- Email: [email protected]
FER-Type catalyst: Synthesis and characterization
I. Kiricsi, G. P~il-Borb61y, 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.
FER-TYPE
CATALYST:
SYNTHESIS
77
AND CHARACTERIZATION
F. Cosentino a, A. Katovic a, G. Giordano a, P. Lentz b and J. B.Nagy b Dipartimento di Ingegneria Chimica e dei Materiali, Universit/t della Calabria, via P. Bucci, 1-87030 RENDE (CS), Italy a
b Laboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix rue de Bruxelles 61, B-5000 NAMUR, Belgium
This work deals with the synthesis and characterisation of zeolite FER from aluminosilicate hydrogels containing cetyltrimethylammonium bromide (CTAB), a low cost organic compound. The range of the experimental conditions was found to be rather narrow, but once the zeolite FER has been formed it does not transform into other zeolitic or dense phases. The production of MTBE, a compound used in new gasoline formula in order to substitute the aromatics, is limited by the iso-olefin availability. Recently, it has been shown that the zeolite FER is an excellent catalyst for olefin (C4, C5) isomerization [ 1].
1. INTRODUCTION The resolution of the pollution problems connected with emission of exhaust from motor vehicles has been attempted by changing the gasoline formula, firstly by introducing aromatics as high octane additive and afterwards replacing them with less toxic compounds, i.e. branched oxygenated alkanes such as methyl ethyl tert-buthyl ether (MTBE). The MTBE production is based on the direct reaction between methanol and an iso-olefin (C4). The limiting factor in supplying MTBE is the availability of the iso-butene. The most interesting way for the production of the aforementioned feedstock seems to be the direct isomerization of n-butene. On the other hand, the medium-pore zeolites are among the most suitable catalysts in the isomerization processes [2]. The medium-pore high silica zeolite FER has been claimed to be both highly selective and stable in the catalytic isomerization of n-butene. The production of C5+ compounds is drastically reduced when this type of zeolite is used as catalyst [ 1]. The zeolite FER, as well as almost all zeolite types, can be obtained in the presence of a variety of organic compounds, i.e. diaminoalkanes, pyrrolidine, cyclic amines, quaternary ammonium compounds (TMA) [3]. In this study, the choice of the organic compound, cetyltrimethylammonium bromide (CTAB), was based on its low price and the possibility to confront the final physical and chemical properties of FER zeolite samples obtained from
78 reaction systems prepared with two different organic compounds, one long(Ctr) and bulky (-N+(CH3)3 end group) and the other short (C2) with two amino groups, the diaminoethane. CTAOH was previously used as an organic template to synthesize zeolite ZSM-35 [4]. However, the obtained phases, both as synthesized and calcined one, were not pure crystalline FER phases. In order to obtain more information on the constituents of the tetrahedral framework (structural units Si(nAl); Si and AI site types) as well as the state of the compensating cations (inorganic and organic) in the zeolite structure, the FER samples were characterized by MAS NMR techniques [5-7].
2. EXPERIMENTAL The molar composition of the starting hydrogels containing cetyltrimethylammonium bromide was: x Na20 : y CTAB : z AI203 : 1 SiO 2 : 50 H20 where x = 0.12 - 0.2, y = 0.035 - 0.32 and z = 0.00625 - 0.05. A reference sample of zeolite FER was prepared in the presence of ethylenediamine (C2DN) and according to [8]: 0.1 Na20:1.3 C2DN :0.065 AI203 : 1 SiO2 : 39 H20. The hydrogels were obtained by slowly adding a colloidal silica type Ludox AS-40 (DuPont) to a previously prepared homogeneous solution consisting of the organic compound (Aldrich), sodium hydroxide (Carlo Erba), sodium aluminate (Carlo Erba) and distilled water. The hydrothermal syntheses were performed under static conditions at 170 ~ whereas the crystallisation time varied as a function of the initial hydrogel composition depending mainly on the aluminium content.
Table 1. Recording conditions for the MAS NMR spectra.
Chemical shift standard Frequency (MHz) Recycle time (s) Pulse width (~ts) Contact time (ms) Spinning rate (kHz) Number of scans
13C
23Na
27A1
29Si
TMS 100.61 4 4.5 5 3.8 10-20000
Na(H20)6 + 105.84 0.1 1.66 10 8192
AI(H20)63+ 104.26 0.1 1 10 5000
TMS 39.74 6 6 3.8 4800
79 The solid products were recovered in a usual manner and checked by powder X-ray diffraction (XRD). The zeolite FER samples were further characterised by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), thermal analysis and sorption measurements. 29Si, 27A1, 23Na and 13C MAS-NMR spectra of representative as-made FER and H-FER samples were recorded either on a Bruker MSL 400 or a Bruker CXP 200 spectrometer and the recording conditions are given in Table 1.
3. RESULTS AND DISCUSSION 3.1. Synthesis conditions and samples characterisation The preparation of pure zeolite FER (see Figure 1) from hydrogels containing CTAB is possible in a very narrow range of experimental conditions (Table 2). The undesired products during the FER synthesis are mordenite, a zeolite with a similar Si/AI ratio, and the dense phases, cristobalite and quartz. Although the zeolite FER is formed at relatively low CTA§ ratios, it is necessary to increase the organic content in order to prevent the contamination of the zeolite product by dense phases. The co-crystallisation of zeolite MOR is observed when the aluminium content in the starting hydrogel is relatively high and the sodium hydroxide content is higher than 0.2 (sample n ~ 4). It is necessary to increase the organic amount to avoid the MOR formation from the hydrogels having a low Si/AI ratio. This results in an increase of the crystallisation time that cannot be reduced by increasing the alkalinity of the initial hydrogel. On the other hand, if the amount of CTAB in the hydrogel is too high, the final product contains only the FER zeolitic phase but its yield remains less than 100 % even aider long reaction periods.
I/I0
5
10
15
20
25
30
35
40
2 theta (~ Figure 1. XRD patterns ofFER sample n ~ 10 (bottom spectrum: as-made sample and upper spectrum: calcined sample)
80 Table 2. Products obtained from the hydrogels containin[~ CTAB. N~ (Si/Al)g~ (CTA§ ~ to, days Products 1 2 3 4 5 6 7 8
40 0.07 40 0.16 40 0.2 20 0.07 20 0.28 12.5 0.2 10 0.2 10 0.32 cr = cristobalite; Q= quartz
6 7 7 12 16 9 12 30
FER + cr FER + cr FER + cr FER + MOR + Q + cr FER FER FER FER + amorphous
When the only product of the synthesis is FER (samples n ~ 5 - 7), no transformation of phases is observed after prolonged crystallisation times (> 20 days). It can be seen from Table 3 that the sample of zeolite FER obtained from a hydrogel containing CTAB has a similar value of the (Si/Al)zeoliteratio as the sample prepared from the reaction mixture in the presence of C2DN. Thus it is possible to obtain FER crystals with high Al/u.c. values even if the choice of the starting conditions is limited. In fact, it seems that in the presence of CTA § ions more aluminium is incorporated in the zeolite framework starting from a hydrogel with a definitely higher Si/AI ratio. The plate-like crystals that are typical of zeolite FER are observed for both crystallisation systems but with significant differences in size. Smaller FER crystals having a slightly higher amount of aluminium per unit cell are obtained in the presence of CTAB. The values of the specific surface area and the micropore volume obtained by BET measurements confirm that the longer and bulkier organic molecule is also completely removed from the zeolitic channels after the thermal treatment. The thermal stability of the zeolite FER was ascertained from the XRD patterns recorded after the calcination of the samples at 600 ~ overnight.
3.2. MAS NMR Spectroscopy Two zeolite FER samples with a similar aluminium content per unit cell (see Table 3) were chosen for further characterisation by MAS NMR spectroscopy, one prepared in the presence of CTAB (n ~ 6, Table 2) and the reference sample obtained from a hydrogel containing C2DN, as well as the corresponding H-forms of these samples
Table 3. Physicochemical characterisation of two FER samples. Organic (Si/Al)gel A1/u.c. d, lain s.s.a., m2/g CTAB (n~ C2DN
12.5 7.7
4.91 4.10 ....
1-2 15
362 358
micropore volume 0.19cm3/g 0.13 cm3/~
81
B
"" " ' ~ " " ;&"""" ~""" "~,""" "k;'" : ~ " "-;.~ "~;,;.,~
.... & ; ' " :;&,"-i~ i;0"~
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I~
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~
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Figure 2.27A1 NMR spectra of samples synthesised from gels containing CTAB (A: as-made; B H-form) and C2DN (C as-made; D: H-form).
3.2.1.27A! NMR For the as-synthesised form of both FER samples only one NMR line corresponding to the tetrahedral framework aluminium is detected (Figure 2), respectively at 53.9 ppm for the sample synthesised in presence of C2DN and 51.5 ppm in the case of CTAB. The spectra of the H-forms display a second M R line at 0 ppm (C2DN) and at 1.1 ppm (CTAB) related to extra-framework octahedral Al sites. At the same time the broadening of the Altet line towards higher fields (shoulder at -~ 30 ppm) is related to the deformation of some tetrahedral aluminium sites. These sites are still tetrahedral aluminium sites, where one A1-O-Si bond has been broken. All this indicates that the dealumination during the thermal treatment of the NH4-FER occurs in about equal amount for both samples.
82
3.2.2. ZVSi NMR Figure 3. shows the 29Si NMR spectra of the as-made and the calcined samples synthesized from the hydrogels containing CTAB or C2DN. The complex pattern observed in the spectra of the as-made sample is the result of the superimposition of lines stemming from silicon atoms in the four crystallographic sites of the FER, having zero or one aluminium atom as second neighbour. The lines due to Si atoms having an AI in their second coordination sphere are always shifted to higher field values and mix with the lines stemming from defect silanol groups. In our case, a rather high amount of defect groups are present at -99 ppm and -105 ppm in the C2DN-FER sample, the amount of which decreases during calcination. The -99 ppm defect groups are probably of the Si(OH)2 type, while the -105 ppm line is due to SiOH defect groups. The CTAB-FER as-made sample contains less defect groups.
r;l
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|
-80 |
.-80 ,
.
-90 ,
|
9 ,,
-I~)0
-~10
.
-100 ,
"110 |
.
-110 ,
9
-I~K)
,
-120 ,
9
!
-II30 ;pPm)
' -~o ' -~o ' -~o ' -~o'
-130 (pore) , ,.
. ~ o ,
-~ 0 .
-1~ 0 .
-110 ,
!
-;2o
.
-120 ~
-~o
.
?o~ml
-1h .0 (ppm)
Figure 3.29Si N]V[R spectra of samples synthesised from gels containing CTAB (A" as-made; B" H-form) and C2DN (C as-made; D H-form).
83
9
r
"
&
"
~,
"
~,
"
&
"
&
'
~L
'~
Figure 4. tsC NMR spectrum of the as-made sample synthesised from the gel containing CTAB. After calcination, the 29Si NMR spectra are almost identical, which indicates that the product characteristics regarding the silicon environment does not depend on the nature of the organic compound used for the synthesis. The drastic decrease in intensity in the region from -105 to -l 10 ppm is explained first by the decrease of the amount of defect groups, but mostly by the dealumination process (Figure 3).
3 . 2 . 3 . tSC N M R
The ~3C NMR spectrum of the as-synthesised FER zeolite prepared in the presence of C2DN consists of only one sharp line at 40.1 ppm and it is in good agreement with the chemical shift determined for the pure liquid ethylenediamine (44.3 ppm). This indicates that the organic molecule remains intact within the channels of the zeolite. On the contrary, the ~3C NMR spectra of the FER sample synthesised with CTAB compared to the spectra of pure CTAB show some differences (Figure 4). The line stemming from the -N+(CH3)3 group at 54 ppm is very small and the 69 ppm line due to CH2-N+- groups is also very small. In addition, new lines appear at 60.4, 57.8, 55.6, 47.0 and 45.7 ppm. This spectral modification shows unambiguously the partial deamination of the CTA + ions during the hydrothermal treatment. Indeed, the line at 45.7 ppm can be attributed to (CH3)3N and that at 55.6 ppm to (CH3)4N+.
3.2.4. 23Na NMR
The 23Na NMR spectra put in evidence the role of the sodium as countercation in asmade samples The position (- l I ppm) and the narrowness of the resonance line indicates
84 the presence of the Na § in hydrated state. In case of the H-FER samples it can be seen that the remaining sodium cations are in quite different chemical environment after the thermal treatment. The NMR line is shifted to the higher field (ca. at -22 ppm) and it becomes quite broad and asymmetrical with a shoulder at - 10 ppm. As the organic molecules are completely removed from the zeolitic framework (shown by the high s.s.a, and micropore volume), it is likely that the remaining sodium cations are bonded to extraframework aluminium species.
4. CONCLUSIONS FER zeolite samples have been successfully synthesised in the presence of both diaminoethane and cetyltrimethylammonium bromide. While the as-made samples contain quite different amounts of defect groups, the calcined samples are almost identical. The framework tetrahedral aluminium of the as-made sample is partially eliminated as extraframework octahedral species during calcination, while most of the aluminium atoms either remain in the tetrahedral framework position or turn into deformed tetrahedral species. On the other hand, the presence of the CTA § ions in the initial hydrogel favours the incorporation of aluminium into the framework while the crystals of the so-obtained zeolite FER are small (1 - 2 lam). Since the size of the crystals has an important role in the zeolitecatalysed processes, the smaller zeolite crystals, provided that their thermal stability is satisfactory, are preferred in these applications [9, 10].
ACKNOWLEDGEMENTS P. Lentz gratefully acknowledges financial support from FRIA, Belgium.
REFERENCES
1. H.H. Mooiwer, K.P. De Jong, B. Kraushaar-Czametzki, W.M.J. Stork, B.C. Krutzen, Stud. Surf. Sci. Catal. 84 (1994)2327. J. Weitkamp, U. Weil3, S. Ernst, Stud. Surf. Sci. Catal. 94 (1995) 363. 3. P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 220. 4. R.B. Borade, A. Clearfield, Zeolites 14 (1994) 458 5. G. Engelhardt, Stud. Surf. Sci. Catal. 58 (1991) 285. 6. J.B. Nagy, Z. Gabelica, G. Debras, P. Bodard, E.G. Derouane, P.A. Jacobs, J. Mol. Catal. 20 (1983) 327. 7. C.J. Plank, E.J. Rosinski, M.K. Rubin, US Patent 4,016,245 (1977). 8. J.B. Nagy, P. Bodard, I. Hannus, I. Kiricsi, Synthesis, Characterization and Use of Zeolitic Microporous Materials, DecaGen Ltd., Szeged, 1998. 9. A. Corma, Chem. Rev. 95 (1995) 559. 10. M.A. Camblor, A. Corma, A. Martinez, F.A. Mocholi, J. Perez-Pariente, Appl. Catal. 55(1989)65. .
FER-TYPE
CATALYST:
SYNTHESIS
77
AND CHARACTERIZATION
F. Cosentino a, A. Katovic a, G. Giordano a, P. Lentz b and J. B.Nagy b Dipartimento di Ingegneria Chimica e dei Materiali, Universit/t della Calabria, via P. Bucci, 1-87030 RENDE (CS), Italy a
b Laboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix rue de Bruxelles 61, B-5000 NAMUR, Belgium
This work deals with the synthesis and characterisation of zeolite FER from aluminosilicate hydrogels containing cetyltrimethylammonium bromide (CTAB), a low cost organic compound. The range of the experimental conditions was found to be rather narrow, but once the zeolite FER has been formed it does not transform into other zeolitic or dense phases. The production of MTBE, a compound used in new gasoline formula in order to substitute the aromatics, is limited by the iso-olefin availability. Recently, it has been shown that the zeolite FER is an excellent catalyst for olefin (C4, C5) isomerization [ 1].
1. INTRODUCTION The resolution of the pollution problems connected with emission of exhaust from motor vehicles has been attempted by changing the gasoline formula, firstly by introducing aromatics as high octane additive and afterwards replacing them with less toxic compounds, i.e. branched oxygenated alkanes such as methyl ethyl tert-buthyl ether (MTBE). The MTBE production is based on the direct reaction between methanol and an iso-olefin (C4). The limiting factor in supplying MTBE is the availability of the iso-butene. The most interesting way for the production of the aforementioned feedstock seems to be the direct isomerization of n-butene. On the other hand, the medium-pore zeolites are among the most suitable catalysts in the isomerization processes [2]. The medium-pore high silica zeolite FER has been claimed to be both highly selective and stable in the catalytic isomerization of n-butene. The production of C5+ compounds is drastically reduced when this type of zeolite is used as catalyst [ 1]. The zeolite FER, as well as almost all zeolite types, can be obtained in the presence of a variety of organic compounds, i.e. diaminoalkanes, pyrrolidine, cyclic amines, quaternary ammonium compounds (TMA) [3]. In this study, the choice of the organic compound, cetyltrimethylammonium bromide (CTAB), was based on its low price and the possibility to confront the final physical and chemical properties of FER zeolite samples obtained from
78 reaction systems prepared with two different organic compounds, one long(Ctr) and bulky (-N+(CH3)3 end group) and the other short (C2) with two amino groups, the diaminoethane. CTAOH was previously used as an organic template to synthesize zeolite ZSM-35 [4]. However, the obtained phases, both as synthesized and calcined one, were not pure crystalline FER phases. In order to obtain more information on the constituents of the tetrahedral framework (structural units Si(nAl); Si and AI site types) as well as the state of the compensating cations (inorganic and organic) in the zeolite structure, the FER samples were characterized by MAS NMR techniques [5-7].
2. EXPERIMENTAL The molar composition of the starting hydrogels containing cetyltrimethylammonium bromide was: x Na20 : y CTAB : z AI203 : 1 SiO 2 : 50 H20 where x = 0.12 - 0.2, y = 0.035 - 0.32 and z = 0.00625 - 0.05. A reference sample of zeolite FER was prepared in the presence of ethylenediamine (C2DN) and according to [8]: 0.1 Na20:1.3 C2DN :0.065 AI203 : 1 SiO2 : 39 H20. The hydrogels were obtained by slowly adding a colloidal silica type Ludox AS-40 (DuPont) to a previously prepared homogeneous solution consisting of the organic compound (Aldrich), sodium hydroxide (Carlo Erba), sodium aluminate (Carlo Erba) and distilled water. The hydrothermal syntheses were performed under static conditions at 170 ~ whereas the crystallisation time varied as a function of the initial hydrogel composition depending mainly on the aluminium content.
Table 1. Recording conditions for the MAS NMR spectra.
Chemical shift standard Frequency (MHz) Recycle time (s) Pulse width (~ts) Contact time (ms) Spinning rate (kHz) Number of scans
13C
23Na
27A1
29Si
TMS 100.61 4 4.5 5 3.8 10-20000
Na(H20)6 + 105.84 0.1 1.66 10 8192
AI(H20)63+ 104.26 0.1 1 10 5000
TMS 39.74 6 6 3.8 4800
79 The solid products were recovered in a usual manner and checked by powder X-ray diffraction (XRD). The zeolite FER samples were further characterised by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), thermal analysis and sorption measurements. 29Si, 27A1, 23Na and 13C MAS-NMR spectra of representative as-made FER and H-FER samples were recorded either on a Bruker MSL 400 or a Bruker CXP 200 spectrometer and the recording conditions are given in Table 1.
3. RESULTS AND DISCUSSION 3.1. Synthesis conditions and samples characterisation The preparation of pure zeolite FER (see Figure 1) from hydrogels containing CTAB is possible in a very narrow range of experimental conditions (Table 2). The undesired products during the FER synthesis are mordenite, a zeolite with a similar Si/AI ratio, and the dense phases, cristobalite and quartz. Although the zeolite FER is formed at relatively low CTA§ ratios, it is necessary to increase the organic content in order to prevent the contamination of the zeolite product by dense phases. The co-crystallisation of zeolite MOR is observed when the aluminium content in the starting hydrogel is relatively high and the sodium hydroxide content is higher than 0.2 (sample n ~ 4). It is necessary to increase the organic amount to avoid the MOR formation from the hydrogels having a low Si/AI ratio. This results in an increase of the crystallisation time that cannot be reduced by increasing the alkalinity of the initial hydrogel. On the other hand, if the amount of CTAB in the hydrogel is too high, the final product contains only the FER zeolitic phase but its yield remains less than 100 % even aider long reaction periods.
I/I0
5
10
15
20
25
30
35
40
2 theta (~ Figure 1. XRD patterns ofFER sample n ~ 10 (bottom spectrum: as-made sample and upper spectrum: calcined sample)
80 Table 2. Products obtained from the hydrogels containin[~ CTAB. N~ (Si/Al)g~ (CTA§ ~ to, days Products 1 2 3 4 5 6 7 8
40 0.07 40 0.16 40 0.2 20 0.07 20 0.28 12.5 0.2 10 0.2 10 0.32 cr = cristobalite; Q= quartz
6 7 7 12 16 9 12 30
FER + cr FER + cr FER + cr FER + MOR + Q + cr FER FER FER FER + amorphous
When the only product of the synthesis is FER (samples n ~ 5 - 7), no transformation of phases is observed after prolonged crystallisation times (> 20 days). It can be seen from Table 3 that the sample of zeolite FER obtained from a hydrogel containing CTAB has a similar value of the (Si/Al)zeoliteratio as the sample prepared from the reaction mixture in the presence of C2DN. Thus it is possible to obtain FER crystals with high Al/u.c. values even if the choice of the starting conditions is limited. In fact, it seems that in the presence of CTA § ions more aluminium is incorporated in the zeolite framework starting from a hydrogel with a definitely higher Si/AI ratio. The plate-like crystals that are typical of zeolite FER are observed for both crystallisation systems but with significant differences in size. Smaller FER crystals having a slightly higher amount of aluminium per unit cell are obtained in the presence of CTAB. The values of the specific surface area and the micropore volume obtained by BET measurements confirm that the longer and bulkier organic molecule is also completely removed from the zeolitic channels after the thermal treatment. The thermal stability of the zeolite FER was ascertained from the XRD patterns recorded after the calcination of the samples at 600 ~ overnight.
3.2. MAS NMR Spectroscopy Two zeolite FER samples with a similar aluminium content per unit cell (see Table 3) were chosen for further characterisation by MAS NMR spectroscopy, one prepared in the presence of CTAB (n ~ 6, Table 2) and the reference sample obtained from a hydrogel containing C2DN, as well as the corresponding H-forms of these samples
Table 3. Physicochemical characterisation of two FER samples. Organic (Si/Al)gel A1/u.c. d, lain s.s.a., m2/g CTAB (n~ C2DN
12.5 7.7
4.91 4.10 ....
1-2 15
362 358
micropore volume 0.19cm3/g 0.13 cm3/~
81
B
"" " ' ~ " " ;&"""" ~""" "~,""" "k;'" : ~ " "-;.~ "~;,;.,~
.... & ; ' " :;&,"-i~ i;0"~
" " i k ; "" ; ~ " ' ~ ; ' " ~
@
|lWWlW
....1509....tO0|.... ~3.... ol.....~, ....... -I~ -I~ '(~)
I~
www#www
1oo
w
w|l
~
lllll~(~
o
-
| l|ll!
'"-;oo-lsoW~l
Figure 2.27A1 NMR spectra of samples synthesised from gels containing CTAB (A: as-made; B H-form) and C2DN (C as-made; D: H-form).
3.2.1.27A! NMR For the as-synthesised form of both FER samples only one NMR line corresponding to the tetrahedral framework aluminium is detected (Figure 2), respectively at 53.9 ppm for the sample synthesised in presence of C2DN and 51.5 ppm in the case of CTAB. The spectra of the H-forms display a second M R line at 0 ppm (C2DN) and at 1.1 ppm (CTAB) related to extra-framework octahedral Al sites. At the same time the broadening of the Altet line towards higher fields (shoulder at -~ 30 ppm) is related to the deformation of some tetrahedral aluminium sites. These sites are still tetrahedral aluminium sites, where one A1-O-Si bond has been broken. All this indicates that the dealumination during the thermal treatment of the NH4-FER occurs in about equal amount for both samples.
82
3.2.2. ZVSi NMR Figure 3. shows the 29Si NMR spectra of the as-made and the calcined samples synthesized from the hydrogels containing CTAB or C2DN. The complex pattern observed in the spectra of the as-made sample is the result of the superimposition of lines stemming from silicon atoms in the four crystallographic sites of the FER, having zero or one aluminium atom as second neighbour. The lines due to Si atoms having an AI in their second coordination sphere are always shifted to higher field values and mix with the lines stemming from defect silanol groups. In our case, a rather high amount of defect groups are present at -99 ppm and -105 ppm in the C2DN-FER sample, the amount of which decreases during calcination. The -99 ppm defect groups are probably of the Si(OH)2 type, while the -105 ppm line is due to SiOH defect groups. The CTAB-FER as-made sample contains less defect groups.
r;l
" | |
|
-80 |
.-80 ,
.
-90 ,
|
9 ,,
-I~)0
-~10
.
-100 ,
"110 |
.
-110 ,
9
-I~K)
,
-120 ,
9
!
-II30 ;pPm)
' -~o ' -~o ' -~o ' -~o'
-130 (pore) , ,.
. ~ o ,
-~ 0 .
-1~ 0 .
-110 ,
!
-;2o
.
-120 ~
-~o
.
?o~ml
-1h .0 (ppm)
Figure 3.29Si N]V[R spectra of samples synthesised from gels containing CTAB (A" as-made; B" H-form) and C2DN (C as-made; D H-form).
83
9
r
"
&
"
~,
"
~,
"
&
"
&
'
~L
'~
Figure 4. tsC NMR spectrum of the as-made sample synthesised from the gel containing CTAB. After calcination, the 29Si NMR spectra are almost identical, which indicates that the product characteristics regarding the silicon environment does not depend on the nature of the organic compound used for the synthesis. The drastic decrease in intensity in the region from -105 to -l 10 ppm is explained first by the decrease of the amount of defect groups, but mostly by the dealumination process (Figure 3).
3 . 2 . 3 . tSC N M R
The ~3C NMR spectrum of the as-synthesised FER zeolite prepared in the presence of C2DN consists of only one sharp line at 40.1 ppm and it is in good agreement with the chemical shift determined for the pure liquid ethylenediamine (44.3 ppm). This indicates that the organic molecule remains intact within the channels of the zeolite. On the contrary, the ~3C NMR spectra of the FER sample synthesised with CTAB compared to the spectra of pure CTAB show some differences (Figure 4). The line stemming from the -N+(CH3)3 group at 54 ppm is very small and the 69 ppm line due to CH2-N+- groups is also very small. In addition, new lines appear at 60.4, 57.8, 55.6, 47.0 and 45.7 ppm. This spectral modification shows unambiguously the partial deamination of the CTA + ions during the hydrothermal treatment. Indeed, the line at 45.7 ppm can be attributed to (CH3)3N and that at 55.6 ppm to (CH3)4N+.
3.2.4. 23Na NMR
The 23Na NMR spectra put in evidence the role of the sodium as countercation in asmade samples The position (- l I ppm) and the narrowness of the resonance line indicates
84 the presence of the Na § in hydrated state. In case of the H-FER samples it can be seen that the remaining sodium cations are in quite different chemical environment after the thermal treatment. The NMR line is shifted to the higher field (ca. at -22 ppm) and it becomes quite broad and asymmetrical with a shoulder at - 10 ppm. As the organic molecules are completely removed from the zeolitic framework (shown by the high s.s.a, and micropore volume), it is likely that the remaining sodium cations are bonded to extraframework aluminium species.
4. CONCLUSIONS FER zeolite samples have been successfully synthesised in the presence of both diaminoethane and cetyltrimethylammonium bromide. While the as-made samples contain quite different amounts of defect groups, the calcined samples are almost identical. The framework tetrahedral aluminium of the as-made sample is partially eliminated as extraframework octahedral species during calcination, while most of the aluminium atoms either remain in the tetrahedral framework position or turn into deformed tetrahedral species. On the other hand, the presence of the CTA § ions in the initial hydrogel favours the incorporation of aluminium into the framework while the crystals of the so-obtained zeolite FER are small (1 - 2 lam). Since the size of the crystals has an important role in the zeolitecatalysed processes, the smaller zeolite crystals, provided that their thermal stability is satisfactory, are preferred in these applications [9, 10].
ACKNOWLEDGEMENTS P. Lentz gratefully acknowledges financial support from FRIA, Belgium.
REFERENCES
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