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Deuteron magnetic resonance studies of ammonia in AgNaY zeolites
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
479
Deuteron Magnetic Resonance Studies of Ammonia in A g N a Y Zeolites
M. Hartmann* and B. Boddenberg
Lehrstuhl ~ r Physikalische Chemic II, Universit~it Dortmund, D-44221 Dortmund, Germany
The adsorption of ammonia is investigated in silver exchanged AgNaY zeolites. Silverincorporation into NaY increases the ammonia adsorption probably due to the formation of silver-diammine-complexes. 2H-NMR spectra show fast isotropic reorientations of the ammonia at room temperature, which transform into a rigid lattice behavior with decreasing temperature. Comparison with Ag(ND3)2+-complexes ion-exchanged into NaY show the same dynamic behavior giving additional evidence for a silverdiammine-complex formation upon adsorption of ammonia.
1. INTRODUCTION Transition metal ions are catalytically active in a variety of chemical reactions [1]. Incorporation of transition metal ions into zeolite cavities or channels may result in catalysts with unique properties. 2H nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the dynamics of adsorbed deuterated molecules as well as their interaction with different adsorption sites. This technique has been successfully used for studies of adsorbed benzene and propene in zeolites [2]. Ammonia may be assumed to interact specifically with the silver sites in zeolites since Ag(ND3)2+-complexes are known in solution, rendering this molecule a well-suited candidate for this study on the locations and the properties of silver ions in Y zeolites.
2. EXPERIMENTAL SECTION Starting from NaY (Union Carbide LZY-52, Si/AI=2.4) silver exchanged zeolites Ag(x)NaY with x = 14, 28, 50 and 100 % were prepared with aqueous solutions of different * Present address: Institute of Chemical Technology I, Universit~it Stuttgart, D-70550
Germany.
Stuttgart,
480 AgNO 3 concentration. The silver contents of the samples were determined by. electron microprobe analysis (EMPA). The samples were dehydrated under vacuum (p < 10-5 hPa) for 18 h at 420 ~ and subsequently oxidized for 6 h at the same temperature. The ammonia adsorption isotherms were measured volumetrically at 298 K. Atter completion of the first adsorption isotherm, the ammonia was pumped off and the sample was evacuated at 298 K for 18 h (p < 10-5 hPa). Subsequently a second isotherm was measured. For the 2H-NMR experiments the samples were pretreated as described above and ammonia-d3 (MSD, Montreal, Canada) was adsorbed up to a pressure of 100 hPa. Then the sample was evacuated overnight, sealed under cooling in liquid nitrogen and stored in the dark. The 2H-NMR spectra were recorded at a resonance frequency 0~0/2rt = 52.72 MHz using a Bruker CXP 100 spectrometer. The spectrometer as well as the measuring procedures applied have been described elsewhere [3]. For comparison an AgfND3)2Y zeolite was prepared by exchanging the complex ion Ag(ND3)2 + into the NaY zeolite. Under dry nitrogen ammonia-d3 was introduced into a solution of AgNO 3 in D20. Under stirring the addition of ammonia was performed until the solution became transparent. Now the AgfND3)2 + complex has been formed in solution. Adding a calibrated amount of NaY and additional stirring in the dark formed an Ag(ND3)2Y zeolite with an exchange degree of 55 %. The zeolite was then separated from the solution and the sample was subsequently carefully dehydrated, sealed and stored in the dark.
3. RESULTS Figure 1 shows the adsorption isotherms at 298 K of ammonia in the zeolites NaY, Ag(14)NaY, Ag(28)NaY, Ag(50)NaY and AgY (Ag(100)NaY). These isotherms were obtained for the zeolites activated as described above. The isotherms obtained after 18 h ambient temperature evacuation are only displayed for the zeolites AgY, Ag(28)NaY and Ag(14)NAY. Generally, for each zeolite the pairs of isotherms steeply increase at low pressure and run almost parallel to each other yielding difference amounts Nirr that are collected in Table 1. In comparison to NaY, Nirr is enlarged considerably up to a factor of about two in the case of the most highly silver exchanged zeolite AgY.
Table 1 Irreversible adsorbed amounts of ammonia at 298 K. sample NaY Ag(la)NaY Ag(28)NaY Ag(50)NaY AgV
Nirr/(NH3/u. c. ) 30 30 30 35 63
481 N/(NH3/u.c.) 120
.
.
.
.
.
.
.
.
.
.
~
'
~__.___@
X
~
100
"
X = 100 50
80
; X=O
60
rption
40
2O
+
NaY
- 4 - Ag(14)NAY 0
-
0
I
10
__1
20
..
!
I
I
-I
I
I
30
40
50
60
70
80
,
1
90
100
p/hPa
Figure 1. Ammonia adsorption isotherms at 298 K in silver exchanged Y zeolites Figure 2 shows the 2H-NMR spectra of ammonia in NaY zeolite at selected temperatures between 293 and 80 K. With decreasing temperature the spectra develop from singlets of increasing width into solid state powder patterns of the Pake-Type with Av = 51 kHz prominent edge splitting. T he spectral shape transition range extends from about 200 K to 100 K. The 2H-NMR spectra of (a) AgY loaded with ammonia (Nirr = 63/u.c.) and (b) Ag(ND3)2Y at selected temperatures between 290 and 80 K are compared in Figure 3. In both cases, the spectra develop in the same fashion with decreasing temperature from singlets of increasing widths into temperature-independent solid state powder pattern. From the edge sI~litting of Av = 53 kHz the deuteron quadrupolar coupling constant is readily calculated as e"~qQ/h = 71 kHz. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bonding except the C3-axis rotation proceed very slowly. The breakdown of the solid state Pake pattern into temperature dependent Lorentzian-type singlets occurs in both samples in the temperature interval between 167 and 235 K.
482
293K
250 K
200 K
167 K
/ .... 150
143K
i .... 100
i . . . . . . . . . . . . . . . . . . . 50 0 -50 -100
-1,50
50
125 K
15o
lOO
(P-Po)/kHz
1~
-so
-~oo
.50
-~
-1~
-1~
80 K
100K
1~
o
~
0
(P-iU'o)/kHz
-50
-1~
-1~
1~
1~
~
0
Figure 2.2H-NMR spectra of ammonia adsorbed in NaY zeolite (Nirr = 30 NH3/u.c. )
483
(.)
I---25 k Hz
(b)
I---! 25 k Hz
L -
, . . , _ , . ,
.
.
.
.
.
.
.
J
_
,
.
.
.
.
.
.
.
_
290K
290K
222K
222K
162K
162K
80K
80K
Figure 3.2H-NMR spectra of ammonia in (a) AgY (63 ND3/u.c.) and (b) Ag(ND3)2Y
484 4. DISCUSSION The adsorption of ammonia in NaY zeolites can be increased by ion-exchange of Ag(I) ions. With increasing silver content the overall adsorption increases (Figure 1). The amount of strong adsorbed ammonia molecules Nirr also increases with the degree of exchange. In NaY 30 NH 3 molecules can not be removed from the sample due to their adsorption on strong adsorption sites. Due to absence of strong Lewis or Broensted acid sites only the sodium cations in the supercages are able to adsorb ammonia strongly [4,5]. The sodium cations are located on site SII and adsorb ammonia with an adsorption enthalpy of about 44 kJ/mol, which is close to typical chemisorption enthalpies [6,7,8]. With increasing degree of silver exchange the amount of strongly adsorbed ammonia molecules also increases, showing that silver cations are able to adsorb more than one ammonia molecule. It is well known that Ag + form silverdiammine-complexes Ag(ND3)2 + in solution and solids [9]. It was shown previously that Cu 2+ and Ni 2+ are also able to form amine complexes in solution [10] and zeolites [11,12]. Therefore, it is very likely that Ag + also forms silverdiamminecomplexes in zeolites [11], but no experimental evidence could be presented so far. The increase in ammonia adsorption in Ag(x)NaY zeolites can very well be explained by the formation of Ag(NH3)2 + complexes in the supercages. At a low level of silver-exchange, the increase is very small, showing that most of the silver-cations are no__!tlocated in the supercage. This is in excellent agreement with X-ray and xenon adsorption data, which show a preference of Ag+ for the SI position in the double six ring [ 13]. It can be concluded from our data that not all silver ions are accessible for ammonia and only some silver ions migrate into accessible sites. Therefore the irreversible adsorbed amount of ammonia Ni~ does not increase linearly with the degree of silver exchange. In AgY 63 ammonia molecules were found to be strongly adsorbed corresponding to about 32 Ag + cations on the SII positions in the supercages. X-ray crystallographic data assign between 25 and 30 silver cations to sites in the supercage [14]. This was also confirmed by xenon adsorption and xenon NMR data [ 15]. The formation of silverdiammine-complexes in zeolites can be confirmed by our NMR data. Investigation of the ion-exchange with silverdiammine-complexes indicate that these complexes can only replace all sodium cations in the supercage, but are not able to enter the 13cage [16]. Therefore, the situation in AgfND3)2Y should be comparable to the AgY after adsorption of 64 ammonia molecules. In fact, we observed that the spectra looked almost identical at all temperatures. In NaY the spectral shape develops very slowly in a temperature interval of about 100 K. This transformation interval shortens with increasing degree of silver exchange down to 68 K for AgY and Ag(ND3)2Y. In all samples a low temperature powder pattern is observed, which is characteristic for an axially symmetric electric field gradient (EFG) tensor being operative. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bond except the C3-axis rotation proceeds very slowly [17]. The effective deuterium quadrupole coupling constant (DQCC) is 69 kHz for NaY and 71 kHz for AgY. Single crystal 2H-NMR measurements of the [Ag(ND3)2]Ag(NO2)2-complex show a DQCC of 71.6 + 0.5 MHz at room temperature [9]. Therefore, it is most likely that at least at low temperatures AgfND3)2+-complexes are also present in zeolites.
485 With increasing temperature some molecular motion runs the spectrum shapes from the slow into the fast oriental exchange limits measured on the time scale XNMR. At present it is still unclear whether this motion involves only the ammonia molecules or the Ag(ND3)2 + complex.
5. CONCLUSIONS The exchange of Ag + ions into NaY zeolites leads to an increase in ammonia adsorption most likely due to the formation of Ag(NH3)2+-complexes in the supercage of zeolite Y. The 2H-NMR-results show that the dynamic properties of the silverdiammin-complexes formed by ion-exchange or adsorption are almost identical. They show a fast isotropic reorientation at room temperature and a rigid lattice behavior at low temperature.
ACKNOWLEDGMENTS
Support of this research is gratefully acknowledged by "Fonds der Chemischen Industrie".
REFERENCES
[ 1] I. E. Maxwell, Adv. Catal. 31 (1982) 1. [ 2] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 488. [ 3] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 480. [ 4] V. Kanazirev and N. Borisova, Zeolites 2 (1982) 23. [ 5] V. P. Shiralkar and S. B. Kulkami, J. Colloid and Interface Sci. 1.08 (1985) 1. [ 6] R. M. Barrer and R. M. Gibbons, Trans. Farad. Soc. 59 (1963) 2569. [ 7] K. Morishige, S. Kittaka and S. Ihara, J. Chem. Soc. Faraday Trans. 1 81 (1985) 2525. [ 8] V. B. Shiralkar and S. B. Kulkarni, J. Colloid. Interface. Sci. 109 (1986) 115. [ 9] H. M. Maurer and A. Weiss, J. Chem. Phys. 69 (1978) 4046. [ 10] A. F. HoUemann and N. Wiberg, Lehrbuch der Anorg. Chemic, Walter de Gryter: Berlin, 1985. [11] B. Coughlan and J. J. McEntee, Proc. A. Ir. Acad. 76B (1975) 473. [12] A. Gedeon, J. L. Bonardet and J. Fraissard, J. Chem. Soc. Faraday Trans. 86 (1990) 413. [ 13] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 11. [ 14] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 85. [ 15] R. Grol3e, J. Watermann, A. Gedeon, J. Fraissard and B. Boddenberg, Zeolites 12 (1992) 909. [ 16] P. Fletscher and R. P. Townsend, J. Chromat. 201 (1980) 93. [ 17] S. W. Rabideau and P. Waldstein, J. Chem. Phys. 46 (1966) 4600.
Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.
479
Deuteron Magnetic Resonance Studies of Ammonia in A g N a Y Zeolites
M. Hartmann* and B. Boddenberg
Lehrstuhl ~ r Physikalische Chemic II, Universit~it Dortmund, D-44221 Dortmund, Germany
The adsorption of ammonia is investigated in silver exchanged AgNaY zeolites. Silverincorporation into NaY increases the ammonia adsorption probably due to the formation of silver-diammine-complexes. 2H-NMR spectra show fast isotropic reorientations of the ammonia at room temperature, which transform into a rigid lattice behavior with decreasing temperature. Comparison with Ag(ND3)2+-complexes ion-exchanged into NaY show the same dynamic behavior giving additional evidence for a silverdiammine-complex formation upon adsorption of ammonia.
1. INTRODUCTION Transition metal ions are catalytically active in a variety of chemical reactions [1]. Incorporation of transition metal ions into zeolite cavities or channels may result in catalysts with unique properties. 2H nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the dynamics of adsorbed deuterated molecules as well as their interaction with different adsorption sites. This technique has been successfully used for studies of adsorbed benzene and propene in zeolites [2]. Ammonia may be assumed to interact specifically with the silver sites in zeolites since Ag(ND3)2+-complexes are known in solution, rendering this molecule a well-suited candidate for this study on the locations and the properties of silver ions in Y zeolites.
2. EXPERIMENTAL SECTION Starting from NaY (Union Carbide LZY-52, Si/AI=2.4) silver exchanged zeolites Ag(x)NaY with x = 14, 28, 50 and 100 % were prepared with aqueous solutions of different * Present address: Institute of Chemical Technology I, Universit~it Stuttgart, D-70550
Germany.
Stuttgart,
480 AgNO 3 concentration. The silver contents of the samples were determined by. electron microprobe analysis (EMPA). The samples were dehydrated under vacuum (p < 10-5 hPa) for 18 h at 420 ~ and subsequently oxidized for 6 h at the same temperature. The ammonia adsorption isotherms were measured volumetrically at 298 K. Atter completion of the first adsorption isotherm, the ammonia was pumped off and the sample was evacuated at 298 K for 18 h (p < 10-5 hPa). Subsequently a second isotherm was measured. For the 2H-NMR experiments the samples were pretreated as described above and ammonia-d3 (MSD, Montreal, Canada) was adsorbed up to a pressure of 100 hPa. Then the sample was evacuated overnight, sealed under cooling in liquid nitrogen and stored in the dark. The 2H-NMR spectra were recorded at a resonance frequency 0~0/2rt = 52.72 MHz using a Bruker CXP 100 spectrometer. The spectrometer as well as the measuring procedures applied have been described elsewhere [3]. For comparison an AgfND3)2Y zeolite was prepared by exchanging the complex ion Ag(ND3)2 + into the NaY zeolite. Under dry nitrogen ammonia-d3 was introduced into a solution of AgNO 3 in D20. Under stirring the addition of ammonia was performed until the solution became transparent. Now the AgfND3)2 + complex has been formed in solution. Adding a calibrated amount of NaY and additional stirring in the dark formed an Ag(ND3)2Y zeolite with an exchange degree of 55 %. The zeolite was then separated from the solution and the sample was subsequently carefully dehydrated, sealed and stored in the dark.
3. RESULTS Figure 1 shows the adsorption isotherms at 298 K of ammonia in the zeolites NaY, Ag(14)NaY, Ag(28)NaY, Ag(50)NaY and AgY (Ag(100)NaY). These isotherms were obtained for the zeolites activated as described above. The isotherms obtained after 18 h ambient temperature evacuation are only displayed for the zeolites AgY, Ag(28)NaY and Ag(14)NAY. Generally, for each zeolite the pairs of isotherms steeply increase at low pressure and run almost parallel to each other yielding difference amounts Nirr that are collected in Table 1. In comparison to NaY, Nirr is enlarged considerably up to a factor of about two in the case of the most highly silver exchanged zeolite AgY.
Table 1 Irreversible adsorbed amounts of ammonia at 298 K. sample NaY Ag(la)NaY Ag(28)NaY Ag(50)NaY AgV
Nirr/(NH3/u. c. ) 30 30 30 35 63
481 N/(NH3/u.c.) 120
.
.
.
.
.
.
.
.
.
.
~
'
~__.___@
X
~
100
"
X = 100 50
80
; X=O
60
rption
40
2O
+
NaY
- 4 - Ag(14)NAY 0
-
0
I
10
__1
20
..
!
I
I
-I
I
I
30
40
50
60
70
80
,
1
90
100
p/hPa
Figure 1. Ammonia adsorption isotherms at 298 K in silver exchanged Y zeolites Figure 2 shows the 2H-NMR spectra of ammonia in NaY zeolite at selected temperatures between 293 and 80 K. With decreasing temperature the spectra develop from singlets of increasing width into solid state powder patterns of the Pake-Type with Av = 51 kHz prominent edge splitting. T he spectral shape transition range extends from about 200 K to 100 K. The 2H-NMR spectra of (a) AgY loaded with ammonia (Nirr = 63/u.c.) and (b) Ag(ND3)2Y at selected temperatures between 290 and 80 K are compared in Figure 3. In both cases, the spectra develop in the same fashion with decreasing temperature from singlets of increasing widths into temperature-independent solid state powder pattern. From the edge sI~litting of Av = 53 kHz the deuteron quadrupolar coupling constant is readily calculated as e"~qQ/h = 71 kHz. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bonding except the C3-axis rotation proceed very slowly. The breakdown of the solid state Pake pattern into temperature dependent Lorentzian-type singlets occurs in both samples in the temperature interval between 167 and 235 K.
482
293K
250 K
200 K
167 K
/ .... 150
143K
i .... 100
i . . . . . . . . . . . . . . . . . . . 50 0 -50 -100
-1,50
50
125 K
15o
lOO
(P-Po)/kHz
1~
-so
-~oo
.50
-~
-1~
-1~
80 K
100K
1~
o
~
0
(P-iU'o)/kHz
-50
-1~
-1~
1~
1~
~
0
Figure 2.2H-NMR spectra of ammonia adsorbed in NaY zeolite (Nirr = 30 NH3/u.c. )
483
(.)
I---25 k Hz
(b)
I---! 25 k Hz
L -
, . . , _ , . ,
.
.
.
.
.
.
.
J
_
,
.
.
.
.
.
.
.
_
290K
290K
222K
222K
162K
162K
80K
80K
Figure 3.2H-NMR spectra of ammonia in (a) AgY (63 ND3/u.c.) and (b) Ag(ND3)2Y
484 4. DISCUSSION The adsorption of ammonia in NaY zeolites can be increased by ion-exchange of Ag(I) ions. With increasing silver content the overall adsorption increases (Figure 1). The amount of strong adsorbed ammonia molecules Nirr also increases with the degree of exchange. In NaY 30 NH 3 molecules can not be removed from the sample due to their adsorption on strong adsorption sites. Due to absence of strong Lewis or Broensted acid sites only the sodium cations in the supercages are able to adsorb ammonia strongly [4,5]. The sodium cations are located on site SII and adsorb ammonia with an adsorption enthalpy of about 44 kJ/mol, which is close to typical chemisorption enthalpies [6,7,8]. With increasing degree of silver exchange the amount of strongly adsorbed ammonia molecules also increases, showing that silver cations are able to adsorb more than one ammonia molecule. It is well known that Ag + form silverdiammine-complexes Ag(ND3)2 + in solution and solids [9]. It was shown previously that Cu 2+ and Ni 2+ are also able to form amine complexes in solution [10] and zeolites [11,12]. Therefore, it is very likely that Ag + also forms silverdiamminecomplexes in zeolites [11], but no experimental evidence could be presented so far. The increase in ammonia adsorption in Ag(x)NaY zeolites can very well be explained by the formation of Ag(NH3)2 + complexes in the supercages. At a low level of silver-exchange, the increase is very small, showing that most of the silver-cations are no__!tlocated in the supercage. This is in excellent agreement with X-ray and xenon adsorption data, which show a preference of Ag+ for the SI position in the double six ring [ 13]. It can be concluded from our data that not all silver ions are accessible for ammonia and only some silver ions migrate into accessible sites. Therefore the irreversible adsorbed amount of ammonia Ni~ does not increase linearly with the degree of silver exchange. In AgY 63 ammonia molecules were found to be strongly adsorbed corresponding to about 32 Ag + cations on the SII positions in the supercages. X-ray crystallographic data assign between 25 and 30 silver cations to sites in the supercage [14]. This was also confirmed by xenon adsorption and xenon NMR data [ 15]. The formation of silverdiammine-complexes in zeolites can be confirmed by our NMR data. Investigation of the ion-exchange with silverdiammine-complexes indicate that these complexes can only replace all sodium cations in the supercage, but are not able to enter the 13cage [16]. Therefore, the situation in AgfND3)2Y should be comparable to the AgY after adsorption of 64 ammonia molecules. In fact, we observed that the spectra looked almost identical at all temperatures. In NaY the spectral shape develops very slowly in a temperature interval of about 100 K. This transformation interval shortens with increasing degree of silver exchange down to 68 K for AgY and Ag(ND3)2Y. In all samples a low temperature powder pattern is observed, which is characteristic for an axially symmetric electric field gradient (EFG) tensor being operative. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bond except the C3-axis rotation proceeds very slowly [17]. The effective deuterium quadrupole coupling constant (DQCC) is 69 kHz for NaY and 71 kHz for AgY. Single crystal 2H-NMR measurements of the [Ag(ND3)2]Ag(NO2)2-complex show a DQCC of 71.6 + 0.5 MHz at room temperature [9]. Therefore, it is most likely that at least at low temperatures AgfND3)2+-complexes are also present in zeolites.
485 With increasing temperature some molecular motion runs the spectrum shapes from the slow into the fast oriental exchange limits measured on the time scale XNMR. At present it is still unclear whether this motion involves only the ammonia molecules or the Ag(ND3)2 + complex.
5. CONCLUSIONS The exchange of Ag + ions into NaY zeolites leads to an increase in ammonia adsorption most likely due to the formation of Ag(NH3)2+-complexes in the supercage of zeolite Y. The 2H-NMR-results show that the dynamic properties of the silverdiammin-complexes formed by ion-exchange or adsorption are almost identical. They show a fast isotropic reorientation at room temperature and a rigid lattice behavior at low temperature.
ACKNOWLEDGMENTS
Support of this research is gratefully acknowledged by "Fonds der Chemischen Industrie".
REFERENCES
[ 1] I. E. Maxwell, Adv. Catal. 31 (1982) 1. [ 2] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 488. [ 3] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 480. [ 4] V. Kanazirev and N. Borisova, Zeolites 2 (1982) 23. [ 5] V. P. Shiralkar and S. B. Kulkami, J. Colloid and Interface Sci. 1.08 (1985) 1. [ 6] R. M. Barrer and R. M. Gibbons, Trans. Farad. Soc. 59 (1963) 2569. [ 7] K. Morishige, S. Kittaka and S. Ihara, J. Chem. Soc. Faraday Trans. 1 81 (1985) 2525. [ 8] V. B. Shiralkar and S. B. Kulkarni, J. Colloid. Interface. Sci. 109 (1986) 115. [ 9] H. M. Maurer and A. Weiss, J. Chem. Phys. 69 (1978) 4046. [ 10] A. F. HoUemann and N. Wiberg, Lehrbuch der Anorg. Chemic, Walter de Gryter: Berlin, 1985. [11] B. Coughlan and J. J. McEntee, Proc. A. Ir. Acad. 76B (1975) 473. [12] A. Gedeon, J. L. Bonardet and J. Fraissard, J. Chem. Soc. Faraday Trans. 86 (1990) 413. [ 13] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 11. [ 14] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 85. [ 15] R. Grol3e, J. Watermann, A. Gedeon, J. Fraissard and B. Boddenberg, Zeolites 12 (1992) 909. [ 16] P. Fletscher and R. P. Townsend, J. Chromat. 201 (1980) 93. [ 17] S. W. Rabideau and P. Waldstein, J. Chem. Phys. 46 (1966) 4600.