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Adsorption calorimetric and spectroscopic studies on isomorphous substituted (Al, Fe,In, Ti) MFI zeolites
H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)
Catalysis by Microporous Materials
108
Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
Adsorption calorimetric and spectroscopic studies on isomorphous substituted (AI, Fe, In, Ti) MFI zeolites J. J:~chen,'* G. Vorbeck,' H. Stach, b B. Parlitz, ~ J.H.C. van Hooff' ' Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Analytik Umwelttechnik und Forschung GmbH, Rudower Chaussee 5, D-12489 Berlin, Germany c Institut fiir Angewandte Chemie Berlin-Adlershof e.V., Rudower Chaussee 5, D-12489 Berlin, Germany
The catalytically active sites of isomorphous substituted MFI structures have been characterized by infrared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH3 adsorption, the NH~ TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from A1 > Fe > In > > silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample.
1. INTRODUCTION The properties of catalytically active sites in zeolites can be tailored by various methods. One is changing the chemical composition of the zeolite lattice. According to the concept of the next nearest neighbours the acid site strength of a given Al rich zeolite (FAU, MOR) depends on the A1 content of the lattice. 1 Consequently, dealumination is a proper method to change acid site strength of such materials. If the A1 content is already low as in MFI where the maximum Al concentration cannot rise above a limiting value, derived from the topology of MFI, acidic strength can be changed significantly by isomorphous substitution. 2 Furthermore, this method allows to change the kind of active sites as in TS-1 for instance. 3 Therefore, we have studied the influence of the isomorphous substitution of triand tetravalent ions in MFI on the strength and kind of the active sites by TPD, IR, microcalorimetry' using ammonia and deuterated acetonitrile as probe and by chemical methods. * Present address: Analytik Umwelttechnik Forschung GmbH, Rudower Chaussee 5, D- 12489 Berlin, Germany
109 2. EXPERIMENTAL SECTION All samples have been synthesized hydrothermally in teflon lined static autoclaves under autogenous pressure at 443 K for 48 h after carefully preparation of the appropriate starting gel. As templating agent an aqueous solution of tetrapropylammonium bromide was used. Sodium aluminate for the AI-Sil, iron sulphate for the Fe-Sil and indium nitrate for the In-Sil have been added to the sodium silicate solution to get the corresponding isomorphous substituted MFI structures. As reference pure silicalite has been made from the same silica source. For more details of the synthesis procedure and the following treatments (ion exchange, calcination) see ref. 5. TS-1 has been made in a similar way according to method 1 in ref. 6 using tetraethyl orthosilicate, tetraethyl orthotitanate and tetrapropylammonium hydroxide. The temperature programmed desorption of ammonia was performed at normal pressure in a flow reactor with He as carder gas. The flow rate was 1 cm 3 s1, the NH3 concentration 3 Vol. % and the heating rate amounted to 10 K min -~. Sample weights of 200 mg were used. IR spectra were recorded with a Bruker spectrometer IFS 113v at room temperature coadding 500 scans which was equipped with a heatable vacuum cell. The samples were pressed into 7.5 mg cm 2 disks. Before measurement of the unloaded sample and CD3CN dosage the samples were activated for 1 h at 723 (In-Sil at 673 K) in high vacuum. The adsorption calorimetric measurements were carded out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h.
3. RESULTS AND DISCUSSION
According to the XRD pattern all samples are well crystallized and show the typical feature of the MFI structure. Its largely pure formation is confirmed by the results of nhexane adsorption. The values of the micropore volume (at pips = 0.5) are fairly close to the theoretical ones calculated for an ideal MFI-structure (0.19 cmVg, see Table 1). Table 1 gives the Si/Me ratios of the framework as further characteristic data. An equal concentration of Me *+ in the lattice have been strived for. However, the results of the chemically determined Me 3+ concentration and the ammonium ion exchange capacity disagree especially for the InSil. 7 It is less pronounced for Fe-Sil. Therefore the creation of extra-framework species in InSil and Fe-Sil has to be considered which do not contribute to the Br~nsted acidity but to other kinds of acidic sites. This is in agreement with the results of the TPD measurements. Figure 1 shows the TPD profiles of ammonia desorption for some MFI zeolites. A relatively high contribution to the low-temperature peak in particular for Fe-Sil is found? It is generally accepted that the low-temperature peak is due to weak acid sites such as weak Lewis and Bronsted sites or cations.' The position of this peaks in Figure 1 differ slightly only, other than the position of the high-temperature peak. The latter vary by about 50 K from Al-, Fe- to In-Sil indicating differences in strength of this strong acidic sites. A small peak for silicalite/a confirms the AI impurity found in this sample. From the area of the hightemperature peak the concentrations of the Bronsted sites have been estimated. The values are
110 Table 1 Some characteristic data of the samples used Sample
A1-Sil Fe-Sil In-Sil TS-1 Silicalite/a Silicalite/b
Si/Me ratio overall" framework 44 25 34 35 2000 oo
52 32 88 40 oo
Microp.vol. (cm3/g) 0.18 0.17 0.20 0.20 0.18
Concent. Bronsted sites (mmol/g) ion exch. TPD calorim. 0.32 0.50 0.06 -
0.28 0.41 0.20 0.03 -
0.35 0.50 0.15 0 0.05 0
"From chemical analysis, compared in Table 1 with results of the ion exchange capacity. Where as a good agreement for AI- and Fe-Sil is found the comparison for In-Sil shows a difference. This indicates that some strong Lewis sites which do not have ion exchange properties are contributing to the high-temperature peak of the TPD profile. However, microcalorimetry allows a more accurate determination of the strength and the number of acidic sites. Figure 2 shows the differential heats of adsorption of ammonia for the zeolites under discussion. Typically shaped curves for MFI zeolites are found. 8 The heat curves of A1- and Fe-Sil first decrease slightly from low to higher coverage and drop sharply until a plateau is reached with heats of about 80 kJ/mol. This is less pronounced for In-Sil. Heats of adsorption < 80 kJ/mol are due to physisorption of the ammonia with cations, terminal OH groups and likely weak Lewis s i t e s . 9 The curve of silicalite drops sharply from the beginning (130 kJ/mol) till heats of about 60 kJ/mol. This very small number of sites with high heats of adsorption is due to AI impurities in the lattice which create some Bronsted acidity. Because of the absence of cations and extraframework material in the silicalite the heat of ammonia adsorption < 60 kJ/mol represents most likely the interaction of silanol groups with the ammonia. Consequently, the part of the heat curves of the Me-Sil samples in the 80-60 kJ/mol region should be due to adsorption of the ammonia with the formed ammonium ions (former Br~msted sites) and the extraframework material such as Mehydroxide or weak Lewis sites. The following part of the heat curve at high loading than represents the interaction of the silanol groups with the ammonia as found for silicalite. Going back to the beginning of the heat curves which represents the chemisorption of the ammonia with the Br~nsted sites it can be seen that the initial heats differ for the Me-Sil investigated. The first derivative of the heat curves, dQ/da, as function of the loading, a, gives maxima at about 140 kJ/mol (AI-Sil), 128 kJ/mol (Fe-Sil) and 100 kJ/mol for In-Sil. These decreasing characteristic values represent decreasing acidic strength of the bridging OH as can be expected from the results of quantum chemical calculations. 1~This is in line with results in the literature based on IR, catalytic investigations, TM i, or adsorption calorimetric measurements of propane and butane on AI, Ga and Fe MFI zeolites. 13 In Table 1 (last column) are listed the concentrations of the strong acidic sites (Q > 80 kJ/mol, Br~nsted type) which correlate rather good with the ion exchange values again with exception of the In-Sil. As mentioned above, this can be interpreted in terms of some Lewis acidity based on IR measurements which are discussed in the following paragraphs.
111
513 AI I
', I
648
t
t I !
j
703 513
l I
I i
Figure 1. TPD profiles of ammonia chemisorbed on MFI zeolites; from top to bottom: Fe-Sil (---), A1-Sil ( - - ) , In-Sil (-"-), silicalite/a ( .... ).
o^,.
,7It ,'/493
',/ ~.
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673". '~
J4~'3"5;3 673 773 "~" '~ T (K)
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v
9O
0 7O
,,-% 50
3O 0.00
1
i
0.50
I
1.00
i
1.50
a (mmol/g)
Figure 2. Differential molar heats of chemisorption of ammonia at 423 K on MFI zeolites as function of the amount adsorbed; II AI-Sil, 9 Fe-Sil, * In-Sil, 9 silicalite/a.
112
2301
1.00
0.80
I,-
_
2265 0.60
_
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,,2300 I
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_
I
0.20
0.00
-
I
2000
I
21 O0
I
2200
2300
Wave~ber
2400
2500
cm- 1
Figure 3. IR difference spectra of the CN stretching region of CD3CN adsorbed at I mbar and 295 K on" 1, Fe-Sil; 2, AI-Sil; 3, In-Sil and 4, silicalite/a. Figure 3 displays the difference infrared spectra in the CN stretching region when CD3CN is adsorbed on the three isomorphous substituted Me-Sil and silicalite. Table 2 shows the stretching and bending wavenumbers of the Brons~ted sites before and after acetonitrile adsorption. Acetonitrile is an attractive probe molecule since it allows to discriminate between Lewis and Br~nsted acidity and to determine their acid strength, x'' 15 A number of bands can be detected in the range 2265-2330 cm -~ (Figure 3) due to the CN stretching mode of the adsorbed deuterated acetonitrile. The band at 2114 cm 1 originates from the symmetric CD stretching mode. Its position is shifted by a few cm ~ compared to the gas-phase value due to a small interaction of the molecule with the zeolite framework. 1' The band at 2265 cm 1 is due to the CN stretching mode of acetonitrile also weak bonded to the zeolite walls. The band at 2277 cm -1 is assigned to the CN stretching of acetonitrile interacting with the terminal Si-OH Table 2 Wavenumbers (in cm -I) of the Bronsted OH stretching, v(OH), and OD bending modes, di(OD), and their shifts upon adsorption of CD3CN
Sample
v(OH)
AI-Sil Fe-Sil In-Sil
3610 3631 3640
after adsorption v(OH)'
2500 2600 .
.
.
6(OD)
after adsorption di(OD) shift
894 865
988 950 .
" Center of gravity of two subbands divided by Evans window, see ref. 15.
94 85
113 groups. Both bands disappear (first the 2265 cm-~ band) after pumping off of the weakly bonded base at room temperature. As the 2277 cm" band disappear the OH stretching mode at 3745 cm-~, found for all zeolites investigated, appear again (not shown). The remaining peaks are due to stronger sites. As demonstrated in ref. 15 the signal at 2300 cm -~is characteristic for the Bronsted complex of the acetonitrile. This can be clearly seen for the A1-Sil and, due to the A1 impurity, silicalite/a (traces 2 and 4 in Figure 3). A small band at about 2330 cm~ is due to AI based Lewis sites. 15Because of the large amounts of extraframework materials in Fe-Sil and In-Sil bands for Lewis sites can be expected. However, because of their dissimilar chemical properties (ion radius and ionisation potential) ~' this bands may be found at different positions and in superposition with the Br(Jnsted band. This is indeed the case. In-Sil shows a band at 2306 cm~ due to a Lewis complex of the acetonitrile. After some desorption a small shoulder at about 2290 cm-1 can be found indicating weak Br(Jnsted acidity. On the other hand the comparable quite high intensity of the "Bronsted band" (Fe-Sil) point out that another mode contributes to this band. Desorption leads in this case to reduction of intensity on the high-frequency side and a low-frequency shift of the band to 2299 cm -~. This is not observed for A1-Sil. Obviously, the Lewis band of the Fe-Sil has a wavenumber somewhat higher than 2301 cm-~and weaker in strength than the Bronsted complex. This interpretation would be in line with the results of the adsorption calorimetric measurements (see Figure 2). Not only the CN region gives information about the acidity but also the shift of the OH stretching modes of the zeolites upon adsorption of a base (Table 2). The high-frequency shift of the v(OH) from 3610-3640 cm-~ after isomorphous substitution of the lattice corresponds with the decreasing heat of ammonia adsorption (AI> Fe>In). But more important the decreasing heats combine with a lower shift of the v(OH), see column 2 and 3 in Table 2. This is in agreement with results in ref. 16 where A1 and Ga MFI have been investigated by IR and adsorption of CO. Unfortunately, the concentration of the Bronsted sites in In-Sil is to low to detect values for the shift. Further, a lower shift of the bending modes can be detected too confn-ming the lower acidic strength of the Fe-Sil compared with A1-Sil. For this of course, it is necessary to deuterate the zeolites to make bending modes of the bridging OH observable. ~7 The substitution of Si4+ by Ti~+ in the lattice gives no strong acidic sites as could be expected. This can be concluded from the comparison of the heat curves in Figure 4. Figure 4 presents the heat curves of ammonia adsorbed on AI-Sil (with Bronsted site), TS-1 and a pure silicalite which contains terminal silanol groups alone. Consequently, only weak heats of ammonia adsorption (< 60 kJ/mol) in silicalite are found. Heats lower than 40 kJ/mol are due to physisorption on the zeolite walls which is about 15 kJ/mol higher than the heat of condensation of the ammonia at the boiling point. Incorporation of Ti into the MFI lattice leads to extra adsorption sites which give heats for ammonia between 100 and 60 kJ/mol. The amount of this sites corresponds approximately to the Ti concentration in the sample. Because of the missing Bronsted sites this sites should be due to Lewis centres which bond the ammonia coordinatively. In accordance with such an assumption Zecchina at. al reported recently ~8 in a XAFS study that Ti which is fourfold coordinated in the lattice expands its coordination sphere number by adsorption of ammonia and other polar molecules. Figure 5 shows the IR difference spectra of TS-1 after adsorption of different amounts deuterated acetonitfile which also can evidence Lewis acidity. In the OH stretching region only silanols disappear and a broad band due to the disturbed Si-OH appear upon adsorption of the base. Because of the weak acidity the shift of the Si-OH amounts to 340 cm-~compared to about 1000 cm ~ in the case of the bridging OH of zeolites (see Table 2). In the CN
114 150
130
110 o E .--) 'z
90
...,.,,,.
-,,,,.,,
0 70
v, 9
~
50
.
9
0
i
i
i
i
0.00
I
!
i
i
i
0.50
1.00
a (mmol/g)
Figure 4. Differential molar heats of chemisorption of ammonia at 423 K on M F I zeolites
as function of amount adsorbed; 9 A1-Sil, 9 TS-1, 9 silicalite/b 0.40 2265
0.30
23O0
II
0.20
o
2275 0.10
~t
2283
0.00
_~
~
.
_
-0.10 I
1500
I
I
I
I
2000
2500
3000
3500
Waventrnber cm-
4000
1
Figure 5. IR difference spectra of TS-1 with decreasing amount adsorbed of CD3CN; from top to bottom: adsorption at 1 mbar and 0.05 mbar, desorption at room temperature (15 min.), 353 K (1 h) and 573 K (1 h).
115 stretching region four bands appear with increasing loading of the base. The first very small band (2283 cm-1), due to some stronger sites, can not yet be identified. The following bands at 2300 and 2275 cm-1 can be assigned to coordinatively bonded acetonitrile in accordance with the XAFS results and the complex with the Si-OH, respectively. The remaining signal at 2265 cm-1 appears last due to the weak physisorbed acetonitrile as described above. Summarizing it can be concluded that incorporation of trivalent cations with increasing ion radius into the MFI lattice results in decreasing acidic Bronsted centres. The introduction of Ti4+ into MFI leads to sites which bond bases coordinatively.
ACKNOWLEDGEMENT
The preparation of the TS-1 sample by Arjan van der Pol (TU Eindhoven) and the kind experimental support in the IR by Jos van Wolput (TU Eindhoven) are gratefully acknowledged.
REFERENCES
1. D. Barthomeuf, Mat. Chem. Phys., 17 (1987)64. 2. J.C. Vedrine, in Zeolites Chemistry and Catalysis ed. P.A. Jacobs, N.I. Jaeger, L. Kubelkov~i and B. Wichterlov~i, Elsevier, Amsterdam, 1991, p. 25. 3. G. Belussi and V. Fattore, in Zeolites Chemistry and Catalysis ed. P.A. Jacobs, N.I. Jaeger, L. Kubelkov~i and B. Wichterlov~i, Elsevier, Amsterda..m, 1991, p. 79. 4. H.G. Karge, in Catalysis and Adsorption by Zeolites, ed. G. Ohlmann, H. Pfeifer and R. Fricke, Elsevier, Amsterdam, 1991, p. 133. 5. G. Vorbeck, M. Richter, R. Fricke, B. Parlitz, E. Schreier,..K. Szulzewsky and B. Zibrowius, in Catalysis and Adsorption by Zeolites, ed. G. Ohlmann, H. Pfeifer and R. Fricke, Elsevier, Amsterdam, 1991, p. 631. 6. A.J.H.P. van der Pol and J.H.C. van Hooff, Appl. Catal., 92 (1992) 93. 7. G. Vorbeck, J. J/inchen, B. Parlitz, M. Schneider and R. Fricke, J. Chem. Soc. Chem. Commun., 1994, 123. 8. D.J. Parrillo and R.J. Gorte, Catal. Lett. 16 (1992) 17; L.J.Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053. 9. A. Auroux, V. Bolis, P. Wierzchowski, P.C. Gravelle and J.C. Vedrine, J. Chem. Soc. Faraday Trans. 2, 75 (1979) 2544. 10. J. Sauer, Chem. Rev. 89 (1989) 199. 11. M.F.M. Post, T. Huizinga, C.A. Emis, J.M. Nanne and W.H.J. Stork, in Zeolites as Catalysts, Sorbents and Detergent Builders, ed H.G. Karge and J. Weitl~mp, Elsevier, Amsterdam, 1989, p. 367. 12. J. Cejka, A. Vondrov~, B. Wichterlov~i, G. Vorbeck and R. Fricke, Zeolites, 14 (1994) 147. 13. A. Auroux, A. Tuel, J. Bandiera and Y. Ben Taarit, Appl. Catal., 93 (1993) 181. 14. E. Paukshtis, E.N. Yurchenko, Russ. Chem. Rev., 52 (1983) 242. 15. A.G. Pelmenschikov, R.A. van Santen, J. J~_nchen and E. Meijer, J. Phys. Chem. 97 (1993) 11071. 16. I. Mirsojew, S. Ernst, J. Weitkamp and H. Kn6zinger, Catal. Lett. 24 (1994) 235. 17. W.P.J.H. Jacobs, J.H.M.C. van Wolput, R.A. van Santen, and H. Jobic, Zeolites, 14 (1994) 117. 18. S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, C. Lamberti, G. Leofantini, L. Marchese, G. Pertini, G. Vlaic and A. Zecchina, Catal. Lett. 26 (1994) 195.
Catalysis by Microporous Materials
108
Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
Adsorption calorimetric and spectroscopic studies on isomorphous substituted (AI, Fe, In, Ti) MFI zeolites J. J:~chen,'* G. Vorbeck,' H. Stach, b B. Parlitz, ~ J.H.C. van Hooff' ' Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Analytik Umwelttechnik und Forschung GmbH, Rudower Chaussee 5, D-12489 Berlin, Germany c Institut fiir Angewandte Chemie Berlin-Adlershof e.V., Rudower Chaussee 5, D-12489 Berlin, Germany
The catalytically active sites of isomorphous substituted MFI structures have been characterized by infrared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH3 adsorption, the NH~ TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from A1 > Fe > In > > silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample.
1. INTRODUCTION The properties of catalytically active sites in zeolites can be tailored by various methods. One is changing the chemical composition of the zeolite lattice. According to the concept of the next nearest neighbours the acid site strength of a given Al rich zeolite (FAU, MOR) depends on the A1 content of the lattice. 1 Consequently, dealumination is a proper method to change acid site strength of such materials. If the A1 content is already low as in MFI where the maximum Al concentration cannot rise above a limiting value, derived from the topology of MFI, acidic strength can be changed significantly by isomorphous substitution. 2 Furthermore, this method allows to change the kind of active sites as in TS-1 for instance. 3 Therefore, we have studied the influence of the isomorphous substitution of triand tetravalent ions in MFI on the strength and kind of the active sites by TPD, IR, microcalorimetry' using ammonia and deuterated acetonitrile as probe and by chemical methods. * Present address: Analytik Umwelttechnik Forschung GmbH, Rudower Chaussee 5, D- 12489 Berlin, Germany
109 2. EXPERIMENTAL SECTION All samples have been synthesized hydrothermally in teflon lined static autoclaves under autogenous pressure at 443 K for 48 h after carefully preparation of the appropriate starting gel. As templating agent an aqueous solution of tetrapropylammonium bromide was used. Sodium aluminate for the AI-Sil, iron sulphate for the Fe-Sil and indium nitrate for the In-Sil have been added to the sodium silicate solution to get the corresponding isomorphous substituted MFI structures. As reference pure silicalite has been made from the same silica source. For more details of the synthesis procedure and the following treatments (ion exchange, calcination) see ref. 5. TS-1 has been made in a similar way according to method 1 in ref. 6 using tetraethyl orthosilicate, tetraethyl orthotitanate and tetrapropylammonium hydroxide. The temperature programmed desorption of ammonia was performed at normal pressure in a flow reactor with He as carder gas. The flow rate was 1 cm 3 s1, the NH3 concentration 3 Vol. % and the heating rate amounted to 10 K min -~. Sample weights of 200 mg were used. IR spectra were recorded with a Bruker spectrometer IFS 113v at room temperature coadding 500 scans which was equipped with a heatable vacuum cell. The samples were pressed into 7.5 mg cm 2 disks. Before measurement of the unloaded sample and CD3CN dosage the samples were activated for 1 h at 723 (In-Sil at 673 K) in high vacuum. The adsorption calorimetric measurements were carded out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h.
3. RESULTS AND DISCUSSION
According to the XRD pattern all samples are well crystallized and show the typical feature of the MFI structure. Its largely pure formation is confirmed by the results of nhexane adsorption. The values of the micropore volume (at pips = 0.5) are fairly close to the theoretical ones calculated for an ideal MFI-structure (0.19 cmVg, see Table 1). Table 1 gives the Si/Me ratios of the framework as further characteristic data. An equal concentration of Me *+ in the lattice have been strived for. However, the results of the chemically determined Me 3+ concentration and the ammonium ion exchange capacity disagree especially for the InSil. 7 It is less pronounced for Fe-Sil. Therefore the creation of extra-framework species in InSil and Fe-Sil has to be considered which do not contribute to the Br~nsted acidity but to other kinds of acidic sites. This is in agreement with the results of the TPD measurements. Figure 1 shows the TPD profiles of ammonia desorption for some MFI zeolites. A relatively high contribution to the low-temperature peak in particular for Fe-Sil is found? It is generally accepted that the low-temperature peak is due to weak acid sites such as weak Lewis and Bronsted sites or cations.' The position of this peaks in Figure 1 differ slightly only, other than the position of the high-temperature peak. The latter vary by about 50 K from Al-, Fe- to In-Sil indicating differences in strength of this strong acidic sites. A small peak for silicalite/a confirms the AI impurity found in this sample. From the area of the hightemperature peak the concentrations of the Bronsted sites have been estimated. The values are
110 Table 1 Some characteristic data of the samples used Sample
A1-Sil Fe-Sil In-Sil TS-1 Silicalite/a Silicalite/b
Si/Me ratio overall" framework 44 25 34 35 2000 oo
52 32 88 40 oo
Microp.vol. (cm3/g) 0.18 0.17 0.20 0.20 0.18
Concent. Bronsted sites (mmol/g) ion exch. TPD calorim. 0.32 0.50 0.06 -
0.28 0.41 0.20 0.03 -
0.35 0.50 0.15 0 0.05 0
"From chemical analysis, compared in Table 1 with results of the ion exchange capacity. Where as a good agreement for AI- and Fe-Sil is found the comparison for In-Sil shows a difference. This indicates that some strong Lewis sites which do not have ion exchange properties are contributing to the high-temperature peak of the TPD profile. However, microcalorimetry allows a more accurate determination of the strength and the number of acidic sites. Figure 2 shows the differential heats of adsorption of ammonia for the zeolites under discussion. Typically shaped curves for MFI zeolites are found. 8 The heat curves of A1- and Fe-Sil first decrease slightly from low to higher coverage and drop sharply until a plateau is reached with heats of about 80 kJ/mol. This is less pronounced for In-Sil. Heats of adsorption < 80 kJ/mol are due to physisorption of the ammonia with cations, terminal OH groups and likely weak Lewis s i t e s . 9 The curve of silicalite drops sharply from the beginning (130 kJ/mol) till heats of about 60 kJ/mol. This very small number of sites with high heats of adsorption is due to AI impurities in the lattice which create some Bronsted acidity. Because of the absence of cations and extraframework material in the silicalite the heat of ammonia adsorption < 60 kJ/mol represents most likely the interaction of silanol groups with the ammonia. Consequently, the part of the heat curves of the Me-Sil samples in the 80-60 kJ/mol region should be due to adsorption of the ammonia with the formed ammonium ions (former Br~msted sites) and the extraframework material such as Mehydroxide or weak Lewis sites. The following part of the heat curve at high loading than represents the interaction of the silanol groups with the ammonia as found for silicalite. Going back to the beginning of the heat curves which represents the chemisorption of the ammonia with the Br~nsted sites it can be seen that the initial heats differ for the Me-Sil investigated. The first derivative of the heat curves, dQ/da, as function of the loading, a, gives maxima at about 140 kJ/mol (AI-Sil), 128 kJ/mol (Fe-Sil) and 100 kJ/mol for In-Sil. These decreasing characteristic values represent decreasing acidic strength of the bridging OH as can be expected from the results of quantum chemical calculations. 1~This is in line with results in the literature based on IR, catalytic investigations, TM i, or adsorption calorimetric measurements of propane and butane on AI, Ga and Fe MFI zeolites. 13 In Table 1 (last column) are listed the concentrations of the strong acidic sites (Q > 80 kJ/mol, Br~nsted type) which correlate rather good with the ion exchange values again with exception of the In-Sil. As mentioned above, this can be interpreted in terms of some Lewis acidity based on IR measurements which are discussed in the following paragraphs.
111
513 AI I
', I
648
t
t I !
j
703 513
l I
I i
Figure 1. TPD profiles of ammonia chemisorbed on MFI zeolites; from top to bottom: Fe-Sil (---), A1-Sil ( - - ) , In-Sil (-"-), silicalite/a ( .... ).
o^,.
,7It ,'/493
',/ ~.
I\
673". '~
J4~'3"5;3 673 773 "~" '~ T (K)
150 I 130
110 o E --3 _,,%
v
9O
0 7O
,,-% 50
3O 0.00
1
i
0.50
I
1.00
i
1.50
a (mmol/g)
Figure 2. Differential molar heats of chemisorption of ammonia at 423 K on MFI zeolites as function of the amount adsorbed; II AI-Sil, 9 Fe-Sil, * In-Sil, 9 silicalite/a.
112
2301
1.00
0.80
I,-
_
2265 0.60
_
TM
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,,2300 I
0.40
_
I
0.20
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-
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2000
I
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2200
2300
Wave~ber
2400
2500
cm- 1
Figure 3. IR difference spectra of the CN stretching region of CD3CN adsorbed at I mbar and 295 K on" 1, Fe-Sil; 2, AI-Sil; 3, In-Sil and 4, silicalite/a. Figure 3 displays the difference infrared spectra in the CN stretching region when CD3CN is adsorbed on the three isomorphous substituted Me-Sil and silicalite. Table 2 shows the stretching and bending wavenumbers of the Brons~ted sites before and after acetonitrile adsorption. Acetonitrile is an attractive probe molecule since it allows to discriminate between Lewis and Br~nsted acidity and to determine their acid strength, x'' 15 A number of bands can be detected in the range 2265-2330 cm -~ (Figure 3) due to the CN stretching mode of the adsorbed deuterated acetonitrile. The band at 2114 cm 1 originates from the symmetric CD stretching mode. Its position is shifted by a few cm ~ compared to the gas-phase value due to a small interaction of the molecule with the zeolite framework. 1' The band at 2265 cm 1 is due to the CN stretching mode of acetonitrile also weak bonded to the zeolite walls. The band at 2277 cm -1 is assigned to the CN stretching of acetonitrile interacting with the terminal Si-OH Table 2 Wavenumbers (in cm -I) of the Bronsted OH stretching, v(OH), and OD bending modes, di(OD), and their shifts upon adsorption of CD3CN
Sample
v(OH)
AI-Sil Fe-Sil In-Sil
3610 3631 3640
after adsorption v(OH)'
2500 2600 .
.
.
6(OD)
after adsorption di(OD) shift
894 865
988 950 .
" Center of gravity of two subbands divided by Evans window, see ref. 15.
94 85
113 groups. Both bands disappear (first the 2265 cm-~ band) after pumping off of the weakly bonded base at room temperature. As the 2277 cm" band disappear the OH stretching mode at 3745 cm-~, found for all zeolites investigated, appear again (not shown). The remaining peaks are due to stronger sites. As demonstrated in ref. 15 the signal at 2300 cm -~is characteristic for the Bronsted complex of the acetonitrile. This can be clearly seen for the A1-Sil and, due to the A1 impurity, silicalite/a (traces 2 and 4 in Figure 3). A small band at about 2330 cm~ is due to AI based Lewis sites. 15Because of the large amounts of extraframework materials in Fe-Sil and In-Sil bands for Lewis sites can be expected. However, because of their dissimilar chemical properties (ion radius and ionisation potential) ~' this bands may be found at different positions and in superposition with the Br(Jnsted band. This is indeed the case. In-Sil shows a band at 2306 cm~ due to a Lewis complex of the acetonitrile. After some desorption a small shoulder at about 2290 cm-1 can be found indicating weak Br(Jnsted acidity. On the other hand the comparable quite high intensity of the "Bronsted band" (Fe-Sil) point out that another mode contributes to this band. Desorption leads in this case to reduction of intensity on the high-frequency side and a low-frequency shift of the band to 2299 cm -~. This is not observed for A1-Sil. Obviously, the Lewis band of the Fe-Sil has a wavenumber somewhat higher than 2301 cm-~and weaker in strength than the Bronsted complex. This interpretation would be in line with the results of the adsorption calorimetric measurements (see Figure 2). Not only the CN region gives information about the acidity but also the shift of the OH stretching modes of the zeolites upon adsorption of a base (Table 2). The high-frequency shift of the v(OH) from 3610-3640 cm-~ after isomorphous substitution of the lattice corresponds with the decreasing heat of ammonia adsorption (AI> Fe>In). But more important the decreasing heats combine with a lower shift of the v(OH), see column 2 and 3 in Table 2. This is in agreement with results in ref. 16 where A1 and Ga MFI have been investigated by IR and adsorption of CO. Unfortunately, the concentration of the Bronsted sites in In-Sil is to low to detect values for the shift. Further, a lower shift of the bending modes can be detected too confn-ming the lower acidic strength of the Fe-Sil compared with A1-Sil. For this of course, it is necessary to deuterate the zeolites to make bending modes of the bridging OH observable. ~7 The substitution of Si4+ by Ti~+ in the lattice gives no strong acidic sites as could be expected. This can be concluded from the comparison of the heat curves in Figure 4. Figure 4 presents the heat curves of ammonia adsorbed on AI-Sil (with Bronsted site), TS-1 and a pure silicalite which contains terminal silanol groups alone. Consequently, only weak heats of ammonia adsorption (< 60 kJ/mol) in silicalite are found. Heats lower than 40 kJ/mol are due to physisorption on the zeolite walls which is about 15 kJ/mol higher than the heat of condensation of the ammonia at the boiling point. Incorporation of Ti into the MFI lattice leads to extra adsorption sites which give heats for ammonia between 100 and 60 kJ/mol. The amount of this sites corresponds approximately to the Ti concentration in the sample. Because of the missing Bronsted sites this sites should be due to Lewis centres which bond the ammonia coordinatively. In accordance with such an assumption Zecchina at. al reported recently ~8 in a XAFS study that Ti which is fourfold coordinated in the lattice expands its coordination sphere number by adsorption of ammonia and other polar molecules. Figure 5 shows the IR difference spectra of TS-1 after adsorption of different amounts deuterated acetonitfile which also can evidence Lewis acidity. In the OH stretching region only silanols disappear and a broad band due to the disturbed Si-OH appear upon adsorption of the base. Because of the weak acidity the shift of the Si-OH amounts to 340 cm-~compared to about 1000 cm ~ in the case of the bridging OH of zeolites (see Table 2). In the CN
114 150
130
110 o E .--) 'z
90
...,.,,,.
-,,,,.,,
0 70
v, 9
~
50
.
9
0
i
i
i
i
0.00
I
!
i
i
i
0.50
1.00
a (mmol/g)
Figure 4. Differential molar heats of chemisorption of ammonia at 423 K on M F I zeolites
as function of amount adsorbed; 9 A1-Sil, 9 TS-1, 9 silicalite/b 0.40 2265
0.30
23O0
II
0.20
o
2275 0.10
~t
2283
0.00
_~
~
.
_
-0.10 I
1500
I
I
I
I
2000
2500
3000
3500
Waventrnber cm-
4000
1
Figure 5. IR difference spectra of TS-1 with decreasing amount adsorbed of CD3CN; from top to bottom: adsorption at 1 mbar and 0.05 mbar, desorption at room temperature (15 min.), 353 K (1 h) and 573 K (1 h).
115 stretching region four bands appear with increasing loading of the base. The first very small band (2283 cm-1), due to some stronger sites, can not yet be identified. The following bands at 2300 and 2275 cm-1 can be assigned to coordinatively bonded acetonitrile in accordance with the XAFS results and the complex with the Si-OH, respectively. The remaining signal at 2265 cm-1 appears last due to the weak physisorbed acetonitrile as described above. Summarizing it can be concluded that incorporation of trivalent cations with increasing ion radius into the MFI lattice results in decreasing acidic Bronsted centres. The introduction of Ti4+ into MFI leads to sites which bond bases coordinatively.
ACKNOWLEDGEMENT
The preparation of the TS-1 sample by Arjan van der Pol (TU Eindhoven) and the kind experimental support in the IR by Jos van Wolput (TU Eindhoven) are gratefully acknowledged.
REFERENCES
1. D. Barthomeuf, Mat. Chem. Phys., 17 (1987)64. 2. J.C. Vedrine, in Zeolites Chemistry and Catalysis ed. P.A. Jacobs, N.I. Jaeger, L. Kubelkov~i and B. Wichterlov~i, Elsevier, Amsterdam, 1991, p. 25. 3. G. Belussi and V. Fattore, in Zeolites Chemistry and Catalysis ed. P.A. Jacobs, N.I. Jaeger, L. Kubelkov~i and B. Wichterlov~i, Elsevier, Amsterda..m, 1991, p. 79. 4. H.G. Karge, in Catalysis and Adsorption by Zeolites, ed. G. Ohlmann, H. Pfeifer and R. Fricke, Elsevier, Amsterdam, 1991, p. 133. 5. G. Vorbeck, M. Richter, R. Fricke, B. Parlitz, E. Schreier,..K. Szulzewsky and B. Zibrowius, in Catalysis and Adsorption by Zeolites, ed. G. Ohlmann, H. Pfeifer and R. Fricke, Elsevier, Amsterdam, 1991, p. 631. 6. A.J.H.P. van der Pol and J.H.C. van Hooff, Appl. Catal., 92 (1992) 93. 7. G. Vorbeck, J. J/inchen, B. Parlitz, M. Schneider and R. Fricke, J. Chem. Soc. Chem. Commun., 1994, 123. 8. D.J. Parrillo and R.J. Gorte, Catal. Lett. 16 (1992) 17; L.J.Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053. 9. A. Auroux, V. Bolis, P. Wierzchowski, P.C. Gravelle and J.C. Vedrine, J. Chem. Soc. Faraday Trans. 2, 75 (1979) 2544. 10. J. Sauer, Chem. Rev. 89 (1989) 199. 11. M.F.M. Post, T. Huizinga, C.A. Emis, J.M. Nanne and W.H.J. Stork, in Zeolites as Catalysts, Sorbents and Detergent Builders, ed H.G. Karge and J. Weitl~mp, Elsevier, Amsterdam, 1989, p. 367. 12. J. Cejka, A. Vondrov~, B. Wichterlov~i, G. Vorbeck and R. Fricke, Zeolites, 14 (1994) 147. 13. A. Auroux, A. Tuel, J. Bandiera and Y. Ben Taarit, Appl. Catal., 93 (1993) 181. 14. E. Paukshtis, E.N. Yurchenko, Russ. Chem. Rev., 52 (1983) 242. 15. A.G. Pelmenschikov, R.A. van Santen, J. J~_nchen and E. Meijer, J. Phys. Chem. 97 (1993) 11071. 16. I. Mirsojew, S. Ernst, J. Weitkamp and H. Kn6zinger, Catal. Lett. 24 (1994) 235. 17. W.P.J.H. Jacobs, J.H.M.C. van Wolput, R.A. van Santen, and H. Jobic, Zeolites, 14 (1994) 117. 18. S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, C. Lamberti, G. Leofantini, L. Marchese, G. Pertini, G. Vlaic and A. Zecchina, Catal. Lett. 26 (1994) 195.