Correlation of Sorption Behavior of Nitrogen, Oxygen, and Argon with

Ind. Eng. Chem. Res. 1996, 35, 4221-4229

4221

Correlation of Sorption Behavior of Nitrogen, Oxygen, and Argon with Cation Locations in Zeolite X† R. V. Jasra, N. V. Choudary, and S. G. T. Bhat* Research Centre, Indian Petrochemicals Corporation Ltd., Vadodara 391 346, India

Specific retention volume, sorption selectivity, and heat of sorption for nitrogen, oxygen, and argon have been measured in zeolite X having alkali or alkaline-earth metals as extra-framework cations using gas chromatography. These sorption properties show a strong dependence on the nature as well as the extent of extra-framework cation, reflecting that the cations are the principle sites for interactions with the sorbate molecule. Zeolites studied show sorption selectivity toward nitrogen from its mixture with oxygen or argon. The electrostatic interactions between sorbate molecules and the cations are largely responsible for the observed nitrogen selectivity for the zeolite from its mixture with oxygen or argon. The nonlinear dependence of the sorption properties with cation charge density indicates that the interactions of the sorbate molecule depend not only on the cation type but also on the sites it occupies in a zeolite structure. The correlation observed between sorption properties and the cation site occupancy shows that nitrogen can be used as a probe for determining cationic sites in a zeolite structure. 1. Introduction

2. Materials and Methods

Adsorption processes, namely, pressure-swing adsorption (PSA) and vacuum-swing adsorption (VSA), are being increasingly used (Lee and Stahl, 1973; Jasra et al., 1991; Reiss, 1994) for the commercial production of oxygen from air. It is estimated (Reiss, 1994) that, presently, nearly 5% of the world’s oxygen demand is met by this technology. Zeolite-based nitrogen-selective microporous adsorbent is the key component in these processes. The use of zeolites of type A, X, and mordenite has been reported (Jasra et al., 1991) for oxygen production by PSA and VSA. There have been continuous efforts to modify the adsorbents for attaining higher adsorbent productivity by enhancing adsorption capacity and selectivity. Nitrogen adsorption selectivity of 3-5 is normally observed for the conventional zeolites, viz., NaX, CaA, or mordenite. It has earlier been reported that the extra-framework cations in the zeolite structure (Barrer, 1978; Coe et al., 1988; Choudary et al., 1993, 1994) are largely responsible for nitrogen selectivity of the zeolites. Consequently, attempts have been made to increase the number of selective sites in the zeolite by changing the Si/Al ratio (Coe et al., 1988) or using a combination of extra-framework cations (Coe et al., 1988; Coe and Kuznicki, 1984). It is known (Breck, 1974) that extra-framework cations occupy different sites in the zeolite matrix depending on the zeolite type, nature, and number of the cations. Thus, the distribution of cations in zeolites controls the interaction between sorbate molecules and zeolite and is a major factor responsible for adsorption selectivity. Hence, it is imperative to establish the cation distribution-adsorption selectivity relationship in order to be able to design an adsorbent for a given air separation process. In our earlier report (Choudary et al., 1993), we have described nitrogen and oxygen adsorption on zeolite A having varying Na+ and Ca2+ ion distribution. In continuation, we now report the sorption of nitrogen, oxygen, and argon in zeolite X having alkali and alkaline-earth extra-framework cations wherein an attempt is made to correlate the sorption behavior with known cation distribution.

2.1. Materials. Zeolite NaX in the form of 1.5 mm cylindrical pellets containing 20% bentonite clay binder was supplied by the CATAD division of Indian Petrochemicals Corp. Ltd., Thane, India. The Si/Al ratio of the zeolite X was 1.2 [unit cell formula is Na88[(AlO2)88(SiO2)104]‚nH2O]. Ultra-high-purity (99.999%) nitrogen, oxygen, argon, and helium were used in all measurements. The various salts used for cation exchange in zeolites include cesium chloride (99%, Fluka); calcium chloride (99%) and potassium chloride (99.8%, Ranbaxy, India); barium chloride (99.9%, Glaxo Laboratory, India); and strontium chloride (99%, S.D. Fine Chemicals, India). All these chemicals have been used without any further purification. 2.2. Cation Exchange and Characterization of Zeolite X. Ion exchange was carried out by repeatedly refluxing the zeolite pellets at 353 K with the required salt solution (2-10% w/w and solid to liquid ratio of 1/5) followed by thorough washing with hot distilled water. The samples were then dried at 383 K in an air oven. The extent of ion exchange was determined by chemical analysis of sodium and the exchanged cation using automatic atomic absorption spectrophotometer (GBC Model 902, Austria). Crystallinities of ion-exchanged zeolite samples were established by X-ray diffraction analysis of the samples using a Rigaku X-ray diffractometer (Model Dmax3, Japan). Comparison of X-ray diffractions at 2θ values between 5 and 45° for ionexchanged pellets with that of NaX powder showed that the crystalline structure of the zeolite is retained during ion exchange. 2.3. Sorption Measurements. The gas chromatographic (GC) method used for determining sorption is described in detail elsewhere (Choudary et al., 1993). These measurements were made using a Varian Vista 6000 gas chromatograph. The crushed zeolite sample (180-250 µm) was packed in a thoroughly cleaned 60 cm long stainless steel column having 0.63 cm outer and 0.53 cm internal diameters, respectively. The sorbent was activated by raising the column temperature to 673 K under a hydrogen flow which is not retained by the adsorbents studied and keeping the sample at 673 K ((1 K) for 4 h after attaining the temperature. To

†

IPCL Communication No. 305.

S0888-5885(96)00168-6 CCC: $12.00

© 1996 American Chemical Society

4222 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

eliminate/minimize in-situ steaming of the zeolite sample during the activation, the heating rate was kept low (2 K/min) and carrier gas flow was sufficiently high (60 mL/min). After the activation, the column temperature was brought down to 303 K, and the carrier gas flow decreased to 30 mL/min. A pulse (0.5 mL) of the gas mixture consisting of 0.5, 2.0, and 2.0 vol % of oxygen, nitrogen, and helium, respectively, in hydrogen was injected into the column using a gas sampling valve (Shimadzu, Japan) and the retention times of gases were measured. In the case of argon, its mixture (2% argon + 2% helium) in hydrogen was injected. This procedure was repeated at 313, 323, and 333 K. The retention time values were precise within (0.01 min. The effect of varied pulse size was studied in LiNaX-98 and CaNaX-97 zeolites by varying the nitrogen concentration in the mixture from 1 to 20% v/v. The variation in pulse size was observed to have a negligible effect on retention volumes. Corrected retention times were used to calculate the net retention volume, VN, and specific retention volume, Vm, using the equations

VN ) FtRj/(1 - pw/po)T/TR

(1)

Vm ) VN/Ws

(2)

where j is given by

j ) (3/2)[((pi/po)2 - 1)/((pi/po)3 - 1)] and tR is the retention time, pi and po are the column inlet and outlet pressures, pw is the water vapor pressure at room temperature TR, T is the column temperature, and ws is weight of adsorbent in the column. Limiting sorption selectivity of gas A over B, RA/B, and initial heats of sorption, ∆H0, were calculated from specific retention volumes using the following equations:

RA/B ) Vm(A)/Vm(B)

(3)

-∆H0 ) R d ln(Vm/T)/d(1/T)

(4)

where R is the gas constant and T is the GC column temperature in Kelvin. The uncertainties in the values of Vm, RA/B, and ∆H0 as calculated using the method of propagation of errors from the known errors in the experimental parameters are (0.8, (1.6, and (1.8%, respectively. 3. Theoretical Calculations of Heats of Sorption The total energy of physical sorption, φ, is comprised of dispersion φD, polarization φP, field-dipole interactions φFµ, field-quadrupole interactions φFQ, close-range repulsion φR, and sorbate-sorbate interaction φSP. Thus, φ is given by

φ ) -(φD - φR) - φP - φFµ - φFQ - φSP

(5)

The sorbates studied being nonpolar and sorption coverage being small, the terms φFµ and φSP can be ignored. Hence, eq 5 reduces to

φ ) -(φD - φR) - φP - φFQ

(6)

Substituting the terms for various interactions (Hirschfelder et al., 1954), eq 6 can be rewritten as

Figure 1. Zeolite X structure and the various cation locations.

1 ∂F 1 Rs 2 F φ)- Q 4 ∂r 2 k

∑r6 + ∑r12 A

B

(7)

where Q is quadrupole moment, k is a constant (in SI units k ) 9 × 109), F is the field, and Rs is the polarizability of a sorbate molecule. The values of A and B are calculated as described in the literature (Barrer, 1978). As Al and Si are not directly exposed to the sorbate molecules and possess small polarizability values, their interactions with the latter are ignored in the present calculations. Hence, the principle interactions of sorbate molecules with zeolite structure are assumed to be through lattice oxygen atoms and accessible extra-framework cations. Extra-framework cations in zeolite X may be located in the double 6-rings, the β-cages, and the supercages (Figure 1). However, due to steric factors, nitrogen, oxygen, and argon molecules cannot enter the β-cages and thus can interact with cations located only in the supercages. When a cation is located within the supercage, the electric field around the cation is partially shielded and the electrostatic and induction interactions are expected to be lower than that of an isolated ion. Furthermore, the dispersion forces acting on the molecule will be higher as sorbate molecules will also interact with oxygen atoms of the zeolite. The interaction energies between sorbate molecules and the cations located in the 6-ring and 4-ring of the zeolite were calculated using the following assumptions: (1) Electrostatic and polarization interaction energies are primarily affected by the oxygen ring where cation is located. (2) The 4-rings and the 6-rings have been considered as planar rings with a free diameter of 1.98 and 3.1 Å, respectively, with the cations positioned in the middle of the rings. (3) Sorbate molecules are nonrotating and the axis of the quadrupole is presumed to be parallel to the cationmolecule axis. The electrostatic and induction energies are calculated using the reduced field determined by subtracting the field due to oxygen atoms from the field of the cation as shown in the equations given below. The interactions of sorbate molecule with cations in the 6-ring and 4-ring are depicted in Figure 2. For 6-ring:

F)

k [q - 6qo sin3 R] 2 c r

(8)

where qc and qo are cation and oxygen charges in C, respectively. For 4-ring: The cations with diameter larger than the free diameter of the 4-ring are vertically displaced and protrude from the 4-ring into the supercage as shown in Figure 2. Therefore, the field for Sr2+, Ba2+, K+, and Cs+ is given by the following equation.

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4223

Figure 2. Interaction of sorbate molecules with 6-ring and 4-ring zeolite cations.

F)

kqc

kqo 4 sin3 R 2 2 r (r + r′)

(9)

Vm/(cm3 g-1)

However, for ions Li+, Na+, and Ca+2 having diameter less than 1.98 Å, the field is given by

F)

k [qc - 4qo sin3 R] r2

Table 1. Specific Retention Volume of Nitrogen, Oxygen, and Argon in Various Adsorbents at 303 K

(10)

The contribution from the term -(φD + φR) was taken as 6.3 kJ/mol for all the sorbate gases, the value reported by Barrer (Barrer, 1978) for the Ar-NaX system. 4. Results 4.1. Specific Retention Volumes. The specific retention volume (Vm) is a measure of sorbate sorption capacity. These data for nitrogen, oxygen, and argon given in Table 1 show that Vm(N2) values for different zeolites have the following trends at all the temperatures studied:

LiNaX-98 . NaX > KNaX-90 > CsNaX-70 CaNaX-97 > SrNaX-96 . BaNaX-94 It should be noted that LiNaX-98 and CaNaX-97 show exceptionally high Vm values compared to other monoand bivalent cations. The values of Vm(O2) and Vm(Ar) are of lower magnitude than that of nitrogen but show similar trends. A plot of Vm versus cation charge density (Figure 3) shows that the specific retention volume for nitrogen increases sharply with an increase in charge density of the extra-framework zeolitic cation. Furthermore, this change is more pronounced for bivalent cations. The values for Vm(O2) and Vm(Ar) are of similar order (from

sample

unit cell formula

N2

O2

Ar

LiNaX-31 LiNaX-64 LiNaX-74 LiNaX-86 LiNaX-98 NaX KNaX-36 KNaX-61 KNaX-90 CsNaX-22 CsNaX-70 CaNaX-37 CaNaX-54 CaNaX-70 CaNaX-93 CaNaX-97 SrNaX-96 BaNaX-94

Li27Na61X Li56Na32X Li65Na23X Li76Na12X Li86Na2X Na88X K32Na56X K54Na34X K79Na9X Cs19Na69X Cs61Na27X Ca16.5Na55X Ca24Na40X Ca31Na26X Ca41Na6X Ca42.5Na3X Sr42Na4X Ba41.5Na5X

6.41 5.77 5.94 9.00 30.07 8.00 5.32 4.33 4.27 3.51 2.89 8.36 9.89 12.39 23.38 31.82 21.71 8.21

2.39 2.09 2.13 3.18 3.23 2.45 2.54 2.40 2.39 2.07 1.99 2.72 2.75 2.67 2.95 3.74 3.62 2.71

2.25 1.94 2.01 2.87 2.55 2.28 2.25 2.18 2.27 2.12 2.09 2.53 2.42 2.11 2.13 2.86 2.83 2.24

2 to 4) and show little increase with increasing charge density of the extra-framework cations. The dependence of specific retention volumes on the extent of exchange of extra-framework cations for Li+, K+, Cs+, and Ca2+ has been shown in Figure 4. In LiNaX zeolite samples, Vm(N2) decreases with an increase in lithium exchange up to 64% exchange level, after which there is an exponential increase particularly at a Li+ exchange level of 90-98%. This is similar to the observations of Dzhigit et al. (Dzhigit et al., 1979), who have reported the Henry constant values for propane adsorption in NaX, Li56NaX, and Li91NaX zeolites as 0.206, 0.084, and 0.618 mmol g-1 Torr-1, respectively. Oxygen and argon show lower Vm values but follow a similar trend. 4.2. Sorption Selectivities. In general, sorption selectivity from N2, O2, and Ar binary mixtures (eq 3)

4224 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

Figure 3. Specific retention volumes for (a) N2, (b) O2, and (c) Ar as a function of extra-framework cation charge density.

Figure 4. Dependence of specific retention volumes for N2 as a function of percentage ion exchange.

follows the trend

RN2/Ar > RN2/O2 . RO2/Ar The sorption selectivities, R, show a linear dependence on the charge density of extra-framework cations (Figure 5), with lithium-exchanged zeolite being an exception. The LiX sample shows a sharp increase in R values at higher lithium exchange level. Furthermore, there is a distinction in the dependence of nitrogen adsorption selectivity on cation charge density between zeolites having mono- and bivalent extra-framework cations. A dependence of R’s on the degree of cation exchange has been shown for LiNaX, KNaX, CsNaX,

Figure 5. Sorption selectivity for (a) N2/O2 and (b) O2/Ar as a function of extra-framework cation charge density.

and CaNaX zeolites in Figure 6 and is similar to that of specific retention volumes. 4.3. Heats of Sorption. The heats of sorption data on various zeolites were calculated using eq 4 and are given in Table 2. These data have also been plotted against the charge density of the extra-framework cations as given in Figure 7. It is observed that the heat of sorption for nitrogen (Figure 7), like specific retention volumes and selectivities, increases with increasing charge density of the cations, with mono- and bivalent cations showing different slopes. The heat of sorption for the LiX sample is very high compared to zeolites having other monovalent cations. The heats of sorption for oxygen as a function of cation charge density of various zeolites show minima for

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4225 Table 2. Heats of Sorption at Zero Coverage for N2, O2, and Ar in Various X Type Zeolites sample LiNaX-31 LiNaX-64 LiNaX-74 LiNaX-86 LiNaX-98 NaX KNaX-36 KNaX-61 KNaX-90 CsNaX-22 CsNaX-70 CaNaX-37 CaNaX-54 CaNaX-70 CaNaX-93 CaNaX-97 SrNaX-96 BaNaX-94

Figure 6. Dependence of sorption selectivities for N2, O2, and Ar on the extent of extra-framework cations: (a) RN2/O2; (b) RO2/Ar.

l

-∆H0/(kJ mol-1) N2 O2 Ar 16.9 16.2 17.1 18.2 27.0 18.5 16.6 16.2 15.4 14.9 14.1 19.0 19.3 24.0 28.5 28.8 25.2 21.4

11.6 11.0 11.2 12.8 13.4 12.0 11.4 11.6 11.9 11.9 12.2 12.0 11.9 13.9 14.4 15.3 13.9 13.5

10.3 10.4 10.6 10.9 12.4 11.2 11.1 11.2 11.5 11.8 12.3 11.3 11.3 13.2 13.9 13.5 13.5 12.9

cation charge density/ (C m-1 × 10-10)

21.1 15.7 11.6 9.6

32.0 27.2 23.7

dependence of heats of sorption for argon on cation charge density shows a pattern similar to oxygen. The dependence of heats of sorption on lithium-, potassium-, cesium-, and calcium-exchanged zeolites with varying cation content is shown in Figure 8. There is a decrease in the heats of sorption values up to 64% exchange of lithium for LiNaX. Harlfinger et al. (Harlfinger et al., 1983) have also shown such a decrease in the integral heat of adsorption for 1-butene in Li56NaX (56% Li exchange) compared to NaX. The heats of sorption increase with further lithium exchange in the zeolite. However, the rise is very sharp at 97% lithium exchange. For CaNaX zeolites, heats of sorption show a pattern similar to Vm and sorption selectivity. The increase is higher for nitrogen than for oxygen. However, in the case of argon, heats of sorption show little decrease initially, after which it increases with increasing calcium content. Theoretically calculated heats of sorption for various sorbate-sorbent systems (Table 3) show higher values when the cations are located in the 4-ring. The data on cation locations as given in Table 4 show that alkali ions are sited in both the 4- and 6-rings, whereas alkaline-earth cations are sited only in the 6-rings of the supercages. Thus, the heats of sorption for bivalent ions in the 4-rings were not calculated. The values for bivalent cation-exchanged zeolite samples overestimated heats of sorption. This was attributed to two assumptions, viz., the siting of bivalent cation out of the 6-ring plane and the presence of steric hinderance to sorbate molecule in approaching the cation. However, it is reported (Mortier, 1982) that bivalent calcium cations are displaced (0.28 Å) out of the plane of the 6-ring. Furthermore, the distance between sorbate molecule and the cation is proposed to be optimized (Barrer, 1978) from energy calculations to account for steric hindrance. These two factors were incorporated into the calculations by displacing the bivalent cation by 0.28 Å into the sodalite cage and optimizing the distance between sorbate molecules and the cations (0.4 Å) to get agreement between the calculated and experimental heats for bivalent cations.

Figure 7. Heat of sorption at zero coverage for (a) N2, (b) O2, and (c) Ar as a function of extra-framework cation charge density.

5. Discussion

mono- as well as bivalent cations as seen in Figure 7. Lithium- and calcium-exchanged zeolites show higher heats of adsorption than other zeolite samples. The

5.1. Interaction Energies and the Sorption Properties. The specific retention volume, Vm, and heat of sorption, ∆H0, measured in the Henry’s region reflect

4226 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

a

Table 3. Theoretically Calculated Interaction Energies in kJ/mol for Sorbate Molecules with Zeolitic Cations in 4-Rings and 6-Rings nitrogen cation F(F

l

Q)

oxygen

EI -∆H0 F(F

Q)

argon rMa/ 1030Ra/ m3 EI -∆H0 EI -∆H0 nm

Li+ Na+ K+ Cs+ Ca2+ Sr2+ Ba2+

10.6 9.8 26.7 7.6 5.5 19.4 4.9 2.5 13.8 3.3 1.3 10.9 8.1 14.9 29.4 7.2 12.4 25.9 5.9 9.4 21.6

For 6-Ring 1.7 8.6 16.6 9.0 15.5 0.068 1.2 4.9 12.4 5.1 11.4 0.095 0.8 2.2 9.3 2.3 8.6 0.133 0.5 1.1 8.0 1.2 7.5 0.169 1.3 12.6 20.2 13.7 20.0 0.099 1.2 10.4 17.9 11.4 17.7 0.113 1.0 7.9 15.2 8.6 14.9 0.135

Li+ Na+ K+ Cs+

10.1 10.7 27.1 7.2 6.4 20.0 5.1 5.0 16.3 3.7 3.4 13.5

For 4-Ring 1.6 9.5 17.4 1.2 5.7 13.0 0.8 4.4 11.5 0.6 3.0 9.9

1040Qa/C m3 1030Ra/m3

0.029 0.19 0.84 2.44 0.53 0.86 1.56

9.9 16.2 5.9 12.2 4.6 10.9 3.2 9.5

N2

O2

Ar

5.0035 1.74

0.800 1.54

0.0 1.60

a Data for ionic radii of cations, r , polarizability, R, and M quadrupole moment, Q, taken from references (Barrer, 1978; Furuyama and Nagato, 1984).

Table 4. Alkali and Alkaline-Earth Cation Site Population in Zeolite X no. of cations in sites

b

sample

I

I′

II

Na81X(Hyd) 8.6 11.2 21.7 3.8 32.3 30.8 Na81X(Dehyd) Na88X(Hyd) 9.0 8.0 24.0

II′ III unlocated 19.9 6.2

Mortier, 1982 Mortier, 1982

8.0

47.0

Barrer et al., 1968, 1969 Hseu, 1972 Hseu, 1972 Pluth and Smith, 1972 Olson, 1968 Mortier et al., 1972 Mortier and Bosmans, 1971 Mortier et al., 1972 Olson, 1968

9 16 29 18 K70X 9 13 26 38 K87X K87X(Hyd) 8.9 7.2 23.2 13.3 5.0 25.0 6 Ca40X 7.5 17.3 17.3 9.0 Ca43X Ca41Na3X- 10.0 12.1 23.0 (Dehyd) 11.2 7.0 19.5 4.2 Sr43X Ba36Na16X- 7.0 4.7 11.4 3.7 Na (Dehyd)

l

Figure 8. Dependence of heat of sorption at zero coverage for (a) N2 and (b) O2 on the extent of extra-framework cations.

the interactions between the zeolitic surface and the sorbate molecules. Only nitrogen and oxygen, which possess quadrupole moments, are capable of electrostatic interactions with the zeolitic surface, while argon can only interact through van der Waals type forces. Theoretical calculations showed (Table 3) that the major contribution to N2/O2 and N2/Ar sorption selectively is from higher electrostatic interaction of nitrogen

ref

19.6 7.9

with the cations due to its quadrupole moment. The induction and dispersion energies being comparable for N2 and Ar, their contribution toward N2/Ar selectivity is negligible. Though the induction energy for Ar is higher compared to that for oxygen, the higher electrostatic energy for O2 makes it more selective. These energies increase with decreasing zeolitic cation size and correlate well with the trend observed for RN2/O2. However, the magnitude for zeolite exchanged with bivalent cation is higher than those for zeolites having monovalent cations. The trend observed for the specific retention volume is explained in terms of higher electrostatic interactions for nitrogen. Similarly, the variation in Vm for monoand bivalently exchanged zeolites is in line with the dependence of electrostatic energies observed for various adsorbates. 5.2. Sorption Properties and Cation Locations. As the extra-framework cations are the principle sites for interactions with sorbate molecules, the theoretically calculated heats of sorption for nitrogen are plotted against the cation charge densities of isolated cations in Figure 9. These plots show a linear increase in heat of sorption values with an increase in cation charge density. However, experimentally measured data do not show such a dependence (Figure 7). For example, among monovalent cations, lithium shows abnormally high values for heats of sorption compared to other cations. Furthermore, heats of sorption for zeolites

Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4227

l

Figure 9. Theoretically calculated heats of sorption for N2 for isolated cation as a function of cation charge density.

exchanged with bivalent cations show values lower than that expected by extending the heats of sorption versus cation charge density plot for monovalent cation (Figure 7) corresponding to bivalent cation charge density. These observations reflect that sorption properties are dependent not only on the charge density of the extraframework cations but also on the sites they occupy in a zeolite structure. The possible cation sites identified (Breck, 1974) in a zeolite X are as follows: site I, centrally located within the hexagonal prism linking sodalite cages; site I′, within the sodalite cage above a 6-ring; site II, in a supercage adjacent to the 6-ring outside a sodalite cage; site II′, in a sodalite cage adjacent to the 6-ring; site III, in a supercage in the neighborhood of the 4-ring. These sites are depicted in Figure 1. Cations at sites I, I′, and II′ are inaccessible to adsorbate molecules having diameter more than 2.8 Å. Only sites II and III are accessible to the molecules of N2, O2, and Ar. Thus, the cations present on sites I, I′, and II′ will not have direct interactions with the sorbate molecules. In the following, a correlation of the observed sorption properties is made with the cation locations. 5.2.1. Monovalent and Bivalent Cations. To compare the sorption properties of N2, O2, and Ar in different cation-exchanged zeolites, the data on zeolite samples having the highest cation-exchange level were considered. The highest cation exchange observed in CsX was 70%. This corresponds to 56 cations/unit cell, which is equal to the maximum number of monovalent cations which can be present in the supercages of zeolite X with Si/Al ) 1.2. In other words, unlike other monovalent cations, all the Cs+ ions are present only in the supercages and hence are accessible to sorbate molecules. Thus, as far as the number of cations accessible to N2, O2, and Ar molecules is concerned, CsX70, NaX and, KX-90 are equivalent. Thus the sorption data of CsX-70 can be compared with those of NaX and KX-90. As the larger ionic radius of Cs+ (1.70 Å) precludes its entrance to (sites I and I′) sites in sodalite cages, entry to which is through a 6-ring window (diameter of 2.8 Å), the Cs+ ions are present only in the supercages. For example, it has been reported (Barrer et al., 1968, 1969) that during Cs+/Na+ exchange in zeolite X, 14 Na+ ions move from the supercages to the sodalite cages on the introduction of 56 Cs+ ions/unit cell. The fact that the plots showing the dependence of sorption properties of N2 (Figures 3, 5, and 7) on cation charge density for monovalent and bivalent cationexchanged zeolites are not collinear leads to a question whether the accessibility of the two types of cations to N2 molecules is different. For example, CaX, BaX, and SrX show lower heats of sorption than the values expected from the extrapolation of the plot with respect

to monovalent cations (Figure 7). This indicates that the bivalent cations occupy less accessible sites within the zeolite structure. The data on cation locations (Hseu, 1972; Olson, 1968, 1970; Mortier and Bosmans, 1971; Mortier et al., 1972; Pluth, 1971; Pluth and Smith, 1972; Costenoble et al., 1978; Herden et al., 1987) summarized in Table 4 show that monovalent cations occupy two types of sites, viz., sites II and III, which are accessible to sorbate molecules. However, bivalent cations occupy only one accessible site (site II) in zeolite structure. Between the two accessible sites, i.e., II and III, the cations present in site III, which is on the surface of the large cage residing over the 4-ring, are more accessible. This is because, the free diameter of the 4-ring (1.98 Å) being smaller than that of the 6-ring (3.1 Å), the cations having diameter larger than 1.98 Å are displaced vertically and protrude from the 4-ring into the supercage. Furthermore, the screening effect of oxygen of the ring is less on the cations present in the 4-ring (site III) than those present in the 6-ring (site II). Thus, the cations when present in site III exhibit higher interactions with the sorbate molecules. Furthermore, Na+, K+, and Cs+ among monovalent cations and Ca+2, Sr+2, and Ba+2 among bivalent cations occupy similar sites within the zeolite structure. Compared with other monovalent cations, accessibility of lithium ions to nitrogen molecules is higher even when present at the same sites (sites II and III). 5.2.2. LiNaX Zeolites. The trend in the sorption properties observed for zeolite samples having different lithium contents (Figures 4, 6, and 8) can also be explained in terms of the sites Na+/Li+ cations occupy in zeolite structure. The distribution of Na+ ions (Table 4) is of 4 type I sites, 32 type I′ sites, 31 type II sites, and 8 type III sites, with 6 ions unlocated. The ions which are accessible to sorbate molecules are 31 (site II) and 8 (site III). Lithium ions have been reported (Mortier et al., 1972; Mortier and Bosmans, 1971) to occupy preferentially the sites I′ and II in zeolite X, with the site I position in the hexagonal prism being energetically unfavorable. The Li+ ion, because of its smaller size (0.68 Å), resides almost in the plane of the six-member ring, while the larger Na+ ion (0.95 Å) is held well above the plane in trigonal-pyramidal coordination to the O2 oxygens. Consequently, it is reasonable to assume that the site II lithium ions are less accessible to sorbate molecules than site II sodium ions. Obviously, this would lead to less pronounced sorption properties of the sorbate. However, when all the site II sodium ions are replaced, site III and unlocated 6 Na+ ions will begin to be replaced. On the basis of 7Li-NMR spectra (Herden et al., 1987), it has been shown that for zeolite LiNaX (Na14.3Li67.4(AlO2)81.7(SiO2)110.3), 32 Li+ ions/unit cell are in site I′, 32 Li+ ions are in the site II position, and 4 Li+ ions with 14 Na+ ions/unit cell are in site III in the supercage. NMR lines corresponding to Li+ ions at site III are only observed when Li+ exchange is higher than 64 Li+ ions/unit cell. This clearly shows that site III Na+ ions are replaced with Li+ at nearly 80% Li+ exchange. This means that the increase in the sorption properties of N2 and O2 observed near this exchange level is due to the site III Li+ ion interactions with sorbate molecules. However, the sharp rise in sorption properties at lithium exchange above 90% indicates that, at those Li+ exchange values, the four site I or six unlocated Na+ ions start getting removed. However, as the site I position is energetically unfavorable (Mortier et al., 1972; Mortier and Bosmans, 1971; Herden et al., 1987), the in-coming Li+ ions occupy site III instead of

4228 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996

site I. Thus, there is an increase in the number of accessible site III Li+ ions corresponding to those Na+ ions which are removed from site I/unlocated sites. The abnormally high values of sorption properties observed for LiX-98 compared to NaX, KX-90, and CsX-70 (Figures 3, 5, and 7) can be explained in terms of the availability of these additional site III Li+ ions compared to other monovalent ions. 5.2.3. CaNaX Zeolites. Sorption properties for N2 in calcium-exchanged zeolites (Figures 4, 6, and 8) show a small increase in their values up to approximately 4045% followed by a steady increase with Ca2+ ion exchange. These observations can be explained in terms of occupancy of cation sites by Na+/Ca2+ ions. In the first step, the Ca2+ ions substitute Na+ from the inaccessible sites I and I′. This is an energetically favorable exchange and causes little effect on the sorption characteristics. The total number of Na+ ions at sites I and I′ is 36 ions/unit cell corresponding to 18 Ca2+ ions/unit cell or about 45% Ca2+ exchange. Subsequent exchange will replace site II and site III Na+ ions from the supercages. However, Ca2+ ions are only located at site II (Table 4) which are accessible to sorbate molecules. The cation charge density of Ca2+ being higher, this replacement results in enhanced interactions and hence an increasing trend in the magnitude of sorption properties. A similar distribution of Ca2+ ions in zeolite X has been shown (Jacobs et al., 1973) from IR spectroscopy of CO2 adsorption and heats of adsorption for ethylene (Amelitcheva et al., 197778). Theoretical calculations of Madelung’s potential at different sites (Dempsey, 1969; Dempsey and Olson, 1970) and X-ray diffraction (Pluth and Smith, 1972) of Faujasite zeolite with bivalent cations also support such a cation distribution. Angell and Schaffer (1966) have shown that heat of sorption of CO in NaX zeolite remains unchanged up to 35% calcium exchange, suggesting only the ions in the inaccessible sites were being replaced. With all the Na+ present at sites II and III, 39 Na+ ions/unit cell in the supercages are replaced by Ca2+ ions, further (>90%) Ca2+ exchange replaces 6-7 Na+ ions from unlocated sites, to occupy sites II. The sharp rise in the sorption properties observed at this exchange level indicates that the unlocated 6-7 Na+ ions are present at sites which are less accessible to sorbate molecules compared to site II. Thus, for zeolites having Ca2+ exchange beyond 90%, availability of these additional accessible Ca2+ ions results in a steep rise in sorption properties. 5.2.4. KNaX Zeolites. The heats of sorption data for N2 show (Figure 8) an initial sharp decrease until 36% of K+/Na+ exchange (30 K+ ions/unit cell) followed by a steady decrease. On the other hand, the heats of sorption for oxygen and argon show a marginal increase beyond 36% potassium exchange after an initial decrease. These observations show that, up to 36% K+ exchange, the Na+ from site II of the supercages may be getting substituted with K+ ions. As the charge density of K+ ions is lower than that of Na+, there is a decrease in the electrostatic interaction energy with the sorbate molecules which is reflected in N2 heat of sorption data. Beyond 36% exchange, the K+ ions occupy sites III (Table 4). Due to their larger size, K+ ions present at sites II and III are expected to be more accessible to sorbate molecules compared to Na+ ions. The arrest in the decrease in heats of sorption of N2 beyond 36% K+ exchange may be due to the enhanced interaction with the N2 molecules after this exchange value. An increase in heat of sorption of oxygen and

argon at a higher K+ exchange level (Figure 8) also indicates higher accessibility of K+ ions in site III compared to site I or I′. However, in contrast to nitrogen, the increase in heat of sorption for oxygen and argon is due to the enhanced contributions from dispersion and polarization energy components for zeolite samples having bulkier K+ ions. Similar observations are reported for the propane-KNaX system by Amelitcheva et al. (1977-78). The Henry constant values for Na0.94X, K0.66Na0.26X, K0.80Na0.21X, and K1.00Na0.03X are reported to be 0.206, 0.241, 0.423, and 0.556 mmol g-1 Torr-1, respectively, which clearly show that, for a nonpolar molecule like propane, the dispersion force increases with increasing potassium content of the zeolite. 5.2.5. CsNaX Zeolites. There is a decrease in the heat of sorption value for N2 with Cs+ ion exchange in NaX. The Cs+ ion occupies the supercage sites II and III only. As the charge density of Cs+ is lower than that of the Na+ ion, its electrostatic interactions with the sorbate molecule will be smaller than those of the Na+ ion and hence the observed trend in sorption properties. Water sorption studies (Avgul et al., 1971) on CsX show that site II and site III cations show little difference energetically mainly because the larger size of the Cs+ ion. Heats of sorption of oxygen in different cesiumexchanged zeolite samples show a trend similar to that of KX zeolites. However, in the case of argon, the heat of sorption is higher than that in NaX for all Cs+exchanged samples. This is due to expectedly higher dispersion interactions for Ar-CsX compared with ArNaX or Ar-KX. 5.2.6. Nitrogen Molecule as a Probe for Cation Location. As the heats of sorption in the present study are measured in the Henry region, these essentially reflect the sorbate-sorbent interactions. As discussed earlier, extra-framework cations and the oxygen atoms of the zeolite are the major sites contributing toward sorbate-sorbent interactions. From the calculated heats of sorption data (Table 3), it is observed that predominant contribution toward heat of sorption of nitrogen is from its electrostatic interactions with extraframework cations. On the other hand, for oxygen and argon, dispersion energy due to their interactions with zeolitic oxygens is significant compared to their electrostatic contribution. Consequently, the heat of sorption values for nitrogen will show a higher dependence on the nature and location of extra-framework cations than oxygen and argon, reflecting the possibility of using nitrogen sorption as a probe for cation locations. In fact, the present data for heats of sorption of nitrogen for mono- and bivalent cations clearly show occupancy of bivalent cations at less accessible sites. In fact, crystallographic data (Table 4) show that bivalent cations only occupy site II of the zeolite X. Unlike monovalent cations, these are not located at more accessible sites III. The increase in the heat of sorption for N2 observed near 80% Li+ exchange reflects the occupancy of more accessible site III. Such an occupancy of site III is shown from 7Li NMR spectra (Herden et al., 1987) where NMR lines corresponding to Li+ ions at site III are only observed at this Li+ exchange value. As discussed in the previous sections, similar correlations have been observed for CaNaX, KNaX, and CsNaX zeolites having varying amounts of cations. These observations indicate that nitrogen can be used as a probe for assessing the location of cation sites in a zeolite and also nitrogen is a better probe molecule than an inert gas like argon.

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6. Conclusions The following can be concluded from the present study: There is a strong dependence of sorption properties studied on the nature of extra-framework cations, reflecting thereby that these cations form the principle sites of interactions with sorbate molecules. However, the nonlinear dependence of the sorption properties with the charge density of the extra-framework cations clearly shows that sorbate-sorbent interactions depend not only on the cation charge density but also on the sites these cations occupy in a zeolite structure. Furthermore, cation site occupancy in a zeolite structure depends on its content in the zeolite. Lithium and calcium among alkali and alkaline-earth cations, respectively, show higher interactions toward sorbate molecules in a fully exchanged zeolite. The sorption selectivity for nitrogen from its mixture with oxygen or argon is predominantly because of higher electrostatic interactions of nitrogen molecule with the electric field of the cations. The correlation observed for nitrogen sorption properties and the locations of alkali and alkaline-earth cations particularly lithium and calcium in the zeolite X structure strongly indicates that nitrogen can be used as a probe molecule for assessing the cation sites in a zeolite. Acknowledgment We thank the management of Indian Petrochemicals Corporation Ltd., Vadodara, India, for permission to publish this work and Mr. R. D. Parte for technical assistance for GC measurements. Literature Cited Amelitcheva, T. M.; Bezus, A. G.; Bogomolova, L. L.; Kiselev, A. V.; Shoniya, N. K.; Shobayva, M. A.; Zhadanov, S. P. Adsorption of ethane and ethylene by zeolite MgNaX and CaNaX with different degrees of ion exchange. J. Chem. Soc., Faraday Trans. 1 1977-78, 306. Angell, C. L.; Schaffer, P. C. Infrared spectroscopic investigations of zeolites and adsorbed molecules. J. Phys. Chem. 1966, 70, 1413. Avgul, N. N.; Bezus, A. G.; Dzhigit, O. M. In Molecular Sieve Zeolites, Vol. II; Gould, R. F., Ed.; Advances in Chemistry Series 102; American Chemical Society: Washington, DC, 1971; p 185. Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. Barrer, R. M.; Davies, J. A.; Rees, L. V. C. Thermodynamics and thermochemistry of cation exchange in zeolite Y. J. Inorg. Nucl. Chem. 1968, 30, 3333. Barrer, R. M.; Davies, J. A.; Rees, L. V. C. Comparison of the thermodynamics and thermochemistry of cation exchange in zeolite Y. J. Inorg. Nucl. Chem. 1969, 31, 2599. Breck, D. W. Zeolite Molecular Sieves: Structure Chemistry and Use; Wiley: New York, 1974. Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Adsorption of a nitrogen-oxygen mixture in NaCaA zeolite by elution chromatography. Ind. Eng. Chem. Res. 1993, 32, 548. Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. In Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H. G., Holdrich, W., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1994; Vol. 84, p 1247. Coe, C. G.; Kuznicki, S. M. An improved polyvalent ion exchange adsorbent for air separation. U.S. Patent 4,481,018, 1984. Coe, C. G.; Kuznicki, S. M.; Srinivasan, R.; Jenkins, R. J. In Perspectives in Molecular Sieve Science; Flank, W. H., Whyte,

T. E., Jr., Eds.; ACS Symposium Series 368; American Chemical Society: Washington, DC, 1988; p 478. Costenoble, M. L.; Mortier, W. J.; Uytterhoeven, J. B. Location of cations in synthetic zeolites -X and -Y. J. Chem. Soc., Faraday Trans. 1 1978, 74, 477. Dempsey, E. The calculation of Madelung potentials for faujasite type zeolites. J. Phys. Chem. 1969, 73, 3660. Dempsey, E.; Olson, D. H. Relationships between divalent cation distribution and residual water content in metal cation faujasite type zeolites. J. Phys. Chem. 1970, 74, 305. Dzhigit, O. M.; Kiselev, A. V.; Rachmanova, T. A.; Zhdanov, S. P. Influence of Li+, Na+ and K+ cation concentrations in X and Y zeolites on isotherms and heats of adsorption of propane and water. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2662. Furuyama, S.; Nagato, M. Sorption of argon, oxygen, nitrogen, nitric oxide and carbon monoxide by magnesium, calcium and barium mordenites. J. Phys. Chem. 1984, 88, 1735. Harlfinger, R.; Hoppach, D.; Quaschik, U.; Quitzsch, K. Adsorption of C4 hydrocarbons on X zeolites containing Li+, Na+, K+, Rb+ and Cs+ cations. Zeolites 1983, 3, 123. Herden, H.; Einicke, W.-D.; Scho¨llner, R. Investigation of the arrangement and mobility of Li ions in X and Y-zeolites and the influence of mono- and divalent cations in it. J. Inorg. Chem. 1987, 43, 2538. Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; John Wiley and Sons: New York, 1954. Hseu, T. Crystal Structure of 1,1,4,4-tetramethyl-1,4-diaza-2,5diboracyclohexane and structural studies of some faujasite type zeolites. Ph.D. Thesis, University of Washington, Seattle, WA, 1972. Jacobs, P. A.; Cauwelaert, F. H. V.; Vansant, E. F. Surface probing of synthetic faujasites by adsorption of carbon dioxide. J. Chem. Soc., Faraday Trans. 1 1973, 69, 2130. Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Separation of gases by pressure swing adsorption. Sep. Sci. Technol. 1991, 26, 885. Lee, H.; Stahl, D. E. Oxygen-rich gas from air by pressure swing adsorption process. AIChE Symp. Ser. 1973, 69 (134), 1. Mortier, W. J. In Proceedings of the 6th International Zeolite Conference; Olson, D., and Bisio, A., Eds.; Butterworths: Surrey, U.K., 1982; p 734. Mortier, W. J.; Bosmans, H. J. Locations of univalent cations in synthetic zeolites of the Y and X type with varying silicon to aluminium ratio: Part 1. Hydrated potassium exchanged form. J. Phys. Chem. 1971, 75, 3327. Mortier, W. J.; Bosmans, H. J.; Uytterhoeven, J. B. Locations of univalent cations in synthetic zeolites of the Y and X type with varying silicon to aluminium ratio: Part 2. Dehydrated potassium exchanged form. J. Phys. Chem. 1972, 76, 650. Olson, D. H. X-ray evidence for residual water in calcined divalent cation faujasite type zeolites. J. Phys. Chem. 1968, 72, 1400. Olson, D. H. Reinvestigation of the crystal structure of the zeolite hydrated NaX. J. Phys. Chem. 1970, 74, 2758. Pluth, J. J. Crystal structures of several ion-exchanged forms of faujasite. Ph.D. Thesis, University of Washington, Seattle, WA, 1971. Pluth, J. J.; Smith, J. V. Positions of cations and molecules in zeolites with the faujasite-type framework. VII. Dehydrated calcium-exchanged X. Mater. Res. Bull. 1972, 7, 1311. Pluth, J. J.; Smith, J. V. Positions of cations and molecules in zeolites with the faujasite-type framework. VIII. Hydrated calcium-exchanged X. Mater. Res. Bull. 1973, 8, 459. Reiss, G. Status and development of oxygen generation processes on molecular sieve zeolites. Gas Sep. Purif. 1994, 8, 95.

Received for review March 25, 1996 Revised manuscript received July 23, 1996 Accepted July 30, 1996X IE960168M

X Abstract published in Advance ACS Abstracts, October 1, 1996.