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129Xe NMR of AgX, CuX, ZnX and CdX zeolites. Comparative study of nd10 element-xenon interactions
18 March 1994
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 219 ( 1994) 440-444
129XeNMR of AgX, CuX, ZnX and CdX zeolites. Comparative study of yld” element-xenon interactions A. Gedeon, J. Fraissard Laboratoire de Chimie des Surfaces, assock! au CNRS, URA 1428, UniversitP Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Received 4 October 1993; in final form 4 January 1994
Abstract ‘29XeNMR spectroscopy of Xe adsorbed on silver-, copper-, zinc- and cadmium-exchanged sodium X zeolites is used for determining the location of these cations (inside or outside the supercages) and the nature of Xe-cation interactions.
1. Introduction lz9Xe NMR spectroscopy, introduced by one of us in the early 1980s as a means of characterizing microporous materials, has proved to be a useful tool for the determination of the void space, the structural and chemical properties of zeolites and the location and electrostatic effects of cations therein [ 1-5 1. In previous papers we have observed that the chemical shift of xenon adsorbed on AgX (Ag+: [ Kr]4d’O) was much less than that of NaX and even less than that of the quasi-isolated Xe atom [ 6-8 1. We attributed this exceptional result to the formation of an unstable Ag+-Xe complex whose lifetime was however, long enough for the instantaneous increase in the electron density, due to 4d”-5d0 transfer from Ag+ to Xe, to cause this variation of the chemical shift. Recently, we have shown that in dehydrated CuY zeolites, where the copper in the supercages is in the form of Cu+ with similar electronic structure to Ag+ (cu+: [ Ar] 3d’O), the chemical shifts of adsorbed xenon were also lower than in NaY [ 91. These results were attributed to the specific 3d’O-5d0 interaction between Cu+ and Xe.
We wished to extrapolate these results to all species with a d’O electronic structure (Ag+, Cu+, Zn2+ and Cd2+). In the present work, comparative results on the application of the ‘29Xe NMR technique to the study of AgX, CuX, ZnX and CdX are reported.
2. Experimental Starting with NaX zeolite (Linde 13X, Si/Al ratio= 1.2) exchanged samples (AgX, CuX, ZnX and CdX) were prepared conventionally by refluxing the zeolite with a 0.1 M aqueous solution of silver, copper, zinc or cadmium nitrate at 80°C for 12 h at pH 6. The solution was then filtered, the solid washed in distilled water and then dried at 100°C. The Ag+, Cu+, Zn2+, Cd2+ and Na+ cations were quantitatively analysed by atomic absorption spectroscopy. The samples prepared correspond to 86 k 1% cation exchange. The solids were evacuated at 26°C for 12 h, and then slowly heated to 400°C. The zeolites were held at this temperature for 12 h and then brought back to ambient temperature. The adsorption isotherms were measured volumetrically at 26°C. The
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van der Waals xenon diameter of 4.4 8, limits its sorption in faujasite-type zeolites to the supercages. The amount of xenon adsorbed is expressed as the number of Xe atoms, N, per gram of anhydrous solid. ‘29Xe NMR was carried out on a Bruker CXP 100 spectrometer operating at 24.9 MHz. For each sample, single-line NMR spectra were taken and the chemical shifts determined by using xenon gas at vanishing pressure as the reference. Depending on the nature of the sample and the xenon concentration, the chemical shift can be positive (i.e. downfield or high-frequency) or negative.
3. Results Fig. 1 shows the adsorption isotherms of xenon in fully dehydrated NaX, AgX, CuX, ZnX and CdX. NaX exhibits the usual type of xenon adsorption isotherm for monovalent-ion-exchanged X- and Y-type zeolites with an extended linear region at low pressure [2]. The adsorption of xenon on dehydrated AgX is strongly enhanced, especially at low pressures, and the isotherms exhibit well developed saturation behaviour. In the case of CdX, the adsorp-
tion capacity is still higher than that of NaX but lower than that of AgX over the range of pressures studied. However, the adsorption isotherm of CdX increases steeply at low pressure and runs almost parallel to that of AgX at higher pressures. Compared to NaX, the adsorption isotherms of xenon in CuX and ZnX are distinctly lower. Apart from a slightly concave initial portion, the adsorption isotherm for ZnX is practically linear. Conversely, in the pressure range studied the slope of the CuX isotherm decreases steadily with the increase of P. Fig. 2 shows the isotropic chemical shifts 6 of xenon in NaX, AgX, CuX, ZnX and CdX as a function of the xenon concentration, N. Compared to NaX with the linear 6 versus N dependence of monovalent-ion-exchanged X- and Y-type zeolites [2], the shifts in dehydrated AgX and CuX are distinctly lower over the range of concentrations studied. For AgX, the chemical shift decreases with concentration down to negative values in the range of about - 20 ppm at low xenon concentration. For CuX, the chemical shift is positive but always less than that for NaX at the
2.5E+21
2E+21
5E+2C ,
0 o,..,.,....s.,.-1.
0
200
400
600
“I”’
800
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P (Ton-) Fig. 1. Xenon adsorption isotherms at 26°C for the zeolites: ( A ) NaX; (+) AgX; (0) CdX, (m) ZnX; (Cl) CuX.
5E+20
1E+21
N (Xe atoms/g) Fig. 2. ‘*%e NMR chemical shift versus number of xenon per gram of anhydrous solid for the zeolites: (A ) NaX; (e) AgX; (0) CdX; (w) ZnX, (0) CuX.
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same xenon concentration. At low concentration, this shift is about 35-40 ppm. In contrast to the silver- and copper-exchanged zeolites the lz9Xe chemical shifts for zeolites ZnX and CdX are higher than that for NaX. For ZnX, the curve of 6=f(N) passes through a minimum at low xenon concentration and becomes roughly parallel to that for NaX at high xenon concentrations. The chemical shift S,=, extrapolated to zero concentration is about 82 ppm. In the case of CdX, the shifts are much higher than the previous ones. For example a,=, = 150 ppm for CdX, and the value of 6 at N= 1O*Ofor CdX is 140 ppm, greater than that for ZnX, 80 ppm.
4. Discussion The chemical shift of xenon adsorbed in a zeolite is the sum of the terms corresponding to the various perturbations to which this probe is subjected [ 21,
a,,,=0 is the reference (xenon gas at zero pressure). & is characteristic of the xenon-wall interaction; it depends on the form and the dimensions of the void space and on the ease of xenon diffusion. Ss, refers to the interaction of xenon with the strong adsorption sites (SAS) of the zeolites if there are any, and Sx, corresponds to xenon-xenon interactions. This latter term accounts for the dependence of S on the xenon concentration, N, leading to an increase of 6 with N. SE is due to the electric field created by the cations. S, expresses the contribution of the magnetic fields created by the paramagnetic compensating cations. Hence, the extrapolation of 6 to zero xenon concentration, r&,=0, should yield the sum 8s + 8sAs. It has been shown [ 21 that in NaY zeolites, S,, can be neglected at 26°C. The shift, S, is roughly independent of the %/Al ratio (S decreases by about 4 ppm as the Si/Al ratio increases from 1.2 to 54 [ lo] ), and, therefore, &Co= Ss. Consequently, for cation-exchanged Y zeolites, deviations of c&.=, from the corresponding data for NaY may be considered to represent the influence of the interaction of xenon with the exchanged cation (Js,). Generally, when the solid has adsorption sites of different strengths (e.g. Na+ and SAS) the residence time of xenon on the
SAS is relatively long, especially at low concentration; the electron cloud deformation is large and, consequently, the resulting chemical shift is much greater than for Nay. However, when the number of xenon atoms increases there is more and more xenon adsorbed on the weak sites. As a result of rapid exchange between the xenon atoms adsorbed on the various sites the chemical shift decreases, goes through a minimum and then increases with N when Xe-Xe interactions become important. Such variations due to the presence of SAS have been observed, for example, with zeolites containing alkaline earth cations [ 21, Ce3+ and La3+ ions [ 111 or supported metal particles (Pt, Rh, etc.) [ 21. The chemical shift of xenon adsorbed on these systems is always greater, at least for the lowest concentrations, than that of xenon adsorbed on NaY and NaX. It should be noted that this effect (increase of 6 and the existence of a minimum in the S=f(N) plot, more or less pronounced, depending on the SAS concentration) is much enhanced by the presence of paramagnetic cations, e.g. Ni*+ [ 12,131. The large positive shift and the parabolic behaviour of the S=f( N) curves in the case of divalent cations has been attributed first by Fraissard and Ito [ 21 to the high polarization of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2 + cations. Later, Cheung et al. [ 14 ] proposed a model to explain the strong adsorption of xenon in zeolites with 2+ cations (Ca*+, Mg*+, Ba*+ ). It consists of extending the electron attraction described above to the point where an electron is transferred from the xenon to the cation. This model suggests the formation of a partial bond between the xenon atom and the 2 + cation formed by donation of a xenon 5p electron to the empty s-orbital of the 2+ cation. Bond formation introduces low-lying electronic excited states which lead to a large paramagnetic contribution in the chemical shift (6> 0). This model can be applied only to the divalent cations which have electron affinity, M*++M+, comparable to the ionization potential of Xe+Xe+. A similar model concerning electron transfer from Xe was proposed by Fraissard et al. [ 15 ] to explain the high shift of S in platinum supported on NaY zeolite. It is clear that what is stated here must be modified by the secondary effects, amongst other things, the
A. Gedeon, J. Fraissard/ ChemicalPhysicsLetters219 (I 994) 440-444
location of cations in the supercages and the charge density of the AlO; groups in the zeolite cages. To date, only Ag+X, Ag+Y [ 6-81 and Cu+Y [ 91 zeolites have shown chemical shifts either negative or at least less than those for NaX or Nay. Let us consider the case of AgX. In previous papers, arguments were put forward that the feature of the chemical shifts of xenon in silver-exchanged X and Y zeolites, namely, the displacement of the resonances to lower frequencies than in the respective sodium forms, is due to a specific interaction of xenon with the silver cations in the supercage, especially silver cations in SIII sites [ 8 1. For steric reasons such cations allow the xenon atoms closer contact than Ag+ on SII sites. This could explain the greater efficiency of 4d,-5& back-donation from Ag+ to xenon involving the silver 4d and the xenon 5d orbitals. This mechanism is considered to be responsible for the observed low-frequency shifts. The inability of the SII silver cations to bring about appreciable resonance shifts may also be due to the special hybridized state of these ions [ 16 ] due to their strong interaction with the zeolite matrix. Moreover, the high xenon adsorption capacity of AgX zeolites, as shown in the adsorption isotherms (Fig. 1) indicates that there is a strong interaction between xenon and several silver cations. Consider now the results obtained for the zeolite CuX. Fig. 2 shows that the chemical shift decreases monotonically with N and is always lower than that in NaX. As in the case of CuY [ 9 1, these values of 6 and the absence of a minimum in the 6=f(N) plot lead us to conclude that there is no Cu*+ in the supercages (paramagnetic centres). This suggests that during dehydration at 400°C many Cu*+ ions have migrated towards sodalites and prisms and the Cu*+ ions residing in the supercages have been transformed by autoreduction to Cu+. These results are in agreement with the literature, which shows that the dehydration of Cu-faujasites is accompanied by autoreduction of cupric to cuprous ions [ 17,18 1. This result is also confirmed in the case of Cu*+-rho zeolites in which the Cu*+ ion is in contact with the xenon [ 19 1. The observed signal width is about 100 kHz whereas the lines detected in the CuX sample are not wider than 200 Hz. Line broadening due to the paramagnetic ions has also been shown in the case of the Ni*+Y zeolite [ 13 1. In a recent report [ 91 devoted to CuY zeolites, we
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have shown that the parallel between exchanged copper and silver is due to d” configurations of both Ag+ and Cu+ cations located on SIII sites allowing the back-donation mechanism to come into play. In this Letter, we show that in CuX zeolites we have the same behaviour. The shielding observed indicates the formation of an unstable short-lived Cu+-Xe complex, due to 3di0-5d0 donation from Cu+ to Xe. Buckingham and Kollmann [ 201 have shown that in the case of Xe-O2 or Xe-NO a contact shift can also be detected even if the lifetime of these complexes is short. The adsorption isotherm of CuX (Fig. 1) which is lower than that of NaX confirms that the cations Cu*+ migrate towards the small cages during the heating treatment. Then there is a cationic defect in the supercages (compared to NaX) which leads to a decrease in the adsorption capacity of the zeolite. At this level of cation exchange (86O/b), the number of Na+ initially located in the sodalites and prisms is too small to compensate for this migration. In the case of zeolites CdX and ZnX, the evolution of the 6 versus N curves is completely different from those of AgX and CuX (Fig. 2). The form of the curves, in particular the presence of a minimum, proves that the xenon interacts with strong adsorption sites which can only be Zn*+ or Cd*+ and shows that the nature of these interactions is different from the previous ones with Ag+ and Cu+ even though all these cations have the same d” electronic structure. The reasons which can be advanced to explain this difference are: - Location of the Zn*+ and Cd*+ cations: we have shown in previous studies [ 8 ] on analogous systems that only cations residing on crystallographic SIII sites could give rise to k-d, back-donation. We suggest here that the remaining Zn*+ and Cd*+ cations in the supercages after dehydration interact strongly with the zeolite matrix and are situated on SII sites which prevent such d, donation [ 2 11. - The predominance of the charge effect due to the high polarization of the xenon by the divalent cation: as the ionization potential of Xe ( 12.1 eV) is not too different from that for Zn+ +Zn*+ ( 17.9 eV) or that for Cd+ *Cd*+ ( 16.9 eV), there is a high probability, according to Cheung’s model [ 141, that an electron can be transferred from the xenon atom to the 2 + cation. This bond formation tends to a high and positive shift.
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For ZnX, the small positive deviation from linearity for N< 2 X 10” testifies to the presence at this high level of exchange of a few Zn2+ cations in the supercages. This result is in agreement with the literature which indicates the presence of a limited number of Zn in the supercages [ 22 ] and shows that these cations tend to migrate towards the small cages during the dehydration treatment [ 231. The ZnX isotherm confirms the decrease of the charges in the supercages due to this migration. In the case of CdX, and as expected for numerous strong adsorption sites, the 6 and N coordinates of the minimum are considerably higher than those for ZnX. This result suggests that at the same cationic exchange level, the number of Cd2+ located in the supercages is higher than that of the cations in ZnX. This conclusion is also confirmed by the small slope of the part of the 6 versus N curve before the minimum and by the fact that the xenon adsorption capacity for CdX is greater than that for ZnX (Fig. 1).
5. Conclusion
These results confirm that ‘29Xe NMR is a useful tool for studying at least qualitatively the location and the charge of Ag+, Cu+, Zn2+ and Cd’+ cations in X zeolites. We have shown that Ag+ and Cu+ can interact specifically with xenon (&O-e donation). In the case of the divalent Zn2+ and Cd’+ cations, this interaction could be masked by the charge effect. The positive chemical shift observed may be due either to the high polarization of xenon or to an electron transfer from a xenon 5p orbital to the empty orbital of the divalent cations.
References [ 1] T. Ito and J. Fraissard, in: Proceedings of the 5th International Conference on Zeolites ( 1980) p. 5 10. [2] J. Fraissard and T. Ito, Zeolites 8 (1988) 350. [ 31 J. Fraissard, A. Gedeon, Q.J. Chen and T. Ito, in: Zeolite chemistry and catalysis, eds. P.A. Jacobs, N.I. Jaeger and L. Kubelkova (Elsevier, Amsterdam, 1991) p. 46 1. [4] P.J. Barrie J. Klinowski, Progr. NMR Spectry. 24 ( 1992) 91. C. Dybowski, N. Bansal and T.M. Duncan, Ann. Rev. Phys. Chem. 42 ( 199 1) 433. R. Grosse, R. Burmeister, B. Boddenberg, A. Gedeon and J. Fraissard, J. Phys. Chem. 95 (1991) 2443. A. Gedeon, R. Burmeister, R. Grosse, B. Boddenberg and J. Fraissard,Chem. Phys. Letters 179 (1991) 191. R. Grosse, A. Gedeon, J. Watermann, J. Fraissard and B. Boddenberg, Zeolites 12 ( 1992) 909. [9] A. Gedeon, J.L. Bonardet and J. Fraissard, J. Phys. Chem. 97 (1993) 4254. [lo] Q.J. Chen, M.A. Springuel-Huet and J. Fraissard, Zeocat 90: catalysis and adsorption by zeolites (Elsevier, Amsterdam, 199 1) p. 129, and references therein. [ 111 Q. Chen, T. Ito and J. Fraissard, Zeolites 11 ( 199 1) 239. [ 121 N. Bansal and C. Dybowski, J. Phys. Chem. 92 (1988) 2333. [ 131 A. Gedeon, J.L. Bonardet, T. Ito and J. Fraissard, J. Phys. Chem. 93 (1989) 2563. [ 141 T.T.P. Cheung, C.M. Fu and S. Wharry, J. Phys. Chem. 92 (1988) 5170. [ 151 T. Ito, L.C. de Menorval and J. Fraissard, J. Chim. Phys. 80 (1983) 573. [ 161 J. Texter, T. Gonsiorowski and R. Kellerman, Phys. Rev. B 23 (1981) 4407. [ 171 G. Coudurier, T. Decamp and H. Praliaud, J. Chem. Sot. Faraday Trans. 178 (1982) 2661. [ 18 ] P.A. Jacobs, W. de Wilde, R. Schoonheydt, J.B. Uytterhoeven and H. Bayer, J. Chem. Sot. Faraday Trans. 172 (1976) 1211. [ 191 A. Gedeon and J. Fraissard, unpublished results. [20] A.D. Buckingham and P.A. Kollmann, J. Phys. Chem. 32 (1972) 65. [21] R.V. Hercigonja, V.M. Radak and I.J. Gal, Zeolites 12 (1992) 54. [22:] T.A. Egerton and S. Stone, J. Chem. Sot. Faraday Trans. I 69 (1973) 22. 12381K. Otsuka, J. Manda and A. Morikawa, J. Chem. Sot. Faraday Trans. I 77 ( 198 1) 2429.
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 219 ( 1994) 440-444
129XeNMR of AgX, CuX, ZnX and CdX zeolites. Comparative study of yld” element-xenon interactions A. Gedeon, J. Fraissard Laboratoire de Chimie des Surfaces, assock! au CNRS, URA 1428, UniversitP Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Received 4 October 1993; in final form 4 January 1994
Abstract ‘29XeNMR spectroscopy of Xe adsorbed on silver-, copper-, zinc- and cadmium-exchanged sodium X zeolites is used for determining the location of these cations (inside or outside the supercages) and the nature of Xe-cation interactions.
1. Introduction lz9Xe NMR spectroscopy, introduced by one of us in the early 1980s as a means of characterizing microporous materials, has proved to be a useful tool for the determination of the void space, the structural and chemical properties of zeolites and the location and electrostatic effects of cations therein [ 1-5 1. In previous papers we have observed that the chemical shift of xenon adsorbed on AgX (Ag+: [ Kr]4d’O) was much less than that of NaX and even less than that of the quasi-isolated Xe atom [ 6-8 1. We attributed this exceptional result to the formation of an unstable Ag+-Xe complex whose lifetime was however, long enough for the instantaneous increase in the electron density, due to 4d”-5d0 transfer from Ag+ to Xe, to cause this variation of the chemical shift. Recently, we have shown that in dehydrated CuY zeolites, where the copper in the supercages is in the form of Cu+ with similar electronic structure to Ag+ (cu+: [ Ar] 3d’O), the chemical shifts of adsorbed xenon were also lower than in NaY [ 91. These results were attributed to the specific 3d’O-5d0 interaction between Cu+ and Xe.
We wished to extrapolate these results to all species with a d’O electronic structure (Ag+, Cu+, Zn2+ and Cd2+). In the present work, comparative results on the application of the ‘29Xe NMR technique to the study of AgX, CuX, ZnX and CdX are reported.
2. Experimental Starting with NaX zeolite (Linde 13X, Si/Al ratio= 1.2) exchanged samples (AgX, CuX, ZnX and CdX) were prepared conventionally by refluxing the zeolite with a 0.1 M aqueous solution of silver, copper, zinc or cadmium nitrate at 80°C for 12 h at pH 6. The solution was then filtered, the solid washed in distilled water and then dried at 100°C. The Ag+, Cu+, Zn2+, Cd2+ and Na+ cations were quantitatively analysed by atomic absorption spectroscopy. The samples prepared correspond to 86 k 1% cation exchange. The solids were evacuated at 26°C for 12 h, and then slowly heated to 400°C. The zeolites were held at this temperature for 12 h and then brought back to ambient temperature. The adsorption isotherms were measured volumetrically at 26°C. The
0009-2614/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00098-B
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van der Waals xenon diameter of 4.4 8, limits its sorption in faujasite-type zeolites to the supercages. The amount of xenon adsorbed is expressed as the number of Xe atoms, N, per gram of anhydrous solid. ‘29Xe NMR was carried out on a Bruker CXP 100 spectrometer operating at 24.9 MHz. For each sample, single-line NMR spectra were taken and the chemical shifts determined by using xenon gas at vanishing pressure as the reference. Depending on the nature of the sample and the xenon concentration, the chemical shift can be positive (i.e. downfield or high-frequency) or negative.
3. Results Fig. 1 shows the adsorption isotherms of xenon in fully dehydrated NaX, AgX, CuX, ZnX and CdX. NaX exhibits the usual type of xenon adsorption isotherm for monovalent-ion-exchanged X- and Y-type zeolites with an extended linear region at low pressure [2]. The adsorption of xenon on dehydrated AgX is strongly enhanced, especially at low pressures, and the isotherms exhibit well developed saturation behaviour. In the case of CdX, the adsorp-
tion capacity is still higher than that of NaX but lower than that of AgX over the range of pressures studied. However, the adsorption isotherm of CdX increases steeply at low pressure and runs almost parallel to that of AgX at higher pressures. Compared to NaX, the adsorption isotherms of xenon in CuX and ZnX are distinctly lower. Apart from a slightly concave initial portion, the adsorption isotherm for ZnX is practically linear. Conversely, in the pressure range studied the slope of the CuX isotherm decreases steadily with the increase of P. Fig. 2 shows the isotropic chemical shifts 6 of xenon in NaX, AgX, CuX, ZnX and CdX as a function of the xenon concentration, N. Compared to NaX with the linear 6 versus N dependence of monovalent-ion-exchanged X- and Y-type zeolites [2], the shifts in dehydrated AgX and CuX are distinctly lower over the range of concentrations studied. For AgX, the chemical shift decreases with concentration down to negative values in the range of about - 20 ppm at low xenon concentration. For CuX, the chemical shift is positive but always less than that for NaX at the
2.5E+21
2E+21
5E+2C ,
0 o,..,.,....s.,.-1.
0
200
400
600
“I”’
800
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P (Ton-) Fig. 1. Xenon adsorption isotherms at 26°C for the zeolites: ( A ) NaX; (+) AgX; (0) CdX, (m) ZnX; (Cl) CuX.
5E+20
1E+21
N (Xe atoms/g) Fig. 2. ‘*%e NMR chemical shift versus number of xenon per gram of anhydrous solid for the zeolites: (A ) NaX; (e) AgX; (0) CdX; (w) ZnX, (0) CuX.
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same xenon concentration. At low concentration, this shift is about 35-40 ppm. In contrast to the silver- and copper-exchanged zeolites the lz9Xe chemical shifts for zeolites ZnX and CdX are higher than that for NaX. For ZnX, the curve of 6=f(N) passes through a minimum at low xenon concentration and becomes roughly parallel to that for NaX at high xenon concentrations. The chemical shift S,=, extrapolated to zero concentration is about 82 ppm. In the case of CdX, the shifts are much higher than the previous ones. For example a,=, = 150 ppm for CdX, and the value of 6 at N= 1O*Ofor CdX is 140 ppm, greater than that for ZnX, 80 ppm.
4. Discussion The chemical shift of xenon adsorbed in a zeolite is the sum of the terms corresponding to the various perturbations to which this probe is subjected [ 21,
a,,,=0 is the reference (xenon gas at zero pressure). & is characteristic of the xenon-wall interaction; it depends on the form and the dimensions of the void space and on the ease of xenon diffusion. Ss, refers to the interaction of xenon with the strong adsorption sites (SAS) of the zeolites if there are any, and Sx, corresponds to xenon-xenon interactions. This latter term accounts for the dependence of S on the xenon concentration, N, leading to an increase of 6 with N. SE is due to the electric field created by the cations. S, expresses the contribution of the magnetic fields created by the paramagnetic compensating cations. Hence, the extrapolation of 6 to zero xenon concentration, r&,=0, should yield the sum 8s + 8sAs. It has been shown [ 21 that in NaY zeolites, S,, can be neglected at 26°C. The shift, S, is roughly independent of the %/Al ratio (S decreases by about 4 ppm as the Si/Al ratio increases from 1.2 to 54 [ lo] ), and, therefore, &Co= Ss. Consequently, for cation-exchanged Y zeolites, deviations of c&.=, from the corresponding data for NaY may be considered to represent the influence of the interaction of xenon with the exchanged cation (Js,). Generally, when the solid has adsorption sites of different strengths (e.g. Na+ and SAS) the residence time of xenon on the
SAS is relatively long, especially at low concentration; the electron cloud deformation is large and, consequently, the resulting chemical shift is much greater than for Nay. However, when the number of xenon atoms increases there is more and more xenon adsorbed on the weak sites. As a result of rapid exchange between the xenon atoms adsorbed on the various sites the chemical shift decreases, goes through a minimum and then increases with N when Xe-Xe interactions become important. Such variations due to the presence of SAS have been observed, for example, with zeolites containing alkaline earth cations [ 21, Ce3+ and La3+ ions [ 111 or supported metal particles (Pt, Rh, etc.) [ 21. The chemical shift of xenon adsorbed on these systems is always greater, at least for the lowest concentrations, than that of xenon adsorbed on NaY and NaX. It should be noted that this effect (increase of 6 and the existence of a minimum in the S=f(N) plot, more or less pronounced, depending on the SAS concentration) is much enhanced by the presence of paramagnetic cations, e.g. Ni*+ [ 12,131. The large positive shift and the parabolic behaviour of the S=f( N) curves in the case of divalent cations has been attributed first by Fraissard and Ito [ 21 to the high polarization of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2 + cations. Later, Cheung et al. [ 14 ] proposed a model to explain the strong adsorption of xenon in zeolites with 2+ cations (Ca*+, Mg*+, Ba*+ ). It consists of extending the electron attraction described above to the point where an electron is transferred from the xenon to the cation. This model suggests the formation of a partial bond between the xenon atom and the 2 + cation formed by donation of a xenon 5p electron to the empty s-orbital of the 2+ cation. Bond formation introduces low-lying electronic excited states which lead to a large paramagnetic contribution in the chemical shift (6> 0). This model can be applied only to the divalent cations which have electron affinity, M*++M+, comparable to the ionization potential of Xe+Xe+. A similar model concerning electron transfer from Xe was proposed by Fraissard et al. [ 15 ] to explain the high shift of S in platinum supported on NaY zeolite. It is clear that what is stated here must be modified by the secondary effects, amongst other things, the
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location of cations in the supercages and the charge density of the AlO; groups in the zeolite cages. To date, only Ag+X, Ag+Y [ 6-81 and Cu+Y [ 91 zeolites have shown chemical shifts either negative or at least less than those for NaX or Nay. Let us consider the case of AgX. In previous papers, arguments were put forward that the feature of the chemical shifts of xenon in silver-exchanged X and Y zeolites, namely, the displacement of the resonances to lower frequencies than in the respective sodium forms, is due to a specific interaction of xenon with the silver cations in the supercage, especially silver cations in SIII sites [ 8 1. For steric reasons such cations allow the xenon atoms closer contact than Ag+ on SII sites. This could explain the greater efficiency of 4d,-5& back-donation from Ag+ to xenon involving the silver 4d and the xenon 5d orbitals. This mechanism is considered to be responsible for the observed low-frequency shifts. The inability of the SII silver cations to bring about appreciable resonance shifts may also be due to the special hybridized state of these ions [ 16 ] due to their strong interaction with the zeolite matrix. Moreover, the high xenon adsorption capacity of AgX zeolites, as shown in the adsorption isotherms (Fig. 1) indicates that there is a strong interaction between xenon and several silver cations. Consider now the results obtained for the zeolite CuX. Fig. 2 shows that the chemical shift decreases monotonically with N and is always lower than that in NaX. As in the case of CuY [ 9 1, these values of 6 and the absence of a minimum in the 6=f(N) plot lead us to conclude that there is no Cu*+ in the supercages (paramagnetic centres). This suggests that during dehydration at 400°C many Cu*+ ions have migrated towards sodalites and prisms and the Cu*+ ions residing in the supercages have been transformed by autoreduction to Cu+. These results are in agreement with the literature, which shows that the dehydration of Cu-faujasites is accompanied by autoreduction of cupric to cuprous ions [ 17,18 1. This result is also confirmed in the case of Cu*+-rho zeolites in which the Cu*+ ion is in contact with the xenon [ 19 1. The observed signal width is about 100 kHz whereas the lines detected in the CuX sample are not wider than 200 Hz. Line broadening due to the paramagnetic ions has also been shown in the case of the Ni*+Y zeolite [ 13 1. In a recent report [ 91 devoted to CuY zeolites, we
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have shown that the parallel between exchanged copper and silver is due to d” configurations of both Ag+ and Cu+ cations located on SIII sites allowing the back-donation mechanism to come into play. In this Letter, we show that in CuX zeolites we have the same behaviour. The shielding observed indicates the formation of an unstable short-lived Cu+-Xe complex, due to 3di0-5d0 donation from Cu+ to Xe. Buckingham and Kollmann [ 201 have shown that in the case of Xe-O2 or Xe-NO a contact shift can also be detected even if the lifetime of these complexes is short. The adsorption isotherm of CuX (Fig. 1) which is lower than that of NaX confirms that the cations Cu*+ migrate towards the small cages during the heating treatment. Then there is a cationic defect in the supercages (compared to NaX) which leads to a decrease in the adsorption capacity of the zeolite. At this level of cation exchange (86O/b), the number of Na+ initially located in the sodalites and prisms is too small to compensate for this migration. In the case of zeolites CdX and ZnX, the evolution of the 6 versus N curves is completely different from those of AgX and CuX (Fig. 2). The form of the curves, in particular the presence of a minimum, proves that the xenon interacts with strong adsorption sites which can only be Zn*+ or Cd*+ and shows that the nature of these interactions is different from the previous ones with Ag+ and Cu+ even though all these cations have the same d” electronic structure. The reasons which can be advanced to explain this difference are: - Location of the Zn*+ and Cd*+ cations: we have shown in previous studies [ 8 ] on analogous systems that only cations residing on crystallographic SIII sites could give rise to k-d, back-donation. We suggest here that the remaining Zn*+ and Cd*+ cations in the supercages after dehydration interact strongly with the zeolite matrix and are situated on SII sites which prevent such d, donation [ 2 11. - The predominance of the charge effect due to the high polarization of the xenon by the divalent cation: as the ionization potential of Xe ( 12.1 eV) is not too different from that for Zn+ +Zn*+ ( 17.9 eV) or that for Cd+ *Cd*+ ( 16.9 eV), there is a high probability, according to Cheung’s model [ 141, that an electron can be transferred from the xenon atom to the 2 + cation. This bond formation tends to a high and positive shift.
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For ZnX, the small positive deviation from linearity for N< 2 X 10” testifies to the presence at this high level of exchange of a few Zn2+ cations in the supercages. This result is in agreement with the literature which indicates the presence of a limited number of Zn in the supercages [ 22 ] and shows that these cations tend to migrate towards the small cages during the dehydration treatment [ 231. The ZnX isotherm confirms the decrease of the charges in the supercages due to this migration. In the case of CdX, and as expected for numerous strong adsorption sites, the 6 and N coordinates of the minimum are considerably higher than those for ZnX. This result suggests that at the same cationic exchange level, the number of Cd2+ located in the supercages is higher than that of the cations in ZnX. This conclusion is also confirmed by the small slope of the part of the 6 versus N curve before the minimum and by the fact that the xenon adsorption capacity for CdX is greater than that for ZnX (Fig. 1).
5. Conclusion
These results confirm that ‘29Xe NMR is a useful tool for studying at least qualitatively the location and the charge of Ag+, Cu+, Zn2+ and Cd’+ cations in X zeolites. We have shown that Ag+ and Cu+ can interact specifically with xenon (&O-e donation). In the case of the divalent Zn2+ and Cd’+ cations, this interaction could be masked by the charge effect. The positive chemical shift observed may be due either to the high polarization of xenon or to an electron transfer from a xenon 5p orbital to the empty orbital of the divalent cations.
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