Isomorphous Substitution of Boron into Zeolites ZSM-5 and Y with

J. Phys. Chem. 1994,98, 41 18-4124

4118

Isomorphous Substitution of Boron into Zeolites ZSM-5 and Y with Aqueous NH4BF4 S. Han,' K. D. Schmitt, S. E. Schramm, P. T. Reischman, D. S. Shihabi, and C. D. Chang Mobil Research and Development Corporation, Central Research Laboratory, P. 0. Box 1025, Princeton, New Jersey 08543-1025 Received: December 14, 1993'

Treatment of ZSM-5 and Y zeolites with aqueous NH4BF4 (secondary synthesis) yields [B]ZSM-5 and [BIY. Tetrahedral B3+ in the zeolite framework was verified by boron M A S NMR (B3+ shift: ZSM-5 = -3.6 ppm; Y = -2.7 ppm). B3+ chemical shift was found to correlate with the S i - O S i bond angle (from silicon M A S N M R ) , further demonstrating that boron has been inserted in the framework. X R D and S T E M showed increasing crystallinity losses with increasing amounts of framework B3+, and this is believed to be due to partial and random crystal collapse to accommodate the smaller B3+ ion. The NH4BF4 reaction is believed to insert B3+ via a metathesis pathway involving formation of various aluminum fluorides. X R D identified one product as (NH&AlF6, and aluminum M A S NMR (decoupling and saturation comb techniques) suggests a second product as [Al(H20)6] [AlFs]. This work demonstrates the versatility of the aqueous metal fluoride treatment for zeolite isomorphous substitution, particularly for previously difficult-to-prepare substituted zeolites.

Introduction Isomorphous substitution of zeolites, i.e. replacement of existing T-atom sites with various metal ions, has been of significant interest since the new materials produced may exhibit novel catalytic properties.' In general, isomorphous substitution in zeolites isachieved through two synthetic routes: (1) incorporation of the desired metal through crystallization under hydrothermal conditions or (2) modification of an existing framework through various post-synthesis treatments (secondary synthesis). Both synthetic pathways offer materials which do not occur naturally. The insertion of trivalent boron into zeolite frameworks has been an area of copious research activity. There have been numerous reports of B3+ insertion into medium pore ZSM-5type zeolites under hydrothermal synthesis conditions,2-12 but the direct synthesis of boron-containing larger pore zeolites has been less successful. Barrer and Freund initially reportedI3 that attempts to directly synthesize boron-containing faujasite-type zeolites resulted in materials containing borate trapped inside sodalite cages with no framework incorporation. More recently, the preparation of boron-containing beta,zJ4and mordenite,l5 by hydrothermal synthesis was reported. We report here the preparation of [B]ZSM-5 and [BJYzeolites by post-synthesis treatment using dilute aqueous solutions of ammonium tetrafluoroborate, NH4BF4. Post-synthesis modification through treatment with aqueous metal fluorides has been shown to be a versatile method for preparing isomorphously substituted zeolites. Skeels and Breck reported16 that the replacement of framework AI3+with Si4+using (NH4)zSiFs was possible with zeolites, producing highly stable, silicon-enriched Y. Chang et al. subsequently reported" the reverse reaction, i.e. substitution of Al3+ into high silica ZSM-5 frameworks by replacement of Si4+using aqueous aluminum fluoride solutions. The versatility of this treatment has been demonstrated'*-19 with the incorporation of Fe3+,Ti4+,Sn2+,and Cr3+ into various zeolite frameworks using the respective aqueous metal fluoride salts. Recently, this method was extended to insert Be2+ into ZSM-5 by using aqueous (NH4)2BeF4 solution.20 The reaction is believed to proceed via a metathesis pathway wherein the desired metal ion inserts into a T-atom vacancy created by the formation of

* To whom correspondence should be addressed. @

Abstract published in Advance ACS Abstracts, March 15, 1994.

0022-365419412098-4118$04.50/0

soluble aluminum16or silicon17fluorides. A previous report2' on the reaction of Y zeolite with aqueous NH4BF4 resulted only in dealumination with no boron substitution under the acidic conditions used. Previous workon post-synthesis preparation of boron-containing Y zeolite is limited. Klinowski et aLZ2and Gaffney et al.ls have reported insertion of B3+ by prior dealumination using a complexing agent and subsequent treatment with aqueous KOH/ B2O3 solutions. Other insertion methods tried, including treatment with aqueous boric acid, sodium metaborate, and B2O3, were ~ n s u c c e s s f u l . ~[B]Y ~ prepared by KOH/B203 treatment inserted minor amounts of B3+into the framework, withultrastable Y zeolite yielding slightly more framework boron. Although [B]ZSM-5 prepared by this route has been claimed not to affect zeolite structural integrity,23it was unclear whether base treatment had deleterious effects on the structure of Y, particularly since strong base treatment may dissolve zeolite framework ~ilica.2~ Our studies on boron insertion into zeolites centered on examining the versatility of aqueous fluoride treatments with metal ions having smaller radius ratios [r(BS+)/r(OZ-) = 0.201 than those metal ions traditionally inserted into T-atom sites, namely Si4+,A13+,Ti4+,Fe3+,and Ga3+. We attempted to further examine this system under moderate pH conditions which may favor substitution chemistry and in addition determine the stability of zeolite Y in the presence of framework B3+ using X-ray diffraction (XRD), magic-angle spinning (MAS) NMR, and scanning transmission electron microscopy (STEM).

Experimental Procedures [B]ZSM-5. In a plasticvessel, 1.Og of NHd+-exchanged ZSM-5 (Si02/A1203 = 57) was slurried with 50 mL of a solution 0.2 M in NH4BF4 buffered to pH = 7.3 with ammonium acetate. This mixture was heated for 18 h at 85 OC. The resultant product was filtered and washed with distilled water. The catalyst was then exchanged with NH4N03 and washed. All wash and exchange solutions were kept at pH = 8-9 by addition of NH40H. [BIY. The following reaction sequence was used throughout. In a plastic bottle, 3.0 g of a 65% NH4+-exchanged Y zeolite was reacted with an NH4BFd solution adjusted to pH = 7.1 by the addition of ammonium acetate. The bottle was shaken vigorously and heated for 18 h at 85 "C. The product was then filtered, washed, and exchanged with NH4N03. All wash and exchange 0 1994 American Chemical Society

Isomorphous Substitution of Boron into Zeolites

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4119

solutions were adjusted to pH = 8-9 by addition of N H 4 0 H . The product ID numbers, volumes, and tetrafluoroborate concentrations are as follows: [BIY-1 50 mL of a 0.10 M NH4BF4solution (0.0050 mol BF4-) [BIY-2

00 mL of a 0. IO M NH,BF4 solution (0.010 mol BF;)

[BIY-3

50 m L of a 0.10 M NH4BF4solution (0.015 mol BF4-)

[BIY-4 100 mL of a 0.20 M NH4BF4solution (0.020 mol BF;) [BIY-5 150 mL of a 0.25 M NH4BF4solution (0.038 mol BF;) MAS NMR. All samples studied were in the NH4+ form before calcination, and all N M R spectra were obtained on a 4.7-T spectrometer. Spectra were taken on a JEOL FX-200 with Chemagnetics magic-angle spinning probe. For llB MAS NMR, the [BIZSM-5 product was quantitated using 2.5-ps pulses while quantitations for the [B]Y products were done at 4.5-ps pulses. For [BIY, between 50 and 1000 scans and 11-Hz of line broadening were needed for a good signal to noise ratio. The chemical shift reference for boron is boron trifluoride etherate, BS.O(CzHs)z. For the [B]Y products,Z9SiMAS NMR spectra were obtained using 60° pulses a t 1 5 4 intervals without proton decoupling. The silicon spectra were simulated using Gaussian curves. The Si/Al ratios were determined from the relative peak areas of the simulated spectra. 27AlMAS NMR spectra were obtained with 5.7-kHz spinning to sufficiently remove contributions from the side bands to the center bands. Short pulse excitation was used to assure uniform excitation. Good signal to noise was obtained with 1000 scans and 23-Hz line broadening. Pulses of 5 2 ps were used where the solution 90° pulse was 6 ps. Aluminum spectra were quantitated in an absolute sense by comparison of areas (after being normalized for scans and sample weight) to either solid Al(N03)3-9HzO or corundum. A pH = 1 solution of Al(N03)3 was used as the chemical shift reference. STEM. The STEM data were taken on a Vacuum Generators HB 50 1 microscope using an annular dark field detector. Images were edge-enhanced. Samples for STEM analyses were prepared using two techniques: (1) a 200 mesh carbon-coated copper grid was dusted with the zeolite by contacting the zeolite powder with the grid, and (2) zeolite powder was embedded in epoxy and then sliced into 100-200-nm thin sections. In both cases a series of annular dark field images was obtained.

-

Results and Discussion [BIZSM-5. Figure 1 shows the boron MAS NMR spectrum for [BIZSM-5 prepared by NH4BF4 treatment. The spectrum shows the boron peak at -3.6 ppm, and this chemical shift is characteristic of tetrahedral framework boron, as observed previously for [B]ZSM-5.25 Quantitation of this peak gives boron content at 0.068 wt %, which agrees well with the boron content from elemental analysis (0.073 wt 76). The final SiOz/ A1203+B203 ratio in the product zeolite was 58. This quantity of substituted boron is less than that observed for [BIZSM-5 prepared hydrothermally,2-12 and no attempt was made to optimize the preparation to increase boron insertion.

sample parent NH4Y [BIY-1 [BIY-2 [BIY-3

[BIY-4 [B]Y-5

B/Si+AP treatment severity 0.10 0.20 0.30 0.40 0.75

wt % Bb

% crystallinity

0 0.014 0.020

100

0.039

34 20 9

0.13 0.16

64

44

a Moles of BF4- in reactant solution per mole of Si+Al in zeolite. Percent tetrahedral framework B’+ in sample as quantitated by boron MAS NMR. Basedon average intensitiesof four strongestpeaks between 20 and 3 5 O , 29.

The preparation of [BIZSM-5 by NH4BF4 treatment was used to illustrate feasibility of this reaction for isomorphous substitution of B3+ into zeolites. [BIY. Treatment of zeolite Y with aqueous NH4BF4 under nonacidic conditions results in incorporation of B3+into the lattice but also results in the destruction of major portions of the framework. Framework incorporation of boron will be shown by NMR, and evidence from STEM, XRD, and NMR will be presented to show that this attack proceeds randomly in thecrystal with rejection of silicon as an amorphous high silica/alumina phase and rejection of aluminum as hexaaquo- and hexafluoroaluminate species. Data from the boron MAS N M R studies and the relative crystallinity of the samples are summarized in Table 1. Results from the silicon and aluminum MAS NMR studies for the samples are given in Table 2. Table 3 gives data on octahedral aluminum concentrations with and without fluorine decoupling in the solid samples as determined by AI MAS NMR. Figure 2 gives the boron MAS NMRspectrumofthe treatedproduct [BIY-5, which is representative of all the Y materials. Figure 3 shows the correlation between Si4+ and B3+ shifts for [BIZSM-5, [BIY, [BIB, prepared analogously to literature preparations,2J4and [B]mordenite.15 Figure 4 shows XRD patterns of treated samples [BIY-3 and [BIY-4 versus the parent NH4Y zeolite. The other treated zeolite patterns are similar. The silicon spectra and simulations of all the samples in the study are given in Figure 5. Figures 6 and 7 show STEM pictures of the parent and selected products. Figures 8 and 9 give results of the A1 spin lattice relaxation studies. Treatment of zeolite Y under conditions where only the ratio of BF4- to zeolite increased resulted in increasing amounts of tetrahedral framework boron (Table 1). The chemical shift of the single detectable boron species by boron MAS NMR is -2.7 ppm. This is significantly different from that observed for

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The Journal of Physical Chemistry, Vol. 98, No. IS, 1994

I l l 1

I

l

20.00

l

I l l 1

I l l /

10.00

I l l ,

0.00

I l l /

I I I I

-10.00

1 1 1

-20.00

PPm

Figure 2. The 64.0-MHz IIB MAS NMR spectrum of [B]Y-5 product

from NHdBF4 treatment obtained using 5-s recycle, 976 scans, and 11-

Hz line broadening.

10

20

30

40

50

60

Degree8 28

I

j .2.7 -2.9-

.3.1

Framework Boron

ShiR

-

-3.3!

-3.7-

[B]ZSM-5

-I

amorphous borosilicates or aluminosilicate gels26and is not from the starting reagent since its shift was found to be -1.7 ppm in the solid state, the same as had been reported in solution.27 Framework tetrahedral B3+ has a chemical shift of -3.2 ppm in [BIB and -3.6 in [B]ZSM-5. The range of boron shifts for Y, 8, and ZSM-5 is believed to reflect the range of average T-0-T angles characterizing these zeolites. The correlation between Si4+ chemical shift and Si0-Si bond angle for silicates is well-known.28-32 Similar correlations have been established for A1-0-P in aluminum phosphates,33 P - 0 4 in phosphorus containing ~ilicates,3~ and shifts of Al3+ in aluminosilicates.35 The correlation between Si4+ and B3+shifts for ZSM-5,@,mordenite,l5 and Y zeolites is evidence that B3+ has been inserted into the Y framework (Figure 3). Insertion of B3+ into the zeolite Y framework is not without penalty. Increasing theseverity of treatment results in increasing levels of boron insertion by N M R but decreasing crystallinity by XRD (cf. Table 1 and Figure 4). Silicon MAS N M R confirms the XRD findings and shows that the Y zeolite is undergoing both dealumination and destruction of its framework structure. The silicon spectra and simulations of the parent NH4Y and NH4BF4-treated Y samples are shown in Figure 5. The spectrum of NH4Y is characteristic of a faujasite36 with a peak at -105.6 ppm for Si(OAl), -100.7 ppm for Si( lAl), -95 ppm for Si(2Al), -89.5 ppm for Si(3Al), and-88.9 ppm for Si(4Al). The notation Si(nA1) represents a silicon linked ton A1 and (4- n) Si neighbors. The treated materials have an additional peak at -1 11 ppm which is not attributable to a faujasite. Its shift is similar to the Si (4%)

Figure 4. X-ray diffraction patterns for parent NH4Y and [BIY-3 and

[BIY-4 products. shifts seen in amorphous ~ilicas.3~Since it represents nonfaujasitic silicon, its contribution to the silicon spectrum must not be included if a true picture of the chemistry of the remaining zeolite is to emerge. With this in mind, the Si/A1 ratios given in Table 2 were calculated from the faujasitic peak intensitiesg8 determined by the simulation. The ratios increase with increasing tetrahedral B3+ so that dealumination of the remaining zeolite framework accompanies formation of non-zeolitic silica. The increasing amounts of amorphous silica determined are directionally consistent with the decreasing crystallinity as measured by XRD, but the absolute numbers are not in agreement due to factors such as zoning, collapse of particular zeolitic regions, etc. Powder and thin section micrographs for the parent NH4Y and [BIY-4 products (Figures 6 and 7) also indicate extensive loss of crystallinity in the zeolite crystals, as noted by material loss. In some crystals the loss was largely in the interior, in others largely on the exterior, and still others the loss was uniform throughout. Thus, fromobservation of individual zeolitecrystals, theNH4BF4 treatment may not cause random loss of crystallinity, but from considerations of the bulk sample, the crystallinity loss is indeed random. It is believed that these overall random crystallinity losses are due to random Al3+ siting in Y zeolite along with the ability of the BF4- moiety to enter the pores of the zeolite. We postulate the following to explain the crystallinity loss. Initially, a BF4- species may react with framework Al3+ to dealuminate and form aluminum fluoride. Boron insertion is followed by collapse of various regions where Al3+ was sited in the zeolite. Boron is responsible for the crystallinity loss, not fluoride. As a base case, we treated Y with solutions of NH4F up to 5 times more concentrated in fluoride under similar conditions and found no loss in crystallinity. Boron incorporation causes the crystallinity loss, but silicon and aluminum N M R data (Table 2) show that much more Al3+ is removed from the framework than is replaced by B3+. The aluminosilicate pore system of Y is unable to accommodate smaller framework B3+ ions to a significant extent; however, the N M R evidence clearly suggests some B3+ ions are incorporated. This agrees well with previous conclusions12 that B3+ is stabilized in smaller pore zeolite systems (usually those with higher Si/Al ratios) than faujasites. We conclude that partial lattice collapse in Y is necessary to stabilize the remaining framework boron. The reaction scheme is complete if the fate of the aluminum is known. The XRD patterns (Figure 4) showed new peaks for

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4121

Isomorphous Substitution of Boron into Zeolites

:2A .90

A

8\

NH,Y Parent

-110

-100

-120

-130

-80

-90

[BIY-1

-100

-110

-120

-130

PPm

ppm

tBIY-3

-80

-90

-100

PPm

-110

-120

-130

-110

-120

-130

PPm

-80

-90

-100 PPm

PPm

Figure 5. The 39.4-MHz 29Si MAS N M R spectra and simulations for NH4Y and [B]Y products.

TABLE 2 lBlY

Silicon and Aluminum MAS NMR Results for

The previously characterized, partly soluble hexafluoroaluminate species is from the following reaction:

samDle

Si/AP

SilAlb

% amorphous Sic

NH4Y parent [BIY-1 [BIY-2 [BIY-3 [BIY-4 [BIY-5

2.65 3.19 3.38 3.91 4.63 4.41

2.62 5.15 7.03 9.02 24.15 51.80

0 10.37 17.63 26.33 46.34 66.25

AlF, hydrate

+ 3NH,F

-

(NH,),AlF,

(2)

However the only way to arrive a t (2) from (1) would be to leave 2 mol of AlF3 uncomplexed. An “[Al][AlF6]” species satisfies the stoichiometry and is the only possible analog of the AMF6 type in this reaction.

0 Calculated by silicon MAS N M R for zeolite portion of product. Calculated by aluminum MAS N M R for the entire product. Determined by silicon MAS NMR.

the products. These peaks correspond to two known compounds. The first was identified as (NH&AlFs by match with peaks [d space(A),1/1,,,]: 5.135,100;4.449,30.2; 3.153,54.2. (NH& AlF6 is an expected product in this fluoride exchange reaction with framework A P . The second set of peaks at d spaces 4.086, 4.162,2.944,2.405, and 2.084 were matched with the compound CaSnF6. A suitable compound isostructural with the AMF6 type is believed to be ”[Al][AlF6]”, which would be formed in the following manner. The specific reaction for isomorphous substitution is

-

[ A l l y + NH,BF, [B]Y (unstable)

+ AlF, hydrate + NH,F

(1)

The hexaaquo species, [Al(H20)6][AlF6], or some variant of a [AlF,( H20)&,] 3-x species formed from aqueous solutions of A1F3,39would be expected to give two types of octahedral Al3+ in the aluminum MAS N M R of the zeolite samples. Two octahedral Al3+ species were detected by NMR, but the evidence is indirect.

Aluminum MAS N M R of each of the treated samples showed the expected tetrahedral zeolitic Al3+ at -58 ppm and a broad octahedral signal at -0.1 ppm. Proton decoupling narrowed the octahedral signal to 80 Hz. This narrowing is exactly comparable to that observed by us for hexaaquo aluminum in Al(N03)3, A12(S04)3,and Al(C104)3. Proton decoupling did not change the area of the signal, only its width. Fluorine decoupling, on the other hand, increased both the signal intensity and width. The width increased to 650 Hz for the fluorine decoupled line. It was

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The Journal of Physical Chemistry, Vol. 98, No. 15, 1994

Han et al.

-

-

5,000 A

2,000 A

Figure 6. STEM annular dark field, edge-enhanced images-powder. Top: parent NH4Y. Bottom: [BIY-4. Magnification = 50 OOOX.

not possible to simultaneously decouple both fluorine and protons on our spectrometer. The aluminum MAS NMR decoupling results are exactly as expected for [Al(H20)6] [A1F6]. It should have a hexaaquo ion whose NMR line should be broadened by nearby protons. Since the protons are in relatively mobile water molecules, the broadening should be small. The AlF63- ion has directly bonded fluorines which can be expected to broaden the aluminum signal so strongly that no signal is seen at all without high-power fluorine decoupling. The A13+ in AlFb3- appears to have a broader line even with fluorinedecoupling than the hexaaquo ion does without, which explains the increase in line width in the fluorine decoupled spectrum. The observed [AlF6]/ [Al(H20)6] ratio is close to 1:1, at least for the samples treated with the largest amounts of BF4per gram of zeolite (Table 3). An increase in peak area is not as convincing as observing two separated peaks might be for the existence of two overlapping types of A13+, so further evidence was sought from relaxation behavior. The saturation comb technique4 was used to determine T I ,the spin-lattice relaxation time, of the A13+. The results from the T I measurements with H decoupling and F decoupling are shown in Figure 8. The T1recovery curves for the tetrahedral

Figure 7. STEM annular dark field, edge-enhanced images-thin section. Top: parent NH4Y. Bottom: [BIY-4. Magnification = 50 OOOX.

TABLE 3 Octahedral AI3+ Concentrations by Aluminum MAS NMR sample

wt % octahedral A P , no F decoupling

wt % octahedral A P , with F decoupling

[BIY-1 [BIY-2 [BIY-3 [BIY-4 [B]Y-5

0.08 0.18 0.67 2.62 2.47

0.28 1.86 2.20 4.96 5.3 1

and octahedral AP+ obtained with H decoupling are shown in Figure 9. A typical zeolite T Ivalue of 6 ms was determined for the tetrahedral A P . The octahedral A13+had a longer TI value of 0.2 s. The proton decoupled octahedral A13+was fit very well with a single relaxation time, but with F decoupling, the TI measurements do not fit a single relaxation time, thus presenting further evidence that more than one species is present. The stoichiometry predicted from (2) and (3) above was confirmed experimentally by simply adding 2 extra moles of NH4F per mole of NH4BF4 in the preparative step. This would supply the extra NH4F required to complex all of the AlF3. XRD analysis of this product showed disappearance of the five peaks attributed to [ A K H ~ O )w~ I6 i .

,

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 4123

Isomorphous Substitution of Boron into Zeolites

r-----l

750

1 "

20

10

30

40

50

I

I

Bo

70

Tlmr After SaturatlonComb In Yllllrecondr

160.0

80.0

0.0 PPm

-80.0

-160.0

5ooo

Poak

Hrlght 2500

-0

0.2

0.4

0.8

I

I

Tlmr Aftar Saturatlon Comb In Socondr .

. . . . . . . . . 10.0 80.0 0.0 -80.0 -160.0 PPm

Figure 8. The 52.0-MHz *'AI MAS N M R spectra of [BIY-S product obtained using saturation comb pulse sequence to measure T I . Top: with proton dccoupling. Bottom: with fluorine decoupling.

Conclusions Treatment of zeolites ZSM-5 and Y with aqueous NH4BF4 introduces B3+ into the framework by isomorphous substitution with Al3+. This work further demonstrates the versatility of aqueous metal fluorides for framework exchange by secondary synthesis and its potential for making metal-containing zeolites heretofore difficult to prepare. [B]Y prepared in this manner has been shown to contain framework B3+ by boron and silicon MAS NMR, but collapse of the framework was observed. This work is an example of a strained system due to a small metal ion in a large pore system which results in partial and random (interior vs exterior) crystal collapse to accommodatethe metal ion. NMR data indicate that the tetrahedral B3+ shift is related to the Si4+ shift (noAl3+ neighbors) in varying zeolites,which further justifies the case of B3+ in the zeolite framework. The B3+ is believed to substitute for A P , as various aluminum fluoride products are characterized from the reaction. Acknowledgment. The authors wish to acknowledge C. T.-W. Chu, J. B. Higgins, R. D. Partridge, and A. B. Schwartz for helpful discussions and comments. We thank S. L. Lawton for running the JCPDS search program and E. Bowes for providing some of the catalysts used in this study. The expert technical assistance of D. M. Mitko and P. T. Isabella is greatly appreciated.

Figure 9. Recovery curves for T I relaxation of tetrahedral (above) and octahedral (below) Al3+ obtained with proton decoupling.

References and Notes (1) Tielen, M.; Geelen, M.;Jacobs, P. A. Acra Phys. Chem. 1985, 31, 1. (2) Taramasso, M.;Perego, G.; Notari, B. In Proceedings of the F i f h Internaiional Zeolite Conference;Rees, L. V., Ed.; Heyden: London, 1980; p 40. (3) Gabelica, 2.;Nagy, J. B.; Bodart, P.; Debras, G. Chem. Lerr. 1984, 1059. (4) Gabelica, Z.; Debras, G.; Nagy, J. B. In Caralysis on d e Energy Scene; Kaliaguine, S.,Mahay, A,, Eds.; Elsevier Science: Amsterdam, 1984; p 113. (5) Derouane, E. G.; Baltusis, L.; Dessau, R. M.; Schmitt, K.D. Srud. Surf. Sci. Caral. 1985, 20, 135. (6) Chu, C. T.-W.; Chang, C. D. J. Phys. Chem. 1985,89, 1569. (7) Chang, C. D.; Hellring, S.D.; Miale, J. N.; Schmitt, K. D.; Brigandi, P. W.; Wu, E. L. J. Chem. Soc., Faraday Trans. I 1985,81, 2215. (8) Howden, M. G. Zeolires 1985, 5, 334. (9) Ione, K.G.; Vostrikova, L. A,; Mastikhin, V. M. J. Mol. Catal. 1985, 31, 355. (10) Bodart, P.; Nagy, J. B.; Gabelica, 2.;Derouane, E. G. Appl. Catal. 1986, 24, 315. (11) Coudurier, G.; Vedrine, J. C. Pure Appl. Chem. 1986.58, 1389. (12) Jansen, J. C.; Biron, E.; van Bekkum, H. Srud. Surf. Sci. Coral. 1988, 37, 133. (13) Barrer, R. M.; Freund, E. F. J. Chem. SOC.,DaIfon Trans. 1974, 1049. (14) Derewinski, M.; Di Renzo, F.; Espiau, P.; Fajula, F.; Nicolle, M.-A. Srud. Surf.Sci. Caral. 1991, 69, 127.

(15) Gaffney, T. R.; Pierantozzi, R.; Seger, M.R. In ACS Symposium Series; Occelli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 398, p 374. (16) Skeels,G. W.;Brcck,D. W. InProceedingsoftheSixfh Internafional Zeolire Conference;Olson,D. H., Bisio, A., Eds.; Butterworths: Surrey, 1983; p 87. (17) Chang, C. D.; Chu, C. T.-W.; Miale, J. N.; Bridger, R. F.; Calvert, R. B. J. Am. Chem. Soc. 1984, 106, 8143. (18) Skeels, G. W.; Flanigen, E. M. In ACS Symposium Series; Occelli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 398, p 420. (19) Skeels, G. W.; Flanigen, E. M. Srud. Surf. Sci. Catal. 1989, 49A, 331.

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