Acidity of cloverite

H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)

Catalysis by Microporous Materials 124

Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.

Acidity of cloverite A. Janin 1, J.C. Lavalley 1., E. Benazzi 2, C. Schott-Darie 3, H. Kessler 3 1URA CNRS 414 Catalyse et Spectrochimie, ISMRA-Universit~ 6, boulevard du Mar~chal Juin, 14050 Caen Cedex, France 21nstitut Fran~ais du P~trole, 1 & 4 Avenue de Bois Pr~au, 92506 RueiI-Malmaison Cedex, France 3Laboratoire de Mat~riaux Mineraux CNRS URA 428, ENSCMu, 3 rue A. Werner, 68093 Mulhouse Cedex, France IR spectra of cloverite activated at 723 K present three bands at 3700, 3673 and 944 cm -1 characterizing free hydroxyl groups. The bands at 3673 cm -1 and 944 cm -1 are assigned to the stretching and bending modes, respectively, of terminal P-OH groups. The band at 3700 cm -1 may be attributed to Ga-OH groups. Use of CO and C2H4 probes evidences the medium BrSnsted acidity (Ho = -6) of P-OH groups and the weak acidity (Ho = -3) of hydroxyls characterized by the 3700 cm -1 band. Pyridine adsorption reveals the presence of Lewis acid sites. Use of NH3 or H20 in large quantities leads to the collapse of the structure as shown by the concomitant study of the structural bands. 1. INTRODUCTION

Cloverite is a molecular sieve with a 20-membered ring pore opening, which is the largest ring size obtained for any molecular sieve. This gallo-phosphate presents a three dimensional pore system with an unusual shape : four terminal hydroxyl groups protrude into opening ;this may lead to highly specific shape-selective catalysts providing these OH groups are acidic. To our knowledge, only two papers have been devoted to the acidic characterization of cloverite (1,2). Bedard et al (1) assigned to P-OH and Ga-OH hydroxyl groups a broad band at 3163 cm -1. It was found unsensitive to acid-base titration by NH3 and HCI leading to the conclusion that interrupted framework hydroxyl groups are essentially neutral. By contrast, Barr et al. (2) reported that after activation at 800 K, only two sharp v(OH) bands, at 3700 and 3670 cm -1, persisted in the v(OH) range. They were assigned to two kinds of P-OH groups ; benzene adsorption evidenced that those characterized by the 3670 cm -1 band corresponded to BrSnsted acidic sites. The aim of the present paper is to clarify the assignment of the v(OH) bands observed by IR spectroscopy on the activated material and to determine the acidity of the corresponding hydroxyl groups by using several probe molecules like CO, ethylene, pyridine, ammonia and H20.

125

2. EXPERIMENTAL The cloverite sample studied in this work was prepared according to (3). Infrared spectra were recorded on a Nicolet MX-1 spectrometer with a resolution of 2 cm -1 . Activation and adsorption of probes were performed on cloverite deposited on a silicon disk of 100 #m thickness. This technique allows one to study the spectra in the whole 4000-400 cm -1 wavenumber range. Pyridine was introduced at room temperature (r.t.) then immediately evacuated at the same temperature. Stepwise desorption was performed up to 723 K. NH3 and H20 were introduced in small quantities then under a 250 Pa pressure at room temperature. CO and C2H4 were adsorbed at 100 K and 200 K respectively on an activated self-supported wafer.

3. RESULTS AND DISCUSSION 3.1 Thermal decomposition of the template

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o ob

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Figure 1. Spectrum of cloverite supported on a silicon disk after calcination under 02 and evacuation at, a) 443 K, b) 573 K, c) 673 K, d) 723 K. The spectrum of quinuclidine adsorbed at 573 K on HY is inserted.

126

Quinuclidine is used as template. Since the synthesis also involves HF in solution, quinuclidinium fluoride is formed. Adsorption of quinuclidine on a HY zeolite leads to quinuclidinium species characterized, after desorption at 573 K, by main bands at 3002, 2957, 2890 and 1464 cm -1. The spectrum of the as-synthesized cloverite supported on the silicon disk, activated at 443 K, presents corresponding bands at 3160, 2946, 2883 and 1487 cm -1 (fig. 1). Their intensity decreases by heating at 573 K, whereas no free hydroxyl groups are recovered, which agrees with the assumption that a large part of quinuclidine is under the form of quinuclidinium fluoride. The quinuclidinium bands disappear after calcination at 673 K, and, even better at 723 K. Such high temperatures generate sharp bands at 3700, 3673 and 944 cm -1 assigned to free hydroxyl groups. Their appearance is not due to the cloverite framework destruction since the structural bands at near 1100, 650 and 490 cm -1 are not affected. This result suggests that the free OH groups were previously affected by quinuclidine. All the acidity measurements presented below are relative to a sample calcined at 723 K under oxygen.

312, LOW temperature CO and C2H4 adsorption Spectra obtained by adsorption at increasing pore filling of CO and 02H4 at 100 K and 200 K are reported on Fig.2 and 3 respectively. 3496

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a ............ " - 7

, #oo

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I~InvEI, IUHBER Figure 2. Spectra resulting from CO adsorption at 100 K on cloverite activated at 723 K following the amount of CO introduced : b) 100 ~mol g-1) ; c) 200 #mol g-1) ; d) 400 ~mol g-1) ; e) Pe ~ 100 P a ; f ) Pe ~ 500 P. Comparison with the spectrum of the activated cloverite (a).

.............

1

:3750 :3500 .32S0 I,,IRVENUHBER Figure 3. Spectra resulting from C2H4 adsorption at 200 K on cloverite activated at 723 K following the amount introduced : b).400 #mol g-1) ; c) 800 ~mol g-1) ; d) Pe ~ 100 Pa. Comparison with the spectrum of the activated cloverite (a).

127 These two weakly basic probes interact specifically with OH groups characterized by the v(OH) band at 3676 cm -1 (initially at 3673 and shifted up at low temperature) which is shifted to 3496 cm -1 with CO and 3404 cm -1 with C2H4. CO species are also characterized by v(CO) bands at 2168 and 2138 cm -1. The latter is attributed to liquid like CO in small cavities (4). It generally arises for high pressures of CO. In the cloverite case, it appears as soon as the first doses of CO are introduced showing a strong confinement effect in small cavities of cloverite. The perturbed v(OH) and v(CO) bands are symmetric and their wavenumbers do not vary during filling showing that acidic sites are homogeneous. Larger amounts of CO or C2H4 perturb the hydroxyls characterized by the 3704 cm -1 band (initially at 3700 cm -1) which shifts to 3610 and 3539 cm -1, respectively. The corresponding v(CO) is overlapping with the v(CO) bands.

3.3. Pyridine adsorption Pyridine adsorption on a self-supported wafer or on the solid deposited on a silicon slide, are compared. Results are similar and Fig. 4 summarized the spectra obtained by stepwise desorption of pyridine from cloverite deposited on silicon. At room temperature, pyridine interacts with hydroxyls at 3673 and 3700 cm -1. The 944 cm -1 band is also perturbed confirming that it is a vibrational band due to hydroxyl groups. The pyridine species formed are characterized as pyridinium species (1545 cm -1 band) and coordinated species (1450 and 1610 cm -1 bands).

II

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f

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Figure 4. Spectra of species formed by pyridine adsorption on activated cloverite after evacuation at b) r.t., c) 423 K, d) 523 K, e) 623 K, f) 723 K. Comparison with the spectrum of activated cloverite (a).

128

By evacuation at increasing temperature, the bands at 3673 and 944 cm -1 are regenerated concomitantly whereas the 1545 cm -1 band due to pyridinium species disappears. This confirms that the bands at 3673 and 944 cm -1 occur from the same hydroxyls which protonated pyridine. After desorption at 523 K there is no more pyridinium species but the 3700 cm -1 band is hardly recovered. After pyridine desorption at 523 K, bands are noted at 1456, 1495, and 1621 cm -1. Their high wavenumber is in favor of the presence of strong Lewis acid sites.

3.4 NH3 and H20 adsorption These two probes give rise to different results according they are introduced in small quantities (ca 3 #mol) or under an equilibrium pression of 250 Pa. We report in Fig. 5 the spectra resulting from adsorption of NH3 on activated cloverite deposited on silicon. When NH3 is introduced in small quantities, it appears that the bands characteristic of hydroxyls are not perturbed. Spectra in the 8(NH) rancje show a band at 1614 cm -1 attributed to coordinated species. The band at 1460 cm-' is then very small showing that few NH3 molecules are protonated. All these species disappear by evacuation at 423 K, whereas the spectrum of the activated cloverite is recovered. When NH3 is introduced under a 250 Pa equilibrium pressure, irreversible transformations occur:

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3750

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I

3550 1700 1~00 LIRVENUHBEB

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820

Figure 5. NH3 adsorption on activated cloverite supported on a silicon disk. Spectra obtained by either b) introducing 3 l~mol of NH3, c) followed by evacuation at 423 K or d) introducing a large amount of NH3 (Pe ~ 250 Pa), e) followed by evacuation at 423 K. Comparison with the spectrum of the activated cloverite (a).

129

i) bands at 3700, 3673 and 944 cm -1 disappear and are not regenerated after evacuation at 423 K. Note that the 8(NH) bending band intensity at 1614 cm -1 is then very strong. ii) irreversible transformations can be deduced from the observations in the range 1300-400 cm -1 . In particular, the decrease of the structural band intensity at 490 cm1 suggests a collapse of the structure. Similar results have been obtained for H20 adsorption. No interaction between hydroxyls and small quantities of H20 occurs. By contrast when a H20 pressure is introduced the structure is destroyed. 4. DISCUSSION

4.1. v(OH) assignment By comparison with results obtained on phosphated alumina samples, the 3673 and 944 cm -1 bands can be assigned to stretching and bending vibrational modes of P-OH groups (5). Adsorption and desorption of probe molecules confirm they characterize the same OH groups. Assignment of the 3700 cm -1 band is more difficult. Barr et al. assigned it to another type of POH groups. However we were not able to detect a corresponding 8(OH) vibration in the 1000-950 cm -1 range. Moreover, probe adsorption shows that the acidity of such OH is much weaker. Another possibility is to assign the 3700 cm -1 band to free GaOH groups, although the spectrum of Ga203 in H-ZSM-5 presents a v(OH) band at 3640 cm -1 (6). The v(GaOH) band in a cristalline gallosilicate with pentasil structure has been localized at 3660 cm -1 (7).

4.2. Acidity Pyridine adsorption at room temperature on activated cloverite evidences the presence of Lewis and Br6nsted acid sites. Pyridinium species hardly persists after evacuation at 423 K, showing that the BrSnsted acidity is not very strong. Spectra analysis confirms that pyridinium species occur from the interaction with P-OH groups since the 3673 cm -1 and 944 cm -1 bands are regenerated by heating at the expense of the pyridinium species. The 3700 cm -1 v(OH) band hardly reappears by thermal evacuation suggesting that an irreversible reaction occurs during this treatment. Note that pyridine adsorption on phosphated alumina leads to protonation (8) due to interaction with free POH hydroxyls. CO and C2H4 adsorption specifically perturbs the P-OH groups. The observed shifts •v(OH) = 275 and 180 cm -1 for C2H4 and CO respectively, are of the same order than those found for silica-alumina samples (9). The shift observed with CO is similar to those reported by Makarova et al on a SAPO-37 (10) and for P-OH hydroxyls on an ALPO sample (11). Introduction of larger amounts of CO or C2H4 perturbs the hydroxyl groups giving rise to the 3700 cm -1 band. The Av(OH) shifts are 94 and 165 cm -1 respectively which corresponds to the same proton affinity as found for SiO2 (12). In a recent work (9), the Av(OH) shift due to CO and C2H4 adsorption was correlated to Ho values determined by Umansky et al. (13) using colored indicators. From the scale so obtained, the acidity of POH groups of cloverite can be estimated to Ho = - 6 ; that of hydroxyls characterized by the 3700 cm -1 band is much weaker (Ho -- - 3). After template elimination under oxygen, cloverite has a good stability in an anhydrous atmosphere. By contrast, presence of H20 or NH3 provokes the destruction of the framework. Surprisingly, these probes do not interact with OH groups when they are introduced in small quantities. Reversible adsorption then occurs with formation of coordinated species. Some authors have shown that the

130

P-O bonds in SAPO or ALPO can be reversibly opened with bases and that the structure of some alumina phosphates and AIPO4-based materials are very sensitive to water at room temperature after template removal (14). 5. CONCLUSION The simultaneous study of spectra in the whole 4000-400 cm -1 IR range, allows us to assign bands at 3673 and 944 cm -1 to stretching and bending vibrational modes respectively of free P-OH groups. These attributions are supported by i) Interactions with basic probes evidencing that the two bands at 3673 and 944 cm -1 characterize the same hydroxyl groups. ii) The acidic strength of corresponding hydroxyls equal to that of P-OH groups in SAPO or ALPO. NH3 and H20 undergo a chemical reaction with the structural P-O-Ga bonds. This interaction is reversible when introducing small amounts of these probes but becomes irreversible for higher amounts and leads to the collapse of the structure. The 3700 cm -1 band assignment is not straitforward yet. We suggest it can characterize free GaOH groups, since such groups are expected from the zeolite structure (3). However the 3700 cm -1 wavenumber is higher than that observed on Ga203 in ZSM-5 zeolites or in gallosilicates. REFERENCES

1. R.L. Bedard, C.L. Bowes, N. Coombs, A.J. Holmes, T. Jiang, S.J. Kirkby, P.M. Macdonald, A.M. Malek, G.A. Ozin, S. Petrov, N. Plavac, R.A. Ramik, M.R. Steele, D. Young, J. Am. Chem. Soc., 115 (1993) 2300. 2. T.L. Barr, J. Klinowski, Heyong He, K. Alberti, G. MQller, J.A. Lercher, Nature, 365 (1993) 429. 3. A. Merrouche, J. Patarin, H. Kessler, M. Soulard, L. Delmotte, J.L. Guth, J.F. Joly, Zeolites, 12 (1992) 226. 4. S. Bordiga, E. Garrone, C. Lamberti, A. Zecchina, C.O. Arean, V.B. Kazansky, L.M. Kustov, J. Chem. Soc. Faraday Trans., 90 (1994) 3367. 5. A. Mennour, C. Ecolivet, D. Cornet, J.F. H~midy, J.C. Lavalley, L. Mariette, P. Engelhard, Mater. Chem. Phys., 19 (1988) 301. 6. P. Meriaudeau and C. Naccache, Appl. Catal., 73 (1991) L.13. 7. A.Uy Khodakov, L.M. Kustov, T.N. Bondarenko, A.A. Dergachev, V.B. Kazansky, Kh.M. Minachev, G. Borbely, H.K. Beyer, Zeolites, 10 (1990) 603. 8. F. Abbattista, A. Delmastro, G. Gozzelino, D. Mazza, M. Vallino, G. Busca, V. Lorenzelli, J. Chem. Soc. Faraday Trans., 86 (1990) 3653. 9. A. Janin, J.C. Lavalley, Unpublished results. 10. M.A. Makarova, K.M. AI-Ghefaili, J. Dwyer, J. Chem. Soc. Faraday Trans., 90 (1994) 383. 11. L. Kubelkova, S. Beran, J. Lercher, Zeolites, 9 (1989) 589. 12. T.P. Beebe, P. Gelin, J.T. Yates Jr, Surf. Sci., 148 (1984) 526. 13. B. Umansky, J. Engelhardt, W.K. Hall, J. Catal., 127 (1991) 128. 14 R. Vomscheid, M. Briend, M.J. Peltre, P. Massiani, P.P. Man, D. Barthomeuf, J. Chem. Soc., Chem. Com. (1993) 544.