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Sorption of Nitrogen, Oxygen, and Argon in Silver-Exchanged Zeolites Jince Sebastian and Raksh Vir Jasra* Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), G. B. Marg, Bhavnagar-364 002, Gujarat, India
Sorption of nitrogen, oxygen, and argon on silver-exchanged zeolites, A, X, Y, mordenite, BEA, L, and ZSM-5 at 288.2 and 303.0 K were studied. The nitrogen adsorption capacity, selectivity, and heat of adsorption in the low-pressure region are very high for silver-exchanged zeolites compared to other cation-exchanged samples, showing strong interactions between nitrogen molecules with the silver cations. Heat of nitrogen adsorption decreases with the increase in the adsorption equilibrium pressure in all zeolites except zeolite A. However, zeolite AgA shows N2 adsorption capacity of 20.8 molecules per unit cell for nitrogen at 101.3 kPa and N2/O2 selectivity in the range of 5-14.6 at 303 K, the highest known so far for any zeolite A type of adsorbent. Other zeolites also show increased adsorption capacities for nitrogen on silver exchange, but these are smaller compared to those observed for zeolite A. Furthermore, unlike other cation-exchanged zeolites which show small oxygen selectivity over argon, silver-exchanged zeolites display argon selectivity over oxygen. The stronger interaction of nitrogen molecules with silver cations present inside zeolite cavities is attributed to π-complexation of N2 molecules with silver cations. Selective adsorption of argon is explained in terms of its interaction with silver cations through Ar(pσ)-Ag(dσ) bonding molecular orbital. Introduction Adsorption processes for the separation of oxygen and nitrogen from air gained relevance and significance in the early 1970s with the commercial realization of the concept of pressure swing adsorption (PSA). Alkali and alkaline earth metal cation1-7 exchanged zeolites are used as N2-selective adsorbents for oxygen enrichment from air. The adsorption capacity and selectivity for N2 significantly influence the economic viability of an adsorption process. For example, low silica zeolite X having more than 90% of the extraframework cations exchanged with lithium ions is reported4-7 to be the best adsorbent for oxygen production due to its high N2 adsorption capacity (∼30 cm3 g-1 at 101.3 kPa and 303.0 K) and nitrogen-oxygen selectivity (∼10). The advent of LiX has made adsorption processes economical compared to cryogenic fractionation up to 200 tons per day of oxygen. However, the maximum attainable purity by adsorption processes using these adsorbents is still around 95% with separation of 0.934 mol % argon present in the air being a limiting factor to achieving 100% oxygen purity. Adsorption techniques provide an advantage over energy-intensive alternative cryogenic fractionation for oxygen enrichment of air, particularly at low to moderate throughput. Therefore, research efforts are in progress to develop adsorbents with higher N2 adsorption capacity as well having argon-oxygen selectivity. In the case of zeolites, the extraframework cations present in the cavities are the principal sites for interactions with the adsorbate molecules. Nitrogen molecules have higher electrostatic interactions with the zeolite extraframework cations than O2 and Ar mol* To whom correspondence should be addressed. Fax: +91 278 2567562. Tel.: +91 278 2471793. E-mail: rvjasra@ csmcri.org
ecules due to nitrogen’s higher quadrupole moment and, thereby, show selective adsorption from air. In our recent studies, the potential of silver-exchanged zeolite A for air separation was shown.8 Silver-exchanged zeolite A shows strong adsorption of N2 as well as argon. The N2 adsorption capacity (22.5 cm3 g-1 at 101.3 kPa and 303.0 K) and nitrogen-oxygen selectivity (5-15) observed9 for fully silver exchanged zeolite A are the highest reported for any zeolite A type adsorbent. Silverexchanged zeolites of type X10,11 and mordenite12 were also reported to have argon selectivity over oxygen. Silver-exchanged zeolite X was also reported to show higher adsorption capacity for N2 compared to NaX. To understand the strong adsorption of nitrogen and argon in silver-exchanged zeolites, sorption of N2, O2, and Ar has been studied in zeolite A, zeolite X, zeolite Y, mordenite, zeolite L, BEA, and ZSM-5 at 288.2 and 303.0 K. The stronger interaction of nitrogen in silverexchanged zeolites has been discussed in terms of the number and locations of silver cations inside zeolite cavities/channels and the π-complexation of nitrogen with coordinately unsaturated silver cations. Experimental Section Materials. Zeolites A and X from Zeolites and Allied Products, Bombay, India, zeolite Y (SiO2/Al2O3 ) 5.5) from Su¨d-Chemie AG, Germany, zeolite L, ZSM-5, mordenite, and BEA from Zeocat, Uetikon, Switzerland, and silver nitrate (99.9%) from Ranbaxy Fine Chemicals Ltd., New Delhi, India, were used as the starting materials. The zeolite samples in powder form were used for the adsorbent preparation. The chemical composition and surface area of the zeolites studied are given in Table 1. Oxygen (99.99%), nitrogen (99.99%), argon (99.99%), and helium (99.99%) from Hydrogas India Pvt. Ltd., Bombay, India, were used for the adsorption isotherm measurements.
10.1021/ie050442p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/10/2005
Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005 8015 Table 1. Properties and Chemical Composition of the Zeolites chem composition on anhydrous basis, wt % zeolite NaA NaX NaY 5.5 zeolite L BEA Na mordenite 060 Na mordenite 510 NaZSM-5 (25) NaZSM-5 (40) NaZSM-5 (100) NaZSM-5 (400) NaZSM-5 (900)
BET surf. area,
m2
g-1
542 810 312 612 50 450 371 379 348 334 315
Silver Ion Exchange. Silver ions form mononuclear species with appreciable stability in aqueous solution. These cations were introduced into the highly crystalline sodium form of zeolite by conventional cation exchange from aqueous AgNO3 solution. Typically, the zeolite was refluxed with aqueous AgNO3 solutions, containing 1.5 times excess Ag+ ions over the quantity of base Na+, in the solid/liquid ratio 1:80 at 353 K. The residue was filtered, washed with hot distilled water until the washings were free from Ag+ ions as tested with sodium chloride solution, and dried at 353 K. All the activities were carried out in the absence of direct contact with light. The ion exchange of Ag+ into zeolite is highly facile, is complete in a single stage,13 and can occur even at ambient conditions due to the high exchange selectivity of Ag+ over Na+. The extent of silver exchange in zeolites was determined from the analysis of sodium/ silver in the original and filtrate solutions by using a Shimadzu AA-680 atomic absorption spectrometer. X-ray Powder Diffraction. X-ray powder diffraction at ambient temperature was collected using a Philips X’pert MPD system in the 2θ range of 5-65 using Cu KR1 (λ ) 1.540 56 Å). Scanning Electron Microscopy (SEM). Microscopic analysis of the sodium- and silver-exchanged zeolite samples were carried out using a LEO 1430 VP variable-pressure scanning electron microscope. XPS Analysis. X-ray photoemission spectroscopic studies were carried out using a VG ESCA 3 (II) spectrometer. Binding energies for Ag 3d electrons were determined by scanning from 362 to 379 eV in 0.050 eV steps. DRIFT IR Spectroscopy. A diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic study of the adsorbed N2 molecules was carried out at various temperatures ranging from 303 to 673 K using a PerkinElmer Spectrum GX after in situ activation at 673 K under nitrogen flow (30 cm3 min-1) equipped with The Selector DRIFT accessory (Graseby Specac, P/N 199900 series) incorporating an environmental chamber (EC) assembly (Graseby Specac, P/N 19930 series). The spectra were recorded using self-supported zeolite wafer samples in the range of 400-4000 cm-1. The spectra were recorded at room temperature under dry nitrogen flow (30 cm3 min-1) and after in situ heating at different temperatures (423-723 K) using an automatic temperature controller (Graseby Specac, P/N 19930 series) connected with the EC at a heating rate of 10 K min-1. Typically, 30 scans were co-added at a resolution of 4 cm-1. Adsorption Isotherm Measurements. The presence of water in a zeolite significantly affects the validity
Na2O 27.3 24.6 3.0 0.1 1.2 7.5 0.04 3.0 1.5 1.8 1.5 1.5
K2O
17.5
Al2O3
SiO2
21.1 19.0 22.0 18.3 7.8 12.4 7.8 6.4 3.8 1.8 0.4 0.2
51.5 56.4 71.0 64.2 91.0 80.1 92.1 90.8 94.7 90.1 98.0 98.1
of the adsorption isotherms. Therefore, 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 mmHg) for 8 h before the sorption measurements. The adsorption of nitrogen, oxygen, and argon was measured at 288.2 and 303.0 K using a static volumetric system (Micromeritics Instrument Corporation, USA, Model ASAP 2010). The adsorption temperature was maintained ((0.1 K) by circulating water from a constanttemperature bath (Julabo F25, Germany). The requisite amount of the adsorbate gas was injected into the volumetric setup at volumes required to achieve a targeted set of pressures ranging from 0.1 to 850 mmHg. Three pressure transducers of capacities 1 mmHg (accurate within 0.12% of the reading), 10 mmHg (accurate within 0.15% of the reading), and 1000 mmHg (accurate within 0.073% of full scale) were used for the pressure measurements. The adsorption and desorption were completely reversible, and it is possible to remove the adsorbed gases by simple evacuation. The isotherms were fitted in the virial equation
ln(p/q) ) A + Bq + Cq2 + ...
(1)
where q is the amount of gas adsorbed per unit weight of the adsorbent and A, B, and C are the first, second, and third virial coefficients, respectively. Values of Henry’s constant, K, were determined from the virial coefficient using the equation
K ) exp(-A)
(2)
The pure component selectivity of two gases X and Y was calculated from the adsorption isotherms by using the equation
RX/Y ) [VX/VY]P,T
(3)
where VX and VY are the volumes of gases X and Y, 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
∆H°ad ) R{[∂ ln p]/[∂(1/T)]}θ
(4)
where R is the universal gas constant; θ is the fraction of the adsorbed sites at a pressure p and temperature T. A plot of ln p against 1/T gives a straight line with slope of ∆H°ad/R.
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Table 2. Colors of the Adsorbents adsorbent
color of hydrated form
color after vacuum dehydration at 673 K
% crystallinity
AgA AgX AgY 5.5 AgL AgBEA Ag mordenite 060 Ag mordenite 510 AgZSM-5 (25) AgZSM-5 (40) AgZSM-5 (100) AgZSM-5 (400) AgZSM-5 (900)
white slightly gray slightly gray white white slightly gray white white white white pale yellow pale yellow
brick red yellow dark yellow gray gray tan gray gray gray gray gray dark brown dark brown
97 98 99 98 99 98 99 99 99 100 100 100
The errors in the Henry’s constant, adsorption selectivity, and heat of adsorption estimated from the propagation of error method were 0.5%, 0.4%, and 0.4%, respectively. Results The diffraction patterns of the starting materials show that these are highly crystalline samples showing reflections in the range 5-35 typically observed for zeolites. The structures of the zeolites were retained after cation exchange because no loss of crystallinity was observed during silver exchange. The percent crystallinity of the silver-exchanged zeolites was evaluated from the X-ray diffraction patterns by considering 10 major peaks, and the values are given in Table 2. However, intensities of the peaks corresponding to 2θ values of 10.16, 12.46, 14.37, 16.10, 20.4, and 32.55 have changed for zeolite A. SEM pictures of AgA and AgX are given in Figure 1. EDX measurements also confirmed the absence of Na+ ions in the silver-exchanged zeolites. Comparison of AgA and AgX morphologies with starting NaA and NaX showed that cubic morphology and crystallize size are retained on silver exchange in these zeolites. Activation and Color Changes Observed. Silverexchanged zeolite A reversibly changed its color from white to brick red upon vacuum dehydration at 623 K. Silver-exchanged zeolite X became yellow and zeolite Y (silica-alumina ratio 5.5) became dark yellow at 623 K under vacuum. These color changes were reversible with respect to the adsorption and desorption of water molecules. However, silver-exchanged ZSM-5’s with silica/alumina ratios of 400 and 900 irreversibly change their color to dark brown during vacuum dehydration at 623 K. Other zeolites studied turn gray during the vacuum dehydration process. Colors observed for the hydrated and vacuum dehydrated forms of the silverexchanged zeolites are given in Table 2. Ralek et al. first reported that the white hydrated silver form of zeolite A exhibits a red color after dehydration at 623 K,14 which has been later confirmed by many authors.15-26 The dehydrated silver-exchanged zeolite A has been reported to adsorb visible light at
Figure 1. SEM images of AgA and AgX.
500 nm. The color changes observed for AgA on heating under vacuum has been attributed15-26 to the formation of Agnx+ clusters inside the sodalite cavity of zeolite A. It is reported23 that on vacuum dehydration silver ions migrate and undergo autoreduction to form Ag0, which interacts with silver ions to form clusters. Various types of clusters varying from linear Ag+-Ag0-Ag+ to Ag86+, Ag54+, and Ag128+ have been reported depending on the zeolite type and the extent of silver exchange.15-25 In the case of zeolite AgA, the yellow color observed at lower temperatures ( Na+ (31.5 kJ mol-1) > Ag+ (19.7 kJ mol-1). As discussed in an earlier section, silver-exchanged zeolites A, X, and Y on vacuum dehydration at higher temperature form clusters that possess charge higher than +1. The electrostatic interactions of these clusters with adsorbed N2 molecules will be higher than those with isolated Ag+ ions, which might be responsible for higher heats of adsorption for N2 in silver-exchanged zeolites. Under prolonged evacuation at higher temperatures (>523 K), some of the Ag+ ions present inside the supercage undergo reversible intrazeolite autoreduction to Ag0 by extraction with lattice oxygen, with desorption of oxygen as reported24 from thermal studies. Ag0 migrates inside the beta cage and interacts with Ag+ present there to form silver clusters. If this occurs, positively charged structural defects would result on the zeolite surface inside the zeolite cavities that also might
have electrostatic interaction with N2 molecules. However, this explanation for stronger interactions of N2 molecules with silver-exchanged zeolites is not tenable due to the following reasons. The silver clusters in zeolites A and X have been reported15-26 to be present in the sodalite cage, thereby ruling out the possibility of direct interaction of the N2 molecules with silver clusters because N2 molecules due to their higher kinetic diameter (2.67 Å) cannot enter the sodalite cage, which has smaller pore openings (2.2 Å). Though N2 is observed to have stronger interactions with silver-exchanged zeolites, the same cannot be said for oxygen molecules despite the fact that oxygen also has a quadrupole moment albeit smaller in value. The heats of adsorption observed for oxygen molecules in the zeolites and Ar selectivity over oxygen in silverexchanged zeolites do not support the formation of positively charged holes on evacuation in the samples studied by us. Their presence should have resulted in higher heats of adsorption due to chemisorption of oxygen atoms in the lower pressures to repair the oxygen-deficient structural defects. This observation shows that factors other than electrostatic interactions are responsible for stronger N2 interactions in silverexchanged zeolites. The higher heats of adsorption for N2 observed in all silver-exchanged zeolites can be explained in terms of π-complexation of nitrogen molecules with silver ions present inside the zeolite supercage. The electronic configurations on N2 [KK (σ2s)2 (σ2s*)2 (σ2px)2 (π22py)2 (π22pz)2 (π22py*)0 (π22pz*)0] and Ag+ [Kr] 4d10 5s0 show that the highest occupied and lowest unoccupied molecular orbitals in the N2 molecule are the bonding π2p orbital and the antibonding π2p* orbital, respectively. Ag+ ions have the completely occupied highest energy 4d orbital along with the unoccupied 5s orbital. The energy difference between the lowest unoccupied molecular orbital Ag+ ions present in zeolite X and the highest occupied molecular orbital of nitrogen molecule is reported32 to be around 8 eV. This facilitates electron transfer by both σ-donation (electron transfer from the bonding π2p orbital of N2 molecules to the 5s orbital of Ag+ ions) and d-π2p* back-donation (electron transfer from the completely occupied 4d orbital of Ag+ ions to the unoccupied π2p* orbital of N2 molecules). This π-complexation of nitrogen molecules with silver ions of the zeolites results in stronger interaction between silver-exchanged zeolite and nitrogen molecules. However, in the O2 molecule electronic configuration [KK (σ2s)2 (σ2s*)2 (σ2px)2 (π22py)2 (π22pz)2 (π22py*)1 (π22pz*)1], two antibonding π2p orbitals are occupied by one electron each, thus making it difficult for π-complexation. This is also reflected32 in the higher energy difference (11 eV) between the lowest unoccupied molecular orbital of Ag+ ions present in zeolite X and the highest occupied molecular orbital of oxygen molecule. Therefore, the increase observed for N2 adsorption in silver-exchanged zeolites is not seen for O2. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic study of the adsorbed N2 molecules also supports stronger interaction of nitrogen molecules with extraframework silver ions present in the zeolite. N2 molecules, being totally symmetric, do not absorb IR radiation. However, N2 molecules adsorbed in zeolites experience an induced dipole moment, which varies during vibration, and an induced band is observed.40,41
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The magnitude of the induced dipole moment depends on the strength of the interaction of the nitrogen molecules with the extraframework cations. At 303 K, N2 molecules adsorbed on AgA give the adsorption band at 2085 cm-1. Transition metal complexes are reported42 to interact with N2 through end-on coordination (Ms NtN). This M-N2 bonding is interpreted in terms of the σ-donation and π-back-donation. It is further reported42 that ν(NtN) shifts in the range 2220-1850 cm-1 on π-complexation with transition metals from 2331 cm-1 expected for free N2 molecule. For example, NtN stretching frequency in dinitrogen complexes with [Ru(N2)(NH3)5]Br2, Co(N2)(PPh3)3, [Os(N2)(NH3)5]Cl2, and Ir(N2)Cl(PPh3)2 is observed at 2105, 2093, 2022, and 2105 cm-1, respectively. The NtN stretching frequency at 2085 cm-1 observed for N2 molecule adsorbed in zeolite AgA is closer to these values, confirming the π-complexation between nitrogen molecule and Ag+ cations of the zeolites. Argon molecules show higher interactions with Ag+ ion exchanged zeolites, and oxygen molecules do not show a marked increase in adsorption values. Consequently, small argon sorption selectivity is observed over oxygen in silver-exchanged zeolites. From the pure rotational spectra of the complexes of Ar with NaCl and AgCl, the Ar-Ag bond length was found43 to be considerably shorter than the Ar-Na bond length. In addition, the Ar-Ag bond energy was estimated using ab initio calculations to be ∼23 kJ mol-1 in Ar-AgF, which is significantly larger than the corresponding value of 10 kJ mol-1 for Ar-NaCl. These higher bond energies in Ar-AgF have also been supported by the electron density contour plots of some valence molecular orbitals which show significant overlap between Ar and Ag metal, particularly for the Ar(pσ)-Ag(dσ) bonding orbital. Significant electron donation up to ∼0.11 electron from Ar to metal halide is observed from Mulliken orbital populations. A similar explanation is put forth by Grosse et al.44 from their studies on adsorption of xenon using 129Xe NMR spectroscopy in zeolites X and Y. Unlike other cations, silver-exchanged zeolites have been reported to display the displacement of 129Xe chemical shifts to lower values, i.e., upfield with respect to the corresponding sodium forms of these zeolites. These unusual upfield shifts and the distinctly higher isosteric heats of adsorption45 (26.5 and 31.2 kJ mol-1) for Xe in Ag+ sites compared to the sodium form (18.5 kJ mol-1) of zeolite Y at low xenon concentration are attributed to specific interactions (dπ-dπ) presumably between the 4d orbital of Ag+ cations and the 5d orbital of xenon molecules. Munakata et al. studied the adsorption of noble gases such as Kr and Xe on silverexchanged mordenite46 at 273 K. Their experimental results also indicate that the silver mordenite has a higher adsorption capacity of adsorption of noble gases. These studies on Xe and Ar clearly show the existence of special interactions of the Ar(pσ)-Ag(dσ) bonding orbital for Ar and Ag+ and (dπ-dπ) between the 5d orbital of xenon and the 4d orbital of Ag+ cations. The argon selectivity observed for silver-exchanged zeolites in our case can be explained from the special interactions, Ar(pσ)-Ag(dσ), between bonding molecular orbitals. The variations observed for N2 adsorption in terms of the Henry’s constants, heats of adsorption, and N2 adsorption capacity and selectivity for different silverexchanged zeolites (Tables 4, 5, and 6) can be explained
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. Extraframework cation positions are labeled with roman numerals.
in terms of the difference in number of accessible/ coordinately unsaturated Ag+ cations present in these zeolites as discussed in the following sections. Adsorption in AgA. The adsorption N2 capacity and the RN2/O2 adsorption selectivity are much higher (Table 5) in fully exchanged AgA compared to NaA in the pressure range studied. For example, AgA shows equilibrium N2 adsorption capacity of 20.8 molecules per unit cell at 303 K and 101.3 kPa compared to 4.6 molecules per unit cell in NaA. Similarly, RN2/O2 selectivity values at 3.33 kPa are 3.1 and 15.4 for NaA and AgA, respectively. The dependence of heats of adsorption of N2 on the adsorption coverage given in Figure 4 shows that the decrease in heat value with adsorption coverage is less in AgA compared to other zeolites. In AgA, N2 heats of adsorption decrease from 36.5 kJ mol-1 at 1 molecule per unit cell to 35.8 kJ mol-1 at 19 molecules per unit cell coverage. On the other hand, in AgX it decreases form 32.9 kJ mol-1 at 1 molecule per unit cell to 19.1 kJ mol-1 at 12 molecules per unit cell adsorption coverage. In the case of silver mordenite, the N2 heat of adsorption decreases from 32.9 kJ mol-1 at 0.2 molecule per unit cell to 28 kJ mol-1 at 1 molecule per unit cell. This reflects the presence of a higher number of N2-selective sites inside AgA cavities, compared to other zeolites. The dependence of N2 heats of adsorption on percentage silver exchange in zeolite A (Figure 5) shows an exponential rise at around 70% silver exchanges. These sharp increases in heat value show that N2 selectivity in zeolite A arises only after 70% sodium cations are exchanged with silver cations. In Ag12A, it has been reported47 that three Ag+ ions present within the sodalite unit in hydrated AgA move closer to the planes of the nearest 6-rings upon dehydration. Simultaneously, the Ag+ ions present at the 4-ring site and at the 8-ring sites undergo reduction, and these Ag+ ions become nearly zero coordinate, 2.9 Å from the nearest framework oxide ions. The sum of the ionic radii of Ag+ and O2- is 2.6 Å, and the other Ag-O bond distance in the zeolite structure ranges from 2.2 to 2.5 Å.47 Therefore, the Ag+ ions present at the 4-ring site, and at the 8-ring sites with Ag-O bond distance of 2.9 Å, are least adequately coordinated Ag+ ions and are the energetically potential adsorption sites in the supercage. In terms of the accessibility of Ag+ ions, cations located at the 6-ring, 4-ring, and 8-ring are expected to interact with N2 molecules as normally observed for sodium or calcium cations present at these locations (Figure 6). However, the factor that makes N2 molecule interaction with silver cations stronger and higher is
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Figure 7. Framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The numbers 1-4 indicate the different oxygen atoms. Silicon and aluminum atoms alternate the tetrahedral intersections, except that Si substitutes for about 4% of the Al atoms. Extraframework cation positions are labeled with roman numerals.
the presence of coordinately unsaturated Ag+ ions at 4-ring and 8-ring locations, which can have stronger π-complexation with nitrogen molecules as explained in an earlier section. Heats of adsorption similar to that observed of NaA for NaAgA samples having less than 70% Ag+ exchange can be explained in terms of locations of Ag+ in partially silver exchanged zeolite A. It has been reported48 that in partially silver exchanged zeolite Na4.4Ag7.6A, dehydrated under vacuum at 643 K, three sodium ions occupy 8-ring sites, the remaining 1.4 sodium and 6.6 silver ions are located at 6-ring sites, and one reduced silver atom per unit cell is located in the sodalite cage. Thus, there are no Ag+ ions present at 4-ring and 8-ring sites, which due to their unsaturated coordination react strongly with N2 molecules. Adsorption in AgX and AgY. The cation locations of Ag+ ions in zeolites AgX and AgY have been reported by Lee et al.49 and Eulenberger et al.50 respectively from X-ray diffraction studies. In AgX, 32 cations are located (Figure 7) at either site I (center of the hexagonal prism connecting the sodalite ages) or site I′ (near the 6-ring window of the prism on the inside of sodalite cage). The other 32 Ag+ ions are located at site II (on either side of the single 6-ring window between the sodalite and supercage). The distance between these Ag+ cations and framework oxygen is reported to be 2.273 Å, similar to that between Ag+ cation at the 6-ring and framework oxygen in Ag12A. Twenty-three Ag+ ions are located at three different III′ sites (opposite the 4-ring near the wall of the supercage or the edge of the 12-ring). These Ag+ ions located at three different III′ sites have Ag-O distances of 2.702, 2.31, and 2.45 Å. Cations present at site I or I′ being present inside hexagonal prisms are not accessible to N2 molecules and will not contribute to N2 adsorption. Of the other accessible cations at sites II and III′, the cations located at site III′ with Ag-O distance of 2.702 Å will be coordinately unsaturated ones as cations located at other sites (II or other III′) are strongly interacting with framework oxygen of the zeolite as observed from Ag-O distances of 2.27-2.45 Å. Therefore, in AgX, the Ag+ cations located at site III′ with Ag-O distance of 2.702 Å being coordinately unsaturated will strongly interact with N2 molecules through π-complexation. Hutson et al.33 also explained the stronger N2 adsorption in AgX in terms of more accessible Ag+ cations located at the site termed site II* by them from neutron diffraction studies.
In AgY, 10.7 Ag+ cations at the center of the hexagonal prism (site I), 10.7 Ag+ cations inside the sodalite cage at the 6-ring forming the base of hexagonal prism (site I′), and 28.3 ( 0.6 Ag+ cations at the 6-ring facing toward the supercage (site II) are reported50 with an Ag-O distance of 2.32 Å. Similar Ag+ ion locations have been reported from neutron diffraction studies. Silver cations located at sites I and I′ are not accessible to N2 molecules for interaction and therefore would not contribute to N2 adsorption. Ag+ cations present at site II having an Ag-O distance of 2.32 Å do not show unsaturated coordination as observed for AgA and AgX. In view of this, no increase of N2 adsorption is expected on exchanging Ag+ in NaY as reported by Hutson et al.33 However, as seen from Tables 4-6, we have observed a 2 times increase on N2 adsorption capacity as well as 15 and 2 times increases of the Henry’s constant and initial heats of adsorption, respectively, on exchanging silver in NaY. The difference of the Si/Al ratio of the sample studied by Hutson et al.33 and Eulenberger et al.50 was 2.4, which is much lower than the Si/Al ratio value of 5.5 for our sample. Furthermore, in our case (Table 2), the yellow color of the sample was observed on vacuum dehydration at 623 K, reflecting the formation of a weakly interacting silver cluster inside sodalite cages as reported for AgX. Yellow color formation on vacuum dehydration in AgY has been reported by Gellens et al.51 The number of extraframework silver cations in our sample is 30 compared to 56 in the samples studied by Hutson et al.33 Though in the literature data on Ag+ cation locations are not available for AgY with the number of cations equal to 30 or near this value, it seems that some of the Ag+ cations present in supercage sites in such samples are coordinately unsaturated and are interacting strongly with N2 molecules, which is being confirmed by further experiments. Adsorption in Mordenite and ZSM-5 Zeolites. Zeolite mordenite has elliptical, straight cylindrical onedimensional channels, running parallel to the c-axis38 with minor and major axes as 5.8 and 7.0 Å, respectively. The main channels are circumscribed by 12-ring oxygen atoms of the framework and open in the b direction into smaller side channels circumscribed by 8-rings having a minimum free diameter of 3.9 Å and leading toward the next main channel. However, halfway to the neighboring large channel, each side channel branches through two distorted 8-rings of 2.8 Å minimum free diameters into two similar side channels opening into the next main one. Of the total eight sodium ions per unit cell, four are located at the center of each distorted 8-ring. This effectively isolates the main channels from one another and leaves each main channel lined with two rows of side pockets. It is reported52 that molecules as large as argon can access to these pockets. The remaining four sodium ions are reported53 to occupy random sites of the 8-ring and 12ring positions in the main channel of the zeolite. Nitrogen heat of adsorption in sodium mordenite (Figure 3) does not show variation with adsorption coverage, reflecting the energetically uniform surface. In silver-exchanged mordenite, a high heat of adsorption for N2 is observed at lower coverage (1 molecule per unit cell). However, at adsorption coverage higher than 1 molecule per unit cell, the heat of adsorption value does not vary with coverage and is closer to the value observed for sodium mordenite. This is also seen in N2
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Figure 8. Variation of heat of adsorption with number of cations in ZSM-5.
adsorption isotherms for silver and sodium mordenite (Figure 3), wherein the nitrogen adsorption sharply increases with pressure in the low equilibrium pressures (
