Ind. Eng. Chem. Res. 1991,30, 2523-2530
2523
MATERIALS AND INTERFACES Adsorption Characteristics of High-Exchange Clinoptilolites Mark W.Ackleyt and Ralph T. Yang* Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
Clinoptilolite, the most abundant natural zeolite, has been modified by ion exchange t o fully exchanged forms of the monovalent cations K+, Na+, and H+and highly exchanged forms of the bivalent cations Ca2+(89%)and Mg2+(72%). The Dubinin-Astakhov volume filling model was applied to the supercritical isotherm data to predict pore volume at the normal boiling temperatures of N2 and CH4-temperatures too low for practical isotherm measurement due to the slow diffusion of these gases. Adsorption isotherms and energetic heterogeneity have been discussed in terms of the various cations and their locations in the clinoptilolite channels. Adsorption characteristics have been greatly altered through cation manipulation to produce a range of CH4:N2selectivity ratios exceeding 1 order of magnitude. In view of the difficulty of the CH4/N2separation, these results suggest excellent potential for tailoring clinoptilolite for separation of other specific gas mixtures.
Introduction Interest in noncryogenic separation of nitrogen/methane mixtures for the purpose of upgrading aging natural gas wells has prompted recent developments in new materials (Baksh et al., 1990). Ackley and Yang (1990) have demonstrated the use of carbon molecular sieve (CMS) for separation of Nz/CH4 mixtures in pressure swing adsorption (PSA) processes but have also shown that the potential for CMS to achieve the desired pipeline quality (90% methane) is doubtful. A process using 4A molecular sieves for the same separation was developed by Habgood (1958), but this process was limited to low temperatures (273 K)and high methane feed content (290%). A calcium-exchanged clinoptilolite showed promise in a PSA process (Frankiewicz and Donnelly, 19831, but the product was below pipeline quality. Although separation can be significantly improved through process optimization, maximum performance is limited by the adsorption characteristics of the sorbent, as discovered in the process studies cited above. These factors contribute to our continued interest in CH4/Nz separation. Further, the potential of clinoptilolite in gas separations has motivated the current research in determining fundamental adsorption characteristics of modified clinoptilolites. Zeolites can be tailored through ion exchange to affect desired gas separation by exploiting equilibrium and/or kinetic properties. The success of such tailoring, however, is largely dependent upon understanding the interrelationship between the zeolite structure and the gas adsorption/diffusion characteristics. A notable series of studies estimated the electrostatic contributions of dispersion and quadrupole and dipole moment interaction forces with the framework of Na+- and H+-exchanged clinoptilolites (Barrer and Makki, 1964; Barrer and Coughlan, 1968; Barrer and Murphy, 1970). Adsorption characteristics were measured for three gases representing nonpolar (Kr), quadrupole moment (CO,), and dipole moment (H20)contributions to the gasaorbent interaction 'Current address: Linde Division, Union Carbide Industrial Gases Inc., P.O.Box 44, Tonawanda, NY 14151-0044.
potential. Galabova and Haralampiev (1980) attempted to relate the selectivity of Nz of various alkaline exchanged forms of clinoptilolite in air separation to the properties of the predominant resident cation. The mixture of cations contained in natural clinoptilolites, as well as in most exchanged clinoptilolites described in the literature, complicates any attempt to associate structural/adsorption/diffusioncharacteristics of the sorbent. An intent of this research was to produce fully exchanged clinoptilolites, each containing a single cation type. The gases CHI (nonpolar) and N2 (small quadrupole moment) served as probe molecules of similar kinetic diameter but different shape. Both diffusion (Ackley and Yang, 1991) and adsorption characteristics of these gases with the modified clinoptilolites were determined. This article addresses only the adsorption characteristics. Fully exchanged clinoptilolite sorbents were achieved for the monovalent cations K+, Na+, and H+, while highly exchanged (89% and 72%) materials resulted for the bivalent cations Ca2+and Mg2+,respectively. A purified form of the raw natural zeolite was also included in the study. Isotherms were measured at 300 and 323 K for both N2 and CHI. The volume filling model of DubininAstakhov was applied to these supercritical isotherms in order to estimate adsorption capacities and the corresponding approximate pore volumes at the normal boiling points. Molecular sieving was confirmed for CH, in the Na+ and Ca2+clinoptilolites and for N2 in Ca2+clinoptilolite. Nitrogen adsorbed in the bivalent forms of the zeolite displayed the greatest energetic heterogeneity. The range of equilibrium selectivity ratios of CH,/N2 resulting from this group of modified clinoptilolites spanned more than 1 order of magnitude.
Structure and Composition of Clinoptilolite Clinoptilolite is the most abundant of the natural zeolites (Mumpton and Ormeby, 1976). Many applications have been suggested and investigated for clinoptilolite, but few have been commercialized. Composition and purity vary widely among the many deposita found throughout the world. Clinoptilolite has been successfully synthesized 0 1991 American Chemical Society
2524 Ind. Eng. Chem. Res., Vol. 30,No. 12, 1991 ’
Table I. Channel Characteristics and Cation Sites i n Clinoptilolite tetrahedral approximate ring channel size/channel cation major dimensions, channel axis site cations nm X nm 0.72 X 0.44 M(1) Na, Ca A 1o/c 0.47 X 0.41 M(2) Ca, Na R 8/c K 0.55 X 0.40 M(3) C 8/a A 1o/c Mg 0.72 X 0.44 M(4)
.”
E
50
I
40
Y
30
+
U
20
L
l 0o0.0
0.1
0.2
0.3
Amount Adsorbed
0.4
0.5
mmol/g
10 01 0.0
0.2
0.4
0.6
Amount Adsorbed
0.8
J 1.0
mmol/g
N2for modified clinopti-
Figure 6. Isosteric heat of adsorption of CHI for modified clinoptilolites.
adsorbing n-hexane vapor on the K+ clinoptilolite to saturation at 300 K. The resulting pore value of 0.138 cm3/g (as determined from the weight of vapor adsorbed and its liquid density at 300 K) is in good agreement with the value of 0.129 cm3/g estimated from the CH4 adsorption data. Pore volumes estimated from CH, isotherms are in close agreement for the PUR, H+, Mg2+,and K+ clinoptilolites. Since the framework of various forms of clinoptilolite is stable (Bish, 1984), the only variations in pore volume should be due to experimental error and/or the change in pore space occupied by cations of differing size and number. The cation volume (based upon the crystal ionic radii of the cations) ranges from 0.002 to 0.022 cm3/g for the materials of this study. The significantly lower pore volumes estimated for the Na+ and Ca2+clinoptilolites is clear evidence of molecular sieving of CHI. Barrer and Makki (1964) estimated a pore volume of 0.135 cm3/g by measuring the amount of water vapor adsorbed on Na+-exchanged clinoptilolite at a relative pressure ratio of PIPo = 0.5. Assuming 24 water molecules/unit cell, Barrer (1982) gives the porosity of Na+ clinoptilolite as 0.34 cm3/ (cm3of crystal). This porosity is equivalent to a theoretical pore volume (based upon the assumed water content) of 0.186 cm3/g. The pore volumes estimated from the N2 data are higher and more variable than those determined from CHI isotherms. Breck (1974) has indicated that nitrogen always projects higher pore volumes in zeolites than other adsorbates of similar molecular size. Molecular sieving of N2 by the Ca2+clinoptilolite is evidenced by the substantially lower pore volume for this material. The CHI data provides a more reliable estimate of the physical pore volume in this study. C. Isosteric Heats of Adsorption. The combination of either methane (nonpolar) or nitrogen (small quadrupole moment) with the relatively low cation density clinoptilolites is expected to produce relatively weak interactions. It is worth noting that the adsorbate density at the highest adsorption levels in this study correspond to only about 1.0 and 2.0 molecules/unit cell for N2 and CH,, respectively. Conversely, the cation densities vary from 3 bivalent cations/unit cell to 6 monovalent cations/unit cell in fully exchanged clinoptilolites. Thus, interactions are predominantly sorbate-cation and sorbate-framework rather than sorbate-sorbate. Significant differences in energetics were observed among the various modified clinoptilolites. The isosteric heats computed from two isotherms (300 and 323 K) are given for N2 and CHI in Figures 5 and 6. The highest interaction energies occur between N2 and the Mg2+and Ca2+clinoptilolites, as illustrated in Figure
5. The higher more concentrated charge of Ca2+and Mg2+
Figure 5. Isosteric heat of adsorption of lolites.
is apparently required for this increased interaction with the weak quadrupole moment of N,. Barrer and Coughlan (1968) have shown a similar behavior for C02,which has a quadrupole moment almost three times that of N2,with monovalent Na+ and H+ clinoptilolites. The isosteric heats for N2 adsorbed on the monovalent clinoptilolites suggest more homogeneous energetics. This is likely due to the more uniform charge distribution created in the structure by the higher density of singly charged cations. The decrease in the overall isosteric heat in the order PUR > K+ > Na+ > H+ suggests a corresponding decrease in the N2 interaction energies with these different cations. Each cation coordinates energetically with the framework according to its location, however, and the effect of this coordination upon the internal electrostatic fields cannot be overlooked. The purified sample is a mixture of 74% monovalent and 26% bivalent cations (mole basis). The effects of molecular sieving are also manifested in the isosteric heats of adsorption. The sharp and continuous decline for Ca2+ clinoptilolite appearing in both Figures 5 and 6 is likely the result of a smaller number of possible interactions due to pore blockage, i.e. many of the Ca2+charge centers are inaccessible to both N2 and CHI. The values of qBtin Figure 6 for the Na+ clinoptilolite are much higher than can be reasonably expected. Quantitatively, these values result from the significant decrease in CHI capacity as temperature increases from 300 to 323 K. The pore volumes in Table V indicate molecular sieving of CH4in this zeolite. All M(1) and M(2) sites are occupied in the fully exchanged Na+ clinoptilolite-effectively blocking all eight-membered-ring (B, C) channels and partially blocking the 10-membered-ring A channel (Koyama and Takeuchi, 1977). Galabova (1979) has suggested that heating may result in a migration of cations in the structure. Only a small change in the position of the Na+ cation in channel A is required to completely bock this channel to CH,. Since the CHI capacity of the Na+ clinoptilolite is reduced to nearly that of the Ca2+clinoptilolite at 323 K, it appears that the degree of molecular sieving has increased with the temperature increase for the Na+ clinoptilolite. Thus, the CH, isosteric heats for Na+ clinoptilolite are anomalous in that the isotherms at 300 and 323 K effectively represent different zeolite structures. One might argue that a greater degree of blockage is expected with Na+ clinoptilolite compared to that of Ca2+ clinoptilolite simply due to the greater cation density, but this is contrary to the CHI adsorption capacities shown in Figure 4. Na+ and Ca2+cations are nearly the same size and occupy the same sites, as indicated in Table I. Because sites M(1) and M(2) are located at the intersections of A/C
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2529 function of pressure in Figure 7 for the modified clinoptilolites of this study. The selectivity ratio varies for this class of adsorbents by more than 1 order of magnitude and includes a reversal of preference from CHI (H+, K+, M8+, PUR) to N2 (Na+, Ca2+). Selectivity favoring N2 has been induced by molecular sieving of CH4. This range of selectivities is quite remarkable given the limited differences in properties of the two gases which can be exploited for separation. Since restricted diffusion is also characteristic of this group of materials with either N2 or CHI, potential separation processes must acknowledge both equilibrium and kinetic selectivities. The diffusion characteristics of N2 and CHI for these modified clinoptilolites is the subject of a future article. 0 0.0
0.2
0.4
0.6
0.8
1.0
Partial Pressure atm Figure 7. Selectivity ratio CHI/NB for modified clinoptilolites.
and B/C channels, respectively, Na+ and Ca2+are very efficient pore-blocking cations-cation density being less important than location. There are only four M(1) and M(2) combined sites/unit cell, however, and the Na+-exchanged material produces an excess of approximately 2 cations/unit cell for which the locations are not well-defined. These cations may in fact be the suspected migrating cations referred to above. Since the bivalent cations (3 cations/unit cell) do not completely saturate M(1) and M(2) sites, why is the molecular sieving of CHI at 300 K more complete in the Ca2+clinoptilolite? Recalling that the Ca2+clinoptilolite is only 89% Ca2+-exchanged, the secondary cation appears to be Mg2+from the composition data in Table 11. Mg2+occupies site M(4), which is located at the center of channel A (Koyama and Takeuchi, 1977), completely blocking this channel to CHI. Barrer (1966) has correlated the initial heat of adsorption for simple nonpolar molecules with their polarizability for a number of different sorbents. The heat of adsorption is equated to the dispersion, repulsion, and polarization contributions of these simple nonpolar molecules. The initial heats of adsorption of CH4adsorbed on K+, H+, and Mg2+clinoptilolites are about 10% higher than would be predicted from the limited results for Kr adsorbed on H+ clinoptilolite and the polarizability correlation (Barrer and Coughlan, 1968). Following the same correlation,the larger polarizability of CH, (26.0 X cm3) compared to that of N2 (17.6 X cm3) may even offset the quadrupole moment interaction effect of Nz so that CH, initial heats of adsorption can be equivalent to or greater than those for N2 for the same sorbent. As seen from comparison of the results of Figures 5 and 6, K+ clinoptilolite may be representative of such an effect. This reasoning is uncertain, however, without more detailed knowledge of the internal electrostatic fields and the individual contributions to the adsorbate-cation-framework interaction potentials. D. Selectivity. The selective preference by a sorbent for one gas over another is the basis for many separation processes. In particular, a wide range of pressure swing adsorption (PSA) processes have been developed to exploit specific equilibrium and/or kinetic properties of various sorbents (Yang, 1987). As an example of the versatility of PSA for CH4/N2separation, Baksh et al. (1990) have suggested a process based upon the equilibrium selectivity of CHI over N2provided by a Moo2-impregnatedactivated carbon, while Ackley and Yang (1990) developed a process utilizing the higher kinetic selectivity of CMS for Nz. The selectivity ratio, defined here as the amount of CH, adsorbed per amount of N2 adsorbed, is shown as a
Conclusions Clinoptilolite has been modified by ion exchange to produce a group of sorbents characterized by a wide range of selectivities that can be exploited to affect the difficult separation of nitrogen and methane. Fundamental adsorption properties such as capacity and heat of adsorption have been related to distinguishing structural properties such as cation type and location. Internal pore volumes have been estimated from the Dubinin-Astakhov equation and used to verify molecular sieving. By relating adsorption characteristics to the less complex structures of fully and highly exchanged clinoptilolites, it should now be possible to manipulate further the clinoptilolite pore structure through the use of mixed cations to affect desired gas separations. Acknowledgment This work was supported by NSF Grant CTS 8914754. Nomenclature A = differential molar work of adsorption, kJ/mol b = van der Waals covolume, cm3/mol E = characteristic free energy of adsorption, kJ/mol f = fugacity, atm M, = molecular weight, g/mol n = structure parameter in D-A equation P = pressure, atm PUR = purified but without ion exchange q = adsorbate concentration, mmol/(g of solid) qo = limiting (saturation) adsorbate concentration, mmol/ (g of solid) R = gas constant, R = 8.314 J/(mol K) T = temperature, K or "C ,Wo = pore volume, cm3/(g of solid) Greek Symbols a =
thermal coefficient of limiting adsorption, K-'
p
= angle between a- and c-coordinate axes of unit cell, rad
p
= density, g/cm3
Subscripts
b = normal boiling condition c = critical condition s = saturation condition
Registry No. CHI, 74-82-8; N,,7727-37-9.
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2530 Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 Ackley, M. W.; Yang, R. T. Kinetic Separation by Pressure Swing Adsorption: Method of Characteristics Model. AIChE J . 1990, 36,1229. Ackley, M. W.; Yang, R. T. Diffusion in Ion-Exchanged Clinoptilolites. AIChE J., 1991,in press. Ames, L. L., Jr. The Cation Sieve Properties of Clinoptilolite. Am. Mineral. 1960,45,689. Ames, L. L., Jr. Cation Sieve Properties of the Open Zeolites Chabazite, Mordenite, Erionite and Clinoptilolite. Am. Mineral. 1961, 46, 1120. Ames, L. L., Jr. Some Zeolite Equlibria with Alkali Metal Cations. Am. Mineral. 1964a,49, 127. Ames, L. L., Jr. Some Zeolite Equilibria with Alkaline Earth Metal Cations. Am. Mineral. 1964b,49, 1099. Baksh, M. S. A.; Kapoor, A.; Yang, R. T. A New Composite Sorbent for Methane-Nitrogen Separation by Adsorption. Sep. Sci. Technol. 1990,25,845. Barrer, R. M. Specificity in Physical Sorption. J . Colloid Interface Sci. 1966,21,415. Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. Barrer, R. M.; Makki, M. B. Molecular Sieve Sorbents from Clinoptilolite. Can. J . Chem. 1964,42,1481. Barrer, R. M.; Coughlan, B. Molecular Sieves Derived from Clinoptilolite by Progressive Removal of Framework Charge: Characterization by Sorption of COz and Krypton. Molecular Sieves; Society of Chemical Industry: London, 1968,p 141. Barrer, R. M.; Murphy, E. V. T. Influence of Decationation and Dealumination on Sorption by Mordenite and Clinoptilolite. J . Chem. SOC.A 1970,2506. Barrer, R. M.; Townsend, R. P. Transition Metal Ion Exchange in Zeolites. Part 2. Ammines of Co3+,Cu2+and Zn2+in Clinoptilolite, Mordenite and Phillipsite. J . Chem. Soc., Faraday Trans. 1 1976,2650. Barrer, R. M.; Papadopoulos, R.; Rees, L. V. C. Exchange of Sodium in Clinoptilolite by Organic Cations. J. Inorg. Nucl. Chem. 1967, 29,2047. Bish, D. L. Effects of Exchangeable Cation Composition on the Thermal Expansion/Contraction of Clinoptilolite. Clays Clay Miner. 1984,32,444. Breck, D. W. Zeolite Molecular Sieves; John Wiley: New York, 1974. Chi, C.-H.; Sand, L. B. Synthesis of Na- and K-Clinoptilolite Endmembers. Nature 1983,304,255. Dubinin, M. M. Physical Adsorption of Gases and Vapors in Micropores. In Progress in Surface and Membrane Science; Cadenhead, D. A., et al., Eds.; Academic Press: New York, 1975. Faires, L. M. Inductively Coupled Plasma Atomic Emission Spectroscopy. Metals Handbook, 9th ed., Materials Characterization; ASM: Metals Park, OH, 1986;Vol. 10. Frankiewicz, T. C.; Donnelly, R. G. Methane/Nitrogen Gas Separation over the Zeolite Clinoptilolite by the Selective Adsorption of Nitrogen. In Industrial Cas Separations; Whyte, T. E., Jr., et al., Eds.; American Chemical Society: Washington, DC, 1983. Galabova, I. M. Relationship Between New Structural Data on Clinoptilolite and Its Behaviour in Ion-Exchange and Heating. Z. Naturforsch. 1979,34a,248. Galabova, I. M.; Haralampiev, G. A. Oxygen Enrichment of Air on Alkaline Forms of Clinoptilolite. In The Properties and Appli-
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