5132
J. Phys. Chem. 1993,97, 5132-5135
Phase Discrimination with *%SiMAS NMR in EMT/FAU Zeolite Intergrowths J. A. Martens,' Y. L. Song, E. J. P. Feijen, P. J. Grobet, and P. A. Jacobs Centrum voor Opperolaktechemie en Katalyse, KU Leuven, Kard. Mercierlaan 92, 8-3001 Heverlee, Belgium Received: January 5, 1993; In Final Form: February 23, I993
In the Z9Si MAS N M R spectra of Na-exchanged FAU/EMT zeolite intergrowths, the signals from the FAU and EMT phases coincide. However, after Li ion exchange of such zeolites, an upfield shift of the FAU signals by 2-3 ppm occurs and, consequently, partial resolution of the resonance signals is possible. Thus the combination of 29Si MAS NMR spectra of the Na- and Li-exchanged forms of an EMT/FAU zeolite allows one to determine the Si,Al distribution in the FAU and EMT phases separately as well as the relative amounts of each phase in the sample. The method is applicable to EMT/FAUsamples containing Si(4Al) environments and is illustrated for ZSM-2 and ZSM-3.
Introduction Zeolite Y is a synthetic form of the mineral faujasite, the structure of which is cubic and denoted with the three letter code FAU in the zeolite at1as.I It consists of sheets of truncated octahedra (sodalite cages) interconnected through double sixmembered rings and stacked in ABCABC sequence. Upon stacking of the elementary sheets of truncated octahedra in an ABAB sequence, a structure with hexagonal symmetry appears.2 By alternative systematic or even random A,B,C stacking of the elementary sheets, an infinite number of new structures may be generated. CSZ-1,3 CSZ-3: ZSM-2,$ZSM-3,6 ZSM-20,' and ECR-308are code names for materialsidentified as such structural composites. A pure hexagonal end-member was synthesized by Delprato et al. from an aluminosilicatehydrogel containing 18crown-6 etherq9 In the Atlas of Zeolite Structure Types the topology of this hexagonal phase is denoted with EMT.' A convenient technique for the characterization of the FAU and EMT phases in FAUIEMT polytype materials is not available. In powder X-ray diffraction patterns, many of the line profiles are asymmetric and broadened. Severe overlap of the diffraction peaks of the two phases makes structure determination difficult. The diffraction patterns of some materials (CSZ-1,3Jo ZSM-20," and ECR-308)can beindexed in the hexagonal system. Based on X-ray diffraction studies, Kokotailoand Ciric proposed that ZSM-3 is a random stacking of FAU and EMT layers.I2No structure has yet been advanced for ZSM-2. High-resolution electron microscopy (HREM) with nearatomic resolution in combination with image-simulationmethods have allowed one to detect twinning of a zeolite Y crystal across its [ 1111crystallographic~1anes.l~ Thus the existenceofdiscrete FAU and EMT regions was demonstrated in individual crystals of ZSM-20.I4 However, with the HREM technique only a very limited number of crystals from a sample can be examined, and it remains uncertain to what extent they are representative for the whole sample. 29SiMAS NMR has already proven to be extremely valuable for the study of the silicon nearest neighbors in fa~jasites.~s It has allowed one to confirm the Lbwenstein ruleI6 and provided a powerful quantitative tool for the calculationof zeolite framework compositions. 29SiMAS NMR spectra of FAU materials exhibit a splitting into five distinct signals corresponding to the number, n, of A1 atoms in the first four neighboring sites.I5 The position of the peak center for a Si(nA1) signal also depends on the presence of aluminum in second nearest neighbor positions of the framework." This explains the dependence of the peak position of the Si(nA1) signals on the framework composition. The chemical shift is further influenced by the crystallographic structure and nature 0022-365419312097-5 132$04.00/0
I
A
-80
. . .I . . . & . . . . l
I . . .
-100 PPM
I . ..I....I....I..
-80
.
-100 PPM
Figure 1. 29SiMAS NMR spectra of Na-ZSM-2 and Li-ZSM-2 (the supercripts E and F refer to EMT and FAU, respectively).
of the charge compensating cations, which through their mordination with framework oxygens can distort silicon tetrahedra.Is In this new method, the shift dependence on the nature of the exchangeablecationsis exploited for discriminating between FAU and EMT topologies. In the present paper, we show that 29Si MAS NMR can be used to quantify the amount of FAU and EMT phases in Al-rich FAUIEMT intergrowths and todetermine the Si/Al ratio in the two phases individually.
Experimental Section 29SiMAS NMR spectra were recorded on a Bruker 400 MSL instrument at 79.7 MHz, with a pulse length of 4 MS, a pulse interval of 5 s, and a spinning rate of 4 kHz. The number of scans exceeded 10 000 in all experiments. A lithium tetramethylammonium-ZSM-2 sample was prepared according to the synthesis recipe of Barrer and Sieber.19 The as-synthesized sample was ion exchanged 2 times with 0.5 M aqueous solutions of LiCl to obtain fully exchanged Li-ZSM-2 and 6 times with 0.5 M aqueous NaCl solution to obtain NaZSM-2. A ZSM-3 sample was synthesized according to a published recipe.6 Li-ZSM-3 and Na-ZSM-3 forms of this zeolite were prepred by repeated ion exchange with aqueous LiCl and NaCl solutions, respectively. A siliceous FAU/EMT intergrowth was synthesized from a hydrogel with the following composition: (Si02)1~(A1203)(Na20)2.4 15-crown-5ether)o.19~( 18-crown-6ether)o.776(H20) 135. 0 1993 American Chemical Society
29SiMAS NMR in EMT/FAU Zeolite Intergrowths
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5133
TABLE I: B i Chemical Shifts,Si(&) Distributions, Phase Com ition, and Si/Al Ratios in the FAU and EMT Domains of ZSM-2 and ZSM-3 As Determined by %i MAS NMR on Li- and EExchanged Samples Si(nA1) distribution
Na-ZSM-2 Li-ZSM-2 (FAU) (EMT) Na-ZSM-3 Li-ZSM-3 (FAU) (EMT)
-85.9 -82.8 -85.9 -85.6 -82.8 -86.1
16 9.5 7 12 7 5
-90.2 -87.0 -90.4 -89.8 -87.7 -90.2
35.5 18.5 16.5
-94.7 -91.9 -95.2
31.5 15.5 14.5
-100.3 -96.6 -99.2
12 5.5 8
31.5 20
-94.5 -92.1 -94.6
33.5 16 16
-99.5 -96.2 -99.1
19 10 IO
11
-103.8 -101.8 -105.2 -103.9 -101.6 -105.1
4.5 4.0 1.5 3.5 3
I
~~
Si/AI atomic ratio ~~
overall' ZSM-2 ZSM-3
FAU phase
EMTuhase
A
B
1.6 1.7
1.7 1.8
1.6 1.8
1.6 1.7
(Si ZSM-2 ZSM-3
+ A1)F/(Si + A1)E+F(7%)
(Si
+
+ Al)E+F(76)
53 57
47 43
From the decomposition of the 29Si N M R spectrum of A, the Li-exchanged sample and B, the Na-exchanged sample.
Crystallization was performed at 372 K over 7 days. The overall Si/A1 ratio of the sample was equal to 3.6. The sodium form of this sample was obtained by calcination at 823 K to remove the ethers. It was ion exchanged into its lithium form by contacting the sample with 1 N LiCl solution under reflux conditions. The ion exchange with LiCl was repeated 3 times.
Li-ZSM-2
Results and Discussion The 29Si MAS NMR spectra of hydrated Na-ZSM-2 and Li-ZSM-2 samples are given in Figure 1. The Na-ZSM-2 sample shows a five-line spectrum, representingat decreasing resonance values the five Si(nA1) environments, with n equal to 0, 1,2, 3, or 4, respectively. Obviously, in the "Si MAS NMR spectrum of Na-ZSM-2, the signals referring to the respective FAU and EMT environments are not resolved. On the contrary, seven maxima can be distinguished in the 29Si NMR envelope of the Li-ZSM-2 sample (Figure 1). One of the marked changes in the 29SiNMR spectrum of the ZSM-2 sample upon Li exchange is the Occurrence of a signal at -82.8 ppm. Chemical shifts at the same position are observed for the Si(4Al) environment in LiX and LiY zeolites (spectra not shown). The signal at -82.8 ppm is therefore assigned to Si(4Al) sites in FAU parts of the structure, further denoted as Si(4Al)F. The better dispersion of the signals in the 29SiNMR spectrum of the Li form of ZSM-2 can be explained by assuming a systematic upfield shift of all Si(nA1)Flines by 2 to 3 ppm compared to the Si(nA1)E lines. A similar shift of the 29SiNMR signals in zeolite Li-A with respect to Na-A has been observed previously.20 The chemical shift of the EMT signals in ZSM-2 seems to be much less susceptibleto the nature of the exchangeablecations. Based on these considerations,the assignment of all Si resonance lines in Li-ZSM-2 is possible, as shown in Figure 1. The signals at -105.2 ppm, -101.8 ppm, and -99.2 ppm are assigned to Si(OAI)F,Si(OAl)E,and Si( lAl)Eenvironments, respectively. The maximum in the resonance envelope at ca. -88 ppm represents the signal for Si(3Al)Foverlapping now with Si(4AI)E. Si(2Al)F and Si(3Al)E environments give rise to a peak at ca. -91 ppm, Si(lA1)F and Si(2Al)E to a peak at ca. -96 ppm. Using this assignment, the intensity of the individual peaks [Si(nA1)Fand Si(nA1)EI can be obtained as follows. Decomposition of the 29Si NMR spectrum of Na-ZSM-2 (Figure 1) using Gaussian envelopesforthe five lines [Si(nAl)E+Tispossible. Decomposition of the seven-line spectrum of Li-ZSM-2 (Figure 1) is equally
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.
.
.
.
l
.
.
.
.
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.
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F i p e 2. Decomposition of the 29SiMAS N M R spectrum of Li-ZSM-2 using 10 Gaussian components with predetermined position and intensity: (a) experimental spectrum; (b) sum of the intensities of theindividual components.
possiblevia the same procedure. Thisprovides a set of 12equations and 10 unknown parameters, linking the nature of the Si environment to its intensity in the 29Si NMR spectra. A new decomposition of the 29SiNMR spectrum of Li-ZSM-2 can now be made using 10Gaussianlines, each with predetermined position and intensity. The line widths arevaried until reasonable agreement of the experimentaland simalatedresonance envelope is reached. Figure 2 shows this decomposition into the 10 individual environments. Excellent agreement exists between the experimental resonance line and that obtained upon addition of the 10 individual components.
5134 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
Martens et al.
Na-ZSM-3
Li-ZSM-3
Figure 3. 29SiMAS NMR spectra of Na-ZSM-3 and Li-ZSM-3.
The chemical shifts and relative intensities of the different signals in the Na-ZSM-2 and Li-ZSM-2 spectra are listed in Table I. Reasonable agreement exists between the intensity of Si(nAl)E+Fsignalin Na-ZSM-2 and the sum of intensities of the corresponding Si(nA1)Eand Si(nAI)Fsignals observed in the Li form of the sample (Table I). Based on the decomposed spectra, the Si/Al ratio of the whole sample, as well as of the individual FAU and EMT parts of the sample, was calculated (Table I). Thus it seems that the ZSM-2 crystals investigated are composed of nearly equal amounts of FAU and EMT domains, having a Si/AI ratio of 1.6 and 1.7, respectively. The 29Si MAS NMR spectra of hydrated Li-ZSM-3 and NaZSM-3 are shown in Figure 3. A partial resolution of the FAU and EMT signalswas obtained by ion exchangeof the Na-ZSM-3 sample into its Li form (Figure 3). The procedure for decomposition and assignment of the spectra of Na-ZSM-3 and LiZSM-3 is the same as that explained for ZSM-2. It is illustrated in Figure 4. The chemical shifts for the different Si environments, the phase composition, and the Si/Al ratios are given in Table I. In the ZSM-3 sample the FAU phase dominates (57% FAU), has a Si/Al ratio of 1.7, and is slightly more aluminous than the EMT phase, which has a Si/Al ratio of 1.8. The two examples of ZSM-2 and ZSM-3 illustrate the power of the method for aluminous samples. We experienced that Li exchange of siliceous samples, having low cation exchange capacities, does not result in a sufficient dispersion of FAU and EMT signals. This is illustrated in Figure 5 for a siliceous FAU/ EMT intergrowth that was synthesized from a gel containing 18-crown-6ether. A trace amount of 15-crown-5ether was added to the gel in order to provoke the intergrowth of FAU in the EMT crystals.2' The 29SiMAS NMR spectrum of the Na- and Liforms of siliceous EMT/FAU intergrowths are very similar (Figure 5 ) . It is concluded that in order to be able to apply the method, the zeolite sample must be aluminum rich and contain Si(4Al) environments. Comparison and decomposition of 29Si MAS NMR of the lithium and sodium forms of aluminous FAU/EMT polytype materials will be extremely useful to follow their crystallization kinetics, to determine the siting of Si and AI in their framework, and to attribute their catalytic and sorptive properties to specific environments. The method will probably be useful for the study of other zeolite polytype materials with a single T-atom site as well.
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. . . . , . . . . I . . . . , . . . . I . ,
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-110
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Figure 4. Decomposition of the 29SiMAS NMR spectrum of Li-ZSM-3 using 10 Gaussian components with predetermined position and intensity: (a) experimentalspectrum;(b) sum of theintensities of theindividual components.
Conclusions The phase composition and Si,Al distribution in the individual phases of FAU/EMT zeolite intergrowths can be determined by decomposition of the 29Si MAS NMR of the Na- and Liexchanged forms of the sample. In this paper, the method is applied to ZSM-2 and ZSM-3. The ZSM-2 sample investigated contains 53% FAU and 47% EMT phase, with a Si/Al ratio of 1.6 and 1.7, respectively. ZSM-3 contains 57% FAU phase with a Si/Al ratio of 1.7. The EMT layers in the ZSM-3 sample have a Si/A1 ratio of 1.8. For EMT/FAU zeolites with higher Si/Al
29SiMAS NMR in EMT/FAU Zeolite Intergrowths
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5135
LI-FAU/EMT
(2) Moore, P. B.; Smith, J. V. Mineral. Mag. 1964, 35, 1008. (3) Barrett, M. G.; Vaughan, D. E. W. US.Patent 4309313, 1982. (4) Vaughan, D. E. W.;Barrett, M. G. US. Patent 4 333 859, 1982. ( 5 ) Ciric, J. US.Patent 3 41 1 874, 1968. (6) Ciric, J. US.Parent 3 415 736, 1968. (7) Ciric, J. U.S. Parent 3 972 983, 1976. (8) Vaughan. D. E. W. Eur. Pat. Appl. 315 461, 1988. (9) Delprato, F.; Guth, J. L.; Anglerot, D.; Zivkov, C. Eur. Pat. Appl. 364 352, 1990. (10) Millward, G. R.; Thomas, J. M.; Ramdas, S.;Barlow, M. T. In Proceedings of the 6th Interwtional Zeolite Conference;Olson, D., Bisio, A., Eds.; Butterworths: Surrey, 1984; p 793. (1 1) Derouane, E.; Dewaele, N.; Gabelica, 2.;Nagy, J. B. Appl. Caral. 1986, 28, 285. (12) Kokotailo, G. T.; Ciric, J. In Molecular Sieve Zeolites- r; Flanigen, E. M., Sand, L. B., Eds.; Advanced Chemistry Series 101;American Chemical Society: Washington, DC, 1971; p 109. (13) Audier, M.; Thomas, J. M.; Klinowski, J.; Jefferson, D. A,; Bursill, L. A. J. Phys. Chem. 1982,86 (4) 581. (14) Vaughan, D. E. W.; Treacy, M. M. J.; Newsam, J. M,; Strohmaier, K.G.; Mortier, W. J. In Zeolite Synthesis; Occelli, M. L., Robson, H.E., Eds.;ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989; p 545. (1 5 ) Engelhardt, G.; Michel, D. High Resolution Solid Stare NMR of Silicates and Zeolites; John Wiley & Sons: Chichester, New York, Brisbane, Toronto, Singapore, 1987. (16) Lijwenstein, W . Am. Mineral. 1954, 39, 92. (17) Melchior, M. T.; Newsam, J. M. In Zeolites: Facts, Figures, Future; Jacobs, P . A,, van Santen, R. A., Eds.; Elsevier: Amsterdam, Oxford, New York, Tokyo, 1989; p 805. (18) Grobet, P. J.; Mortier, W. J.; Van Genechten, K . Chem. Phys. Lett. 1985, I19 (4) 361. (19) Barrer, R. M.; Sieber, W. J . Chem. Soc.,Dalton Trans. 1977, 1020. (20) Engelhardt, G.; Michel, D. High Resolurion Solid Stare NMR of Silicates andzeolites; John Wiley & Sons: Chichcster, New York, Brisbane, Toronto, Singapore, 1987; p 257. (21) Anderson, M. W.; Pachis, K.S.;Prkbin, F.; Carr, S.W.; Terasaki, 0.;Ohsuna, T.; Alfreddson, V. J. Chem. Soc., Chem. Commun. 1991, 1660.
Na-FAUIEMT
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Figure 5. 29SiMAS N M R spectra of the Li and N a forms of a siliceous FAU/EMT intergrowth synthesized in the presence of crown ethers.
ratios, thus lacking Si(4Al) environments, Li-exchange does not result in a sufficient resolution of FAU and EMT signals.
Acknowledgment. J.A.M. and P.J.G. acknowledge the Flemish NFWO for research positions as Senior Research Associate. E.F. is grateful to IWONL for a research grant. This work has been sponsored in part by the Air Products company (Allentown, USA) and in part by a Belgian National Center of Excellence Programme on Supramolecular Catalysis. The authors are indebted to H. Geerts for her skillful help in generating the NMR data and to C. Coe for his constructive comments and enthusiasm. References and Notes (1) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, 3rd rev. ed.;Zeolites, Vol. 12; 1992, p 5 .
