Temperature effects on formation and transformation of alkali-metal

Langmuir 1992,8,1750-1756

1750

Temperature Effects on Formation and Transformation of Alkali-Metal Ionic Clusters in y-Irradiated Zeolites Xinsheng Liu and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dane, Indiana 46556 Received February 5, 1992. In Final Form: April 24, 1992

Temperature effects on ionic clusters created in the alkali-metal ion-exchangedzeolitessuch as sodalite, A, X, and Y during yirradiation were investigated using the electron spin resonance technique. The results revealed that temperature is the key factor which determines the nature of the ionic clusters being observed. The ionic clusters such as Na2+, Na32+, KS2+,and N43+were observed from the corresponding zeolites. Transformation between Na2+ and Nas2+was found on increasing temperature, indicating delocalization of the unpaired electrons. The overall electron acceptability of the zeolites, represented by the Sanderson electronegativity, is considered to be one factor which governs the formation and transformation on the ionic clusters.

Introduction Studies of the photoinduced electron transfer between adsorbed aromatic molecules and electron-accepting sites of zeolites1I2and the formation of charged clusters such as Nan("-')+ ( n = 2-6) and Kn("-l)+( n = 3 or 4)3-20 in zeolite cavities upon photoirradiation and chemical treatment have been active areas in research in recent years. Related to these are y-irradiation studies of zeolites where structural defects are created in zeolites and their catalytic activities altered.21-26 Although several explanations such as iron impurities, solid defect sites, strained T-O-T links,27and ionic clusters2 have been proposed for the

* To whom correspondence should be addressed.

(1)Iton, L. E.; Turkevich, J. J. Phys. Chem. 1978,82,200. (2)Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1991,95,506. (3)Kaaai, P. H. J. Chem. Phys. 1965,43,3323. (4)Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday SOC.1966,41,328. (5)Kasai, P. H.; Bishop, R. J. In Zeolite Chemistry and Catalysis; ACS Monograph Series 171;Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976;p 350. (6)Barrer, R. M.;Cole, J. F. J. Phys. Chem. Solids 1968, 1755. (7)Edwards,P.P.; Harrison,M. R.; Klinowski, J.; Ramdas,&;Thomas, J. M.; Johnson, D. C.; Page, C. J. J.Chem. SOC.,Chem Commun. 1984, 982. (8)Harrison, M. R.; Edwards, P. P.; Klinowski, J.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984,54,530. (9)Westphal, U.;Geismar, G. 2. Anorg. Allg. Chem. 1984,508,165. (10)Smeulders, J. B. A. F.; Hefni, M. A.; Klassen, A. A. K.; de Boer, E.; Westphal, U.; Geismar, G. Zeolites 1987,7,347. (11)Breuer, R. E. H.; de Boer, E.; Geismar, G. Zeolites 1989,9, 336. (12)Yoon, K. B.; Kochi, J. K. J. Chem. SOC.,Chem. Commun. 1988, 510. (13)Martens, L. R.M.; Grobet, P. J.; Jacobs, P. A. Nature 1985,315, 568. (14)Martens, L.R. M.; Grobet, P. J.; Vermerien, W. J. M.; Jacobs, P. A. Proc. Znt. Zeolite Conf.. 7th 1986.935. (15)Martens, L.R. M.; Vermerien; W. J. M.; Grobet, P. J.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1987. 31. 531. (16)Andekon, P. A,; Edwards, P.P. J.Chem. Soc., Chem. Commun. 1991,915. (17)Anderaon, P. A.; Singer, R. J.;Edwards, P. P. J.Chem. Soc., Chem. Commun. 1991,914. (18)Xu,B.; Kevan, L. J. Chem. SOC.,Faraday Tram. 1991,87,2843. (19)Liu, X.;Thomas, J. K. Chem. Phys. Lett., in press. (20)Kuranova, G. A. High Energy Chem. (Engl. Transl.) 1991,25,91. (21)Wichterlova, B.; Novakova, J.; Prasil, Z.Zeolites 1988,8, 117. (22)Nakazato, C.; Murayama, R.; Aoki, M.; Masuda, T. Bull. Chem. Soc. Jpn. 1987,60,2775. (23)Aboul-Gheit, A. K.; Summan, A. M. React. Solids 1990,8,179. (24)Vedrine, J. C.;Abou-Kais, A.; Massardier, J.; Dalmai-Imelik, G. J. Catal. 1973, 29,120. (25)Abou-Kais, A.; Vedrine, J. C.; Massardier, J. J.Chem. Soc., Faraday Trans. 1 1976, 71,1679. (26)Abou-Kais, A. J. Chem. SOC.,Faraday Trans. 1 1974,70, 1039. (27)Shih, S.J. Catal. 1983,79,390.

electron-accepting sites in zeolites, the dependence of the state and the very nature of the sites in zeolites on changing conditions are not established. Recently we reportedlg the formation of new ionic clusters such as Na2+ and Na32+in zeolites y-irradiated at 77 and 196 K and their transformations. Here we present further studies, using the electron spin resonance (ESR) technique, on electron-transfer phenomena that occur in alkali-metal-ion-exchanged(Li+, Na+, K+, Rb+, and Cs+) zeolites such as A, X, Y, and sodalite, under y-irradiation at different temperatures. Attention is focused particularly on effects of temperature on the ionic clusters created in the zeolites and on their transformations. From the results obtained, the very nature which governs the formation of the ionic clusters is proposed.

Experimental Section A. Sample Preparation. The Na+ zeolites were obtained from Aldrich except for Na sodalite, which was synthesized hydrothermally following the procedures given in the literature.% The Li+,K+,Rb+,and Cs+partially ion-exchanged zeolites were obtained by ion exchange with aqueous solutions of the corresponding cation chloride The ion exchange was carried out twice followed by calcination at 150 O C . The degree of ion exchange for the samples in this study was limited to 50-95% (analyzedby using the atomic absorption technique), depending on the nature of the cations. The exact numbers are given in Table I. The crystallinities of the partially ion-exchanged zeolites were checked using X-ray diffraction and confirmed to be similar to those of the original samples. The samples were calcined at 550 O C prior to transferringtoESR tubes, dehydrated further in the ESR tubes at 200 OC under vacuum (1X Torr) for 8 h, and then sealed with a hot torch. The ESR tubes were made of quartz with a low concentration of paramagnetic impurities. The samples were irradiated for 1h using a 10 OOOCi W o y-source witha dose rate of approximately 14.5krad/min (1rad = 100 erg/g = 6.242 X 1013eV/g) at three temperatures: 295 K (roomtemperature),196 K (acetone-dryice bath), and 77 K (liquidNz). For the ionic transformation studies,the samples were first y-irradiated at 77 K, then warmed to 196 K in a dry ice-acetone bath for 30 min, and finally warmed to 295 K at ambient conditions (30 min). B. ESR Measurements. ESR spectra were recorded on a Varian E-line Century Series electron spin resonance spectrometer operated at about 9.3 GHz with 100-kHz frequency modulation. For the samples irradiated at room temperature, (28)Barrer, R. M.;White, E. A. D. J . Chem. SOC.1952, 1561. (29)Breck, D. W. Zeolite Molecular Sieues: Structure, Chemistry and Use; John Wiley & Sons: New York, 1974; p 529.

0743-746319212408-1750$03.00/0 0 1992 American Chemical Society

Langmuir, Vol. 8, No. 7, 1992 1751

Alkali-Metal Ionic Clusters in 7-Irradiated Zeolites Table I. ESR Data of the Ionic Clusters Observed from the Zeolites ?Irradiated at Different Temperatures zeolite

Li7Nad LbNagX

NQ sodalite Nad

NaeoX

cluster observed Na2+ (77 K) Nm2+ (196 K) - (295 K) Na2+ (77 K) - (196 K) - (295 K) Na32+ (77

K)

Nas2+(196 K) - (295 K) Na2+ (77 K) Na32+ (196 K) - (295 K) N43+ (77 K)

N43+ (196 K)

- (295 K) Na55Y

N43+ (77 K) N 4 3 + (196 K) N43+ (295 K)

K76N&X

K32+(77 K)

Ks2+(196 K)

- (295 K)

no. of lines

g

7 10

1.9983 2.0003

A (G) M ( G ) 100 65

-

-

7 -

2.0063

85.0

25

1.9988 1.9987 2.0050 2.0026 1.9990 1.9983

30 33

11 11

-

10 10 7 10

-

13 13

-

13 13 13 10 10

-

-

1.9990 1.9990 1.9992 1.9983 1.9983 -

-

43 20

-

-

72 39.5

-40 20

28 27.5 30.0 30.0 32.5 12.5 12.8

12 12 15 15 15 6 6

the ESR spectra were recorded at room temperature, while the ESR measurements of those at low temperatures were made at 77 K. The g values were evaluated within 0.0005 accuracy using an internal DPPH standard (g = 2.0036). To avoid saturation of ESR signals, particularly for those of the ionic clusters, a microwave power as low as 0.01 mW was used. For the quantitative measurements of the spins, the method given by Algerm was followed.

Results Samples Irradiated at 296 K. Figure 1 shows three types of ESR spectra observed from the y-irradiated zeolite samples at 295 K: (1)a spectrum having a central signal (g = 2.0031) with two small signals symmetrically located on each side (separated by 74 G) (this type of spectrum was observed for the samples N d , K76Na4X, KwNaaY, RbdazzX, and Cs~Na30X(Figure la)); (2) a spectrum with a type 1signal and a signal having a g value of 2.0123 (this type of spectrum was observed for the samples NQ sodalite, L i ~ N a dNa12A, , K 4 N 4 , and Li46NaMX (Figure lb)); (3) a spectrum with a type 1signal together with a signal having hyperfine structure due to formation of ionic clusters in the zeolite cavities3-16 (the spectrum was only observed for the sample Na55Y (Figure IC)).The signals of type 1 (see Figure la), which have been extensively studied and ascribed to a defect "E' center" (g = 2.003) for the strongest signa131 and a "hydrogen-associated defect center" for the tiny doublet separated by 74 G32(see Figure la), result partially from the defects created in the supersil tube used. Contributions to the signals from zeolite defects can also be assigned to these centers that provide possible traps for the escaped electrons. Due to the overlap of the signals with the signals from the quartz tube, these signals will not be addressed further in the following discussion. The signal having a g value of 2.0123 (Figure lb) has been assigned to an electron hole on a framework oxygen atom2192446 (model I or model 11). The absence of the hyperfine structure of the signal indicates that the electron hole on the oxygen does not interact with A1 nuclei or (30)Alger, R.S.Electron Paramagnetic Resonance: Techniques and Applications; Wiley-Interscience: New York, 1968; p 184. (31)Pfeffer, R. L.J. Appl. Phys. 1985, 57, 5176. (32)Tsai, T. E.; Griecom, D. L.; Friebele, E. J. Phys. Rev. Lett. 1988, 61, 444. (33) Tsai, T. E.; Griscom, D. L. J . Non-Cryst. Solids 1987, 91, 170.

alkali-metal cations, in contrast to what was observed for H+-form zeolites, HZSM-5 and H-Y12lpz6where hyperfine structures indicating interactions with A1 were observed (model 11). The electron hole on the framework oxygen ESi-OO-SiE ESi-OO-Ale model I model I1 in the present case may interact only with the Si atom present in the forms of =Si-O"-Si= (model I) and =Si-Oo-, which do not give hyperfine structure (%Si, natural abundance 95.3 % ,I = 01,and may be highly delocalized over several framework at0ms.~l93~ If there were any strong interactions between the hole and the A1 or alkali-metalatoms, a signal with hyperfine structure would be observed. From a comparison of the samples, it follows that formation of the electron hole during y-irradiation is not only dependent on the compositional Si/A1 ratio of the sample, but also on the nature of the exchanged cations. The most intense signal of the electron hole was observed from samples with lower Si/A1 ratios (see Figure lb) and for the zeolite X with different exchanged cations, i.e., from the samples Li~Na34Xand K76Na&. However, the escaped electrons trapped by cations in the zeolites were only observed for the sample NabsY at 295 K. In the Na55Y, an electron was trapped by four Na+ cations, giving a hyperfine structure with 13 line^^-^ P N a , natural abundance loo%, I = 3/2, 2 n I + 1 = 2 X 4 X 3/2 + 1= 13,n is the number of Na nuclei) (see Figure IC)and a pink color, which is consistent with the observations reported in the l i t e r a t ~ r e . ~ -The l ~ species observed and the data obtained from the spectra are summarized in Table I. Samples Irradiated at 196 K. Samples -y-irradiated at 196 K gave typical spectra shown in Figure 2. Three types of spectra are also observed: spectra typical of the electron hole located on the framework oxygen (Figure 2a,b), Li46Na34X, RbMNazzX, Cs~Na3oX,and K 4 N 4 belong to this category; spectra of the alkali-metal cation clusters (Figure 2c,d), K76Na& and N-X belong to this group; and complex spectra of electron holes and metal cation clusters (Figure 2-1, L i 7 N 4 , NQ sodalite, NalzA, and Nas5Ygive this type of spectrum. In addition to these signals, a weak doublet separated by about 503 G is also observed, which is due to hydrogen atoms20123-25 created during y-irradiation (see Figure 2b,c for the small signals on either side of the central signals). Irradiation of the NQ sodalite and Nal2A a t 196 K gave spectra which are different from those seen for the Na43+ clusters. The spectra are composed of hyperfine structures with 10 lines. The hyperfine coupling constants are 33 G for the NQ sodalite and 39.5 G for the NalzA which are bigger than that observed for Na43+(Figure2f,g). Patterns of 10lines are expected for an unpaired electron interacting with 3 equiv of Na+ ions (I = 3/2, natural abundance 100%, 2nI+ 1= 2 X 3 X 3/2 + 1 = 10). Therefore, we assign the patterns to Na32+clusters. The line widths are 11G for the Nas sodalite and 20 G for the Nal2A. Besides signals due to ionic clusters, the spectrum of the NQ sodalite sample also contains other very broad signals at the low magnetic field side, which overlap with signals of the ionic clusters (see Figure 20. Very recently, Anderson et al.35 have reported an observation of Nas2+ ionic clusters in partially Li+ ion-exchanged zeolites X and Y which were treated with solvated electrons. Compared with their hy(34)Wichterlova, B.; Beran, S.; Novakova, J.; Prasil, Z.Acta Phys.

Chem. - .. . 1985.31. 45. -- - - - I

- - I

(35)Anderson, P. A.; Barr, D.; Edwards,P. P. Angew. Chem.,Znt. Ed. Engl. 1991, 30, 1501.

Liu and Thomas

1752 Langmuir, Vol. 8, No. 7, 1992

t 5 I

U

Figure 2. ESR spectra of zeolites y-irradiated at 196 K: (a) KdNad; (b) LiMNaUX;( c ) NmX; (d) KwNFQX;(e) Li7NasA; (f) Nae sodalite; (g) Na& (h) Na55Y.

perfine coupling constant (a = 45 G),35 the hyperfine coupling constants observed for the zeolites NalZA and Na sodalite in this study are smaller (see Table I). At 196 K, however, the irradiated N-X and Na55Y gave ionic clusters with patterns of 13 lines (Figure 2c,h),

which are characteristic of Na3+. The hyperfine coupling constants of the clusters in these samples are 27.5 G for the N-X and 30 G for the Na55Y. The line widths are 12 and 15 G, respectively. From Figure 2, it is seen that the spectrum of the sample

Alkali-Metal Ionic Clusters in y-Irradiated Zeolites Li7NasA (Figure 2e) also exhibits a pattern of 10 lines with a hyperfine coupling constant A = 65 G and a line width AH = 20 G. Because the sample is partially Li+ ion-exchanged, the ionic clusters Li32+ and Nas2+ or a mixture of both could be formed in this sample, since both 7Li (natural abundance 92.57 5% ) and 23Nanuclei have I = 3/2. However, on the basis of four facts ((1)the coupling constant of an unpaired electron with 7Linuclei should be small, as observed from measurements of F centers of irradiated LiCl and LiF crystal^;^ (2) the total spin density on the ns orbital of the cluster calculated on the basis of the isotropic coupling of the free Li atom36is 136% for Li32+,which is clearly too large; (3) formation of sodium ionic clusters Na32+ in Li+ ion-exchanged zeolites X and Y has been observed;35 and (4) theoretical calculations by and Dimukhambetov et al.38predict that the electron affinity of Li+ cation in Li+ ion-exchanged zeolites is very weak compared to that of Na+),we assign the spectrum of the sample to the Na32+ cluster. The unpaired electron in the LiyNad, therefore, interacts with 3 equivof Na+ions. Under the same conditions, LiaNa34X and Li27Na28Y do not show spectra characteristic of ionic clusters (Figure 2b). The spectrum of sample K76Na& exhibits a pattern of 10 lines with a hyperfine coupling A = 12.8 G (Figure 2d) and a line width AH = 6 G. A pattern of 10lines is expected for an unpaired electron interacting with 3 equiv of K+ ions (39K and 41K,natural abundance 93.08% and 6.91 5% , both I = 3/2, 2nI + 1 = 2 X 3 X 3/2 + 1 = lo), suggesting that Ka2+clusters are formed in the zeolite cavities. Very recently, the new potassium cluster K32+was reported17J8 with samples prepared by treatment of K+-A and K+-X with potassium vapor and the hyperfine splitting a = 12.8 G was observed. The pattern and the hyperfine coupling constant observed for the y-irradiated sample are consistent with those reported, indicating that the same species is obtained. Under the same conditions, K 4 N a and KmNa5Y do not show formation of ionic clusters. Samples Irradiated at 77 K. Samples irradiated at even lower temperature (77K) gave three common features in their ESR spectra (Figure 3): (1)the electron hole was observed from all the samples except for K76Na.& and N a d , as exemplified by the spectra of Figure 3; (2) the atomic hydrogen was observed in all samples (see the spectra in Figure 3a-c as examples) (the signals are about 2 orders of magnitude more intense than those at 196 K); and (3) new ionic clusters such as Na2+ in L i ~ N a dand LiaNa34X (Figure 3a,c) and Naz+ in NalzA (Figure 3b) were observed. The creation of atomic hydrogen is due to the presence of OH groups and residual water molecules. The residual water or OH groups at 77 K evidently act as electron traps and then decompose into atomic hydrogen. However, at higher temperatures (196 and 295 K), either the protons cannot function as effective electron traps, as demonstrated by the ESR spectra of the samples at these temperatures, or the H atoms react. At 77 K, it is obvious that the electron hole is much more stable than at high temperatures, as indicated by its signal in most of the samples. The significant enhancement in intensity of the atomic hydrogen also reflects this point. The ionic clusters formed at this temperature reflect the effects of temperature on the movement of the escaped electrons. Li7NasA and Li46Na34X gave spectra having (36) Holton, W. C.; Blum, H. Phys.Reo. 1962,125, 89. (37) Beran, S. J. Phys. Chem. Solids 1982,43, 221. (38) Dimukhambetov, E. E.; Lygin, V. I.; Chadiarov, E. G. Khim. 1988,62,226.

Zh. Fiz.

Langmuir, Vol. 8, No. 7, 1992 1753

I

1. Y

l

I

w

!

! 1

/,".,

"

n

iai I(

I

a,G I

I

I

I

l

l

1

"

1

12.50

1

1

I

l

l

I O I,".,

Figure 3. ESR spectra of zeolites y-irradiated at 77 K (a) Li7N a d ; (b) Na12A; (c) L i d & ; and (d) K78NaX. The asterisk denotes the signal due to the defect from the quartz tube. The symbol H denotes signals from atomic hydrogen.

hyperfine structures with seven lines (see Figure 3a,c). A pattern of seven lines is expected for an unpaired electron interacting with 2 equiv of alkali-metal cations. The hyperfine coupling constants are 100 G for the Li7Na& and 85 G for the Li~Na34X. The line widths are larger for both samples compared to those of other ionic clusters (see Table I), 43 G for the Li7NasA and 25 G for the Li46Na34X. For the same reasons as those mentioned for the samples irradiated at 196 K, the spectra with hyperfine structure are assigned to NaZ+clusters. However, the Li27NazaY samples does not show a hyperfine structure typical of the ionic clusters. Nas sodalite irradiated at 77 K showed the same spectrum as that seen at 196 K, having a hyperfine structure characteristic of the ionic clusters NaS2+. Na12A, however, showed a spectrum indicating formation of ionic cluster NaZ+. Its hyperfine structure has seven lines with a hyperfine coupling constant A = 72 G and a line width of -40 G (Figure 3b). The pattern is similar to what has been seen by Kuranova,2° but the hyperfine coupling constant is smaller (83.1G was obtained by the authorm). N a d and Na55Y irradiated a t this temperature gave the spectra similar to those obtained at 196 K, showing the characteristics of the Na43+ clusters. For the K+-form samples, only the K76Na& (Figure 3d) gave a spectrum having hyperfine structure, which shows formation of K32+ionic clusters. For the R b d a D X and CsaoNa3oX samples, no spectra of ionic clusters were observed, similar to what has been seen for the corresponding 295 K samples. The species observed and the corresponding data obtained from the spectra are given in Table I. Formation of the Ionic Clusters. Formation of the ionic clusters Nh3+ in the zeolites was monitored as a function of irradiation time for the sampleN m Y irradiated a t 295 K. Figure 4 shows changes in concentration of the clusters (spins) in the sample as a function of irradiation time. It is seen that the concentration of the clusters in the sample is linear only over the time range shorter than 10 min. A t times longer than 10 min the slope changes, showing a slower increase of trapped electrons with

Liu and Thomas

1754 Langmuir, Vol. 8, No. 7, 1992

0

10 20 30 Gamma-Irradiation

40

50

60

Time (min.)

Figure 4. Changes in concentration of the N43+cluster as a

function of y-irradiation time.

irradiation time. After 50 min of irradiation, the concentration of the clusters reaches 3.5 X mol/g, corresponding to 2.1 X 1017 clusters/g. The yield (G value) estimated from this experiment using the data within 10 min is 0.9. Transformation of Ionic Clusters with Increasing Temperature. Transformation studies of ionic clusters were performedby irradiating the samples at 77 K followed by warming them to 196 and 295 K without further irradiation. The results revealed that clusters suchas Na2+, in the corresponding zeolites Nal2A and Li7Na&, underwent a change to the corresponding Na32+ clusters on increasing the temperature to 196 K. The number of nuclei with which the unpaired electron interacts increased from two to three. The transformation between these pairs of clusters is irreversible. The same experiments performed on other zeolites did not show such changes. The clusters such as Na32+in the zeolites Nas sodalite and Na3+in the N e and Na65Y are maintained without change a t both temperatures. Further warming of the samples to 295 K causes the signals of these clusters to disappear except for the Na3+ in the Na55Y. The spectra of the samples were similar to those seen at 295 K. Figure 5 shows temperature effects on the NaY sample y-irradiated a t 77 K. At 77 K, three kinds of species were observed: atomic hydrogen (the sharp signals separated by -503 G), electron hole (the signal denoted by an arrow in the figure), and Na3+ ionic cluster (the signals with hyperfine structure). After warming of the sample to 196 K for 4 min and then cooling it back to 77 K, the spectrum changessignificantly. The signals of the atomic hydrogens disappear, the intensity of the signals of the electron holes decreases, and so does that of the ionic clusters. These changes indicate that the back electron transfer from the atomic hydrogens and ionic clusters to the electron holes takes place on increasing temperature. During this process, about 359% of the ionic clusters lose their electrons. After further warming of the sample to 295 K for 1 min and then cooling it back to 77 K, the intensity of the signals of the ionic clusters further decrease and the signals of the electron holes almost completely disappear, showing further the back electron transfer. The ionic clusters, after this process, further decrease by 25%. Due to the back electron transfer reaction, it is understood that only the ionic clusters are observed at room temperature. Discussion The zeolites used in this study are structurally related. The basic building unit of the zeolites can be considered to be a sodalite cage. The sodalite structure is built up of sodalite cages only through sharing of their four-

77K _ 1

196k

-

4 mins.

295K 1 minutc c

Figure 5. Temperature effects on the N-Y

y-irradiated at 77 K. The arrow denotes the signals due to the electron hole. For other signals, see text.

membered rings along the x , y, and z directions. The cage formed by this connection is only the sodalite cage. The structure of zeolite A is formed by connecting the sodalite cages also through their four-membered rings along the x , y, and z directions but via oxygen bridges. The structures of zeolites X and Y (both are isostructural) are built up by connecting the sodalite cages through their sixmembered rings in a tetrahedral manner via oxygen bridges. The framework Si/A1 ratios, which determine the numbers of charge compensating cations, are 1.0 for the zeolites sodalite and A, 1.4 for the zeolite X, and 2.5 for the zeolite Y, which correspond to 96 M+ cations for the sodalite and zeolite A (normalizedto the compositions of zeolites X and Y),80 M+ for the zeolite X, and 55 M+ for the zeolite Y (M = alkali-metal cations). From the results shown above, it is understood that the events occurring during y-irradiation of the zeolites involve creation and transfer of the electrons and holes in the zeolites. The main processes are ESi-O-SiE u ESi-OO-SiE ESi-O-AlE u di-OO-AIE =Si+ + e- u =si* =Si+ + H + e- c,ESi-H' Z-OH

-

(H,O)

+

+ e-

H'

+ Z-0-

+ e-

+ e-

(1) (2)

(3) (4)

(Z = zeolite) (5)

nM+ e- M,(n-l)+( n = 2-4; M = cation) (6) Obervation of the trapped species such as structural defects, atomic hydrogen, and electrons trapped by the cations, i.e., the ionic clusters, depends on the stability to back electron transfer after irradiation at the temperatures used. Lowering the irradiation temperature slows

Langmuir, Vol. 8, No. 7, 1992 1755

Alkali-Metal Ionic Clusters in y-Irradiated Zeolites back electron transfer considerably. The escaped electrons are effectivelytrapped by alkali-metal cations and protons at low temperature and consequently give observable ESR spectra, whereas the concomitant electron holes are also stabilized at lower temperature, i.e., 77 K. Temperature effects on species such as electron holes, alkali-metal ionic clusters, and atomic hydrogen lead us to conclude that temperature determines the rates of the back electron transfer, and hence determines whether or not the species are observed (see Figure 5). At 77 K, the unpaired electronsare localized and interact either with one nucleus such as H, or with two, three or four cations in their vicinity. When temperature increases (either the irradiation temperature or the subsequent temperature after irradiation), the unpaired electrons become delocalized, transfer back to the holes, or interact with more cations. As a consequence, atomic hydrogen is no longer significant, and the number of cations participating in the electron trapping increases. Further increase of temperature (to 295 K) causes further delocalization of the unpaired electrons and for most samples leads to a disappearance of the ionic clusters. From the 295 K spectra, it is clear that the most stable species created during y-irradiation are framework defects. The temperature effect studies provide a detailed picture for formation of the ionic clusters in zeolites and transformation of the clusters. The general trend for the formation of ionic clusters found from present studies is that samples with lower Si/ A1 ratios (higher cation content) tend to localize the unpaired electrons among a small number of cations at low temperature, i.e., form ionic clusters with a small number of cations, and that the trapped electrons are delocalized or back transfer with increasing temperature (see Table I). However, for samples with higher SUA1 ratios such as Na55Y the ionic cluster is readily formed and temperature (in the range of 295-77 K) does not have a significant effect on the unpaired electron in the ionic cluster. To delocalize or destroy the electrons in the clusters requires a higher t e m ~ e r a t u r e . ~ , ~ To understand this general trend, particularly for the ionic clusters, the factors which govern the stability of the ionic clusters need to be considered. There are two factors which play important roles, i.e., the residual charge of the cations, and the geometrical arrangement of cations in the cavities. This first factor depends on cationframework interactions, while the second depends on site preference of the cations. Since the cation-framework interactions are of long-range chara~ter,~+~O we use the Sanderson electronegativity4l concept to account for the overall electron-trapping ability of the zeolites. The Sanderson electronegativity has already been successfully applied to zeolites for their physicochemical and a very recent theoretical work by Haug et aL40 on the absorption spectrum of an electron solvated in sodalite showed the usefulness of this concept. The interactions of a cation with its surroundin framework were indeed of long-range character (-40 ) and could only be well described by the electronegativity equalization model (Sanderson’s model) compared with four other models.40 On the basis of this model, it is not unreasonable to

x

(39) Mortier, W. J.;Schoonheydt, R. A. Prog. Solid State Chem. 1985, 16, 63. (40) Haug,K.; Srdanov, V.; Stucky, G.;Metiu, H.J. Chem.Phys. 1992, 95, 3495. (41) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic Press: New York, 1971. (42) Jacobs, P. A,; Mortier, W. J.; Uytterhoven, J. B. J. Inorg. N u l . Chem. 1978,40, 1919.

Table 11. Sanderson Electronegativities of the Zeolites (S.)

sample

S,

sample

S,

LbNd

2.3587 2.3237 2.2984

LtNmX N G

2.4494 2.4188 2.3550 2.2969 2.2525

NalzA &Na&

K76NG RbdazzX CswNmX

sample

S,

Li27Na~Y 2.6056

N-Y KwNasY

2.5857 2.5390

correlate the electron-trapping ability of the cations in different zeolites with the related structures. The electronegativities of the zeolites (S,) are obtained using the following Sanderson’s equation:41 S, = (M’Al’Sil92-10384)1/(X+X+(lg2-X)+3&1)

M = cation. The values calculated for all the zeolites used in this study are given in Table 11. It is seen from the values that the ability of the zeolites to stabilize an unpaired electron (electronegativity) decreases in the sequence of Li > Na > K > Rb > Cs, and that for the zeolites with the same cation decreases in the order of Y > X > A = sodalite. The expected trend for these zeolites is in agreement with what has been seen for the formation of the ionic clusters: for the zeolites A, the ionic clusters were observed for Li+ and Na+ samples but not for the K+ sample;for the zeolites X, the ionic clusters were observed for Li+, Na+, and K+ samples but not for Rb+ and Cs+ samples; and for the different zeolites with the same kind of cation, the ionic clusters are more easily formed in the zeolite Y. At room temperature, the ionic clusters were only observed in zeolite Na55Y but not in other zeolites, NQ sodalite, NalZA, and NmX. The trend seen in this study was also observed in studies of electron transfer between adsorbed aromatic molecules and ze0lit.e~:~where the zeolite Y shows a much stronger electron-accepting ability than zeolite X. The overall electron-accepting ability of the zeolites governs the electron trapping by the cations. Introduction of other cations such as Li+, K+, Rb+, and Cs+ into the Na+-form zeolites disturbs the formation of the ionic clusters. These “guest” cations function differently: Li+in the Li+partially ion-exchangedsamples tends to make the residual Na+ interact with the unpaired electron stronger compared to those without Li+. The Na2+ and Naz+ clusters formed in these samples with Li+ exhibit large coupling constants (see Table I). However, K+,Rb+,andCs+tend to prevent the formation of ionic clusters. Partially ion-exchanged zeolites A, X,and Y with these cations do not generally give ionic clusters after y-irradiation. The effects of the cations must be due to their interactions with the zeolite frameworks and, consequentially,to the effects of the cations on the charge of the Na+. In addition to the overall electron-acceptingability, the geometricalarrangement of the cations in the cavities must also be an important factor in affecting formation of the ionic clusters,44and in determining the number of cations in an ionic cluster. Using the known locations of cations in the the possible electron-trapping sites in the zeolites are tentatively proposed and schematicallyshown in Figure 6. For sodalite, only one site is possible which is composed of three B site cations (see ref 45 for the definition of the B site) located in the sodalite cage (see sodalite in Figure 6). For zeolite A, two sites are in its big (43) Liu, X.; Thomas, J. K. Manuscript in preparation. (44) Ogama, K.; Nitta, M.; Aomura, K. J.Phys. Chem. 1978,82,1655. (45) Motier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth Scientific Limited Guildford, England, 1982.

Liu and Thomas

1756 Langmuir, Vol. 8, No. 7, 1992

Sodalite

Zeolite A

I

300

500

700

Wavelength (nm)

Figure 7. Reflectance spectra of the y-irradiated N a Y at 295 K before (dash line) and after (solid line) introductionof toluene (8 mbar of vapor). Zeolite X and Y

Figure 6. Schematic representation of the locations of ionic clusters in the zeolites. The larger empty circle denotes the trapped electron, the small solid circle denotes Na+ cation, and the small shadowed circle also denotes the Na+ cation but at a level different from that of the small solid circle.

cage (a-cage): one is composed of one A site cation and one E site cation, and the other is composed of two A site cations and one E site cation. The former could transform to the latter under the conditions which allow the trapped electron to move from the center of the former site to the center of the latter site (see zeolite A in Figure 6). For zeolitesX and Y, there are two possible sites for the trapped electrons (see zeolites X and Y in Figure 6): the site composed of four C site cations in its sodalite cage and the site composed of four G site cations in its supercage, as shown in Figure 6. The location of the N843+ clusters in y-irradiated zeolite Y was first proposed by Kasai3 to be in the supercage of the zeolite. Later Edwards et al.'-* proposed that the location of the Nh3+clusters was in the

sodalite cage for their Na vapor treated samples. To know where the Na3+clusters are located in y-irradiated zeolites X and Y, toluene was introduced into the y-irradiated zeolite. The electron affinity of toluene is -1.3 eV,48and its size (-6 A) is much bigger than the entry aperture of the sodalite cage (2.6 A). Toluene can only enter the supercage and take the electron from the Nm3+ cluster in the supercage. UV-visible reflectance studies of the sample before and after introducing toluene molecules show a disappearance of the absorption band of the N a 3 + cluster around 500 nm in the spectrum (see Figure 7) upon introducing toluene molecules into the sample. This clearly indicates that the Na3+clusters present in the y-irradiated NaY are located inside the supercages. Acknowledgment. We thank the EPA for financial support of this work. (46) Birk, J. B. Photophysics of Aromatic Molecules; WileyInterscience: New York, 1970; p 462.