J. Phys. Chem. 1995, 99, 7002-7008
7002
27Al and 29SiMAS NMR Study of Zeolite MCM-22 Waclaw Kolodziejski, Claudio Zicovich-Wilson, Catalina Corell, Joaquin Perez-Pariente, and Avelino Corma* Instituto de Tecnologia Quimica UPV-CSIC, Universidad Politkcnica de Valencia, Avda. de 10s Naranjos s/n, 46022 Valencia, Spain Received: October 28, 1994; In Final Form: February 15, 1995@
Zeolite MCM-22 was studied by 27Al and 29Si NMR with magic angle spinning (MAS). Bloch-decay and cross-polarization spectra were compared and assigned for as-synthesized and calcined samples with various A1 content. The interpretation was assisted by 27Alquadrupole nutation NMR. It was found that the average Si-0-Si angles corresponding to inequivalent framework tetrahedral sites vary from 138" to 164". The as-synthesized and calcined materials both contain two kinds of framework tetrahedral Al, but the calcined zeolite also contains two kinds of extraframework octahedral Al. Such results can be interpreted on the basis of the MCM-22 structure proposed by Leonowicz et al. (Science 1994, 264, 1910). However, the detailed assignment of the Si(OA1) peaks is still open to discussion.
Introduction Zeolite MCM-22, invented in 1990, already has interesting catalytic application^.'-^ Thermally stable up to 1198 K, this novel material has high BET surface area ('400 m21g) and unusually large sorption capacity (typically ca. 15 wt %) for water, cyclohexane, and n-hexane. MCM-22 containing Pt or Pd is a useful catalyst for the conversion of paraffins to olefins and/or aromatics.' The Pt/MCM-22 catalyst for hydrocracking is capable of producing high-density, cycloparaffii-richjet fuel.2 When used in combination with a conventional cracking catalyst such as ultrastable zeolite Y, MCM-22 containing Cu or rare earth metals reduces CO and NODI02 emissions from refineries to the ecologically acceptable concent~ations.~Catalytic test^^-^ show that MCM-22 behaves like both 10- and 12-memberring zeolites. During the preparation of this paper Unvemcht et aL6 published fragmentary NMR data on MCM-22 with the SUA1 ratios of 11 and 21 for as-synthesized and S i c 4 dealuminated samples, respectively. They presented some questionable line deconvolutions, which will be discussed here in view of our results. Leonowicz et al. have recently proposed two structures of MCM-22 derived from high-resolution electron micrographs and synchrotronX-ray diffraction (XRD) powder data.7 The zeolite is composed of interconnected {43t663[43])building units forming a three-dimensional P61mmm (1st structure) or Cmmm (2nd structure) lattice, which contains two independent channel systems with rings of 10 and 12 tetrahedral (T) atoms. The largest pore has a diameter of 7.1 A, and its height is 18.2 A. The unit cell contains either 8 (1st structure) or 13 (2nd structure) inequivalent T-sites. The authors7 opt rather for the second structure, because the first one contains two linear T-0-T links. Now is the time to examine thoroughly the structure of MCM-22 by spectroscopic methods. Experimental Section Crystallization of zeolite MCM-22 was canied out in a stainless-steel rotating autoclave over 7 days at 400-423 K under autogeneous pressure from a gel containing hexameth* T o whom correspondence should be addressed at the Instituto de Tecnologfa Quimica UPV-CSIC, Universidad Politknica de Valencia, Avda. de 10s Naranjos s/n, 46022 Valencia, Spain. Phone: 34-6-387 78 00. Fax: 34-6-387 78 09. Abstract published in Advance ACS Abstracts, April 15, 1995. @
TABLE 1: Molar Compositions of the Synthesis Gels" sample SiOdA1203 OH-/Si02 H201Si02 WSi02 M-15 M-25 M-50
30
0.30
40
50 100
0.14 0.12
45 45
0.50 0.35 0.50
R stands for hexamethyleneimine used as a template.
yleneimine used as a template.' The samples M-15, M-25, and M-50 (Table 1) were designated according to the total SUA1 ratios determined by the elemental analysis before calcination. The zeolite was calcined in air for 3 h at 853 K and characterized by powder XRD (Figure 1). Before the NMR measurements were made, the samples were stored for several days at room temperature in an atmosphere of 80% relative humidity. Solid-state 27Aland 29SiNMR spectra were recorded under magic angle spinning (MAS) at ambient temperature on a Varian VXR400S WB spectrometer at 104.2 and 79.5 MHz, respectively. The rotors were driven by dry air, and the magic angle was set precisely by observing the 79Br resonance of KBr. A conventional Bloch-decay (BD) technique and single-contact cross-polarization (CP) from protons were used. The 29Si spectra were recorded with a 5 mm Doty probe, with instrumental spectral resolution, dependent on shimming, magic angle setting, and spinning stability, of better than 5 Hz, as determined from the I3C CPMAS spectrum of adamantane. 27Al NMR was measured with a 7 mm Varian probe. The acquisition parameters are given in Table 2. The 29Si spectra were deconvoluted using standard Varian software. The Hartmann-Hahn condition for 'H 27AlCP, involving only the central (-'I* -t1/2) transition of 27Al (selective excitation), was established and optimized on k a ~ l i n i t e . ~ . ~ Quadrupole nutation spectral0 were recorded with MAS at 7 kHz using a radio frequency (rf) field w4f/2Z of 50 lcHz and a recycle delay of 3 s. A total of 36 data points collected in the tl dimension in increments of 1 ps. A sine-bell-shifted apodization and zero filling were used in both dimensions, and the FIDs were doubly Fourier transformed in the magnitude mode.
-
-
Results and Discussion Conventional 27AlBD spectra of MCM-22, compared within either the as-synthesized or calcined sample series, are similar as concerns the number and positions of the peaks. The typical
0022-365419512099-7002$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 18, 1995 7003
*'Ai and 29Si MAS NMR Study of Zeolite MCM-22
[countrl
-
IBOO-
-
1600-
-
1400-
12001000-
-
800BOO
-
400-
200 0
Figure 1. X-ray diffraction pattern (Cu K a radiation) of calcined M-25. The pattern agrees well with the previously published TABLE 2: MAS and CPIMAS NMR Acquisition Parameters MAS pulse flip angle recycle delay MAS rate resonance us) (rad) (SI (MZ) 27 Al 0.3 27/20 0.5 7 29Si 3.0 32718 60 6 n/2 pulse resonance 27Al 29Si
us)
9.0 6.5
CPIMAS contact time (ms)
recycle delay
MAS rate
(S)
0.8
3
(Mz) I
4.0
3
5
TABLE 3: Integral Intensity of the *'AI BD NMR Peaks of MCM-22 Expressed in Arbitrary Units and Scaled to the Same Sample Weight" intensity samule SdA1 ratiob Iv VI M-15 as-made 15 849 calcined 439 91 (189)e M-25 as-made 25 466 calcined 363 80 M-50 as-made 50 203 calcined 100 9 Roman numbers denote the A1 coordination. Elemental analysis. MCM-22 treated with acetylacetone.
56
/56\
-
+/
calcined
I"
120
spectra are shown in Figure 2. The relative NMR intensities approximately reflect the SUA1 ratios of the as-synthesized samples (Table 3). The as-synthesized material gives a peak from 4-coordinated framework Al, which has a maximum at 56-57 ppm and looks like an unresolved asymmetric doublet. The calcined zeolite gives two peaks. The peak of 4-coordinated A1 with a maximum at 56-57 ppm resembles an unresolved asymmetric triplet. Calcination creates extraframework species, which give a peak of 6-coordinated A1 at ca. 0-1 ppm. The
NMR on MCM-22
''AI
,
EO
,
40
,
0
-
-40
,
ppm from AI (H,O);+ Figure 2. Bloch-decay (BD) and cross-polarization (CP) 27Al MAS NMR spectra of sample M-15. intensity ratio of the apparent components of the 4-coordinated Al resonance depends, for both the as-synthesized and calcined material, on the A1 content and sample history. The comparison of the total intensities for the same sample before and after
~
Kolodziejski et al.
7004 J. Phys. Chem., Vol. 99,No. 18, 1995
27AIQUADRUPOLE NUTATION ON AS-SY NTHESIZED MCM-22 SAMPLE M-15
120
-
58
54
50
46
42
38
F2 I
'
I
'
I
'
1
'
'
ppm from AI (H,O)?' 180 140 100 60 kHz Figure 3. 27AlNMR quadrupole nutation spectrum of as-synthesized M-15recorded with a radio frequency field od2n of 50 kHz.
27AI QUADRUPOLE NUTATION ON CALCINED MCM-22 SAMPLE M-15
A 48.1
F, cross-sections
Fi 40
--
60 80 N
5
58
100 120
50
50 kHz
135 kHz
42
ppm
F, cross- sections
140
50 kHz
160
135 kHz
53.5 ppm
48.1 ppm
180
58
54
50
46
42
38
F2 yc1.1-l-'
ppm from AI (li20):' 180 140 100 60 kHz Figure 4. 27Alquadrupole nutation NMR spectrum of calcined M-15, showing an expanded region of 4-coordinated Al. The spectrum was recorded with a radio frequency field W&JC of 50 kHz. calcination (Table 3) indicates that a large portion of A1 becomes Nh4R "invisible" after calcination, Le., gives extremely broad lines. Indeed, the acetylacetone treatment" of calcined M-15 doubles the intensity of the 6-coordinated AI peak, but the overall spectrum intensity is still 26% lower than that of assynthesized M-15. The 'H 27AlCP peaks have practically the same positions as their counterparts in the BD spectra (Figure 2). The 4-coordinated AI peak in the CP spectrum of the as-synthesized sample, after applying appropriate line broadening (not shown), has a shape similar to that of the corresponding BD peak. However, the BD and CP shapes of the 4-coordinated A1 peak from the calcined material seem to be different, thus implying that the latter peak may be composed of resonances from inequivalent framework tetrahedral sites. Since such a way of
-
reasoning can be misleading,12J3we have attempted to resolve the 27Al peaks using the quadrupole nutation spectroscopy1° (Figures 3-5). This two-dimensional NMR technique permits 27Alenvironments with different quadrupole coupling constants to be resolved along the F1 axis and the asymmetry of the relevant environments to be estimated, considering that stronger quadrupolar interaction (less symmetric chemical environment) corresponds to a higher line intensity at 3w,.&n within the same F1 cross-section. Because of the sensitivity reason we have studied only the sample with the highest A1 content. First we recorded several nutation spectra of as-synthesized M- 15, using various rf fields. We found that the 4-coordinated A1 peaks are best resolved with w,42n = 50 kHz. Then this rf field was applied to calcined M-15.
J. Phys. Chem., Vol. 99, No. 18, 1995 7005
*'A1 and 29Si MAS NMR Study of Zeolite MCM-22
"AI QUADRUPOLE NUTATION ON CALCINED MCM-22
110-
130
29Si
-
0.4 ppm
'!'Si BD NMR on calcined MCM-22
NMR on MCM-22
-111 2
Sample M-15
-113.2 .110.3 Sample
-1052
I -1199
Q *g
3
-110.7 kll3.l
L -80
-90
-100
-110
-120
-130
-140
I
-80
ppm from TMS
Figure 6. Bloch-decay (BD) and cross-polarization (CP) 29Si MAS NMR spectra of sample M-15: (a) as-synthesized; (b) calcined.
For as-synthesized M-15 we observed two peaks, at 54 and 47 ppm, both spread between o d and 3 o d in the F1 dimension (Figure 3). The peak at 54 ppm has relatively higher intensity
I
I
I
I
I
I
-90
-100
-110
-120
-130
-140
ppm from TMS
Figure 7. 29SiBD NMR spectra of calcined MCM-22 having various A1 content.
at o d and lower intensity at 3wd than the peak at 47 ppm, so the latter one corresponds to the site with a high quadrupolar coupling constant and a less symmetric environment. For
7006 J. Phys. Chem., Vol. 99, No. 18, 1995
Kolodziejski et al.
MCM-22
"Si BD NMR
Space group R / m m m
6
3 8
l
- 85
~
'
"
- 100
l
4; ~
- 105
'
'
'
l
~
'
. 110 -115 ppm from TMS
~
~
- 120
l
~
'
- 125
~
'
l
~
"
'
l
Figure 8. Comparison of the experimental 29SiBD N M R spectrum of calcined M-50, having a total SUA ratio of 50, with the bar plots expressing theoretical peak positions and intensities for the model crystal structure' of zeolite MCM-22 attributed to the space group P6/mmm. The T-site numbers are given over the bars. The bar lengths reflect relative populations of the T-sites, and the chemical shifts 6 are related to the T-0-T angles a: (a) 6 calculated from average a according to ref 17; (b) 6 calculated from average [cos COS a - I)] according to the equation for ZSMJ (room temperature) given in ref 18. calcined M-15 we observed two 4-coordinated A1 peaks, at 54 and 48 ppm, which seem to have similar intensity distributions in the F1 dimension (Figure 4), so both sites have similar quadrupolar coupling constants and consequently similar environmental symmetry. The reversed relative intensity of those peaks in the nutation spectrum (cf. the F2 cross-sections in Figure 4) comparing to the conventional BD spectrum (Figure 2) probably results from large differences in the relaxation time Tze between the tetrahedral sites. A pronounced effect of TQ on the 27Alquadrupole nutation has already been investigated.l4 Here, we assume that Tze is appreciably shorter for the site, giving less intensive peaks in the nutation spectrum of calcined M-15. Our nutation spectra were measured under the same acquisition conditions, with the same rf field levels in particular. Therefore, it is possible to compare the peaks from 4-coordinated A1 between the as-synthesized and calcined samples (Figures 3 and 4). It tums out (cf. F1 cross-sections) that upon calcination the higher and lower frequency peaks become more and less spread toward 3wd2n, respectively. At the same time, the former shifts from 54.2 to 53.5 ppm and the latter from 47.2 to 48.1 ppm (cf. F2 cross-sections), probably indicating an increase and a decrease of the second-order quadrupolar interaction, respectively. It follows that upon calcination the tetrahedral Al-sites, giving the peaks at ca. 54 and 48 ppm, become more and less distorted, respectively. The significant differences between the nutation spectra of as-synthesized and calcined samples, as concems tetrahedral Al, indicate that the template removal and dealumination considerably affect the MCM-22 framework. We also note that the specious triplet from 4-coordinated A1 in calcined M-15 (Figure 2) consists of two peaks (Figure 2), so at least one of them is evidently asymmetric, which is intrinsic of its quadrupolar shape. Considering this
and the behavior of both peaks in the F1 dimension, either from as-synthesized or calcined M-15, we conclude that the deconvolution of the 27Al spectra into several symmetric peaks, presented in ref 6, is inappropriate. The presence of two kinds of tetrahedral A1 can be explained either by chemical or crystallographic nonequivalence, which may affect both isotropic chemical shift and quadrupolar parameters, and thereby line position^.'^ For example, in TPAZSM-5 the doublet splitting of the tetrahedral A1 resonance is due to the chemical nonequivalence caused by the interaction of the zeolite framework with TPA+ and Na+ cations. We consider this explanation as less probable for MCM-22, because the NazO content in our samples, according to the elemental analysis, is below 0.1 wt %. On the other hand, in highlysiliceous Z S M J the splitting originates from the crystallographic nonequivalence, and this is probably our case. We suggest that the two 4-coordinated A1 peaks, detected for MCM-22 by the quadrupole nutation, are from two sets of the framework tetrahedral sites, differentiated by their location in small and large atom rings and/or by their structural relation to two distinct pore systems proposed for this zeolite.' Such assignment deserves verification by the double-rotation technique. Inspect then the nutation spectrum from octahedral A1 of calcined M-15 (Figure 5 ) . There are two peaks, at 0.4 and -1.0 ppm. Although both are centered at wrf in the F1 dimension, the smaller one seems to be more spread toward 3wd, so it corresponds to a higher quadrupolar coupling constant and a less symmetric environment. The peaks are either from two different extraframework species or from extraframework material of the same type, but deposited in two different channel systems of zeolite MCM-22. The typical 29Sispectra are shown in Figures 6 and 7. Note the significant differences in the peak positions between the
~
~
'
~
J. Phys. Chem., Vol. 99, No. 18, 1995 7007
27Al and 29SiMAS NMR Study of Zeolite MCM-22
MCM-22
"SI BO NMR Space group Cmmm
4
- 95
- 100
-
105
- 110
- 115
- 120
-125
pptri from TMS Figure 9. Comparison of the experimental 29SiBD NMR spectrum of calcined M-50, having a total SUA1 ratio of 50, with the bar plots expressing theoretical peak positions and intensities for the model crystal structure' of zeolite MCM-22 attributed to the space group Cmmm. The T-site numbers are given over the bars. The bar lengths reflect relative populations of the T-sites, and the chemical shifts 6 are related to the T-0-T angles a: (a) 6 calculated from average a according to ref 17; (b) 6 calculated from average [cos COS a - l)] according to the equation for ZSMJ (room temperature) given in ref 18.
as-synthesized and calcined samples (Figure 6). This is caused by the template removal and dealumination. After calcination the peaks move to a lower chemical shift and become better resolved (Figure 7). CP works differently for the as-synthesized and calcined samples, because the proton pool is different (Figure 6). In the as-synthesized samples, polarization is transferred from the template protons and the 29Si peaks are affected almost similarly (Figure 6a). In the calcined samples polarization is transferred from the hydroxyl protons, so CP strongly and selectively enhances the peaks from hydroxylated sites and other silicons in their proximity (Figure 6b). Thus, for the calcined samples the peak at -98 ppm comes, at least in part, from the Si(OSi)30H sites, which are usually detected at ca. -100 ppm.I5 The peak of calcined M-15 at -105 ppm must have some contribution from the Si(OA1) sites located in the vicinity of surface silanols. For the as-synthesized M-15 the peak at -99 ppm is mainly from the Si(lA1) sites, because it loses intensity on calcination (cf. the BD spectra in Figure 6). For the calcined samples with high A1 content the Si(OSi)30H and Si(lA1) peaks are probably overlapped, which has already been observed for other zeolite^.'^ For the highlysiliceous calcined samples the peak at -98 ppm is exclusively from the Si(OSi)30H sites (Figure 7). Deconvolution of the 29Si NMR spectra of MCM-22 into individual components is not trivial. Unverricht et a1.6 divided their spectra into two sets of five Gaussian lines, the sets being assigned to Si(OA1) and Si(lA1) sites. It was concluded that MCM-22 has at least five inequivalent framework T-sites. We found that the deconvolutions shown in ref 6 do not accurately reproduce the background. This is probably a general problem,
mentioned also to us by others.16 We have come upon it, when deconvoluting our 29Si spectra of this and other zeolites. In order to get reasonable fitting one has to use non-Gaussian (Le., partially Lorentzian) lines or purely Gaussian lines with a broad signal simulating the background. By the former method we deconvoluted the spectrum of calcined M-25, resembling that from the dealuminated sample in ref 6, into six components, and by the latter procedure we obtained nine components plus the background signal (not shown). We admit that the deconvolution using Gaussian lines is more appropriate from the theoretical point of view, but the broad background signal was definitely too intensive considering the good crystallinity of our sample. We conclude that the deconvolution of such a complex 29Si spectrum, if it is not sufficiently resolved, can be quite ambiguous. Instead, we offer another approach. In the 29SiBD spectrum of calcined M-50 (Figure 7) the peaks in the range from -104 to -120 ppm all come from the Si(OAl) sites. We calculated that the average Si-0-Si angles corresponding to inequivalent Si(OA1) sites vary from 138" to 164" (cf. an equation of Engelhardt and Radeglia,I7 vide infra). Then, we constructed schematic 29Si spectra for the structural models of MCM-22 proposed by Leonowicz et aL7 (Figures 8 and 9). The chemical shifts 6 were calculated from two experimental relationships, both reported with high correlation coefficients r for numerous Si(OA1) sites giving NMR peaks in a wide frequency range. The equation of Engelhardt and Radeglia17 was obtained with r = 0.974 for 21 silica polymorphs and zeolites: 6 = -0.61928 - 18.68. The equation of Fyfe et a1.18 was obtained with I = 0.969 for 24 inequivalent T-sites in highly siliceous ZSM-5 using high-accuracy room-temper-
7008 J, Phys. Chem., Vol. 99, No. 18, 1995
Kolodziejski et al.
ature crystallographic data refined from X-ray diffraction ex21.44. Both periments: 6 = -287.6 coscl/(cos a - 1) equations predict similar bar patterns (Figures 8 and 9), except for some differences in the order of closely located lines. Note that most of the lines, whose positions were calculated from the equation of Fyfe et al. (Figures 8b and 9b), are shifted too far toward the higher frequency. Some correspondence of chemical shifts occurs between the experimental spectrum and the bar pattern in Figure 9a, calculated from the equation of Engelhardt and RadegliaI7 for the Cmmm model. However, the latter model places three strong lines of sites 6 , 13, and 5 in the spectral region, where there is only minor absorption in the experimental spectrum. On the other hand, we found that the appropriate adjustment of the parameters in the equation 6 = aa ib considerably improves the bar positions in Figure 8a for the P6lmmm model. None of the bar diagrams adequately fits the experimental spectrum.
+
Conclusions Zeolite MCM-22 contains two kinds of framework tetrahedral Al, Al"(A) and Al"(B), resonating at ca. 54 and 47-48 ppm, respectively. In the as-synthesized material AIIV(A) resides in a more symmetric environment than Al"(B). Calcination in air for 3 h at 853 K dealuminates the zeolite framework. Upon the calcination the environments of Al"(A) and Al"(B) become more and less distorted, respectively. This renders them more similar in the calcined material. We conclude that the template removal and dealumination appreciably affect the MCM-22 framework. The calcination creates two kinds of extraframework octahedral Al, Alw(A) and Al"I(B), resonating at 0.4 and -1.0 ppm, respectively. AlW(A) resides in a less symmetric environment than Alm(B). Both tetrahedral and octahedral Al species exist in two forms, which can be explained in accordance with the structure of MCM-22, proposed by Leonowicz et aL7 Thus, Al"(A) and Al"(B) may correspond to two sets of the framework tetrahedral sites, differentiated by their location in small and large atom rings and/or by their structural relation to two distinct channel systems. Then, AIV1(A) and AlW(B) may correspond to extraframework material of the same type, but deposited in two differential channel systems. We have assigned 29Si NMR spectra as follows. The Si(1Al) sites of as-synthesized zeolite give the peak at -99 ppm. The peak from the Si( 1Al) sites can be overlapped for calcined zeolite with the peak at -98 ppm from the Si(OSi)30H sites, provided that the A1 content is not very low. For the calcined material the peak at -105 ppm has some contribution from the
Si(OA1) sites located in the vicinity of surface silanols. In highly-siliceous calcined MCM-22 the peaks in the range from -104 to -120 ppm all come from the Si(OA1) sites. This implies that the average Si-0-Si angles corresponding to inequivalent Si(OA1) sites vary from 138" to 164". The schematic 29Si spectra constructed for the structural models of Leonowicz et al.? using the equations from refs 17 and 18, do not fit the experimental spectrum of highly-siliceous MCM-22. It turns out that the models and/or the equations have to be revised. The concept of two pore systems containing 10and 12-member rings is generally correct, since it has already been confirmed by the catalytic test^^-^ and it is consistent with our 27AlNMR results. Acknowledgment. We are grateful to the Spanish Comision Interministerial de Ciencia y Tecnologia (MAT94-0359-C0201) for financial support. References and Notes (1) (a) Rubin, M.; Chu, P. U.S.Patent 4 954 325, 1990. (b) Dessau, R. M.; Partridge, R. D. U.S. Patent 4 962 250, 1990. (2) Kirker, G. W.; Mizrahi, S.; Shih, S. US.Patent 5 000 839, 1991. (3) Absil, R. P. L.; Bowes, E.; Green, G. J.; Marler, D. 0.;Shihabi, D. S.; Socha, R. F. U.S.Patent 5 085 762, 1992. (4) (a) Coma, A.; Corell, C.; Llopis, F.; Martinez, A.; Pirez-Pariente, J. J. Appl. Catal. A 1994, 115, 121. (b) Coma, A.; Corell, C.; Martinez, A.; Pkrez-Pariente, J. Zeolites and Related Microporous Materials: State of Arl1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surjace Science and Catalysis 84; Elsevier: Amsterdam, 1994; p 859. (5) Souverijns, W.; Verreist, W.; Vanbutsele, G.; Martens, I. A,; Jacobs, P. A. J. Chem. SOC., Chem. Commun. 1994, 1671. (6) Unvemcht, S.; Hunger, M.; Emst, A.; Karge, H. G.; Weitkamp, J. Zeolites and Related Microporous Materials: State ofArt 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Studies in Surface Science and Catalysis 84; Elsevier: Amsterdam, 1994; p 37. (7) Leonowicz, M. E.: Lawton, J. A,; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (8) Rocha, J.; Liu, X.; Klinowski, J. Chem. Phys. Lett. 1991,182,531. (9) Kolodzieiski. W.: Lalik. E.: Lerf. A,: Klinowski. J. Chem. Phvs. Lett. 1992, 194, i29. (10) Samoson, A.; Liuumaa, E. J. Maan. Reson. 1988. 79, 255. (11) Grobet, P. J.; G&k, H.; Martens, J. A.; Jacobs, P. A. J . Chem. SOC.,Chem. Commun. 1987, 1688. (12) Hayashi, S.; Hayamizu, K. Chem. Phys. Lett. 1993, 203, 319. (13) Banie, P. J. Chem. Phys. Lett. 1993, 208, 486. (14) Janssen, R.; Tijink, G. A. H.; Veeman, W. S. J. Chem. Phys. 1988, 88, 518. (15) Engelhardt, G.; Michel, D. High-Resolution Solid-state NMR of Silicates and Zeolites; J. Wiley & Sons: Chichester, 1987. (16) Akporiaye, D. Private communication. (17) Engelhardt, G.; Radeglia, R. Chem. Phys. Lett. 1984, 108, 271. (18) Fyfe, C. A.; Feng, Y.; Grondey, H. Microporous Mater. 1993, 1 , 393. JP9429 170
