Sorption of N2, O2, and Ar in Mn(II)-Exchanged Zeolites A and X Using

Ind. Eng. Chem. Res. 2007, 46, 6293-6302

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SEPARATIONS Sorption of N2, O2, and Ar in Mn(II)-Exchanged Zeolites A and X Using Volumetric Measurements and Grand Canonical Monte Carlo Simulation Jince Sebastian,† Renjith S. Pillai, Sunil A. Peter, and Raksh V. Jasra* Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, BhaVnagar-364 002, India

The adsorption of nitrogen, oxygen, and argon has been studied in Mn(II)-cation-exchanged zeolite A and X powders and pellets at 288.2 and 303 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. Mn(II)-exchanged zeolite A (RO2/Ar ) 1.2-1.4) and X (RO2/Ar ) 1.4-1.8) showed small selectivity for oxygen over argon. In zeolite A, the adsorption capacity for nitrogen, oxygen, and argon increases with the increase in Mn(II) exchange levels. In the case of zeolite X, the adsorption capacity for nitrogen increases at lower adsorption pressures upon Mn(II) cation exchange, but at higher equilibrium pressures, the nitrogen adsorption capacity is slightly lower than that of NaX in the granular form. However, Mn(II)-exchanged zeolite X powder showed increased adsorption capacity for nitrogen, oxygen, and argon with increase in Mn(II) exchange levels, indicating that the binder in zeolite X affects the adsorption capacities of these gases. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with Mn(II) cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra-framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Mn-exchanged zeolites than argon molecules. This is reflected in isosteric heat data of oxygen in Mn(II)-exchanged zeolite A (15.115.4 kJ/mol for O2 and 13.3-14.3 kJ/mol for Ar) and Mn X (15.8-17.2 kJ/mol for O2 and 13.9-15.2 kJ/mol for Ar). Adsorption isotherms and heats of adsorption were also calculated using grand canonical Monte Carlo simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra-framework sodium and manganese cations. Introduction Zeolites, crystalline inorganic solids with pore diameters in the range 1-10 Å, are of immense interest in gas and chemical industries as molecular sieve adsorbents for purification and separation.1 The extra-framework cations invariably present in zeolites play a significant role in determining their adsorptive behavior.2 Separation of nitrogen and oxygen from air is one of the major adsorptive separation processes used in many commercial situations.3-5 However, nitrogen and oxygen adsorption studies in zeolites are largely confined to alkali and alkaline-earth-cation-exchanged zeolites,7-11 with limited studies reported for transition-metal-ion-exchanged zeolites.12-13 This is despite the fact that transition metal ions, due to the presence of the d-shell, can coordinate to adsorbate molecules and could show different adsorption behavior compared to filled-shell cations.12-18 For example, silver12-14 and cobalt16 cations in zeolites are reported to interact strongly with the adsorbed nitrogen molecules. Similarly, cerium-exchanged zeolite X was reported to show oxygen selectivity over nitrogen and argon in the low-pressure region.17,18 Scarce adsorption data on transitionmetal-ion-exchanged zeolites could be due to the difficulty in * To whom correspondence should be addressed. Fax: +91 278 2567562/2566970. Tel.: +91 278 2471793. E-mail: [email protected]. † Present address: Separation and Process Research Centre, Korea Institute of Energy Research, Daejeon, Republic of Korea.

exchanging the transition metal ions into zeolites, while retaining the zeolite structure, particularly at higher cation exchange. Molecular simulation of adsorption phenomena in zeolite is emerging as a rapid, cost-effective method for the evaluation of potential adsorbents. The grand canonical Monte Carlo simulation technique is particularly adapted to calculate the equilibrium adsorption isotherm. For example, Razmus and Hall19 used Monte Carlo simulation to reproduce the experimental single component equilibrium adsorption isotherms of nitrogen, oxygen, and argon in zeolite 5A. Watanabe et al.20 used grand canonical Monte Carlo simulation method to study the air separation properties of zeolite A, X, and Y. Hutson et al.21 used both canonical and grand canonical Monte Carlo simulation techniques to study the influence of residual water on the adsorption of nitrogen, oxygen, and argon in Li-LSX zeolite. Richards et al.22 studied both single component and binary mixture adsorption in Li-X, using grand canonical Monte Carlo simulation technique. The present study reports the adsorption of nitrogen, oxygen, and argon in manganese(II)-exchanged A and X type zeolite powder as well as pellets at 288.2 and 303 K. An attempt is also made to correlate the experimental data with the simulated adsorption data of sodium form of zeolite A, X and their manganese(II)-exchanged form, using grand canonical Monte Carlo simulation technique.

10.1021/ie070067w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007

Table 1. Lennard-Jones Parameters Used for Adsorbate-Adsorbate and Adsorbate-Zeolite Interactions atom type

σ, Å

, kcal mol-1

N O Ar Si Al Oz Na Mn

3.318 3.050 3.403 0.076 1.140 3.040 1.746 1.339

0.0724 0.1008 0.2376 0.0370 0.0384 0.3342 0.1001 1.0589

Experimental Section Materials. Zeolite A with chemical composition Na96Al96Si96O384‚208H2O and zeolite X with chemical composition Na88Al88Si104O384‚220H2O with 15 wt % binder in the granular forms (3 mm) (Zeolites and Allied Products, Bombay, India), zeolite X powder with chemical composition Na88Al88Si104O384‚220H2O (M/s Zeocat, France), and manganese nitrate (E Merck India Ltd., Bombay, India) were used as the starting materials for the adsorbent preparation. Oxygen (99.99%), nitrogen (99.99%), argon (99.99%), and helium (99.99%) (Hydrogas India Pvt. Ltd., Bombay, India) were used for the adsorption isotherm measurements. Cation Exchange. Manganese cations were introduced into the highly crystalline sodium form of zeolites by the conventional cation exchange from aqueous solution. Typically, the zeolites were treated with 0.05 M aqueous solution of manganese nitrate in the solid/liquid ratio 1:80 at 353 K for 4 h. The residue was filtered and washed with hot distilled water, until the washings were free from ions and dried in air at room temperature. Zeolite samples having different degrees of manganese exchange were prepared by repeated ion exchange into the zeolites. The degree of cation exchange of these samples was determined by ICP-AES analysis (Perkin-Elmer Instruments, Optima 2000DV) of the filtrate of the washed cationexchanged samples. The number in the sample name shows the percentage degree of sodium cations present in the zeolites is exchanged with manganese cation (e.g., MnA 41 means 41% of the sodium cations present in NaA are exchanged with manganese cations). X-ray Powder Diffraction. X-ray powder diffraction studies at ambient temperature were carried out using a PHILIPS X’pert MPD system in the 2θ range of 5-65° using Cu KR1 (λ ) 1.54056 Å). The diffraction patterns of the starting materials show that they are highly crystalline showing the reflections in the range 5-35° typical of zeolites. The structures of the zeolites

were retained after the cation exchange. Percentage crystallinities of manganese-exchanged zeolites were determined from the X-ray diffraction pattern by considering the intensity of 10 major peaks. The sodium form of the zeolite was considered as an arbitrary standard for the calculations. The unit cell parameters were determined by the Visser indexing program supplied by Philips Analytical. Activation and Isotherm Measurements. Prior to adsorption measurements, the samples were initially dried at 353 K for 24 h. The samples were further activated in situ by increasing the temperature (at a heating rate ) 1 K min-1) to 673 K under vacuum (5 × 10-3 mm Hg), and the temperature and vacuum were maintained for 8 h before the sorption measurements. Nitrogen, oxygen, and argon adsorption was measured at 288.2 and 303.0 K using a static volumetric system (Micromeritics ASAP 2010). The adsorption data obtained are fitted in the Langmuir equation and Virial equation.16 The values for the Langmuir constant and Henry’s constant were determined from these data. Isosteric heat of adsorption and adsorption selectivity were determined from the adsorption data at 288.2 and 303.0 K. The Langmuir equation follows:

P/qP0 ) 1/bqm + P/qmP0

(1)

The Virial equation follows:

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

(2)

Henry’s constant, K, was determined from the first Virial coefficient using the following equation:

K ) exp(-A)

(3)

where q is the amount of gas adsorbed per unit weight of the adsorbent, qm is the monolayer capacity of the adsorbent, b is Langmuir constant, P is the equilibrium pressure, P0 is the saturation vapor pressure, and A, B, and C are the first, second, and third Virial coefficients, respectively. Adsorption capacity, selectivity, and isosteric heat of adsorption were determined from the adsorption isotherms measured at 288.2 and 303.0 K. The pure component selectivity of gases A and B was calculated by using the equation

Figure 1. X-ray powder diffraction patterns of (a) NaA and MnNaA and (b) NaX and MnNaX zeolites.

RA/B ) [VA/VB]P,T

(4)

Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6295 Table 2. Unit Cell Composition, Crystallity, and Surface Area of Mn(II)-Exchanged Zeolites adsorbent

unit cell composition (on anhydrous basis)

NaA MnA 41 MnA 59 MnA 74 MnA 89 MnA 99 Na X MnX 47 MnX 60 MnX 71 MnX 79 MnX 90 MnX 100

% crystallinity

BET surface area, m2 g-1

external surface area, m2 g-1

100 97 95 95 92 89 100 98 96 93 92 91 88

316 426 489 526 522 542 575 564 562 530 520 524

12 30 35 41 44 24 35 37 46 47 48 48

Na96Al96Si96O384 Mn19.5Na57Al96Si96O384 Mn27Na42Al96Si9 6O384 Mn35.5Na25Al96Si96O384 Mn42.5Na11Al96Si96O384 Mn47.5Na1Al96Si96O384 Na88Al88Si104O38 4 Mn20.5Na47Al88Si104O384 Mn26.5Na35Al88Si104O384 Mn31Na26Al88Si104O384 Mn35Na18Al88Si104O384 Mn39.5Na9Al88Si104O384 Mn44Al88Si104O3 84

Table 3. Equilibrium Adsorption Capacity for N2, O2, and Ar on Mn(II)-Exchanged Zeolites

Table 5. Virial Coefficient and Henry’s Constant at 303.0 K on Mn(II)-Exchanged Zeolites

equilibrium adsorption capacity, molecules/unit cell at 101.3 kPa 288.2 K

adsorbent nitrogen oxygen argon

303.0 K

adsorbent

nitrogen

oxygen

argon

nitrogen

oxygen

argon

NaA MnA 41 MnA 59 MnA 74 MnA 89 MnA 99 NaX MnX 47 MnX 60 MnX 71 MnX 79 MnX 90 MnX 100

6.55 8.02 8.70 9.06 9.14 9.29 7.71 6.13 6.18 6.26 6.32 6.37 6.42

2.12 2.62 2.98 3.10 3.14 3.22 2.43 2.02 2.14 2.23 2.33 2.37 2.40

1.84 2.19 2.38 2.58 2. 64 2.69 2.20 1.58 1.51 1.50 1.52 1.53 1.56

4.58 5.65 6.23 6.48 6.53 6.66 5.54 4.63 4.75 4.74 5.82 4.88 4.93

1.53 1.97 2.17 2.30 2.32 2.38 1.80 1.64 1.65 1.67 1.65 1.63 1.66

1.42 1.69 1.74 1.81 1.84 1.86 1.67 1.19 1.18 1.21 1.19 1.18 1.20


4.55 4.20 3.99 3.94 3.93 3.91 4.30 3.88 3.55 3.47 3.25 3.52 3.25

adsorbent

nitrogen

oxygen

argon

nitrogen

oxygen

argon

adsorbent


0.012 0.024 0.028 0.030 0.034 0.035 0.014 0.065 0.078 0.088 0.089 0.093 0.097

0.013 0.023 0.022 0.020 0.019 0.018 0.010 0.030 0.035 0.039 0.043 0.044 0.081

0.017 0.024 0.021 0.020 0.022 0.021 0.014 0.026 0.026 0.024 0.023 0.022 0.021

0.111 0.308 0.448 0.441 0.565 0.562 0.160 1.064 1.695 2.000 2.022 2.241 2.500

0.037 0.087 0.091 0.086 0.082 0.075 0.034 0.086 0.118 0.137 0.144 0.146 0.309

0.047 0.080 0.079 0.079 0.076 0.075 0.044 0.060 0.056 0.055 0.053 0.047 0.045


3.1 4.1 4.3 4.5 4.7 4.6 3.4 7.2 9.0 9.2 10.0 10.3 10.6

(5)

where R is the universal gas constant, and θ is the fraction of the adsorbed sites at a pressure P and temperature T. The errors in the Henry constant, adsorption selectivity, and heat of adsorption estimated from propagation of error method were 0.5%, 0.4%, and 0.4%, respectively. Surface areas of the cation-exchanged zeolites were determined from N2 adsorption data at 77.35 K. The nitrogen adsorption at 77.35 K was measured using Micromeritics ASAP 2010, after activating the sample at 673 K under vacuum.

nitrogen

oxygen

argon

7.93 11.21 13.85 14.65 14.72 14.90 10.22 15.49 21.53 23.27 29.05 29.27 29.53

2.47 3.18 3.52 3.72 3.86 3.92 2.98 3.94 4.00 4.04 4.00 3.98 4.01

2.32 2.53 2.67 2.88 2.91 2.98 2.86 1.91 1.84 1.78 1.76 1.76 1.78

N2/O2 3.33 kPa

-∆adH° ) R[∂ ln P/∂(1/T)]θ

5.78 5.69 5.60 5.56 5.52 5.50 5.57 5.97 6.01 6.04 6.05 6.05 6.04

adsorption selectivity

Langmuir constant b

where VA and VB are the volumes of gases A and B, respectively, adsorbed at any given pressure P and temperature T. Isosteric heats of adsorption were calculated from the adsorption data collected at 288.2 and 303.0 K using the Clausius-Clapeyron equation

5.72 5.46 5.36 5.31 5.27 5.25 5.53 5.31 5.23 5.22 5.52 5.62 5.34

Table 6. Adsorption Selectivity on Mn(II)-Exchanged Zeolites at 303.0 K

Table 4. Langmuir Fitting Data at 303.0 K on Mn(II)-Exchanged Zeolites slope

Henry’s constant K, 10-5 cc g-1 Pa-1

Virial coefficient A

N2/Ar

O2/Ar

101.99 kPa

3.33 kPa

101.99 kPa

3.33 kPa

101.99 kPa

3.0 3.1 2.9 2.9 2.7 2.8 3.1 3.0 2.8 2.7 2.4 2.7 2.7

3.4 4.8 5.6 5.4 5.9 6.3 3.7 9.8 14.1 15.9 16.4 17.6 18.9

3.2 3.7 3.7 3.5 3.5 3.6 3.3 3.9 4.1 3.8 3.6 4.0 4.1

0.9 1.2 1.3 1.2 1.3 1.4 1.1 1.4 1.6 1.5 1.7 1.6 1.8

0.9 1.2 1.3 1.2 1.3 1.3 1.1 1.2 1.3 1.3 1.4 1.4 1.4

Micropore surface areas of the various samples were determined from t-plots. In this method, the amount of nitrogen adsorbed is replotted against t, the corresponding multilayer thickness for the adsorption of nitrogen on the nonporous reference solid.23 The slope of the t-plot is directly proportional to the surface area of the sample. Any upward deviations from linearity are due to the capillary condensation inside the pores, and the initial slope is proportional to the micropore surface area. Computational Methodology The Cerius2 suite of software (Accelrys Software Inc.), utilizing grand canonical ensemble (fixed pressure) Monte Carlo simulation methods, was used for computation of adsorption data. The “crystal builder” module was used to construct the crystal structure of the zeolites, and the “sorption” module was used for the gas adsorption simulation studies. The simulations

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Figure 2. Experimental and simulated adsorption isotherms of N2, O2, and Ar in (a) NaA, (b) MnA60, (c) MnA90, and (d) MnA99 at 303 K.

were performed on a Silicon Graphics Fuel workstation running on an IRIX v.6.5 platform. Construction of Zeolite Models. The crystallographic data of Pluth et al.24 was used to construct the crystal structure of the sodium form of zeolite A. For the crystal structure of fully manganese-exchanged zeolite A, crystallographic data of pseudocell given by Yanagida et al.25 was used, and eight such cells were used to construct the super crystal of MnA for the gas adsorption simulation studies. The crystallographic data26 of NaX with 92 aluminum atoms was used to construct the initial structure, and from this unit cell four aluminum atoms were converted randomly to silicon atoms in order to get the desired number of 88 aluminum atoms per unit cell. The fully manganese(II)-exchanged zeolite X structure was prepared using the crystallographic data given by Jang et al.27 In this case, also the number of aluminum atoms and manganese ions were adjusted to 88 and 44 from 92 and 46, respectively. For partially Mn(II)-exchanged zeolite structures, sodium ions are replaced with the requisite number of manganese ions according to cation positions reported in the literature.24-27 Furthermore, as it is well-established experimentally and theoretically that nitrogen, oxygen, and argon molecules cannot access sodalite cages, due to their kinetic diameters being larger than the pore opening of the sodalite cage. Therefore, dummy atoms with appropriate van der Waals radii were placed in these cages, in order to avoid any introduction of adsorbate into these cages during simulation. Simulation Details. The force field used in the simulations was a modified version of the Cerius2 Watanabe-Austin

potential energy model.20,28 The total energy of the zeolite framework and adsorbed molecules (U) is expressed as the sum of the interaction energies between the adsorbate and zeolite (UAZ) and that between the adsorbate (UAA) molecules.

U ) UAZ + UAA

(6)

Both UAZ and UAA are written as the sums of pairwise additive potentials, uij, in the form

[( ) ( ) ] ( )

uij ) 4ij

σij rij

12

-

σij rij

6

+

qiqj rij

(7)

where the first term is the repulsion-dispersion Lennard-Jones (LJ) potential with ij, σij corresponding to the parameter sets for species “i” and “j” and the second term represents the Coulombic interaction potential between point charges qi and qj of species “i” and “j” separated by a distance rij. The LJ parameter, σij, for adsorbate-zeolite interaction is given by the mixing rule21 as

σij )

(

)

σi + σj 2

(8)

The σj values for the zeolite lattice are related to the van der Waals radius, Rj, of the respective ions by

Rj ) 21/6σj

(9)

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Figure 3. Experimental and simulated adsorption isotherms of N2, O2, and Ar in (a) NaX, (b) MnX 60, (c) MnX 90, and (d) MnX 100 at 303 K.

and ij is assumed to be the a geometric combination of i and j.

ij ) (ij)0.5

(10)

The force field LJ parameters used in the simulations are given in Table 1.20,22,29 Existing Cerius2 models were used for all the adsorbates. In these models, the quadrupole moments of nitrogen (-1.2 × 10-26 esu) and oxygen (-0.4 × 10-26 esu) were assigned by the use of three-site point charge models. For nitrogen,22 the central point charge was given as +0.810 e with outer neutralizing charges of -0.405 e, and for oxygen,30 the central point charge was given as +0.224 e with outer neutralizing charge of -0.112 e. The neutral spherical model was selected for argon. The partial charges for silicon and oxygen of zeolite framework were fixed at usually considered values29,31 (i.e., +2.4 e and -1.2 e, respectively). The partial charges on sodium and manganese were taken as +0.8 e and +1.6 e, respectively, to get experimental isosteric heat for both type of zeolites.29-33 The values of partial charges on aluminum of sodium form of zeolites and manganese-exchanged zeolite are taken as +1.6 e, in order to get the total charge on the zeolite systems to be zero. Absolute adsorption isotherms were then computed using a grand canonical Monte Carlo calculation algorithm. The simulations were performed at a temperature of 303 K using one unit cell of each zeolite for 3-5 million simulation steps. The Lennard-Jones potential for the adsorbate-zeolite interactions and both the Lennard-Jones and Coulombic terms of the

adsorbate-adsorbate interactions were calculated using the minimum image convention34 with a real space potential cutoff distance of 12.0 Å. The Coulombic term for the adsorbatezeolite interactions was evaluated using the Ewald summation method.34 Results and Discussion The X-ray powder diffractions of the manganese-exchanged zeolite samples were studied, and the patterns show (Figure 1) the retention of the zeolite structure after the manganese ion exchange as the major 2θ diffractions typically observed for zeolite A (7.2, 10.2, 12.4, 16.4, 21.6, 24.0, 27.1, 30.0, and 34.2) and X (6.1, 10.0, 15.5, 20.1, 23.4, 26.7, 29.3, 30.5, 31.0, and 32.1) are retained. The unit cell values for both zeolites A and X did not show any significant change from 24.62 and 24.92 Å observed for zeolites NaA and NaX, respectively, on manganese exchange. The percentage crystallinity for the various manganeseexchanged zeolite samples are given in Table 2. Complete exchange of the sodium ions with manganese ions was observed only after several cycles of cation exchange. Unit cell chemical composition and surface area of the various manganese-exchanged zeolites are given in Table 2. The surface area of the zeolites increases on manganese exchange. This may be due to the decrease in the number of extra-framework cations while replacing monovalent sodium ions with divalent manganese ions. Like cobalt-exchanged zeolite X,16 the external surface area of the manganese-exchanged zeolites particularly with samples exchanged with a higher amount of manganese

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Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 Table 7. Isosteric Heat of Adsorption on Mn(II)-Exchanged Zeolites at 1 Molecule/Unit Cell Coverage heat of adsorption, kJ mol-1 experiment al

Figure 4. Simulated and experimental adsorption isotherms of (a) nitrogen, (b) oxygen, and (c) argon in both MnX 90 powder and granules at 303 K.

also increases with the increase in the percentage of manganese exchange. This could be due to the strong interaction of bivalent manganese ions to the zeolite structure which can result in some structural defects or the formation of amorphous phase, probably due to dealumination during the cation exchange or vacuum dehydration process. This is also evidenced by the decrease in the crystallinity of zeolite samples exchanged with higher manganese content. The equilibrium adsorption capacities for the adsorption of nitrogen, oxygen, and argon on zeolites A and X (granular) containing different amounts of manganese ions are determined from the adsorption isotherms, and the number of molecules of nitrogen, oxygen, and argon adsorbed per unit cell of various

simulated

adsorbent

nitrogen

oxygen

argon

nitrogen

oxygen

argon


21.3 22.2 24.1 25.6 26.5 28.4 21.5 29.2 32.6 35.2 37.3 40.0 40.2

15.3 15.1 15.2 15.3 15.4 15.4 15.1 15.8 16.2 16.4 16.6 16.7 17.2

13.6 13.3 13.4 13.6 13.9 14.3 14.6 13.9 14.0 14.2 14.5 14.8 15.2

22.9 23.2 25.2 25.9 26.8 27.3 22.7 26.6 28.4 29.1 29.9 30.8 30.9

13.8 13.9 14.1 14.3 13.7 14.0 11.9 13.9 14.8 14.9 15.0 15.1 15.2

11.1 11.2 11.4 11.3 11.6 11.5 10.7 10.5 10.3 10.4 10.6 10.8 11.1

manganese-exchanged zeolites A and X at 101.3 kPa and 288.2 and 303.0 K are given in Table 3. In manganese-exchanged zeolite A, the nitrogen, oxygen, and argon adsorption capacity increases with increase in manganese exchange. In the case of manganese exchanged zeolite X, the adsorption capacity of oxygen increases marginally with manganese percentage, while those of nitrogen and argon decrease slightly at 101.3 kPa on manganese exchange. The adsorption data obtained at 288.2 and 303.0 K were fitted into the Langmuir equation, and the values of the slope and the Langmuir constant for the adsorption of nitrogen, oxygen, and argon on different amounts of manganese-ion-exchanged zeolite A and zeolite X at 303.0 K are given in Table 4. In manganeseexchanged zeolite A, the values of slope and Langmuir constant b increase linearly for nitrogen with an increase in manganese percentage, while those for oxygen and argon decrease. In manganese-exchanged zeolite X, the values of slope and Langmuir constant b increase linearly for nitrogen and oxygen with an increase in manganese percentage, while those for argon increase on manganese exchange and remain almost unaffected by the exchange level. The nitrogen, oxygen, and argon adsorption data obtained at 288.2 and 303.0 K were also fitted in the Virial equation, and the values for the Virial coefficient A and Henry’s constant K at 303.0 K are given in Table 5. The Henry’s constant values for the adsorption of nitrogen and oxygen increase in manganeseexchanged zeolites. The magnitude of increase in the value of Henry’s constant is higher for zeolite X compared to zeolite A. The Henry’s constant for nitrogen adsorption increases from 7.9 to 14.7 in zeolite A on increasing the amount of manganese from 0 to 99%, while that in zeolite X increases from 10.22 to 29.53 on increasing the manganese exchange level from 0 to 100%. Henry’s constant for oxygen adsorption also increases from 2.47 to 3.92 in zeolite A and 2.98 to 4.01 in zeolite X. The Henry’s constant for argon adsorption also increases with manganese exchange in zeolite A from 2.32 to 2.98, while it decreases from 2.86 to 1.76 in zeolite X. Pure component adsorption selectivity for nitrogen and oxygen at different equilibrium pressures was calculated, and the values at 303.0 K are given in Table 6. The values of nitrogen/oxygen, nitrogen/argon, and oxygen/argon selectivity increase on manganese exchange. The increase in the nitrogen selectivity values is high in the low-pressure region, and the selectivity value decreases with an increase in the equilibrium adsorption pressure. The nitrogen/oxygen selectivity increases from 3.1 to 4.7 in the low-pressure region, on exchanging sodium ions of zeolite A with manganese ions. In zeolite X,


Figure 5. Experimental and simulated heats of adsorption of N2, O2, and Ar in (a) NaA, (b) MnA, (c) NaX, and (d) MnX at different coverage.

the nitrogen/oxygen selectivity increases from 3.4 to 10.6 in the low-pressure region. However, in the high-pressure region the nitrogen/oxygen selectivity decreases that of the sodium form. Nitrogen/argon selectivity also increases on manganese exchange on both the zeolites in the low-pressure region. The sodium forms of the zeolites show similar adsorption behavior toward oxygen and argon. On exchanging the extra-framework sodium ions with manganese ions, the zeolites became oxygen selective over argon, and the values of the oxygen/argon selectivity increase with an increase in the manganese exchange and equilibrium adsorption pressure. In view of the significance of oxygen-argon separation, these obtained selectivity values are highly interesting. Simulation of the adsorption of nitrogen, oxygen, and argon in zeolites A and X, exchanged partly and fully with Mn(II) cations, were carried out at 303 K and at different pressures. Figure 2 shows both experimental and simulated adsorption isotherms of nitrogen, oxygen, and argon in zeolite A at 303 K, exchanged with Mn(II) cations at different exchange levels. It is clear from the figure that simulation of nitrogen adsorption isotherm predicts experimental results well. There is a large difference between the simulated and experimental data for argon and oxygen adsorption isotherms. The slightly higher values for the simulated nitrogen adsorption isotherms at higher pressures for the zeolite A with higher manganese content are due to the fact that, at higher manganese exchange levels, there was decrease in the crystallinity of zeolite

A as shown in Table 2. Furthermore, the adsorbent samples used for the experiments would have minor quantities of other phases formed during the synthesis of zeolite powders, and we were using a single unit cell of the zeolite for the simulation study. The simulated adsorption isotherms of oxygen and argon also show higher values than the experimental data; for oxygen the adsorption capacity is higher around 1 molecule/unit cell (uc) at 1 atm pressure, and for argon it is less than 1 molecule/ uc at 1 atm. Figure 3 shows both experimental and simulated adsorption isotherms of nitrogen, oxygen, and argon in zeolite X at 303 K, exchanged with Mn(II) cations at different exchange levels. It can be seen from the figure that the simulated adsorption isotherm results match very well with experimental data for NaX, but for manganese-exchanged zeolite X, experimental values are lower than theoretically predicted values. This may be due to the following reasons: loss of crystallinity during cation exchange, underestimated parameter selection for simulation, and/or the presence of 15 wt % binder in adsorbent pellets which may interfere during the ion exchange process. In the case of NaA, the binder, mainly kaolin, is converted to zeolite A during the process of granule making. Therefore, these are essentially binderless adsorbent pellets. But, in the case of NaX, bentonite clay is mainly used as a binder which does not get converted to zeolite during the granule making process, and it may interfere during the cation exchange process. If the binder did not affect the cation exchange process, the decrease in the adsorption capacity would be due to the presence of 15 wt %

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Figure 6. Framework structure of zeolite A. Near the center of each line segment is an oxygen atom. Silicon and aluminum atoms alternate the tetrahedral intersections. Extra-framework cation positions are labeled with roman numerals.

inert binder. But in our case, the adsorption capacity of nitrogen in Mn(II)-ion-exchanged zeolite X pellets is very low compared to that of a simulated adsorption isotherm, and therefore, during the cation exchange process, a portion of Mn(II) ions may be associating with the binder. To confirm further the effect of binder, we exchanged NaX in the powder form (with out binder) with Mn(II) cation repeatedly and measured the adsorption isotherms of nitrogen, oxygen, and argon in these zeolite X powders with different Mn(II) content. It was noted that the adsorption capacities of all these gases increase with increasing Mn(II) cation content in the zeolite X in powder form. Figure 4 shows the adsorption isotherms of nitrogen, oxygen, and argon in approximately 90% Mn(II)-exchanged zeolite X powder as well as granules along with simulated results of the adsorption nitrogen, oxygen, and argon in MnX 90 at 303 K. From the figure, it can be seen that the adsorption capacity for nitrogen is much higher for MnX 90 powder than granules, clearly showing the effect of binder on reducing adsorption capacity of the zeolite X pellets. However, it is still lower than the predicted capacity by simulation. This may be due to the loss of crystallinity during repeated exchange of Mn(II) cations. In the case of Mn(II) exchange zeolite X powder, the crystallinity decreased to 81% after repeated cation exchange. Furthermore, for simulation study, we used one unit cell of the zeolite X sample, and in the case of zeolite sample used for experimental measurements, it may contain minor quantities of other phases formed during the synthesis of zeolite X powder. The oxygen and argon adsorption capacities also increase with an increase

in Mn(II) cation content in zeolite X powder, from 1.66 to 3.06 molecule/uc for oxygen and from 1.20 to 2.5 molecule/uc for argon in MnX 90 at 303 K and 101.3 kPa. Therefore, it is clear that a discrepancy between theoretical and experimental adsorption data for MnX zeolite pellets is mainly due to the presence of 15 wt % inert binder which could affect the exchange of Mn(II) ions into the zeolite structure by associating part of the manganese ions into the binder during cation exchange process. The isosteric heats of adsorption for nitrogen, oxygen, and argon in manganese-exchanged zeolite A and X granules with different manganese exchange levels, calculated from both experimental data and simulation at coverage of 1 molecule/ uc, are given in Table 7. The nitrogen heat of adsorption increases on manganese exchange in zeolites A and X. The magnitude of the increase in nitrogen heat of adsorption in zeolite X is much higher than that in zeolite A. In zeolite A, the N2 heat of adsorption value increases from 21 to 28 kJ mol-1, while that in zeolite X increases from 21 to 40 kJ mol-1. In the case of MnX 90 powder, the heat of adsorption of nitrogen is around 35 kJ mol-1 at 1 molecule/uc coverage. The heat of adsorption values for oxygen and argon remain almost the same as that of the parent sodium form of the zeolite. Figure 5 shows both experimental and simulated heats of adsorption of nitrogen, oxygen, and argon in NaA, MnA, NaX, and MnX at different coverages. In both type of zeolites, similar behavior was observed. The simulated isosteric heats of adsorption of nitrogen, oxygen, and argon are also in good agreement with the experimental data, except for MnX, which may be due to the presence of binder. In the case of the powder form of MnX (binderless), the heat of adsorption of nitrogen (35 kJ/mol) is less than that (40 kJ/mol) of the granular form of MnX (with binder) at a coverage of 1 molecule/uc. Adsorbate molecules can interact with the zeolite surface through lattice oxygen atoms and accessible extra-framework cations and Al and Si atoms. The Al and Si atoms present at the center of tetrahedra are not directly exposed to the sorbate molecules. Consequently, their interactions with the sorbate molecules are negligible. Therefore, the principal interactions of the sorbate molecules with the zeolite surface are through lattice oxygen atoms and extra-framework cations. The electrostatic interactions between the sorbate molecules and the extra-framework cations of the zeolite depend on the quadrupole moments of the sorbate molecules and are expected to follow the order N2 > O2 > Ar in agreement with quadrupole moment

Figure 7. Unit cell structure with nitrogen adsorbed at 101 kPa and 303 K in (a) NaA and (b) MnA (yellow for silicon, pink for aluminum, red for oxygen, green for sodium ions, brown for manganese ions, and blue for nitrogen molecules).


Figure 8. The framework structure of zeolite X, near the center of the each line segment is an oxygen atom. The numbers 1-4 indicate the different oxygen atoms. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that Si substitutes for Al at about 4% of the Al positions. Extra-framework cation positions are labeled with roman numerals.

values of -1.2 × 10-26 esu, -0.4 × 10-26 esu, and zero for N2, O2, and Ar, respectively. The pseudocell framework structure of zeolite A is given in Figure 6. There are 12 negative charges that are balanced by cations in each pseudocell. For the NaA pseudocell (Na12Al12Si12O48), 8 Na+ ions are located at site I at the center of the six-member ring, 3 at site II at the eight-member aperture directly obstructing the entrance, and 1 at site III near the fourmember ring inside the main cavity. In the case of the unit cell structure given by Pluth and Smith,24 some of the main cavities were inaccessible to these gases for adsorption due to the blocking of sodium ions located at site II. These cavities were blocked by putting dummy atoms with appropriate van der Waals radii to ensure that the adsorbates were excluded from these locations during the adsorption simulations. In the crystal structure of dehydrated MnA,25 the framework bond angles of dehydrated MnA are also very similar to those in NaA. Thus, the adsorption behavior of both the sodium and manganese form of zeolite A is expected to have same adsorption properties. The small variations in the adsorption behavior toward nitrogen, oxygen, and argon are due to the difference in the interaction of sodium and manganese ions with these adsorbate molecules. The manganese ions are located on the 3-fold axis close to the planes of the six ring windows.25 The manganese ions are recessed 0.108 Å into the sodalite unit from the [111] plane and are triagonally coordinated to three framework oxygen

atoms at a distance 2.11 Å. These Mn2+ ions sitting on the six ring windows can interact with the guest molecules through the windows. The increase in the nitrogen heat of adsorption in manganese(II) exchanged zeolites compared to NaA is due to the difference in the coordination environment. All the main cavities in MnA were accessible to these adsorbate molecules as there are no blocking cations at the entrance of eight-member rings. Figure 7 shows the unit cell structures of sodium and manganese zeolite A adsorbed with nitrogen at 101 kPa and 303 K. In the case of sodium zeolite A, nitrogen molecules are sitting close to the sodium cations located in all the sites I, II, and III. Therefore, we can find nitrogen molecules well inside the main cavity of sodium zeolite A. But in the case of manganese zeolite A, the nitrogen molecules are inside the supercage of the zeolite close to the manganese cations located at site I at the center of the six-member ring. Zeolite X is a synthetic aluminum-rich analogue of the naturally occurring mineral faujasite (Figure 8). The 14-hedron with 24 vertices known as the sodalite cavity or β-cage may be viewed as its principal building block. These β-cages are connected tetrahedrally at six-rings by bridging oxygen to give double six-rings (D6Rs, hexagonal prisms) and, concomitantly, an interconnected set of even larger cavities (super cage) accessible in three dimensions through 12-ring (24-membered) windows. The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms but are displaced from those points to give near-tetrahedral angles about Si and Al. Single six-rings (S6Rs) are shared by sodalite and supercage structures, and may be viewed as the entrances to the sodalite units. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeolite cavities.26 They are usually found at the following sites shown in Figure 8: site I at the center of the D6R, I′ in the sodalite cavity on the opposite side of one of the D6Rs sixrings from site I, II′ inside the sodalite cavity near a S6R, II at the center of the S6R or displaced from this point into a supercage, III in the supercage on a 2-fold axis opposite a fourring between two 12-rings, and III′ somewhat or substantially off III (off the 2-fold axis) on the inner surface of the super cage. In the case of the crystal structure of NaX26 used for the simulation study, 30 Na+ ions are located in site I′, 32 Na+

Figure 9. Unit cell structure with nitrogen adsorbed at 101 kPa and 303 K in (a) NaX and (b) MnX (yellow for silicon, pink for aluminum, red for oxygen, green for sodium ions, brown for manganese ions, and blue for nitrogen molecules).

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ions in site II, and the remaining 26 Na+ ions are located in site III′. In a fully dehydrated Mn46X,27 16 Mn2+ ions fill site I at the center of D6R and the remaining 30 Mn2+ ions are located at the 32-fold site II in the supercage. Mn2+ ions at site I at the centers of D6Rs are each coordinated octahedrally by six framework oxygen atoms. Of the 32 site II positions, 30 are occupied. Each Mn2+ ion coordinates at 2.130 Å to three framework oxygen atoms. No manganese ions are present in site III or site III′, which can interact directly with the adsorbate molecules. Figure 9 shows the unit cell structures of sodium and manganese zeolite X adsorbed with nitrogen at 101 kPa and 303 K. In the case of NaX, nitrogen molecules are sitting close to sodium ions located in site II, at the center of the six member ring and also in site III′, inside the supercage as can be seen from the Figure 9a. But in the case of MnX, nitrogen molecules can interact with manganese ions located in site II only. Therefore, we can find a nitrogen molecule inside the supercage, only near to the manganese ions located in site II, at the center of the six member ring as can be seen from the Figure 9b. Conclusions Volumetric measurements of the sorption of nitrogen, oxygen, and argon are performed in manganese exchanged zeolite A and X granules with varying manganese content. Nitrogen sorption is observed to show higher (2-3 times) sorption capacity than oxygen and argon in all these zeolite samples. Isosteric heat of sorption data show stronger interactions of nitrogen molecules with Mn(II)-exchanged zeolites. Mn(II)exchanged zeolites A and X showed slight selectivity for oxygen over argon (RO2/Ar ) 1.2-1.8). Higher interaction of oxygen with Mn(II)-exchanged zeolites is also reflected in isosteric heat data of oxygen and argon in Mn(II)-exchanged zeolites A and X. Simulation of the sorption of nitrogen in sodium and manganese zeolites A and X shows that the adsorbed nitrogen molecules sit closely to the extra-framework cations accessible through the supercage. Simulation of adsorption isotherms and heats of adsorption of nitrogen, oxygen, and argon in sodium and fully manganese-exchanged zeolite A and X also match very well with the experimental results, except for MnX. In the case of MnX, it is clear from the simulation that the binder in NaX granules affects the exchange of Mn(II) ion and thus causes the decrease in the adsorption capacity for nitrogen than the starting NaX sample at higher pressures. This could be due to the association of Mn(II) ion to the binder during the cation exchange of zeolite X granules. This is also confirmed from the adsorption isotherms data of nitrogen measured in Mn(II) exchanged zeolite X powder. Acknowledgment We thank the Department of Science and Technology, New Delhi, and the Council of Scientific and Industrial Research, New Delhi, for financial assistance, and Dr. P. K. Ghosh, Director, CSMCRI, for his encouragement to do this work. We would like to thank Dr. James Wescott, Accelrys Inc., for helpful discussions.

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ReceiVed for reView January 11, 2007 ReVised manuscript receiVed June 22, 2007 Accepted July 9, 2007 IE070067W

Sorption of N2, O2, and Ar in Mn(II)-Exchanged Zeolites A and X Using

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