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Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
861
Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy Béatrice Moulin1, Laetitia Oliviero1, Françoise Maugé1, Jean-François Groust2, Jean-Marc Krafft2, Guylène Costentin2, Pascale Massiani2 1
LCS, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal Juin, F-14050 Caen LRS, Université Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05
2
Abstract Three protic probe molecules were used to characterize by FTIR spectroscopy the strength, concentration and environment of basic sites in series of faujasites (FAU structure) in which either the Al content (Y, X and LSX) or the nature of the framework charge compensating alkali cation (Na, K, Cs) was varied. The frequency shifts observed for adsorbed MBOH and adsorbed methylacetylene well account for variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites. Lastly H2S dissociation brings quantitative information on the amount of basic sites. The results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. Keywords: basicity, faujasite, zeolite, MBOH, methylacetylene, H2S.
1. Introduction Brønsted organic acids have been first used as probe molecules to characterize surface basicity [1]. They were most often strong acids able to react with both strong and weak basic surfaces, not allowing discrimination between sites of different basic strengths. This explains the frequent use of CO2 as probe molecule for basicity characterization even if reactive adsorption on strong basic sites limits its application. Weaker Brønsted acids such as water, alcohols, alkenes and alkynes that do not interact specifically with basic sites but rather with acid/base pairs can then be used. Also, some authors have proposed others interesting probes to study basic sites, among which 2-methyl-3-butyn2-ol (MBOH) [2], H2S [3] or methylacetylene [4]. Thus, MBOH decomposes into acetone and acetylene on basic sites whereas acidic and amphoteric surfaces lead to different reaction products. In the same way, adsorbed H2S and methylacetylene lead to dissociated and non-dissociated species depending on the basic properties of the surface. In this study, the efficiency of these three probe molecules towards characterization by IR spectroscopy of basic sites in zeolites is compared. To this aim, we chose the faujasite structure (FAU) as a representative zeolite in which basicity is known to vary when changing the Al content and/or the nature of the charge compensating alkali cation [5].
862
B. Moulin et al.
2. Experimental The faujasite samples were two commercial NaY (Si/Al~2.5) and NaX (Si/Al~1.3) zeolites, and a low silica X (LSX, Si/Al=1) that was synthesized and exchanged in solution by various alkali ions (M=Na, K, Cs) according to known procedures. Table 1 reports the chemical compositions for all samples as well as the resulting mean negative charges on framework oxygens evaluated using the Sanderson electronegativity method [6]. Adsorption of the probe molecules was followed by DRIFT operando at 413 K in the case of MBOH, after pre-treatment at 773K under O2 flow, and by transmission IR experiments in the cases of H2S and methylacetylene by adsorbing increasing small doses (7.5 μmol.g-1) of the probe at room temperature on pellets of zeolites previously evacuated at 673 K. Before introduction of H2S and methylacetylene into the IR cell, the gas was submitted to cryogenic trapping to remove traces of water. Zeolites NaY NaX NaLSX KLSX CsLSX
Chemical composition Na55Al55Si137O384 Na85Al85Si107O384 Na96Al96Si96O384 K83Na13Al96Si96O384 Cs58Na38Al96Si96O384
Si/Al ratio 2.5 1.3 1.0 1.0 1.0
δO -0.359 -0.420 -0.440 -0.484 -0.496
Table 1 – Chemical compositions and mean negative charges on framework oxygens of the zeolites under study
3. Results and discussion 3.1. Activated zeolites After activation, the IR spectra of all samples present different residual vibration bands. Bands detected at 1481 and 1429 cm-1 correspond to carbonate species in a quasisymmetric configuration. These carbonates species are eliminated almost completely by evacuation of the zeolite at higher temperature (773 K) or by treating the sample under O2 at 673 K. In addition, a sharp OH band at ~3689 cm-1 is detected as well as two broad bands centred at 3385 and 3235 cm-1. The bands in the 3400-3200 cm-1 region are associated to O-H vibrations of water in interaction by H-bonding with a framework oxygen atom. The band at 3689 cm-1 should correspond to water coordinated to charge compensating alkali ions. The presence of non-dissociated water is also confirmed by the detection of a weak band at ~1650 cm-1 characteristic of water δ(HOH) vibration. Note that water was always detected on the surface of these basic zeolites, although present to a very small extent. 3.2. MBOH adsorption Previous studies [7] proposed various adsorption modes for MBOH on basic oxides. By examining the vibration frequencies of the adsorbed species formed during MBOH adsorption on zeolitic surfaces, it has been possible to show that MBOH is adsorbed mainly either via the MBOH alcohol function interacting with both a framework oxygen atom and a cation (fig 1 - type I), or via a direct interaction between the acetylenic hydrogen and framework oxygen atom (fig 1 - type II). Both adsorption modes require a strong basic O2- Lewis site. Note that type I adsorption implies the acid-base pair Mδ+/O2δ- of the faujasite while type II adsorption implies only the framework oxygen atom.
Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy
863
For the three studied faujasites (NaY, NaX, NaLSX), analogous vibration bands are observed which agree with the MBOH adsorption modes described above. However, both the intensities and the wavenumbers of the vibration bands vary, depending on the sample. It is now well established (Lavalley, 1996) that displacements of IR bands due to the adsorption of molecules on basic sites can be used to assess the strength of basic centres of oxides. In the present study, the displacements of both the ν(OH) and ν(≡CH) vibrations of MBOH related to type I and type II adsorptions, respectively, allow us to rank the basicity of the three faujasites. The ν(OH) wavenumber shifts from 3644 cm-1 in the gas phase to 3504 cm-1 when adsorbed on NaY and down to 3465 cm-1 when interacting with NaLSX. As for the ν(≡CH) vibration, its frequency shifts from 3313 cm-1 in the gas phase to 3278 cm-1 when adsorbed on NaY and down to 3262 cm-1 when interacting with NaLSX (Fig. 2A). Hence, the lower the ν(OH) and the ν(≡CH) vibrations, the greater the basicity of the zeolites. This leads to a first classification of the zeolites that is in line with the basicity order NaY < NaX < NaLSX expected from the respective Si/Al ratios of the samples and consequently mean negative charges on oxygens (Table 1).
Figure 1. Modes of MBOH adsorption on basic zeolites
3.3. Methylacetylene and H2S adsorption Methylacetylene and H2S were used to precise various basic features in the LSX family (experiments have also been done on NaX and NaY but will not be discussed here). It is known [4] that methylacetylene can be adsorbed dissociatively or not. The dissociation of methylacetylene is characterized by the appearance of a typical ν(OH) vibration. Therefore, the absence of any zeolitic ν(OH) band for the different LSX samples indicates that methylacetylene adsorbs mainly without dissociation. The presence of non-dissociated adsorbed methylacetylene is also evidenced by the detection of specific ν(C≡C) and ν(≡CH) vibrations band. As expected, their wavenumbers decrease with increasing basicity. Moreover, the complex shape of the ν(≡CH) band reveals different environments of basic sites. H2S adsorption has already been used in our group to probe basic sites of Y zeolites [3]. H2S can adsorb dissociatively or non-dissociatively. H2S dissociates on a strong basic framework oxygen atom and cationic sites, leading to the formation of zeolitic OH groups, whereas undissociated H2S can interact either by coordination with the cation or by H-bonding. On the whole series of LSX zeolites, H2S dissociates leading to the appearance of a ν(OH) band that is shifted from 3640 cm-1 on NaLSX (less basic), to 3667 cm-1 on KLSX and finally to 3678 cm-1 on CsLSX (more basic). In addition, the intensity of this band strongly increases from Na to Cs (Fig 2B), indicating that the fraction of basic framework oxygen atoms presenting a basicity high enough to dissociate H2S increases from Na to Cs, in line with the increase of the mean basicity determined by the Sanderson electronegativity method.
864
B. Moulin et al.
NaY
-1
-1
3490
500
3276
NCH (cm )
NaX
NOH (cm )
3500
3280
A
3272
3480
3268
3470
3264
3460 0,35
NaLSX 0,37
0,39
0,41
DOZ-
0,43
3260 0,45
I N(OH) (cm-1)
3510
CsLSX
B
400 300 200
KLSX
100
NaLSX
0 0,4
0,42
0,44
0,46
-
0,48
0,5
0,52
DOZ
Figure 2. (A) ν(OH) () and the ν(≡CH) () wavenumbers of MBOH adsorbed on zeolites as a function of Sanderson electronegativity; (B) Intensity of ν(OH) band created by dissociated H2S as a function of Sanderson electronegativity
Thus, methylacetylene and H2S lead to a classification of the zeolite basicity: NaLSX < KLSX < CsLSX that again, well accounts for the change of the negative charge of the basic framework oxygen atoms with change of the nature of the cations.
4. Conclusion This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites.
5. Acknowledgments The authors thank the Agence Nationale de la Recherche for its financial support (ANR BASICAT NT05-3_44329). J.L.Paillaud is acknowledged for his advices for the synthesis of LSX.
References [1] [2] [3] [4] [5] [6] [7]
J.C. Lavalley, Catal. Today, 27(3-4) (1996) 377. H. Lauron-Pernot, F. Luck, J.M. Popa, Appl. Catal., 78 (1991) 213. F. Maugé, A. Sahibed-Dine, M. Gaillard, M. Ziolek, J. Catal., 207 (2002) 353. D. Mordenti, P. Grotz, H. Knozinger, Catal. Today, 70 (2001) 83. D. Barthomeuf, Catal. Rev., 4 (1996) 521. R.T. Sanderson, Electronegativity and bond energyJ. Am. Chem. Soc., 105(8) (1983) 2259. N.E. Fouad, P. Thomasson, H. Knozinger, Appl. catal., 194-195 (2000) 213.
861
Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy Béatrice Moulin1, Laetitia Oliviero1, Françoise Maugé1, Jean-François Groust2, Jean-Marc Krafft2, Guylène Costentin2, Pascale Massiani2 1
LCS, ENSICAEN, Université de Caen, CNRS, 6 Bd Maréchal Juin, F-14050 Caen LRS, Université Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05
2
Abstract Three protic probe molecules were used to characterize by FTIR spectroscopy the strength, concentration and environment of basic sites in series of faujasites (FAU structure) in which either the Al content (Y, X and LSX) or the nature of the framework charge compensating alkali cation (Na, K, Cs) was varied. The frequency shifts observed for adsorbed MBOH and adsorbed methylacetylene well account for variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites. Lastly H2S dissociation brings quantitative information on the amount of basic sites. The results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. Keywords: basicity, faujasite, zeolite, MBOH, methylacetylene, H2S.
1. Introduction Brønsted organic acids have been first used as probe molecules to characterize surface basicity [1]. They were most often strong acids able to react with both strong and weak basic surfaces, not allowing discrimination between sites of different basic strengths. This explains the frequent use of CO2 as probe molecule for basicity characterization even if reactive adsorption on strong basic sites limits its application. Weaker Brønsted acids such as water, alcohols, alkenes and alkynes that do not interact specifically with basic sites but rather with acid/base pairs can then be used. Also, some authors have proposed others interesting probes to study basic sites, among which 2-methyl-3-butyn2-ol (MBOH) [2], H2S [3] or methylacetylene [4]. Thus, MBOH decomposes into acetone and acetylene on basic sites whereas acidic and amphoteric surfaces lead to different reaction products. In the same way, adsorbed H2S and methylacetylene lead to dissociated and non-dissociated species depending on the basic properties of the surface. In this study, the efficiency of these three probe molecules towards characterization by IR spectroscopy of basic sites in zeolites is compared. To this aim, we chose the faujasite structure (FAU) as a representative zeolite in which basicity is known to vary when changing the Al content and/or the nature of the charge compensating alkali cation [5].
862
B. Moulin et al.
2. Experimental The faujasite samples were two commercial NaY (Si/Al~2.5) and NaX (Si/Al~1.3) zeolites, and a low silica X (LSX, Si/Al=1) that was synthesized and exchanged in solution by various alkali ions (M=Na, K, Cs) according to known procedures. Table 1 reports the chemical compositions for all samples as well as the resulting mean negative charges on framework oxygens evaluated using the Sanderson electronegativity method [6]. Adsorption of the probe molecules was followed by DRIFT operando at 413 K in the case of MBOH, after pre-treatment at 773K under O2 flow, and by transmission IR experiments in the cases of H2S and methylacetylene by adsorbing increasing small doses (7.5 μmol.g-1) of the probe at room temperature on pellets of zeolites previously evacuated at 673 K. Before introduction of H2S and methylacetylene into the IR cell, the gas was submitted to cryogenic trapping to remove traces of water. Zeolites NaY NaX NaLSX KLSX CsLSX
Chemical composition Na55Al55Si137O384 Na85Al85Si107O384 Na96Al96Si96O384 K83Na13Al96Si96O384 Cs58Na38Al96Si96O384
Si/Al ratio 2.5 1.3 1.0 1.0 1.0
δO -0.359 -0.420 -0.440 -0.484 -0.496
Table 1 – Chemical compositions and mean negative charges on framework oxygens of the zeolites under study
3. Results and discussion 3.1. Activated zeolites After activation, the IR spectra of all samples present different residual vibration bands. Bands detected at 1481 and 1429 cm-1 correspond to carbonate species in a quasisymmetric configuration. These carbonates species are eliminated almost completely by evacuation of the zeolite at higher temperature (773 K) or by treating the sample under O2 at 673 K. In addition, a sharp OH band at ~3689 cm-1 is detected as well as two broad bands centred at 3385 and 3235 cm-1. The bands in the 3400-3200 cm-1 region are associated to O-H vibrations of water in interaction by H-bonding with a framework oxygen atom. The band at 3689 cm-1 should correspond to water coordinated to charge compensating alkali ions. The presence of non-dissociated water is also confirmed by the detection of a weak band at ~1650 cm-1 characteristic of water δ(HOH) vibration. Note that water was always detected on the surface of these basic zeolites, although present to a very small extent. 3.2. MBOH adsorption Previous studies [7] proposed various adsorption modes for MBOH on basic oxides. By examining the vibration frequencies of the adsorbed species formed during MBOH adsorption on zeolitic surfaces, it has been possible to show that MBOH is adsorbed mainly either via the MBOH alcohol function interacting with both a framework oxygen atom and a cation (fig 1 - type I), or via a direct interaction between the acetylenic hydrogen and framework oxygen atom (fig 1 - type II). Both adsorption modes require a strong basic O2- Lewis site. Note that type I adsorption implies the acid-base pair Mδ+/O2δ- of the faujasite while type II adsorption implies only the framework oxygen atom.
Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy
863
For the three studied faujasites (NaY, NaX, NaLSX), analogous vibration bands are observed which agree with the MBOH adsorption modes described above. However, both the intensities and the wavenumbers of the vibration bands vary, depending on the sample. It is now well established (Lavalley, 1996) that displacements of IR bands due to the adsorption of molecules on basic sites can be used to assess the strength of basic centres of oxides. In the present study, the displacements of both the ν(OH) and ν(≡CH) vibrations of MBOH related to type I and type II adsorptions, respectively, allow us to rank the basicity of the three faujasites. The ν(OH) wavenumber shifts from 3644 cm-1 in the gas phase to 3504 cm-1 when adsorbed on NaY and down to 3465 cm-1 when interacting with NaLSX. As for the ν(≡CH) vibration, its frequency shifts from 3313 cm-1 in the gas phase to 3278 cm-1 when adsorbed on NaY and down to 3262 cm-1 when interacting with NaLSX (Fig. 2A). Hence, the lower the ν(OH) and the ν(≡CH) vibrations, the greater the basicity of the zeolites. This leads to a first classification of the zeolites that is in line with the basicity order NaY < NaX < NaLSX expected from the respective Si/Al ratios of the samples and consequently mean negative charges on oxygens (Table 1).
Figure 1. Modes of MBOH adsorption on basic zeolites
3.3. Methylacetylene and H2S adsorption Methylacetylene and H2S were used to precise various basic features in the LSX family (experiments have also been done on NaX and NaY but will not be discussed here). It is known [4] that methylacetylene can be adsorbed dissociatively or not. The dissociation of methylacetylene is characterized by the appearance of a typical ν(OH) vibration. Therefore, the absence of any zeolitic ν(OH) band for the different LSX samples indicates that methylacetylene adsorbs mainly without dissociation. The presence of non-dissociated adsorbed methylacetylene is also evidenced by the detection of specific ν(C≡C) and ν(≡CH) vibrations band. As expected, their wavenumbers decrease with increasing basicity. Moreover, the complex shape of the ν(≡CH) band reveals different environments of basic sites. H2S adsorption has already been used in our group to probe basic sites of Y zeolites [3]. H2S can adsorb dissociatively or non-dissociatively. H2S dissociates on a strong basic framework oxygen atom and cationic sites, leading to the formation of zeolitic OH groups, whereas undissociated H2S can interact either by coordination with the cation or by H-bonding. On the whole series of LSX zeolites, H2S dissociates leading to the appearance of a ν(OH) band that is shifted from 3640 cm-1 on NaLSX (less basic), to 3667 cm-1 on KLSX and finally to 3678 cm-1 on CsLSX (more basic). In addition, the intensity of this band strongly increases from Na to Cs (Fig 2B), indicating that the fraction of basic framework oxygen atoms presenting a basicity high enough to dissociate H2S increases from Na to Cs, in line with the increase of the mean basicity determined by the Sanderson electronegativity method.
864
B. Moulin et al.
NaY
-1
-1
3490
500
3276
NCH (cm )
NaX
NOH (cm )
3500
3280
A
3272
3480
3268
3470
3264
3460 0,35
NaLSX 0,37
0,39
0,41
DOZ-
0,43
3260 0,45
I N(OH) (cm-1)
3510
CsLSX
B
400 300 200
KLSX
100
NaLSX
0 0,4
0,42
0,44
0,46
-
0,48
0,5
0,52
DOZ
Figure 2. (A) ν(OH) () and the ν(≡CH) () wavenumbers of MBOH adsorbed on zeolites as a function of Sanderson electronegativity; (B) Intensity of ν(OH) band created by dissociated H2S as a function of Sanderson electronegativity
Thus, methylacetylene and H2S lead to a classification of the zeolite basicity: NaLSX < KLSX < CsLSX that again, well accounts for the change of the negative charge of the basic framework oxygen atoms with change of the nature of the cations.
4. Conclusion This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites.
5. Acknowledgments The authors thank the Agence Nationale de la Recherche for its financial support (ANR BASICAT NT05-3_44329). J.L.Paillaud is acknowledged for his advices for the synthesis of LSX.
References [1] [2] [3] [4] [5] [6] [7]
J.C. Lavalley, Catal. Today, 27(3-4) (1996) 377. H. Lauron-Pernot, F. Luck, J.M. Popa, Appl. Catal., 78 (1991) 213. F. Maugé, A. Sahibed-Dine, M. Gaillard, M. Ziolek, J. Catal., 207 (2002) 353. D. Mordenti, P. Grotz, H. Knozinger, Catal. Today, 70 (2001) 83. D. Barthomeuf, Catal. Rev., 4 (1996) 521. R.T. Sanderson, Electronegativity and bond energyJ. Am. Chem. Soc., 105(8) (1983) 2259. N.E. Fouad, P. Thomasson, H. Knozinger, Appl. catal., 194-195 (2000) 213.