2443
J. Phys. Chem. 1991,95, 2443-2441
'*@XeNMR of Silver-Exchanged X- and Y-Type Zeolites R. Crosse, R. Burmeister, B. Boddenberg,* Lehrstuhl fur Physikalische Chemie II, Universitat Dortmund, Otto- Hahn-Strasse, 0-4600 Dortmund 50, Federal Republic of Germany
A. Cedeon, and J. Fraissard Luboratoire de Chimie des Surfaces, Uniuersite Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, Cedex 05, France (Received: October 19, 1989; In Final Form: September 17, 1990)
The adsorption isotherms and the 129XeNMR chemical shifts of xenon sorbed in the supercages of partially and fully silver exchanged zeolites NaX and NaY after vacuum dehydration as well as after subsequent oxygen treatment were determined. The unusual displacements of the chemical shifts upfield from those found for the sodium forms are attributed to specific interactions of xenon with silver cations in the supercages. It is concluded that after vacuum dehydration at 400 and 500 O C no silver clusters are formed in the supercages with the possible exception of the fully silver exchanged zeolite AgX.
Introduction It has been shown by a variety of techniques that silver clusters can be formed in the zeolites A, X, and Y on dehydration at elevated temperatures. In type A zeolites low-nuclearity silver clusters are generated which reside in the &cages.'-" In X- and Y -type zeolites such clusters, residing in the D6RIB-cage units, are produced as well,3*5*89'2-'7 but recent optical and far-infrared measurements suggest the formation of higher nuclearity clusters J ~ dehydration of the AgnZ+( n = 5-13) in the s u p e r c a g e ~ . ~The faujasites may further result in the formation of metallic-like silver particles on the outside of the c r y ~ t a l l i t e s , ' ~or,~ ~as~has - ~ ~also been suggested,12 in the supercages. All types of high-nuclearity clusters were found to be destroyed by oxygen treatment in the 300-500OC temperature range. Only Tsutsumi and TakahashiZ0 have reported a different behavior. 129XeN M R is a convenient and sensitive tool for the investigation of the physical and chemical properties of the intracrystalline void systems of ~eolites.~'-*~ Since the bulky xenon atoms (1) Kim, Y.;Seff, K. J . Am. Chem. Soc. 1978, la0, 175. (2) Gellens, L. R.; Mortier, W. J.; Schoonheydt, R.A.; Uytterhoeven, J. B. J. Phys. Chem. 1981,85, 2783. (3) Gellens, L. R.;Mortier, W. J.; Uytterhoeven, J. B. Zeolites 1981, I, 11. (4) Narayana, M.; Kevan, L. J . Chem. Phys. 1982, 76, 3999. ( 5 ) Beyer, H.K.; Jacobs, P.A. In Studies in Surjace Science and Caralysis, Jacobs, P.A., et at., Eds.; Elsevier: Amsterdam, 1982; Vol. 12, p 95.
(6) Karge, H. G. In Studies in Surjace Science and Catalysis; Jacobs, P. A., et al., Eds.; Elsevier: Amsterdam, 1982; Vol. 12, p 103. (7) Ozin, G. A.; Baker, M. D.; Godber, J. J . Phys. Chem. 1984,88,4902. (8) Baker, M. D.; Ozin, G. A.; Gcdber, J. J . Phys. Chem. 1985,89, 305. (9) Baker, M.D.; Godber, J.; Ozin, G. A. J. Phys. Chem. 1985,89,2299. (IO) Michalik, J.; Kevan, L. J . Am. Chem. Soc. 1986, 108, 4247. ( I I ) Texter, J.; Kellerman, R.;Gonsiorowsky, T. J. Phys. Chem. 1986,90, 2118. (12) Kellerman, R.;Texter, J. J . Chem. Phys. 1979, 70, 1562. (13) Gellens, L. R.;Mortier, W. J.; Uytterhoeven, J. B. Zeolites 1981, I , 85. (14) Ozin, G. A.; Hugus, F.; Mc Intosh, D.F.; Mattar, S.In Intrazeolite
Chemistry; Stucky, G.. Dwyer, F. G., Eds.; America1 Chemical Society: Washington, DC, 1983; ACS Symposium Ser. No. 218, p 409. (IS) Ozin, G. A.; Hugues, F.; Mattar, S.M.; McIntosh, D. F. J . Phys. Chem. 1986, 90, I 129. (16) Ozin,G. A.; Hugues, F. J . Phys. Chem. 1983, 87, 94. (17) Schoonheydt. R.A.; Hall, M. B. Lunsford, J. H. Inorg. Chem. 1983, 22, 3834. (18) Brown, D. R.; Kevan, L. J . Phys. Chem. 1986. 90, 1129. (19) Gellens, L. R.;Schoonheydt, R.A. In Studies in Surface Science and Catalysis; Jacobs, P.A., et al., Eds.; Elsevier: Amsterdam, 1982; Vol. 12, p 87. (20) Tsutsumi, K.; Takahashi, H. Bull. Chem. Soc. Jpn. 1972,45,2332. (21) Ito, T.; Fraissard, J. Proc. 5th Int. Conf. Zeolites, Naples 1980, 510. (22) Ito, T.; Fraissard, J. J . Chem. Phys. 1982, 76, 5225. (23) Fraissard, J.; Ito, T. Zeolites 1988,8, 350.
TABLE I: Ionic Compositions of Zeolites AgNaX and AgNaY zeolite sample Yl%" no. of Agt/uc
AgNaX
NaX 20AgNaX 40AgNaX 60AgNaX 8OAgNaX AgX
0
0 17.2
20 40 60 80
34.4 51.6 68.8 86
100
AgNaY
NaY IOAgNaY AgY
0
0
10
5.6 56
100
'Degree of ion exchange. TABLE 11: Water Content of Silver-Exchanged Y Zeolites after Dehydration a t 26 OC no. of no. of no. of H,O re1 water zeolite samDle AR!+/UC H,O/uc at saturn concn
lOAgNaY(26) AgY(26) 'Reference 42.
5.6 56
135 130
250' 236'
0.54 0.55
(4 0.44 nm) cannot penetrate into the &cages, the supercages of the faujasite type X and Y zeolites can selectively be sensed by this technique. As an application, the present contribution aims at exploring the state of silver in the supercages of X and Y zeolites under various pretreatment conditions. Experimental Section The commercial zeolites NaX (Linde 13X,Si/Al = 1.2)and NaY (LZY 52,Si/AI = 2.4)were treated with aqueous AgN03 solutions according to the procedure described el~ewhere?~ yielding zeolites with degrees of ion exchange (y) between 10 and 100% (Table I). The preparation of the zeolites yAgNaX and yAgNaY was performed under exposure to daylight and in the dark, respectively. A further sample of the fully silver exchanged X zeolite, AgX, was prepared under dark conditions. After preparation and storage, the zeolites prepared in the dark were white whereas the illuminated samples were faintly gray. Various samples of these zeolites were dehydrated under vacuum a t 26 O C and a t slowly rising temperatures up to 400-500 OC. Oxidation of samples of the latter pretreatment was performed at the final dehydration temperatures by an 8-h contact with oxygen under a pressure of 300 mbar. Subsequently, the oxygen was pumped off and the (24) Boddenberg, B.;Burmeister, R. Zeolites 1988, 8, 480.
0022-3654191l2095-2443%02.50/0 0 1991 American Chemical Societv
2444 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 a
Grosse et al.
h
\
uo
p / mbar
Figure 1. Adsorption isotherms of xenon in vacuum-dehydrated (400 "C)
silver-exchanged NaX zeolites. Samples prepared under daylight with degrees of silver exchange (%): 0 (X); 20 (+); 40 ( 0 ) ;60 ( 0 ) ;80 (A); 100 (0). AgX after oxidation at 450 O C ( 0 ) .Sample prepared in the dark: AgX (e). samples cooled to ambient temperature. The samples prepared are designated as e.g. yAgNaX(8,ox) where y is the degree of silver for sodium exchange (in a),8(in "C)is the temperature at which the final vacuum dehydration was carried out, and "ox" indicates treatment with oxygen at 6 OC following the dehydration. The residual water of the dehydrated zeolite samples lOAgNaY(26) and AgY(26) were determined to be 54 and 55% of the saturation values, respectively (Table 11). It was observed that the zeolites with the lowest silver content studied, lOAgNaY and 20AgNaX, had turned almost black at the dehydration temperature 400 OC in close similarity with the observation reported in the literature.I2 After treatment with oxygen at both 400 and 500 O C , these samples were white. AgY was gray after dehydration at 400 OC,as has also been reported by other a u t h ~ r s , and ~ J ~turned white and faintly yellow after the oxidation at 400 and 500 OC, respectively. The samples yAgNaX with ~ 3 4 0 % were yellow after dehydration at 400 "C. A sample of AgX that was dehydrated a t 400 OC retained its yellow coloration on subsequent oxidation at 450 O C . The xenon adsorption isotherms were measured volumetrically at 26 OC. The lZ9XeN M R measurements were carried out at 26 OC using a Bruker CXP 100 spectrometer operating at the resonance frequency 24.9 MHz. All samples investigated yielded single m n a n c e lines. Their displacements relative to the reference is expressed conventionally as b = ( u - U , , ~ ) / L J , , ~ Results Figure 1 shows the adsorption isotherms (26 "C) of xenon in the dehydrated zeolites yAgNaX(400) of various degrees of silver for sodium ion exchange y between 0 and 100%. With the exception of the fully exchanged zeolite AgX at the higher pressures, the amount of xenon adsorbed increases with the silver content at any given p over the pressure range studied. For y up to 80% the isotherms exhibit linear portions at low xenon concentrations with slopes that increase with y . The adsorption isotherm for AgX(400) exhibits a much different behavior with very steep increase initially and a pronounced saturation behavior. It is seen that the adsorption isotherm of this zeolite is independent of whether the silver exchange and the subsequent dehydration have been performed under daylight or dark conditions. Dehydration and subsequent oxidation of AgX at 450 O C increase the amount of xenon adsorbed with respect to AgX(400) (Figure 1). Figure 2 shows the lSXe isotropic chemical shifts 8 (reference is xenon gas a t zero pressure) as a function of the xenon concentration N (number of xenon atoms/supercage) for the zeolites yAgNaX(400) with the same y values as in Figure 1. With increasing silver content the 13 vs N curves are displaced to lower shift values Over most of the concentration range investigated. For the zeolites with y up to 80% they have positive slopes and are concave to the N axis, and at higher xenon concentration they approach the straight line of NaX. The curve for AgX, on the
N / atoms per supercage F i i 2. 129XeNMR chemical shifts of xenon in silver-exchanged NaX zeolites dehydrated in vacuo at 400 OC. Samples prepared under daylight with degrees of silver exchange (56): 0 (X); 20 (+); 40 (0); 60 ( 0 ) ;80 (A);100 ( 0 ) . AgX after oxidation at 450 O C ( 0 ) . Sample prepared in the dark AgX (e).
I
I
I
400
800
1200
I 1600
Figure 3. Xenon adsorption isotherms in silver-exchanged NaY zeolites. (+) lOAgNaY(400); ( 0 )lOAgNaY(400,ox);(e) IOAgNaY(500,ox); (X) AgY(400); (m)AgY(500,ox); (0) AgY(400,ox);(A) lOAgNaY(26);
(VI AgY(26).
other hand, is much different: it has almost zero slope initially, turns steeply upwards, and merges into a straight line crossing the NaX curve. It is observed that the chemical shifts of AgX(400) prepared under dark conditions are the same as those of the sample obtained under daylight illumination. By oxidation at 450 OC the shifts of the dehydrated zeolite AgX(400) are displaced by about 10 ppm to lower values. The shift a t zero xenon concentration, tiNIO,is considered to reflect the interaction of the xenon atoms with the zeolite framework and the cations in the supercages (see Discussion section). This shift is obtained by extrapolation of the 6 vs N curves (Figure 2) to zero xenon concentration. When at low xenon concentrations the measured shifts exhibit a linear dependence on N (yAgNaX(400), with y = 0, 20, and 40%), ,6, is found by linear extrapolati~n.~~ If such a situation is not prevailing, the extrapolation to zero concentration cannot unambiguously be carried out since the 6 vs N curve may depend on several factors such as the relative numbers of various sites and the lifetimes of the atoms thereon,40 and information on these are not available a priori. The extrapolations for the zeolites 80AgNaX(400), AgX(400), and AgX(400) oxidized 450 OC are self-suggesting under the prevailing circumstances (Figure 2). In the case of the zeolite 60AgNaX(400) the suggested linear extrapolation (Figure 2) is believed to hold true approximately. The shifts aNIO thus obtained systematically decrease (the resonances go to lower frequency, i.e., upfield) with increasing silver content and become even negative with respect to the reference chosen here, e.g. dNIO = -50 ppm for AgX(400,ox). Figure 3 shows the xenon adsorption isotherms of the zeolites lOAgNaY and AgY under different pretreatment conditions. The following characteristic features may be recognized. (i) Over the
2445
Silver-Exchanged X- and Y-Type Zeolites
li p
\
80: 60-
M
40:
.o x As '20x4 e 4 0 x 4 .60%4
20 0-
-20
.8oXbg
-
-40-
lOOX4 (0) I
I
I
1
dN /
I
Figure 4. IBXe NMR chemical shifts of silver-exchanged NaY zeolites. (+) lOAgNaY(400); ( 0 ) IOAgNaY(400,ox); (X) AgY(400); (0) AgY(400,ox); (A) lOAgNaY(26); (V) AgY(26). Dashed line: NaY from ref 21. Dotted line: partially dehydrated NaY from ref 25.
pressure range studied the amount of xenon adsorbed in the zeolites dehydrated at 400 O C increases with the silver content as in the case of yAgNaX(400) (Figure 1). (ii) The adsorption isotherms of the zeolites dehydrated at 400 and 500 O C are practically the same before and after the oxygen treatment. (iii) The isotherms of xenon in the partially dehydrated zeolites lOAgNaY(26) and AgY(26) are markedly different from each other, although the water content of both samples is practically the same (Table 11). The isotherms of AgY(26) and AgY(400) start with different slopes at zero pressure whereas for the corresponding zeolites IOAgNaY the isotherms are practically identical initially. (iv) A comparison of Figures 1 and 3 reveals that the adsorption isotherms of xenon in 80AgNaX(400) and AgY(400) lie close together. Figure 4 shows the IBXe chemical shifts as function of the xenon concentration N for the zeolites lOAgNaY and AgY under various pretreatment conditions. lOAgNaY(400) exhibits a linear 6 vs N relationship with intercept o6, and slope that are slightly higher and lower, respectively, in comparison to Nay2' (dashed line in Figure 4). The oxygen treatment of this sample pulls down 6 at low concentrations, but leaves the shifts unchanged for N >, 1 molecules/supercage. The chemical shifts obtained for AgY(400) and AgY(400,ox) are about 65 ppm upfield from N a y . Over the whole concentration range measured not even the slightest effect of the treatment with oxygen is observed. At low xenon concentration the chemical shifts practically coincide with those of the zeolite 80AgNaX(400) (see Figure 2). In contrast to the silver-exchanged zeolites dehydrated at 400 or 500 OC the IBXe chemical shifts for the zeolites lOAgNaY(26) and AgY(26) are now downfield from dehydrated NaY and are similar to those of hydrated Nay2$with roughly the same water content (dotted line in Figure 4).
Discussion Fraissard et al.23have shown that the chemical shift of xenon adsorbed in a zeolite is the sum of terms corresponding to the various perturbations to which this probe is subjected. Consequently, the chemical shift of xenon in a zeolite is 6 = bmf + 6s + 6c + 6xc (1) 6,f is the reference, usually taken as the chemical shift of xenon gas extrapolated to zero pressure. & characterizes the xenon-wall interaction: it depends on the form and the dimensions of the void space and on the ease of xenon diffusion. bC refers to the interaction of xenon with the cations of the zeolite, and axe = tixtxe pXcrwhere pxc is the local density of xenon in the void spaces, describes the shift contribution due to xenon-xenon collisions. In the realm of the additive interaction scheme expressed by eq 1 (25) Gedeon. A,; Ito, T.; Fraissard, J. Zeolites 1988. 8,176.
l
I
10" atoms / g framework
id Pa
dP
N / atoms per supercage
I
Figure 5. Plot of '29Xechemical shift extrapolated to zero xenon concentration vs initial slope of the adsorption isotherms for xenon in the silver-exchanged NaX zeolites dehydrated at 400 OC.
this latter term only accounts for the dependence of 6 on the xenon concentration N leading to an increase of 6 with N. Hence, the extrapolation of 6 to zero xenon concentration, should yield the sum of the first three terms of eq 1 It has been shown that in the sodium forms of X and Y type zeolites 6c can be neglected and therefore 6N.0 N + 6s in these types of zeolite^.^^,^' Consequently, deviations of 6N=o for cation-exchanged X and Y zeolites from the corresponding data of NaX and NaY may be considered to represent approximately the interaction of xenon with the exchanged cations ( 1 5 ~ ) . It is usually observed that the '29Xe resonance spectra at ambient temperature of xenon in zeolites consist of a single line in spite of the existence of two or more sites for xenon adsorption where the shifts are different. This is explained as being due to rapid exchange of the xenon atoms among these sites. For the simple two-site model, for instance, applied to the shifts a t zero xenon concentration, the observed averaged shift is I
where pi and 6 b , (i = a, b) are the fractional occupation and the chemical shift on the sites a and b, respe~tively.~~ In the rapid exchange regime the rate of the exchange (l/s)of the xenon atoms between the sites a and b fulfills the condition 1/7 >> uo16k=o 6h=ol,where yo is the resonance frequency. The most striking feature of the presently studied silver-exchanged X- and Y-type zeolites after high-temperature dehydration is the displacement of the 129Xechemical shifts to lower values, i.e. upfield, with respect to the corresponding sodium forms of these zeolites. With any other ion for sodium exchange studied so far21-23v2628 comprising H, alkaline, and multivalent cations including paramagnetic transition metals, the shifts have invariably been found to be downfield from the sodium forms; at least they remain unchanged. Similar downfield shifts are observed for the zeolites containing water mo1ecules25 or platinum p a r t i ~ l e s . ~ f ~ " ~ These statements refer especially to the shifts extrapolated to zero xenon concentration. The discussion to follow concentrates on two main questions which are intimately related to each other. (i) What is the state of silver in the supercages of the presently studied X- and Y-type zeolites? (ii) What type of xenon cage interaction is responsible for the unprecedented upfield l2 Xe N M R chemical shifts observed? In this context the chemical shifts extrapolated to zero
4
(26) Bansal, N.;Dybowski, C. J. Phys. Chem. 1988, 92, 2333. (27) Cheung, T.T.P.; Fu, C. M.; Wharry, S . J . Phys. Chem. 1988, 92,
5 170.
(28) Gedeon, A.; Bonardet. J. L.; Ito, T.; Fraissard, J. J . Phys. Chem. 1989,93, 2563. (29) Ito, T.;de Menorval, L. G.; Fraissard, J. J . Chim. Phys. 1983,80,
-5 .1.
(30)de Menorval, L. C.; Fraissard, J.; Ito, T. J . Chem. Soc.. Faraday
Trans. I 1982. 78,403.
(31) Hermerschmidt, D.;Haul, R. Ber. Bunsen-Ges. Phys. Chem. 1980,
84. 902.
2446 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
xenon concentration will mainly be considered because at finite concentration N the Xe-Xe interactions come into play.21 Consider first the results obtained for the fully dehydrated zeolites X, yAgNaX(400) (Figures 1 and 2). The increase of the xenon adsorption with the degree of silver exchange indicates that strong adsorption centers for xenon are introduced in the supercages which add to and/or replace weakly adsorbing centers characteristic for NaX. This increasingly strong adsorption becomes apparent most clearly in the increase of the initial slopes of the isotherms and the greater tendency to show saturation behavior. Figure 5 shows that the chemical shifts dNLo and the initial slopes of the adsorption isotherms of xenon in yAgNaX(400) taken from Figures 1 and 2 are correlated. This result in conjunction with the observed single line character of the 129XeN M R spectra suggests that the observed shifts upfield from the NaX are the result of the rapid exchange of xenon among the weak and strong adsorption centers with a negative value of 6 for xenon in contact with the latter. In the case of AgX a special situation seems to be prevailing where probably some strong adsorption centers are different from those in the partially exchanged zeolites. It is noted that the correlation seen in Figure 5 is quite unusual. In nickel-exchanged Y zeolite, for instance, the adsorption isotherms change at most slightly with the degree of ion exchange but the chemical shifts are influenced considerably.26*28Likewise, the presence of platinum in the supercages has only a slight effect on the adsorption isotherm29 but strongly affects the chemical ~hifts.~~JO From these qualitative consideration, it is, of course, not possible to draw any conclusions about the nature of these strong adsorption sites for xenon in the zeolites yAgNaX(400) as well as in yAgNaY(400) 0, = 10 and 100) where similar relationships between the adsorption isotherms and the chemical shifts show up. Consider now the results obtained for the zeolites IOAgNaY and AgY after high-temperature dehydration and oxidation (Figures 3 and 4). According to the literature the vacuum dehydration of low silver content Y zeolites at elevated temperatures leads to a disapperance of silver cations from the intracrystalline cavitiesI8 to yield high-nuclearity silver clusters located outside the zeolite crystallite^'^*'^-^^ or, as has been suggested by other authors,8J2Js inside the supercages. The black or dark gray color of the dehydrated zeolites has been ascribed to these clusters. Behind these processes is the autoreduction of Ag+ to Ago leaving O H groups and/or Lewis acid and ZO- sites on the framework32 and subsequent migration of Ago to form the stable clusters. The oxidation reverts the clusters to isolated Ag+ ions in the zeolite structure.I8 The presently obtained 129XeN M R results for xenon in lOAgNaY (Figure 4) are in full accordance with these findings if the displacements of 6N.0 downfield from NaY after vacuum dehydration and subsequently upfield after oxidation are attributed to the interaction of xenon with framework sites and/or silver clusters in the supercages generated by the autoreductive process, and with Ag+ ions in the supercages, respectively. Actually, it has been shown3)that the presence of framework sites of the types mentioned before leads to downfield shifts of the INXe resonances. Thus, this discussion gives some hint that the shift upfield from NaY is due to the interaction of Xe with Ag+ ions within the zeolite framework. The observed complete ineffectiveness of the treatment with oxygen at 400 OC (chemical shifts and adsorption isotherms) and 500 OC (adsorption isotherms) of the zeolite AgY dehydrated at these temperatures (Figure 4) is strong indication that in this zeolite the physical and chemical properties of the supercages are essentially the same before and after oxidation. Since virtually all authors agree that after the oxidative treatment silver is present as Ag+ (at least in the supercages) it is concluded that the same is also true for the zeolites after the high-temperature dehydration. (32) Jacobs, P. A.; Uytterhoeven, J. B.; Beyer, H. K. J . Chem. Soc., Faraday Trans. I 1979, 75, 56. (33) Barrage, M. C.;Bonardet, J. L.; Fraissard, J. J . Coral. Leu., in pres.
Grosse et al.
0
4 20
LO
NA8 /*tame
60
80
par un~rs.11
Figure 6. Plot of Iz9Xechemical shifts 15"~ versus the overall silver content of silver-exchanged zeolites X (circles) and Y (squares). Open symbols: dehydrated zeolites, closed symbols: zeolites dehydrated and subsequently oxidized. For details see text.
This conclusion agrees with X-ray results which indicate that the treatment with oxygen affects essentially only the silver distribution in the fi-~ages.'~ It further supports the interpretation given before that the '29Xe N M R shifts upfield from NaY are due to the interaction of xenon with silver cations. The notion, however, that the interaction of xenon with paramagnetic silver species (e.g., clusters) causes the unusual upfield shifts observed cannot be discarded a priori. On the basis of the information available from the literature, namely the silence of ESR'6J8*34*3S with vacuumdehydrated as well as oxygen-treated silver X and Y zeolites, and, on the other hand, the observations that X- and y-irradiati~n,'~*~* the reduction with or the interaction with 0 atom@ of such zeolites is the prerequisite to generate paramagnetic silver species in such zeolites, it is not very probable that magnetic couplings of xenon with unpaired electronic spins play a role. Even if the existence of paramagnetic silver species were considered, the Fermi contact coupling should result in downfield s h i f t ~ , 2 * , ~ ~ , ~ and the dipolar pseudocontact shifts can be estimated (using the relevant formula4' with gll and g, values characteristic for Ag2+37) to lead to negative 6 values of at most several ppm, taking into account the large radius of the xenon atom. In conclusion, the presently obtained results give some evidence that in vacuum-dehydrated AgY a t 400 OC Ag+ ions only are present in the supercages, and the unusual '29Xe N M R chemical shifts of the silver exchanged zeolites are due to specific xenon/silver cation interactions. The observation that in the partially dehydrated zeolites lOAgNaY(26) and AgY(26) the chemical shifts are downfield from fully dehydrated NaY and similar in magnitude to partially dehydrated NaY supports this view. Here the hydration shell of Ag+ prevents the direct access of xenon to the cation and the specific xenon/Ag+ interaction cannot come into play. The differences of 6 between partially dehydrated AgY and NaY may be due to a different structure of the hydration species of these cations and/or a different distribution of the (34) Chmelka, 8. F.;Ryoo, R.; Lin, S.-B.;de Menorval, L. C.; Radke, C. J.; Peterson, E. E.; Pines, A. J. Am. Chem. Soc. 1988, 110, 4465. (35) Narayana, M.; Kevan, L. J . Chem. Phys. 1985,83,2556. (36) A h - K a i s , A.; Vedrine, J. C.; Naccache, C. J . Chem. Soc., Faraday
Trans. 2 1978, 74, 959. (37) Kanzaki, N.; Yasumori, I. J . Phys. Chem. 1978,82. 2351. (38) Hermerschmidt, D.;Haul, R. Eer. Bunsen-Ges. Phys. Chem. 1981, 85, 739. (39) Buckingham, A. D.;Kollman, P. A. Mol. Phys. 1972, 23, 65. (40) Gedeon, A.; Bonardet, J. L.; Fraissard, J. J. Chlm. Phys. 1988,85* 871. (41) Jesson, J. P. In NMR ofParamagnetic Molecules; La Mar, G . N., Horrocks, W. De W., Holm, R. H., Eds.; Academic Press: New York, 1973; P 1.
Silver-Exchanged X- and Y-Type Zeolites residual water between the supercages and the &cages. The foregoing arguments concerning the xenon/cation interaction are believed to be also valid for the silver-exchanged X zeolites with exchange degree up to 80% because the chemical shifts of the vacuumdehydrated silver-exchanged X and Y zeolites exhibit similar dependencies on the silver content as well as on the xenon concentration. In this context it is of interest to consider the dependence of the chemical shift dNIo as function of the overall silver content (Figure 6). It is seen that (i) the shifts drop more rapidly the higher the silver content, and (ii) the shifts of AgY and 8OAgNaX are almost the same although the overall silver content is distinctly different. The first of these observations suggests that with increasing overall silver content the silver for sodium exchange initially occurs preferentially at nonsupercage sites, Le., in the D6 rings (site I) and in the @-cages (site 1', 11') whereas the supercage sites are noticeably populated only at higher silver content attaining 19.5 Ag+/uc on SI1 sites in the case of AgYe3 This conclusion is in accordance with literature data for hydrated4z and dehydrated'* low silver content Y zeolites where the preference of site I in the initial stage of the silver for sodium exchange was observed. On the basis of the arguments pursued up to here the second of the above statements suggests that the zeolites AgY and 8OAgNa X exhibit almost the same concentration of Ag+ in the supercages. This conclusion seems to be reasonable on the following grounds. In the oxidized zeolite 95AgNaX 20.5 Ag+/uc are found to be located on SI1 and further 14.4 Ag+/uc on SI11 sites.3 The latter figure just corresponds to the surplus of silver in 95AgNaX over the presently studied 80AgNaX. So, the assumption that 80AgNaX is identical with 95AgNaX bereft of the silver cations on SI11 immediately explains the present findings. It goes even further by stating that AgY and 80AgNaX exhibit not only the same concentration but also the same distribution of the silver cations over the sites available in the supercages; in both zeolites the SI1 sites are populated with concentration of about 20 Ag+/uc. This latter far-extending notion gives the key for a possible explanation of the very unusual behavior of the zeolite AgX with respect to both the chemical shifts and the adsorption isotherms. The initial very steep increase of the latter indicates the presence of adsorption centers on which the lifetime of adsorbed xenon atoms is very much longer than on the sites present in AgY and . reasonably, these centers are identified yAgNaX with ~ 1 8 0 Most with Ag+ cations on SI11 sites which according to the previous reasoning start to come into play not before the 80% exchange level of the X zeolite has been surpassed. This entails that, as long as these sites are not nearly fully covered by xenon, the Iz9Xe N M R shifts should exhibit no or at most a slight dependence on the xenon concentration. This is, in fact, observed experimentally a t low xenon loadings (Figure 1). It follows that dNIO can be identified practically with the shift of xenon in contact with Ag+ (42) Costenoble, M.; Maes, A. J. Chem. Soc., Faraday Trans. I 1978.74, 131.
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2447 on a SI11 site, It should be pointed out that the condition of rapid exchange of the xenon atoms among all types of sites must still hold since a single resonance was observed at all concentrations. The fact that the chemical shifts of AgX after oxidation decrease in comparison to the dehydrated state suggests that the number of Ag+ on SI11 sites becomes larger. It is imaginable that this increase results from the oxidation of silver clusters residing in the supercages. Such silver clusters should lead to a restriction of the volume accessible to xenon entailing a reduction of the saturation volume and a larger slope of the 6 vs N curve at higher xenon concentration." Actually, both effects are observed experimentally (Figures 1 and 2). Since the oxidation destroys, at least partially, the silver clusters, the saturation volume should be larger and the d vs N slope lower in the oxidized than in the dehydrated state of the AgX. Also these subtleties are reflected in the experimental results. This explanation implies that the interaction of xenon with silver clusters is not the source of strong adsorption and upfield shifts. In fact, it has been shown that after reduction with hydrogen both AgX(400) and AgY(400,ox) exhibit considerably reduced xenon adsorption, and xenon chemical shifts d,, that are downfield from NaY by about 30-120 ppm depending on the type of zeolite and reduction condition~.~~*"
Conclusions The present study was intended to inquire into the feasibility of silver-cluster formation in the supercages of the synthetic Xand Y-type zeolites on vacuum dehydration at elevated temperatures (400-500 "C). Some evidence has been obtained that under these pretreatment conditions in both types of zeolites only Ag+ ions are present in the supercages with the possible exception of the fully silver exchanged zeolite AgX. The preliminary results obtained in this work have shown that lBXe NMR is a very useful tool for the study of the interesting and important silver-in-zeolite system. The very unusual upfield shifts observed have been concluded to be due to a specific interaction of xenon with the Ag+ cations in the supercages. Tentatively, we suggest that this shielding might be due to a d,d, back-donation from Ag to Xe involving the silver 4d- and xenon 5d-orbitals. Similar phenomena have been recently observed for the "Se resonance in the presence of silver cations4s and formerly in the spectroscopy of other nuclei such as 29Si.46 Acknowledgment. This work was supported in the realm of the FrenchlGerman project PROCOPE. B.B. acknowledges financial support by Deutsche Forschungsgemeinschaft. Registry No. Ag, 7440-22-4; Xe, 7440-63-3. (43) Grosse, R.; Boddenberg, B.; Watermann, J.; Geilke, T. Manuscript in preparation. (44) Burmeister, R.; Gedeon, A.; Grosse, R.; Boddenberg, 8.;Fraissard, J. Unpublished results. (45) Dieden, R.; Hevesi, L. Bull. Magn. Reson. 1989, 11, 193. (46) Harris, R. K.; Kennedy, J. D.; McFarlane, W. In NMR and the Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic Press: London, 1978, p 309.
