Comparison of CO, CD3CN, and C5H5N Probe Molecules - American

most selective probe, not only being able to differentiate Brønsted (BAS) from Lewis acid sites (LAS), but. BAS with varying acid strength as well. H...
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Langmuir 2001, 17, 6233-6239

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An FTIR Study of the Surface Acidity of USY Zeolites: Comparison of CO, CD3CN, and C5H5N Probe Molecules W. Daniell,† N.-Y. Topsøe,‡ and H. Kno¨zinger*,† Department Chemie, Physikalische Chemie, Ludwig-Maximilians-Universita¨ t, Haus E, Butenandtstr. 5-13, Mu¨ nchen, D-81377, Germany, and Haldor Topsøe A/S, Nymøllevej 55, P.O. Box 213, Lyngby 2800, Denmark Received March 6, 2001. In Final Form: July 2, 2001 A comparison of three basic probe molecules of varying strength, namely pyridine, acetonitrile, and carbon monoxide, was made, and their suitability toward the characterization of surface acidity in zeolitic materials was assessed. Two test samples were employed: both ultrastable Y (USY) zeolites but with differing silica:alumina ratios and degree of extraframework material. Carbon monoxide proved to be the most selective probe, not only being able to differentiate Brønsted (BAS) from Lewis acid sites (LAS), but BAS with varying acid strength as well. However, its weak basicity did not allow interaction with OH groups situated within the sodalite cages, this only being achieved with the two more basic probes. OH groups interacting with extraframework material also failed to form H-bonds with CO. Both pyridine and acetonitrile interacted with all OH groups, and pyridine was protonated by the most acidic bridged species. Although LAS could be told apart from BAS, differentiation of different types of LAS and BAS among themselves was not directly possible with these two molecules.

1. Introduction Dealuminated or ultrastable Y (USY) zeolites are of vital industrial importance in the refining of petroleum and are used extensively as components of fluidized cracking catalysts.1-3 Their structure and acidic properties are greatly influenced by the dealumination process4,5 which generates extraframework alumina possessing Lewis acidity and inducing enhanced Brønsted acidity within the material.6 Cairon et al. have even demonstrated that this extraframework material possesses a nonnegligible Brønsted acidity of its own.7 Though still not fully understood and a point of much debate in the literature, the generation of this extraframework material has been shown to greatly modify the catalytic properties of the zeolite.6,8 Williams et al.9 recently proposed a link between hydrocarbon cracking activity in dealuminated Y zeolites and the method of dealumination (steaming cf. leaching). Hence, the importance of being able to characterize the acidity within dealuminated Y zeolites is clearly seen. To further enhance our understanding of the process and so allow improvements in catalytic activity to be achieved, it is essential that both the type and †

Ludwig-Maximilians-Universita¨t. Haldor Topsøe A/S. * To whom correspondence should be addressed: e-mail [email protected], Fax (+49) 89 2180 7605. ‡

(1) Wojciechovski, B. W.; Corma, A. Catalytic Cracking, Catalysis, Chemistry and Kinetics; Marcel Dekker: New York, 1986. (2) Magee, J. S.; Mitchell, M. M., Jr., Fluid Catalytic Cracking: Science and Technology; Elsevier: Amsterdam, 1993; Vol. 76. (3) Scherzer, J. Octane-Enhancing Zeolite FCC Catalysts: Scientific and Technical Aspects; Marcel Dekker: New York, 1990. (4) Beyerlein, R. A.; Choi-Feng, C.; Hall, J. B.; Huggins, B. J.; Ray, G. J. Top. Catal. 1997, 4, 27. (5) Barthomeuf, D. Mater. Chem. Phys. 1987, 17, 49. (6) Beyerlein, R. A.; McVicker, G. B.; Yacullo, L. N.; Ziemak, J. J. J. Phys. Chem. 1988, 92, 1967. (7) Cairon, O.; Chevreau, T.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1998, 94, 3039. (8) Fritz, P. O.; Lunsford, J. H. J. Catal. 1989, 118, 85. (9) Williams, B. A.; Miller, J. T.; Snurr, R. Q.; Kung, H. H. Microporous Mesoporous Mater. 2000, 35-36, 61.

strength of generated acid sites, as well as the accessibility of those sites, can be determined. The investigation of surface acidity through IR spectroscopy of adsorbed probe molecules has proved to be a powerful tool and has been the topic of numerous reviews,10-12 many of which have highlighted the features of particular probe molecules and the relevant criteria necessary for selection of the appropriate molecule.13,14 Of the most suitable and frequently used, we have for this work chosen carbon monoxide, acetonitrile and pyridine, which not only differ in their size but also in their proton affinity (598, 783, and 912 kJ mol-1, respectively12). Carbon monoxide (CO) is a small and soft (in terms of basicity) molecule which can bond to both protic and aprotic sites.10,11 It forms weak hydrogen bonds (H-bonds) with Brønsted acid sites (BAS) which require measurements to be made at low temperature, under which conditions CO is totally unreactive. The formation of OHCO complexes gives rise to a low-frequency shift of the O-H stretching mode and a simultaneous high-frequency shift of the C-O stretching mode (away from that recorded in the gas phase at 2143 cm-1).10 Both the ∆ν(CO) and ∆ν(OH) shifts increase with the acid strength of the proton, thus allowing discrimination of BAS according to their acid strength. Pyridine also undergoes coordination to aprotic Lewis acid sites (LAS) and H-bonding with weak acidic groups but can be protonated by stronger acidic sites to form the pyridinium ion (PyH+). The spectra of adsorbed pyridine are usually analyzed using the so-called 8a and 19b ring vibration modes (according to the nomenclature of Kline (10) Kno¨zinger, H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 2, p 707. (11) Lercher, J. A.; Grundling, C.; Eder-Mirth, G. Catal. Today 1996, 27, 353. (12) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723. (13) Kno¨zinger, H. In Elementary Reaction Steps in Heterogeneous Catalysis; Joyner, R. W., van Santen, R. A., Eds.; Kluwer Academic Publishers: Dordrecht, 1993; p 267. (14) Paukshtis, E. A.; Yurchenko, E. N. Russ. Chem. Rev. 1983, 52, 42.

10.1021/la010345a CCC: $20.00 © 2001 American Chemical Society Published on Web 08/31/2001

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and Turkevich15), which respond most sensitively to intermolecular interactions through the nitrogen lone pair electrons. These modes allow the vibrations from coordinated pyridine compounds, PyH+, and H-bonded pyridine to be distinctly separated.16 Acetonitrile is the least commonly used of the three probe molecules examined in this work. It is a relatively soft base and sterically less crowded than pyridine in the vicinity of the lone pair electrons on the nitrogen atom, which are used when coordinatively bonded to aprotic sites. With BAS, acetonitrile either forms strong hydrogen bonds or can become protonated, depending on the strength of the acid site.17 A drawback of using acetonitrile is that interpretation of the C-N stretching mode can become complicated for coordinated or H-bonded species because of Fermi resonance between the ν2 stretching mode and the (ν3 + ν4) combination mode.18 This is overcome experimentally by use of perdeuterated acetonitrile (CD3CN), which displays only a single ν(CtN) band at 2263 cm-1 in the liquid phase. In this paper we highlight the problems of selection of a suitable probe molecule for determination of surface acidity, by comparing carbon monoxide, (perdeuterated) acetonitrile, and pyridine, with respect to their ability to characterize the acidic properties of two USY zeolite samples. The properties concerned with here are as follows: (i) the acid strength of the different OH groups, i.e., supercage, sodalite cage and extraframework; (ii) the nature of the acid site, i.e., Brønsted and Lewis acid; (iii) the strength of different acid sitesspossible heterogeneity in Brønsted acid sites; (iv) the accessibility of interaction with the acid sitessgeometric consideration and basicity of probe molecules. 2. Experimental Section The ultrastable Y-zeolite samples obtained from Zeolyst International were products CBV760 (SiO2:Al2O3 ) 50.90; 2 wt % Na2O) and CBV712 (SiO2:Al2O3 ) 11.46; 4 wt % Na2O), designated as USY-H (high silica:alumina ratio) and USY-L (low silica:alumina ratio) in this work, respectively.19 Both samples have comparable surface areas (720 and 730 m2 g-1) and unit cell dimensions (24.24 and 24.35 Å, respectively). FTIR spectra of adsorbed CO (Linde, 99.999%) and CD3CN (Aldrich, 99.95% D) were recorded on a Bruker IFS66 spectrometer (MCT detector) in transmission mode with a resolution of 2 cm-1 over 128 scans. Powder samples were pressed into self-supporting disks (F ) 10 mg cm-2) and mounted into a Pt-Ir frame. This was supported within a purpose-built, in-situ IR cell (CaF2 windows)20 connected to a vacuum/gas dosing apparatus and attached to a quartz reactor in which pretreatment was carried out. All samples were treated under vacuum (ca. 10-3 Pa) at 723 K for 16 h, before cooling to the adsorption temperature. CO admission was carried out at 85 K with the gas being initially admitted in increments of 0.1 mbar and then in larger steps of 1-10 mbar until saturation of all surface acid sites was achieved. Admission of acetonitrile was carried out using the same procedure but with adsorption performed at ambient temperature (298 K). Pyridine (Fluka, spectroscopic grade) was dried over an activated Linde 5A molecular sieve and further degassed by the conventional freeze-pump-thaw technique. After the abovedescribed evacuation pretreatment, the self-supporting wafer was cooled to 423 K where 1 mbar of pyridine was admitted to the cell. After 1 h of adsorption, the excess and weakly adsorbed (15) Kline, C. H.; Turkevich, J. J. Chem. Phys. 1944, 12, 300. (16) Parry, E. P. J. Catal. 1963, 2, 371. (17) Thibault-Starzyk, F.; Travert, A.; Saussey, J.; Lavalley, J.-C. Top. Catal. 1998, 6, 111. (18) Kno¨zinger, H.; Krietenbrink, H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 2421. (19) Linsten, O. M.; Frested, A. O ¨ . U.S. Patent 5,059,567, 1991. (20) Kunzmann, G. Doctoral Thesis, University of Munich, 1987.

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Figure 1. Comparison of the OH stretching regions of the two zeolites (upper, USY-H; lower, USY-L) after pretreatment under vacuum (623 K, 16 h). pyridine was removed by evacuation at the same temperature for 30 min to below 10-5 mbar. The spectra were recorded in the transmission mode (4 cm-1 resolution over 256 scans) on a BioRad FTS60 spectrometer equipped with a MCT detector.

3. Results 3.1. FTIR Characterization of Pretreated Samples. USY-H. The spectrum of the USY-H sample after pretreatment revealed three hydroxyl bands in the O-H stretching region at 3739, 3632, and 3567 cm-1 (Figure 1). These were assigned to terminal hydroxyl groups (SiOH), bridging OH’s in the supercages (HF, high frequency), and bridging OH’s in the sodalite cages (LF, low frequency), respectively.21 The intensity of the silanol band compared with the other two bands is a reflection of the high silicato-alumina ratio. USY-L. After pretreatment the USY-L sample revealed a more complex spectrum than that of USY-H, containing six bands in the O-H stretching region (Figure 1). In addition to bands assigned to silanol groups (3744 cm-1), HF (3632 cm-1), and LF (3567 cm-1) species, were three less intense bands at 3673, 3603, and 3525 cm-1. These last three bands were attributed to hydroxyl groups present on extraframework material (EF), HF groups interacting with this extraframework material (HF′), and LF groups involved in similar interaction (LF′).7,22 The differences between the two materials is clearly highlighted in Figure 1, the USY-L sample exhibiting, first. a relatively less intense silanol band in relation to the HF and LF bands and, second, the presence of extraframework material within the pore structure of the zeolite, created through dealumination. 3.2. Adsorption of CO. USY-H. On adsorption of 0.1 mbar of CO the HF band was seen to decrease in intensity, and a new band at 3280 cm-1 appeared. This corresponded to a shift ∆ν(OH) ) 352 cm-1, a typical value for USY zeolites.7,23 On adsorption of 0.2 mbar of CO the HF band disappeared completely, and the shifted band reached its maximum intensity, implying the saturation of all these sites with CO (Figure 2A). Higher pressures of CO resulted in the shift in intensity of the silanol band from 3739 to ca. 3660 cm-1, a shift of ∼80 cm-1 typical for terminal (21) Zecchina, A.; Otero Arean, C. Chem. Soc. Rev. 1996, 187. (22) Barthomeuf, D. Zeolites. 1990, 10, 131. (23) Makarova, M. A.; Garforth, A.; Zholobenko, V. L.; Dwyer, J.; Earl, G. J.; Rawlence, D. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier: Amsterdam, 1994; p 365.

FTIR Study of USY Zeolites

Figure 2. (A) FTIR spectra of USY-H depicting the shift of OH bands on adsorption of CO at 85 K: (a) before adsorption, (b) 0.1, (c) 0.2, (d) 0.3, (e) 1.0 mbar of CO. (B) Evolution of the C-O stretching region of USY-H with increasing CO pressure at 85 K: (a) before adsorption, (b) 0.1, (c) 0.2, (d) 0.3, (e) 1.0, (f) 5.0 mbar of CO.

silanol groups.24 These silanol bands were saturated on addition of 100 mbar of CO (spectra not shown), but at no stage did the LF species interact with CO, and the band at 3567 cm-1 remained unaffected throughout. In the carbonyl stretching region of the spectra (Figure 2B), bands were observed at 2180 and 2157 cm-1, corresponding to the interaction of CO with HF and silanol groups, respectively.21 The former saturated upon addition of 0.2 mbar of CO and the latter not until admission of 100 mbar. The band growing in at 2141 cm-1 is assigned to physisorbed CO.24 Coordinated CO to LAS potentially within USY-H were not detected. USY-L. Adsorption of 0.1 mbar of CO on USY-L resulted in a large shift (ca. 400 cm-1) of the HF band. Closer examination, however, reveals that in fact two shifted bands are apparent at 3284 cm-1 and a shoulder at 3198 cm-1, corresponding to shifts of 348 and 434 cm-1, respectively (Figure 3A). This implies a certain degree of heterogeneity within the HF groups and a variation in their acidity. Further addition of CO resulted in the shift of the silanol band by ∼85 cm-1 to 3657 cm-1, with saturation occurring on admission of 50 mbar of CO. Note that even at these pressures no visible interaction between CO and the LF and LF′ species was observed. Moreover, no interaction with the HF′ groups (3603 cm-1) was detected either, though the band at 3284 cm-1 shifted to ca. 3240 cm-1 due to solvent effects of physisorbed CO at such high partial pressures.24 The presence of physisorbed CO was confirmed by an intense band at 2143 cm-1.24 (24) Beebe, T. P.; Gelin, P.; Yates, J. T. Surf. Sci. 1984, 148, 526.

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Figure 3. (A) FTIR spectra of USY-L depicting the shift of OH bands on adsorption of CO at 85 K: (a) before adsorption; (b) 0.1, (c) 0.5, (d) 5, (e) 50 mbar of CO. (B) Evolution of the C-O stretching region of USY-L with increasing CO pressure at 85 K: (a) before adsorption; (b) 0.1, (c) 0.5, (d) 5, (e) 50 mbar of CO.

The corresponding spectra in the ν(CO) region are shown in Figure 3B. On adsorption of 0.1 mbar of CO three bands at 2229, 2179, and 2170 cm-1 are observed. The former is assigned to tetrahedrally coordinated Al3+ Lewis acid sites25 present in extraframework material, whereas the weak band at 2170 cm-1 is attributed to Na+ cationic impurities.26 As with USY-H, the band at 2179 cm-1 is assigned to the bridging HF species, its saturation coinciding with the disappearance of the 3632 cm-1 band. With increasing CO pressure two further bands at 2157 and 2143 cm-1 become apparent. These are assigned to interaction through H-bonding to weak Brønsted acid sites (silanol groups) and physisorbed CO, respectively. 3.3. Adsorption of CD3CN. USY-H. The first bands to appear in the CtN region (Figure 4A) on adsorption of CD3CN (0.1 mbar) were at 2332 and 2302 cm-1, corresponding to coordinated acetonitrile and acetonitrile H-bonded to bridging OH groups, respectively.27,28 The former was readily saturated by admission of a further 0.1 mbar of CD3CN, whereas the intensity of the second band increased with increasing pressure of acetonitrile, until admission of ca. 0.5 mbar. In spectra taken after admission of 0.3 mbar of CD3CN, a third band at 2274 cm-1 is also apparent. This belongs to weakly H-bonded (25) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497. (26) Kno¨zinger, H.; Huber, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2047. (27) Rouxhet, P. G.; Sempels, R. E. J. Chem. Soc., Faraday Trans. 1 1974, 70, 2021. (28) Medin, A. S.; Borovkov, V. Y.; Kazansky, V. B.; Pelmenschikov, A. G.; Zhidomirov, G. M. Zeolites 1990, 10, 668.

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Figure 4. (A) Evolution of the C-N stretching region of USY-H with increasing CD3CN pressure at 298 K: (a) initial; (b) 0.1 c) 0.2, (d) 0.3, (e) 0.5, (f) 1, (g) 2, (h) 5 mbar of CD3CN. (B) FTIR spectra of USY-H on adsorption of CD3CN at 298 K: (a) initial; (b) 0.1 c) 0.2, (d) 0.3, (e) 0.5, (f) 1, (g) 2, (h) 5 mbar of CD3CN.

acetonitrile which interacts with the weaker silanol BAS.29 The band at 2116 cm-1 is assigned to the νs(CD3) vibration together with a very weak band at 2250 cm-1, which is the corresponding antisymmetric νas(CD3) stretch.30 The corresponding changes in the OH stretching region are shown in Figure 4B. On adsorption of 0.1 mbar of CD3CN small decreases in the intensity of the HF and LF bands are observed, until after admission of 0.5 mbar when both species are saturated. This coincides with the saturation of the band at 2302 cm-1. This H-bonding interaction with strong BAS also leads to the formation of broad bands with maxima at ca. 2700 and 2400 cm-1. This is common for strong H-complexes in vapors, liquids, and solids.30,31 These maxima represent the so-called A and B bands of the typical ABC pattern seen for interaction of acetonitrile with strong acids, the two bands actually being defined by a minimum (an Evans window) at ca. 2600 cm-1 which breaks up the overall increase in intensity into sections.30,31 The silanol bands are not saturated until admission of 10 mbar of CD3CN and are shifted from 3739 to ca. 3420 cm-1, a shift of ca. 320 cm-1, similar to those previously reported in the literature.32 USY-L. On adsorption of up to 0.5 mbar of CD3CN (Figure 5A), three bands at 2113, 2300, and 2325 cm-1 are (29) Scokart, P. O.; Declerck, F. D.; Sempels, R. E.; Rouxhet, P. G. J. Chem. Soc., Faraday Trans. 1 1977, 73, 359. (30) Pelmenschikov, A. G.; van Santen, R. A.; Ja¨nchen, J.; Meijer, E. J. Phys. Chem. 1993, 97, 11071. (31) Clydon, M. F.; Sheppard, N. Chem. Commun. 1969, 1431. (32) Kno¨zinger, H. In The Hydrogen Bond-Recent Developments in Theory and Experiments; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; Vol. 3, p 1265.

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Figure 5. (A) FTIR spectra of USY-L depicting the evolution of the C-N stretching region on adsorption of CD3CN at 298 K: (a) initial, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.5, (f) 1, (g) 2, (h) 3, (i) 5 mbar of CD3CN. (B) Evolution of the O-H stretching region of USY-H with increasing CD3CN pressure at 298 K: (a) initial, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.5 mbar of CD3CN.

visible, which are attributed to νs(CD3), acetonitrile H-bonded to bridging OH groups, and coordinated acetonitrile, respectively. The formation of strong H-bonds with the bridging OH groups is accompanied by the development of two very broad bands with maxima at ca. 2700 and ca. 2400 cm-1 (Figure 5B), respectively, as already mentioned above for USY-H. At higher pressures a band at 2262 cm-1 belonging to CD3CN H-bonded to SiOH groups becomes apparent, the intensity of which grows with coverage and corresponds with a decrease in intensity of the silanol peak at 3744 cm-1. Figure 5B shows the development of the OH region with increasing pressure and it is clear to see that all six OH types are involved in interaction (spectra a-e). It appears that by admission of 0.5 mbar (spectrum e) the EF, HF, LF, and LF′ species are saturated, leaving behind only silanol and HF′. On admission of 5 mbar of CD3CN the silanol bands are shifted from 3740 to ca. 3450 cm-1 and are almost saturated, at which point the HF′ species are also saturated (spectrum not shown). This corresponds to a shift in the position of the band at 2300 to 2288 cm-1 (Figure 5A), which indicates H-bonding to potentially weaker bridging OH groups. Pelmenschikov30 reported bands at 2300 and 2284 cm-1 and assigned them to HF and LF species in USY, respectively. Though that does not appear to be the case in this situation, a degree of heterogeneity does appear to have been detected within the acidic bridging OH’s. 3.4. Adsorption of C5H5N. USY-H. Adsorption of C5H5N produced two sets of bands in the 3000-2500 and

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and 3567 cm-1 representing the bridging OH groups have now undergone an almost total loss of intensity, and a broad band with a maximum at ca. 2800 cm-1 is now apparent. This is indicative of shifts of ca. 800 and 750 cm-1 for the HF and LF species, respectively. Kubelkova et al.33 have reported similar shifts in HY and HX zeolites and assigned them to the formation of PyH+...-O(Zeo) ionpair complexes. The sharp bands in the region 31503000 cm-1 are due to C-H stretching vibrations of pyridine. USY-L. Adsorption of pyridine on USY-L (Figure 6A,B) gave very similar spectra to those recorded on USY-H. Bands were assigned as with the previous sample, with frequencies at 1634 and 1543 cm-1 indicative of PyH+, and bands at 1620 and 1454 cm-1 assigned to pyridine coordinated to cationic aluminum LAS present within extraframework material. A broad band with maximum at ca. 2800-2900 cm-1 represents a shift of bridging OH groups which have formed a PyH+...-O(zeo) ion pair [∆ν(OH) ) ca. 750-800 cm-1]. All OH bandsssilanol, EF, HF, HF′, LF, and LF′shave interacted to a greater or lesser degree with pyridine at this coverage. Most likely the more acidic bridged OH species have led to protonation to form the PyH+ ion. 4. Discussion

Figure 6. (A) FTIR spectra showing the 4000-2500 cm-1 region of pyridine desorption at 423 K on (a) USY-H and (b) USY-L (stippled and solid spectra are recorded before and after pyridine adsorption). (c) and (d) are the subtracted spectra showing the spectral changes due to pyridine on USY-H and USY-L, respectively. (B) The corresponding spectra in the 1650-1450 cm-1 region are shown.

1700-1400 cm-1 regions, respectively (Figure 6A,B). The former were associated with various C-H stretches and the latter with the 8a and 19b ring vibration modes of the pyridine molecule.10 An examination of the OH region showed that all three types of OH groups interacted with pyridine at 423 K, whereas in the ring vibration region, five new bands are apparent. The bands were assigned to the 8a and 19b ring vibration modes of the pyridinium ion (at 1635 and 1545 cm-1) and pyridine coordinated to triply coordinated Al3+ LAS (1622 and 1454 cm-1), respectively. The coordinated pyridine frequencies match those reported by Morterra et al.25 on transitional aluminas, whereas those of protonated pyridine (PyH+) match those reported by Parry.16 He also reported that the fifth band, at 1490 cm-1, can be indicative of not only protonated or coordinated pyridine, but H-bonded (e.g., to weak Brønsted acid sites) pyridine as well. However, the latter gives rise to a weak band at this frequency, as does coordinated pyridine, whereas PyH+ gives a very strong absorption. Therefore, this band is also assigned to the PyH+ ion. No bands are assigned to weak H-bonding pyridine, as it is probable that these species have been removed through the vacuum treatment. Figure 6A shows that at this coverage of pyridine the terminal silanol band (3739 cm-1) is affected slightly. Hence, we conclude that only slight interaction with pyridine has occurred, which could lead to weak H-bonding. The HF and LF hydroxy species have, on the other hand, interacted with pyridine and are apparently acidic enough to have led to protonation of pyridine. The peaks at 3632

4.1. Adsorption of Carbon Monoxide. In both samples we see interaction with terminal silanol bands and the bridged OH groups found in the supercages (HF). However, in neither sample did CO interact with the LF species, the bridged OH groups positioned within the sodalite cages. CO is far too large to fit inside the cage when adsorbed at low temperature21 and too weak, in terms of its bascity, to abstract the H from the OH group situated at the cage mouth. The advantages of CO were (i) that due to the sensitivity of the C-O stretching mode, LAS and BAS could be differentiated (even though the peaks were very close to one another), and (ii) BAS of similar strength could be distinguished, not only using the ν(C-O) frequencies but from the red shifts induced in the OH bands. In both samples the weak BAS in the form of silanol groups could be separated from the strongly acidic BAS from bridging hydroxyls within the supercages. In the USY-L sample heterogeneity within the HF species was also detectable. According to many authors,6,34,35 EF material leads to enhanced Brønsted acidity in USY zeolites. This can be observed in the USY-L sample, though the presence of EF material also causes interaction with the bridging OH groups, leading to HF′ and LF′ species, which prevent their interaction with CO. Hence, using CO as a probe, their acidity cannot be determined. Whether this is due to physical blocking of the pores by this extraframework material is unclear. These results are in contrast to those reported by Makarova et al.,34 however, who, using a USY zeolite sample of Si/Al ) 3.0, observed enhanced Brønsted acidity in those OH (HF) groups perturbed through interaction with extraframework LAS. The discrepancies between these results highlight, first, the strong dependence of the acidity in USY zeolites on the dealumination process and resultant Si/Al ratio and, second, the ability of CO to assess the accessibility of those generated acidic sites. (33) Kubelkova, L.; Kotrla, J.; Florian, J. J. Phys. Chem. 1995, 99, 10285. (34) Makarova, M. A.; Al-Ghefaili, K. M.; Dwyer, J. J. Chem. Soc., Faraday Trans. 1994, 90, 383. (35) Mirodatos, C.; Barthomeuf, D. J. Chem. Soc., Chem. Commun. 1983, 39.

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In the case of the USY-L sample, CO was also able to detect LAS belonging to Na+ cationic impurities as well as Al3+ sites present in extraframework material. The latter assignment was made on the basis that Al3+ within the lattice occupies tetrahedral Si4+ sites and is, therefore, coordinatively saturated. Within extraframework material, however, the likelihood of coordinatively unsaturated Al3+ ions, which are accessible to CO, is higher. A recent paper by Ma et al.,36 on the other hand, has suggested that tricoordinated framework aluminum species are generated by dealumination in MCM-22, which can serve as Lewis acid centers. No aprotic sites were detected within USY-H, which contrasts with the results obtained through acetonitrile and pyridine adsorption (see following section). 4.2. Adsorption of Acetonitrile. Because of its stronger proton affinity, acetonitrile interacted with the LF species in both samples. It is accepted that at the adsorption temperature acetonitrile is not capable of entering into the sodalite cages, but yet interaction is clearly evident. It would seem, therefore, that from its position of nearest approach to the OH groups (being that of the cage entrance), acetonitrile is able to induce an interaction. The disappearance of the bridged LF species in the OH region of the spectra coincides with saturation of the band at ∼2300 cm-1 in both samples. However, this occurs simultaneously with the removal of the HF species, thus denying the possibility of discriminating between the two bridged species. Interaction with extraframework material in USY-L was also observed via the disappearance of the OH bands EF, HF′, and LF′, but again there was a lack of corroborating evidence in the C-N stretching region in order to differentiate them from HF or LF species. These strongly acidic bridging OH groups could, however, be distinguished from the weaker silanol bands and from acetonitrile coordinated to LAS. It should be noted that the interpretation can become complicated by the fact that one cannot tell protonated and Lewis sites apart, both possessing the same wavenumber in CtN stretching region.17 In a study of acetonitrile adsorption on various zeolites, Thibault-Starzyk et al. proposed that protonation only occurs above 625 K with HY zeolites,17 and so we assign the bands at 2332 (USY-H) and 2325 cm-1 (USY-L) to coordination of acetonitrile to LAS. It is readily apparent that the band visible in the USY-L sample (2325 cm-1) displays a greater intensity than the bands assigned to acetonitrile interaction with bridging and terminal OH groups, whereas the band at 2332 cm-1 in sample USY-H is relativley small in comparison to the two corresponding bands. If the intensity of these bands is taken as a measure of the relative quantity of the Lewis acid, bridging OH, and terminal OH sites available, then we can propose that USY-L has more Lewis acid sites available than USYH, most likely present within extraframework alumina. The lower wavenumber in the USY-L sample could, therefore, be a reflection of the presence of weaker LAS present in this extraframework material. This explanation could also apply to the band at 2262 cm-1 attributed to weak H-bonding. In USY-H the band is at 2274 cm-1 and attributed to interaction with SiOH groups.29 The lower wavenumber in USY-L may be indicative of weaker AlOH groups present on the EF material. 4.3. Adsorption of Pyridine. As with acetonitrile, pyridine was able to interact with the LF BAS as well as with the silanol and HF groups in both samples. In fact, (36) Ma, D.; Deng, F.; Fu, R.; Han, X.; Bao, X. J. Phys. Chem. B 2001, 105, 1770.

Daniell et al.

in the USY-L sample all six OH species were found to interact with the probe molecule, though interaction with the weak silanol bands after pumping at 423 K was only slight. One drawback of pyridine, however, is that due to its strong basicity, it is impossible to distinguish BAS of similar strength, either through shift of the OH stretching frequency or by a shift of a ring vibration mode. Coordinated pyridine, however, gives rise to vibration modes at different frequencies from H-bonded pyridine and protonated pyridine, PyH+, and therefore allows these species to be discerned. Hence, in USY-L and USY-H, where H-bonding was minimal, the LAS and BAS were clearly distinguishable by comparing the normal modes of coordinated pyridine and the PyH+ ion, respectively. LAS were detected in both the samples, even though CO failed to detect their presence in the USY-H sample, but whereas CO could be used to differentiate between Na+ and Al3+ ions, this was not possible using pyridine. Indeed, the spectra of the two samples in the ring vibration region were virtually identical, despite the fact that when using CO or acetonitrile differences relating to the different nature and relative abundances of the various acid sites were observed. 4.4. Specificity vs Basicity of Probe Molecules. In this work three probe molecules of varying proton affinity have been used to characterize the acidity of two USY zeolites. In general, it can be seen that the least basic probe (CO) is the most site specific, being able to distinguish between (i) LAS and BAS (though not in USYH, where it detected no LAS), (ii) BAS sites of similar strength, and (iii) different cations. However, exactly this softness prevents it from interacting with sites with which, despite its small size, it cannot physically come into contact with when adsorbed at low temperature. This is, on the other hand, achieved by using a stronger base, e.g., acetonitrile or pyridine. Their strength allows them to induce an interaction, despite spatial separation from the OH groups. The drawback here, however, is plain to see. Enhanced basicity which allows interaction with all acid sites reduces the specificity and, hence, the ability to distinguish between sites. This is more extreme the stronger the base. From this dilemma we can draw two conclusions: (i) there is no universal probe molecule for the characterization of acidity in zeolites, and (ii) one needs to clearly define the goals from the outset of such analysis, to make the tradeoff that is basicity vs specificity. Conversely, this tradeoff can be seen as advantageous. For the catalytic chemist knowledge of the type and strength of acid sites is only important for those sites which are feasibly involved in the reaction. Hence, a probe molecule that can selectively differentiate between sites such as, in this case, the LF and HF species can be of great worth, particularly when studying cracking catalysts in which only the BAS located in the supercages are accessible for alkane sorption.37 It should be noted, however, that direct correlations between spectroscopic measurements (such as shifts of the OH stretching frequencies) and observed catalytic activity cannot always be achieved. In a recent study of probe molecule adsorption on different zeolite materials, Kotrel et al.38 established that a relationship between the ∆ν(OH) observed on adsorption of CO and the activtiy toward n-hexane cracking was only obtained for materials of similar structure, i.e., where factors such as pore size did not have an overriding influence. (37) Eder, F.; Stockenhuber, M.; Lercher, J. A. J. Phys. Chem. B 1997, 101, 5414. (38) Kotrel, S.; Lunsford, J. H.; Kno¨zinger, H. J. Phys. Chem. B 2001, 105, 3917.

FTIR Study of USY Zeolites

5. Conclusions We have shown that the amount of information obtained from probing an acidic surface greatly depends on the basicity of the probe molecule. A trade-off is required between basic strength, which can allow interaction with acid sites despite spatial separation, and specificity. CO has been shown to be almost ideal in allowing BAS and LAS of similar strength to be determined but displays a certain insensitivity toward LAS compared with acetonitrile or pyridine and is incapable of interaction with LF species when measurements are performed at low temperature. These conditions are required, however, to see interactions with BAS, whereas pyridine and acetonitrile can be used at temperatures closer to actual reaction temperatures. Pyridine and acetonitrile, on the other hand, show very little specificity in terms of BAS, since they react with all BAS present and allow no discrimination between sites. Su and Barthomeuf39 used benzene at room temperature as a probe molecule for the characterization (39) Su, B. L.; Barthomeuf, D. Zeolites 1993, 13, 626.

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of acid properties of a series of zeolites built from sodalite cages and hexagonal prisms, namely HY, LZY-82, HSAPO37, and HEMT. Similar to CO, benzene is a very soft base. Its molecular size, however, is comparable to that of pyridine, and it is consequently hindered by spatial constraints when used to titrate protons. Although these spacial constraints do not exist in HEMT, benzene again does not titrate all protons, presumably because it is too weak a base to attract all protons located in the sodalite cages. It would appear, therefore, that no single probe molecule is capable of providing all the desired information and that use of several probe molecules is necessary in order to attain a more detailed understanding of the acidic properties of zeolites. Acknowledgment. The work done in Munich was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and the Fonds der Chemischen Industrie. LA010345A