are small enough to access all the micropores of MOR and because their structural simplicity enables a more straightforward interpretation of the roles that cation location and zeolite structure have on adsorption properties. Surprisingly, there have been relatively few experimental adsorption studies for the Na form of MOR (NaMOR). Nitrogen adsorption measurements have been conducted most often for material characterization purposes.1 A few studies involving rare gases have been carried out, including argon isotherm studies2 and nuclear magnetic resonance (NMR)/isotherm studies with xenon.3 * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) See, for example, Barrer, R. M.; Peterson, D. L. Proc. R. Soc. A 1964, 280, 466. (2) Takaishi, A. Y.; Amakasu, F. J. Chem. Soc., Faraday Trans. 1971, 67, 3565. (3) Springuel-Huet, M. A.; Fraissard, J. P. Zeolites 1992, 12, 841.
Adsorption isotherms of light hydrocarbons have been measured by Satterfield and co-workers4 and Choudhary et al.,5 whereas Xu and co-workers have used NMR to examine the siting of light hydrocarbons in Na-MOR.6,7 Several groups have employed Monte Carlo (MC) methods to examine adsorption on MOR. Smit and den Ouden performed a Metropolis MC study of small molecules in Na-MOR, obtaining siting information and heats of adsorption.8 Later studies by Nivarthi and co-workers used grand canonical MC (GCMC) to compute adsorption isotherms of Xe in Na-MOR.9,10 GCMC studies of single and multicomponent adsorption of spherical molecules have been performed by Clark and co-workers, in which complex and nonideal adsorption behavior was identified.11 Macedonia and Maginn used GCMC to examine the single and multicomponent adsorption behavior of light hydrocarbons in MOR.12 Highly nonideal adsorption behavior was seen between smaller (C1,C2) and larger (C3) species. The nonideality is due to two distinct adsorption sites (4) Satterfield, C. N.; Frabetti, A. J., Jr. AIChE J. 1967, 13, 731. (5) Choudhary, V. R.; Mayadevi, S.; Pai Singh, A. J. Chem. Soc., Faraday Trans. 1995, 91, 2935. (6) Xu, Q.; Eguchi, T.; Nakayama, H.; Nakamura, N. J. Chem. Soc., Faraday Trans. 1995, 91, 2949. (7) Xu, Q.; Eguchi, T.; Nakayama, H.; Nakamura, N. J. Chem. Soc., Faraday Trans. 1996, 92, 1039. (8) Smit, B.; den Ouden, C. J. J. J. Phys. Chem. 1988, 92, 7169. (9) Nivarthi, S. S.; Van Tassel, P. R.; Davis, H. T.; McCormick, A. V. Zeolites 1995, 15, 40. (10) Nivarthi, S. S.; Van Tassel, P. R.; Davis, H. T.; McCormick, A. V. J. Chem. Phys. 1995, 103, 3029. (11) Clark, L. A.; Gupta, A.; Snurr, R. Q. J. Phys. Chem. B 1998, 102, 6720. (12) Macedonia, M. D.; Maginn, E. J. Proceedings of the 12th International Zeolite Conference; Treacy, M. M. J., Marcus, B. K., Bisher, M. E., Higgins, J. B., Eds., Materials Research Society: Warrendale, PA, 1999; pp 363-370.
10.1021/la9912500 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000
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in MOR, one of which is accessible only to small molecules. Bates and co-workers examined the energetics and siting of larger hydrocarbon (> C3) molecules in MOR in infinite dilution studies and found that these molecules could only access the larger of the two sites.13,14 Most of these simulation studies have utilized a purely siliceous representation of MOR because of computational convenience, the relative uncertainty of cation positioning, and force field availability. Not surprisingly, the quantitative agreement between the results of simulations in which cations were neglected and experimental measurements has been generally poor. For the simulation studies of Na-MOR in which cations were included,9,10 agreement between simulated and experimental isotherms was also quite poor. This poor agreement was probably due to an inadequate force field and an oversimplified model for the charge distribution and cation location within the MOR lattice. The goal of this work is to use a combination of experimental measurements and GCMC simulations to gain insight into the adsorption behavior of small molecules in Na-MOR. We wish to examine the impact that crystal structure, cation location, and cation loading have on adsorption isotherms and isosteric heats. We also hope to be able to develop a force field and structural representation for the sorbates and zeolite that enable molecular simulations to yield adsorption results that are in quantitative agreement with experimental results. The remainder of the paper is organized as follows. We begin by providing an overview of the crystallographic data on MOR that is available in the literature. We then discuss the characterization of our MOR samples and the details of our cryogenic and ambient adsorption experiments. Next, we discuss details concerning the selection and justification of models for the zeolite and sorbates employed in our GCMC simulations, compare computed and measured sorption results, and present a brief set of conclusions. 2. Crystallography of Mordenite Zeolites The locations of both the framework and nonframework atoms of MOR zeolites have been studied by a large number of workers using X-ray diffraction techniques. In an early study, Meier reported the structure of a hydrated sample of naturally occurring Na-MOR with unit cell formula Na8Al8Si40O96‚24H2O.15 It was concluded that the structure could be refined with Cmcm symmetry, although the true symmetry was probably Cmc21. The structure consisted of a 1-dimensional, 12-ring pore system in [001], with small 8-ring “side pockets” bordering this channel. The positions of all framework atoms, as well as half of the nonframework (Na) atoms, were determined. The location of these Na atoms is (0, 0.5, 0) in Cmcm, and has come to be known as the position Na(I) (Na(I) is referred to as site “A” in the compilation of cation sites by Mortier16). Na(I) is located midway between the 12-ring pores in both the [100] and [010] directions (see Figure 1a). This area of the zeolite is very constricted, and thus only accessible to very small atomic species, such as cations. Because Na(I) cations are stationary, even in a hydrated sample, they could be detected in the diffraction study. The remaining Na atoms could not be located because they (13) Bates, S. P.; van Well, W. J. M.; van Santen, R. A.; Smit, B. J. Am. Chem. Soc. 1996, 118, 6753. (14) Bates, S. P.; van Well, W. J. M.; van Santen, R. A.; Smit, B. J. Phys. Chem. 1996, 100, 17573. (15) Meier, W. M. Z. Kristallografiya 1961, 115, 439. (16) Mortier, W. J. Compilation of Extraframework Sites in Zeolites, Butterworth Scientific Ltd.: Guildford, Surrey, U.K., 1982.
Macedonia et al.
Figure 1. (a) Cmcm structure of mordenite viewed along [001]. Sodium ions at position Na(I) are included as determined by Meier.15 The side pockets are located in the areas above and below the Na(I) positions. (b) Pbcn structure of mordenite viewed along [001]. Sodium ions at positions Na(I), Na(IV), and Na(VI) are included as determined by Schlenker and co-workers.21 Apparent close proximity of cations in positions Na(IV) and Na(VI) is due to [001] projection.
reside as hydrated ions in the 12-ring channel. The presence of large amounts of water in this channel tends to make cations mobile and disordered, thus impossible to detect in a diffraction study. This Cmcm structure of MOR can be seen in Figure 1a. A more recent singlecrystal study of MOR, performed by Alberti and coworkers, confirmed a symmetry of Cmcm for the framework atoms and suggested that reduction to Cmc21 was likely due to cation ordering upon dehydration.17 Sherman and Bennett detected other symmetries, such as Imcm, Cmmm, and Immm, when examining several different samples of MOR.18 It was concluded, however, that these symmetries were the result of intergrowths or c-axis stacking faults of the Cmcm structure. A hybrid Cmcm structure, which included these stacking faults, was proposed by Mortier and co-workers during a study of dehydrated H-MOR.19 Only 2% of the zeolite was observed to have stacking other than Cmcm. The hybrid solution thus included 2% of T-atoms shifted by 0.5 in the c-direction. A similar c-axis fault plane model was later proposed by Rudolf and Garces to account for differences in X-ray patterns between various samples.20 Schlenker and co-workers performed a detailed study of dehydrated Na-MOR to locate the cations not solved in earlier studies.21 Several interesting results were gained from this study. First, it was observed that the symmetry of the zeolite was reduced from Cmcm to Pbcn upon dehydration of the sample. The need to refine in Pbcn arose because of diffuse reflections that violated the C-centering of the cell. These reflections disappeared upon rehydration of the sample. Second, the positions of all Na cations were located in the dehydrated sample. Consistent with the earlier studies, it was found that approximately half of the Na atoms resided at the position Na(I). Cations were also found to reside in the 12-ring channel at positions designated as Na(IV) and Na(VI). Na(IV) is located near the opening of the side pocket in the main channel, whereas Na(VI) is located further out into the 12-ring channel. The structure of this Pbcn MOR may be seen in Figure 1b. The occupancies of the Na(IV) and Na(VI) sites are ≈2.5 and ≈1.5 per unit cell (UC), respectively. (17) Alberti, A.; Davoli, P.; Vezzalini, G. Z. Kristallografiya 1986, 175, 249. (18) Sherman, J. D.; Bennett, J. M. Adv. Chem. Ser. 1973, 121, 52. (19) Mortier, W. J.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1975, 10, 1319. (20) Rudolf, P. R.; Garces, J. M. Zeolites 1994, 14, 137. (21) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 751.
Adsorption of Methane, Ethane, and Argon in Zeolite Mordenite
Because of the very small differences in X-ray scattering signatures of silicon (Si) and aluminum (Al) atoms, it has not been possible to pinpoint the locations of Al atoms by a diffraction study. From 29Si magic angle spin (MAS) NMR studies, Debras and co-workers have suggested that the Al atoms are located almost exclusively in the 4-ring structures of MOR.22 More recently, Takaishi et al. developed a method that can predict positions of these Al atoms that are consistent with the 29Si MAS NMR results.23 A schematic for the positions of the Al atoms in MOR is included in ref 23. Another key issue in the structure of MOR is the impact of the Si/Al ratio. The lowest Si/Al ratio is seen in naturally occurring samples and corresponds to the maximum amount of Al in the structure (Si/Al ) 5). Direct sythesis methods can access Si/Al ratios of 5-24.24 The Si/Al ratio can be raised by chemically treating the sample with acids to accomplish isomorphous substitution of framework Al with Si. The distribution of Al is then very important to understanding cation distributions as a function of Si/Al ratio. In the Si/Al ) 5 form, Al is distributed relatively evenly throughout the lattice.23 However, after mild dealumination, it has been observed that Al atoms in the walls of the 12-ring channel are preferentially substituted with Si in both natural and synthetic samples.25 This observation is consistent with structural considerations, especially in view of the fact that chemical leaching agents would have a far easier time accessing the 12-ring sites than sites in the 8-ring channels. The 12-ring channel Al atoms are located in secondary building units of the zeolite, and thus their substitution is accomplished with minimal structural damage. Variations in the crystal structure, such as the emegence of mesopores, are observed upon extensive dealumination (Si/Al > 11) because primary building units are affected.25 The positions and stability of cations in the zeolite are strongly related to its Al distribution. Naturally, cations will tend to locate in areas of the lattice with high electronegativity. These areas correspond directly to areas where Al atoms (group III) reside instead of Si atoms (group IV). Tyburce and co-workers were able to show, using structural information and electrostatic modeling, that the area surrounding Na(I) had the highest electronegativity in MOR, and thus was the most likely position for Na ions.26 They also concluded that Na(IV) and Na(VI) were highly probable Na ion sites. However, upon removal of Al atoms from the walls of the 12-ring channel during mild dealumination (Si/Al ≈ 11), the electronegativity of Na(IV) and Na(VI) should be greatly reduced, while Na(I) should remain relatively unchanged. It can then be concluded that in samples with Si/Al ) 5, all three Na sites are populated, whereas only Na(I) is occupied in samples with Si/Al ) 11. 3. Experimental Section 3.1. Materials Characterization. Two Na-MOR samples, designated Na600 and Na640, were obtained from Tosoh Corporation (Tokyo, Japan). Using X-ray fluorescence measurements, the unit cell formula for the Na600 sample was determined to be Na8Al8Si40O96, and that for Na640 was Na4.8Al4.8Si43.2O96. These measurements indicate the Si/Al ratios for Na600 and (22) Debras, G.; Nagy, J. B.; Gabelica, Z.; Bodart, P.; Jacobs, P. A. Chem. Lett. 1983, 199. (23) Takaishi, T.; Kato, M.; Itabashi, K. Zeolites 1995, 15, 21. (24) Jacobs, P. A.; Martens, J. A. Synthesis of High-Silica Aluminosilicate Zeolites, Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1987, pp 321-329. (25) Olsson, R. W.; Rollmann, L. D. Inorg. Chem. 1977, 16, 651. (26) Tyburce, B.; Kappenstein, C.; Cartraud, P.; Garnier, E. J. Chem. Soc., Faraday Trans. 1991, 87, 2849.
Langmuir, Vol. 16, No. 8, 2000 3825 Table 1. Physical Properties of Mordenite Samples compound
Si/Al
SA (BET), m2/g
micropore, cm3/g
Na600 Na640
5 9
165 220
0.07 0.10
Na640 are 5 and 9, respectively. To independently check the Si/Al ratios, we compared X-ray diffraction peak intensities for the two samples, following the method described by Itabashi and co-workers.27 From these results, we estimated that Si/Al ) 5.1 for Na600 and Si/Al ) 10.8 for Na640, which is consistent with the fluorescence data. The X-ray diffraction patterns are similar to accepted patterns for Na-MOR, and scanning electron microscopy (SEM) images indicate well-formed crystals. Nitrogen adsorption was performed with a Micromeritics 2405 at 77 K. The surface area was determined by Brunauer-Emmett-Teller (BET) analysis, with the micropore volume calculated from the t-plot. Physical properties of the experimental samples are summarized in Table 1. 3.2. Cryogenic Argon Adsorption. Cryogenic experiments were performed on a Micromeritics 2010 analyzer. This instrument has been modified to include a 1 Torr transducer to provide high quality data at very low pressures. All analyses were performed at liquid argon temperature (87.3 K). The samples were first outgassed at 573 K under a rough pump vacuum for 12 h. The samples were then transferred to the analyzer in a sealed sample container and outgassed a second time at 423 K utilizing the molecular drag pump of the Micromeritics 2010. When the rate of pressure change during an adsorption experiment was
