Synthesis of MFI titanosilicates from methylamine—TPABr media

ELSEVIER

Synthesis of MFI titanosilicates methylamine-TPABr media Masashi Shihata Facultks Universitaires de Namur, D@atiement

from

de Chimie, Namur, Belgium

Zelimir Gabelica Universite’ de Haute Alsace, ENSCMu, 3, Rue A. l44rne~, F-68093 M&house,

Cedex, France

A new synthesis route for MFI-type titanosilicates (TS-1) using methylamine along with tetrapropylammonium bromide (TPABr), silica or silicon tetrachloride and titanium tetrachloride, thus avoiding the use of costly TPAOH and Ti-alkoxide, has been investigated. The presence of titanium ions in the reaction gel slowed down the crystallization of MFI zeolites, while addition of hydrogen fluoride as a co-mineralizer into the gel dramatically improved the crystallization kinetics. Ti was readily incorporated into the zeolite framework, as confirmed by IR, XRD (unit cell expansion), and UV-vis spectroscopy. Reduced initial amounts of methylamine in the gel led to an increase of the apparent crystallization rate of the zeolite and to a remarkable decrease of the amount of framework Ti, suggesting that high concentration of methylamine is needed for the Ti incorporation into the growing MFI crystallites. 0 Elsevier Science Inc. 1997 Keywords: Titanosilicate; TS-1; synthesis; methylamine;

hydrogen fluoride; TPABr; silica; titanium tetrachloride

INTRODUCTION MFI-type titanosilicate materials, also designated TS-1, have received considerable interest since their first synthesis was reported in 1983.’ Such materials show excellent performances in the epoxidation and hydroxylation of olefins with hydrogen peroxide.2s As the presence of Na+ cations in the precursor gel favors the formation of extra-framework titanium species, microporous titanosilicates have always been synthesized in alkali-free conditions.’ In the original patent,’ tetraethyl orthotitanate was used as the Ti source, and a large quantity of TPAOH was necessary to bring the pH of the reaction gel to basic values needed for rapid crystallization. Kumar et al. suggested the use of tetrabutyl orthotitanate and acetylacetone that readily form stable complexes with titanium ions, to prepare well-crystalline TS-1 materials involving high Ti concentrations4 However, the major drawback of all these conventional syntheses still consists in using costly alkali-free TPAOH templates. To avoid using TPAOH, Miiller et a1.j proposed the use of ammonia to fix the pH to basic values favorable for zeolite syntheses. They reported that TS-1 crystals were obtained from the mixture of titanium tetraisoreprint requests to Dr. Shibata at KAO Corp., Tokyo Research Laboratories, l-3, Burka P-Chome, Sumidaku, Tokyo 131, Japan. Received 10 March 1997; revised 2 June 1997; accepted 5 June 1997

Address

Zeolites 19:246-252, 1997 0 Elsevier Science Inc. 1997 655 Avenue of the Americas,

New York, NY 10010

propoxide, alkali-free TPAbr, H,O,, ammonia and water; however, the actual role of ammonia in such TS-1 syntheses was not investigated more in depth. The synthesis method for MFI zeolites in neutral or acidic media using fluoride ions as mineralizing agents along with alkali-free TPABr6 has been successfully applied for TS-1 syntheses’-“; however, the catalytic performancles of such materials were reported to be inferior to those prepared in basic media with TPAOH.” It is possibly because of the relative small framework Ti content and/or the presence of extra-framework byproducts, (TiO,) . Recently, we reported that methylamine had a potential mobilizing and complexing ability toward not only silica species but also many metallic ions such as boron, aluminum, or galliun~.‘O-ls For example, alkali-free MFI alurninosilicate, gallosilicate, and alumino-gallobimetallosilicate could be synthesized using both alkalifree TPABr as a template and methylamine as a mineralizing agent. In these syntheses, methylamine was shown tc) form medium-strong complexes with both mobilizAl’+ and Gas+, and thus proved an excellent ing agent toward these species. In addition, a slight excess of methylamine was used to bring the final pH of mixture to basic values, which does not require the use of TPAOH. lo The purpose of this work was to examine the potential of methylamine to mobilize titanium species for TS-1 synthesis in alkali-free media without using TPAOH. The possibility of using SiCl, as a Si source,

0144.-2449/97/$17.00 PI SOl44-2:449(97)00078-X

Synthesis of MN titanosilicates:

which formed Si-O-B gel complexes SiO, in MFI borosilicate synthesis,13

more readily than was also examined.

EXPERIMENTAL Synthesis of MFI-titanosilicates In all the syntheses, alkali-free tetrapropylammonium bromide (TPABr, from Janssen Chimica) was used as a template for the MFI structure. Titanium tetrachloride (Merck) was the main titanium source. Either solid silica (Aerosil 200 from Degussa) or silicon tetrachloride (Merck) was used as a Si source. Methylamine (40% aq. solution from Fluka) was the mineralizer (mcr bilizing). Some other ingredients (hydrogen fluoride, hydrogen chloride, oxalic acid, acetylacetone, or hydrogen peroxide) known to make complexes with titanium were optionally added to the system as co-mineralizers. Four different procedures were used to prepare the reaction gels.

Procedure I: Preparation of TS-1 from SiO, and TX& TiCl, was added to an aqueous solution of TPABr pre-cooled at 5°C. A complexing agent (hydrogen fluoride, hydrogen chloride, or oxalic acid) was optionally added to that solution, then solid SiO, was added. The final mixture was stirred for at least 3 11 at 20°C. The resulting homogeneous gel was admixed with methylamine, stirred for 1 h, and then transferred to a Teflon autoclave. The gel molar composition was: (96x)SiO,

- x TiCI_, - y CHsNH,

- z complexing -

agent

- 25 TPABr

3500 H,O

Procedure II: Preparation Ti- acac complex

of TS-1 from

SiO,

a,nd

SiO, was added to the aqueous solution containing TPABr and Ti acetylacetonate (Ti-acac from Merck). The gel was stirred for 1 hr then admixed with methylamine and stirred for another 1 h before being transferred to the autoclave. The gel preparation was entirely achieved at about 20°C. The gel molar composition was: 92 SiO,, - 4(Ti-acac) -

25 TPABr

-

- 300 CHsNHs

3500 H,O

Procedure III: Preparation of TS-1 from SiO, and Tit& in the presence of H202 TiCl, was added to an aqueous solution of H,O, pre-cooled at 5°C. The resulting pale-red solution was added to the cold methylamine solution until the formation of a pale-yellow suspension. The latter was then admixed with SiO, and stirred for 1 h at 20°C before being transferred to the autoclave. The gel molar composition was: 92 SiOs - 4 TiCI4 - 300 CHSNHS -

25 TPABr

-

3500 H,O

- 4 H,O,

M. Shibata and Z. Gabelica

Procedure IV.. Preparation of TS-1 from SiCl, and TiCl, SiCl, was added to an aqueous solution containing TPABr and, optionally, hydrogen fluoride pre-cooled at 5°C. The resulting gel was also cooled to 5°C before the addition of TIC],, then admixed with methylamine and stirred for another 1 h before being transferred to the autoclave. The gel molar composition was: 92 Sic&

- 4 Tic&

- 50 TPABr

-

- 3500

600 CHsNH,

- x HF

H,O

The autoclaves were heated at 185’C for various periods of time under static conditions. The final solids were filtered, washed thoroughly with cold water, and dried at 100°C. The crystals were further submitted to an ultrasonic cleaning to remove the unreacted gel from products not being 100% crystalline, prior to IR, EDX, UV-vis, or XPS measurements. All the products were calcined under nitrogen (heating rate: lO”C/min) from 20°C to 550°C; the samples freed from TPA+ were maintained at 550°C for 6 h in flowing air to burn off the residual coke stemming from the nonoxidative TPA thermal decomposition.

Characterization All products were checked for their nature and purity by SEM (Philips XL 20 microscope). Spot Energy Dispersive X-ray analyses (EDX) of Si and Ti on selective areas (core and edge) across the individual crystallite were performed using an EDAX P.V. 9800 Phillips analyzer coupled with SEM. TS-1 samples with known Si/Ti ratios were used as standards. X-ray powder diffraction (XRD) patterns were recorded on a Phillips P.W. 1349/30 diffractometer (Cu-Kcw radiation) using a-alumina as internal standard. ET-IR spectra were obtained using a Bio-Rad FTS-6OA spectrometer with KBr pellets containing 1 wt% sample. The UV-vis spectra were recorded on Shimadzu UV-3100PC spectrometer. XPS (X-ray photoelectron spectroscopy) spectra were recorded on a JEOL JPSSO instrument using an Mg Kol X-ray source. The binding energy of 133.3 eV for Si(2p) was chosen as an internal reference. Chemical analyses of the products were carried out by inductively coupled plasma spectroscopy (ICP) on a Shimadzu ICPS-1OOOOIV instrument.

RESULTS AND DISCUSSION Crystallization behavior Fi,pre 1 shows the crystallization curves of TS-1 and silicalite-1 (metal-free MFI silicate) samples synthesized from methylamine-TPABr media, as coded in Tabb 1. The crystallinity of products was estimated by comparing the intensities of eight characteristic diffraction peaks of MFI zeolites. Pure silicalite-1 crystals were obtained after 2-d heating at 18O”C,‘” and this sample was arbitrarily considered as being 100% crystalline. In contrast to the crystallization of silicalite-1, no crystalline phase was detected in the reaction gel after 14-d heating in the presence of TiCl, (sample 1). This shows that, as in the case of Al”+ and Gas’ ions,iO the

Zeolites

19:246-252,

1997

247

Synthesis

of off

tjtanosiiicates:

M. S~j4ata and 2. Gabelica

80‘;i ?L .G .-r = a

GO-

6

4o

J

2

4

6

8

10

12

Reaction Time (days) Figure 1 Crystallization curves of TS-7 and silicate-l in methylamine-TPABr media. (0) Sample ?, 2, 7, 9 and 10; (A) sample 4; (A)sample 6; (El) sample 8; (mf sample 11; (0) silicalite-1.

presence of Ti4+ impedes the growth of MFI zeolites. The absence of any XRD peaks corresponding to TiO, (anatase) indicates that the amorphous phase is still composed of stable Ti-0-Si oligomers.14 Indeed, we have checked that CHsNH, and TiCl, admixtures heated under similar conditions but without silica yielded pure anatase. The effect of some potential Ti-complexing agents on the crystallization of TSl in methylamine-TPABr was examined. Adding either HCl, oxalic acid, or H,O, did not improve the crystallization kinetics, the reaction mixtures being still amorphous after at least 10-d heating (samples $7, and 9). When Ti-acac was used as the Ti source, low-crystalline (32%) MFI zeolite was obtained (sample 8). In contrast, the presence of hydrogen fluoricle in the reaction gels markedly increased the kinetics of crystallization. A highly crystalline (84%) MFI product was obtained after 10-d heating (sample 4). Fluoride ions are excellent solubilizing agents toward silica, and silicalite-1 can be synthesized using HF or NH,F even in neutral or acidic media.” Fluoride ions probably inTable 1

TS-1 sample synthesized

Sample name (number)

Procedure

I

1 2 3 4 5 6 7 8 9 10 11

OX:

248

in CH,NH,-TPABr

I I

I I I I II III IV IV oxalic acid; acac: acetylacetonate.

Zeolites 1!3:246-252,

1997

crease the solubility of both the Si species and the Ti species and, hence, improve the crystallization kinetics of TS-1. Conversely, in methylamine alone, the stable “K-O-Si oligomeric species may not readily transform into crystalline TS-1, in contrast to Ti-free silicate species. Indeed, we have demonstrated elsewhere%* that once such T&-O--Si oligomeric species are formed in methylamine media but in the absence of fluoride ions, TS1 crystals were difficult to grow even in the presence of seed crystals. SiCl, was expected to co-hydrolyze more readily along with TiCl, and make Si-O-Ti oligomers than SiO,, as shown in the case of the borosilicate synthesis.13 The reaction gel in which both SiCl, and TiCl, were co-hydrolyzed did not form any MFI crystals after 14-d heating (sample lo), in contrast to the borosilicate synthesis in which 100% crystallinity was attained in 5 d under the identical conditions.13 Adding HF again accelerates the crystallization (sample ll), as in the case of the synthesis from SiO, (sample 4). The Ti concentration in the reaction gel also drastically affects the crystallization kinetics, fiere 2 shows the crystallization curves for various Ti-containing gels in the presence of hydrogen fluoride. Clearly, the larger the titanium content in the gel, th:e slower the crystallization, indicating that Ti4’- ions tend to impede the nucleation of TS1. Under these synthesis conditions, the c~~llini~ of high Ti4+-containing sample (4 or 6 Ti/96 T atoms; T = Si + Ti) did not reach 100% crystallinity after 10-d heating. SEM and E
media CH,NH, (mot) 300 300 300 300 300 100 300 300 300 600 600

Complexing agent (mol)

Si source

300 HCI 100 l-IF 100 I-IF 100 HF 100 HF 20 ox (acac) 4.0 H,O, 50 HF

sio SiOz SiO, SiO, SiO, SiO SiOz SiO, SiOz SiCI, SiCI,

Ti source (mol) 4.0 4.0 2.0 4.0 6.0 4.0 4.0 4.0 4.0 4.0 4.0

TiCI, TiCI, TiCI, TiCI, TiCI, TiCI, TiCI, Ti-acac TiCI, TiCI, TiCI,

Synthesis of MFI titanosilicates:

M. Shibata and 2. Gabetica

80 .z? C z yj

60 40

2

4

6

8

10

Reaction Time (days) Figure2 Crystallization curves of TS-1 grown from the reaction gels containing variable Ti concentration. (A) Silicalite-I (0 TV96 7); (Cl) sample 3 (2 Ti/% T); (A,) sample 4 (4 Ti196 T); (0) sample 5 (6 TV96 T).

for ?-S-l crystallization, because fonvard formation of silicalite-1

it leads to the straigbtcrystals (not TS-1).

Characterization of titanosilicates T&de 2 describes some physiochemical

properties of MFI crystals synthesized in methylamine-TPABr media. All the crystallites exhibited a homogeneous size and shape (Qure J), as often observed in various MFI m~tallosilicates obtained under the identical conditions.“‘-‘9 TSl crystals synthesized in the presence of hydrogen fluoride as a co-mineralizer (samples 3, 4, and 5) exhibited a prism-like morphology (Z+$~W 3); the overall crystallite size decreased as the amount of titanium in the gel increased. Although samples 4 and 5 contained unreacted gel around the crystals, the gel phase could be easily separated and removed from the crystals through the ultrasonic cleaning. F‘~Pw 4 shows the IR spectra of some products. Sample 4, which was TSl synthesized in the presence of HF, showed a relatively well-resolved LR band at 960 ctC ’ that is not detected in silicafite-I synthesized under the same conditions. This IR band has been a good proof (fingerprint) of the existence of framework metallic ions, and particularly of framework Ti in TS1.l.’ The unit cell volume that was calculated from XIU) data provides important information on the metallic ions existing in the metallosilciate framework. An incorporation of M I’+ ion into the zeolite framework would cause a unit cell volume variation. It would expandicont~act depending on the relative size of the M “+ with respect to Si”+. The Ti-0 bond being longer than Si-0, the unit cell volume of TS-1 would be larger than that of silicalite-1.” The unit cell volumes of a series of samples, as calculated from XRD data, are presented in T/;a& 2. Silicalite-1 synthesized from methylamine-TPABr media had a unit cell volume nearly identical to the one synthesized in basic media.’ The unit cell volume

Figure 3 Scanning electron mi~rographs sample 6; and (Cl sample 11.

of (A) sample 4; (Bf

of sample 4 showed a marked increase (5368 A”) with respect to that. of silicalite-1 (5339 A”). Samples 3 and 5, which crystallized from a different Ti-containing gel under the same synthesis conditions with sample 4, also showed the expansion of the unit cell volnme. These results strongly support the IR data,

Zeoiites

19246-252,

1997

249

Synthesis

of MN titanosilicates:

Table 2

Physiocochemical

Sample name (number)

M. Shibata

properties

of some MFI zeolites prepared from CH,NH,-TPABr

Initial Ti content (atom/96 T)

Reaction time (d)

2.0 4.0 6.0 4.0 4.0 0.0

6 10 10 3 10 2

3 4 5 6 11 silicalite-I

and Z. Gabelica

Average crystal size (pm) 20 x 16 x 16 x 60 x 60x 48 x

media Ti content (atom/96T)

Symmetry”

Unit cell volume (A3)

Total (ICP)

Frameworkb

Framework Til total Ti ratio

0 0 0 M M M

5362 5368 5370 5340 5342 5339

1.7 2.1 4.9 2.0 1.0 -

1.3 1.6 1.7 0.1 0.2 -

0.76 0.76 0.35 0.05 0.20 -

8 8 8 12 16 8

aM: monoclinic; 0: orthorhombic. bCalculated from the unit cell volume.

which suggested that titanium ions were indeed incorporated into the framework of TS-1 synthesized in methylamine-TPABr media in the presence of HF. A relationship between the unit cell volume of TSl and the quantity of framework Ti has been proposed.‘” Using this relation, we have evaluated that the quantities of the framework Ti of samples 4 and 5, which, respectively, involve 4 Ti/96 T and 6 Ti/96 T in the initial gels, were very close. This suggests that the amount of lattice titanium seems to saturate to a value

I

1400

I

I

I

1200

1000

800

I

600

1 400

Wave number (cm-l) Figure 4

IR spectra of (a) silicalite-1; (b) sample 4; (c) sample 6; and (d) sample 11.

250

Zeolites

19:246-252,

1997

corresponding to 1.7 Ti/96 T under these synthesis conditions. This value is actually smaller than the one obtained. using the conventional method involving Ti and Si alkoxides and TPAOH (namely 2.5 Ti/96 T in the original patent).’ Although samples 4 and 5 were estimated to contain almost the same amount of framework Ti (calculated from the unit cell volume), ICP data showed that larger amount of Ti existed in sample 5 than in sample 4 (Table 2). This suggests that some extra-framework titanium could exist in sample 5. Diffusle-reflectance UV-vis is an efficient technique to evaluate qualitatively the presence omfframework and extra-framework Ti in TS-1. As shown in Figure 5, samples 3 and 5 showed a neat LN band at about 230 nm, corresponding to isolated framework titanium in tetrahedral coordination.17 The presence of this band could be considered as another proof for Ti incorporation into the zeolite framework. Sample 5 showed, in addition to the 230-nm band, another band at 300 to 350 nm that was not detected in sample 3. This

300

250

350’

400

Wave length (nm) Figure 5 Diffuse-reflectance (b) sampl,e 5.

UV-vis spectra of (a) sample 3 and

Synthesis of MN titanosiiicates: Table 3 Surface Ti quantities of some TS-1 synthesized presence of HF Sample name (number) 3 4 5

in the

Total Ti content (EDX) (atom196 T)

Framework Ti/total Ti ratio (XPS)

1.0 2.0 2.2

0.77 0.75 0.41

broad band has been attributed to anatase.” This suggests that any excess of Ti in the reaction gel, with respect to the amount likely to be incorporated in TS-1 framework, would yield some Ti-bearing extraframework phase. The state (coordination) of titanium on the surface of crystals was semiquantitatively examined by XPS. The Ti 2ps,, peak was separated into two peaks (mixture of Lorentzian and Gaussian curves), centered at BE values of 459.8 ? 0.2 eV and 458.3 t 0.2 eV, respectively. The former peak has been assigned to framework Ti in tetrahedral coordination. The latter has been attributed to an extra-framework Ti phase. l8 The ratio of framework Ti to the total Ti (framework + extra-framework) on the surface of TS1, calculated from the intensity of those two peaks, are reported in l’ubk 3. The ratios of samples 3 and 4 were almost equal (0.77 and 0.75, respectively) and that of sample 5 was fairly lower (0.41). These findings confirmed the results of UV-vis, ICP, and XRD. Accordingly, it was concluded that the maximum value of framework Ti was about 1.7 Ti/96 T and that a too large amount of titanium (probably more than 4 Ti/96 T) ‘in the initial gel leads to the formation of the extra-framework Ti. Sample 6, whose initial methylamine quantity was l/3 that of sample 4, did not clearly show the IR band (Figure 4). Moreover, its unit cell volume at 960 cm-’ was almost the same as that of silicalite-1, suggesting that the amount of framework Ti was very small (if any). These findings indicate that Ti atoms are difficult to incorporate into the zeolite framework when the methylamine quantity is not sufficient. Note that the initial quantities of HF in samples 6 and 4 were the same. It was, therefore, considered that HF may not play a dominant role for the Ti incorporation, although it was shown to accelerate the crystal growth (Figure 1). It can be assumed that the respective “mobilizing (mineralizing) )I roles of methylamine and fluoride ions seem fundamentally different. The former probably plays a major role in forming the appropriate Si-0-Ti oligomeric species needed for the ‘Ti incorporation into the growing crystallite. The latter would solubilize them to accelerate the crystallization. These crystallization mechanisms will be discussed more in depth elsewhere.14 Sample 11, which was synthesized from SiCl, in the presence of HF, had a large needle-like shape similar to that of silicalite-1 prepared from SiO,, and the untreated gel phase was also observed on the surface of the crystals (@me 3). In contrast to sample

M. Shibata and Z. Gabeiica

4, this gel could not be removed even by ultrasonic washing. Sarnple 11 showed the IR band at 960 cm-’ (Fipre 4); however, its intensity was weaker than in sample 4, suggesting that only a small amount of Ti was incorporated into the framework. Indeed, from the unit cell volume increase, this amount was estimated to be about 0.2 Ti/96 T. It was considered that the synthesis involving SiCl, leads to the formation of an inactive intermediate (such as p-titanic acid or highly polymerized Ti-0-Si species), which are not readily “solubilized” or “complexed” by methylamine through the hydrothermal treatment. This suggests that SiCl, is not an appropriate Si source for TS-1 synthesis, in contrast to t.he borosilicate synthesis.‘”

CONCLUSION TS-1 was synthesized by a new procedure using methylamine and TPABr. Silica and titanium tetrachloride could be used as the Si source and the Ti source, respectively. The advantage of this procedure over the conventional method is that high-crystalline TS-1 can be obtained without using costly TPAOH, Si-alkoxide, and Ti-alkoxide. Although the presence of Ti ions in the reaction gel slowed down the crystallization of MFI zeolites, the crystallization rate could be increased by further addition of F- ions as a co-mineralizer. In the case of the samples synthesized in the dual HF-methylomine media, IR and UV-vis, along with the lattice constant expansion, confirmed the incorporation of titanium into the MFI framework. The use of SiCl,, which efficiently forms StO-B gel complex in borosilicate synthesis, leads to a slow crystallization and low-framework Ti incorporation, suggesting that SiCl, is not an appropriate Si source for TS-1 synthesis. Decreasing the quantity of methylamine in the gel improved the apparent crystallization rate; however, the amount of lattice titaniurn in the crystals dramatically decreased. These findings indicated that a larger quantity of methylamine is required to generate the appropriate Ti-0-Si oligomers that would potentially form the TS-1 framework; when the amount of methylamine is not sufficient, only silicalite-1 would crystallize.

REFERENCES Taramasso, M., Perego, G., and Notari, B., U.S. Patent, 4410501 (1983) Perego, G., Bellussi, G., Corno, C., Taramasso, M., Buonomo, F., and Esposito, A. Stud. Surf. Sci. Caral. 1986, 28,129 Clerici, M.G. and Ingallina, P. J. Catal., 1993 71, 140 Kumar, R., Raj, A, Kumar S., and Ratnasamy, P., Stud. Surf. Sci. Catal. 1994, 84, 109 Miiller, U. and Steck, W. Stud. Surf. Sci. Catal. 1994,84,203 Guth, .J.L., Kessler, H. and Wey, R. in Proceedings of the 7th international Zeolite Conference (Eds. Y. Murakami, A. lijima and J.W. Ward) Kodansh, Tokyo, 1986, p. 121

Zeolites

19:246-252,

1997

251

Synthesis of MN titanosilicates: 7 8

9 10

11

252

M. Shibata and Z. Gabelica

Shilun, Q., Wenqin P. and Shangqing, Y. Stud. Surf Sci. Catal. 1989, 49, 133 Dwyer, J., Zhao, J. and Rawlence, D. in Proceedings of the 9th international Zeolite Conference (Eds. R. von Ballmoos, M.M.J. Treaty and J.B. Higgins) Butter-worth and Heinemann. London, 1993, p. 155 Dartt, C.B., Khouw, C.B., Li H.-X. and Davis, M.E. Microporous Mater. 1994,2,425 Gabelica, Z., Giannetto, G., DOS Santos, F.C., Monque R. and Galiarso, R. in Proceedings of the 9th /ntemationa/Zeo/ite Conference (Eds. R. von Ballmoos, M.M.J. Treaty and J.B. Higgins) Butterworth and Heinemann, London, 1993, p.231 Gianneto, G., DOS Santos, F., Monque, R., Galiasso, R. and Gabelica. 2. Zeolites 1995, 15, 719

Zeolites

19:246-252,

1997

12 13 14 15 16 17

18

Gabelica, 2. in Proc. 8” Seminarion Brasileiro de Catalise vol. 7, Anais, Rio de Janeiro, Brazil, 1995 p. 1 Shibata, M. and Gabelica, Z. Appl. Catal. A, in press Gabelica, Z., Shibata, M., Gerard, J., Wiame, M. and Valange, S. manuscript in preparation Boccuti, M.R., Rao, K.M., Zecchina, A., Leofanti, G. and Petrini, G., Stud. Surf. Sci. Catal. 1989, 48, 133 Millini, R., Massara, E.P., Perego, G. and IBellussi, G., J. Catal. 1992, 137, 497 Zecchina, A., Spoto, G., Bordiga, S., Ferrero, A., Petrini, G.. Leofanti, G. and Padovan, M., Stud. Surf. Sci. Catal. 1991 69, 251 Blasco, T., Camblor, M.A., Fierro, J.L.G. and Perez-Pariente, J. Microporous Mater. (1994), 3, 259