Infrared spectroscopy, thermoprogrammed desorption, and nuclear magnetic resonance study of the acidity, structure, and stability of zeolite MCM-22

UTTERWORTH I N E M A N N

Infrared spectroscopy, thermoprogrammed desorption, and nuclear magnetic resonance study of the acidity, structure, and stability of zeolite MCM-22 A. Corma, C. Corell, V. Forn6s, W. Kolodziejski, and Joaquin P6rez-Pariente Instituto de Tecnologla Quimica uPv-CsIC, Universidad Polit~cnica de Valencia, Valencia, Spain Zeolite MCM-22 has been studied by infrared (i.r.) spectroscopy, thermoprogrammed desorption (t.p.d.) of NH 3, and 27AI n.m.r, with magic angle spinning (MAS). It has been found that zeolites MCM-22 and ZSM-5 both have framework i.r. bands at about 1,245 and 550 cm-% and zeolites MCM-22 and Y both have two pore opening i.r. bands at about 380 and 317 cm -1. In MCM-22 there are two kinds of bridged hydroxyls (3,620 and 3,575 cm - 1) and two kinds of internal silanols (3,500 and 3,700 cm-1). The latter two i.r. frequencies are similar to those in zeolites ZSM-5 and Y, respectively. 27AI n.m.r, reveals two kinds of framework tetrahedral AI. The spectroscopic similarities and the dual appearance of the relevant i.r. bands/n.m.r, peaks seem to indicate that MCM-22 probably has two distinct pore systems containing 10- and 12-member rings, which is in accordance with our recently published results of catalytic tests. Quantitative results on Brcnsted and Lewis acidity are reported from the i.r. study of pyridine adsorption and from t.p.d, of NH 3. The Br0nsted sites are strongly acidic and accessible for pyridine. MCM-22 is very sensitive to the calcination conditions, being more dealuminated on heating in air than in a vacuum o r N 2. The dealumination occurs even during grinding in a mortar with a concomitant decrease in the acidity of the final sample. Keywords: Zeolite MCM-22; i.r.; solid-state n.m.r.; acidity; structure

INTRODUCTION Zeolite MCM-22 is a novel molecular sieve invented in the laboratories of Mobil. 1 High thermal stability (up to 1,198 K), an elevated surface area (over 400 In" g - l ) , and a very large sorption capacity (about 15 wt%) for water and small organic molecules render this material very interesting for catalysis. MCM-22 has been proposed as a catalyst for the conversion of paraffins to olefins and aromatics, for cracking, isomerization, and alkylation. 1'2 In particular, the zeolite is remarkably useful for the production of highdensity, cycloparaffin-rich jet fuel. 2b MCM-22 containing Cu or rare earth metals is also promising from the ecological point of view because when it is added to a conventional USY catalyst, it considerably reduces CO and NO/NO 2 emissions from refineries. ~ The catalysis science community is searching now for new applications of MCM-22, prompted by the very interesting structural conclusions published reAddress reprint requests to Prof. Corma at the Instituto de Tecnologfa Qufmica UPV-CSIC, Universidad Polit6cnica de Valencia, Avda. de los Naranjos, s/n.-46022 Valencia, Spain. Received 3 November 1994; accepted 13 February 1995 Zeolites 15:576-582, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010

cently by Leonowicz et al. 4 This zeolite is composed of interconnected {435663[43]} building units forming a three-dimensional dodecasil-1 H-like lattice belonging to the P6/mmm or Cmmm space group. It is of great importance that the structural models contain two independent channel systems with rings of 10 and 12 tetrahedral (T) atoms and large supercages, which are 7.1 A wide and 18.2 • long. There are already chemical clues concerning the structure of MCM-22. Indeed, we found by testing isomerization/disproportionation of m-xylene on the acidic zeolite and hydroisomerization of n-decane on the bifunctional Pt/zeolite catalyst that MCM-22 behaves like both 10- and 12-member ring zeolites. 5 The same conclusion was drawn by Souverijns et al., 6 who investigated isomerization and hydrocracking of n-decane. A spaciousness index of about 8 also locates MCM-22 in the intermediate region between 10- and 12-member ring zeolites. 7 To gain further insight into the catalytic action of MCM-22 one has to examine and control (e.g. by dealumination) its acidic properties. This is the aim of our present work, which studies thermal decomposition of the template and the creation of acid sites and also provides new infor0144-2449/95/$10.00 SSDI 0144-2449(95)00015-X

Zeolite MCM-22: A. Corma et aL

Table 1 Molar compositions of synthesis gels. R stands for hexamethlyleneimine used as a template Sample M-15 M-25 M-50

SiO2/ AI203

OH - I SiO2

H20/ SiO2

R/SiO2

30 50 100

0.30 0.14 0.12

40 45 45

0.50 0.35 0.50

° °

Z

05

~

o

CO nO

mation on the structure and stability of this unique zeolite.

EXPERIMENTAL Crystallization of zeolite MCM-22 was carried out in a stainless steel rotating autoclave over 7 days at 400423 K under autogenous pressure from a gel containing hexamethyleneimine used as a template.~ The Si/ Al ratios of the as-synthesized samples were found by elemental analysis to be the same as those in the synthesis gels. We synthesized three samples (Table 1), designated M-15, M-25, and M-50, where the numbers correspond to the total Si/A1 ratios of the assynthesized samples. Calcination in air for the i.r. and XRD studies was carried out for 6 h at 823 K, and for the n.m.r, studies it was done for 3 h at 853 K. Calcination was carried out in a vacuum i.r. cell (see next i.r. details). Room temperature i.r. spectra were measured with a Nicolet 710 FT i.r. spectrometer. Only the framework spectral region was recorded from zeolite/KBr pellets. In all other cases, the i.r. spectra were recorded from zeolite wafers (10 mg cm -2) mounted into the vacuum cell and pretreated in a vacuum (10 -2 Pa) under various conditions. For the study of template decomposition the zeolite wafers, made of as-synthesized material, were successively heated in a vacuum over 1 h at 373, 473, 573, 673, and 723 K with the i.r. spectrum being recorded after each calcination period. When we were interested in the pyridine and/or hydroxyl spectral region, the wafers,

4000

3400

2800

2200

1600

cm -1

WAVENUMBER

Figure 2 Infrared spectra of M-15 recorded after consecutive calcinations in a vacuum at increasing temperatures (373, 473, 573, 673, and 773 K). The inset shows the expanded hydroxyl spectral region after treatment at 773 K.

made of as-synthesized or air-calcined material, were heated in a vacuum at 673 K overnight. Hereafter, such samples are designated as vacuum-calcined and air-calcined and vacuum-activated, respectively. For the

1455

i

1620

< >I--

Z

U4

I-

_z

I 0

I

I

10

I

I

20 DEGREES 2 0

I

1650 30

40

Figure 1 XRD pattern (CuKo radiation) of calcined MCM-22. The sample was calcined in air for 6 h at 823 K.

i

I

1450

WAVENUMBER Cnil Figure 3 infrared spectra in the NH/CH region of hexamethyleneimine: (a) neutral; (b) protonated with sulfuric acid.

Zeolites 1 5 : 5 7 6 - 5 8 2 , 1995

577

Zeolite MCM-22." A. Corma et aL

adsorption/desorption studies, after the wafer activation, pyridine vapor (5 torr) was passed to the cell and adsorbed onto zeolite. The excess pyridine was removed in a vacuum over three consecutive 1-h periods of heating at 523, 623, and 673 K, each of them followed by the i.r. measurement. All of the spectra in the pyridine region (1,750-1,350 cm-1) were scaled according to the sample weight. T e m p e r a t u r e p r o g r a m m e d desorption experiments were done in a Micromeritics 2900 apparatus. The as-synthesized samples (100 mg) were preactivated in an oxygen stream over 2 h at 723 K, and then NH 3 was adsorbed at 373 K. Desorption was carried out at a heating rate of 10 K min-1. Solid-state 27A1 Bloch decay n.m.r, with magic angle spinning (MAS) was done at ambient temperature on a Varian VXR-400S WB spectrometer at 104.2 MHz. We used a 7-mm Varian probe with zirconia rotors driven by dry air at 7 kHz, a rr/20 flip angle, and a recycle delay of 0.5 s.

RESULTS A N D D I S C U S S I O N First, we have examined the samples by XRD (Figure 1). The peak positions and intensities agree reason-

UJ

O

m

•

b

a

C

o

t 3700

.L___.__ ~ 3500

3700

.~._.--L 3700

3500

WAVENUMBER

J 3500

cn~1

Figure 5 Infrared spectra in the hydroxyl region of air-calcined and vacuum-activated MCM-22 with various Si/AI ratios: (a) M-15; (b) M-25; (c) M-50o The samples were first calcined in air for 6 h at 823 K and then activated overnight at 673 K in the vacuum i.r. cell.

ably with those in the literature, l proving that pure MCM-22 has been obtained. In our i.r. spectra of M-15 (Figure 2) there are the following template bands: 3,215 and 3,055 c m - 1 (NH stretching), 1,610 cm-1 (NH bending), 2,935, 2,860, 1,470, and 1,455 c m - I (CH2 vibrations).S Note that the NH bending band is strong from protonated hexamethyleneimine (Figure 3b) and from the template (Figure 2) but weak from pure hexamethyleneimine (Figure 3a). It follows that the template consists mostly of the (CHz)6NH ~- cations. Upon the zeolite calcination in a vacuum all of the template bands decrease with the temperature of the treatment (Figure 2). At 673 K we detect only residual bands of the original template, whereas a new band at 2968 cm-1 (CH~ stretching) s indicates that some imine rings are cracked, and consequently aliphatic chains are formed. Since the CH2 bands lose more intensity than the NH bands we believe that the aliphatic chains are still bound to the >NH~- moiety. At

tl~, 0

a

m
UJ O Z ,< g0

o

b

3900

u~o

U3

U) m

u3u~ 0

3500 I

WAVENUMBER

crr; 1

1700

I

I

I

1300 WAVENUMBER

Figure 4 Infrared spectra in the hydroxyl region of M-15: (a) vacuum calcined; (b) air calcined and then vacuum activated. The calcination in air and the vacuum treatment were carried out for 6 h at 823 K and overnight at 673 K, respectively.

578

Zeolites 1 5 : 5 7 6 - 5 8 2 , 1995

900

500

cm "1

Figure 6 Infrared spectrum in the framework region of calcined M-15, recorded from a KBr pellet. The sample was calcined in air for 6 h at 823 K.

Zeolite MCM-22: A. Corma et aL Table 2 Infrared frequencies (in cm -1) of MCM-22 and zeolites Y and ZSM-5. MCM-22 was calcined in air for 6 h at 823 K, and the spectrum (Figure 6) was recorded from a KBr pellet. The assignment was done according to Refs. 12-15 T - 0 asym. stretching

T - 0 sym. stretching Internal T - 0 bend

Sample

Si/AI ratio

External

Internal

External

Internal

Double ring

MCM-22 Y ZSM-5

15a 2.45 20

1,245 1,140 1,220

1,092b 1,022 1,098

790/810

740 720

660/595/550/500

790 793

754

Pore opening

450 459 452

652/580/505 616/542

380/317 385/315

a Si/AI ratio of the synthesis gel. b 1,080 cm -1 in the as-synthesized material.

773 K the template bands above 3,000 cm-1 disappear, although a minor band at 1,610 cm-1 may indicate that the template is not completely removed even at such a high temperature. Consider now the hydroxyl groups, which are created during the calcination. A broad band at 3,510 c m - 1 (Figure 2), which is best seen after the calcination at 573 K, corresponds to internal silanols. These groups are lost at higher temperatures (673 and 773 K), probably because of condensation, whereas two new bands appear at 3,747 cm-1 (external silanols) and at 3,620 cm-1 (bridged Si(OH)A1 hydroxyls). The latter one has a shoulder at about 3,575 c m - I (Figures 2 and 4), indicating that the bridged hydroxyls are located at inequivalent positions in the MCM22 structure. The template can be removed completely by calcination in air at 823 K, but such treatment results in partial framework dealumination. Accordingly, we have observed (Figure 4) a considerable decrease of the Si(OH)A1 band and detected a new band at 3,670 cm -~, assignable to hydroxyls from extraframework AI (EFA1). For the air-calcined and vacuum-activated samples, the concentration of different hydroxyl groups depends on the A1 content, which can be assessed from the intensity of the respective i.r. bands (Figure 5). With an increasing Si/A1 ratio, the populations of bridged and EFA1 hydroxyls decrease, considering the bands at 3,620 and 3,670 c m - 1, respectively. It is obvious that dealumination destroys the bridged hydroxyls. There are external silanols at the crystal surface (3,747 cm -1) and internal silanols residing in defects (about 3,700 and 3,500 cm-a). The internal silanols are either less (3,700 cm-1) or more (3,500 cm-~) involved in molecular interactions, for example, with cavity walls. Considering the structure of MCM-22, one could infer that the former silanols reside in the framework defects located in the large 12-member ring channels or cavities, and the latter occur in the narrower 10-member ring channels. Indeed, the internal silanols in zeolite Y absorb at 3,700 cm-~, whereas those in ZSM-5 give a band at 3,500 c m - ~.0-~ ~ Then, the relative intensity of those bands from MCM-22 could possibly provide information on the relative number of hydroxyl nests generated in both types of channels during synthesis and/or during activation. In sample M-50 the external silanols are outnumbered by the internal silanols from the

framework defects (see the band at 3,747 cm-1 versus those at about 3,700 and 3,500 c m - 1 in Figure 5). With an increasing Si/AI ratio the population of the external and internal silanols decreases and increases, respectively. This may indicate that the MCM-22 crystals with the lower AI content are bigger but have more defects (subject to our future studies by scanning transmission electron microscopy). Infrared spectroscopy features of the framework vibrations are shown in Figure 6 and Table 2. The intrinsic vibrations of the T O 4 tetrahedra (T = Si, A1) are supposed to be structure msens~tlve. According to Flanigen et al. i2 they give "internal" bands at about 1,100, 800, and 450 cm -1. However, we noticed an effect of dealumination: the band at 1,080 cm-1 was shifted by 12 cm-~ to the high frequency upon the MCM-22 calcination in air (Table 2). The adjacent tetrahedra mutually vibrate with specific frequencies, dependent on framework topology, and give "external" bands, which are useful in structural studies. For example, these bands are claimed to identify the following structural features: double rings (about 650550 c m - 1),l~ five-member ring chains (1,200 c m - x) and blocks (550 c m - l ) in ZSM-V5,14'1"~motion of oxygen atoms which share sodalite and double-six rings •

3620

•

-

12

a

b

0 I 3900

3500

1750

1550

1350

WAVENUMBER cm -1 Figure 7 Hydroxyl (a) and absorbed pyridine (b) i.r. bands of vacuum-calcined M-15: (al) before pyridine adsorption; (a2) after pyridine adsorption and desorption at 523 K; (a3) difference spectrum• (bl), (b2), and (b3) correspond to pyridine that remained adsorbed at 523, 623, and 673 K, respectively• Before the pyridine adsorption the sample was calcined overnight at 673 K in a vacuum i.r. cell.

Zeolites 1 5 : 5 7 6 - 5 8 2 , 1995

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Zeolite MCf/I-22: to. Corma et al.

Table 3 Acidity of air-calcined and vacuum-activated MCM-22 measured in p,mol of adsorbed pyridine/g of the zeolite at increasing temperatures. Before the pyridine adsorption the samples were calcined in air for 6 h at 823 K and then activated overnight at 673 K in the vacuum i.r. cell. Absorption coefficients of the pyridine i.r. bands were taken from Ref. 16 Lewis

BrCnsted Sample 2

3900

3500

2

1750

1550

1350

WAVENUMBER cm-1 Figure 8 Hydroxyl (a) and adsorbed pyridine (b) i.r. bands of air-calcined and vacuum-activated M-15: (al) before pyridine adsorption; (a2) after pyridine adsorption and desorption at 523 K; (a3) difference spectrum. (bl), (b2), and (b3) correspond to pyridine that remained adsorbed at 523, 623, and 673 K, respectively. Before the pyridine adsorption the sample was calcined in air for 6 h at 823 K and then activated overnight at 673 K in a vacuum i.r. cell.

in zeolite Y (580 c m - l) and large-pore rings of eight and more tetrahedra (420-300 cm-1).]2 It turns out (Table 2) that there are some similarities between the external band frequencies of MCM-22 and ZSM-5 (1,245 and 1,220 c m - 1 , 5 5 0 and 542 c m - l ) as well as between those of MCM-22 and faujasite (660 and 652 cm-1, 595 and 580 cm-1, 500 and 505 c m - l , and particularly the bands below 400 c m - ~). All of this is consistent with the recently published structure of MCM-22. 4 Acidity distribution has been monitored using pyridine adsorption/desorption (Figures 7 and 8, Table 3). The fingerprint pyridine bands corresponding to Lewis and BrCnsted sites appear at 1,450 and 1,540 cm-1, respectively.16 Considering the intensities of the 3,670 c m - 1 (Figure 7 al) and 1,450 c m - 1 bands (Figure 7b) for vacuum-calcined M-15 we found that the amount of EFAI, either hydroxylated or that bearing Lewis sites, is very small. We conclude that dealumination is negligible when the sample is activated in vacuum up to 673 K. On the other hand, the BrCnsted acidity is strong, since the 1,540 c m - ~ band decreases only slightly with the temperature rise (Figure 7b). Practically all of the bridged hydroxyls are accessible for pyridine (see the 3,620 and 3,575 c m - 1 bands in Figure 7a). Comparing with the vacuumcalcined material, air-calcined and vacuum-activated M-15 (see Figures 7 and 8) has considerably less bridged hydroxyls (the 3,620 and 3,575 c m - 1 bands), more hydroxylated EFAI (the 3,670 c m - 1 band), and considerably more Lewis sites (the 1,450 c m - 1 band). The 1,540 cm-~ band decreases more with the temperature rise, indicating that the air-calcined and vacuum-activated zeolite has weaker BrCnsted acidity than the vacuum-calcined zeolite. It turns out that calcination in air at 823 K dealuminates the MCM-22 zeolite. Quantitative results for the pyridine adsorption on air-calcined and vacuum-activated MCM-22

580

Zeolites 1 5 : 5 7 6 - 5 8 2 , 1995

M-15 M-25 M-50

523 K

623 K

673 K

523 K

623 K

673 K

54 36 30

36 24 18

27 10 9

26 18 12

23 14 9

23 14 9

(Table 3) show that the total acidity increases with the A1 content and so does the fraction of strong BrCnsted sites, which remain occupied by pyridine at 673 K. As we have already shown, calcination of MCM-22 in air results in certain zeolite dealumination. In vacuum the dealumination seems to be much smaller and the same we have found in the case of nitrogen. We stress that for MCM-22 the effect of the calcination atmosphere on the zeolite dealumination is more profound than the A1 content of the starting zeolite. The stability of MCM-22 is even dependent on sample grinding. For example, the manual grinding in a mortar for 10 min reduces the total acidity to 50%, as measured at 523 K by the pyridine adsorption technique. This makes the method of sample preparation crucial for the reproducibility of XRD, t.p.d., i.r., or MAS n.m.r, results. The original t.p.d, graphs of all of our samples show three distinct peaks at 460, 635, and about 850 K (Figure 9a). However, the blank experiment done without previous adsorption of N H 3 (Figure 9b) gives the peak at about 850 K, probably corresponding to dehydroxylation. Unfortunately, our t.p.d, apparatus does not work on line with a mass spectrometer, so we have not been able to clarify further this point. Recently, Unverricht et al. 7 have published t.p.d, pattern of MCM-22 with the peak at 834 K assigned to strong Lewis sites. This peak is completely missing from our difference graph (Figure 9c). Thus, our t.p.d, experiments detect only two types of acid sites of different strength. The total acidity decreases with the decreasing A1 content (Table 4). T h e t.p.d. method itself does not allow one to assign the peaks. On the basis of the former i.r. results we suggest that both BrCnsted and Lewis centers contribute to each peak, but the latter sites participate in the overall pattern to a lesser extent. Acidity of MCM-22 measured by t.p.d, in cm a of adsorbed NHa/g of the zeolite. The as-synthesized samples (100 rag) were preactivated in an oxygen stream over 2 h at 723 K,

Table 4

and then NH 3 was adsorbed at 373 K. Desorption was carried out at a heating rate of 10 K min -1 Sample M-15 M-25 M-50

Total acidity

Weak acidity

Strong acidity

37 16 9

22 10 5

15 6 4

Zeolite MCM-22: A. Corma et aL

57

460 Z

~3

0

l-

M -50 I

I

468

I

I

668

I

( ~'-'~'--'[~v~

868

0

1068 K

Figure 9 Thermoprogrammed desorption/NH3 graphs for MCM-22 (a) original; (b) from the blank experiment; (c) difference pattern. Before the NH3 adsorption as-synthesized MCM22 was heated in an oxygen stream for 2 h at 723 K. NH3 was adsorbed at 373 K.

27A1 n.m.r. MAS spectra of MCM-22, either assynthesized (Figure 10) or air-calcined (Figure 11), are similar as concerns the number and positions of the peaks. T h e relative n.m.r, intensities for the assynthesized samples approximately reflect the total Si/AI ratios (Table 5). The as-synthesized material gives a signal from four-coordinated framework A1, which has a maximum at 56-57 ppm and a shoulder at 49-50 ppm. The air-calcined zeolite gives two signals: at 57 ppm from four-coordinated framework A1 and at about 0-1 ppm from six-coordinated EFA1.

M -25

M -15 I

I

120

80

I

I

40

0

ppm from Al

(H20)63+

I

-40

Figure 11 27AI n.m.r. MAS spectra of calcined MCM-22 with various total AI/Si ratios. The samples were calcined in air for 3 h at 853 K.

57 The asymmetric shape of the four-coordinated AI signal can originate from the overlap of different resonances and/or quadrupolar effects. Considering that i.r. has detected two types of BrCnsted sites, we would expect two types of tetrahedral AI and two respective n.m.r, peaks. These are best resolved for the assynthesized samples (Figure 10). The total spectrum intensity for the same sample decreases upon calcination (Table 5), indicating that a large portion of A1 becomes n.m.r. "invisible," that is, gives extremely broad lines that are beyond the n.m.r, detection. 17

49

Table 5 Integral intensity of the 27AI MAS n.m.r, peaks of MCM22 expressed in arbitrary units and scaled to the same sample weight. Total Si/AI ratios were found by elemental analysis. Calcination was carried out in air for 3 h at 853 K. Roman numbers denote the AI coordination

I°

~,~

M -15 I

I

120

80

I

40

ppm from AI

Intensity Sample

i

i

0

-40

(H20)63*

Figure 10 27AI n.m.r. MAS spectra of as-synthesized MCM-22 with various total AI/Si ratios.

M-15 As-made Calcined M-25 As-made Calcined M-50 As-made Calcined

Total Si/AI ratio

IV

VI

15

849 439

91

466 363

80

203 100

9

25 50

Zeolites 1 5 : 5 7 6 - 5 8 2 , 1995

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Zeolite MCM-22: A. Corma et al.

CONCLUSIONS

REFERENCES

Zeolite MCM-22 has some common framework features with zeolite ZSM-5 (i.r. bands at 1,245 and 550 c m - 1) and with larger-void zeolites (i.r. bands below 400 c m - 1 as in faujasites). The i.r. study of the template decomposition indicates that the majority of the imine molecules are in the cationic form. In the hydroxyl i.r. region we have found two bands from the bridged Si(OH)AI hydroxyls, at 3,620 and 3,575 cm-1, both from strongly acidic BrCnsted centers accessible for pyridine and located at inequivalent positions in the MCM-22 structure. This is consistent with the n.m.r, results for framework tetrahedral A1. Calcination in air causes partial dealumination of the MCM-22 framework, which can be deduced from the appearance of Lewis acidity (i.r. results) and octahedral EFA1 (27A1 n.m.r, results). The dealumination is less severe when the calcination is done in a vacuum or in an N 2 atmosphere. The calcination creates internal silanols (defect sites), which give hydroxyl bands at 3,700 and 3,500 cm-1. Again, these frequencies resemble those for zeolites Y and ZSM-5, respectively. Zeolite MCM-22 should be handled with special caution. In fact, even grinding in a mortar can cause dealumination, with the extent increasing with the time of the grinding.

1 (a) Rubin M.K. and Chu, Po US Pat. 4 954 325 (1990). (b) Dessau, R.M. and Partridge, R.D. US Pat. 4 962 250 (1990) 2 (a) Huss, A., Jr., Kirker, G.W., Keville, K.M. and Thomson, R.T. US Pat. 4 992 615 (1991). (b) Kirker, G.W., Mizrahi, S. and Shih, S. US Pat. 5 000 839 (1991). (c) Del Rossi, K.J. and Huss, A., Jr. US Pat. 5 107 047 (1992) 3 Absil, R.P.L., Bowes, E., Green, G.J., Marler, D.O., Shihabi, D.S. and Socha, R.F. US Pat. 5 085 762 (1992) 4 Leonowicz, M.E., Lawton, J.A., Lawton, S.L. and Rubin, M.K. Science 1994, 264, 1910 5 Corma, A., Corell, C., Llopis, F., Martfnez, A. and PdrezPariente, J. Appl. Catal. A 1994, 115, 121 6 Souverijns, W., Verreist, W., Vanbutsele, G., Martens, J.A. and Jacobs, P.A.J. Chem. Soc. Chem. Commun. 1994, 1671 7 Unverricht, S., Hunger, M., Ernst, A., Karge H.G. and Weitkamp, J. Studies in Surface Science and Catalysis, Vol. 84, Zeolites and Related Microporous Materials: State of the Art 1994 (Eds. J. Weitkamp, H.G. Karge, H. Pfeifer and W. Holderich) Elsevier, Amsterdam, 1994, p. 37 8 Bellamy, L.J. The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 1975 9 Jacobs, P.A. and Uytterhoeven, J.B.J. Catal. 1971, 22, 193 10 Scherze,J. and Bass, J.L.J. CataL 1973, 28, 101 11 Woolery, G.L., Alemany, L.8., Dessau, R.M. and Chester, A.W. Zeolites 1986, 6, 14 12 Flanigen, E.M., Khatami, H. and Szymanski, H.A. Adv. Chem. Ser. 1971, 101,201 13 Jacobs, P.A., Beyer, H.K. and Valyon, J. Zeolites 1981, 1,161 14 Coudurier, G., Naccache, C. and Vedrine, J.C.J. Chem. Soc. Chem. Commun. 1982, 1413 15 Jansen, J.C., van der Gaag, F.J. and Bekkum, H. Zeolites 1984, 4, 369 16 Emeis, C.A. J, Catal. 1993, 141,347 17 Engelhardt, G. and Michel, M. High Resolution Solid-State NMR of Silicates and Zeolites, Wiley, New York, 1987

ACKNOWLEDGMENT We are grateful to Comision Interministerial de Ciencia y Tecnologia (MAT94-0359-C02-01) for financial support.

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