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Characterisation of acid sites in decationated zeolites: Study of NH3 sorption by frequency-response technique and FTIR spectroscopy
H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)
Catalysis by Microporous Materials
116
Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All fights reserved.
Characterisation of acid sites in decationated zeolites: Study o f N H 3 sorption by frequency-response technique and FTIR spectroscopy Gy. Onyestyfika, D. Shenb and L. V. C. Reesb aCentral Research Institute for Chemistry of Hungarian Academy of Sciences, P.O.Box 17, H- 1525 Budapest, Hungary bDepartment of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, U.I~ A novel technique is suggested for measuring and interpreting data on kinetics of ammonia adsorption-desorption processes characterising acidic sites in zeolite catalysts. The frequency-response results were compared with the bands observed in FTIR spectra of numerous zeolites studied. The frequency-response method has been shown to be capable of distinguishing the different strengths and concentrations of acid sites present in various zeolites under equilibrium sorbate pressures which are more closely related to real reaction conditions. The method, therefore, has an advantage over techniques which require high vacuum conditions or low probe molecule pressures to obtain the relevant information. Adspecies detected by frequency-response technique (FRT) surely plays a role in a dynamic system Combination of this technique and other methods is necessary in order to reach at a complete description of zeolitic acid sites.
1. INTRODUCTION A "rate spectrum" characteristic of a gas/solid surface dynamic phenomenon can be obtained by the frequency response (FR) technique. Analogous to a spectroscopic method various rate processes which occur simultaneously (e. g. diffusion in micro- and macropores; adsorption and desorption on different sites; complex reactions invot~g multisteps) can be investigated and distinguished [1]. The frequency response method has been successfully applied recently to the study of mass transfer kinetics in zeolites and has become one of the most powerful experimental methods for studying intracrystalline [2] and intercrystalline [3] diffusional resistances. The technique of frequency-response chemisorption is not well known. In spite of advantages of this method, only a few papers have been published. First Naphthali and Polinski [4,5] demonstrated the usefulness of this method. Yasuda [6,7] further perfected the technique. Marcelin et. al. [8,9] and Li et. al. [10] applied this method to study chemisorption on supported catalysts. However the characterisation of the chemisorption
117 properties of catalytically active sites in zeolites has not, until now, been studied by this new dynamic method. The growing importance of zeolites in sorption and catalytic applications has induced a wide variety of techniques to be used to characterise these materials. In catalysis an important property of a zeolite is its acidity. Consequently, zeolitic acid sites have been extensively studied both in the absence and in the presence of probe molecules and reactants. Techniques for the characterisation of acid sites in zeolites have progressed considerably in the past decade [ 11-14]. In spite of all the research carried out in the area of zeolites over the last thirty years, there are still many unanswered questions concerning the nature of their acidity. The results show that in this complex field a single experimental technique or calculation covers only a small segment of the relevant chemistry. Using a multi-technique approach is recommended for the characterisation of acid sites. The use of the FK method in this field could give information on the dynamic behaviour of chemisorption sites. In principle, the frequency-response method is capable of measuring reaction rates in complex systems and after early examples [1,16,17] we recommend the adoption of this powerful method for studying reaction kinetics. The aim of this paper is study the application of the FK method to chemisorption in zeolites using ammonia as the probe molecule for the characterisation of acidic sites because of its convenience in FKT.
2. EXPE IM NTAL The theoretical solutions of the frequency response method have been comprehensively developed for the kinetic behaviour of a gas-surface system [1,6]. The frequency-response parameters (phase lag and amplitude) are derived from the equivalent fundamental sinewave perturbations by a Fourier transformations of the volume and pressure square waves. The experimental FR data, the "FR spectra" of a system, are described by the in-phase (real) and out-of phase (imaginary) characteristic functions [1]: (PB/PZ)COS~Z.B- l=E~:jr,_j/(r,..j2+to2)
(PBIPz)sin~Z.B=gr,jto/(r,.j2+m2)
(1) (2)
where ~:j/~j=(6Aj/6P)eRT/Ve, which is correlated to a gradient of an adsorption isotherm stemming from Aj; ~j.is the time constant of ad/desorption process for adsorbate on the site J; PB and PZ are the pressure responses to the +1% volume perturbations in the absence (B) and presence (Z) of sorbent and ~Z-B is the difference between phase lags. The in-phase function (Sin) tends to ~:j/~j in the lower frequency region. The phase difference and the out-phase function (8out) maxima appear at perturbation frequencies of the resonance, which are dependent on the type and strength of adsorption sites, the temperature and the pressure. The associated dynamic parameters of the FR spectra (~:'/~" and ~j which J determines local maxima on a curve of the out-of phase component) could ~e determined by fitting the characteristic functions generated by an appropriate theoretical model. The adsorption and desorption rate constants can be determined from the pressure dependence of ~j values.
118 The principle of the FR technique has been described previously [15]. The frequency window used in this study was 0.01-10Hz. ~50mg zeolite sample were placed into a sorption chamber and outgassed at 723K or lower temperature for 14 hours before carrying out ammonia sorption experiments. The NH 3 sorbate was admitted to pretreated samples and allowed to come to pressure equilibrium at 0.4; 0.7; 1.0; 1.5 and 2.0 Torr in a temperature range of 373-7231(. Measurements were carried out in the presence and absence of sorbent zeolite samples to obtain the difference of the respective FR parameters. The H-zeolite samples in the form of self-supporting wafers with ~5mg/cm2 thickness were also investigated using a Nicolet 5PC FTIR spectrometer. The IR spectra were recorded without sorbate after pretreatment in vacuum to observe the OH-bands and in presence of adsorbed ammonia after the sample had been evacuated at different temperatures to establish the NH-bands. The ammonia sorbate was from ARGO International. The ammonium form of zeolite samples were obtained after 5 times repeated exchange at reflux temperature in 1 M NH4C1 solution and the characteristics of the samples may be seen in Table 1. Table 1. Ammonium-exchanged zeolite catalysts Zeolite Si/AI NH4 + meq/g % of CEC*
A 1.1 5.48 98
X,FAU
Y,FAU
ERI
C,HEU
MOR
MFI
1.2
2.6
3.2
4.5
5.8.
33.5
5.10 97
3.22 96
2.86 93
2.10 84
1.90 90
0.43 88
* CEC=cation exchange capacity The sodium forms of zeolites were obtained mostly by courtesy of WOLFEN; A-zeolite was provided by BAYER and H-ZSM-5 by DEGUSSA from Germany. ICI U.I~ provided EU-1 samples with different Si/A1 ratio (29, 121 and 712). Clinoptilolite (C,HEU) was microcrystalline zeolitic tuff from sedimentary deposit (Horseshoe Dam, Arizona, U.S.A.) and proved to be quite pure (about 96%).
3. RESULTS AND DISCUSSION In Figure 1 the frequency response rate spectra of ammonia sorption and the FTIR spectra in the hydroxyl region of calcined and evacuated H-forms and in the 1800-1300cm-1 region in presence of adsorbed ammonia may be compared for eight different zeolite samples. Ordinates of IR spectra were prepared on the same scale (2 absorbance unit) for purpose of easier comparison. In the case of FR spectra four times (X and Y,FAU) and two times larger (Clinoptilolite) scales were necessary with the same sample mass (-50mg). The samples were pretreated at 723K in vacuum with the exception of A-zeolite (423K); Xfaujasite and erionite (523K) which are sensitive for dehydroxilation under 7231(.
119
In Figure 1 the FR spectra were recorded at 523K excepted A-zeolite (423K). However frequency-responses were well detectable for various zeolite structure of different Si/A1 ratios over a wide temperature range of 373-723I~ The FR spectra showed differencies which were of the different types of zeolites. The appearance of a peak at -~10Hz was a common feature of all samples. The response spectra of ammonia in various zeolite structures can be associated to adsorption/desorption processes on acidic sites because: (a)
(b)
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,
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.
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Fig. 1 Comparison of(a)the FR spectra ([]) (PB/Pz)cos~z.B-1 and (O) (PB/Pz)sin~Z.B for ammonia sorption at 1 Torr with (b) OH and (c) NH bands in the FTIR spectra of eight different zeolite structures. (i) the shape of response curves is characteristic of a rate-determined sorption process and the experimental data points can be fitted only by a sorption model (the intersection of inphase and out-phase characteristic functions is at the maximum of the out-phase component
121 and the maximum value of out-of-phase function is half of the maximum value of in-phase function as seen in the spectra where only one peak was observed [ERI, MOR and MFI]; (ii) if microp ore diffusion is the rate- controlling step in the crystals the response will contain only a single peak with different positions which depend strongly on the zeolite structure; (iii) if pure diffusion is a dominant process the intensities of the response functions should decrease continuously with increasing temperature whereas no great variation occured in the spectra with change of temperature. It seems that at the ammonia pressures used in these studies diffusion is not rate-determining which is not the case with Fomi et. ars ammonia TPD investigations [ 18]. The FK spectra show no indication of peaks at frequencies higher than 100Hz, i.e. there are no ad-/desorption processes with dynamic time constant smaller than 0.01s. Most probably our frequency window is wide enough to investigate all interactions, but lower frequencies should be studied. However our apparatus is limited to a maximum 100s dynamic time constant. Time constants greater than 100s are probably only important for Al-rich zeolites (A and X,FAU) which are very sensitive to dehydroxylation and are not so important in catalysis. The FR results are not correlated with the OH bands observed in the FTIK spectra of the numerous zeolites studied (second column in Fig. 1). For example X- and Y-faujasites have the same structure and OH-bands, but their FR spectra are quite strikingly different. The FK method seems to be more sensitive to distinguishing Br6nsted acid sites with different strengths especially when the dependence on temperature of the FR spectra is taken into consideration. In the third column of Fig. 1 the FTIK spectra of adsorbed ammonia in the 1800-1300 cm"1 region are shown. NH-bands at--1680cm -1 and-~1450cm -1 are assigned to ammonium ions, namely adsorbed ammonia on Br6nsted acid sites. The band at -~1630cm"1 indicates the presence of Lewis acid rites. The spectra of adsorbed ammonia are presented on A- and X-zeolites in the presence of 1 Torr of ammonia at 298K. On these two samples the intensities of the NH bands decrease on evacuation and increasing temperature. With other H-zeolites spectra show only small differences at 298K with ammonia in the cell and at 373K without ammonia in the gas phase. When the other spectra recorded under these conditions are compared the NH-bands in the IK spectra are more sensitive to interaction between ammonia and Br6nsted acid sites with different strengths; the spectra are more complex and some correlation can be observed with the FK spectra. Erionite, X- and Yfaujasites have similarities in their structure; they all contain 6-rings and have very similar IK spectra in the hydroxyl region. Erionite shows only one NH4 + IK peak and one FK peak; in the case of Y,FAU we can distinguish at least two; on X,FAU at least three peaks appear in both IK and FK spectra. Most probably the peaks in the FR spectra can be assigned to Br6nsted acidity. On A-zeolite three peaks can be observed in the FK spectra and the NH4 + IK band is quite wide and could contain three peaks. Clinoptilolite shows quite different distributions of the three peaks in the FK spectra compared with A- or X-zeolites. Clinoptilolite is of natural origin and contrary to the all other samples contains divalent cations (Ca 2+ and Mg 2+) which were not exchanged completely with NH4 +. Br6nsted acid sites can be generated by the heterolitic dissociation of water in the electric field of these cations, which can appear in the FK spectra.
122 It should be emphasised that the intensity of the FK spectrum is not proportional to the amount of the surface species as given by the intensity of the bands in the FTIR spectra. The intensity of the FK signals depends on the number of sorption sites too, but fundamentally is determined and correlated by the gradient of the adsorption isotherm on the specific sorption site. The manifestation of this fact can be observed if we compare the FK spectra of various H-zeolites. For example MOR and MFI show FR spectra with practically identical intensity in spite of the greatly different Si/A1 ratio, OH and chemisorbed ammonia concentrations. The FR technique seems to be a very sensitive method for investigation of Br6nsted acid sites. When three different H-EU-1 samples are compared the intensity of silanol OH (3745 cm"1) does not decrease while the Br6nsted acid OH decreases with the Sj/A1 ratio (29---~121---~712) and is hardly detectable in the case ofthe last sample. By the FR method the same responses were qualitatively observed with 10 times smaller intensity on the Si/AI=712 sample. This decreasing signal can be balanced with increasing sample mass.
41
3( 62
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i
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,
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0.1
1
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.
.
.
.
.
.
100 Wavenumbers
Fig. 2 Comparison of the effect of calcination temperature on the FK and IR spectra of HX,FAU. In general the Br6nsted acid sites in H-zeolites are quite stable over a wide temperature range. This fact is shown in the similar FR, spectra recorded on Y,FAU after pretreatment at 523, 623 and 723I~ The H-form of A-zeolite cannot be prepared and its FK spectrum can be recorded only below 473K. X-faujasite is also very sensitive to dehydroxylation as can be seen in Fig. 2. Increasing the pretreatment temperature from 523 to 623K produces a great change in the FK and IR spectra. This figure indicates also that with the FK method in the frequency window employed that the dynamic behaviour of Br6nsted acid sites can be
123 investigated. The changes in the FR peaks and NH4 + IR bands parallelly follow the dehydroxylation of H-X,FAU.
4. CONCLUSIONS In this work the FR technique proved to be a powerful method for the investigation of adsorbed species on different chemisorption sites. The FR method has been shown to be capable of giving information on the dynamic behaviour of sorption in zeolites. Adspecies detected by the FR technique surely play a role in this dynamic phenomenon. This new technique is proposed for measuring and interpreting data on kinetics of ammonia ad/desorption processes characterising acidic sites in zeolite catalysts. A comparison between the results of FR and FTIR "spectroscopy" is presented on eight different H-zeolites. These are only the initial exploratory studies of the application of FR method on this novel and complicated field. A study of the ~equency responses, temperature dependence of FR spectra and determination of dynamic parameters of various H-zeolites will be presented in later papers. ACKNOWLEDGEMENTS Gy6rgy Onyesty~ik would like to acknowledge the PHARE Programme Implementation Unit, the Hungarian National Committee for Technological Development (OMFB), for the fellowship (Contract No.:H9112-0169), which made this study possible. REFERENCES 1. Y. Yasuda, Heterog. Chem. Rev. 1 (1994) 103. 2. L. V. C. Rees and D. Shen, Proc. Charact. Porous solids, France, Marseilles, 1993. p563 3. Gy. Onyestyak, D. Shen and L. V. C. Kees, J. Chem. Faraday Trans. 91 (1995) 000. 4. L. M. Naphtali and L. M. Polinski, J. Phys. Chem. 67 (1963) 369. 5. L. M. Polinski and L. M. Naphtali, Advances in Catalysis 19 (1969) 241. 6. Y. Yasuda, J. Phys. Chem. 80 (1976) 1867. 7. Y. Yasuda, J. Phys. Chem. 80 (1976) 1870. 8. G. Marcelin and J. E. Lester, React. Kinet. Catal. Lett. 28 (1985) 281. 9. G. Marcelin, J. E. Lester and S. F. Mitchell, J. Catal. 102 (1986) 250. 10. Y. Li, D. Willcox and 1L D. Gonzalez, AIChE Journal 35 (1989) 423. 11. J. A. Rabo and G. J. Gajda, Catal. Rev. Sci. Eng. 31 (1989-90) 385. 12. G. Marcelin, J. Chem. Soc. Catalysis 10 (1993) 13. P. A. Jacobs, (ed. F. Delannay), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York and Basel, 1984. p367. 14. H. G. Karge, (eds. G. 0hlmarm et. al.), Catalysis and Adsorption By Zeolites, Elsevier, Amgerdam, 1986, p915. 15. N.G. van Begin and L. V. C. Rees, (eds. P. A. Jacobs and 1L A. van Santen), Zeolites: Facts, Figures, Future, Elsevier, Amgerdam, 1986, p915. 16. Y. Yasuda, J. Phys. Chem. 97 (1993) 3314. 17. Y. Yasuda and I~ Nomura, J. Phys. Chem 97 (1993) 3319 18. L. Fomi, F. P. Vatti and E. Ortoleva, Zeolites 12 (1992) 101.
Catalysis by Microporous Materials
116
Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All fights reserved.
Characterisation of acid sites in decationated zeolites: Study o f N H 3 sorption by frequency-response technique and FTIR spectroscopy Gy. Onyestyfika, D. Shenb and L. V. C. Reesb aCentral Research Institute for Chemistry of Hungarian Academy of Sciences, P.O.Box 17, H- 1525 Budapest, Hungary bDepartment of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, U.I~ A novel technique is suggested for measuring and interpreting data on kinetics of ammonia adsorption-desorption processes characterising acidic sites in zeolite catalysts. The frequency-response results were compared with the bands observed in FTIR spectra of numerous zeolites studied. The frequency-response method has been shown to be capable of distinguishing the different strengths and concentrations of acid sites present in various zeolites under equilibrium sorbate pressures which are more closely related to real reaction conditions. The method, therefore, has an advantage over techniques which require high vacuum conditions or low probe molecule pressures to obtain the relevant information. Adspecies detected by frequency-response technique (FRT) surely plays a role in a dynamic system Combination of this technique and other methods is necessary in order to reach at a complete description of zeolitic acid sites.
1. INTRODUCTION A "rate spectrum" characteristic of a gas/solid surface dynamic phenomenon can be obtained by the frequency response (FR) technique. Analogous to a spectroscopic method various rate processes which occur simultaneously (e. g. diffusion in micro- and macropores; adsorption and desorption on different sites; complex reactions invot~g multisteps) can be investigated and distinguished [1]. The frequency response method has been successfully applied recently to the study of mass transfer kinetics in zeolites and has become one of the most powerful experimental methods for studying intracrystalline [2] and intercrystalline [3] diffusional resistances. The technique of frequency-response chemisorption is not well known. In spite of advantages of this method, only a few papers have been published. First Naphthali and Polinski [4,5] demonstrated the usefulness of this method. Yasuda [6,7] further perfected the technique. Marcelin et. al. [8,9] and Li et. al. [10] applied this method to study chemisorption on supported catalysts. However the characterisation of the chemisorption
117 properties of catalytically active sites in zeolites has not, until now, been studied by this new dynamic method. The growing importance of zeolites in sorption and catalytic applications has induced a wide variety of techniques to be used to characterise these materials. In catalysis an important property of a zeolite is its acidity. Consequently, zeolitic acid sites have been extensively studied both in the absence and in the presence of probe molecules and reactants. Techniques for the characterisation of acid sites in zeolites have progressed considerably in the past decade [ 11-14]. In spite of all the research carried out in the area of zeolites over the last thirty years, there are still many unanswered questions concerning the nature of their acidity. The results show that in this complex field a single experimental technique or calculation covers only a small segment of the relevant chemistry. Using a multi-technique approach is recommended for the characterisation of acid sites. The use of the FK method in this field could give information on the dynamic behaviour of chemisorption sites. In principle, the frequency-response method is capable of measuring reaction rates in complex systems and after early examples [1,16,17] we recommend the adoption of this powerful method for studying reaction kinetics. The aim of this paper is study the application of the FK method to chemisorption in zeolites using ammonia as the probe molecule for the characterisation of acidic sites because of its convenience in FKT.
2. EXPE IM NTAL The theoretical solutions of the frequency response method have been comprehensively developed for the kinetic behaviour of a gas-surface system [1,6]. The frequency-response parameters (phase lag and amplitude) are derived from the equivalent fundamental sinewave perturbations by a Fourier transformations of the volume and pressure square waves. The experimental FR data, the "FR spectra" of a system, are described by the in-phase (real) and out-of phase (imaginary) characteristic functions [1]: (PB/PZ)COS~Z.B- l=E~:jr,_j/(r,..j2+to2)
(PBIPz)sin~Z.B=gr,jto/(r,.j2+m2)
(1) (2)
where ~:j/~j=(6Aj/6P)eRT/Ve, which is correlated to a gradient of an adsorption isotherm stemming from Aj; ~j.is the time constant of ad/desorption process for adsorbate on the site J; PB and PZ are the pressure responses to the +1% volume perturbations in the absence (B) and presence (Z) of sorbent and ~Z-B is the difference between phase lags. The in-phase function (Sin) tends to ~:j/~j in the lower frequency region. The phase difference and the out-phase function (8out) maxima appear at perturbation frequencies of the resonance, which are dependent on the type and strength of adsorption sites, the temperature and the pressure. The associated dynamic parameters of the FR spectra (~:'/~" and ~j which J determines local maxima on a curve of the out-of phase component) could ~e determined by fitting the characteristic functions generated by an appropriate theoretical model. The adsorption and desorption rate constants can be determined from the pressure dependence of ~j values.
118 The principle of the FR technique has been described previously [15]. The frequency window used in this study was 0.01-10Hz. ~50mg zeolite sample were placed into a sorption chamber and outgassed at 723K or lower temperature for 14 hours before carrying out ammonia sorption experiments. The NH 3 sorbate was admitted to pretreated samples and allowed to come to pressure equilibrium at 0.4; 0.7; 1.0; 1.5 and 2.0 Torr in a temperature range of 373-7231(. Measurements were carried out in the presence and absence of sorbent zeolite samples to obtain the difference of the respective FR parameters. The H-zeolite samples in the form of self-supporting wafers with ~5mg/cm2 thickness were also investigated using a Nicolet 5PC FTIR spectrometer. The IR spectra were recorded without sorbate after pretreatment in vacuum to observe the OH-bands and in presence of adsorbed ammonia after the sample had been evacuated at different temperatures to establish the NH-bands. The ammonia sorbate was from ARGO International. The ammonium form of zeolite samples were obtained after 5 times repeated exchange at reflux temperature in 1 M NH4C1 solution and the characteristics of the samples may be seen in Table 1. Table 1. Ammonium-exchanged zeolite catalysts Zeolite Si/AI NH4 + meq/g % of CEC*
A 1.1 5.48 98
X,FAU
Y,FAU
ERI
C,HEU
MOR
MFI
1.2
2.6
3.2
4.5
5.8.
33.5
5.10 97
3.22 96
2.86 93
2.10 84
1.90 90
0.43 88
* CEC=cation exchange capacity The sodium forms of zeolites were obtained mostly by courtesy of WOLFEN; A-zeolite was provided by BAYER and H-ZSM-5 by DEGUSSA from Germany. ICI U.I~ provided EU-1 samples with different Si/A1 ratio (29, 121 and 712). Clinoptilolite (C,HEU) was microcrystalline zeolitic tuff from sedimentary deposit (Horseshoe Dam, Arizona, U.S.A.) and proved to be quite pure (about 96%).
3. RESULTS AND DISCUSSION In Figure 1 the frequency response rate spectra of ammonia sorption and the FTIR spectra in the hydroxyl region of calcined and evacuated H-forms and in the 1800-1300cm-1 region in presence of adsorbed ammonia may be compared for eight different zeolite samples. Ordinates of IR spectra were prepared on the same scale (2 absorbance unit) for purpose of easier comparison. In the case of FR spectra four times (X and Y,FAU) and two times larger (Clinoptilolite) scales were necessary with the same sample mass (-50mg). The samples were pretreated at 723K in vacuum with the exception of A-zeolite (423K); Xfaujasite and erionite (523K) which are sensitive for dehydroxilation under 7231(.
119
In Figure 1 the FR spectra were recorded at 523K excepted A-zeolite (423K). However frequency-responses were well detectable for various zeolite structure of different Si/A1 ratios over a wide temperature range of 373-723I~ The FR spectra showed differencies which were of the different types of zeolites. The appearance of a peak at -~10Hz was a common feature of all samples. The response spectra of ammonia in various zeolite structures can be associated to adsorption/desorption processes on acidic sites because: (a)
(b)
LTA
(c)
LTA
LTA
SilAI=1.1
"~.
~
""
~ ~
3630 B
~"
4
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~
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....
/
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,
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.
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i
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.
,
120
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Wavenumbers
Fig. 1 Comparison of(a)the FR spectra ([]) (PB/Pz)cos~z.B-1 and (O) (PB/Pz)sin~Z.B for ammonia sorption at 1 Torr with (b) OH and (c) NH bands in the FTIR spectra of eight different zeolite structures. (i) the shape of response curves is characteristic of a rate-determined sorption process and the experimental data points can be fitted only by a sorption model (the intersection of inphase and out-phase characteristic functions is at the maximum of the out-phase component
121 and the maximum value of out-of-phase function is half of the maximum value of in-phase function as seen in the spectra where only one peak was observed [ERI, MOR and MFI]; (ii) if microp ore diffusion is the rate- controlling step in the crystals the response will contain only a single peak with different positions which depend strongly on the zeolite structure; (iii) if pure diffusion is a dominant process the intensities of the response functions should decrease continuously with increasing temperature whereas no great variation occured in the spectra with change of temperature. It seems that at the ammonia pressures used in these studies diffusion is not rate-determining which is not the case with Fomi et. ars ammonia TPD investigations [ 18]. The FK spectra show no indication of peaks at frequencies higher than 100Hz, i.e. there are no ad-/desorption processes with dynamic time constant smaller than 0.01s. Most probably our frequency window is wide enough to investigate all interactions, but lower frequencies should be studied. However our apparatus is limited to a maximum 100s dynamic time constant. Time constants greater than 100s are probably only important for Al-rich zeolites (A and X,FAU) which are very sensitive to dehydroxylation and are not so important in catalysis. The FR results are not correlated with the OH bands observed in the FTIK spectra of the numerous zeolites studied (second column in Fig. 1). For example X- and Y-faujasites have the same structure and OH-bands, but their FR spectra are quite strikingly different. The FK method seems to be more sensitive to distinguishing Br6nsted acid sites with different strengths especially when the dependence on temperature of the FR spectra is taken into consideration. In the third column of Fig. 1 the FTIK spectra of adsorbed ammonia in the 1800-1300 cm"1 region are shown. NH-bands at--1680cm -1 and-~1450cm -1 are assigned to ammonium ions, namely adsorbed ammonia on Br6nsted acid sites. The band at -~1630cm"1 indicates the presence of Lewis acid rites. The spectra of adsorbed ammonia are presented on A- and X-zeolites in the presence of 1 Torr of ammonia at 298K. On these two samples the intensities of the NH bands decrease on evacuation and increasing temperature. With other H-zeolites spectra show only small differences at 298K with ammonia in the cell and at 373K without ammonia in the gas phase. When the other spectra recorded under these conditions are compared the NH-bands in the IK spectra are more sensitive to interaction between ammonia and Br6nsted acid sites with different strengths; the spectra are more complex and some correlation can be observed with the FK spectra. Erionite, X- and Yfaujasites have similarities in their structure; they all contain 6-rings and have very similar IK spectra in the hydroxyl region. Erionite shows only one NH4 + IK peak and one FK peak; in the case of Y,FAU we can distinguish at least two; on X,FAU at least three peaks appear in both IK and FK spectra. Most probably the peaks in the FR spectra can be assigned to Br6nsted acidity. On A-zeolite three peaks can be observed in the FK spectra and the NH4 + IK band is quite wide and could contain three peaks. Clinoptilolite shows quite different distributions of the three peaks in the FK spectra compared with A- or X-zeolites. Clinoptilolite is of natural origin and contrary to the all other samples contains divalent cations (Ca 2+ and Mg 2+) which were not exchanged completely with NH4 +. Br6nsted acid sites can be generated by the heterolitic dissociation of water in the electric field of these cations, which can appear in the FK spectra.
122 It should be emphasised that the intensity of the FK spectrum is not proportional to the amount of the surface species as given by the intensity of the bands in the FTIR spectra. The intensity of the FK signals depends on the number of sorption sites too, but fundamentally is determined and correlated by the gradient of the adsorption isotherm on the specific sorption site. The manifestation of this fact can be observed if we compare the FK spectra of various H-zeolites. For example MOR and MFI show FR spectra with practically identical intensity in spite of the greatly different Si/A1 ratio, OH and chemisorbed ammonia concentrations. The FR technique seems to be a very sensitive method for investigation of Br6nsted acid sites. When three different H-EU-1 samples are compared the intensity of silanol OH (3745 cm"1) does not decrease while the Br6nsted acid OH decreases with the Sj/A1 ratio (29---~121---~712) and is hardly detectable in the case ofthe last sample. By the FR method the same responses were qualitatively observed with 10 times smaller intensity on the Si/AI=712 sample. This decreasing signal can be balanced with increasing sample mass.
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Fig. 2 Comparison of the effect of calcination temperature on the FK and IR spectra of HX,FAU. In general the Br6nsted acid sites in H-zeolites are quite stable over a wide temperature range. This fact is shown in the similar FR, spectra recorded on Y,FAU after pretreatment at 523, 623 and 723I~ The H-form of A-zeolite cannot be prepared and its FK spectrum can be recorded only below 473K. X-faujasite is also very sensitive to dehydroxylation as can be seen in Fig. 2. Increasing the pretreatment temperature from 523 to 623K produces a great change in the FK and IR spectra. This figure indicates also that with the FK method in the frequency window employed that the dynamic behaviour of Br6nsted acid sites can be
123 investigated. The changes in the FR peaks and NH4 + IR bands parallelly follow the dehydroxylation of H-X,FAU.
4. CONCLUSIONS In this work the FR technique proved to be a powerful method for the investigation of adsorbed species on different chemisorption sites. The FR method has been shown to be capable of giving information on the dynamic behaviour of sorption in zeolites. Adspecies detected by the FR technique surely play a role in this dynamic phenomenon. This new technique is proposed for measuring and interpreting data on kinetics of ammonia ad/desorption processes characterising acidic sites in zeolite catalysts. A comparison between the results of FR and FTIR "spectroscopy" is presented on eight different H-zeolites. These are only the initial exploratory studies of the application of FR method on this novel and complicated field. A study of the ~equency responses, temperature dependence of FR spectra and determination of dynamic parameters of various H-zeolites will be presented in later papers. ACKNOWLEDGEMENTS Gy6rgy Onyesty~ik would like to acknowledge the PHARE Programme Implementation Unit, the Hungarian National Committee for Technological Development (OMFB), for the fellowship (Contract No.:H9112-0169), which made this study possible. REFERENCES 1. Y. Yasuda, Heterog. Chem. Rev. 1 (1994) 103. 2. L. V. C. Rees and D. Shen, Proc. Charact. Porous solids, France, Marseilles, 1993. p563 3. Gy. Onyestyak, D. Shen and L. V. C. Kees, J. Chem. Faraday Trans. 91 (1995) 000. 4. L. M. Naphtali and L. M. Polinski, J. Phys. Chem. 67 (1963) 369. 5. L. M. Polinski and L. M. Naphtali, Advances in Catalysis 19 (1969) 241. 6. Y. Yasuda, J. Phys. Chem. 80 (1976) 1867. 7. Y. Yasuda, J. Phys. Chem. 80 (1976) 1870. 8. G. Marcelin and J. E. Lester, React. Kinet. Catal. Lett. 28 (1985) 281. 9. G. Marcelin, J. E. Lester and S. F. Mitchell, J. Catal. 102 (1986) 250. 10. Y. Li, D. Willcox and 1L D. Gonzalez, AIChE Journal 35 (1989) 423. 11. J. A. Rabo and G. J. Gajda, Catal. Rev. Sci. Eng. 31 (1989-90) 385. 12. G. Marcelin, J. Chem. Soc. Catalysis 10 (1993) 13. P. A. Jacobs, (ed. F. Delannay), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York and Basel, 1984. p367. 14. H. G. Karge, (eds. G. 0hlmarm et. al.), Catalysis and Adsorption By Zeolites, Elsevier, Amgerdam, 1986, p915. 15. N.G. van Begin and L. V. C. Rees, (eds. P. A. Jacobs and 1L A. van Santen), Zeolites: Facts, Figures, Future, Elsevier, Amgerdam, 1986, p915. 16. Y. Yasuda, J. Phys. Chem. 97 (1993) 3314. 17. Y. Yasuda and I~ Nomura, J. Phys. Chem 97 (1993) 3319 18. L. Fomi, F. P. Vatti and E. Ortoleva, Zeolites 12 (1992) 101.