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Adsorption of Nitrogen, Oxygen, and Argon in Cobalt(II)-Exchanged Zeolite X Jince Sebastian, Sunil A. Peter, and Raksh V. Jasra* Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364 002, India Received June 3, 2005. In Final Form: September 16, 2005 Adsorption of nitrogen, oxygen, and argon on cobalt(II)-exchanged zeolite X at 288.2 and 303.0 K was studied. The nitrogen and oxygen adsorption capacities increase upon cobalt ion exchange up to 71%, beyond which it shows a decreasing trend because of the partial degradation of the zeolite structure during the cation exchange and high-temperature vacuum dehydration processes. The magnitude of the increase in the adsorption capacities for nitrogen is much higher than that of oxygen. The nitrogen/oxygen as well as nitrogen/argon selectivities in the low-pressure region increase with an increase in cobalt exchange. Marginal oxygen selectivity over argon is observed for zeolite samples with higher cobalt exchange. The heats of adsorption values for nitrogen and oxygen increase and that for argon remain unaffected by cobalt exchange in zeolite X. The very high nitrogen adsorption capacity, selectivity, and heat of adsorption in the low-pressure region for cobalt-exchanged zeolite X compared to the parent sodium form of the zeolite show stronger interaction between nitrogen molecules with the extraframework cobalt cations of the zeolite. This stronger interaction has been explained in terms of the π-complexation between nitrogen molecules and cobalt cations of the zeolites, as confirmed by diffuse reflectance infrared Fourier transform spectroscopy, wherein the NtN stretching frequency at 2099 cm-1 is observed for N2 molecules adsorbed in NaCoX.
1. Introduction Zeolites, crystalline inorganic solids with pore diameters in the range of 1-10 Å, are of immense interest in gas and chemical industries as molecular sieve adsorbents for purification and separation.1 The extraframework cations invariably present in zeolites play a significant role in determining their adsorptive properties.2 Separation of nitrogen and oxygen from air is one of the major adsorptive separation processes used in many commercial situations.3-5 However, nitrogen and oxygen adsorption studies in zeolites are largely confined to alkali and alkaline earth cation exchanged zeolites,7-10 that too for zeolites A and X, with limited studies reported for transition metal ion exchanged zeolites.11,12 This is despite the fact that transition metal ions, due to the presence of the d-shell, can coordinate to adsorbate molecules and could show different adsorption behavior compared to filled-shell cations.11-16 For example, silver cations in zeolites are reported to interact strongly with the adsorbed nitrogen molecules.11-14 These strong interactions were explained * Corresponding author. Tel.: +91 278 2471793. Fax: +91 278 2567562. E-mail:
[email protected]. (1) Yang, R. T. Adsorbents: Fundamentals and Applications; WileyInterscience: New York, 2003. (2) Breck, W. Zeolites Molecular Sieves: structure, chemistry and use; Wiley-Interscience: New York, 1974. (3) Jasra, R. V.; Choudary, N. V.; and Bhat, S. G. T. Sep. Sci. Technol. 1991, 26, 885-930. (4) Yang, R. T. Gas Separation by Adsorption Process; Imperial College Press: London, 1997. (5) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; Wiley-VCH: New York, 1994. (6) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Ind. Eng. Chem. Res. 1996, 35, 4221-4229. (7) Chao, C. C. U.S. Patent 4859217, 1989. (8) Chao, C. C.; Sherman, J. D.; Mullhaupt, J. T., Bolinger, C. M. U.S. Patent 5174979, 1992. (9) Coe, C. G.; Kirner, J. F.; Pierantozzi, R.; White, T. R. U.S. Patent 5152813, 1992. (10) Rege S. U.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 53585365. (11) Sebastian, J.; Jasra, R. V. Chem. Commun. 2003, 268-269. (12) Sebastian J.; Jasra, R. V. U.S. Patent No. 6572838, PCT Patent WO 03/080236 A1, UK Patent 2 386 889 A, 2003.
in terms of the electron transfer by both σ-donation from the bonding π2p molecular orbital of N2 molecules to the 5s orbital of Ag+ ions and d-π2p* back-donation from the completely occupied 4d orbital of Ag+ ions to the unoccupied π2p* antibonding molecular orbital of the N2 molecule. Similarly, due to the presence of empty f orbitals, cerium-exchanged zeolite X shows oxygen selectivity over nitrogen and argon in the low-pressure region.15,16 However, scarce adsorption data on transition metal ion exchanged zeolites could be due to the difficulty in exchanging the transition metal ions into zeolites and retaining the zeolite structure, particularly at higher cation exchange. The present study reports on the adsorption of nitrogen, oxygen, and argon in cobaltexchanged zeolite X with different exchange level. An attempt is made to correlate the adsorption data with the locations of the cations inside the zeolite and its coordination with the framework oxygen atoms. 2. Materials and Methods 2.1. Materials. Zeolite X (with chemical composition Na88Al88Si104O384‚220H2O) (Zeolites and Allied Products, Bombay, India) and cobalt nitrate (S. D. Fine Chemicals Ltd., Bombay, India) were used as the starting materials 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. 2.2. Cation Ion Exchange. The cobalt cations were introduced into the highly crystalline sodium form of zeolite by conventional cation exchange from aqueous solution. Typically, the zeolite was refluxed with a 0.05 M aqueous solution of cobalt nitrate with a solid/liquid ratio 1:80 at 353K for 4 h. The residue was filtered, washed with hot distilled water, until the washings were free from ions, and dried in air at room temperature. Zeolite (13) Chen, N.; Yang, R. T. Ind. Eng. Chem. Res. 1996, 35, 4020-4027. (14) Hutson, N. D.; Reisner, D. A.; Yang, R. T.; Toby, B. H. Chem. Matter 2000, 12, 3020-3031. (15) Jasra, R. V.; Chudasama C. D. U. S. Pat. Pub. No. 2005/0090380 A1, 2005. (16) Jayaraman, A.; Yang, R. T.; Bhat, S. G. T.; Choudary, N. V. Adsorption 2002, 8, 271-278.
10.1021/la051460e CCC: $30.25 © 2005 American Chemical Society Published on Web 10/22/2005
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Table 1. Chemical Composition and Structural and Textural Properties of Cobalt(II)-Exchanged Zeolites t-plot
adsorbent
unit cell formula (on dry basis)
% crystallinity
unit cell dimension, Å
BET surface area, m2g-1
micropore surface area, m2g-1
external surface area, m2g-1
NaX NaCoX 46 NaCoX 71 NaCoX 88 NaCoX 93
Na88Al88Si104O384 Co20.5Na47Al88Si104O384 Co31.5Na25Al88Si104O384 Co38.75Na10.5Al88Si104O384 Co41Na6Al88Si104O384
100 84 80 79 72
24.94 24.91 24.91 24.91 24.90
542 676 696 662 669
518 623 616 541 508
24 53 80 121 161
samples having different degrees of cation exchange were prepared by subjecting repeated ion exchange into the zeolites. The extent of the cobalt ion exchange on different zeolite X samples was determined by complexometric titration with EDTA using murexide indicator. The number in the sample name indicates the percentage of ion exchange (e.g. NaCoX 46 means 46% of Na+ ions are replaced with Co2+ ions), as shown in Table 1. The chemical compositions of various cobalt-exchanged zeolite X samples are also given in Table 1. 2.3. 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 ) 1.540 56 Å). The diffraction patterns of the zeolite samples show the reflections in the range 5-35° typical of zeolites. Percentage crystallinity of the cobalt ion exchanged zeolites was determined from the X-ray diffraction pattern by summation of the intensity of 10 major peaks at 2θ values 6.1, 10.0, 15.5, 20.1, 23.4, 26.7, 29.3, 30.5, 31.0, and 32.1. The sodium form of the zeolite was considered as a reference sample for these calculations. 2.4. Activation and Isotherm Measurements. 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 O2. The adsorption selectivities of the pure components were calculated at different pressures, and the values are given in Table 6. The nitrogen/oxygen as well as nitrogen/argon selectivities in the low-pressure region increase from 3.4 to 21.8 and 3.7 to 28.6, respectively, with an increase in cobalt exchange. Marginal oxygen selectivity over argon is observed for zeolite samples with higher cobalt exchange. NaCoX 71 shows the maximum oxygen/argon selectivity value of 1.57 at 101.99 kPa. The isosteric heats of adsorption for nitrogen, oxygen, and argon adsorption on cobalt-exchanged zeolite X were calculated using the Clausius-Clapeyron equation and are given in Table 7. The heats of adsorption values for nitrogen and oxygen increase and that for argon remains unaffected upon cobalt exchange in zeolite X. The variation in the nitrogen heats of adsorption with the amount of N2 adsorbed on NaX and various cobaltexchanged zeolite X is shown in Figure 3. The nitrogen heat of adsorption decreases with an increase in adsorption coverage and finally became similar to that of the NaX. The decrease in the heat of adsorption of nitrogen with adsorption coverage shows that the number of Co2+ sites that can strongly interact with nitrogen molecules is limited only. It was also observed that the number of strongly nitrogen interacting centers increases with an increase in the cobalt exchange, while oxygen and argon molecules have feeble interaction with these sites, due to lower quadrupole moment values, and do not show significant variation with adsorption coverage. Zeolites are a class of materials that have crystallographically well-defined channels and cavities.2 The framework structure of zeolites is composed of a threedimensional network of basic structural units consisting
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Table 4. Virial Coefficient and Henry’s Constant at 303.0 K on Cobalt(II)-Exchanged Zeolites Henry’s constant, K (10-5 cm3 g-1 Pa-1)
virial coefficient, A adsorbent
nitrogen
oxygen
argon
nitrogen
oxygen
argon
NaX NaCoX 46 NaCoX 71 NaCoX 88 NaCoX 93
4.296 3.971 2.571 2.784 2.652
5.528 6.230 5.608 5.862 5.830
5.568 5.927 5.908 6.260 6.104
10.22 14.15 37.37 46.33 52.90
2.98 1.48 1.75 2.14 2.20
2.86 2.00 1.84 1.73 1.68
Table 5. Dubinin-Astakhov Fittings Data at 303.0 K on Cobalt(II)-Exchanged Zeolites energy, kJ mol-1
slope adsorbent nitrogen oxygen NaX NaCoX 46 NaCoX 71 NaCoX 88 NaCoX 93
-0.940 -0.788 -0.545 -0.523 -0.484
-0.988 -1.091 -0.961 -0.950 -0.936
argon -0.985 -0.985 -0.986 -0.987 -0.990
nitrogen oxygen argon 12.19 14.59 21.19 21.89 23.39
10.38 10.46 10.82 11.08 11.36
10.63 10.74 10.80 12.56 12.64
Table 6. Adsorption Selectivities on Cobalt(II)-Exchanged Zeolites adsorption selectivity RN2/O2
RN2/Ar
RO2/Ar
adsorbent
3.33 kPa
101.99 kPa
3.33 kPa
101.99 kPa
3.33 kPa
101.99 kPa
NaX NaCoX 46 NaCoX 71 NaCoX 88 NaCoX 93
3.4 7.7 20.7 20.6 21.8
3.1 3.3 4.1 3.7 3.6
3.7 8.3 30.3 28.7 28.6
3.3 3.7 6.4 5.3 4.9
1.08 1.08 1.46 1.39 1.31
1.07 1.12 1.57 1.45 1.42
Table 7. Isosteric Heats of Adsorption of N2, O2, and Ar on Cobalt(II)-Exchanged Zeolites heat of adsorption, kJ mol-1 a adsorbent
nitrogen
oxygen
argon
NaX NaCoX 46 NaCoX 71 NaCoX 88 NaCoX 93
19.4 34.5 36.4 38.2 40.7
15.1 17.2 17.9 18.1 18.5
13.4 13.3 13.9 14.1 14.1
a
At 1.0 cm3 g-1 coverage.
of SiO4 and AlO4 tetrahedrons linked to each other by sharing apical oxygen atoms. The resulting aluminosilicate structures, which are generally highly porous, possess three-dimensional pores, the access to which is through molecular-sized windows. Most zeolites contain exchangeable extraframework cations in their channels and cavities, as needed to balance the anionic charges of their frameworks. They may also contain easily replaceable guest molecules such as water or organic molecules.2,19
Adsorbate molecules can interact with the zeolite surface through lattice oxygen atoms, accessible extraframework cations, and Al and Si atoms. The Al and Si atoms present at the center of tetrahedra are not directly exposed to the sorbate molecules. Consequently, their interactions with the sorbate molecules are negligible. Therefore, the principal interactions of these sorbate molecules with the zeolite surface are with lattice oxygen atoms and extraframework cations.19 Zeolite X is a synthetic, aluminum-rich analogue of the naturally occurring mineral faujasite (Figure 4). The 14hedron with 24 vertexes, known as the sodalite cavity or β-cage, may be viewed as its principal building block. These β-cages are connected tetrahedrally at six-rings by bridging oxygen to give double six-rings (D6Rs, hexagonal prisms) and, concomitantly, an interconnected set of even larger cavities (supercage) accessible in three dimensions through 12-ring (24-membered) windows.20 The extra framework cations are usually found at the sites shown in Figure 4. The crystal structures of cobalt-exchanged zeolite X are reported in the literature.21 Co2+ ions occupy sites I′, II, and III′ in zeolite X. In the sample with 46 Co2+ ions per unit cell, cobalt ions are found at five different crystallographic sites and sodium ions at a single site, as shown in Table 8. The spectroscopic studies of the cobaltexchanged zeolites22 show that the cobalt ion migrates toward smaller cavities and occupy well-defined cation sites. In zeolite X, the extraframework cations in sites I and I′ are not accessible to the oxygen, nitrogen, and argon molecules. The cation sitting in site II and II′ can interact with the adsorbate molecules through the six-memberring windows, and the cations in site III and III′ can interact directly with these adsorbate molecules. The Co-O distances in NaCoX (Table 8) are longer compared to the sum of the ionic radii of Co2+ and O2(0.74 + 1.32 ) 2.06 Å, and 2.130 Å in Co-O). The coordinately unsaturated Co2+ ions located at sites II′ and III′ can interact strongly with the nitrogen molecules by both σ-donation (electron transfer from bonding π2p orbitals of N2 molecules to 4s orbital of Co2+ ions) and d-π2p* back-donation (electron transfer from the 3d orbital
Figure 3. Variation of heats of adsorption of (a) nitrogen and (b) oxygen and argon with adsorption coverage in NaX and various cobalt(II)-exchanged zeolite X samples.
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Figure 4. The framework structure of zeolite X; near the center of the each line segment is an oxygen atom. The numbers 1-4 indicate the different oxygen atoms. Extraframework cation positions are labeled with Roman numerals. site I is at the center of the D6R, I′ in the sodalite cavity on the opposite side of one of the D6Rs six-rings from site I, II′ inside the sodalite cavity near a S6R, II at the center of the S6R or displaced from this point into a supercage, III in the supercage on a 2-fold axis opposite a four-ring between two 12-rings, and III′ somewhat or substantially off III (off the 2-fold axis) on the inner surface of the supercage. Table 8. Cation Site Occupancies and M-O Distances in NaCoX17 cation location
no. of cations
M-O distance, Å
site I site I′ (2 different) site II site II′ site III
8 (Na+) 16 (Co2+) 19 (Co2+) 1 (Co2+) 10 (Co2+)
2.623 2.149 and 2.32 2.129 2.33 2.30 and 2.27
of Co2+ ions to the unoccupied π2p* of the N2 molecule). This π-complexation of nitrogen molecules with Co2+ ions in the zeolite results in a high value for heat of adsorption, Henry’s constant, adsorption selectivity, and adsorption capacity for nitrogen in the low-pressure region. The DRIFT spectroscopic study of the adsorbed N2 molecules also supports the stronger interaction of nitrogen molecules with extraframework cobalt ions present in the zeolite. N2 molecules being totally symmetric do not absorb IR radiation. However, N2 molecules adsorbed in zeolites experience an induced dipole moment, which varies during vibration, and an induced band is observed.23,24 The magnitude of the induced dipole moment depends on the strength of the interaction of the nitrogen molecules with the extraframework cations. At 303 K, N2 molecules adsorbed on NaCoX 88 give the adsorption band at 2099 cm-1, as shown in Figure 5. The adsorption bands observed at 3400 and 1643 cm-1 show the presence of a small amount of moisture in the zeolite, despite in situ activation of the zeolite sample and a dry nitrogen purge. (19) De Lara, E. C.; Delaval, Y. J. Chem. Soc. Faraday Trans. 2 1978, 74, 790-797. (20) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves: Academic Press: London, 1973. (21) Olson, D. H. Zeolites 1995, 15, 439-443. (22) Base, D.; Seff, K. Microporous Mesoporous Mater. 1999, 33, 265280. (23) Verberckmoes, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. Microporous Mesoporous Mater. 1998, 22, 165-178. (24) Geobaldo, F.; Lamberti, C.; Ricchiardi, G.; Bordiga, S.; Zecchina, A.; Palomino, G. T.; Aren, C. O. J. Phys. Chem. 1995, 99, 11167-11177.
Figure 5. Diffuse reflectance infrared Fourier transform spectra of NaCoX 88 with and without adsorbed N2 molecules measured at 303 K.
Transition metal complexes are reported25 to interact with N2 through end-on coordination (M-NtN). This M-N2 bonding is interpreted in terms of the σ-donation and π-back-donation. It is further reported25 that ν(NtN) shifts in the range 2220-1850 cm-1 on π-complexation with transition metals from 2331 cm-1 expected for free N2 molecule. For example, NtN stretching frequency in dinitrogen complexes [Ru(N2)(NH3)5]Br2, Co(N2)(PPh3)3, [Os(N2)(NH3)5]Cl2, and Ir(N2)Cl(PPh3)2 are observed at 2105, 2093, 2022, and 2105 cm-1, respectively. The NtN stretching frequency at 2099 cm-1 observed for the N2 molecule adsorbed in zeolite NaCoX 88 is closer to these values, confirming the π-complexation between the nitrogen molecule and the cobalt cations of the zeolite. The IR spectrum of N2 molecules adsorbed in LiLSX zeolite showed adsorption bands at 2320-2340 cm-1.26 These results also support the proposal that a shift of the adsorption band toward lower frequencies in NaCoX is caused by the stronger interaction of the N2 molecules with the extraframework cobalt cations via π-complexation 4. Conclusions In this work, the adsorption of nitrogen, oxygen, and argon on the cobalt-exchanged zeolite X with different cobalt content was investigated. It was observed that adsorption capacity, Henry’s constant, Langmuir constant, Dubinin-Astakhov energy, heat of adsorption for nitrogen adsorption, and adsorption selectivities over oxygen and argon in the low-pressure region increase with an increase in cobalt exchange. Marginal oxygen selectivity over argon was also observed for zeolite samples with higher cobalt exchange level. The stronger interactions of the nitrogen molecules were explained in terms of the π-complexation between nitrogen molecules and cobalt cations of the zeolites. Acknowledgment. We acknowledge the financial assistance and support from Department of Science and Technology, Council of Scientific and Industrial Research, and Dr. P. K. Ghosh, Director, CSMCRI. LA051460E (25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds-sPart B: Applications in Coordination, Organometallic and Bioinorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1997; pp 173-177. (26) Coe, C. G., Structural Effects of the Adsorptive Properties of Molecular Sieves for Air Separation, in Access in Nanoporous Materials; Pinnavaia, T. J., Thorpe, M. F., Eds.; Plenum Press: New York, 1995; pp 213-229.
