J . Phys. Chem. 1989, 93, 1401-1404 nificant discrepancies with rate constants from the literature are avoided in the present mechanism, and all rate constants optimized here are in good agreement with the literature values where available. The present study also provides a clearer insight into the role of the component processes in the different stages in the dynamical behavior of the system. The simplified negative feedback (M9) made possible by the higher order dependence on H+ should lead to a clearer understanding of some derivatives of the EOE system, such as the H202-S032--Fe(CN)64- reaction," in which no
1401
"Dushman-type" reaction is possible. Acknowledgment. This work was supported by National Science Foundation Grants CHE-8419949 and CHE-8800169. We thank Gyula Ribai and Kenneth Kustin for many helpful discussions. Registry No. IO3-, 15454-31-6; SO3*-, 14265-45-3; Fe(CN)64-, 3408-63-4. (18) Ribai,
Gy.; Kustin, K.; Epstein, I. R., submitted for publication.
29SiMagic Angle Spinning NMR Spectra of Alkali Metal, Alkaline Earth Metal, and Rare Earth Metal Ion Exchanged Y Zeolites Kuei-Jung Chao* and Jer-Young Chern Department of Chemistry, Tsinghua University, Hsinchu, Taiwan, Republic of China (Received: November 12, 1987; In Final Form: August 1 1988) ~
The variation of the extraframework cation location in groups IA and IIA metals and rare earth metal (RE) Y zeolites as a function of the dehydration and the rehydration is monitored by 29SiMAS NMR. Unheated hydrated zeolites give similar 29Sispectra as they present the similar cation distributions. Upon dehydration a high-field shift is observed which correlates with the distortion of bond angles in silicon-oxygen tetrahedra. The line shapes of 29Sispectra depend on the nature and the location of the exchangeable cations and the occupancy of the different sites in dehydrated and rehydrated states. The correlation between the line shape of 29Sispectra and the migration of cations from the supercages to the sodalite cages after heating treatment was studied. The results of 29SiNMR agree with the known structure data.
Introduction Zeolites are microporous crystalline aluminosilicates of general with x / y I 1. The net formula M,,,[(A102)x(Si02),]~mH20 charge of the tetrahedral aluminosilicate framework is neutralized by exchangeable cations M& of valence n; the void space of greater than 50% of the crystal volume is occupied by m molecules of water.' Due to the similar X-ray scattering powers of Si and AI atoms, the complete order of Si,Al in the lattice cannot be extracted from X-ray diffraction. Recently high-resolution solid-state 29SiN M R with magic angle spinning (MAS) has been shown to be effective in determining the Si,Al ordering in zeolites by providing direct information of the immediate local environment of Si atom^.^-^ As 29Sipeak position shifts either to lower field by an increase of the number of AIO, tetrahedra connected to Si04tetrahedron2s3 or to higher field by increasing the average Si-0-Si or Si-0-AI bond angle^,^,^ 29SiMAS N M R was shown to be successful in determining Si/AI ratio2 and Si,Al ordering of the tetrahedral aluminosilicate framework,6 monitoring the structural reorganization of TO, ( T = Si4+or A13+) unit in dealurnination,' and detecting temperature-induced phase transition in ~ilicalite.~ (1) Breck, D. W. Zeolife Molecular Sieves; Wiley: New York, 1974; pp 319-425. (2) Fyfe, C. A.; Thomas, J. M.; Klinowski, J.; Gobbi, G. C. Angew. Chem., lnt. Ed. Engl. 1983, 22, 259. ( 3 ) Lippmaa, E.; Magi, M.; Samoson, A,; Engelhardt, G.; Grimmer, A. R. J . Am. Chem. SOC.1980, 102,4889. (4) Ramdas, S.;Klinowski, J. Nature (London) 1984, 308, 521. (5) Engelhardt, G.; Radeglia, R. Chem. Phys. Lett. 1984, 3, 271. (6) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J . Am. Chem. SOC.1982, 104, 4859. (7) Grobet, P. J.; Mortier, W. J.; Van Genechten K. Chem. Phvs. Left. 1985, 119, 361. (8) Engelhardt, G.; Lohse, U.; Samoson, A,; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolifes 1982, 2, 59.
0022-3654/89/2093-1401$01.50/0
TABLE I: Composition and Dehydration Temperature of Exchanged Na-Y' no. of cations/u.c.
symbol 78La-Y 88Ca-Y 9 1Sr-Y 77Ba-Y 52Ce-Y 99K-Y
Na 12.5 6.7
dehydb temp, OC 350 RTCto 650 350 350 350
others 14.6 La 24.8 Ca 25.6 Sr 21.7 Ba 9.8 Ce 56.3 K
5.1
13.0 27.0
300
Na-Y composition: Na56.3(Alo2)S6,3(Si02),3~,,.~H20. The temperature was raised slowly (1 OC/min) to the desired value and kept in the temperature range of room temperature 350 OC for 6-16 h or at 650 OC for 24 h under continuous outgassing. CRoomtemperature.
-
The local environment of the silicate ions in an aluminosilicate framework also depends on the cation distribution and the nature of absorbed molecule in the extraframework ~ p a c e . ~ It J ~was reported by Melchior et a1.I0 that the 29SiN M R spectra of dehydrated LiNa-A, Li-A, and Na-A samples exhibited a different chemical shift with respect to their hydrated state. On the basis of 29Si MAS N M R study of hydrated and dehydrated Ca-Y zeolite, Grobet et aL7 suggested that the effect of the cations was more localized in the dehydrated state and the 29SiNMR spectrum of the dehydrated zeolite was more complicated than that of the hydrated state. In this paper we describe a detailed study of dehydration and rehydration of a series of alkali metal, alkaline earth metal, and rare earth metal ion exchanged Y zeolite samples (IA-, IIA-, RE-Y). The chemical shift and line shape of 29Si peaks were found to depend strongly on the degree of dehydration ~~
~
~~
~
~
~
(9) Klinowski, J.; Carpenter, T. A.; Gladden, L. F. Zeolites 1987, 7, 73. (10) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J.; Pictroski, C. E. Proc. 6th lnt. Zeolite Conf., Reno; 1984; 684-693.
0 1989 American Chemical Society
1402 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Chao and Chern CaY
TABLE 11: %i Chemical Shifts of Hydrated, Dehydrated, and Rehydrated Y Zeolites chemical shift 6 sample Na-Y 99K-Y 88Ca-Y 9 1Sr-Y 77Ba-Y 78La-Y 52Ce-Y
1st hyd -94.1 -95.9 -94.9 -96.0 -94.8 -94.9 -94.0
intense peak dehyd rehyd -95.7 -94.1 -97.2 -95.7 -97.7 -95.2 -99.3 -96.6 -98.1 -95.7 -99.4 -97.4 -100.2" -97.0"
2nd hyd -99.6 -100.4 -100.4 -101.7 -100.0 -100.4 -98.9
A
intense peak dehyd rehyd -100.0 -99.2 -102.5 -101.3 -101.6 -100.5 -103.4 -101.9 -102.3 -101.7 -102.7 -101.9
dehyd. dehyd dehyd
P\
:i
1 0 0 ° C dehyd.
"The chemical shift of the spectrum center.
and on the occupation of exchangeable cation sites after heat treatment. Comparison with X-ray diffraction results further provided insight on how the cation location is influenced by dehydration and rehydration.
\ A h
Experimental Section
1
,
1 -
-90-IO0
A binder-free zeolite Na-Y with Si:A1 ratio of 2.41 was obtained from Strem Chemical Co. To avoid possible deficiency in cations, it was further washed with 1 N NaCl solution, then with deionized water, and dried at room temperature. IA-, IIA-, and RE-Y zeolites were prepared by exchanging a portion of the throughly washed Na-Y zeolite with metal chloride solutions. In the following, the percentage of metal ion exchanged in the zeolites has been used as the notation for the samples, e.g., Ca2+exchanged level of 88% was obtained on the 88Ca-Y sample. The 99K-Y was prepared from a sample of 5 g of Na-Y by exchanging 3 times with 200 mL of 2 N KCI a t room temperature for 24 h. The 88Ca-Y, 91Sr-Y, and 77Ba-Y were prepared from 10 g of Na-Y by three 4-h exchanges with their respective chloride solution (500 mL at 1 N concentration) at 86 O C . The 43La-Y was prepared from 5 g of Na-Y by exchange once with 0.1 N LaC1, solution at room temperature. The 78La-Y was prepared from 5 g of Na-Y by two 3-day exchanges with 1000 mL of 0.1 N LaC1, solution at 86 OC. The 52Ce-Y was prepared from 10 g of Na-Y exchanged 3 times with 200 mL of 0.1 N CeC1, solution at room temperatures. Excess salt of each compound was removed by washing with deionized water. The zeolites were air-dried and stored over saturated NH4Cl solution at room temperature. The elemental compositions of zeolite samples were determined by induced-coupling plasma emission atomic spectroscopy (ICP).'] Following the pretreatment procedures similar to that used for X-ray diffraction analysis, dehydrated samples were first evacuated at temperature range from room temperature to 650 OC. After Torr) they were stored in a helium cooling under vacuum ( atmosphere. A summary of the pretreatment conditions is given in Table I. Immediately preceding the NMR measurement, the sample was introduced into a double-bearing cylindrical rotor and sealed in a glovebag under helium atmosphere. The 29SiMAS N M R spectra were acquired at 4.73 T with a Bruker MSL-200 spectrometer with a commercially available magic angle spinning probe. The rotors were spun at 3-4 kHz. Typically, more than 500 free induction decays, with 10-s interval for the dehydrated sample and 5 s for the hydrated sample, were accumulated per sample. Chemical shifts were measured in ppm from tetramethylsilane with high-field shifts being negative. The precision of the chemical shift measurement is 10.2 ppm. Results and Discussion
The 29Sispectra of the hydrated samples (Figures 1-5a) are very similar to that of hydrated Na-Y where only four lines with the Si(OA1)4 line missing were observed, and the chemical shift did not differ much among all IA-, IIA-, 43La-Y, and 52Ce-Y zeolites (Table 11).
R T dehyd.
-110
PPm
Figure 1. 29SiMAS N M R spectra of dehydrated 88Ca-Y samples. Chemical shifts are given in ppm from tetramethylsilane. (a) Hydrated; (b) dehydrated at room temperature; (c) dehydrated at 100 OC; (d) dehydrated at 150 OC;(e) dehydrated at 350 OC; (f) dehydrated at 650 OC.
The 29Sispectra of 88Ca-Y at different dehydration temperatures are shown in Figure 1. After dehydration at increasing temperatures, the peak position of the two most intense Si peaks shifted to high field and the line shapes of the 29SiN M R spectra changed gradually. Outgassing at e150 O C did remove the loosely bonded water but did not alter the location of c a t i o n ~ . ' ~ J ~ Dehydration at 300-650 "C removed most of the zeolite water molecules] and at the same time caused redistribution of the exchangeable cations. For hydrated zeolites, cations in the large cavities are surrounded by liquidlike water molecules' and cations in the smaller cavities as sodalite cages or D6R are bound to a cluster of water molecules and the framework oxygens.' The cation-dipole interactions between cations and water molecules and the cation-anion interactions between cations and framework play an important role in the location of cations and the occupancy of cation sites. In dehydrated zeolites, the site occupancies probably correspond to the minimum electrostatic potential when both the intercation repulsions and the cation-anion attractions are taken into account. The absence of the water molecules results in a less homogeneous charge distribution of the exchangeable cations. The 29SiN M R spectral changes in Figures 1-4 are attributed to either the cation movement or the relocation of cations as a result of dehydration. In addition to the change in the cation distribution and water content, the bond angle of T-O-T (Si-0-Si or A1-O-Si) also increases by dehydration (Table 111). According to Engelhard et aL5, there is a quantitative correlation between 29Sichemical shift and mean T-O-T bond angle of silica polymorphs and zeolitic aluminosilicates. Hence, the distortion of the bond angles in silicon-oxygen tetrahedra is probably responsible for the significant upfield changes of the 29Si N M R spectra of dehydrated Y zeolites as shown in Table 11. The difference in line shapes of the NMR spectra of dehydrated Y zeolites corresponds to the various rearrangements of the cations in IA-, IIA-, and RE-Y after dehydration treatment. The distribution of the exchangeable cations in a zeolite depends on the presence or absence of adsorbed water molecules, the nature of the cation, and the heat treatment. Since all these parameters affect the environment of Si nuclei, they are important for a n a lyzing the 29Sispectra. By taking 29SiN M R spectra in hydrated, dehydrated, and rehydrated states, three distinct types of spectral (12) Dendooven, E.; Mortier, W. J.; Uytterhoven, J. B. J . Phys. Chem.
~~
~
( 1 1 ) Chao, K. J.; Chen, S. H.; Yang, M. H. Fresenius 2. Anal. Chem. 1988, 331, 418.
1984, 88, 1916.
(13) Costenoble, M. L.; Mortier, W. J.; Uytterhoeven, J. B. J. Chem. SOC., Faraday Trans. 1 1978, 74, 466.
29SiMAS N M R Spectra of Y Zeolites
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1403
TABLE 111: Population of the Cation-Exchanged Sites in Y Zeolites" K-Y Ca-Y La-Fauj site hyd (20) dehyd (21) hyd (14) dehyd (13) rehyd (12) hyd (1 8) SI 1.3 (K) 5.4 (K) 7.8 (Ca) 3.3 (La) SI' 13.3 (K) 18.1 (K) 9.7 (Ca) 12.0 (Ca) 15 (Ca) SI1 20.0 (K) 26.8 (K) 3.1 (Ca) 10.6 (Ca) 4.8 (Ca) SII' 30.7 (H20) 12.8 (H2O) 31.4 (0-) 28.3 (0) 15.5 (Ca) 10.3 (La) 20.1 (K) 4.4 (K) unlocated and SV 144.3 138.4 143.8 141.4 mean T-O-Tb 138.7
La-Y dehyd (1 3) 14.9 (La) 9.4 (Na) 23.2 (H2O)
Ce-Fauj hyd (1 5 ) dehyd (1 5 ) 3.4 (Na) 18.0 (Na) 11.5 (Ce) 26.0 (H,O) 10.7 (Na) 32.0 (H,O) 16.0 (H2O) 12 (Ce) 141.2
144.3
OThe numbers in parentheses in the column heads are reference numbers. * I n degrees. Cay
KY
A
1
-90
-100
-110
w m
Figure 2. 29SiMAS NMR spectra of 99K-Y samples: (a) hydrated; (b)
J
I
-90
-100
-110
w m
Figure 3. 29SiMAS NMR spectra of 88Ca-Y samples: (a) hydrated; (b) dehydrated at 350 "C; (c) rehydrated at room temperature.
dehydrated at 300 OC; (c) rehydrated at room temperature. La Y
changes were obtained as illustrated in Figures 2-5. In one type the spectra of the hydrated and rehydrated states are identical, while the spectrum of the dehydrated sample is shifted over a few ppm. The second type also has identical spectra in the hydrated and rehydrated state, but the line shape of the dehydrated samples is more complex. In the third type the spectrum of the rehydrated sample is different from that of the original hydrated form. The following examples indicate that these three situations are observed for IA-, IIA-, and RE-Y zeolites, respectively. Spectra of all 99K-Y samples (Figure 2) are essentially the same, but all the peaks in the dehydrated state shifted approximately 1-2 ppm to higher field relative to the hydrated and rehydrated states. This chemical shift difference corresponds to the average T U T angle change (of -6') between the presence and absence of water. In 88Ca-Y zeolites, the hydrated and rehydrated samples have similar 29SiN M R profiles, while the 29Si spectrum of the dehydrated sample is rather complicated (Figure 3). According to X-ray data, the preferred cation coordination sites are in sodalite cages (site 1') and D6R (site I) for the dehydrated Ca-Y zeolites. Upon hydration they are in supercages (site I1 and V and unlocalized cations).I2-l5 The loss of water was accompanied by the occupation of Ca2+ on the hexagonal prism site, I (Table III), which possesses the strongest cationframework interaction in the dehydrated state. Site I cations can pull the oxygen atoms in D6R toward a suitable bond length and induce distortions in the hexagonal prisms. Hence, occupancy of site I by small di- and trivalent cations distorts the tetrahedral framework as suggested by Smith.16 Similar 29SiN M R profiles were found in hydrated and rehydrated 88Ca-Y samples and not in dehydrated 88Ca-Y. It is rather reasonable that a substantial amount of Ca ions migrated toward the sodalite cages and D6R during dehydration and returned to the supercages upon readsorption of water. In the dehydrated state, the effect of the cations is more localized. The Si on the framework could be strongly (14) Costenoble, M. L.; Mortier, W. J.; Uytterhoven, J. B. J . Chem. SOC., Faraday Trans. I 1976, 72, 1817. (IS) Olson, D. H.; Kokotailo, G . T. Mafer. Res. Bull. 1969, 4, 343. (16) Smith, J. V. Adu. Chem. Ser. 1971, 101, 171.
A
from 86.c exchg 43 LaY from RT exchg
-90
-100
-110
ppm
Figure 4. 29SiMAS NMR spectra of La-Y samples: (a) hydrated 43La-Y; (b) hydrated 78La-Y; (c) dehydrated 78La-Y at 350 "C; (d) rehydrated 78La-Y at room temperature.
influenced by the calcium ions populated on I' or I site in small cages and weakly influenced by the calcium ions in supercages that may induce Si sites as strongly influenced by cations and weakly influenced ones, whereas all the Si sites are influenced more homogeneously by monovalent ions in IA-Y zeolites. We may thus conclude that the spectrum of dehydrated IIA-Y consists of a superposition of two sets of Si(nA1) lines with a difference in chemical shift." The cation location in RE-Y zeolites in the presence and absence of water has been described in the l i t e r a t ~ r e . ' ~ J ~In- ~ ~ (17) Chern, J. Y. Ph.D. Thesis, University of Tsinghua, ROC, 1988. (18) Bennett, J. M.; Smith, J. V. Mafer. Res. Bull. 1969, 4, 343.
1404
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 CeY
n 1 \\\, k
h
rehyd. y
d
b
-90
-100
-110
ppm
Figure 5. "Si MAS N M R spectra of 52Ce-Y samples: (a) hydrated; (b) dehydrated a t 350 "C; (c) rehydrated a t room temperature.
unheated hydrated RE-Y zeolite, all the R E cations bonded to water molecules are in the supercages. At or above 60 "C, R E ions in the supercages start stripping their hydration sphere and migrating into small sodalite cages where each R E ion is cooidinated to framework oxygens and hydroxyl oxygens. Hence there are different occupancies of cation sites by La3+species in 43La-Y and 78La-Y obtained from room temperature and 86 "C ion exchange, respectively. Upon dehydration at 350 "C all rare earth ions in 78La-Y move into the sodalite cages and form the stable sodalite cage complexes with lattice 0 ~ y g e n s . lDue ~ to the formation of a stable sodalite cage complex, La3+ is locked in site 1', and the trivalent ion distribution cannot be changed by rehydration at room temperature. Because of the irreversible migration of RE ions from the supercage to the sodalite cage, the line shape of 29SiN M R spectra of 78La-Y and 52Ce-Y in their dehydrated and rehydrated states are different from that in their hydrated state. Although the distributions of trivalent ions in dehydrated and rehydrated states are similar, their difference in water content can affect the 29Sispectra. The line shape and
Chao and Chern chemical shift of 29Sisignals on hydrated, dehydrated, and rehydrated 78La-Y and 52Ce-Y are different as expected. The spectral change indicated that the perturbation is mainly caused by the interaction of trivalent cations with the framework and zeolite water. The influence of the cations in sodalite cages on the electronic property of framework Si nuclei is different from that in supercages. Moreover, it is interesting to observe that 29Si MAS NMR bands are quite broad for dehydrated and rehydrated 52Ce-Y zeolites (Figure 5). In unheated hydrated 52Ce-Y zeolite, all hydrated Ce3+ ions are surrounded by highly mobile water molecules in supercages. The influence of Ce3+ ions on the 29Sisignal does not differ much from that of Na+ ions. This is attributed to rapid site exchange of all cations in supercages that in turn removes the dipolar interaction of unpaired 4f electron Ce3+.19 After dehydration treatment, most of cerium ions are locked in sodalite cages and bonded to the localized water molecules and framework oxygens so that the dipole interaction between cerium ion at site I' and silicon atom in framework is more pronounced. The broadening in line width for dehydrated and rehydrated 52Ce-Y samples is presumed to be caused by residual dipole interaction between the cerium ions at site I' and silicon nuclei on the framework. This is probably an indication that the magnetic property of Si atoms on the aluminosilicate framework is a function of the nature of trivalent cations in small cages regardless of the presence or absence of water. We were able to correlate the ?Si NMR profile with the change of T-0-T bond angle and cation distribution in zeolites caused by heating, dehydration or rehydration. In addition to the framework structure effects, 29SiMAS N M R spectra were also found to be influenced by the distribution of extraframework cations and the adsorption of water molecules. Acknowledgment. This work was supported in part by the National Science Council of Republic of China. (19) von Ammon, R.; Fisher, R. D. Angew. Chem., Int. Ed. Engl. 1972,
I!, 675. (20) Mortier, W. J.; Bosmans, H. J. J . Phys. Chem. 1971, 75, 3327. (21) Mortier, W. J.; Bosmans, H. J. Uytterhoven, J. B. J . Phys. Chem. 1972,76,650.
