J. Phys. Chem. 1995,99, 12278-12282
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Luminescence Studies on Intrazeolitic Migration of Tb(1II) Ions by Thermal Treatments. 2. LTA-Type Zeolites Suk Bong Hong,*jt Eun Woo Shin: Sang Heup Moon: Chong-Hong Pyun,? Chang-Hong Kim,? and Young Sun Uht Korean Institute of Science and Technology, P.O.Box 131, Cheongryang, Seoul 130-650, Korea, and Department of Chemical Engineering, Seoul National University, Seoul 151 -742, Korea Received: February 21, 1995; In Final Form: June 6, 1 9 9 9
Analysis of the luminescence spectra obtained from TbNa-LTA zeolites with different framework Si/Al ratios treated as a function of temperature reveals that the Tb(II1) ions located at site S5 in supercages migrate irreversibly to site S2' in sodalite cages by heating at 373 K. In particular, the cations in LTA zeolites with SUA1 ratios higher than unity are rearranged inside the sodalite cages when treated at temperatures higher than 573 K. This can be qualitatively correlated to differences in local environment of Tb(II1) ion sites in sodalite cages.
Introduction In our accompanying report we demonstrate the use of luminescence spectroscopy to monitor the intrazeolitic migration of Tb(II1) ions arising from thermal treatments in FAU-type zeolites.' The luminescence measurements reveal that the intrazeolitic migration pathway of Tb(II1) ions in FAU zeolites is strongly dependent on the Si, A1 ordering in the zeolite framework. Zeolite A (LTA topology) is one of the most widely studied and commercially important zeolites. The LTA structure is built from sodalite cages linked in a cubic array via double 4-rings (D4R). The connection of the sodalite cages results in a central cavity referred to as the supercage. This central cavity is connected to six similar cavities by an 8-ring window with an opening diameter of 4.2 A. LTA-type zeolites can be synthesized at a range of framework SUA1 ratios between 1 and 3, using Na alone or together with tetramethylammonium (TMA) ion. The purpose of this work is to investigate a series of TbNaLTA zeolites with different SUA1 ratios treated in the temperature region 300-773 K and to elucidate the influence of framework SUA1 ratio on the intrazeolitic location and migration behavior of the Tb(II1) ions in LTA-type zeolites.
Experimental Section Three LTA zeolites with different SUA1 ratios were prepared using Na alone or together with TMA according to the procedures described e l ~ e w h e r e . ~The . ~ LTA zeolites synthesized in the presence of TMA were calcined in flowing 0 2 at 823 K for 12 h to remove the TMA occluded. The calcined samples were then refluxed twice in 1.0 M NaN03 solutions for 6 h. The Tb(II1) ion exchange was performed by stirring zeolite powder in 0.05 M Tb(N03)3 solutions at room temperature for 24 h. The pH of the Tb(N03)3 solutions containing zeolite powder was adjusted to 6.5, in order to avoid the formation of any insoluble Tb species. After heating under flowing NZat the desired temperatures for 6 h, the TbNa-LTA samples were fully rehydrated over saturated N&Cl solution at room temperature for 2 days.
* To whom correspondence should be addressed. f @
Korea Institute of Science and Technology. Seoul National University. Abstract published in Advance ACS Abstracts, July 15, 1995.
0022-365419512099-12278$09.00/0
The analytical methods employed here are the same as described in our accompanying paper.'
Results and Discussion X-ray Analyses. X-ray powder diffraction patterns of all three LTA zeolites prepared in this work show that they have the LTA structure; no reflections other than those from these zeolites are observed. The crystallinity of each material remains unchanged during the Tb(II1) ion exchange, thermal treatment, and rehydration steps. Chemical Compositions of Zeolites. The chemical compositions of LTA zeolites before and after Tb(II1) ion exchange are given in Table 1. Note that the three LTA zeolites synthesized in this study span the SUA1 ratio range from 1.02 to 2.33. The unit cell compositions of TbNa-LTA samples reveal that as the zeolite has a higher SUA1 ratio, the number of Tb(II1) ions per unit cell decreases accordingly. From bulk chemical analysis, the percentages of Na(1) exchanged with Tb(111) can be calculated and are also listed in Table 1. All three TbNa-LTA zeolites have essentially the same percentage of Na(I) exchanged with Tb(II1). However, their percentages of Na(I) exchanged with Tb(1II) are much smaller than the values obtained from TbNa-FAU zeolite^.'^^ This can be attributed to the small aperture diameter (4.2 A) of LTA zeolites. The water contents of LTA zeolites studied in this work were determined from TGA and are converted to molecules per unit cell. The values listed in Table 1 show that the more Tb(II1) ions the zeolite has, the more water molecules it has. This indicates that the water coordination number of Tb(II1) ions in hydrated LTA zeolites is higher than that of Na(1) ions. Emission Spectra. Figure 1 shows the emission spectra of the three TbNa-LTA zeolites dried at room temperature after ion exchange. All the spectra give four emission bands at 491, 547, 587, and 624 nm, which can be assigned to the 5D4 7Fj transitions of the Tb(II1) ion. The emission band positions for these unheated TbNa-LTA zeolites are quite similar to those obtained from the TbNa-FAU z e o l i t e ~ . ' - ~ % ~ There is a slight but visible dissimilarity in the three emission spectra shown in Figure 1. For example, the electric-dipole 5D4 lF5 transition band at 547 nm is inhomogeneously broadened with increasing SUA1 ratio. In particular, the magnetic-dipole 5D4 7F4 transition band, which is observed
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J. Phys. Chem., Vol. 99, No. 32, I995 12279
Intrazeolitic Migration of Tb(1II) Ions, Part 2
TABLE 1: Chemical Compositions of LTA Zeolites Studied in This Work anhydrous unit molecular wt .% cell composition per unit cell SUA1 exchanged sample" NA-LTA(1) Na95A195Si970384 13 620 1.02 Nasi 7A181.7Si110.30384 13 320 1.35 Na-LTA(I1) Na~7.7A157.7Si134.30384 12 800 2.33 Na-LTA(II1) 1.02 19 Tb6.zNa76.4A195Si970384 14 170 TbNa-LTA(I) 1.35 20 Tb~~Na65.2A1~1.7Si110.30384 13 820 TbNa-LTA(I1) 2.33 18 Tb3.5Na47.zA157.7Si134.30384 13 110 TbNa-LTA(II1) a Fully hydrated at room temperature before or after Tb(II1) ion exchange. From TGA.
5D,-
? mi
n
g HzO/g
solidb
H20 molecules per unit cell
0.265 0.262 0.267 0.292 0.289 0.290
20 1 194 190 230 222 21 1
'F, D ' ,-
'F,
h
Y
(C)
400
450
500
550
600 Wavelength (nm)
650
700
Figure 1. Emission spectra of TbNa-LTA zeolites with different Si/ A1 ratios dried at room temperature after ion exchange. The SUA1 ratios of the framework are (a) 1.02, (b) 1.35, and (c) 2.33. The excitation wavelength used is 232 nm.
0
400
L
1
1
1
1
450
500
550
600
650
Wavelength (nm)
Figure 2. Emission spectra of TbNa-LTA zeolites with different Si/ A1 ratios heated at 773 K. The SUA1 ratios of the framework and the excitation wavelength used are the same as those in Figure 1.
.., ..,
10
at 587 nm in Figure la, splits into two bands at 585 and 592 nm in Figure IC. Such spectral differences can be attributed to differences in the vibrational interaction between the Tb(II1) ion and the zeolite f r a m e w ~ r k .This ~ indicates that an increase in the SUA1 ratio of LTA zeolites is sufficient to show matrix effects on the specific Tb(II1) emission band. The rare-earth ions are known to be predominantly placed at the center of aperture in the supercages of Na-A and Na-Y zeolites during the ion exchange step. The cations in the Na-Y are fully hydrated and thus are not coordinated by zeolite framework oxygem6 However, the rare-earth ions in Na-A cannot be fully hydrated because of the aperture diameter (4.2 A) of Na-A being much smaller than that (7.4 A) of Na-Y.6-8 This suggests that the vibrational interaction between the Tb(III) ion and the zeolite framework must be stronger in Na-A than in Na-Y. Therefore, it is most likely that the crystallographic site population and local environment of the Tb(II1) ions exchanged into LTA zeolites are dependent primarily on the local Si, A1 ordering in the framework, although no X-ray crystallographic studies have identified rare-earth ion sites of LTA-type zeolites with Si/Al ratios higher than 1.0. This is because the Tb(II1) ions exchanged into the supercages of LTA zeolites used in this study are coordinated to framework oxygens as well as water molecule^.^^^ The emission spectra obtained from the three TbNa-LTA zeolites heated at 773 K are shown in Figure 2. No significant differences in the emission band shape and position of 5D4 'F, transitions are observed, when compared to the emission spectra in Figure 1. This trend is different from that obtained from the emission spectra of TbNa-FAU zeolites treated as a function of temperature.'^^ Excitation Spectra. To more accurately examine changes in the location of Tb(II1) ions caused by thermal treatments, we have performed excitation measurements on a series of TbNa-LTA zeolites treated as a function of temperature. Figure
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. ... *,
(h)
200
* 217
j 22 1
220
240
0
Wavelength (nm) Figure 3. Corrected excitation spectra of TbNa-LTA(1)zeolites fully rehydrated after thermal treatments at different temperatures: (a) 300, (b) 373, (c) 423, (d) 473, (e) 523, (0 573, (g) 673, and (h) 773 K. The Si/AI ratio of the framework is 1.02, and the emission wavelength used is 545 nm.
3 shows the excitation spectra of TbNa-LTA(1) zeolites fully rehydrated after thermal treatments at different temperatures. Figure 3a was obtained from TbNa-LTA(1) dried at room temperature. This spectrum shows a broad band at 221 nm, which can be assigned to the 'F 'D transition between the 4f and 5d levels. The band position for this transition is quite
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12280 J. Phys. Chem., Vol. 99, No. 32, 1995
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Figure 4. LTA structure with possible cation positions.
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similar to that obtained from TbNa-Y.4 When TbNa-LTA(1) was heated at 373 K, the 7F 7D band shifts from 221 to 217 nm. No further shift of this band is observed by heating up to 773 K, as seen in parts b-h of Figure 3. However, the intensity of the 7F 7D band shifted to 217 nm becomes stronger with elevating treatment temperature. Re-exchange of the TbNaLTA(1) samples treated at 373-773 K yields no noticeable intensity changes in the 7F 7D band at 217 nm, indicating that the Tb(II1) ions are placed at nonexchangeable sites after thermal treatments. It is well-established that the band positions of 4f" 4f"-'5d transitions for rare-earth ions in inorganic glasses or crystals are very sensitive to variations in local envir~nment.~+'O The different cation sites in the LTA structure are illustrated in Figure 4. Site S2 is located at the hexagonal faces, and sites S2' and S2* are shifted away from the hexagonal faces into the sodalite cage and supercage, respectively, along a 3-fold axis perpendicular to the hexagonal face. Site S5 is the center of the 8-ring window of supercages, and site S3 is near the square faces located in supercages. The most probable candidate for the site represented by the band at 221 nm in Figure 3a may be site S5 rather than site S2* in supercages, since trivalent rare-earth ions in hydrated zeolites prefer a high water coordination number.6 On the other hand, the band appearing at 217 nm in parts b-h of Figure 3 can be assigned to the Tb(II1) ions located at site S2' in sodalite cages. Site S2* in supercages cannot be representative of the band at 217 nm, because the intensity of the band at 217 nm is not lowered by re-exchange with Na(1). In addition, sites S2 and S3 appear to be too small to accommodate the hydrated Tb(II1) ion because of steric constraints. Therefore, it can be concluded that the Tb(II1) ions located at site S5 in the supercages of Na-LTA(1) migrate to site S2' in sodalite cages when heated at 373 K. A shift of the 7F 7D band to a higher energy region indicates that the Tb(111) ions migrated to sodalite cages have excitation energy higher than the cations in supercages. Figure 5 shows distinct changes in the excitation spectra of TbNa-LTA(III) samples, which are caused by thermal treatments up to 773 K. The spectrum given in Figure 5a was obtained from TbNa-LTA(II) dried at room temperature. In the spectrum, the 7F 7Dband is observed at 224 nm. This value is slightly longer in wavelength than that obtained from an unheated TbNaLTA(1) sample. This indicates that the local environment of Tb(II1) ions in unheated TbNa-LTA(I1) is not exactly the same as that of the cations in an unheated TbNa-LTA(1) sample. Recall that the SUAl ratio (1.35) of TbNa-LTA(I1) is higher than that (1.02) of TbNa-LTA(1). Therefore, it is most likely that the 7F 7D band position is sensitive to differences in SUA1 ratio (vide infra). With elevating the treatment temperature to 373 K the 7F 7D band at 224 nm shifts to 219 nm
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.: :. . . i, j a
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Wavelength (nm)
Figure 5. Excitation spectra of TbNa-LTA(I1) zeolites with a SUA1 ratio of 1.35 treated in the temperature region of 300-773 K. The treatment temperatures and the emission wavelength used are the same as those of Figure 3.
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(Figure 5b). This 7F 7Dband shift to a higher energy region can be attributed to the migration of Tb(III) ions from supercages to sodalite cages in the Na-LTA(II) sample, as stated earlier. An interesting observation is that the 7F 7D band shifted to 219 nm shows a tendency to revert to the original position when the treatment temperature is higher than 573 K. As seen in Figure 5h, the TbNa-LTA(I1) samples heated at 773 K gives the 7F 7D band at 223 nm. Re-exchange of this sample with Na(1) yields no noticeable decrease in the intensity of the 7F 7D band at 223 nm. This indicates that the Tb(II1) ions are still placed in sodalite cages. Therefore, it appears that there exist at least two different Tb(II1) sites in the sodalite cages of the Na-LTA(I1) sample. Figure 6 illustrates the excitation spectra of TbNa-LTA(II1) zeolites treated as a function of temperature. Figure 6a was obtained from TbNa-LTA(II1) dried at room temperature after ion-exchange. The spectrum shows the 7F 7D band at 225 nm. Therefore, it is clear that the 7F 7D band position for TbNa-LTA zeolites differs significantly according to the framework SUA1 ratio of LTA zeolites. When TbNa-LTA(1II) was treated in the temperature region 373-573 K, the 'F 7D band observed at 225 nm in Figure 6a shifts to a higher energy region (222 nm), as demonstrated in spectra b-f of Figure 6. This is not unexpected since the same trend was observed in the excitation spectra of TbNa-LTA(1) and TbNa-LTA(I1) samples. Further thermal treatments of TbNa-LTA(II1) at temperatures higher than 573 K result in the 7F 7D band shift again to lower energy region, 225 nm (parts g and h of Figure 6). Effect of Si, A1 Orderings on the Excitation Spectra of TbNa-LTA Zeolites. Many physicochemical properties of zeolites such as thermal stability or acidity are known to be dependent primarily on the location of Si and A1 atoms in the zeolite framework. The excitation spectra presented in this work clearly show that the band position of the 7F 7D transition for TbNa-LTA(1) is not changed with thermal treatments at temperatures higher than 573 K, whereas that from TbNa-LTA(11) or TbNa-LTA(II1) shifts to a lower energy region. To +
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J. Phys. Chem., Vol. 99, No. 32, 1995 12281
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(a)
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Figure 7. Two possible sodalite cage arrangements which minimize A1-0-Si-0-AI linkages for a SUA1 ratio of 1.40: (a) meta and (b) para arrangements of A1 atoms in the eight 6-rings. The open and solid circles represent Si and A1 atoms, respectively.
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TABLE 2: 'F 'D Transition Band Positions for TbNa-LTA Zeolites with Different SUA1 Ratios
C
-2
4
L,. nm
.-%!
-m L
h
2
,, ..
, . * ,
0
200
220
I I
1
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Wavelength (nm) Figure 6. Excitation spectra of TbNa-LTA(II1) zeolites with a SUA1 ratio of 2.33 treated at different temperatures. The treatment temperatures and emission wavelength are the same as those of Figure 3.
elucidate the precise reason for this observation, therefore, it is of interest to consider the Si, A1 ordering of the sodalite cages in LTA zeolites with different SUA1 ratios. Among the various analytical methods, solid-state 29SiNMR spectroscopy has great potential in the direct determination of local Si, Al orderings." Until now, considerable progress has been made in elucidating the Si, Al distributions of a wide variety of zeolites using this technique. It is well-known that the Si, A1 distributions of LTA and FAU zeolites with SUA1 = 1.0 show strict alteration of Si and A1 atoms in the T sites of either framework because of Loewenstein's rule." This indicates that all eight 6-rings in the sodalite cage of these zeolites contain three A1 atoms. Therefore, it is most likely that there must be no significant differences in the local environment of Tb(II1) ions located at site S2' in the sodalite cages of LTA zeolites with SUAl = 1.0, although the local environment of Tb(II1) ions at site S2' depends on various factors such as level of hydration or the amount of Na(1) ions present together with Tb(III) in sodalite cages. Recall that the SUAl ratio of our TbNaLTA(1) sample is very close to unity. Therefore, the consistency of the 7F 7D band position in spectra b-h of Figure 3 can be attributed to the similarity in local environment of all possible sites S2' inside the sodalite cages of TbNa-LTA(1) zeolites. For a composition range 1.0 < SUA1 5 3.0, the 29SiNMR spectra of LTA and FAU zeolites with the same or very similar SUM ratios show significant differences in the relative intensities of the five local Si environments Si(OSi),(OA1)4-,, with n = 0-4."-14 This indicates that the Si, A1 distribution in LTA zeolites is significantly different from that in FAU zeolites. The Si, A1 distribution in FAU-type zeolites can be understood in terms of some ordered structural subunits, although the size of the substructures considered to be fully ordered is different for the various models and procedures used in the simulations of the Si, A1 distributions. To the contrary, no strong evidence of Si, A1 order within certain domains or structural subunits of the LTA framework has been found yet. However, the Si, A1 distribution in LTA zeolites is reported to exhibit a significant next nearest Al, A1 neighbor constraint which is comparable to that of FAU ze01ites.I~ This suggests that LTA-type zeolites
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sample
SUAl
300K
423K
523K
773K
TbNa-LTA(1) TbNa-LTA(I1) TbNa-LTA(II1)
1.02 1.35 2.33
221 224 225
217 219 222
217 219 222
217 223 225
show short-range statistical order, which can be rationalized in terms of a competition between the statistics of the sodalite cage formation step and the tendency of the system to avoid Al, A1 next nearest neighbor interactions. Further support for this speculation has been obtained from the Monte Carlo simulation study reported by S o u k ~ u l i s . 'Figure ~ 7 illustrates two possible Si, A1 ordering schemes of the sodalite cage which minimize the number of AI-0-Si-0-A1 linkages for a SUA1 ratio of 1.40. Melchior claims that for a particular composition range the Si, A1 ordering of LTA zeolites can, in principle, be described by ordered D4R units that are combined randomly within the framework under the restriction of Loewenstein's rule, if the resulting ratio of meta to para arrangements of A1 atoms in the eight 6-rings formed by the combination of six D4R units is properly controlled. l 4 As demonstrated in Figure 7, however, the sodalite cage arrangement for a SUA1 ratio of 1.40 can be expressed only by a preference for either meta or para arrangements of Al atoms in the eight 6-rings. This implies that the Al, Si distribution in LTA zeolites is very complicated, because the combination of meta- and para-Al-substituted 6-rings in a single sodalite cage is not possible. Although the sodalite arrangements given in Figure 7 cannot be regarded as the structural units which describe the experimentally derived populations of Si and A1 atoms in the LTA framework, they clearly show that the local environment of site S2' inside the sodalite cage can be crystallographically different in the Si, Al ordering of 6-rings. Notice that the SUA1 ratio of these two sodalite cages is quite similar to that of the TbNa-LTA(I1) sample studied in this work. We speculate that the state of the trivalent Tb(II1) ions located at site S2' shifted away from the 6-rings containing three A1 atoms in the sodalite cage may be more thermodynamically stable than that of the cations at site S2' above the meta- or para-Al-substituted 6-rings because of differences in the cation-framework interaction. If this speculation is correct, the 7F 7D band shift to a lower energy region, which is observed in parts g and h of Figure 5 , could be attributed to the rearrangement of Tb(1II) ions inside the sodalite cages of the TbNa-LDA(I1) zeolite. We did not carry out the model consideration for the Si, A1 ordering in the sodalite cage of which the SUA1 ratio is quite similar to that of TbNa-LTA(III), because the number of possible sodalite cage arrangements is much greater at higher SUA1 ratios. However, we believe that spectral changes observed from the excitation spectra of TbNa-LTA(II1) can be understood in a manner similar to those from TbNa-LTA(II). Table 2 lists the 7F 'D band positions for all TbNa-LTA zeolites studied in this work. The values listed show that TbNa-
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LTA zeolites with different SUA1 ratios give the 7F 'D bands at different positions, when they were treated at the same temperature. Our accompanying report demonstrates that as the 7D SUA1 ratio of FAU zeolites becomes higher, the 'F transition band from the Tb(II1) ions placed inside the same cage shifts to a higher energy region.' Unlike the case of TbNaFAU zeolites, however, the 7F 7Dtransition band from TbNaLTA zeolites appears at a lower energy region with increasing SUA1 ratio.' Further study is necessary to understand the precise reason for this behavior. In conclusion, it is observed from the luminescence measurements that most of the Tb(II1) ions in the supercages of LTA zeolites prepared in this work migrate irreversibly to sodalite cages by heating at 373 K. The rearrangement of Tb(1II) ions inside sodalite cages is caused by thermal treatments at high temperatures ('573 K), when the zeolite has a SUA1 ratio higher than unity. This can be attributed to crystallographicdifferences in the Tb(II1) ion sites present inside sodalite cages.
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Acknowledgment. We are grateful to Korea Institute of Science and Technology for financial support of this work.
References and Notes (1) Hong, S. B.; Shin, E. W.; Moon, S. H.; Pyun, C.-H.; Kim, C.-H.; Uh, Y.S. J . Phys. Chem. 1995, 99, 12274. (2) Kostinko, J. A. ACS Symp. Ser. 1983, 218, 3. (3) Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. W. ACS Symp. Ser. 1983, 218, 267. (4) Hong, S. B.; Seo, J. S.; Pyun, C.-H.; Kim, C.-H.; Uh, Y,S. Catal. Lett. 1995, 30, 87. ( 5 ) Tanguay, J. F.; Suib, S. L. Catal. Rev.-Sei. Eng. 1987, 29, 1. (6) Suib, S. L.; Zerger, R. P.; Stucky, G . D.; Momson, T. I.; Shenoy, G. K. J . Chem. Phys. 1984, 80, 2203. (7) Hazenkamp, M. F.; van der Veen, A. M. H.; Blasse, G. J . Chem. Soc., Faraday Trans. 1992, 88, 133. (8) Hazenkamp, M. F.; van der Veen, A. M. H.; Feiken, N.; Blasse, G. J . Chem. Soc., Faraday Trans. 1992, 88, 141. (9) Brixner, L. H.; Ackerman, J. F.; Foris, C. M. J . Lumin. 1981, 26, 1. (10) Dujardin, C.; Moine, B.; Pedrini, C. J . Lumin. 1993, 54, 259. (1 1) Engelhardt, G.; Michel, D. High-Resolution Solid-Stare NMR of Silicates and Zeolites; Wiley: New York, 1987. (12) Bennett, J. M.; Blackwell, C. S.; Cox, D. E. J . Phys. Chem. 1983, 87, 3783. ( 13) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak,M.; Magi, M. Z. Anorg. Allg. Chem. 1981, 482, 49. (14) Melchior, M. T. ACS Symp. Ser. 1983, 218, 243. (15) Soukoulis, C. M. J . Phys. Chem. 1984, 88, 4898. Jp9504985
