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Correlation of Sorption Behavior of Nitrogen, Oxygen, and Argon with Ca2+ Locations in Zeolite A: A Grand Canonical Monte Carlo Simulation Study Renjith S. Pillai, Sunil A. Peter, and Raksh V. Jasra* Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, BhaVnagar-364 002, India ReceiVed March 21, 2007. In Final Form: May 30, 2007 The adsorption isotherms for nitrogen, oxygen, and argon in various NaCaA zeolite samples were calculated theoretically using the grand canonical Monte Carlo simulation method. The adsorption capacity, selectivity, and heat of adsorption of nitrogen increase with an increasing number of calcium cations in zeolite A. The heat of adsorption of nitrogen showed a sudden increase when the calcium ion exchange to zeolite A was around 60%. Adsorption isotherms, determined experimentally by the volumetric adsorption method, support theoretically predicted isotherms. These observations have been explained in terms of the interaction of the nitrogen molecule with Ca2+ ions and their locations in zeolite A.
Introduction Adsorption processes are being increasingly used1-3 for the commercial production of oxygen from air. The ability of some zeolites to selectively adsorb nitrogen over oxygen and argon is the basis for the effective separation of nitrogen from air. A nitrogen adsorption selectivity of 3-5 is normally observed for conventional zeolites such as mordenite and zeolites X and A, which are commercially used nitrogen-selective adsorbents for the production of oxygen from air by pressure swing adsorption (PSA) or vacuum swing adsorption (VSA).4,5 The selection of a suitable adsorbent with appropriate adsorption capacity and selectivity for an adsorption-based separation process is significant because the efficiency of a PSA process strongly depends on these properties. Adsorption properties of zeolite-based adsorbents can be influenced by their pore size and chemical composition, for instance, the Si/Al ratio and the nature and location of extraframework cations.6-8 Because of their environmental friendliness, an affinity for N2 and stability under vacuum/pressure make zeolites ideal for PSA-type applications.9 It has been reported that narrow pore size zeolites such as NaA8 and CaA10 are suitable for N2/O2 separation. The nitrogen selectivity of these zeolites results from the electrostatic interaction of the nitrogen molecule, which possesses a quadrupole moment, with extraframework cations of the zeolite. The pore size of a zeolite in combination with the location, size, and charge of an * To whom correspondence should be addressed. E-mail: rvjasra@ csmcri.org. Fax: +91 278 2567562/2566970. Tel: +91 278 2471793. (1) Lee, H.; Stahl, D. E. AIChE Symp. Ser. 1973, 69, 1. (2) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Sep. Sci. Technol. 1991, 26, 885. (3) Reiss, G. Gas Sep. Purif. 1994, 8, 95. (4) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Ind. Eng. Chem. Res. 1996, 35, 4221. (5) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: London, 1997. (6) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structures; Structure Commission of the International Zeolite Association; Elsevier: Amsterdam, 1978. (7) Siriwardane, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Energy Fuels 2001, 15, 279. (8) Goj, A.; Sholl, D. S.; Akten, E. D.; Kohen, D. J. Phys. Chem. B 2002, 106, 8367. (9) Savitz, S.; Myers, A. L.; Gorte, R. J. Microporous Mesoporous Mater. 2000, 37, 33. (10) Jacobs, P. A.; Van Santen, R. A. Stud. Surf. Sci. Catal. Elsevier: Amsterdam, 1989; Vol. 49.
extraframework cation determine a zeolite’s effectiveness as an adsorbent.11-15 Our previous studies on N2/O2 adsorption in NaCaA zeolites by elution chromatography16 showed that the divalent Ca2+exchanged zeolite A had more capacity and selectivity for N2 than did the sodium form of zeolite A. During the elution chromatographic study, a steep increase in the heat of adsorption after a particular level of calcium exchange in zeolite NaA was observed. However, a linear increase in the heat of adsorption with increasing calcium content is expected on the basis of the electrostatic interaction of the N2 molecule with higher-chargedensity calcium ions. To understand this anomalous behavior of N2 adsorption in NaCaA, we have carried out a detailed adsorption study experimentally as well as theoretically, employing the volumetric adsorption method and grand canonical Monte Carlo simulations, respectively. Grand canonical Monte Carlo simulations are suitable for establishing a correlation between the microscopic behavior of the zeolite and an adsorbate system with macroscopic properties such as the adsorption isotherm and the heat of adsorption that are measured experimentally.17,18 Experimental Section Materials. Zeolite A with chemical composition Na96Al96Si96O384· 208H2O (Zeolites and Allied Products, Bombay, India) and calcium chloride (E Merck India Ltd., Bombay, India) were used for the adsorbent preparation. Oxygen (99.99%), nitrogen (99.99%), argon (99.99%), and helium (99.99%) (Hydrogas India Pvt. Ltd., Bombay, India) were used for the adsorption isotherm measurements. Cation Exchange. Calcium cations were introduced into the highly crystalline sodium form of zeolites by conventional cation exchange (11) Breck, W. Zeolites Molecular SieVes: Structure, Chemistry and Use; Wiley-Interscience: New York, 1974. (12) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; Wiley-VCH: New York, 1994. (13) Chao, C. C.; Sherman, J. D.; Mullhaupt, J. T.; Bolinger, C. M. U.S. Patent 5,174,979, 1992. (14) Coe, C. G.; Kirner, J. F.; Pierantozzi, R.; White, T. R. U.S. Patent 5,152,813, 1992. (15) Peter, S. A.; Sebastian, J.; Jasra, R. V. Ind. Eng. Chem. Res. 2005, 44, 6856. (16) Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Ind. Eng. Chem. Res. 1993, 32, 548. (17) Nicholson, D.; Pellenq R. AdV. Colloid Interface Sci. 1998, 179, 76-77. (18) Fuchs, A. H.; Cheetham, A. K. J. Phys. Chem. B 2001, 105, 7375.
10.1021/la700821n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/13/2007
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Figure 1. X-ray powder diffraction pattern of calcium-exchanged NaA at different cation exchange levels. Table 1. Lennard-Jones Parameters Used for Adsorbate-Adsorbate and Adsorbate-Zeolite Interactions atom type
σ(Å)
(kcal mol-1)
N O Ar Si Al Oz Na Ca
3.318 3.050 3.405 0.076 1.140 3.040 1.746 1.764
0.0724 0.0884 0.2381 0.0370 0.0384 0.3342 0.0414 0.2719
from aqueous solution. Typically, the zeolites were treated with a 0.01 M aqueous solution of calcium chloride in a solid/liquid ratio of 1:50 at 353 K for 4 h. The residue was filtered, washed with hot distilled water until the washings were free of chloride ions, and dried in air at room temperature. The percent exchange was measured via flame photometer. Zeolite samples having different degrees of calcium exchange were prepared by subjecting the zeolites to repeated ion exchange. X-ray Powder Diffraction. X-ray powder diffraction studies at ambient temperature were carried out using a Philips X′pert MPD system in the 2θ range of 5-65° using Cu KR1 (λ ) 1.54056 Å). The diffraction patterns of the starting materials indicate that they are highly crystalline, showing reflections in the range of 5-35° that is typical of zeolites. The structure of the zeolite was observed to be retained after the cation exchange. The percent crystallinity of the calcium ion-exchanged zeolites was determined from the X-ray diffraction pattern by a summation of the intensities of 10 major peaks. The sodium form of the zeolite was considered to be an arbitrary standard for comparison. Activation and Isotherm Measurements. Nitrogen, oxygen, and argon adsorption was measured at 288.2 and 303.0 K using a static volumetric system (Micromeritics ASAP 2010). Prior to adsorption measurements, the samples were initially dried at 353 K for 24 h. The samples were further activated in situ by increasing the temperature (at a heating rate of O2 > Ar, in agreement with quadrupole moment values of -1.2 × 10-26 esu, -0.4 × 10-26 esu, and zero for N2, O2, and Ar, respectively. Figure 6 shows the pseudocell framework structure of zeolite A. There are 12 negative charges that are balanced by cations in each pseudocell. For the NaA pseudocell (Na12Al12Si12O48), eight Na+ ions are located at site I at the (33) Sircar, S. A. Sep. Sci. Technol. 1988, 23, 437. (34) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (35) Peterson, D. In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W. H., Ed.; ACS Symposium Series 135; American Chemical Society: Washington, D.C., 1980; p 107.
center of the six-membered ring, three at site II at the eightmembered aperture directly obstructing the entrance, and one at site III near the four-membered ring inside the main cavity. In the case of the unit cell structure given by Pluth and Smith,19 some of the main cavities were inaccessible to these gases for adsorption because of the blockage of sodium ions located at site II. These cavities were blocked by dummy atoms with appropriate van der Waals radii to ensure that the adsorbates were excluded from these locations during the adsorption simulations. In the crystal structure of dehydrated CaA,20 the framework bond angles are also very similar to those in NaA. Thus the adsorption behavior of both the sodium and calcium forms of zeolite A is expected to have the same adsorption properties. The variations in the adsorption behavior toward nitrogen, oxygen, and argon are due to the differences in the interactions of sodium and calcium ions with these adsorbate molecules. The interactions between a zeolite and an adsorbed molecule such as nitrogen involve electrostatic, induction, dispersion, and short-range repulsive forces. The electrostatic interaction arises between the adsorbed molecules possessing dipole or quadrupole moments and the permanent electric field of the zeolite. The permanent electric field depends on the nature of the cations and their location in the zeolite. If electrostatic interactions determine only the adsorption behavior, then there should be a linear increase in the heat of adsorption of N2 with increasing calcium exchange. However, an exponential increase is observed in the heat of adsorption from both experimental and simulation data. These observations show that, in addition to the charge and number of extraframework Na+/ Ca2+ cations, their location inside the zeolite can also significantly influence N2 adsorption behavior in NaCaA. During the simulation studies, we observed that there is no significant change in the adsorption capacity and heat of adsorption by using the cation positions mentioned in the literature19,20 or the cations located using the cation locator25 module of Cerius2 in the NaA and CaA unit cell framework structures. The calcium ions are located on the threefold axis close to the planes of the six-ring windows20
Sorption BehaVior of Nitrogen, Oxygen, and Argon
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Figure 9. Molecular graphics showing two types of interatomic distances between extraframework calcium and framework oxygen in 6MR after 60% exchange of calcium (yellow for silicon, pink for aluminum, red for oxygen, and cyan for calcium).
Figure 10. Molecular graphics snapshots of N2 adsorption at 101.3 kPa and 303 K in (a) NaA, (b) NaCaA35, (c) NaCaA60, and (d) NaCaA97 (yellow for silicon, pink for aluminum, red for oxygen, violet for sodium ions, cyan for calcium ions, and blue for nitrogen molecules).
at site I. These Ca2+ ions present in the six-ring windows can interact with the guest molecules through the windows. The increase in the heat of adsorption of nitrogen in Ca2+-exchanged zeolite A compared to that of NaA is due to the difference in the coordination environment. All of the main cavities in CaA were accessible to these adsorbate molecules because there are no blocking cations at the entrance of eight-membered rings. As the percentage of calcium exchange increases, the Na+ ions at the entrance of the eight-membered ring decreases, leading to the rapid accessibility of nitrogen into the main cavities of higher exchanged Ca2+ zeolite A. At lower exchange levels, Ca2+ ions are located very close to the centroid of the 6 MR, but at higher exchange levels, they are shifted more into the main cavity and hence are easily accessible for the adsorption as shown in Figure 7. This may be the reason for the sudden increase in the heat of adsorption of nitrogen, after a particular level of exchange of Ca2+ ions into zeolite A. Ca2+ cations occupied site I, at a distance of 0.406 Å away from the centroid of the 6MR plane toward the
main cavity of zeolite A as shown in the Figure, and their heat of adsorption of nitrogen was also high. Far-IR spectroscopic studies36 have also shown that at higher calcium exchange levels the majority of Ca2+ ions are located in the six-membered ring but are slightly displaced into the R cage. The remaining cations are located in the six-membered ring displaced into the β cage, which is less accessible. The interatomic distance between extraframework Ca2+ cations and framework oxygen decreases as the percentage of Ca2+ exchange increases as shown in Figure 8. But after 60% exchange, only two types of cation positions are observed for Ca2+ ions in the entire framework of each model, which we studied as given in Figure 9. The snapshots of the simulation of nitrogen adsorption at 101 kPa pressure and 303 K temperature in zeolite A with different Ca2+ exchange levels are shown in Figure 10. In the case of NaA, nitrogen molecules (36) Baker, M. D.; Godber, J.; Helwing, K.; Ozin, G. A. J. Phys. Chem. 1988, 92, 6017.
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are sitting close to the sodium cations located at sites I-III. Therefore, we can find nitrogen molecules well inside the main cavity of NaA, but in the case of calcium-exchanged zeolite A, the nitrogen molecules are inside the super cage of the zeolite close to the calcium cations located at site I at the center of the six-membered ring.
Conclusions The adsorption capacity, selectivity, and heat of adsorption of nitrogen increase with increasing calcium content of zeolite A. At higher calcium content, the increase is exponential. This behavior is mainly due to the better interaction of nitrogen with
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Ca2+ ions located at site I (i.e., in the six-membered ring displaced into the R cage). Ca2+ ions occupy only site I when the calcium exchange in the zeolite is approximately above 60%. Oxygen and argon adsorption capacities also increased with increasing calcium content of the zeolite. Acknowledgment. We are thankful for financial assistance and support from the Council of Scientific and Industrial Research, New Delhi, and Dr. P. K. Ghosh, Director, CSMCRI. We also thank Dr. Jince Sebastian, KIER, South Korea, and Dr. S. Venkatraman, Accelrys, India. LA700821N
