Small-Pore Molecular Sieves SAPO-34 and SAPO-44 with Chabazite

Small-Pore Molecular Sieves SAPO-34 and SAPO-44 with Chabazite Structure: .... Milton E. Rivera-Ramos , Gerardo J. Ruiz-Mercado and Arturo J. Hernánd...
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4878

J. Phys. Chem. 1994,98, 4878-4883

Small-Pore Molecular Sieves SAPO-34 and SAPO-44 with Chabazite Structure: A Study of Silicon Incorporation Sunil Ashtekar, Satyanarayana V. V. Chilukuri,+ and Dipak K. Chakrabarty' Solid State Laboratory, Chemistry Department, and Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Bombay 400 076, India Received: November 9, 1993; In Final Form: February 16, 19948

Two small-pore silicon-substituted molecular sieves SAPO-34 and SAPO-44 with varying amount of silicon have been synthesized and characterized by XRD,SEM, FT-IR spectroscopy, and thermal analyses. Their acidity has been measured quantitatively by temperature-programmed desorption of ammonia. The results of these studies along with solid-state MAS NMR results show that the major part of silicon substitutes isolated phosphorus atoms in the structure whereas some amount of it forms small silica patches. The extent of formation of such small patches is more noticeable in SAPO-34 than in SAPO-44.

Introduction Successful synthesis of aluminum phosphate molecular sieves has opened up new possibilities in shape-selective They consist of alternating A104 and PO4 tetrahedra and have no net framework charge. Substitution of silicon will create a charge imbalance, and so far only negativelycharged frameworks could be synthesized, which means that silicon does not substitute isolated aluminum. Two different mechanisms have been postulated for the incorporationof silicon in the aluminophcsphate (AIP04)framework (i) by replacement of a P atom with Si and (ii) by replacement of a pair of A1 and P atoms with a pair of silicon atoms.4 Substitution of type (i) is well-known, and this generates a negatively charged framework that is countered by protons making the material (SAPO) acidic. Substitution of the second type will not generate any framework charge. Such substitution does not take place due to the instability of the Si0-P bond. However,the Si-0-P bond can be avoided if a number of silicon atoms are linked to each other via oxygen where silicon occupies both AI and P sites forming a heterogeneous region known as silica patches. A number of authors have commented on the formation of such silica-richregions in the SAPO materials.5-* Thus, Martens et al.9 believe that such silica-richregions are present in SAPO-5, SAPO-1 1, and SAPO-37. Das et al.,1° on the other hand, have shown that the AlP04-5 and AlPO4-11 framework can accommodate only a small amount of silicon by substitution at the P site. If a higher amount of silicon is added to the synthesis gel, they form an amorphous silica phase. Recently, Man et al." performed a detailed analysis of SAPO-37 topology, and they have shown that both type (i) substitution and silica patches are formed in this material. It is possible that those framework structures that can accommodate only a small amount of silicon (such as A1PO4-5 and AlP04-11) do not form silica-rich regions extensively, but such regions are easily formed in structures like SAPO-37 that can accommodate a large amount of silicon. It will be interesting to study SAPO-34 and SAPO-44, two smallpore molecular sieves with chabazite structure as they are known to accommodate a large amount of silicon and are found to be very selective for the formation of olefins in the methanol dehydration.12-14 In this paper, the synthesis of SAPO-34 and SAPO-44 with varying amounts of silicon is reported. The as-synthesized materials have been identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis. Acidity has been measured by temperature-programmed de-

.

Abstract published in Advance ACS Abstracrs, April l, 1994.

0022-3654f 94f 2098-4878$04.50f 0

sorption of ammonia (TPD). Silicon environment in these materials has been investigated by solid-state MAS NMR spectroscopy.

Experimental Section Hydrothermalsynthesis of SAPO-34 and SAPO-44was carried out as follows. Solution A was prepared by dissolving pseudoboehmite and orthophosphoric acid (85 wt %) in water, and solution B was prepared by adding fumed silica to the organic template in the presence of a small volume of water. Solution B was then added to solution A with vigorous agitation. The resulting gel was transferred to a stainless steel autoclave and heated under autogeneouspressure for 48 h at 200 OC and at 190 OC for SAPO-34 and SAPO-44, respectively. The templates used were morpholine for SAPO-34 and cyclohexylamine for SAPO-44. The gel compositions (in mole) were structure SAPO-34 SAPO-44

A1203

P2O5

Si02

template

H20

1 1

1 1

X X

3.0

60 60

1.9

Threedifferent compositions were prepared in each case by varying the silica content, with x being 0.6,0.8,1 .Ofor SAPO-34 and 0.5, 0.7, 1.0 for SAPO-44. The final products were accordingly designated as SAPO-34f 1, SAPO-34f 2, SAPO-34f 3, SAPO441 1, SAPO-4412, and SAPO-4413, respectively. After the reaction, the crystals were filtered, washed, and dried at 110 OC for 12 h. These were called the 'as-synthesized" samples. Unless stated otherwise, these materials were used for various studies. X-ray powder diffraction patterns were recorded on a Philips PW 1820 instrument using Cu Ka radiation with nickel filter. SEM photographs were obtained on a Cameca-Su-SEM probe, and infrared spectra were recorded using a Nicolet 170 SX FTIR spectrometer. Chemical analysis was carried out after calcining the samples at 500 OC to remove the template. The calcined samples were dissolved in aqua regia for analyzing aluminum and phosphorus. The undissolved residue was fused with lithium metaborate and dissolved in dilute nitric acid. Analysis was carried out by an atomic emission spectrometer with an ICP source (Labtam Plasma Lab 8440). MAS NMR spectra were recorded on a Varian VXR-300s spectrometer with a Doty Scientific CP-MAS probe. The frequencies were 78.15, 121.41, and 59.59 MHz for 27Al,3IP, and 29Si, respectively. Pulses of 45O were used for all measurements with repetition times of 3 s for z7Al and 10 s for ,IP and 29Si. Data were acquired at a MAS speed of 4.5 kHz. 0 1994 American Chemical Society

Small-Pore Molecular Sieves SAPO-34 and SAPO-44

The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4879

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2 9 / dog 1443 Figure 1. X-ray powder diffraction patterns of as-synthesized samples: (a) SAPO-34/2; (b) SAPO-44/ 1.

Tetramethylsilane, 85% phosphoric acid, and aluminum nitrate in water were employed as references. Differential thermal (DTA) and thermogravimetric (TG) analyses werecarried out in air on a Dupont 9900 thermal analyzer at a heating rate of 10 OC/min. TPD was carried out on an apparatus assembled in the laboratory. The calcined samples were initially heated at 450 "C in a flow of helium for 6 h. Ammonia adsorption was carried out on the samples at 100 OC, after which it was desorbed by heating at 10 OC/min. in a flow of helium at 50 mL/min. TPD spectra were resolved into Gaussians using a computer program from which the number of acid sites was calculated.

Results and Discussion Tetraethylammonium hydroxide (TEAOH) is the commonly used template for the synthesis of SAPO-34. We could obtain highly crystallinesamples by using a cheaper chemical morpholine as the template. The highly crystalline nature of the samples is evident from the X-ray diffraction pattern (Figure 1) and SEM photographs (Figure 2). The d-values were in agreement with those reported in the literature. No impurity phase could be detected in the samples when the morpholine to alumina ratio was higher than 2. Pure phases could not be obtained at a lower concentration of silica (SiO*/A1203 X0.4) in the reaction gel, although not all silica in the gel was incorporated in the structure, as may be seen from the chemical analysis results (Table 1). Similarly in the synthesis of SAPO-44, if the template (cyclohexylamine) to alumina ratio in the gel was lower than 1.9, SAPO-5 also crystallized out along with SAPO-44. Young et al.15 also observed the crystallization of SAPO-44 along with SAPO-5 in systems where cyclohexylamine was employed as a template. A minimum amount of silica (SiOz/A1203 > 0.3) was found to be necessary in the gel for obtaining pure SAPO-44. Unlike Chen et al.,*3 we did not notice the formation of any impurity phases at SiOz/A1203 ratios higher than 0.6. The SEM photographs (Figure 2) show that both SAPO-34 and SAPO-44 were crystallized with a cubic morphology and widely varying crystallite sizes. The only difference observed between these two materials is the presence of twinning in SAPO-

Figure 2. Scanning electron micrographs of as-synthesized samples: (a, bottom) SAPO-34/ 1; (b, top) SAPO-44/ 1.

TABLE 1: Chemical Composition and Acidity of the Samples Obtained from Temperature-Programmed Desorption of Ammonia sample

Si

A1

P

SAPO-34/ 1 SAPO-34/2 SAPO-34/3 SAPO-44/ 1 SAPO-44/2 SAPO-44/3

0.164 0.175 0.191 0.116 0.151 0.183

0.480 0.488 0.476 0.501 0.487 0.478

0.354 0.337 0.333 0.383 0.362 0.338

acidity (mmol/g) net charge weak moderate strong -0.126 -0.151 -0.143 -0.118 -0.151 -0.140

0.370 0.331 0.425 0.309 0.375 0.299

0.209 0.253 0.256 0.179 0.217 0.274

0.455 0.639 0.591 0.395 0.551 0.660

44. In fact, this kind of twinning has been reported for its structural analog the natural chabazite.16 For removal of template, SAPO-34 and SAPO-44 should be calcined initially in nitrogen at 500 OC, after which it can be heated in air. Direct calcination in air led to lossof crystallinity. This possibly happened due to the large amount of heat released by the oxidation of the organic template, which destroyed the structure. Infrared Spectral Analysis. Infrared spectra in the framework vibrationar frequency region are shown in Figure 3. The various vibrational frequencies (Table 2) have been assigned, based on the correspondingfrequenciesreported for the naturally occurring aluminosili~ate-chabazite.~~ In both SAPOS, higher frequency shifts were observed for all bands, when compared to the structurally analogous aluminosilicate. This is not unexpected, as the force constant and hence the vibrational frequency is expected to increase as a consequence of shorter P-O bond length and increased electronegativity of P. Two additional bands at 868 and 8 16 cm-I seen for SAPO-34 may belong to the protonated templateoccluded in thesematerials. Most of the bands in SAPO-44 were found to be diffused when compared to SAPO-34. A weak band at 683 cm-I was seen in SAPO-44, which was absent in SAPO-34. This particular band

4880 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 105-

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Figure 3. IR spectra of as-synthesized samples: (a) SAPO-3412; (b) * indicates bands due to protonated amine.

SAPO-4413. The

TABLE 2 IR Bmd Positions in Framework Vibration Repion (1410-370 cm-l) aeym. sym. D-6 T-O sample stretch stretch rings bend SAPO-3412 1105 740 636 524 presentwork ~

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was reported in the literature for chabazite.1' Though it was speculatedin the literature that SAPO-44 has a slightly distorted structure compared to that of SAPO-34, which has the same structure as chabazite,*3 infrared spectral results suggest the opposite. Both infrared and SEM indicate that SAPO-44 is possibly structurally closest to chabazite. Thermal Analysis. DTA, TG, and DTG curves of the as-synthesized SAPO-34 and SAPO-44 are shown in Figure 4. SAPO-34 showed 4.8% mass loss at 100 OC, whereas the same for SAPO-44 was only 2.8%. What is interesting is that the mass loss observed during drying of the calcined samples of SAPO-34 and SAPO-44 was found to be same for both the samples which was 26%. The removal of template occurs in two steps, and the mass loss due to this processis 17.5% for SAPO-34,whichaccounts for 5.6 molecules of morpholineper unit cell (uc). This amounts to 0.23 cm3 of it/g of the sample. This is close to 0.24 cm3/g reported by Flanigen et a1.18 In the case of SAPO-44, the mass loss due to template removal was 19.5% independent of the amount of silica in the sample. This is equivalent to 5.4 molecules of cyclohexylamine per uc, which corresponds to a volume of 0.28 cm3/g. The ratio of tetrahedral units per uc to the number of template molecules per uc is nearly 7, which is in very good agreement with the results of Flanigen et a1.18 The corresponding value for SAPO-34 was 6. All theseobservationsemphasizetheclosestructural similarity of SAPO-34 and SAPO-44. Although DTA shows exothermic peaks at 430 and 610 O C for SAPO-34 and at 450 and 580 O C for SAPO-44, isothermal calcination at 500 OC was found to be sufficient for the removal of the templates. The shift of the DTA peaks to higher temperature is probably related to the subsequent oxidation of the template.

Temperoture ('c)

....... DT G

Figure 4. TG, DTG, and DTA curves of the as-synthesized samples: (a) SAPO-34/2; (b) SAPO-44/2. 185-c

0 E

.-0In L

0 c V

i

100

......_ 260 300 GbO 560

600 I

T e m p e r a t u r e ( 'C) Figure 5.

Temperature-programmeddesorption spectra of ammonia:

(a) SAPO-34/1; (b) SAPO-34/2; (c) SAPO-34/3.

Acidity. Temperature-programmed desorption of ammonia (Figure 5 ) for SAPO-34 gave two distinct peaks at 185 and 447

Small-Pore Molecular Sieves SAPO-34 and SAPO-44

The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4881

14

I

-

-5 Chargehnit cell Figure 7. Variation of acidity with charge per unit cell.

-4

Temperature ('c)

Figure 6. Temperature-programmed desorption spectra of ammonia: (a) SAPO-44/1; (b) SAPO-44/2; (c) SAPO-4413. O C and is similar to that reported by Schnabel et al.19 TPD of SAPO-44 (Figure 6) is similar to that of SAPO-34 (Figure 5 ) . These curves could be resolved into three clear desorption peaks and are shown in their respective figures. The first peak arises due to the weak acid sites present at surface hydroxyls and will be ignored in this discussion. The other two peaks arising possibly from structural acidity will be referred to as moderate and strong acidity. The total structural acidity of our SAPO-34 samples is of the same order as that reported by Schnabel et al.,19 although they did not attempt to resolve the TPD curve. Table 1 shows the net charge as well as acidity obtained through ammonia TPD. Net charge has been calculated based on the difference of the aluminum and phosphorus content of the samples. Since the total phosphorus and silicon is higher than the amount of aluminum in all the samples (Table l), it appears that the entire substitutionofSi is not taking placeaccording tomechanism (i). The highly crystalline nature of the samples as seen from XRD and SEM results suggests that there is very little or no amorphous silica present in these samples. The high silica content then would imply that there is some heterogeneous substitution of silicon as well, leading to silicon-rich regions. Although the substitution by mechanism (i) will generate one acid site per silicon atom, the silicon-rich region will contribute to acidity only by those Si atoms that lie at the border and depends on the number of aluminum atoms linked to each silicon. Hence, it will be more meaningful to relate structural acidity to net negative charge rather than to the silicon content in the sample. It is possible to calculate the net charge per uc knowing that there are 36 T atoms per uc in the chabazite structure. A comparison of these with the number of structural acid sites (moderate strong) shows that although acidity increases with silicon incorporation, the net negative charge is higher than the acidity. This would imply that some of the protons (countering the negativecharge) are not accessible to theammonia molecules. SAPO-34 and SAPO-44 have some 4 - 0 channels, and any protons located inside such narrow pores may not be accessible to the

+

-

ammonia molecules. A plot of acidity versus charge per uc shows a linear rise for both SAPO-34 and SAPO-44 (Figure 7). The two different plots indicate that the distribution of the silicon atoms is not identical in the two structures. 31Pand *'AI MAS NMR Figure 8 shows the 31Pand 27Al MAS NMR spectra of the different SAPO-34 samples. These spectra show a single symmetric peak a t -27.7 ppm that can be assigned to a single P(4A1) environment20.2' irrespective of the amount of silicon present. This is in agreement with the known fact that P-O-Si linkages are absent in the SAPOs.27 A1 shows the typical NMR peak a t 37 ppm due to aluminum in tetrahedral environment. The second peak at 9.4 ppm may arise due to additional coordination of aluminum. Blackwell et al.21 noticed two 27AlNMR peaks a t 34.5 and 3.4 porn in SAPO-34. They assigned the second peak to a higher coc :dination of -.luminum due to interaction with the second coordination sphere. It is interesting to note that in SAPO-34/1 the NMR. peak due to tetrahedral aluminum is much narrower and highly :iLy.nmetric, suggesting the presence of a second peak on the high field side. With higher silicon content, the band broadens. T;.%seems to imply that the tetrahedral aluminum is subject4*c,to further interactions and such interaction increases with silicon content. This causes the broadening so that the peak appears to shift from 37 to 40.8 ppm. In case of SAPO-44 (Figure 9), there is a single line a t -29.2 ppm due to P(4A1) environment. SAPO-44/1 showed two additional weak peaks at -13 and -19 ppm. Briend et a1.22 observed similar peaks in one of their SAPO-37 samples which disappeared on calcining the sample. These two peaks were enhanced upon cross polarization with protons and hence were assigned to a small number of defect atoms. Z7Al spectra show the tetrahedral aluminum at 37.5 ppm, but also an additional weak peak a t 11.7 ppm. When the sample is rehydrated after calcination to remove the template, a very intense peak at -12.7 ppm was observed. Similar observations were made by Blackwell and Patton in the case of AlPO4- 17z3and SAPO-3421. The second peak may arise from additional coordination of aluminum with water molecules, as suggested by Blackwell and Patton for SAPO34. Such enhancement of the second peak was noticed for SAPO37 by Briend et a1.,22 who assigned this to complete structural collapse in presence of water. 29si NMR. Figure 10shows the 29Si MAS NMR of the various SAPO-34 and SAPO-44 samples. All the SAPO-34 samples

.

4882 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994

I

40

.

.

20

.

0

Ashtekar et al.

. -20. . -LO. . -60. . -80. ppm 160

.

I

120

80

LO

0

-LO -80

ppm

Figure 8. 3lP and 27AlMAS NMR spectra of SAPO-34: (a) SAPO-34/1; (b) SAPO-34/2; (c) SAPO-34/3. Ul

R c:

b

v,

z b

I

30 20 10 0 -10 -20 -30-LO-50-60-70 ppm 160 120 80 LO 0 -LO Figure 9. 3‘P and z7Al MAS NMR spectra of SAPO-44: (a) SAPO-44/1; (b) SAPO-44/2; (c) SAPO-44/3.

showed peaks at around -90.6, -94.5, -98.8, -104.9, and -109 ppm. Based on the model of Saldarriage et aL5 Man et al.” showed that the Si(4Al) NMR peak appears at -89.1 ppm in SAPO-37 as compared to -84.7 ppm in the isostructural faujasite. We assigned the peak at -89.9 ppm in SAPO-34 and that at -91.1 ppm in SAPO-44 to a Si(4Al) environment. This is the most intense peak, and it belongs to the silicon atoms substituted by mechanism (i). Each such silicon will generate one framework negative charge and hence one Bronsted acid site. The other peaks belong to the silicon-rich region and can be assigned to Si(3A1)(-94.5 ppm), Si(2Al) (-98.8), Si(A1) (-104.9), and Si(OAl) (-109). It may be seen that the minor NMR peaks are more prominent in SAPO-34, showing that more silica-rich regions are formed. In the case of SAPO-44, there is very little silica-rich region in SAPO-4411 and SAPO-4412 and the silica patches become dominant only at high silicon concentration in the sample. This suggests that in case of SAPO-44 a higher amount of silicon is incorporated by mechanism (i). Deconvolution of the 29SiMAS NMR spectra (Figure 11) gave the relative abundance of silicon in each type of environment. For small silicon patches, the order should be Si(3Al) > Si(OA1) > Si(2Al) > Si(1Al) and it becomes Si(OA1) > Si(3Al) > Si(2Al) > Si(1Al) for silicon-rich regions

-80

ppm

containing moreSi.l1Vz4 The relative intensityof the MAS NMR peaks after deconvolution of our SAPO-34 and SAPO-44 samples appears to agree with the formation of small patches of silica, although the major part of silicon atoms substitutes the isolated phosphorus atoms (Table 3). However, we notice a reversal in the relative intensity of Si(OA1) and Si(2Al). It can be seen from Figure 11 that the deconvoluted peak assigned to Si(2Al) is extremely broad, and hence it is difficult to accept this as a single silicon environment. Barring this peak, the intensity and position of all other peaks are very well explained by the above model. It is not clear whether the above-mentioned broad peak is due to a small zeolitic region as described by Jacobs et aL9925 in which silicon atoms lie in different A1 environments leading to the broadening. Conclusions Small-pore silicon-substituted aluminum phosphate molecular sieves SAPO-34 and SAPO-44 with varying amounts of silicon have been synthesized. They have been characterized by XRD, SEM,FT-IR, thermal, and chemical analyses. Temperature-programmed desorption of ammonia showed the presence of two types of structural acidity in them. The high

The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4883

Small-Pore Molecular Sieves SAPO-34 and SAPO-44

U

a

-30 -50 -70 -90 -110 -DO -150 ppm -30 -50 -70 -90 -110 -130 -150 ppm Figure 10. 29Si MAS N M R spectra: (a) SAPO-34/1; (b) SAPO-34/2; (c) SAPO-34/3; (d) SAPO-44/1; (e) SAPO-44/2; (0 SAPO-44/3.

References and Notes (1) Wilson, S.T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen,

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1

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-75 -80 - 8 5 -90 -95 -100 -105 -110 -115 ppm Figure 11. Deconvoluted 29Si MAS N M R spectra of SAPO-34/1.

TABLE 3 Distribution (%) of Silicon Environments Obtained from Deconvoluted Silicon MAS NMR Spectra sample

Si(4AI)

Si(3AI)

Si(2AI)

Si(IA1)

Si(OA1)

SAPO-3411 SAPO-34/2 SAPO-3413 SAPO-4411 SAPO-4412 SAPO-4413

57.67 46.63 48.40 80.73 73.80 53.89

17.35 20.92 21.37 9.01 9.65 14.46

12.30 14.71 13.29 5.96 10.98 15.91

2.55 5.10 7.13 0.77 1.21 6.35

10.12 12.64 9.69 3.53 4.35 9.37

silica content of the samples and the high proportion of Si(nAl), n < 4, as seen from solid-state MAS NMR spectroscopy, have established that the major part of silicon substitutes phosphorus randomly, while a small amount of it forms small silica patches by heterogeneous substitution. The formation of such small silica patches is more predominant in SAPO-34.

Acknowledgements We are thankful to RSIC, IIT, Bombay for the various characterization facilities. One of the authors (S.A.) is grateful toCSIR, New Delhi, for the award of ajunior research fellowship.

E. M. J. Am. Chem. SOC.1982, 104, 1146. (2) Wilson, S.T.; Lok, B. M.; Flanigen, E. M. US.Patent 4,310,440, 1982. (3) Pellet, R. J.; Coughlin, P. K.; Shamshoum, E. S.;Rabo, J. A. ACS Symp. Ser. 1988, 368, 512. (4) Lok, B. M.; Messina, C. A.; Patton, R. A.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984. 106, 6092. (5) Sierrade Saldarriage, L.; Saldarriaga, C.; Davis, M. E. J . Am. Chem. SOC.1987, 109, 2686. (6) Martens, J. A.; Mertens, M.;Grobet, P. J.; Jacobs, P. A. Inlnnouation in Zeolite Materials Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., Schulz-Ekloff, G., Eds; Stud. Surf.Sci. Catal. Elsevier: Amsterdam, 1988; Vol. 37, p 97. (7) Hasha, D.; Sierra de Saldarriaga, L.; Saldarriage, C.; Hathaway, P. E.; Cox, D. F.; Davis, M. E. J. Am. Chem. SOC.1988, 110, 2127. (8) Jahn, E.; Muller, D.; Becker, K. Zeolites 1990, 10, 151. (9) Martens, J. A,; Grobet, P. J.; Jacobs, P. A. J. Carol. 1990,126,299. (IO) Das, J.; Satyanarayana, C. V. V.; Chakrabarty, D. K.J. Chem. Sm., Faraday Trans, 1992, 88, 3255. (11) Man, P. P.; Briend, M.; Peltre, M. J.; Lamy, A.; Beaunier, P.; Barthomeuf, D. Zeolites 1992, 11, 563. (12) Anderson, M.W.; Sulikowski, B.; Barrie, P. J.; Klinowski, J. J . Phys. Chem. 1990, 94, 2730. (13) Chen, J., Thomas, J. M. Catal. Lett. 1991, !I, 199. (14) Xu,Y.;Grey, C. P.; Thomas, J. M.; Cheetham, A. K. Catal. Lett. 1990, 4, 25 1. (15) Young, D.; Davis, M. E. Zeolites 1991, 10, 277. (16) Gottardi, G.;Galli, E. InNaturalZeolites;Gorsey,A. E., Engelhardt, W. V., Eds.; Springer Verlag: Berlin, 1985. (17) Flanigen, E. M.; Khatami, H.; Szymanski, H. A. Adu. Chem. Ser. 1971, 101, 201. (18) Flanigen, E. M.; Patton, R. L.; Wilson, S.T. In Innovation in Zeolite Materials Science; Grobet, P. J., Mortier, W. J., Vansant, E. F., SchulzEkloff, G., Eds; Stud. Surf.Sci. Catal. Elsevier: Amsterdam, 1988; Vol. 37, p 13. (19) Schnabel, K-H.; Fricke, R.; Girnus, I.; Jahn, E.; Loffier, E.; Parlitz, B.; Peuker, C. J. Chem. Soc., Faraday Trans. 1991,87, 3574. (20) Williams, R. P. J.; Giles, R. G. F.; Posner, A. M. J. Chem. Soc., Chem. Commun. 1981, 1051. (21) Blackwell, C. S.;Patton, R. L. J. Phys. Chem. 1988, 92, 3965. (22) Briend, M.; Peltre, M. J.; Lamy, A.; Man, P. P.; Barthomeuf, D. J. Caral. 1992, 138, 90. (23) Blackwell, C. S.;Patton, R. L. J . Phys. Chem. 1984, 88, 6135. (24) Ashtekar, S.;Satyanarayana, C. V. V.; Chakrabarty, D. K. Proc. Ind. Acad. Sci., Chem. Sci., in press. (25) Martens, J. A.; Janssens, C.; Grobet, P. J.; Beyer, H. K.;Jacob, P. A. In Zeolites: Facts. Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1989; Vol. 49A, p 215.