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33 C O and C O as Sensitive Probe Molecules for 2

Investigating Migration Effects of Cations in Zeolites J. A. MICHELENA, G. PEETERS, E. F. VANSANT, and P. D E BIÈVRE

Downloaded by UNIV LAVAL on September 20, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch033

University of Antwerp (U.I.A.), Dept. of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium

ABSTRACT Adsorption of CO and CO has been studied on zeolite Y i n which Na+ ions have been p a r t i a l l y exchanged for Ca ions. Physisorption of CO and CO indicates that i n NaCaY zeolites with a Ca loading lower than 35% all the Na ions i n i t i a l l y present i n the small cavities as well as the incoming Ca2+ ions remain inaccessible for CO and C O molecules. However, a Na replacement of 35 to 45 % leaves the Ca ions unaffected by CO and C O but causes an important migration of inaccessible Na ions to accessible supercage positions. Further exchange (> 45%) results i n Ca ions occupying locations exposed to the supercages. 2

2+

2

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+

2

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2

2+

Introduction. The sorptive properties of the synthetic faujasite depend on the nature and the number of cations present. The d i s t r i b u tion of the cations i s affected by the l a t t i c e charge, valency and dimensions of the cations and by the presence of ligands. The results of X-ray d i f f r a c t i o n studies suggest that the exchangeable cations may be found i n three different regions of space within the structure of the dehydrated Y zeolite : the hexagonal prisms, the cubooctahedra and the large cavities. The nomenclature adopted i n this paper i s based on that used by Smith ( 1 ) . At roan temperature, ion-exchange with polyvalent cations i s generally incomplete; however, at higher temperatures, the exchange level increases. Moreover, a dehydration of the zeolite containing polyvalent cations results i n a complete redistribution of the cations towards the small cavities (sites I and I ' ) . Because of the specific adsorption of CO and C 0 on zeolites loaded with divalent cations, a number of workers developed usef u l spectroscopic and sorptive methods to localize divalent cations i n Y-type zeolites (2-10) . Egerton et a l . (2) observed a specific interaction of CO with polyvalent cations and used this method to detect polyvalent cations exposed to the supercages (site I I ) . Indeed, CO i s small enough to enter the supercages 2

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In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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but i s too large to enter the socialite units or hexagonal prisms. Infrared measurements of Jacobs et a l . (£,5) indicate a s i milar behaviour of C0 i n calcium and iragnesium exchanged X and Y zeolites. In the Y zeolites the asymmetric stretching vibration of C0 was found to be cation dependent and suggested that CCu vas an indicator of the accessible divalent cations a t sites 11 i n the zeolite. However, no adsorption study i n the Y zeolite was carried out to confirm the specific character of CO2 towards exchangeable cations. In the zeolite X, intense infrared bands due to carboxylate and carbonate species were observed, hut no correlation was found with any structural s i t e . Barrer and Gibbons (13) investigated the adsorption of C 0 i n different exchanged X-zeolites. An important analysis of the total energy of the bond between OO2 and the zeolites was made. In order to compare the s e n s i t i v i t i e s of CO and C0 towards accessible cations i n the zeolite, detailed CO and C0 adsorption experiments were carried out on a number of p a r t i a l l y exchanged CaNaY zeolites. 2

2


2

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Experimental. Materials. The synthetic zeolite Y (SK-40, Lot n° 3606-46-8) vas supplied by Union Carbide Corporation (France). The anhydrous unit c e l l composition was Na^c (A10 ) 55(2102) 137. Calcium ions were exchanged i n the Y zeolite by a œnventional ion-exchange procedure. After the exchange, the zeolite samples were washed several times with d i s t i l l e d water to remove the excess salts and dried at 60°C. Samples with different levels of calcium exchange were prepared and are designated according to the percentage of sodium replaced by calcium. CaY-12 denotes a sample i n which 12% of the sodium ions i n NaY have been exchanged by c a l cium ions. Carbon monoxide and carbon dioxide (J.T. Baker) were stated to have a purity greater than 99% and were used without further purification. Adsorption experiments. Adsorption isotherms were determined up to 110 Torr i n a conventional constant volume system. Adsorption experiments were carried out between 0 and 40°C, the temperature being controlled to within 0.1°C. The zeolite samples weighed between 0.7 and 1 g. To avoid breakdown of the zeolite structure, special care was taken while raising the temperature during outgassing. The zeolite was a c t i vated overnight after heating the sample stepwise from room temperature to 400°C (100°C per hr). 2

Results. Figures 1 and 2 show respectively CO and COo adsorption isotherms at 0°C for NaY and samples of p a r t i a l l y O a exchanged NaY zeolites. Adsorption equilibrium was reached within 45 minutes, arl the isotherms were completely reversible inallccases . 2 +


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of Cations


0.6Γ

P(torr) Figure 1. Adsorption isotherms of CO in CaNaY zeo lites at 0°C. 3 , NaY; €, CaY-9; O , CaY-38; Φ , CaY60; ·, CaY-78.

P(torr) Figure 2. Adsorption isotherms of C0 in CaNaY zeolites at 0°C. O , NaY; Φ , CaY-9; Φ , CaY-24; ·, CaY-28; € , C a Y - 3 5 ; Θ , CaY-60; β , C a Y - 7 8 . 2


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Adsorption experiments at different temperatures were carried out i n order to derive the isosteric heats of adsorption from the tem perature dependence of the adsorption isotherms. Figure 3 represents the dependence of the adsorbed quantity of CO and C0 at 0°C on the calcium content i n the Y zeolite, at an equilibrium pressure of 80 Torr. The adsorption isobar of CO reveals a decrease i n adsorption u n t i l about 35% of the Na " ions have been replaced. At higher C a contents, an increase i n the CO adsorption i s observed. The isobar for the C0 adsorption however decreases continuously from a pure NaY to a pure CaY. In order to obtain more information about the specific C02~ Ca interaction, the dependence of the isosteric heat (Q t) the calcium content i s shown i n figure 4. This figure indicates a decrease of isosteric heat for a C a content below 35 %, but an increase at higher exchange levels. The isosteric heats are estimated to have an accuracy of 0.7 kcal/mole. 2

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Discussion. Site occupation i n calcium exchanged Y zeolites. At a l l temperatures and pressures used, the adsorption of CO and C0 i s not controlled primarily by the available pore vo lume, but i s dominated by forces between the adsorbate molecules and the adsorbent surface. One can expect a stronger adsorption of CO and CO? on calcium exchanged Y compared to the NaY zeolite because of the stronger electrostatic f i e l d associated with diva lent calcium ions, which causes more pronounced polarization, field--dipole and f i e l d gradient-quadrupole interactions. This should be reflected i n a higher isosteric heat of adsorption when divalent cations are accessible to the CO and CO? molecules. A similar behaviour was found by Barrer et a l . (13) i n the zeolite X. Table I shows that this i s indeed the case i n Y zeolite. 2

Table I : Observed heats of adsorption of 00 and 00^ i n NaY CaY zeolites.

and

Heat of adsorption (kcal/mole)

NaY CaY

CO

CO.

6.3 9.3

7.6 11.5

X-ray analyses of Ca exchanged Y zeolites (1) indicate that, after dehydration, the calcium ions are preferentially localized i n the small cavities at the Sj and Sj> positions. The f i r s t Ca ions to be exchanged occupy inaccessible sites inside the small cavities. This i s achieved by a migration, during the de hydration process, of C a ions towards the small cavities. A 2 +

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Figure 3.

Effects of

80 100 %Hm replaced

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80 100 * Na replaced

The refotionship between the adsorbed quantity at 0°C of CO (a) and (b)atan equilibrium pressure of 80 torr

0

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% Na replaced in the NaY Figure 4. The variation of the isosteric heat of adsorption for the CO adsorption with the calcium content in the Y zeolite g


C0

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possible replacement of Na ions to supercage positions can be a stoichiometric migration (i.e. 1 Ca for 2 Na )» Apparently below 35% C a , no change i n adsorption properties of the large cavities should be expected when the adsorbed gas i s cation-specific. However, a decrease i n adsorption of CO below a Ca content of 35% suggests a preferential migration of C a ions towards sites inside the small cages. This implies a redistribution of i n i t i a l Nations inside the small cages and not an intense migration of inaccessible Na ions towards the Sjj positions. This results i n a lower number of exchangeable cations i n the supercages compared to those i n the pure NaY form and explains the lower adsorbed quantity of CO. This effect should be more pronounced by increasing C a content. Since X-ray studies of CaY zeolites have demonstrated absolute preference of C a ions for the sites I and I I , we can suggest that i n p a r t i a l l y c a l cium exchanged NaY zeolites, up to a C a content of 35%, the Na and Ca * ions prefer the exchange positions S j ' and Sj respectively. As far as their effect on the adsorption properties of CO and 00 i s concerned, two different groups of sites can be considered. The f i r s t set of sites are S , S j , and S J J » , locations i n side the small cavities, which have l i t t l e effect on the adsorption phenomena because the CO and C0 molecules are too large to enter these small cages. The second group of sites are the S J J positions which are accessible for CO and C0 molecules. The influence of cations i n the S^, positions on the adsorption i s uncertain; however, i t i s not excluded that these cations w i l l interact with CO and C0 molecules i n the supercages. 2+

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a) The adsorption of carbon monoxide (fig. 3a) The adsorption of CO gradually decreases up to a calcium exchange level of 35%. However, when 35% of the i n i t i a l Na " ions are replaced by C a ions, a change i n the adsorption properties of the zeolite occurs. At a C a level of 35%, a sharp increase i n the adsorbed quantity i s observed. From this C a content, cation positions inside the small cavities apparently start to be disturbed by repulsion forces between cations, which could result i n a migration of cations towards the supercages during the dehydration. Up to a C a exchange level of 45%, migration of Na " ions into the large cavities dominates. At s t i l l higher Ca contents, the Ca " ions, now present i n the supercage, are responsible for the higher adsorption. The linear relationship between the amount of CO adsorbed and the presence of accessible Ca " ions suggests a strong Ca -CO s p e c i f i c i t y ; Interaction energy calculations, which consider d i s persion, polarization, field-dipole and repulsion energy effects, have been carried out for NaY and CaY zeolites interacting with CO molecules (12). These calculations indicate that the a f f i n i t y of CO molecules for C a ions i s higher than for Na+ ions, moreover, a CO/Ca ratio of 2 seems to be energetically favourable i n 4

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a Y zeolite. b) The adsorption of carbon dioxide (fig. 3 b ) . The CO2 molecule does not possess a permanent dipole moment, but has a large quadrupole moment which w i l l interact with the f i e l d gradient i n the zeolite l a t t i c e . I t might be expected that the cation density i n the adsorptive cavities of a zeolite would have effect on the extent of sorption of a quadrupole molecule. Ibis makes C 0 an excellent indicator for Na and Ca + ions a t accessible positions, because the adsorbed quantity i s related to the number of accessible exchangeable cations. The decrement i n the adsorption of C 0 has to be an indication that a smaller number of cations can interact with the CO2 molecules. As shown i n figure 3 b , three different regions can be d i s tinguished i n the relationship between the amount of CO2 adsor bed and the C a content. These three regions correspond exact l y to those observed with the adsorption experiments of CO. For a Ca * " content below 35%, a sharp decrease i n the CO? adsorbed quantity, with increasing Ca + content, can be explained by a decrease i n the Na content i n the large cavities. Therefore, at these exchange levels, a l l the Ca * " ions have migrated during the dehydration process to inaccessible positions inside the small cavities. Therefore, during dehydration, no migration of Na ions from the small cavities towards the large cavities could occur , and they remain inaccessible for the C 0 adsorption. Between 45 and 100% C a , a linear decrease i n the amount of C 0 adsorbed i n the zeolite i s due to a gradual decrease i n the number of exchangeable cations i n the large cavities. No mi gration of C a ions towards the small cavities must be assumed, so that the replacement of 2x Na ions by χ C a ions decreases the number of cations. Between 35 and 45%, a migration of Na ions from inaccessible sites into the large cavities explains the less pronounced de crease i n the adsorbed quantity of C 0 . Furthermore, the change i n isosteric heat of adsorption (Q ) with increasing C a content (figure 4 ) , supports this cat ion localization picture i n calcium exchanged Y zeolites. In deed, between a Ca * * content of 0 and 35%, the isosteric heat de creases due to a decrease i n the Na*" content i n the large c a v i t i e s ; however, between 35 and 45%, the isosteric heat increases as a result of an increase i n the number of Na ions i n the large cavities. From an exchange level of 45%, the isosteric heat changes linearly with the calciun content, which r e f l e c t s the i n crease i n electrostatic f i e l d and the importance of the divalent catlon-quadrupole interactions. In general, the present adsorption results of CO and CO? show a high preference of C a ions for inaccessible sites, probably s i t e I , i n CaNaY zeolites of low degree of exchange. On the other hand, up t i l l a C a content of 35%, no migration of the Na+ ions from positions inside the small cavities to the supercages was 2

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observed and they remain probably a t Sji positions. These data bring a refinement to the conclusions of Egerton e t a l . (2) and Jacobs e t a l . (4,5) with respect to the location of the exchange able Na as well as C a ions i n the CaNaY zeolites. Further more, the infrared absorption experiments, as well as interac tion energy calculations, establish that a combination of CO and C0 i s a powerful method for investigating the location of acces sible cations i n zeolites. +

2 +

2


Literature Cited. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Smith J.V., Adv. Chem. (1971), 101, 171 Egerton T.A. and Stone F.S., Trans. Far. Soc. (1970), 66, 2364 Egerton Τ.A. and Stone F.S., Trans. Far. Soc. (1973),69, 22 Jacobs P.A., Van Cauwelaert F.H., Vansant E.F. and Uytterhoeven J.G., J.C.S. Faraday I (1973), 69 1056 Jacobs P.A., Van Cauwelaert F.H. and Vansant E.F., J.C.S. Faraday I (1973), 69, 2130 Ward J.W. and Habgood H.W., J . Phys. Chem. (1966),70, 2420 Bertsch L. and Habgood H.W., J . Phys. Chem. (1963), 67, 1621 Angell C.L. and Schaffer P.C., J. Phys. Chem. (1969), 47, 3811 Angell C.L. and Schaffer P.C., J. Phys. Chem. (1966), 70, 1413 Rabo J.A., Angell C.L., Kasai P.H. and Schomaker V., Disc. Far. Soc. (1966), 41, 328 Coughlan B., Ph. D. Thesis, Imperial College of Science and Technology, London (1964) Michelena J.A., De Bièvre P. and Vansant E.F., i n preparation (1976) Barrer R.M. and Gibbons R.M., Trans. Far. Soc. (1965), 61, 948

G. Peeters and J.A. Michelena acknowledge a grant from the EEG (European Economic Catmunity). E.F. Vansant wishes to thank the National Science Foundation (N.F.W.O. - Belgium) for their support.


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