Adsorption of Nitrogen, Oxygen, and Argon in Mono-, Di-, and

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Ind. Eng. Chem. Res. 2005, 44, 6856-6864

Adsorption of Nitrogen, Oxygen, and Argon in Mono-, Di-, and Trivalent Cation-Exchanged Zeolite Mordenite Sunil A. Peter, Jince Sebastian, and Raksh V. Jasra* Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, G. B. Marg Bhavnagar-364002, India

Adsorption of nitrogen, oxygen, and argon is studied in zeolite mordenite (Si/Al ) 5.5) with mono-, di-, and trivalent Na+, Li+, K+, Cs+, Ca2+, Sr2+, Ba2+, La3+, and Ce3+ as extraframework cations. Adsorption capacity, selectivity, and heat of adsorption show a strong dependence on the nature as well as the extent of the extraframework cations. The cation-exchanged mordenite samples show adsorption selectivity toward nitrogen from its mixture with oxygen or argon. The electrostatic interaction between adsorbate molecules and the cations are largely responsible for the observed nitrogen selectivity of the mordenite samples because nitrogen has higher quadrupole moment than oxygen or argon. The relationships between the adsorption properties and charge densities of extraframework cations show that the interaction of adsorbate molecules depends not only on the cation type but also on the positions of these cations in the mordenite structure. Introduction Zeolites A, X, and mordenite are used1,2 as nitrogenselective adsorbents in commercial pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) processes for oxygen enrichment of air. Nitrogen adsorption capacity and selectivity of these adsorbents play a significant role in determining the efficiency/ economy of a PSA/VSA process.3 Therefore, there have been continuous efforts to modify the adsorbents to attain higher adsorbent productivity by enhancing the adsorption capacity and selectivity. It was reported earlier that the extraframework cations in the zeolite structure4,5 are largely responsible for the nitrogen selectivity of the zeolite. Consequently, attempts have been made to increase the number of selective sites in the zeolite by changing the Si/Al ratio6 or using a combination of extraframework cations.7 Jasra et al.8-11 have reported the effects of the nature, number, and location of extraframework cations in zeolites on the sorption of nitrogen, oxygen, and argon in zeolites A and X. They have also studied the effect of presorbed water and carbon dioxide, invariably present in air, on the sorption behavior in zeolites A8,9 and X.11 Tryburce et al.12 have studied the effect of exchangeable cations on the adsorption properties of sodium mordenite and also tried to correlate the changes in pore volume measured by nitrogen adsorption with the locations of exchanged cations. Choudary et al.5 have studied the adsorption of nitrogen, oxygen, and argon on mordenite-type zeolite with different Si/Al ratios using elution gas chromatography, and they have arrived at the conclusion that the higher adsorption capacity for nitrogen is the result of the presence of extraframework sodium ions in the zeolite structure. In our study, we exchanged the extraframework cations of Na-mordenite (Si/Al ) 5.5) with monovalent, divalent, and trivalent cations, such as Li+, K+, Cs+, Ca2+, Ba2+, La3+, and Ce3+, to understand the effect of these cations on the adsorption * To whom correspondence should be addressed. E-Mail: [email protected].

capacity, selectivity, and heat of adsorption of nitrogen, oxygen, and argon. We also fitted the pure component isothermal data on Langmuir, Freundlich, and Langmuir-Freundlich isotherm models. Experimental Section Materials. The sodium form of the mordenite, NaMOR (commercial name ZM-060, obtained from M/s Zeocat, France) was used as the starting material in the powder form. The overall and framework Si/Al ratio of this mordenite is 5.5 and corresponds to the chemical composition Na7.4Al7.4Si40O96 ‚27H2O. For the cation exchange, we used the chloride salts of different cations viz, lithium, potassium, cesium, calcium, strontium, barium (s. d. fine- chem., Mumbai, India), lanthanum, and cerium (Sigma-Aldrich). Ultrahigh-purity nitrogen, oxygen, and argon (Hydrogas India Pvt. Ltd., Bombay, India) were used in all adsorption measurements. Cation Exchange and Characterization of Mordenite. Ion exchange was carried out by repeatedly treating the mordenite powder at 353 K with a 0.1 M salt solution (solid-to-liquid ratio 1:80) followed by filtration and washing with a copious amount of hot distilled water until the sample is free from the chloride ion as determined with an AgNO3 solution. The samples were dried overnight at 353 K in an air oven. The degree of cation exchange of these samples was determined by ICP-AES analysis (Perkin-Elmer Instruments, Optima 2000DV). The following terminology is used to describe the ion-exchanged samples: the first letters show the exchanged cation and the numbers at end show the percentage of the sodium cation present in NaMOR exchanged with this cation (e.g., LiNaMOR-45 means 45% of the sodium cation present in the NaMOR is exchanged with the lithium cation). Adsorption Measurements. The isothermal adsorption of nitrogen, oxygen, and argon on the ion-exchanged mordenites was conducted using a static volumetric adsorption system (Micromeritics Instrument Corporation, Model ASAP 2010) at 288.2 and 303.0 K. The adsorption temperatures were maintained constant

10.1021/ie050128v CCC: $30.25 © 2005 American Chemical Society Published on Web 07/21/2005

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6857

Figure 1. SEM image of NaMOR.

((0.1 K) by circulating water from a constant-temperature bath (Julabo F25, Germany). Prior to the adsorption measurement, the samples were activated in situ by heating to 673 K at a rate of 1 K min-1 under vacuum (5 × 10-3 mmHg) for 8 h. The adsorption isotherm data was fitted in the virial equation

ln(P/q) ) A + Bq + Cq2 + ......

q bP ) qm (1 + bP)

(1)

where q is the volume of gas adsorbed per unit weight of the adsorbent, P is the equilibrium pressure, and A, B, and C are the virial coefficients. Henry’s constant, K, was calculated as

K ) exp(-A)

estimated from the propagation of error method were 0.5, 0.4, and 0.4% respectively. The measured adsorption isotherms were fitted into three isotherm models, namely, the Langmuir, Freundlich, and Langmuir- Freundlich equations. The Langmuir isotherm is given by the equation2

(2)

where q is the amount of gas adsorbed on the adsorbent at equilibrium, qm is the monolayer or saturated amount adsorbed, b is the Langmuir constant, and P is the equilibrium pressure of the adsorbate. The quantity qmb equals K. The Freundlich isotherm is given by the equation21

The selectivity of a gas A over gas B in Henry’s law region was calculated as

KA KB

RA/B )

(3)

q ) KPn

[

∂ ln P ∂(1/T)

]

(4)

θ

where R is the gas constant, P is the equilibrium pressure at the particular coverage θ, and T is the temperature in degrees Kelvin. In the Henry’s law region, we could calculate the heat of adsorption as

-∆H ) R

[

]

∂ ln K ∂(1/T)

(5)

where K is the Henry constant. The errors in the Henry constant, adsorption selectivity, and heat of adsorption

(7)

where K and n are fitting parameters. The Langmuir-Freundlich isotherm is given by the equation2

The heats of adsorption data in various cationexchanged mordenites were calculated from adsorption isotherms by the Clausius-Clapeyron equation4,5,13

-∆H ) R

(6)

bPn q ) qm (1 + bPn)

(8)

Results The crystals of sodium mordenite, used as the starting material, are fiber shaped which is clear from the SEM image (LEO 1430 VP Scanning Electron Microscope) shown in Figure 1. X-ray powder diffraction patterns of the cationexchanged samples were collected using the Philips X’pert MPD system at 2θ values between 5 and 65. The structure of the mordenite is retained after cation exchange because the X-ray diffraction patterns of the samples showed the typical peaks of mordenite at 2θ values of 22.3, 25.7, 26.3, 27.9, and 30.8, as shown in

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Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 Table 1. Nitrogen, Oxygen, and Argon Sorption Capacities on Cation-Exchanged Mordenites at 845 mm of Hg and 303.0 K volume adsorbed (cc g-1)

Figure 2. XRD patterns for different-cation exchanged mordenites with high exchanged cation content

Figure 3. (a) Equilibrium adsorption isotherms for N2, O2, and Ar on NaMOR at 303.0 K. (b) Nitrogen adsorption isotherms on different cation-exchanged mordenites at 303.0 K with high exchanged cation content.

Figure 2. The XRD patterns are of mordenite samples with a high exchanged cation content. Adsorption Capacity. Figure 3a shows the adsorption isotherms of nitrogen, oxygen, and argon on NaMOR at 303.0 K. The nitrogen adsorption capacity on NaMOR is high compared to that of oxygen and argon. Oxygen

sample

N2

O2

Ar

NaMOR LiNaMOR-45 LiNaMOR-63 LiNaMOR-76 LiNaMOR-88 KNaMOR-73 KNaMOR-97 CsNaMOR-86 CsNaMOR-95 CsNaMOR-99 CaNaMOR-64 CaNaMOR-78 CaNaMOR-87 SrNaMOR- 89 BaNaMOR-56 BaNaMOR-78 BaNaMOR-93 LaNaMOR-38 LaNaMOR-59 LaNaMOR-76 CeNaMOR-49 CeNaMOR-84 CeNaMOR-98

17.1 17.2 21.1 22.6 21.1 7.0 6.7 3.3 3.2 3.2 19.7 17.1 14.5 20.1 17.4 17.6 18.8 14.7 14.3 14.1 16.5 14.7 12.4

5.8 6.4 7.2 7.2 7.1 4.1 3.7 2.6 2.6 2.5 7.2 6.8 6.2 8.3 7.7 9.0 9.5 5.5 5.5 5.5 6.2 5.7 5.5

6.0 6.5 6.9 6.8 6.7 4.1 4.0 2.8 2.8 2.8 6.5 6.3 5.5 8.0 8.3 9.3 9.6 5.6 5.8 5.6 6.3 5.7 5.4

and argon show similar adsorption capacities. The isothermal adsorption of nitrogen on different cationexchanged mordenites at 303.0 K is shown in Figure 3b. The sorption capacities of different cation-exchanged mordenites at different cation-exchange levels determined from adsorption isotherms, are given in Table 1. From the table, it is clear that lithium-exchanged mordenite has the highest sorption capacity for nitrogen compared to the other cation-exchanged mordenites. The adsorption capacity for nitrogen in lithiumexchanged mordenite increases as the cation-exchange level increases, but in the case of the 88% lithiumexchanged sample, it shows some decrease in the equilibrium nitrogen adsorption capacity compared to the 76% lithium-exchanged mordenite. The nitrogen sorption capacity of CaNaMOR increases first with calcium exchange in NaMOR and then decreases when the Ca2+-exchange level is higher than 78%. This is in contrast to the dependence of the nitrogen adsorption capacity on Li+/Ca2+ exchange in zeolites A and X, where the capacity increases with an increase in percentage of exchange and an exponential rise in adsorption capacity is observed in zeolite X at higher Li+/Ca2+-exchange levels.9,10 In the case of barium exchange, the nitrogen adsorption capacity decreases first, compared to that of sodium, and then increases with the cation-exchange level. Strontiumexchanged mordenite also shows a higher nitrogen adsorption capacity than the sodium form. The oxygen and argon sorption capacity of barium-exchanged mordenite is higher than that of all of the other cationexchanged mordenites. Figure 4 shows the relationship between the cation size and the sorption capacities of nitrogen at 303.0 K in different cation-exchanged mordenites and zeolite X. The nitrogen sorption capacities are for the samples with highest exchanged cation content. From the figure we can see that, for monovalent cations the adsorption capacity for nitrogen in mordenite decreases sharply with an increase in the size of cations. In the case of

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6859

Figure 4. Relationship between the N2 sorption capacity and the size of extraframework cation in (a) mordenite and (b) zeolite X10 at 303.0 K and 845 mm of Hg. Table 2. Henry Constants from the Virial Equation and Selectivity at 303.0 K Henry constant, Ka

selectivityb

sample

N2

O2

Ar

RN2/O2

RN2/Ar

RO2/Ar

NaMOR LiNaMOR-45 LiNaMOR-63 LiNaMOR-76 LiNaMOR-88 KNaMOR-73 KNaMOR-97 CsNaMOR-86 CsNaMOR-95 CsNaMOR-99 CaNaMOR-64 CaNaMOR-78 CaNaMOR-87 SrNaMOR-89 BaNaMOR-56 BaNaMOR-78 BaNaMOR-93 LaNaMOR-38 LaNaMOR-59 LaNaMOR-76 CeNaMOR-49 CeNaMOR-84 CeNaMOR-98

33.3 35.8 53.8 66.2 61.7 10.5 9.5 4.6 4.6 4.5 94.1 85.3 54.3 119.1 45.9 59.3 72.0 30.0 31.2 30.8 38.2 33.4 28.7

8.0 8.6 10.3 9.9 10.0 5.3 4.7 3.7 3.4 3.4 12.4 11.7 10.4 14.1 12.8 14.5 15.9 7.2 7.5 7.2 9.2 8.2 7.4

8.0 8.7 9.3 8.9 8.6 5.3 5.1 3.8 3.9 3.7 11.2 10.5 9.1 11.4 13.2 15.0 16.2 7.2 7.8 7.8 9.1 7.6 7.1

4.1 4.2 5.2 6.8 6.2 2.0 2.0 1.3 1.3 1.3 7.6 7.3 5.3 8.5 3.6 4.1 4.5 4.2 4.2 4.3 4.1 4.1 3.9

4.2 4.1 5.7 7.4 7.3 2.0 1.8 1.2 1.1 1.2 8.4 8.1 5.9 10.4 3.5 4.0 4.4 4.1 4.0 4.0 4.2 4.4 4.1

1.0 1.0 1.1 1.1 1.2 1.0 0.9 0.9 0.9 0.9 1.1 1.1 1.1 1.2 1.0 1.0 1.0 1.0 1.0 0.9 1.0 1.1 1.0

a Calcultated from the virial equation (×10-3 cc g-1 mm of Hg-1). b Calculated using the Henry constant.

zeolite X, the nitrogen capacity decreases exponentially with an increase in the size of monovalent cations. For divalent cations, the curve shows highest nitrogen capacity for the strontium ion, and for trivalent cations, the nitrogen sorption capacities increase with cation size. But, in the case of zeolite X, the nitrogen capacity decrease sharply with the size of the bivalent cation. These observations show that, in addition to the number and nature of the cations, the positions of the exchanged cations also play an important role in the adsorption of nitrogen on mordenite. Adsorption Selectivity. Henry’s constants and the selectivity of nitrogen over oxygen and argon on different cation-exchanged mordenites at different cationexchange levels are given in Table 2. The strontiumexchanged mordenite sample has the highest value for the Henry constant. The dependence of the selectivity of nitrogen over oxygen on charge density is shown in Figure 5. For monovalent cations, the selectivity increases with charge density. In the case of divalent cations, the selectivity also increases with charge density compared to that of sodium mordenite, and strontium-exchanged mordenite shows highest selectivity in the Henry’s law region. For trivalent cations, the selectivity decreases with charge density. These obser-

Figure 5. Sorption selectivity, N2/O2 at higher exchanged cation content as a function of the extraframework cation charge density. Table 3. Heat of Adsorption of N2, O2, and Ar in the Henry’s Law Region heat of adsorptiona N2 O2 Ar

sample NaMOR LiNaMOR-45 LiNaMOR-63 LiNaMOR-76 LiNaMOR-88 KNaMOR-73 KNaMOR-97 CsNaMOR-86 CsNaMOR-95 CsNaMOR-99 CaNaMOR-64 CaNaMOR-78 CaNaMOR-87 SrNaMOR-89 BaNaMOR-56 BaNaMOR-78 BaNaMOR-93 LaNaMOR-38 LaNaMOR-59 LaNaMOR-76 CeNaMOR-49 CeNaMOR-84 CeNaMOR-98 a

In kJ

mol-1. b

26.7 26.1 28.8 30.8 34.0 20.8 22.9 17.3 18.7 19.3 30.8 32.4 37.9 33.0 26.2 29.5 30.4 29.0 25.1 24.5 29.2 26.9 31.2 In C

m-1

18.5 21.9 15.8 19.1 19.9 16.2 22.4 18.3 18.1 18.0 14.5 16.3 23.6 21.7 26.3 25.3 24.5 22.0 19.1 21.5 24.4 25.6 26.3 ×

18.3 18.4 17.2 20.9 22.1 19.7 21.5 18.8 18.7 18.4 18.0 18.0 23.2 23.6 26.0 23.6 23.2 20.3 17.4 18.6 24.2 25.0 27.9

charge densityb 16.9

23.6 12.1 9.5 32.4 28.4 23.7 45.3 46.7

10-10.

vations also show that, apart from charge density, the accessible locations of the extraframework cations also play an important role in the selectivity of nitrogen over oxygen and argon. Heat of Adsorption. Table 3 shows the heat of adsorption for nitrogen, oxygen, and argon on various cation-exchanged mordenites in the Henry’s law region and different cation-exchange levels. Figure 6 shows the heat of adsorption of nitrogen on cation-exchanged mordenites at different cation-exchange levels. In the case of monovalent cations, the heat of adsorption for

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Figure 6. Dependence of the heat of adsorption of N2 on the extent of the extraframework cations. Figure 8. Heat of sorption of N2 as a function of the extraframework cation charge density.

Figure 7. Heat of adsorption of N2 as a function of the adsorption capacity on different cation-exchanged mordenites.

nitrogen decreases with respect to percentage of cation exchange, except for lithium. This may be the result of the high charge density of lithium compared to sodium, which in turn causes the strong interaction of the nitrogen molecule with extraframework lithium ion. In the divalent cation exchange, the heat of adsorption for nitrogen increases compared to that of the sodium form, and the calcium-exchanged mordenite shows the highest heat of adsorption for nitrogen among the other mordenites. In the trivalent cation exchange, the lanthanumexchanged mordenite shows a decrease in the heat of adsorption for nitrogen, and the cerium-exchanged mordenite shows some increase in heat of adsorption with the exchange level. Figure 7 shows the heat of adsorption of nitrogen at different coverages on various cation-exchanged mordenites. From the figure, it is clear that the initial heat of adsorption of nitrogen is higher on calcium-exchanged mordenite and lower on cesiumexchanged mordenite. Figure 8 shows the charge density versus the heat of adsorption of nitrogen on various cation-exchanged mordenites. In the figure, we can see that the heat of adsorption increases with charge density for monovalent, divalent, and trivalent cations at different slopes. Discussion

Figure 9. (a) Mordenite framework viewed along [00l]. (b) Framework unit cell structure of mordenite.

Interaction Energies and the Sorption Properties. The pore system of mordenite, depicted in Figure 9a, consists of main elliptical channels with dimensions of 6.5 × 7.0 Å, oriented along the c axis which are connected by tortuous pores of 3.4 × 4.8 Å oriented in the b direction.14 It also has side pockets in the c direction with dimensions 2.6 × 5.7 of Å formed by elliptical 8 rings. The framework unit cell structure of mordenite is shown in Figure 9b. The roman numerals

denote the positions occupied by the extraframework cations as suggested by Mortier et al.14 In a zeolite framework, SiO2 and AlO2 tetrahedra are connected by sharing oxygen atoms. Al3+ and Si4+ ions are buried in the tetrahedra of oxygen atoms and are not directly exposed to sorbate molecules. Thus, the main interactions of the adsorbate molecules in a zeolite structure are through lattice oxygen atoms and the

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6861 Table 4. Comparison of Heat of Adsorption of N2, O2, and Ar on Zeolites A, X, and Mordenite

Table 5. Different Cation Site Population in Mordenite Zeolite

heat of adsorption, -∆Ha

site occupancies

cation

zeolite

N2

O2

Ar

sample

I

II

III

IV

VI

ref

Na+

MOR Xb Ac MOR Xb MOR Xb MOR Xb MOR Xb Ad MOR X MOR X

26.7 18.5 19.6 34.0 27.0 22.9 15.4 19.3 14.1 37.9 28.8 25.1 30.4 21.4 33.0 25.2

18.5 12.0 13.8 19.9 13.4 22.4 11.9 18.0 12.2 23.6 15.3 14.1 24.5 13.5 21.7 13.9

18.3 11.2 11.9 22.1 12.4 21.5 11.8 18.4 12.3 23.2 13.5 23.2 12.9 23.6 13.5

Na7.4Al7.4Si40.6O96 Na8Al8Si40O96 K8Al8Si40O96 K8Al8Si40O96 Cs8Al8Si40O96 Ca3.3Al7.8Si40.2O96 Ca8Al8Si40O96 (ideal) Ba8Al8Si40O96

4.5 Na 4 Na 4 Cs 1.8 Ca 2 Ca -

4K 3.3 K 2 Ba

0.5 Ca 2/3 Ca 1 Ba

2.2 Na 3 Na 3K 3K 2 Cs 0.5 Ca 2/3 Ca 1 Ba

0.7 Na 1 Na 1K 0.9 K 2 Cs 0.6 Ca 2/3 Ca -

18 12 12 20 12 20 20 12

Li+ K+ Cs+ Ca2+ Ba2+ Sr2+ a

In kJ mol-1. b Ref 10. c Ref 5.

d

Ref 8.

extraframework cations. The various interactions contributing toward the total energy of physical adsorption, Φ, include dispersion, ΦD, polarization, ΦP, field-dipole interactions, ΦFµ, field-quadrupole interactions, ΦFQ, close-range interactions, ΦR, and sorbate-sorbate interactions, ΦSP.

Φ ) -(ΦD - ΦR) - ΦP - ΦFµ - ΦFQ - ΦSP

(9)

At low sorbate coverage, the contributions from fielddipole and sorbate-sorbate interactions are negligible in the case of nitrogen, oxygen, and argon because they are nonpolar, and the total energy of physical adsorption is given by

Φ ) - (ΦD - ΦR) - ΦP - ΦFQ

(10)

Nitrogen, because of its higher quadrupole moment (5.0035 × 10-40 C m3) compared to those of oxygen (0.800 × 10-40 C m3) and argon (0.0), interacts strongly with the extraframework cations in the mordenite, which is evident from the heat of adsorption data given in the Table 3. An argon molecule with a zero quadrupole moment and a size similar to those of nitrogen and oxygen can be presumed15 to mainly interact through dispersion and polarization forces between framework oxygen atoms and the sorbate molecules. Therefore, the difference in the heat of adsorption of argon from that of nitrogen or oxygen (Table 4) will give an idea of the contribution of electrostatic forces to the total interaction energy. Some interesting observations can be made from the Table 4, which gives the comparison of heat of adsorption values of nitrogen, oxygen, and argon on different cation-exchanged mordenites with those on cationexchanged zeolites X and A. The values given in the table are for the zeolite samples with higher cation content. The heat of adsorption values for the three sorbates in the mordenite samples are substantially higher than those of zeolites X and A. The comparison of heat of adsorption values for argon on different cationexchanged mordenites with those on cation-exchanged zeolites A and X reveal that the contributions of dispersion and polarization interactions are also higher for cation-exchanged mordenite. These observations show that adsorbate molecules have higher interactions with extraframework cations as well as lattice oxygen atoms

in the mordenite structure. Hayhurst and Sefcik16 have proposed that in the sodium form of mordenite with a low Si/Al ratio (Si/Al < 5), the side pockets are accessible for nitrogen sorption from the 12-membered ring main channel. However, the shape of the side pocket is such that the adsorbed nitrogen molecules have restricted rotational freedom. Such restriction results in a higher quadrupole-cation field gradient interaction leading to a higher heat of sorption. A similar rotational restriction of adsorbed nitrogen, even in the main mordenite channel, has been proposed by Furuyamma and Nagato.17 One can expect similar rotational restrictions for other sorbate molecules, such as oxygen. On the other hand, adsorbed molecules will not have these rotational restrictions in zeolites X and A as the molecules are adsorbed in the main cavity which have diameters of 13 and 11.4 Å, respectively. The nonspecific interaction of the oxygen atoms of the mordenite with sorbate molecules is expected to be higher as the channels of mordenite are smaller (6.5 and 7.0 Å for the minor and major diameters, respectively) compared to the larger cavities of zeolites X and A. As a result, the adsorbed molecules in mordenite will have higher electrostatic forces as well as nonspecific interactions with zeolitic cations, framework oxygen atoms, or both thereby explaining the observed values for the heat of sorption. Sorption Properties and Cation Locations. Because the extraframework cations play an important role in the adsorption of sorbate molecules on the zeolite surface, it is better to consider the dependence of the charge densities of these cations on the interactions of the sorbate molecules with the zeolite structure. If the sorption properties, such as heat of adsorption, adsorption selectivity, etc., are solely dependent on the charge density, then the plots of the selectivity and heat of sorption versus charge density would show a single linear relationship. But from Figures 5 and 8, it is clear that the trend is not like that. This means that the sorption properties are not only dependent on charge density but also on the location of the cations. From Figure 4, it is clear that for monovalent cations the nitrogen sorption capacity decreases with an increase in cation size, and for divalent and trivalent cations, the nitrogen sorption capacity increases slightly with cation size. This may be the result of the reallocation of cations in the mordenite structure during cation exchange. In the following section, a correlation between the observed sorption properties and the cation locations is made. Monovalent cations. Table 5 shows the probable cationic sites in mordenite structure for different cations. Devautour et al.18 reported the degree of sodium ion occupation for each site in sodium mordenite (Si/Al ) 5.5) obtained via the thermally stimulated depolar-

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ization current (TSDC) method: site I contains 4.5 sodium ions, site IV contains 2.2 sodium atoms, and site VI contains 0.7 sodium atoms. They have also determined,19 using the same method, that the lithium ions, under exchange, substitute for the sodium ions in the same positions. But the size of lithium ion is small compared to that of sodium, therefore there is an increase in the pore volume in mordenite structure, which is one of the reasons for the increase in the sorption capacity for nitrogen in lithium-exchanged mordenite. In potassium-exchanged mordenite, the cationic sites are II, IV, and VI. The simultaneous presence of two potassium ions (size 1.33 Å) at each secondary channel entrance (sites II and IV) makes them inaccessible to sorbate molecules. This can be observed from the decrease in sorption capacities for nitrogen, oxygen, and argon on potassium-exchanged mordenite. In cesiumexchanged mordenite, the ions are located at sites I, IV, and VI. Because of the large size of cesium ion (1.69 Å), the cesium ions situated at the main channel cause a decrease in pore volume inside the main channel. This is confirmed by the sharp decrease in the sorption capacities for nitrogen, oxygen, and argon on cesiumexchanged mordenite. Bivalent and Trivalent Cations. The site occupancies of bivalent cations, viz calcium and barium, are shown in Table 5. In the ideal case of calcium exchange in mordenite, the calcium ions locations are as follows: two ions are at site I and remaining two ions are equally distributed at sites III, IV, and VI. In these, the ions at site III are inaccessible to sorbate molecules, which may be the reason for the decrease in sorption capacity for nitrogen with the increase in cation exchange. In the case of barium-exchanged mordenite, the cations occupy sites II, III, and IV. This means that, during barium exchange, the sodium ions present at site I are removed and there will be some increase in the pore volume inside the side pockets. The observed increase in the sorption capacities for oxygen and argon on bariumexchanged mordenite can be explained by this phenomenon. The strontium ions may occupy sites II, III, and IV, as in the case of barium, because strontiumexchanged mordenite also shows increased nitrogen adsorption capacity similar to that of barium-exchanged mordenite. There is not much data available for the cation locations in mordenite exchanged with trivalent cations, viz lanthanum and cerium. But the observed decrease in the sorption capacities for nitrogen on these mordenites suggest that the cations are located at less accessible sites such as I, II, or III. Isothermal Models and Fittings. The adsorption isotherms of nitrogen on mordenite follow the type I isotherm,2,21 and those of oxygen and argon are almost linear except for the barium- and strontium-exchanged mordenites, which have higher adsorption capacities for oxygen and argon compared to other cation-exchanged mordenites. In the type I isotherm, the amount of gas adsorbed increases with increasing pressure and then saturates at about monolayer coverage. The Langmuir parameters for the nitrogen, oxygen, and argon adsorption isotherms are given in Table 6-8. Tables 9-11 show the Freundlich parameters for the nitrogen, oxygen, and argon adsorption isotherms. Tables 12-14 shows the Langmuir-Freundlich parameters for the nitrogen, oxygen, and argon adsorption isotherms. The values given in the tables are for mordenites with higher exchanged cation content. The Figure 10 shows the

Table 6. Langmuir Parameters for N2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K

303.0 K

sample

Ka

bb

variance

Ka

bb

variance

NaMOR LiMOR KMOR CsMOR CaMOR SrMOR BaMOR LaMOR CeMOR

42.900 75.092 11.723 5.247 55.968 96.317 75.118 34.761 32.690

0.932 1.815 0.316 0.236 1.980 3.098 2.381 0.947 0.940

0.002 0.089 0.000 0.000 0.145 0.283 0.109 0.010 0.014

25.255 41.993 7.475 3.485 29.962 61.377 44.927 21.540 17.921

0.584 1.107 0.220 0.191 1.196 2.229 1.531 0.638 0.552

0.001 0.017 0.000 0.000 0.035 0.131 0.035 0.003 0.001

a

In cc g-1 atm-1. b In atm-1.

Table 7. Langmuir Parameters for O2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K sample

Ka

bb


8.875 11.232 5.892 3.806 11.557 15.895 19.500 8.523 9.062

0.144 0.214 0.122 0.140 0.379 0.524 0.585 0.174 0.184

a

303.0 K variance

Ka

bb

variance

0.000 0.000 0.000 0.000 0.002 0.002 0.001 0.000 0.000

6.045 7.456 3.719 2.602 7.327 10.686 12.029 5.613 5.609

0.148 0.150 0.115 0.126 0.283 0.402 0.378 0.127 0.133

0.000 0.000 0.000 0.000 0.001 0.002 0.000 0.000 0.000


Table 8. Langmuir Parameters for Ar Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K

303.0 K

sample

Ka

bb

variance

Ka

bb

variance


8.836 10.590 6.123 4.192 10.102 14.244 19.419 8.758 8.687

0.137 0.187 0.140 0.147 0.323 0.383 0.612 0.176 0.180

0.000 0.000 0.000 0.000 0.001 0.001 0.002 0.000 0.000

6.053 6.797 4.035 2.877 6.003 9.141 12.326 5.763 5.447

0.111 0.113 0.111 0.123 0.197 0.255 0.394 0.126 0.109

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

a


Table 9. Freundlich Parameters for N2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K sample

Ka

n


22.470 27.237 8.938 4.254 19.188 24.263 22.812 18.056 17.035

0.672 0.565 0.851 0.884 0.544 0.468 0.515 0.684 0.685

a

303.0 K variance

Ka

n

variance

0.238 0.503 0.008 0.002 0.144 0.480 0.439 0.118 0.089

16.125 20.195 6.141 2.932 13.825 19.479 18.071 13.242 11.612

0.758 0.656 0.890 0.903 0.638 0.525 0.596 0.751 0.775

0.060 0.176 0.003 0.001 0.048 0.210 0.175 0.036 0.026

In cc g-1 atm-n.

isotherms of the experimental data and the isotherm model data for the adsorption of nitrogen on lithiumexchanged mordenite at 288.2 K. The Freundlich isotherm fails to fit exactly with the experimental data in all cases of nitrogen adsorption. The Langmuir isotherm gives a somewhat good fit, but the Langmuir-

Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6863 Table 10. Freundlich Parameters for O2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K

303.0 K

288.2 K

sample

Ka

n

variance

Ka

n

variance


7.765 9.269 5.255 3.346 8.405 10.488 12.379 7.274 7.665

0.917 0.890 0.934 0.926 0.824 0.782 0.767 0.908 0.903

0.002 0.003 0.001 0.000 0.003 0.016 0.031 0.002 0.001

5.279 6.490 3.339 2.314 5.722 7.643 8.762 4.984 4.957

0.917 0.919 0.939 0.933 0.861 0.813 0.828 0.931 0.928

0.001 0.001 0.000 0.000 0.001 0.004 0.009 0.001 0.001

a

Table 14. Langmuir-Freundlich Parameters for Ar Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K

In cc g-1 atm-n.

sample

Ka

n

b

NaMOR 8.918 1.006 0.147 LiMOR 10.536 0.997 0.181 KMOR 6.218 1.010 0.158 CsMOR 4.263 1.011 0.167 CaMOR 8.974 0.929 0.174 SrMOR 13.518 0.969 0.312 BaMOR 17.932 0.954 0.488 LaMOR 8.662 0.993 0.163 CeMOR 8.558 0.991 0.162 a

303.0 K variance

Ka

n

bb

variance

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

6.076 7.090 4.039 2.963 5.844 9.052 11.952 5.655 5.686

1.003 1.027 1.001 1.019 0.983 0.993 0.982 0.988 1.028

0.115 0.161 0.112 0.157 0.166 0.243 0.351 0.105 0.157

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

In cc g-1 atm-n. b In atm-n.

Table 11. Freundlich Parameters for Ar Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K sample

Ka

n


7.783 8.935 5.376 3.659 7.656 10.337 12.125 7.461 7.375

0.921 0.903 0.925 0.922 0.845 0.826 0.759 0.908 0.906

a

303.0 K variance

Ka

n

variance

0.002 0.003 0.001 0.000 0.002 0.011 0.031 0.002 0.002

5.459 6.113 3.636 2.564 5.021 7.302 8.878 5.122 4.919

0.936 0.940 0.939 0.934 0.898 0.875 0.823 0.931 0.942

0.001 0.001 0.000 0.000 0.001 0.004 0.010 0.000 0.001

In cc g-1 atm-n.

Table 12. Langmuir-Freundlich Parameters for N2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K

303.0 K

sample

Ka

n

bb

variance

K

n

bb

variance


40.838 56.322 11.664 5.395 33.083 53.331 51.242 30.315 27.445

0.971 0.858 0.997 1.017 0.751 0.739 0.820 0.925 0.905

0.838 1.100 0.309 0.271 0.744 1.240 1.288 0.694 0.625

0.001 0.006 0.000 0.000 0.002 0.012 0.008 0.001 0.001

24.447 35.671 7.665 3.591 21.232 37.359 34.346 19.513 16.798

0.980 0.913 1.016 1.019 0.823 0.768 0.864 0.944 0.962

0.532 0.785 0.251 0.227 0.548 0.946 0.926 0.482 0.454

0.000 0.002 0.000 0.000 0.001 0.004 0.003 0.000 0.000

a


Table 13. Langmuir-Freundlich Parameters for O2 Adsorption Isotherms on Different Cation-Exchanged Mordenites at 288.2 and 303.0 K 288.2 K sample

Ka


8.861 10.866 5.955 3.819 10.040 14.402 18.145 8.383 8.598

a

n

bb

0.999 0.979 1.007 1.002 0.917 0.943 0.958 0.989 0.967

0.142 0.174 0.134 0.143 0.197 0.379 0.474 0.154 0.123

303.0 K variance

Ka

n

bb

variance

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

5.912 7.345 3.875 2.662 6.729 9.369 11.594 5.642 5.543

0.985 0.991 1.026 1.015 0.948 0.923 0.978 1.003 0.993

0.123 0.133 0.162 0.152 0.178 0.230 0.328 0.133 0.119

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000


Freundlich isotherm gives an exact fit with the experimental data. This can be checked from the variance values. If a model fits exactly with the experimental data, then the variance will be zero. The adsorption isotherms of oxygen and argon are almost linear except

Figure 10. Langmuir, Freundlich, and Langmuir-Freundlich isotherm fittings for nitrogen on lithium-exchanged mordenite at 288.2 K.

for the barium- and strontium-exchanged mordenites. The Langmuir, Freundlich, and Langmuir-Freundlich adsorption isotherm models fit well with the experimental data for the oxygen and argon adsorption isotherms. Conclusion The lithium-exchanged mordenite shows a higher adsorption capacity for nitrogen than the other cationexchanged mordenites. The strontium-exchanged mordenite has the highest value for the Henry constant. The strontium-exchanged mordenite also shows a higher selectivity for nitrogen over oxygen and argon in the Henry’s law region. There is a strong dependence of sorption properties studied on the nature of extraframework cations, thereby reflecting that these cations form the principle sites of interactions with the sorbate molecules. However, the relationships between the adsorption properties and the charge densities of the extraframework cations show that the adsorbate-adsorbent interactions depend not only on the cation charge density but also on the sites these cations occupy in the mordenite structure. The observed high heat of sorption in the cationexchanged mordenites compared to that in cationexchanged zeolites X and A is the result of the rotational restrictions of adsorbate molecules inside the smaller cavities of mordenite, compared to the cavities of zeolites X and A.

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Among the adsorption isotherm models studied, the Langmuirn-Freundlich model fits all of the adsorption isotherm data exactly. Acknowledgment We thank CSIR, New Delhi, for financial support in the form of a research fellowship. We also thank to Dr. P. K. Ghosh, Director of CSMCRI, for his support. Literature Cited (1) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Separation of gases by pressure swing adsorption. Sep. Sci. Technol. 1991, 26, 885. (2) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: London, 1997. (3) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; Wiley-VCH: New York, 1994. (4) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978. (5) Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Sorption of nitrogen, oxygen and argon in mordenite type zeolites. Indian J. Chem. 1999, A38, 34. (6) Pamba, M.; Maurin, G.; Devautour, S.; Vanderschueren, J.; Giuntini, J. C.; Di Renzo, F.; Hamidi, F. Influence of framework Si/Al ratio on the Na+/mordenite interaction energy. Phys. Chem. Chem. Phys. 2000, 2, 2027 (7) Coe, C. G.; Kuznicki, S. M. An improved polyvalent ion exchanged adsorbent for air separation. U.S. Patent 4,481,018, 1984. (8) Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Adsorption of nitrogen-oxygen mixture in NaCaA by elution chromatography. Ind. Eng. Chem. Res. 1993, 32, 548. (9) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Effect of presorbed water and temperature on adsorption of nitrogen and oxygen in NaCaA and NaMgA zeolites. Indian J. Chem. 1995, 34A, 15. (10) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Correlation of sorption properties of nitrogen, oxygen and argon with cation locations in cation exchanged zeolite X. Ind. Eng. Chem. Res. 1996, 35, 4221.

(11) 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: Amsterdam, 1994; Vol. 84, p 1247. (12) Tyburce, B.; Kappenstein, C.; Cartraud, P.; Gaenier, E. Effects of exchangeable cations on the adsorption properties of Na+ mordenite. J. Chem. Soc., Faraday Trans. 1991, 87, 2849. (13) Furuyama, S.; Sato, K. Sorption of argon, oxygen, nitrogen, nitric oxide and carbon monoxide by lithium, sodium, potassium and cesium mordenites. J. Phys. Chem. 1982, 86, 2498. (14) Mortier, W. J.; Pluth, J. J.; Smith, J. V. Positions of cations and molecules in zeolites with the mordenite-type framework. II. Dehydrated hydrogen ptilolite. Mater. Res. Bull. 1975, 10, 1319. (15) Barrer, R. M.; Peterson, D. L. Intracrystalline sorption by synthetic mordenites. Proc. R. Soc. London, Ser. A 1964, 280, 466. (16) Hayhurst, D. T.; Sefcik, M. D. Intrazeolite Chemistry; Stucky, G. D., Dwyer F. G., Eds.; ACS Symposium Series, No. 218; American Chemical Society: Washington, DC, 1983; p 333. (17) 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. (18) Devautour, S.; Vanderschueren, J.; Giutini, J. C.; Henn, F.; Zanchetta, J. V.; Ginoux, J. L. Na+/Mordenite interaction energy determined by thermally stimulated depolarization current. J. Phys. Chem. B 1998, 102, 3749. (19) Devautour, S.; Giutini, J. C.; Henn, F.; Douillard, J. M.; Zanchetta, J. V.; Vanderschueren, J. Application of the electronegativity equalization method to the interpretation of TSDC results: Case of mordenite exchanged by Na+ and Li+ cations. J. Phys. Chem. B 1999, 103, 3275. (20) Takaishi, T.; Kato, M. Determination of the ordered distribution of aluminum atoms in a zeolite framework. Part II. Zeolites 1995, 15, 21. (21) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; Wiley-Interscience: New York, 1996.

Received for review February 2, 2005 Revised manuscript received June 20, 2005 Accepted June 21, 2005 IE050128V

Adsorption of Nitrogen, Oxygen, and Argon in Mono-, Di-, and

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