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IR spectra of 18O exchanged HZSM-5
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H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
IR SPECTRA OF 180 EXCHANGED HZSM-5
F.BAUER 1, E.GEIDEL 2, CH.PEUKER 3 1WIP Isotopenchemie, Permoserstr. 15, 04303 Leipzig, Germany 2 Universitat Hamburg, Institut ~ r Physikalische Chemie, Bundesstr. 45, 20146 Hamburg, Germany 3 Humboldt Universitat, FB Chemie, WIP, Rudower Chaussee 6, Geb. 19.5, 12489 Berlin, Germany
INTRODUCTION IR spectroscopy is widely used in zeolite research. Its successful application is depending on a well-founded assignment of the IR bands. Beside empirical assignments some theoretical investigations are published about vibrational spectra of zeolite frameworks. The aim of this study is to obtain more detailed information on the spectrum of HZSM-5 comparing the IR spectra of 180 exchanged HZSM-5 and of HZSM-5 in the framework vibration region and the OH vibration region as well as the combination tones of the framework and the hydroxyl groups of the two samples. The experimental framework spectra have been compared with frequencies calculated for various ditetrahedra (O3Si-O-SiO3) as one of the most simple framework model. EXPERIMENT The commercial, template-free synthesized NaZSM-5 zeolite (Chemiewerke Bad KOstritz GmbI-I, Germany) with a Si/AI ratio of 15 was threefold ion exchanged with aqueous solution ofNH4NO 3 and calcined 12 hours on air at a temperature up to 820 K for yielding the H-form. Hydrothermal route/1/or exchange with 1802 gas at elevated temperatures/2/can be used for the post- synthesis 180-labeUing of zeolites. The zeolite sample was exchanged with H 2180 (95.2% 180, Chemotrade, Germany) in a water/nitrogen stream at 700 K for 1 hour. This way of exchange was chosen because some dealumination of the sample was required for further kinetic studies. The IR measurements were performed on a spectrometer IFS 66 (Bruker). The Praying Mantis DRIFT attachment (Harrick) was connected with a heated vacuum cell (Harfick) to record diffuse reflectance spectra. The samples were measured at temperatures up to 873 K under a dynamical vacuum better than 10-5 mbar. The calculation of vibrations was carried out for various ditetrahedra (O3Si-O-SiO3) by using the method of normal coordinate analysis based on Wilson's GF matrix method/3/. Structural parameters were taken from a single crystal X-ray diffraction study of Olson et al./4/of an assynthesized ZSM-5 (Si/Al=86). According to Blackwell /5/ the so-called bond length scaled force field (BLSF-BR), without adjustment of force constants, was used as model force field.
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RESULTS AND DISCUSSION The bands in the framework spectrum of the 180 exchanged HZSM-5 sample are shifted to lower wavenumbers (1062, 794, 537, 443 cm-1) compared with the HZSM-5 spectrum (1094, 797, 546, 450 cm-1). The calculation of the v~orational spectra of the ditetrahedra O3Sil-O16-Si403
and
O3Si2-O13-Si803 (Olson's indication) with 180 or with 160 atoms gave the following results. The vibration forms and the sequence of the vibrations remain with 180 exchange. All bands shif~ to lower wavenumbers due to 180 substitution. The highest difference of about 40 cm -1 was calculated between wavenumbers of the anti-symmetrical stretching vibrations of the Si-OSi bridge, which seems to be caused by the low coupling of this vibration with others. This value is in good agreement with the experimental one of 32 cm-1. Similar results concerning the band shifts due to 180 exchange were published for tridymite /6/ and quartz/7/. The DRIFT spectra of undiluted samples show shifts of the f~amework combination tone bands from 1988 cm-i to 1943 cmd and from 1874 cm-1 to 1833 cmd due to the 180 exchange, i.e. the combination tone bands shift 45 and 41 cm-1 to lower wavenumbers compared with 32 cm"1 for the band of the fundamental vibrations. The acid OH band shit, s from 3608 cm-1 to 3586 cm-1 due to the 180 exchange, the combination tone band from 4657 to 4625 cm-1. The appearance of a new band at 3657 cm -1 shows that dealumination takes place during 18O exchange. CONCLUSION Shifts of IR bands were proved due to 180 exchange for framework vibrations and for OH vibrations as well as for the combination tones. The calculated value for the shift of the antisymmetrical Si-O-Si stretching vibration is in good agreement with the experimental finding. The analysis of more extended models for the ZSM-5 fi'ameworlc, including periodic boundary conditions, will allow to compare the calculated band shifts due to 180 exchange with the experimental data for all of the framework bands. ACKNOWLEDGEMENT The authors want to thank the DFG for the support of this work. REFERENCES 1. R.von Ballmoos: The 1SO-exchange method in zeolite chemistry; Otto Salle Verlag, Frankfurt, 1981 2. S.Yang, K.D.Park, E.Oldfield; J.Am.Chem.Soc. 111 (1989) 7278 3. E.B.Wilson, Jr., J.C.Decius, P.C.Cross: Molecular Vibrations; McGraw-Hill Book Company, Inc., New York, 1955 4. D.H.Olson, G.T.Kokotailo, S.L.Lawton, W.M.Meier; J.Phys.Chem 85 (1981) 2238 5. C.S.Blackwell; J.Phys.Chem. 83 (1979) 3251; 83 (1979) 3257 6. A.M.Hofmeister et al.; J.Phys.Chem. 96 (1992) 10213 7. R.K.Sato, P.F.McMillan; J.Phys.Chem 91 (1987) 3494
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
IR SPECTRA OF 180 EXCHANGED HZSM-5
F.BAUER 1, E.GEIDEL 2, CH.PEUKER 3 1WIP Isotopenchemie, Permoserstr. 15, 04303 Leipzig, Germany 2 Universitat Hamburg, Institut ~ r Physikalische Chemie, Bundesstr. 45, 20146 Hamburg, Germany 3 Humboldt Universitat, FB Chemie, WIP, Rudower Chaussee 6, Geb. 19.5, 12489 Berlin, Germany
INTRODUCTION IR spectroscopy is widely used in zeolite research. Its successful application is depending on a well-founded assignment of the IR bands. Beside empirical assignments some theoretical investigations are published about vibrational spectra of zeolite frameworks. The aim of this study is to obtain more detailed information on the spectrum of HZSM-5 comparing the IR spectra of 180 exchanged HZSM-5 and of HZSM-5 in the framework vibration region and the OH vibration region as well as the combination tones of the framework and the hydroxyl groups of the two samples. The experimental framework spectra have been compared with frequencies calculated for various ditetrahedra (O3Si-O-SiO3) as one of the most simple framework model. EXPERIMENT The commercial, template-free synthesized NaZSM-5 zeolite (Chemiewerke Bad KOstritz GmbI-I, Germany) with a Si/AI ratio of 15 was threefold ion exchanged with aqueous solution ofNH4NO 3 and calcined 12 hours on air at a temperature up to 820 K for yielding the H-form. Hydrothermal route/1/or exchange with 1802 gas at elevated temperatures/2/can be used for the post- synthesis 180-labeUing of zeolites. The zeolite sample was exchanged with H 2180 (95.2% 180, Chemotrade, Germany) in a water/nitrogen stream at 700 K for 1 hour. This way of exchange was chosen because some dealumination of the sample was required for further kinetic studies. The IR measurements were performed on a spectrometer IFS 66 (Bruker). The Praying Mantis DRIFT attachment (Harrick) was connected with a heated vacuum cell (Harfick) to record diffuse reflectance spectra. The samples were measured at temperatures up to 873 K under a dynamical vacuum better than 10-5 mbar. The calculation of vibrations was carried out for various ditetrahedra (O3Si-O-SiO3) by using the method of normal coordinate analysis based on Wilson's GF matrix method/3/. Structural parameters were taken from a single crystal X-ray diffraction study of Olson et al./4/of an assynthesized ZSM-5 (Si/Al=86). According to Blackwell /5/ the so-called bond length scaled force field (BLSF-BR), without adjustment of force constants, was used as model force field.
111
RESULTS AND DISCUSSION The bands in the framework spectrum of the 180 exchanged HZSM-5 sample are shifted to lower wavenumbers (1062, 794, 537, 443 cm-1) compared with the HZSM-5 spectrum (1094, 797, 546, 450 cm-1). The calculation of the v~orational spectra of the ditetrahedra O3Sil-O16-Si403
and
O3Si2-O13-Si803 (Olson's indication) with 180 or with 160 atoms gave the following results. The vibration forms and the sequence of the vibrations remain with 180 exchange. All bands shif~ to lower wavenumbers due to 180 substitution. The highest difference of about 40 cm -1 was calculated between wavenumbers of the anti-symmetrical stretching vibrations of the Si-OSi bridge, which seems to be caused by the low coupling of this vibration with others. This value is in good agreement with the experimental one of 32 cm-1. Similar results concerning the band shifts due to 180 exchange were published for tridymite /6/ and quartz/7/. The DRIFT spectra of undiluted samples show shifts of the f~amework combination tone bands from 1988 cm-i to 1943 cmd and from 1874 cm-1 to 1833 cmd due to the 180 exchange, i.e. the combination tone bands shift 45 and 41 cm-1 to lower wavenumbers compared with 32 cm"1 for the band of the fundamental vibrations. The acid OH band shit, s from 3608 cm-1 to 3586 cm-1 due to the 180 exchange, the combination tone band from 4657 to 4625 cm-1. The appearance of a new band at 3657 cm -1 shows that dealumination takes place during 18O exchange. CONCLUSION Shifts of IR bands were proved due to 180 exchange for framework vibrations and for OH vibrations as well as for the combination tones. The calculated value for the shift of the antisymmetrical Si-O-Si stretching vibration is in good agreement with the experimental finding. The analysis of more extended models for the ZSM-5 fi'ameworlc, including periodic boundary conditions, will allow to compare the calculated band shifts due to 180 exchange with the experimental data for all of the framework bands. ACKNOWLEDGEMENT The authors want to thank the DFG for the support of this work. REFERENCES 1. R.von Ballmoos: The 1SO-exchange method in zeolite chemistry; Otto Salle Verlag, Frankfurt, 1981 2. S.Yang, K.D.Park, E.Oldfield; J.Am.Chem.Soc. 111 (1989) 7278 3. E.B.Wilson, Jr., J.C.Decius, P.C.Cross: Molecular Vibrations; McGraw-Hill Book Company, Inc., New York, 1955 4. D.H.Olson, G.T.Kokotailo, S.L.Lawton, W.M.Meier; J.Phys.Chem 85 (1981) 2238 5. C.S.Blackwell; J.Phys.Chem. 83 (1979) 3251; 83 (1979) 3257 6. A.M.Hofmeister et al.; J.Phys.Chem. 96 (1992) 10213 7. R.K.Sato, P.F.McMillan; J.Phys.Chem 91 (1987) 3494