Shape Selective

Feb 11, 2005 - Zeolites are porous materials with pore aperture dimensions (2−10 Å) closer to the .... Ltd., Mumbai, India, and tetraethyl orthosil...
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Pore-Size Engineering of Zeolite A for the Size/Shape Selective Molecular Separation Chintansinh D. Chudasama, Jince Sebastian, and Raksh V. Jasra* Silicates & Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar-364 002, India

The size of pore apertures of zeolite A was systematically controlled by silica deposition on the external surface of the zeolite A from a dilute solution of tetraethyl orthosilicate (TEOS) in dry toluene. The silica deposition on the external surface is confirmed by EDX analysis and external surface area measurements. The adsorption properties of these pore-size-engineered zeolites show that these adsorbents are useful for the size-selective separation of molecules of size in the range of 3-4 Å, such as N2/O2/Ar and H2O/CH3OH/C2H5OH. Argon and nitrogen molecules are restricted from entering the zeolite pores after depositing TEOS on the external surface of zeolite A, and their adsorption capacities decrease. Comparatively smaller oxygen molecules can still enter the pores and become adsorbed there. O2/N2 and O2/Ar selectivity increase with increase in the TEOS deposition. The adsorption properties of the pore-size-engineered zeolite A become similar to that of commercially available molecular sieve 3A after depositing around 0.20% TEOS on the external surface of NaA. Introduction Zeolites are porous materials with pore aperture dimensions (2-10 Å) closer to the molecular sizes of most of the compounds. Therefore, these can be employed for selective adsorption of compounds on the basis of their differences in molecular size and shape. Zeolites have been successfully used as molecular sieves for many commercial applications such as drying of gases1 and solvents and separation of normal alkanes from isoalkanes, cycloalkanes, and aromatics.2 Modification of the pore apertures of zeolites, until recently, was largely confined to the exchange of extraframework cations. For example, 3A and 5A molecular sieves are obtained by the cation exchange of NaA with potassium and calcium, respectively. By substitution of potassium over sodium in A-type zeolites forming NaKA, the preferential adsorption of oxygen over nitrogen is observed. The potassium located at the eight-membered oxygen ring gets relocated to the six-membered ring, resulting in steric hindrance to nitrogen molecules, thereby, facilitating adsorption of oxygen. Recent studies3-13 have shown that the pore-opening size of zeolite and other molecular sieves can be controlled to suit desired applications by other postsynthesis modification techniques such as internal or external surface modification by chemical reactions, preadsorption of polar molecules,3,4 chemical vapor deposition5-10 or similar coating processes, and thermal treatment.11-13 It has been reported widely that by calcination of NaA zeolites at 953-1033 K after water vapor adsorption, the adsorption preference toward oxygen over nitrogen is enhanced manyfold, which can be attributed mostly to pore-size shrinkage.14 The calcined zeolite A pellets showed oxygen selectivity below 213 K, while the powder form becomes oxygen selective only below 163 K. Recently, it was shown that the framework of the * To whom correspondence should be addressed. Tel.: +91 278 2471793. Fax: +91 278 2567562. E-mail: rvjasra@ csmcri.org.

titanosilicate ETS-4 could also be systematically contracted through dehydration at elevated temperature to tune the effective size of the pores giving access to the interior of the crystal.12 This so-called molecular gate effect can be used to tailor the adsorption properties of the material to give size-selective adsorbents suitable for separation of gas mixture such as N2/CH4, Ar/O2, and N2/O2 with size in the 3-4 Å range. ETS-4 dehydrated at 543 K allows N2 molecules to adsorb inside the pores. Dehydration at 573 K results in pore contraction to show the signs of nitrogen exclusion, while smaller oxygen molecules can still penetrate into the crystal, resulting in an oxygen-selective adsorbent. Vansant and co-workers15-21 have shown that the pore size and the affinity of a zeolite structure can be modified by a chemical treatment of the zeolite structure using reactants of the form XmYn with X ) Si, B, and Ge and Y ) H, Cl, alkyl, etc. Such a chemical treatment not only alters the free diameter of the zeolite pores but also changes the electric field of the zeolite surface. The silane, borane, or disilane molecules are chemisorbed on the zeolite surface by reacting with the silanol groups of the zeolite. The Si-H bonds, due to electronegative differences between Si and H atoms, have a hydridic reactivity and can be hydrolyzed with the silanol groups of a zeolite. Polar molecules such as water and amines presorbed in the zeolite can also be used to alter the molecular sieve behavior and the interaction toward adsorbate molecules of the zeolite. The strong reaction between the zeolite cation and the dipole moment of the polar adsorbate blocks diffusion by the clustering of the preadsorbed molecules around the cation in the channels. The preloading, which is selectively located on the most energetic sites, also modifies the energy of interaction between the zeolite and the adsorbate molecules. Zeolites KNaA and NaX were incorporated into composite films composed of alkoxysilanes such as MEMO, hydrolyzed with TEOS, and then subjected to oxidative degradation treatments.22 The compositions of the matrix materials are reported22 to be useful to

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tailor the effective pore dimensions of the composite film for subsequent separations or sensor applications. An interesting possibility5-10,23 for the modification of zeolite pores without affecting the internal structure is by chemically depositing large molecules at the exterior of the zeolite mordenite,5 A,6,7 and ZSM-5.23 Silicon derivatives such as Si(OCH3)4, methylchlorosilanes, can be reacted with the hydroxyl groups of the external surface of the zeolite crystals followed by calcination in the presence of air, which yields silicacoated zeolite. The deposit of SiO2 on the external surface results in the modification of the pore size. However, all the reports of silica deposition on zeolites are using vapor-phase reactions at higher temperature. The degree of control of pore aperture by cationexchange technique is limited, as the sites that cations will occupy depend on their interactions with the zeolite surface. Compared to this technique, modification of pore apertures by deposition of silica species on the external surface provides a wider option for pore aperture sizes. However, the main drawback of silica deposition by vapor-phase alkoxide reaction at the external surface is its nonuniform deposition and also deposition limited by the availability of silanol groups on the external surface. With the availability of techniques such as plasma enhanced CVD, uniformity of deposition may be achieved. However, these techniques are expensive and cumbersome to achieve on a large scale commercially. We have demonstrated24,27 the potential of developing molecular sieves by carrying out liquidphase deposition of silica on the external surface of zeolite A. In the present work, we have carried out silica deposition on the external surface of zeolite A by reaction of zeolite silanol groups with tetraethyl orthosilicate (TEOS) from liquid phase. The adsorption of nitrogen, oxygen, argon, methanol, ethanol, and water were then studied on thus-prepared zeolite A molecular sieves to correlate the adsorption selectivity with pore aperture size. Experimental Section Materials and Methods. Commercially available zeolite NaA powder [(Na2O)6‚(Al2O3)6‚(SiO2)12‚wH2O] from Zeolite and Allied Products, Mumbai, India, and toluene from S.D. Fine Chem. Ltd., Mumbai, India, and tetraethyl orthosilicate from E. Merck India Ltd., Mumbai, India, were used for the adsorbent preparation. Oxygen (99.99%), nitrogen (99.99%), argon (99.99%), and helium (99.99%) from Hydrogas India Pvt. Ltd., Mumbai, India, were used for the adsorption measurements. Zeolite NaA powder was activated at 673 K for 4 h and then cooled to ambient temperature under N2 atmosphere. Toluene was dried,28 and a dilute solution of tetraethyl orthosilicate in dry toluene was prepared. The completely dehydrated zeolite powder was then mixed with the tetraethyl orthosilicate solution, and the slurry was stirred for 4-8 h in N2 environment. The solvent was removed by evaporation under reduced pressure, and the orthosilicate species deposited on the zeolite surface was converted into silica by the calcination in air at 823 K for 4 h. Adsorption Measurement. Nitrogen, oxygen, and argon adsorption at 288.2 and 303.0 K were measured using a static volumetric system (Micromeritics Instrument Corporation U.S.A., ASAP 2010), after activating the sample at 673 K under vacuum (5 × 10-3 mmHg) for 8 h. The temperature of the analysis Dewar was

maintained with in (0.1 K by circulating water using a constant-temperature bath (Julabo F25, Germany). Addition of the adsorbate gas was made at volumes required to achieve a targeted set of pressures ranging from 0.1 to 850 mmHg. The pseudo-equilibrium conditions are defined as follows: a minimum equilibrium interval of 2 s with a relative target tolerance of 5.0% of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine equilibrium for each measurement point. Three pressure transducers of capacities 1 mmHg (accuracy within 0.12% of the reading), 10 mmHg (accuracy within 0.15% of the reading), and 1000 mmHg (accuracy within 0.073% of full scale) were used for the measurement. The surface area of different zeolites was calculated from the nitrogen adsorption data measured at 77.35 K. The pure component selectivity of two gases A and B is calculated by using the equation,

RA/B ) [VA/VB]P,T where VA and VB are the volumes of gas A and B adsorbed at any given pressure P and temperature T. Isosteric heat of adsorption was calculated from the adsorption data collected at 288.2 and 303.0 K usingClausius-Clapeyron equation.

{

∆adH° ) R

[∂ ln p] [∂(1/T)]

}

θ

where R is the universal gas constant, θ is the fraction of the adsorbed sites at a pressure p and temperature T. The error in the heat of adsorption estimated from propagation of error method was 0.4% of the calculated value. X-ray Powder Diffraction. In situ X-ray powder diffraction of zeolite NaA and the silica coated NaA samples were collected at various temperatures using PHILIPS X’pert MPD system equipped with Anton Paar high-temperature reaction chamber XRK900. The diffractions in the 2θ ranges of 5-65 are collected using Cu KR1 (λ ) 1.54056 Å) radiation. The temperature was increased from room temperature to 1123 K at a heating rate of 10 K min-1, and the X-ray diffraction was measured at different measurement temperatures in the range 303-1123 K. The temperature was maintained for 1 h at each temperature before collecting the diffraction pattern. Percentage crystallinity of the NaA and TEOS-deposited NaA samples was determined form the X-ray diffraction pattern by considering the peaks at 2θ values 7.18, 10.17, 12.46, 21.68, 24.06, 27.14, 29.97, and 34.21. SEM and EDX. Microscopic analysis of zeolite NaA and the silica-deposited NaA samples were collected using LEO 1430 VP variable pressure scanning electron microscope equipped with INCA Oxford EDX facility. Results and Discussion The silanol groups present on the external surface of the zeolite undergo condensation reaction with tetraethyl orthosilicate by eliminating C2H5OH molecule. Since the size of the tetraethyl orthosilicate molecules (8.9 Å)29 is much larger than the pore-opening size of zeolite A (3.8 Å),25,26 these molecules cannot enter the zeolite pores; hence, the deposition takes place only on the external surface. The vicinal or germinal silanol

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Figure 1. X-ray powder diffraction pattern of NaA and TEOSdeposited NaA. Table 1. Percentage Crystallinity at Various Temperatures XRD measurement temperature, K

NaA

303 373 473 673 873 1073 1123

100 101 103 103 102 87 10

% crystallinity 0.15% TEOS 0.30% TEOS 100 101 103 104 104 89 9

100 101 101 101 101 88 11

groups present on the surface of a zeolite react with tetraethyl orthosilicate in the following manner.

The X-ray powder diffraction pattern of NaA and the silica-deposited NaA samples are given in Figure 1. The crystalline structure of the zeolite A remained unaffected after the silica deposition as demonstrated by the presence of all reflections corresponding to zeolite A after silica deposition. No loss of crystallinity was observed during the silica deposition process (Table 2). In situ X-ray powder diffraction of zeolite NaA and the TEOS-deposited NaA samples collected at various temperatures up to 1123 K shows that the TEOSdeposited NaA samples also possess high thermal stability as NaA does. The percentage crystallinities of NaA, 0.15% TEOS, and 0.30% TEOS at various temperatures were calculated and are given in Table 1. The intensities of the peaks at 2θ values 7.18, 10.17, 12.46, 21.68, 24.06, 27.14, 29.97, and 34.21 were considered for the percentage crystallinity calculations. The total intensity at 303 K was considered as an arbitrary standard for the calculations.

Scanning electron microscopic images of NaA and the silica-deposited NaA samples were recorded, and the SEM images of NaA and 0.20% TEOS-deposited NaA zeolites are given in Figure 2. Energy dispersive X-ray analysis shows that the silicon concentration of the external surface increases on TEOS deposition. The surface Si/Al ratios of NaA and various TEOS-deposited zeolites were calculated from the elemental concentration obtained from the EDX analysis, and the values are given in Table 2. The surface Si/Al ratio increases with increase in the TEOS deposition. These data confirm the silica deposition on the external surface of the zeolite A. At 77.5 K, the pores of NaA are not accessible to nitrogen molecules, and the surface area measured only reflects the amount of external surface area. Due to silica deposition on the external surface, the amount of external surface marginally increases on TEOS deposition. The increase in the surface area values determined using BET method (Table 2) also reflects the silica deposition on the external surface. The nitrogen, oxygen, and argon adsorption isotherms on NaA and various TEOS-deposited NaA samples were measured at 288.2 and 303.0 K. NaA was also subjected to thermal treatment at 673 K for 4 h, 823 K in air for 4 h, and then activated at 673 K for 8 h to prove the oxygen selectivity obtained was not from pore shrinkage on calcination. The adsorption isotherms on thermal treated NaA and 0.10%, 0.20%, and 1.0% TEOSdeposited NaA measured at 288.2 K are shown in Figure 3. As seen from the figure, NaA adsorbs nitrogen more selectively than oxygen and argon. On depositing 0.10% TEOS on NaA, the nitrogen and argon adsorption capacities and N2/O2 and N2/Ar selectivities decrease. The oxygen adsorption capacity remains almost unaffected, and the O2/Ar adsorption selectivity increases. On increasing the deposition of TEOS on NaA to 0.15%, the nitrogen and argon adsorption capacities further decrease, and the oxygen adsorption capacity remains unaffected. The oxygen adsorption isotherm overtakes that of nitrogen and argon, and the adsorbent becomes oxygen selective over nitrogen and argon. In 0.20% TEOS-deposited NaA, the nitrogen and argon adsorption capacities further decrease, while that of oxygen still remains unaffected. The oxygen selectivity over nitrogen and argon increases. In 0.25% and 0.30% TEOS-deposited NaA the nitrogen and argon adsorption capacities further decrease. The oxygen adsorption capacity also decreases slightly on these samples. In 1.0% TEOS loaded NaA in addition to the decrease in the nitrogen and argon adsorption, oxygen adsorption also decreases. However, the magnitude of the decrease in oxygen adsorption is much smaller, and the adsorbent remains oxygen selective. The adsorption capacities of these samples were determined from the adsorption isotherms and the values at 288.2 K and 760 mmHg are plotted as a function of percentage of TEOS deposition and are given in Figure 4. The nitrogen and argon adsorption capacity decreases on depositing TEOS on NaA. In 0.10% TEOSdeposited NaA, the nitrogen adsorption capacity decreases to 3.8 cm3 g-1 form the original value of 10.8 cm3 g-1 in NaA; argon adsorption capacity also decreases drastically, and the oxygen adsorption capacity remains unchanged. When the amount of TEOS deposition further increases, the nitrogen, argon, and oxygen adsorption capacities decrease, but the decrease in the

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Figure 2. SEM images of NaA and 0.20% TEOS-deposited NaA. Table 2. Si/Al Ratio from EDX Analysis and Crystallinity adsorbent

surface Si/Al ratio

% crystallinity

BET surface area, m2 g-1

NaA 0.10% TEOS 0.15% TEOS 0.20% TEOS 0.25% TEOS 0.30% TEOS 1.0% TEOS

1.00 1.69 1.67 1.71 1.75 1.93 4.2

100 102 101 103 100 101 101

0.687 0.729 0.862 1.152 1.528 1.601 2.576

oxygen adsorption capacity is much lower compared to those of nitrogen and argon. Figure 4 shows the variation in the adsorption capacities of nitrogen, oxygen, and argon with percentage of silica deposition. Pure component adsorption selectivity determined for binary mixtures of N2/O2, N2/Ar, and O2/Ar at equilibrium pressures of 100, 400, and 760 mmHg were calculated from adsorption isotherms and are given in Table 3. The selectivity values also change on TEOS deposition. In 0.10% TEOS-deposited NaA, the N2/O2 adsorption selectivity decreases to 1.04 from the original value of 4.0 in NaA, N2/Ar adsorption selectivity also decreases, and the O2/Ar adsorption selectivity increases. When the amount of TEOS deposition increases, the N2/O2 and N2/Ar further decreases, and the adsorbent becomes oxygen selective over nitrogen and argon after 0.15% or more TEOS is deposited on the external surface of zeolite A. O2/Ar selectivity increases with increase in the TEOS deposition. As seen from the adsorption data, after the deposition of around 0.20% TEOS in NaA, the pore aperture size of NaA (3.8 Å) decreases to make it equivalent to commercial 3A (3.0 Å) molecular sieves. The isosteric heat of adsorption for nitrogen, oxygen, and argon were calculated from the adsorption isotherms on 303.0 and 288.2 K. Even though the adsorption capacity and adsorption selectivity were affected on TEOS deposition, the heat of adsorption values for nitrogen, oxygen, and argon were found unaffected on these TEOS-deposited zeolite samples. Isosteric hearts of adsorption values for nitrogen, oxygen, and argon adsorption are given in Table 4. The adsorption selectivities of these adsorbents are a result of steric, kinetic, and equilibrium effects in combination. The pore size distribution is in molecular size range, i.e., 3-4 Å. The kinetic diameter of oxygen, nitrogen, and argon are 2.49, 2.67, and 3.42 Å, respectively. The size difference among these molecules can be exploited for the separation of molecules. Kinetic selectivity results if one of the components diffuses faster than the others inside the pores of the molecular

sieves. If the micropore diameter of the molecular sieve and the molecular diameter of the adsorbate are nearly the same, steric-kinetic effect can occur. The isosteric heat of adsorption depends on the equilibrium effects, which are observed when one of the components interacts more strongly with the adsorbent surface than the other components. Nitrogen molecules have higher electrostatic interactions with the zeolite extraframework cations than O2 and Ar molecules due to the higher quadrupole moment of nitrogen and, thereby, show higher heat of adsorption. For NaA zeolite, all three gases, nitrogen, oxygen, and argon, can enter the pores of zeolite and interact with the zeolite. Nitrogen selectivity over oxygen and argon observed in the case of NaA is equilibrium selectivity resulting from the difference in interactions of these molecules with zeolite surface. However, with silica-deposited zeolites, stereokinetic effects due to the difference in molecular size of oxygen, nitrogen, and argon are responsible for oxygen selectivity over nitrogen. In addition to the adsorption of nitrogen, oxygen, and argon the adsorption of water, methanol, and ethanol on zeolite NaA and the various TEOS-deposited zeolites were studied. The adsorption data are given in Table 5. The adsorption data show that the TEOS-deposited zeolite samples are also useful for the adsorptive dehydration of alcohols. TEOS-deposited NaA shows water adsorption apacity similar to that of NaA, which is higher than that of the commercial 3A. In larger amounts of TEOS deposition on zeolites, ethanol and methanol adsorption capacities are lower than those in commercial 3A, which facilitates the higher recovery of alcohol. The thermal and hydrothermal stability as seen by XRD also remains unaffected in TEOS-deposited zeolites. To understand the effect of nature of extraframework cations and their locations on the adsorption properties of the silica-deposited zeolites, we have exchanged the sodium ions of the 0.25% TEOS-deposited samples with calcium ions. It was observed that 0.25% TEOSdeposited NaA that was oxygen-selective adsorbent, became nitrogen selective on calcium exchange, showing its adsorption behavior similar to that of NaCaA (Figure 5a). This shows that the oxygen selectivity obtained in TEOS-deposited NaA samples is not only because of the silica deposition but also due to the partial pore entrance blockage by the sodium ions sitting at site III. It may be noted that in NaA, three sodium ions occupy the eight-membered ring site and decrease the effective pore diameter from 4.4 to 3.8 Å. However, on sodium ion exchange with calcium, the number of extraframework cations reduces to half, and it is also known that calcium

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Figure 3. Adsorption isotherms on (a) NaA, (b) 0.10%, (c) 0.20%, (d) 1.0% TEOS-deposited NaA at 288.2 K. Table 4. Heat of Adsorption on NaA and Various TEOS-Deposited NaA heat of adsorption in kJ mol-1 zeolite

nitrogen

oxygen

argon

NaA 0.10% TEOS 0.15% TEOS 0.20% TEOS 0.25% TEOS 0.30% TEOS 1.0% TEOS 3A Ca2+exchanged 0.25% TEOS

20.4 20.3 20.4 20.5 20.4 20.3 20.1 18.7 22.5

15.3 15.4 15.2 15.3 15.3 15.0 15.1 14.7 16.0

13.0 13.1 13.1 13.2 13.2 13.2 13.1 13.3 12.3

Table 5. Water, Methanol, and Ethanol Adsorption Capacity for Various Adsorbents at 303 K and 760 mmHg Figure 4. Variation in adsorption capacity with TEOS deposition. Table 3. Adsorption Selectivity on NaA and Various TEOS-Deposited NaA at 288.2K 100 mmHg

400 mmHg

760 mmHg

sample N2/O2 O2/Ar N2/Ar N2/O2 O2/Ar N2/Ar N2/O2 O2/Ar N2/Ar NaA 0.10% 0.15% 0.20% 0.25% 0.30% 1.0% 3A

4.0 1.04 0.97 0.60 0.60 0.36 0.38 0.80

1.0 2.4 6.4 3.9 4.5 11.0 21.6 3.2

4.3 2.5 8.9 2.3 2.7 4.0 8.3 2.0

3.9 1.1 0.98 0.74 0.64 0.58 0.62 0.71

1.1 2.1 2.7 3.5 3.6 5.0 2.9 3.1

4.2 2.2 2.8 2.6 2.3 2.9 1.8 3.2

3.8 1.1 0.98 0.76 0.65 0.60 0.51 0.82

1.1 2.4 2.6 3.3 3.5 4.6 2.6 3.1

4.1 2.5 2.8 2.5 2.3 2.8 1.7 2.8

ions are not located at eight-membered ring site, i.e., site III. As a result, pore entrance of calcium exchanged zeolite A is not blocked and effectively remains at 4.4 Å, allowing N2, O2, and argon to enter into the pores. Adsorption isotherms on 1% TEOS-deposited CaA are given in Figure 5b. The zeolite remains nitrogen selec-

amount adsorbed in wt % sample

methanol

ethanol

water

NaA 0.10% 0.15% 0.20% 0.25% 0.30% 1.0% 3A

18.20 6.17 6.00 5.30 2.40 2.63 2.23 5.37

15.76 5.40 5.19 3.04 2.08 1.92 1.97 3.02

22.21 21.26 21.38 22.17 21.15 21.69 20.06 19.87

tive even after depositing 1% TEOS on its external surface. The 1% TEOS-deposited CaA also shows adsorption behavior similar to that of CaA. Further increase in the TEOS deposition is also not enough to convert TEOS-deposited CaA to oxygen selective, which indicates that the narrowing of the pore entrance size from 4.8 Å in CaA to 3 Å is very difficult to achieve by pore-size engineering using TEOS deposition. This may be because of two reasons, (i) there are fewer silanol

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Figure 5. Adsorption isotherms on (a) Ca2+ exchanged 0.25% TEOS-deposited NaA and (b) 1% TEOS-deposited CaA. Table 6. Cation Site Occupancies in Dehydrated Zeolite A

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.

groups on the external surface of CaA, which form -OSi-O- bonds with the TEOS molecules and (ii) multilayer silica deposition is required for the narrowing of pore entrance size of CaA to make it capable of distinguishing nitrogen, oxygen, and argon on the basis of their size. Zeolite A (Figure 6) may be viewed as an assemblage of truncated octahedra, each composed of 24 tetrahedra (12AlO4 and 12SiO4). These are also referred to as cuboctahedra, sodalite cavities or β-cages. These cuboctahedra are octahedrally joined at four-membered rings by four bridging oxygen atoms, and this results in a central cavity (also known as a supercage) separated from other similar cages by an eight-memberedring opening or window which has a diameter of 4.2 Å. Each supercage is surrounded by eight smaller cavities of 6.6 Å diameter which have pore openings of 2.2 Å. It is only supercages that are accessible to nitrogen, oxygen, and argon molecules for adsorption. However, for water molecules, even the smaller cavities are accessible for adsorption. There are 12 negative charges that are balanced by cations in each unit cell. For Na12A, eight Na+ ions are located at Site I at the center of the six-membered ring, three at site II at the eightmembered aperture directly obstructing the entrance, and one at site III near the four-membered ring inside the cavity as shown in Figure 6. The sodium ions can be replaced by other cations. However, the cation location varies with the nature of the cation depending on its size and charge. On partially replacing (around 50%) Na+ (0.95 Å) with K+ (1.33 Å) ions in NaA, molecular sieve 3Å zeolite is formed, resulting in a smaller effective pore-opening size of 3 Å

zeolite type

site I

NaA CaA KA

8 5 6

cation locations site II site III 3 1 3

1 0 0

others

reference

3

30-32 26 33

due to the occupation of larger potassium ions at site II. On replacing sodium ions with divalent cations such as calcium or magnesium, one Ca2+ or Mg2+ replaces two Na+ ions; therefore, half the cations are present in the zeolite. The occupancy of site II is reduced as seen in Table 6, on exchanging Ca2+ into NaA, resulting in the decrease in obstruction of eight-membered-ring pore opening to the cavities. Consequently, NaCaA with at least 75% sodium exchange results in a molecular sieve with larger pore opening (4.8 Å). The cation site occupancies of sodium, calcium, and potassium ions in dehydrated zeolite A are given in Table 6. This difference in adsorption selectivity of these adsorbate molecules in NaA has been attributed to differences in their interactions with the zeolite surface especially due to higher electrostatic interactions of nitrogen with zeolitic cations compared to oxygen and argon molecules. Therefore, the observed selectivity is based on the energy of interactions of the adsorbate molecules not due to steric factors, as the molecular size of O2 is the smallest. Therefore, at 288.2 K, all three adsorbate molecules can ingress into the pore aperture (eight-ring) of NaA, which is larger (3.8 Å) than molecular dimensions of these gases. On silica deposition in NaA, the size of the eight-ring is expected to decrease, and at certain amount of silica deposition affect the adsorption of this adsorbate. The degree of influence on adsorption is expected to be in the order of their molecular size with argon experiencing more than nitrogen and oxygen. Adsorption isotherms for oxygen, nitrogen, and argon at 288.2 K in zeolite NaA having silica deposition of 0.1, 0.20, and 1.0% are shown in Figure 3. It is observed from these figures that for the NaA samples having silica higher than 0.10%, the adsorbent shows selectivity for oxygen unlike NaA. The observed adsorption selectivities of oxygen over those of nitrogen or argon are due to the difference in their adsorption rates which follow the order O2 > N2 > Ar, in consonance with the order in their molecular sizes. Therefore, the oxygen selectivity observed in silica-deposited zeolites is kinetic selective not equilibrium selective. This is further supported from heats of adsorption data (Table 4) and measurement of adsorp-

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Literature Cited

Figure 7. Adsorption isotherms on 0.20% TEOS-deposited NaA at 288.2 K with a minimum equilibrium interval of 30 min.

tion isotherms by giving longer adsorption equilibrium time (a minimum equilibrium interval of 30 min with a relative target tolerance of 5.0% of the targeted pressure and an absolute target tolerance of 5.000 mmHg were used to determine equilibrium for each measurement point) for silica-coated zeolites where the adsorption behavior for the three adsorbates was similar to that of non-silica-coated zeolite (Figure 7). As seen from the ethanol, methanol, and water adsorption data in Table 5, the 0.20% silica-deposited NaA shows similar adsorption properties to that of commercially used 3A (NaKA). However, as silica deposition is only on the external surface, adsorption capacity for water is not substantially reduced like that with NaKA wherein the larger sized potassium ions are occupying cavity space and thereby reducing the adsorption capacity. Conclusions The pore aperture size of zeolite A is systematically controlled by silica deposition on the external surface of the zeolite. Sorption of nitrogen, oxygen, and argon on various amounts of silica-deposited zeolite A at ambient temperatures was studied. The crystallinity of the zeolite remains unaffected on silica deposition. The adsorption capacity for nitrogen and argon decreases on silica deposition. At higher silica deposition, the adsorption capacity of oxygen also decreases, but the magnitude of the decrease is much higher for nitrogen and argon. The nitrogen-selective zeolite A becomes oxygen selective over nitrogen and argon after 0.2% TEOS is deposited on the external surface. These pore-sizeengineered zeolites are potential adsorbents for the selective adsorption of oxygen from a gaseous mixture with nitrogen and argon. Acknowledgment We are thankful to Department of Science and Technology, New Delhi, Council of Scientific and Industrial Research, New Delhi, and Dr. P. K. Ghosh, Director of CSMCRI for the financial assistance and support. We are also thankful to Mr. Chandrakanth CK. for SEM/EDX measurements.

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Received for review July 27, 2004 Revised manuscript received December 10, 2004 Accepted December 15, 2004 IE049333L