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1H NMR evidence for the bi-pyridinium nature of the pyridine salt of H3PW12O40
Catalysis Communications 6 (2005) 539–541 www.elsevier.com/locate/catcom
1
H NMR evidence for the bi-pyridinium nature of the pyridine salt of H3PW12O40 Nadine Essayem *, Chantal Lorentz, Alain Tuel, Younes Ben Taˆarit Institut de Recherche sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne, France Received 13 December 2004; accepted 17 May 2005 Available online 23 June 2005
Abstract Pyridinium salts of H3PW12O40 and H3PMo12O40 were investigated by 1H NMR and IR spectroscopies. 1H NMR evidenced the formation of bi-pyridinium cation in the pyridinium salt of H3PW12O40, while a classical pyridinium cation was identified in the pyridinium salt of H3PMo12O40. Pyridinium and bi-pyridinium species were clearly differentiated by infra red spectroscopy: The bi-pyridinium cation is characterized by the splitting of the 8b and 19b vibration modes of the pyridinium ring. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Bi-pyridinium cation; 12-Tungstophosphoric acid; 12-Molybdophosphoric acid; Acidity
1. Introduction 12-Tungtophosphoric acid, H3PW12O40, was reported to be the most acidic and the most stable compound among similar heteropolyacids with the Keggin structure. This is at the origin of its high potential as strong solid acid catalyst [1]. Thus, H3PW12O40 is able to catalyse the isomerization of light alkanes at relatively low temperature [2]. Infra red spectroscopy gave also clear evidences of the strong Brønsted acidity of H3PW12O40. By this technique, solid anhydride H3PW12O40 was shown to achieve the protonation of water molecule while it is hardly observed over acidic zeolithe [3]. Whereas, pyridine is certainly the most used probe molecule to characterize the acidic feature of solid catalysts, one can notice that the nature of the species formed via pyridine adsorption within H3PW12O40 is not unambiguously demonstrated. In this field, it is worth noting that the crystal structure of the pyridinium salt of the 12-tungstophosphoric acid was investigated by Misono et al. [4]. They reported that the best refine*
Corresponding author. E-mail address: [email protected] (N. Essayem).
1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.05.006
ment assumes pyridine molecules to be paired via their ˚ , indicating that the N N atoms at a distance of 2.68 A atoms are bounded via hydrogen bonding. Therefore, they concluded that the pyridine pairs may be regarded as the bipyridinium cation (C5H5N H+ NC5H5). In the present work, we attempted to provide additional evidences to confirm the bipyridinium nature of the pyridinium salt of H3PW12O40. Moreover, the pyridinium salt of H3PMo12O40 was investigated as well. Effectively, as regard to the W-based heteropolyacid, the 12-molybdophosphoric acid is a weaker acid [5]. Thus, one could expect the formation of different pyridine species due to the different acidic strength of these heteropolyacids.
2. Results and discussion The H3PW12O40 Æ nH2O and H3PMo12O40 Æ nH2O were purchased from Fluka and recrystallized from a deionized water solution before use. Five grams of each of these solids were dissolved in 20 ml of deionized water and reacted with an acetone solution of pyridine. The pyridine solution was added drop wise under a vigorous
540
N. Essayem et al. / Catalysis Communications 6 (2005) 539–541
the Keggin units (wavenumbers <1200 cm 1) and from the pyridine vibration mode absorptions (between 1450 and 1700 cm 1), that the solids are consistent with the pyridinium salts of the corresponding heteropolyanion. However, at variance with the Mo-based salt, some absorptions bands were splitted into doublets in the case of the W-based salt. This is the case with the 8b and 19b vibration modes of the pyridinium ring (centered at 1610 and 1535 cm 1, respectively). Similarly, the absorptions around 600 and 516 cm 1, ascribed to d(O–P–O) and mS(W–O–W) vibrations of the Keggin anion [6], were splitted in the case of the tungsten-based salt. These differences were interpreted in terms of the formation of a bipyridinium ion, where two pyridine molecules are aligned with the proton in the same plane. The 19b and 8b vibrations mode of one of the pyridine molecules should be coupled with the vibration mode of the other molecule. The splitting of the d(O–P–O) and mS(W–O–W) may arise from a lifted degeneracy due to a lowering of the symmetry of the heteropolyanion resulting from the formation of the bipyridinium complex. In a previous article [7], we showed that the IR vibrations of this bipyridinium complex was not modified by thermal evacuations at increasing temperature. This clearly indicates that the protonated species Py H+ Py do not decompose easily in PyH+ and pyridine. This reinforces the cluster model of two pyridine molecules symmetri-
stirring and the pH was permanently monitored. When the pH value reached 3.5, the neutralization was stopped and the precipitate was filtered off and thoroughly washed with deionized water (the salts are white in the case of the tungsten-based sample and yellow in the case of the molybdenum-based material). The IR spectra of these two solids, obtained from KBr pellets, are reported in Fig. 1. It is clear from the fingerprint absorptions of
Absorbance
0.5
a
b
1800
1600
1400
1200
1000
800
600
-1
Wavenumbers (cm )
15.0
8.3087
5.5974
1.0000
9.1209 8.8487
14.6907
Fig. 1. Infra red spectra of the pyridinium salts of: (a) H3PMo12O40 and (b) H3PW12O40.
14.0
13.0
12.0
11.0
10.0
9.0
8.0 (ppm)
7.0
6.0
5.0
4.0
Fig. 2. 1H NMR spectrum of the pyridinium salt of H3PMo12O40.
3.0
2.0
1.0
15.0
8.3151
541
11.665
1.0000
9.1385 8.8480
14.6920
N. Essayem et al. / Catalysis Communications 6 (2005) 539–541
14.0
13.0
12.0
11.0
10.0
9.0
8.0 (ppm)
7.0
6.0
5.0
4.0
3.0
2.0
1.0
Fig. 3. 1H NMR spectrum of the pyridinium salt of H3PW12O40.
cally coordinated to the proton via bonds of equivalent strengths. The 1H NMR spectra of these two salts in D6 acetone are reported in Figs. 2 and 3. Essentially, the C–H protons, consistent with the pyridinium ring protons, resonate around 8–9 ppm from TMS. The broad signal observed at 15 ppm from TMS is ascribed to the N–H protons. In the case of the Mo-based salt, the ratio of the N–H to the C–H integrals is roughly 1:5.5. By contrast, in the case of the W-based salt, the ratio is 1:11.5. Hence, in the case of the Mo-based salt, the 1:5.5 ratio is in fair agreement with the expected ratio of 1:5 for a classical pyridinium ion. This deviation is likely due to an underestimate of the broad signal integral. This result gives evidence for the monopyridinium nature of the cation. By contrast, the 1:11.5 ratio exhibited by the W-based salt clearly indicates that a single N–H proton is bonded between two pyridine rings. Therefore, these NMR data confirm quantitatively the bipyridinium nature of the cation in the latter case.
This also bears evidence of the highly acidic nature of the 12-tungstophosphoric acid, since its proton is unsatisfied by binding a single pyridine molecule. By contrast, the 12-molybdophosphoric, leads to the formation of the usual monopyridinium cation; that accounts for its lower acidity.
References [1] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [2] N. Essayem, Y. Ben Taˆarit, C. Feche, P.Y. Gayraud, G. Sapaly, C. Naccache, J. Catal. 219 (2003) 97. [3] N. Essayem, A. Holmqvist, P.Y. Gayraud, J.C. Ve´drine, Y. Ben Taˆarit, J. Catal. 197 (2001) 273. [4] M. Hashimoto, M. Misono, Acta Cryst. C50 (1994) 231. [5] I.V. Kozhevnikov, S.Ts. Khankhasaeva, S.M. Kulikov, Kinet. Catal. 29 (1988) 76. [6] C. Rocchiccioli-Deltcheff, R. Thouvenot, R. Franck, Spectrochim. Acta 32A (1976) 587. [7] N. Essayem, A. Holmqvist, G. Sapaly, J.C. Vedrine, Y. Ben Taˆarit, Stud. Surf. Sci. Catal. 135 (2001) 340.
1
H NMR evidence for the bi-pyridinium nature of the pyridine salt of H3PW12O40 Nadine Essayem *, Chantal Lorentz, Alain Tuel, Younes Ben Taˆarit Institut de Recherche sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne, France Received 13 December 2004; accepted 17 May 2005 Available online 23 June 2005
Abstract Pyridinium salts of H3PW12O40 and H3PMo12O40 were investigated by 1H NMR and IR spectroscopies. 1H NMR evidenced the formation of bi-pyridinium cation in the pyridinium salt of H3PW12O40, while a classical pyridinium cation was identified in the pyridinium salt of H3PMo12O40. Pyridinium and bi-pyridinium species were clearly differentiated by infra red spectroscopy: The bi-pyridinium cation is characterized by the splitting of the 8b and 19b vibration modes of the pyridinium ring. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Bi-pyridinium cation; 12-Tungstophosphoric acid; 12-Molybdophosphoric acid; Acidity
1. Introduction 12-Tungtophosphoric acid, H3PW12O40, was reported to be the most acidic and the most stable compound among similar heteropolyacids with the Keggin structure. This is at the origin of its high potential as strong solid acid catalyst [1]. Thus, H3PW12O40 is able to catalyse the isomerization of light alkanes at relatively low temperature [2]. Infra red spectroscopy gave also clear evidences of the strong Brønsted acidity of H3PW12O40. By this technique, solid anhydride H3PW12O40 was shown to achieve the protonation of water molecule while it is hardly observed over acidic zeolithe [3]. Whereas, pyridine is certainly the most used probe molecule to characterize the acidic feature of solid catalysts, one can notice that the nature of the species formed via pyridine adsorption within H3PW12O40 is not unambiguously demonstrated. In this field, it is worth noting that the crystal structure of the pyridinium salt of the 12-tungstophosphoric acid was investigated by Misono et al. [4]. They reported that the best refine*
Corresponding author. E-mail address: [email protected] (N. Essayem).
1566-7367/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.05.006
ment assumes pyridine molecules to be paired via their ˚ , indicating that the N N atoms at a distance of 2.68 A atoms are bounded via hydrogen bonding. Therefore, they concluded that the pyridine pairs may be regarded as the bipyridinium cation (C5H5N H+ NC5H5). In the present work, we attempted to provide additional evidences to confirm the bipyridinium nature of the pyridinium salt of H3PW12O40. Moreover, the pyridinium salt of H3PMo12O40 was investigated as well. Effectively, as regard to the W-based heteropolyacid, the 12-molybdophosphoric acid is a weaker acid [5]. Thus, one could expect the formation of different pyridine species due to the different acidic strength of these heteropolyacids.
2. Results and discussion The H3PW12O40 Æ nH2O and H3PMo12O40 Æ nH2O were purchased from Fluka and recrystallized from a deionized water solution before use. Five grams of each of these solids were dissolved in 20 ml of deionized water and reacted with an acetone solution of pyridine. The pyridine solution was added drop wise under a vigorous
540
N. Essayem et al. / Catalysis Communications 6 (2005) 539–541
the Keggin units (wavenumbers <1200 cm 1) and from the pyridine vibration mode absorptions (between 1450 and 1700 cm 1), that the solids are consistent with the pyridinium salts of the corresponding heteropolyanion. However, at variance with the Mo-based salt, some absorptions bands were splitted into doublets in the case of the W-based salt. This is the case with the 8b and 19b vibration modes of the pyridinium ring (centered at 1610 and 1535 cm 1, respectively). Similarly, the absorptions around 600 and 516 cm 1, ascribed to d(O–P–O) and mS(W–O–W) vibrations of the Keggin anion [6], were splitted in the case of the tungsten-based salt. These differences were interpreted in terms of the formation of a bipyridinium ion, where two pyridine molecules are aligned with the proton in the same plane. The 19b and 8b vibrations mode of one of the pyridine molecules should be coupled with the vibration mode of the other molecule. The splitting of the d(O–P–O) and mS(W–O–W) may arise from a lifted degeneracy due to a lowering of the symmetry of the heteropolyanion resulting from the formation of the bipyridinium complex. In a previous article [7], we showed that the IR vibrations of this bipyridinium complex was not modified by thermal evacuations at increasing temperature. This clearly indicates that the protonated species Py H+ Py do not decompose easily in PyH+ and pyridine. This reinforces the cluster model of two pyridine molecules symmetri-
stirring and the pH was permanently monitored. When the pH value reached 3.5, the neutralization was stopped and the precipitate was filtered off and thoroughly washed with deionized water (the salts are white in the case of the tungsten-based sample and yellow in the case of the molybdenum-based material). The IR spectra of these two solids, obtained from KBr pellets, are reported in Fig. 1. It is clear from the fingerprint absorptions of
Absorbance
0.5
a
b
1800
1600
1400
1200
1000
800
600
-1
Wavenumbers (cm )
15.0
8.3087
5.5974
1.0000
9.1209 8.8487
14.6907
Fig. 1. Infra red spectra of the pyridinium salts of: (a) H3PMo12O40 and (b) H3PW12O40.
14.0
13.0
12.0
11.0
10.0
9.0
8.0 (ppm)
7.0
6.0
5.0
4.0
Fig. 2. 1H NMR spectrum of the pyridinium salt of H3PMo12O40.
3.0
2.0
1.0
15.0
8.3151
541
11.665
1.0000
9.1385 8.8480
14.6920
N. Essayem et al. / Catalysis Communications 6 (2005) 539–541
14.0
13.0
12.0
11.0
10.0
9.0
8.0 (ppm)
7.0
6.0
5.0
4.0
3.0
2.0
1.0
Fig. 3. 1H NMR spectrum of the pyridinium salt of H3PW12O40.
cally coordinated to the proton via bonds of equivalent strengths. The 1H NMR spectra of these two salts in D6 acetone are reported in Figs. 2 and 3. Essentially, the C–H protons, consistent with the pyridinium ring protons, resonate around 8–9 ppm from TMS. The broad signal observed at 15 ppm from TMS is ascribed to the N–H protons. In the case of the Mo-based salt, the ratio of the N–H to the C–H integrals is roughly 1:5.5. By contrast, in the case of the W-based salt, the ratio is 1:11.5. Hence, in the case of the Mo-based salt, the 1:5.5 ratio is in fair agreement with the expected ratio of 1:5 for a classical pyridinium ion. This deviation is likely due to an underestimate of the broad signal integral. This result gives evidence for the monopyridinium nature of the cation. By contrast, the 1:11.5 ratio exhibited by the W-based salt clearly indicates that a single N–H proton is bonded between two pyridine rings. Therefore, these NMR data confirm quantitatively the bipyridinium nature of the cation in the latter case.
This also bears evidence of the highly acidic nature of the 12-tungstophosphoric acid, since its proton is unsatisfied by binding a single pyridine molecule. By contrast, the 12-molybdophosphoric, leads to the formation of the usual monopyridinium cation; that accounts for its lower acidity.
References [1] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [2] N. Essayem, Y. Ben Taˆarit, C. Feche, P.Y. Gayraud, G. Sapaly, C. Naccache, J. Catal. 219 (2003) 97. [3] N. Essayem, A. Holmqvist, P.Y. Gayraud, J.C. Ve´drine, Y. Ben Taˆarit, J. Catal. 197 (2001) 273. [4] M. Hashimoto, M. Misono, Acta Cryst. C50 (1994) 231. [5] I.V. Kozhevnikov, S.Ts. Khankhasaeva, S.M. Kulikov, Kinet. Catal. 29 (1988) 76. [6] C. Rocchiccioli-Deltcheff, R. Thouvenot, R. Franck, Spectrochim. Acta 32A (1976) 587. [7] N. Essayem, A. Holmqvist, G. Sapaly, J.C. Vedrine, Y. Ben Taˆarit, Stud. Surf. Sci. Catal. 135 (2001) 340.