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Incorporation of iron into MFI structure in fluoride media
I. Kiricsi, G. Pfil-BortxSly,J.B. Nagy, H.G. Karge (Editors) Porous Materials in Environmentally Friendly Processes
Studies in Surface Science and Catalysis, Vol. 125 9 1999 Elsevier Science B.V. All rights reserved.
165
Incorporation o f Iron Into MFI Structure In Fluoride Media Flaviano Testa a, Fortunato Crea', Rosario Aiello" and Janos B.Nagyb aDipartimento di Ingegneria Chimica e dei Materiali, Universit/l della Calabria, 87030 Rende (CS), Italy bLaboratoire de R.M.N., Facult6s Universitaires Notre-Dame de La Paix, 5000 Namur, Belgium
The influence of different fluoride salts on the synthesis of iron-silicalite with MFI structure has been investigated. The solubility of the fluoride sources and the pH of the initial mixtures are the parameters affecting mostly the physico-chemical properties of the microporous products and the mechanisms of growth of the crystals.
1. I N T R O D U C T I O N
One of the trends in catalysis to reduce the environmental impact of hazardous catalytical processes concerns the replacement of toxic and corrosive catalysts such as sulfuric and hydrofluoric acids, BF3, and AiC13 with regenerable solid catalysts characterized by high thermal stability and selectivity. In petrochemical industry very good performances as acid catalysts are exhibited by zeolites and analogues metallosilicates. In particular, iron containing zeolites have been tested for production of cumene in vapour phase with very promising results [ 1]. The differences in catalytic activity of iron containing zeolites and iron supported zeolites are very intriguing and several methods of preparation have been developed [2,3]. Fe-silicates with MFI, MOlL BEA and MWW structures [4] have been synthesized in hydroxide media while in fluoride media only the synthesis of iron containing MFI structure ([Fe]-ZSM-5) has been reported in the presence of NH4F as mineralizing agent [5]. In this paper the synthesis of iron-containing MFI structure in the presence of different inorganic fluoride salts as mineralizing agents and their influence on both the crystallization kinetics and the properties of the zeolitic products, are reported.
2. E X P E R I M E N T A L
The reactants were ammonium fluoride (Carlo Erba, RPE), sodium fluoride (Carlo Erba, RPE) potassium fluoride (Carlo Erba, RPE), cesium fluoride (Aldrich, purum), tetrapropylammonium bromide (TPABr, Fluka, purum), iron nitrate (Fe(NO3)a9H20, Merck,
166 purum), fumed silica (Sigma) and distilled water. The molar composition of the starting mixture was: 10SiO2 - x Fe(NO3)39H20 - 15MF - 1.25TPABr - 300H20 where 0.1 > x _>0.3, and M = Nt-I4, Na, K or Cs. The starting mixtures were prepared adding to the distilled water the fluoride salt, iron nitrate, TPABr and the fumed silica in that order. The resulted gels, aRer complete homogenization, were put in PTFE-lined 25 cm 3 stainless-steel autoclaves. The samples were obtained by hydrothermal synthesis at 170 ~ for prefixed times. After quenching of the autoclaves the products were recovered, filtered, washed with distilled water and finally dried at 80~ overnight. The powder x-ray diffraction patterns were collected using CuK~ radiation (Philips Model PW 1730/10 generator equipped with a PW 1050/70 vertical goniometer). The amount of iron in crystals were determined by atomic absorption spectrometry (Shimadzu AA-660). The amount of TPA occluded in the crystals and the mechanism of decomposition of the organic molecule was obtained by t.g. and d.s.c, analyses, respectively. The measurements were carried out with a Netzsch STA 409 between 20~ and 650~ at a heating rate of 10~ in N2 atmosphere with a flow rate of 10 ml/min. The micrographs were collected by a scanning electron microscope (SEM), Jeol JSTM 330A.
3. RESULTS AND DISCUSSION
In this study fluoride salts were used in a range from 3 to 24 moles. The syntheses with 15 moles were chosen because they gave the fastest crystallization kinetics. The complete results will be published in a forthcoming paper. All the syntheses performed in this study led to products with MFI structure. In Table 1 the pH of the initial gel (pH0, the pH of the final mother liquor (pHf), the iron and tetrapropylammonium ion contents (Fe/u.c. and TPA/u.c., respectively) the induction time (t~,t) i.e. the time occurring for the detection of ca. 4% of crystallinity and the crystallization rate (the slope of the linear part of the crystallization sigmoidal curve) are reported. All the prepared gels are white before the heating in the PTFE autoclaves with iron did not precipitate as low-soluble species but it has formed soluble complexes with fluoride ions. This is different from the synthesis of Fe-silicalite in alkaline media where the obtained gel is pale yellow [6, 7]. The pH of the initial gels increases in the order NH4
167 Table 1 Chemical and kinetic parameters for the Fe-silicalite synthesis from systems 10SiO2-xFe(NO3)39H20-15MF- 1.25TPABr-300H20 at 170~ Fe gel, moles
0.1
0.2
0.3
M,
phi
pHf
NH4 Na K Cs
6.9 7.5 8.5 8.5
6.5 7.0 8.0 8.0
1.2 0.7 0.7 1.3
NH4 Na K Cs H4 Na K Cs
7.0 7.5 8.2 8.2 6.5 7.2 8.0 8.8
6.9 7.5 8.0 7.8 6.5 6.8 7.5 8.0
1.9 0.7 1.6 1.8 3.9* 1.6" 1.5 3.4
t ~ (h)
R(%.h"l)
3.0 3.3 3.8 3.8
8.0 7.1 4.2 3.0
4.4 11.3 11.8 22.0
3.8 3.8 3.8 3.5 3.3 3.8 3.4 2.6
24 13.0 7.0 4.6 34 12.0 12 6.0
3.1 9.0 9.5 10.5 3.9 4.6 7.3 10.5
Fe/u.c. TPA/u.c.
*Brown crystals
.
~F
,,,
J
'
' 'g
f
DSC
.
.
.
b
'
g
Cs
E N D 0
v
!
'4OO
.
|~)0
' Temperature,
l
,A
4OO
SO<)
"C
Figure 1. DSC curves of the samples of Fe-siliealite obtained from the systems: 10SiO2xFe(NO3)39H20-15MF-1.25TPABr-300H20 at 170~ with (a) x=0.1, (b) x--0.2; (c) x=0.3.
168
Figure 2. SEM micrographies of Fe-silicalite samples obtained from the system: 10SiO~xFe(NO3)39H20-15MF-1.25TPABr-300H20 at 170~ with (a) NH4; (b) Na; (c) K; (d) Cs. With increasing Fe-content in the gel, Fe/u.c. in the MFI framework also increases. However, in presence of Na + and NH4 + ions, the samples become brownish, showing the presence of extraframework Fe species. For all the other cases, the total amount of the incorporated Fe is in the framework as tetrahedral species. The amount of TPA per unit cell is equal to 3.8 and close to the theoretical maximum value of 4. For small values correspond probably to higher incorporation of the countercations as it was shown previously for aluminum [8]. Patterns of DSC curves (Figure 1) and temperature of decong~osition of TPA depend on the amount of Fe/u.c. in the crystals as previously observed in presence of either boron, ahaninum or gallium [9]. TPA § ions weakly bonded to the structure decompose at a temperature of ca. 405~ Increasing the amount of Fe/u.c. the organic cation is preferentially linked to the structure as countercation of the iron negative charge and its temperature of decomposition is higher (ca. 450~ It is to be noted that the temperature of decomposition of TPA § ions is low compared to that characteristic of aluminum-containing structures, showing that the interaction between TPA § ions and the (Fe-O-Si) negative charges is not so strong as between TPA § and (AI-O-Si) negative charges. Figure 2 shows the SEM micrographs of the crystals obtained at SFFe ratio 50 in the presence of various cations. Morphology of the MFI crystals highly depends on the mount of Fe/u.c. in the structure. Na-containing crystals have prismatic morphology typical of silica rich
169
MFI zeolite. Increasing the amount of iron (see Table 1) morphology becomes first ovoidal (Kand Cs- samples) and then spherical (NH4 samples). The rate of crystallization (R) increases with the increasing of the pH of the initial gel (Figure 3). The importance of the pH was akeady emphasized in F containing media [10]. Indeed, the OH" and F ions are in competition as mineralizing agents. At low pH, the role ofF" is more important, while it decreases at high pH values. The incorporation of Fe in the MFI framework seems also to depend on the MF solub'dity. Indeed, the solubility of the MF in water increases in the order: NaF
o
6
"/
8 PH i
Figure 3. LogR as a function of pHi (initial pH) for the crystallization of Fe-MFI from gels 10SiO2-xFe(NO3)39H~O- 15MF- 1.25TPABr-300H20 at 170~ When log R is plotted as a function of log (1/tin) (the inverse of the induction time i.e. the induction rate) a linear relation is obtained (Figure 3) for NH4§ K§ and Cs+. In particular, the positive slope increases with the increase of iron content in the initial mixture. This increase means that the induction rate is more influenced by the cations than the rate of crystallization. Moreover, the influence of the cation increases the lower the rate of both induction and crystallization (i.e. at higher Fe-content). Interestingly, the Na-systems do not follow the linear behavior. This suggests that more specific interactions should also play a role in the stab'dization of the nuclei and the particles in formation [ 11]. These result emphasize the need for further investigation of both the liquid and the solid phases of the gel. Quite recently in-situ multi NMR measurements were carried out to characterize the liquid phases of gels leading to the formation of AIPO4-CJZ [12]. That way, various fluoro and fluorophosphate complexes of aluminum could he determined. It has to be emphasized that these species could only be evidenced in the presence of the solid phase of the gel. In a clear solution, all the present species showed a fast exchange between each other.
170
Cs NH 4
7 Na
o +
[ 9
0.1Fe I
Cs
~2 NH 4 ~
o +
a
~
I"
0.2Fe I
w
Cs
~'2
Na
o
NH4
I "
1
0
I
i logR
Figure4. Log R vs. log of the inductionrateas a functionof differentcations.
4. CONCLUSIONS Up to 3.4/u.c. Fe in the MFI framework could be introduced in the presence of CsF in F-containing media The amount of Fe/u.c. depends on the amount of Fe(NO3)39H20 in the initial gel. The pH of the initial gels influences both the rate of induction and crystallization. They both increase with increasing pH. The crystal size and morphology depends on the Fe/u.c. and also on the rate of crystallization. The alkali cations exert a similar influence on both the induction and crystallization rate.
171 5. ACKNOWLEDGMENTS
The present work is a part of a project coordinated by A. Zecchina and cofinanced by the Italian MURST (Corm 98, Area 03).
REFERENCES
1. A.V. Smirnov, F. Di Renzo, O.E. Lebedeva, D. Brunel, B. Chichc, A. Tavolaro, B.V. Romanovsky, G. Giordano, F Fajula and I. Ivanova, Stud. Surf. Sci. Catal. 105 (1997) 1325. 2. V.I. Sobolev, G.I. Pavon, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin and K.G. Ionc, J.Catal., 139 (1993) 435. 3. R.V. Joyner and M. Stockenhuber, Proc., Proceedings of the 12tk International Zeolite Conference, Baltimore 1998, M.M.J. Treacy, B. Marcus, J.B. Higgins and M.E. Bisher eds, Materials Research Society, in press. 4. F. Testa, F. Crea, G.D. Diodati, L. Pasqua, R. AieUo, G. Terwagne, P. Lentz and J.B. Nagy, Microporous and Mesoporous Materials, in press. 5. J. Patafin, J.L. Guth, H. Kessler and G. Coududer, F. Raotz, Fr Pat. 8 617 711 (1986). 6. R. Szostak and T.L. Thomas, J. Catal., 100 (1986) 555. 7. P. Fejes, J. B.Nagy, J. Hal~z and A. Oszko, Appl. Catal. A: General, 175 (1998) 89. 8. J.B.Nagy, P. Bodart, H. Collette, Z. Gabelica, A. Nastro and R. Aiello, J. Chem. Sot., Faraday Trans. 1, 85 (1989) 2749. 9. J.B.Nagy, R. Aiello, F. Crea and F. Testa, Proceexiings of the 12~ International Zeolite Conference, Baltin~re 1998, M.M.J. Treacy, B. Marcus, J.B. Higgins and M.E. Bisher r Materials Research Society, in press. 10. A. Tavolaro, R. Mostowicz, F. Crea, A. Nastro, R. Aiello and J. B.Nagy, Zeolites, 12 (1992) 756. 11. F. Crea, R. Mostowicz, F. Testa, R. Aiello, A. Nastro and J. B.Nagy, Proceedings of the 9~ International Zeolite Conference, Montreal 1992, Eds. R. von Ballmoos, J.B. Higgins, M.M.J. Treaty, Butterworth-Heinemann (USA). 12. M. Haouas, C. Gerardin, F. Taulelle, C. Estournes, T. Loiseau and G. Farey, J. Chim. Phys. 95(1998) 302.
Studies in Surface Science and Catalysis, Vol. 125 9 1999 Elsevier Science B.V. All rights reserved.
165
Incorporation o f Iron Into MFI Structure In Fluoride Media Flaviano Testa a, Fortunato Crea', Rosario Aiello" and Janos B.Nagyb aDipartimento di Ingegneria Chimica e dei Materiali, Universit/l della Calabria, 87030 Rende (CS), Italy bLaboratoire de R.M.N., Facult6s Universitaires Notre-Dame de La Paix, 5000 Namur, Belgium
The influence of different fluoride salts on the synthesis of iron-silicalite with MFI structure has been investigated. The solubility of the fluoride sources and the pH of the initial mixtures are the parameters affecting mostly the physico-chemical properties of the microporous products and the mechanisms of growth of the crystals.
1. I N T R O D U C T I O N
One of the trends in catalysis to reduce the environmental impact of hazardous catalytical processes concerns the replacement of toxic and corrosive catalysts such as sulfuric and hydrofluoric acids, BF3, and AiC13 with regenerable solid catalysts characterized by high thermal stability and selectivity. In petrochemical industry very good performances as acid catalysts are exhibited by zeolites and analogues metallosilicates. In particular, iron containing zeolites have been tested for production of cumene in vapour phase with very promising results [ 1]. The differences in catalytic activity of iron containing zeolites and iron supported zeolites are very intriguing and several methods of preparation have been developed [2,3]. Fe-silicates with MFI, MOlL BEA and MWW structures [4] have been synthesized in hydroxide media while in fluoride media only the synthesis of iron containing MFI structure ([Fe]-ZSM-5) has been reported in the presence of NH4F as mineralizing agent [5]. In this paper the synthesis of iron-containing MFI structure in the presence of different inorganic fluoride salts as mineralizing agents and their influence on both the crystallization kinetics and the properties of the zeolitic products, are reported.
2. E X P E R I M E N T A L
The reactants were ammonium fluoride (Carlo Erba, RPE), sodium fluoride (Carlo Erba, RPE) potassium fluoride (Carlo Erba, RPE), cesium fluoride (Aldrich, purum), tetrapropylammonium bromide (TPABr, Fluka, purum), iron nitrate (Fe(NO3)a9H20, Merck,
166 purum), fumed silica (Sigma) and distilled water. The molar composition of the starting mixture was: 10SiO2 - x Fe(NO3)39H20 - 15MF - 1.25TPABr - 300H20 where 0.1 > x _>0.3, and M = Nt-I4, Na, K or Cs. The starting mixtures were prepared adding to the distilled water the fluoride salt, iron nitrate, TPABr and the fumed silica in that order. The resulted gels, aRer complete homogenization, were put in PTFE-lined 25 cm 3 stainless-steel autoclaves. The samples were obtained by hydrothermal synthesis at 170 ~ for prefixed times. After quenching of the autoclaves the products were recovered, filtered, washed with distilled water and finally dried at 80~ overnight. The powder x-ray diffraction patterns were collected using CuK~ radiation (Philips Model PW 1730/10 generator equipped with a PW 1050/70 vertical goniometer). The amount of iron in crystals were determined by atomic absorption spectrometry (Shimadzu AA-660). The amount of TPA occluded in the crystals and the mechanism of decomposition of the organic molecule was obtained by t.g. and d.s.c, analyses, respectively. The measurements were carried out with a Netzsch STA 409 between 20~ and 650~ at a heating rate of 10~ in N2 atmosphere with a flow rate of 10 ml/min. The micrographs were collected by a scanning electron microscope (SEM), Jeol JSTM 330A.
3. RESULTS AND DISCUSSION
In this study fluoride salts were used in a range from 3 to 24 moles. The syntheses with 15 moles were chosen because they gave the fastest crystallization kinetics. The complete results will be published in a forthcoming paper. All the syntheses performed in this study led to products with MFI structure. In Table 1 the pH of the initial gel (pH0, the pH of the final mother liquor (pHf), the iron and tetrapropylammonium ion contents (Fe/u.c. and TPA/u.c., respectively) the induction time (t~,t) i.e. the time occurring for the detection of ca. 4% of crystallinity and the crystallization rate (the slope of the linear part of the crystallization sigmoidal curve) are reported. All the prepared gels are white before the heating in the PTFE autoclaves with iron did not precipitate as low-soluble species but it has formed soluble complexes with fluoride ions. This is different from the synthesis of Fe-silicalite in alkaline media where the obtained gel is pale yellow [6, 7]. The pH of the initial gels increases in the order NH4
167 Table 1 Chemical and kinetic parameters for the Fe-silicalite synthesis from systems 10SiO2-xFe(NO3)39H20-15MF- 1.25TPABr-300H20 at 170~ Fe gel, moles
0.1
0.2
0.3
M,
phi
pHf
NH4 Na K Cs
6.9 7.5 8.5 8.5
6.5 7.0 8.0 8.0
1.2 0.7 0.7 1.3
NH4 Na K Cs H4 Na K Cs
7.0 7.5 8.2 8.2 6.5 7.2 8.0 8.8
6.9 7.5 8.0 7.8 6.5 6.8 7.5 8.0
1.9 0.7 1.6 1.8 3.9* 1.6" 1.5 3.4
t ~ (h)
R(%.h"l)
3.0 3.3 3.8 3.8
8.0 7.1 4.2 3.0
4.4 11.3 11.8 22.0
3.8 3.8 3.8 3.5 3.3 3.8 3.4 2.6
24 13.0 7.0 4.6 34 12.0 12 6.0
3.1 9.0 9.5 10.5 3.9 4.6 7.3 10.5
Fe/u.c. TPA/u.c.
*Brown crystals
.
~F
,,,
J
'
' 'g
f
DSC
.
.
.
b
'
g
Cs
E N D 0
v
!
'4OO
.
|~)0
' Temperature,
l
,A
4OO
SO<)
"C
Figure 1. DSC curves of the samples of Fe-siliealite obtained from the systems: 10SiO2xFe(NO3)39H20-15MF-1.25TPABr-300H20 at 170~ with (a) x=0.1, (b) x--0.2; (c) x=0.3.
168
Figure 2. SEM micrographies of Fe-silicalite samples obtained from the system: 10SiO~xFe(NO3)39H20-15MF-1.25TPABr-300H20 at 170~ with (a) NH4; (b) Na; (c) K; (d) Cs. With increasing Fe-content in the gel, Fe/u.c. in the MFI framework also increases. However, in presence of Na + and NH4 + ions, the samples become brownish, showing the presence of extraframework Fe species. For all the other cases, the total amount of the incorporated Fe is in the framework as tetrahedral species. The amount of TPA per unit cell is equal to 3.8 and close to the theoretical maximum value of 4. For small values correspond probably to higher incorporation of the countercations as it was shown previously for aluminum [8]. Patterns of DSC curves (Figure 1) and temperature of decong~osition of TPA depend on the amount of Fe/u.c. in the crystals as previously observed in presence of either boron, ahaninum or gallium [9]. TPA § ions weakly bonded to the structure decompose at a temperature of ca. 405~ Increasing the amount of Fe/u.c. the organic cation is preferentially linked to the structure as countercation of the iron negative charge and its temperature of decomposition is higher (ca. 450~ It is to be noted that the temperature of decomposition of TPA § ions is low compared to that characteristic of aluminum-containing structures, showing that the interaction between TPA § ions and the (Fe-O-Si) negative charges is not so strong as between TPA § and (AI-O-Si) negative charges. Figure 2 shows the SEM micrographs of the crystals obtained at SFFe ratio 50 in the presence of various cations. Morphology of the MFI crystals highly depends on the mount of Fe/u.c. in the structure. Na-containing crystals have prismatic morphology typical of silica rich
169
MFI zeolite. Increasing the amount of iron (see Table 1) morphology becomes first ovoidal (Kand Cs- samples) and then spherical (NH4 samples). The rate of crystallization (R) increases with the increasing of the pH of the initial gel (Figure 3). The importance of the pH was akeady emphasized in F containing media [10]. Indeed, the OH" and F ions are in competition as mineralizing agents. At low pH, the role ofF" is more important, while it decreases at high pH values. The incorporation of Fe in the MFI framework seems also to depend on the MF solub'dity. Indeed, the solubility of the MF in water increases in the order: NaF
o
6
"/
8 PH i
Figure 3. LogR as a function of pHi (initial pH) for the crystallization of Fe-MFI from gels 10SiO2-xFe(NO3)39H~O- 15MF- 1.25TPABr-300H20 at 170~ When log R is plotted as a function of log (1/tin) (the inverse of the induction time i.e. the induction rate) a linear relation is obtained (Figure 3) for NH4§ K§ and Cs+. In particular, the positive slope increases with the increase of iron content in the initial mixture. This increase means that the induction rate is more influenced by the cations than the rate of crystallization. Moreover, the influence of the cation increases the lower the rate of both induction and crystallization (i.e. at higher Fe-content). Interestingly, the Na-systems do not follow the linear behavior. This suggests that more specific interactions should also play a role in the stab'dization of the nuclei and the particles in formation [ 11]. These result emphasize the need for further investigation of both the liquid and the solid phases of the gel. Quite recently in-situ multi NMR measurements were carried out to characterize the liquid phases of gels leading to the formation of AIPO4-CJZ [12]. That way, various fluoro and fluorophosphate complexes of aluminum could he determined. It has to be emphasized that these species could only be evidenced in the presence of the solid phase of the gel. In a clear solution, all the present species showed a fast exchange between each other.
170
Cs NH 4
7 Na
o +
[ 9
0.1Fe I
Cs
~2 NH 4 ~
o +
a
~
I"
0.2Fe I
w
Cs
~'2
Na
o
NH4
I "
1
0
I
i logR
Figure4. Log R vs. log of the inductionrateas a functionof differentcations.
4. CONCLUSIONS Up to 3.4/u.c. Fe in the MFI framework could be introduced in the presence of CsF in F-containing media The amount of Fe/u.c. depends on the amount of Fe(NO3)39H20 in the initial gel. The pH of the initial gels influences both the rate of induction and crystallization. They both increase with increasing pH. The crystal size and morphology depends on the Fe/u.c. and also on the rate of crystallization. The alkali cations exert a similar influence on both the induction and crystallization rate.
171 5. ACKNOWLEDGMENTS
The present work is a part of a project coordinated by A. Zecchina and cofinanced by the Italian MURST (Corm 98, Area 03).
REFERENCES
1. A.V. Smirnov, F. Di Renzo, O.E. Lebedeva, D. Brunel, B. Chichc, A. Tavolaro, B.V. Romanovsky, G. Giordano, F Fajula and I. Ivanova, Stud. Surf. Sci. Catal. 105 (1997) 1325. 2. V.I. Sobolev, G.I. Pavon, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin and K.G. Ionc, J.Catal., 139 (1993) 435. 3. R.V. Joyner and M. Stockenhuber, Proc., Proceedings of the 12tk International Zeolite Conference, Baltimore 1998, M.M.J. Treacy, B. Marcus, J.B. Higgins and M.E. Bisher eds, Materials Research Society, in press. 4. F. Testa, F. Crea, G.D. Diodati, L. Pasqua, R. AieUo, G. Terwagne, P. Lentz and J.B. Nagy, Microporous and Mesoporous Materials, in press. 5. J. Patafin, J.L. Guth, H. Kessler and G. Coududer, F. Raotz, Fr Pat. 8 617 711 (1986). 6. R. Szostak and T.L. Thomas, J. Catal., 100 (1986) 555. 7. P. Fejes, J. B.Nagy, J. Hal~z and A. Oszko, Appl. Catal. A: General, 175 (1998) 89. 8. J.B.Nagy, P. Bodart, H. Collette, Z. Gabelica, A. Nastro and R. Aiello, J. Chem. Sot., Faraday Trans. 1, 85 (1989) 2749. 9. J.B.Nagy, R. Aiello, F. Crea and F. Testa, Proceexiings of the 12~ International Zeolite Conference, Baltin~re 1998, M.M.J. Treacy, B. Marcus, J.B. Higgins and M.E. Bisher r Materials Research Society, in press. 10. A. Tavolaro, R. Mostowicz, F. Crea, A. Nastro, R. Aiello and J. B.Nagy, Zeolites, 12 (1992) 756. 11. F. Crea, R. Mostowicz, F. Testa, R. Aiello, A. Nastro and J. B.Nagy, Proceedings of the 9~ International Zeolite Conference, Montreal 1992, Eds. R. von Ballmoos, J.B. Higgins, M.M.J. Treaty, Butterworth-Heinemann (USA). 12. M. Haouas, C. Gerardin, F. Taulelle, C. Estournes, T. Loiseau and G. Farey, J. Chim. Phys. 95(1998) 302.