Infrared Spectroscopy of Heterogeneous Catalysts: Acidity and

Aug 13, 2010 - An HY zeolite, an ultrastable HY zeolite (USY), and a rare earth exchanged Y-zeolite (REY) have been characterized by IR spectroscopy o...
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J. Phys. Chem. C 2011, 115, 937–943

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Infrared Spectroscopy of Heterogeneous Catalysts: Acidity and Accessibility of Acid Sites of Faujasite-Type Solid Acids† Tania Montanari, Elisabetta Finocchio, and Guido Busca* Dipartimento di Ingegneria Chimica e di Processo, UniVersita` di GenoVa, P.le J.F.Kennedy, I-16129, GenoVa, Italy ReceiVed: April 20, 2010; ReVised Manuscript ReceiVed: July 21, 2010

An HY zeolite, an ultrastable HY zeolite (USY), and a rare earth exchanged Y-zeolite (REY) have been characterized by IR spectroscopy of adsorbed CO and pivalonitrile. USY zeolite presents the strongest Brønsted and Lewis acid sites. However, HY and REY present a higher concentration of acid sites. HY, which contains relevant amounts of extraframework alumina species, is more reactive toward pivalonitrile. The location, accessibility, and structure of acid sites in the three catalysts are discussed. Introduction Infrared (IR) spectroscopy is a well established and widely used technique for the characterization of solid catalysts and for mechanistic studies of heterogeneously catalyzed reactions. After the very first application of IR spectroscopy to the surfaces of catalytic materials, reported in 1937 by Buswell et al.,1 concerning water adsorbed in montmorillonite, this technique was developed in the 1940s and the early 1950s by Terenin and his pupils at the Leningrad University2 and later by Eischens and co-workers at Texaco laboratories (Beacon, New York)3 and by Sheppard and Yates at the Cambridge University in U.K.4 In spite of this apparent oldness, several new developments and applications of IR techniques have been proposed until now, such as the development of in situ and “operando” techniques.5 Alfons Baiker and his co-workers, among many other very relevant contributions in the catalysis subject, recently gave further push to the field of vibrational spectroscopy of heterogeneous catalysts with the development of dynamic FTIR techniques,6 coupled TG-FTIR7 and PM-IRRAS8 techniques for gas-solid studies, as well as ATR studies at the liquid-solid interface.9,10 IR spectroscopy of adsorbed probe molecules has been largely utilized to characterize site strength and accessibility on solid acids. Low temperature adsorption of carbon monoxide11–13 has been mostly applied to characterize Brønsted acidity, while accessibility has been mainly investigated using substituted pyridines as hindered probes.14 We proposed the use of hindered nitriles to probe the acidity and accessibility of protonic and cationic sites in zeolites.15 Data obtained on H-FER,15 H-MFI,16 H-BEA,17 regular and dealuminated H-MOR,18,19 and H-MCM2220 allowed us to complete the discussion of the catalytic behavior of these industrial solid acid catalysts.21 The excellent activity of protonic zeolites22 is due to two main properties: the strong Brønsted acidity of bridging Si-(OH)-Al sites and the shape selectivity effect. “Confinement effects” implying additional van der Waals interactions of molecules with the cavity walls make the cavities’ unique solvation and reactivity environments and play a relevant role in the catalysis by zeolites. Xu et al.23 concluded that the active sites of protonic †

Part of the “Alfons Baiker Festschrift”. * To whom correspondence should [email protected].

be

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zeolites have very similar strengths, geometric factors playing a central role in differentiating the catalytic behavior. Additionally, real zeolite catalysts are frequently pretreated in various ways such as steaming, to increase activity and selectivity. As a result of several different factors, zeolites are not “perfect” structures: extra-framework species (EF) and, correspondingly, defects in the framework structure may exist. The external surface is also active while the concentrations of active sites in different positions of the solid may be different. In most cases, the role of these factors is still under debate. Faujasite-type zeolites, usually with relatively high Si/Al ratios (H-Y) and frequently stabilized by dealumination (ultrastable Y, USY), are among the most relevant solid acids for their industrial applications. The main component of fluid catalytic cracking (FCC) catalysts today is rare-earth (RE) exchanged FAU zeolite (RE-Y or RE-USY).24 H-Y zeolites are typical components or supports of hydrocracking catalysts, to provide acidity.25 In both cases, the wide dimension of the channels of faujasite allows quite heavy molecules to be cracked. HY is also the main component of catalysts for solid-catalyzed isobutane/butylene alkylation.26 The “molecular” effects of stabilization and dealumination procedures, the nature and accessibility of the most acidic sites, and the possible synergies among framework and extraframework sites as well as among Lewis and Brønsted acid sites of Y zeolites are still the object of investigation and debate in the very recent literature.27–34 Here, we report data concerning the IR study of three different faujasite-type solid acids using both low-temperature CO adsorption and room temperature pivalonitrile adsorption, with the aim to clarify the nature and accessibility of acid sites of these important catalysts. Experimental Section USY (sample A, Si/Al ) 30) catalyst was supplied by Zeolyst (CBV 720H). HY (sample B, Si/Al ) 5) and REY zeolite (sample C; Si/Al ) 2.7, RE2O3 12.2%, mixed rare earths) were supplied by Grace. As reference materials, amorphous silicas from Degussa (aerosil 200, ca. 200 m2 g-1) and commercial silica-alumina (13% Al2O3) cracking catalyst from STREM Chemicals (330 m2 g-1) have been used. The FT-IR spectra were recorded with Nicolet Magna 750 and Nexus instruments with a resolution of 4 cm-1 using pressed

10.1021/jp103567g  2011 American Chemical Society Published on Web 08/13/2010

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Figure 1. Structure of faujasite and the location of crystallographically different oxygen atoms (left) and of extraframework cationic positions (right).

disks of pure zeolite powders, activated by outgassing at 673 or 873 K into the IR cell. A conventional gas manipulation/ outgassing ramp connected to the IR cell was used. The CO adsorption procedure involves contact of the activated sample disk with CO upon cooling the cell with liquid nitrogen, producing a sample temperature of 130 K, and following warming upon outgassing and vapors at increasing pressures and outgassing in steps at r.t. or increasing temperatures. Pivalonitrile (2,2-dimethylpropionitrile) was purchased from Aldrich, and CO cylinders were supplied by Siad. Results and Discussion The ideal faujasite structure (Figure 1) is formed by wide supercages (13 Å diameter) accessed through 12-member silicate rings with 7.4 Å diameter, much smaller sodalite cages accessed through 6-member silicate rings, and hexagonal prisms connecting the sodalite cages. All of the catalytic chemistry of faujasites is supposed to occur in the supercages. Four crystallographically different oxygen atoms exist in the structure, two of which (1 and 4) point into the supercage, one (2) into the sodalite cage, and the fourth (3) into the hexagonal prism. Several possible extraframework cationic positions may be occupied,35 the most common of which are reported in Figure 1, right: site I, in the center of the hexagonal prism; site II, at the center of the hexagonal window between the sodalite cage and the supercage; site III, near the center of the square window between the sodalite cage and the supercage; site IV, at the center of the supercage; site V, at the center of the 12-ring window between supercages. To characterize acid faujasites, we will use CO and pivalonitrile (PN) as probe molecules. As a reference, we will briefly report here the results of CO and PN adsorption on silica and silica-alumina. The adsorption of CO over pure silica (Figure 2, top) causes the partial shift of the sharp band of the free silanols (3745 cm-1) to lower frequencies. The new component, assigned to silanols interacting with CO through the lone pair at the carbon atom, is centered at 3670 cm-1, but a component at 3590 cm-1 is also evident in the subtraction spectrum. This suggests that, in the case of pure silica, in spite of the sharpness of the band of the terminal silanols, some heterogeneity of these sites occurs, some of them being more acidic than others (∆ν ∼ 75 cm-1; ∆ν ∼ 155 cm-1). In the CO stretching region, two νCO bands due to adsorbed carbon monoxide appear. The lower frequency one is observed at 2140 cm-1, i.e., almost at the same position of the band of liquid CO. This band is observed very frequently upon adsorption of CO at low temperature on solids and is assigned to liquidlike CO. The band at 2155 cm-1 is well shifted above, and this indicates that an electron withdrawing center interacts with the

Figure 2. Top: FT-IR spectra of fumed silica activated at 773 K, recorded at 130 K (a), after saturation with CO at 130 K (b), and after further outgassing at 130 K (c). Bottom: FT-IR spectra of silica-alumina activated at 773 K and after saturation with CO at 130 K and outgassing at increasing temperatures of 130-180 K.

carbon atom. This band is usually assigned to CO H-bonded to the silanol groups. A slight shift up of the band to 2157 cm-1 might confirm the multiplicity of such species, in agreement with the multiplicity of the silanol groups discussed above. CO adsorption over amorphous silica-alumina (Figure 2, bottom) causes again the shift of the band of silanol groups from 3745 cm-1 to give at least three weak components, at 3667, 3580, and in the region 3500-3400 cm-1, due to at least three different H-bonded complexes with CO. The spectra of adsorbed CO show a νCO band at very high frequencies (2230 cm-1), a weak broad component near 2200 cm-1, and sharper bands at 2173 and 2156 cm-1. The higher frequency νCO bands must be attributed to CO adsorbed on Lewis acidic Al3+ ions. The bands at 2173 and 2156 cm-1 may be attributed to CO interacting with two different kinds of hydroxy groups, one of them being strongly acidic (2173 cm-1) and the other one weakly acidic and silica-like (2156 cm-1). Because of the absence of other evident features, it is straightforward to attribute the three observed νOH absorptions at 3667, 3580, and 3500-3400 cm-1 to different types of terminal silanol groups that when unperturbed give place to an unresolved band near 3745 cm-1. The first two components are very similar to those observed on pure silica (see Figure 2, top), but the third one (3500-3400 cm-1) is only evident in silica-aluminas. As discussed elsewhere,36 the presence of Al ions gives rise to few silanol groups whose acidity is strongly enhanced: the shift undergone by the band of these OHs upon CO adsorption is ∼250-350 cm-1. In Figure 3, the spectra relative to the adsorption of pivalonitrile (PN) on silica (top) and silica-alumina (bottom) are reported. In both cases, PN interacts with all surface hydroxy groups, shifting the OH stretching band down. In both cases, the main maximum is found around 3400 cm-1, and corresponds to the CN stretching of 2234 cm-1, similar to that observed in the liquid molecule.37 The small peak at 2306 cm-1 observed on silica is due to an overtone absorption of PN. Over silica-alumina, however, additional features are observed. The broad νOH absorption in the region 3400-2500 cm-1, associated with the CN stretching component at 2250 cm-1, is attributed to PN interacting with strongly acidic silanol groups, in agreement with what was previously found for adsorption of

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Figure 3. FT-IR spectra of fumed silica (top) and silica-alumina (bottom) activated at 673 K (full lines) and in contact with PN vapor 3 Torr (dashed lines, vapor phase spectrum subtracted). Figure 5. FT-IR spectra of sample A (USY) after activation at 773 K (a), in contact with CO 20 Torr at 130 K (b), and after outgassing at 150 K (c) and 180 K (d).

Figure 4. FT-IR spectra of acid faujasites after activation at 773 K (νOH region).

acetonitrile.38 The additional CN stretching component at 2296 cm-1 is attributed to PN interacting with Al3+ Lewis acid sites, similar to the strongest sites of alumina.38 Previous studies showed that PN can allow a study of the accessibility of sites of zeolites.15 In fact, it is so hindered to not enter the 8-ring and 10-ring channels of H-FER and H-MFI zeolites. In contrast, PN does actually enter the 12-ring main channels of H-MOR, although perturbing only a small fraction of the internal OHs.18,19 Finally, PN was found to enter the surface “emisupercages” of H-MWW (H-MCM-22) zeolite, with its access to both sinusoidal channels and supercages forbidden or strongly hindered.20 In Figure 4, the spectra of the three faujasite catalysts under study are reported, after activation by outgassing at 500 °C. The spectrum of the catalyst A, characterized by a Si/Al ratio of 30, clearly shows three OH stretching bands, at 3743 cm-1, sharp and strong, 3627 and 3562 cm-1. Both lower frequency bands show components at their lower frequency sides. This spectrum is typical of USY zeolites.39,40 The strong band at 3743 cm-1 is assigned to νOH of free terminal silanols thought to be located at the external surface. The other two peaks are attributed to the two main kinds of “structural” hydroxy groups: the high frequency OH groups (HF) located in the supercages (3627 cm-1) and the low frequency OH groups (LF) located in the sodalite cages (3562 cm-1). Their low frequency shoulders we observe at 3600 and 3550 cm-1 are mostly assigned to HF and LF species, respectively, interacting with residual extraframework species.41 More recent data obtained on low Si/Al ratio

EF-free HY zeolites reported the identification of three and four different families of hydroxy groups in protonic faujasites. According to Romero Sarria et al.,29,30 besides HF and LF, a third band exists, observed at 3501 cm-1, assigned to OHs in the hexagonal prisms. Suzuki et al.32 reported the existence of four different hydroxy groups, absorbing at 3648 cm-1 (on oxygen site 1, see Figure 1, pointing to the supercage), at 3625 cm-1 (on oxygen site 1′ or 4, pointing also to the supercage), at 3571 cm-1 (on oxygen site 2, pointing to the sodalite cage), and at 3526 cm-1 (on oxygen site 3, pointing to the hexagonal prism). Due to the low Al content in our USY sample, virtually free from EF, the assignment of Suzuki et al.32 could be valid for us, although the frequencies we observe for the shoulders are quite different from those reported by these authors. In Figure 5, the spectra of CO adsorbed at low temperature over sample A are reported. Upon adsorption, the band at 3743 cm-1, due to external terminal silanols, is partially perturbed: it decreases strongly in intensity while a new component appears at 3658 cm-1. The moderate shift down of this band when silanols are interacting with CO, ∆ν ) 85 cm-1, is interpreted as evidence of the weak acidity of such groups, similar to those predominant on silica. During low temperature CO adsorption, the LF band seems to be essentially not perturbed, while the HF band fully disappears, being totally shifted down to 3278 cm-1. This is an indication of the location of the two different sites, one in the supercages (those responsible for the HF band) and the others in the sodalite cages, likely unaccessible to CO (the LF band). The strong observed shift down for the HF band, ∆ν ) 350 cm-1, is indicative of the very strong acidity of these groups. We can remark here that the shift down observed on this sample is even stronger than that typically observed on most zeolites, such as, e.g., 300-330 cm-1 in different H-MFI samples.21 The spectrum we observe is similar to that reported for CO adsorption on other USY samples.41 On the other hand, the position of the perturbed band is at lower frequency than that observed on extraframework material free HY zeolites having higher Al content (above 3300 cm-1 for samples with Si/Al ratios of 2.5-5.511,42,43). On the other hand, looking at our spectra, the new band due to νOH of bridging OHs perturbed by CO, a shoulder is apparent at the lower frequency side near 3185 cm-1. This may be due to a small fraction of more acidic supercage OHs, whose OH stretching should undergo a shift down, ∆ν ) 440 cm-1.

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Figure 6. FT-IR spectra of sample A (USY) after activation at 773 K (a), in contact with PN 20 Torr at r.t. (b, vapor phase spectrum subtracted), and after outgassing at r.t. (c), 473 K (d), 523 K (e), and 573 K (f).

In a recent paper, Niwa et al.44 attributed to a new OH absorbing at 3595 cm-1 the enhanced acidity of USY treated with ethylendiamminetetracetic acid. Indeed, we find some negative absorptions near 3600 cm-1 in the subtraction spectrum in Figure 5; thus, we cannot exclude that the shoulder at 3185 cm-1 formed upon CO adsorption is due to the shift of this component. In this case, the shift down of the OH stretching is ∆ν ∼ 415 cm-1, thus also associated with very strong acidity. Our data substantially confirm those of Navarro et al.27 According to these authors, steam-treated Y zeolites with EF aluminum removed by any known method give rise to HF OHs with exceptionally strong acidity (∆νOH 443 cm-1). We also confirm a very partial interaction of sodalite cage OHs with CO (small negative peak at 3562 cm-1 in the subtraction spectrum). The analysis of the right section of Figure 5 shows that the interaction of CO with terminal silanols gives rise, like on silica, to adsorbed CO characterized by νCO at 2156 cm-1, i.e., poorly perturbed with respect to the liquid phase value of near 2140 cm-1 (evident as a shoulder). This is a further confirmation of the weakness of this interaction, i.e., of the weakness of the Brønsted acidity of the external silanols of protonic zeolites. Instead, the interaction of CO with the supercage OHs gives rise to a species characterized by a sharp νCO at 2180 cm-1, thus confirming the strong acidity of the structural OHs of protonic zeolites. We can remark that this experiment does not provide any evidence of Lewis acidity in sample A: in fact, we do not find any significant absorption in the range 2240-2180 cm-1, where νCO of carbon monoxide interacting with Al3+ absorbs. In Figure 6, the spectra relative to the experiment concerning pivalonitrile (PN) adsorption on sample A are reported. PN apparently interacts with all surface hydroxyl groups of sample A. In fact, all three sharp bands observed on the activated samples fully disappear after PN adsorption. After contact with the vapor as well as after outgassing at r.t., the main OH stretching band is centered, very broad, at 3390 cm-1. This feature disappears after outgassing at 200 °C, with the corresponding recovery of the sharp band at 3743 cm-1. This allows us to attribute the broad band at 3390 cm-1 to the interaction of the external terminal silanols absorbing at 3743 cm-1 with PN. This confirms that the acidity of the predominat silanol on sample A groups is similar to that of silica. Additional broad νOH components at 3190 and 3090 cm-1 are observed in contact with the vapor and also after outgassing at room temperature and 473 K but disappear after outgassing at 523 K. These

Montanari et al. features could be associated with H-bonded complexes involving more acidic silanol groups whose OH stretching mode, when free, is almost superimposed to the main band near 3743 cm-1. This indicates that more acidic terminal silanol groups may exist together with the predominant weakly acidic ones. After outgassing at 473 and 523 °C, a couple of broad absorptions is still observed with the minimum at 2577 cm-1, identifying the A and B components of the so-called ABC spectrum typical of strong quasi-symmetrical hydrogen bonds. This feature, also present before outgassing, corresponds to the still complete disappearance of both HF and LF OH bands, thus being assigned to the strong interaction of both sodalite cage and supercage hydroxyl groups with PN. In agreement with this, outgassing at temperatures higher than 523 °C causes the recovery of both HF and LF bands, and the disappearance of the ABC spectrum. However, traces of organic matter resist outgassing at 573 K, thus showing decomposition or transformation of PN at such high temperatures. The analysis of the CN stretching region, expanded in the inset of Figure 6, allows confirmation of these data. Weak H-bonding on silanols (νOH at 3390 cm-1) corresponds to νCN at 2235 cm-1; stronger H-bonding with silanols (νOH at 3190 and 3090 cm-1) corresponds to νCN at 2250 cm-1, with a tail at higher frequency; quasi-symmetrical H-bonding (ABC spectrum) corresponds to νCN at 2277 cm-1; the νCN band at 2297 cm-1 provides evidence of the presence of Lewis acid sites. The modification of the intensities of the νCN bands with outgassing under heating is likely due to deasorption/readsorption and a consequent redistribution of the PN molecules among weaker/ stronger and more/less accessible sites. These data provide additional information indicating that a distribution of Brønsted acidity exists among external terminal silanols. Moreover, the strong acidity of supercage hydroxyl groups is confirmed. Additionally, it is evident that also sodalite cage OHs (those responsible for the LF νOH band) are reversibly but completely perturbed by a large and hindered molecule like PN. The spectrum of sample B in the OH stretching region (Figure 4) is much more complex than that of sample A, with a strong, quite broad component centered at 3550 cm-1 and a shoulder at 3525 cm-1 (split LF band) and at 3622, 3600 cm-1 (split HF band). Additionally, a complex and evident absorption is found in the region 3650-3690 cm-1. The last feature is usually attributed to hydroxyl groups on extraframework material (EF), while the two components at the lower frequency sides of both HF and LF bands (3600 and 3525 cm-1) are attributed either to supercage and sodalite OHs interacting with EF material or, according to Suzuki et al.,32 to OHs on oxygen sites 1′ or 4 and 3, respectively. The component in the range 3750-3730 cm-1, also multiple, is very weak in this sample, showing a small concentration of free silanol groups, likely because part of the EF material is located at the external surface and interacts with silanol groups. The overall spectrum is typical of HY zeolite containing a significant amount of extraframework species.45 In Figure 7, the spectra relative to the low temperature adsorption of CO on sample B are reported. Upon adsorption of CO, two broad features clearly form at 3535 and 3325 cm-1, while all the components above 3600 cm-1 are certainly involved in the interaction, with the corresponding bands being almost or completely disappeared in contact with CO. It is not possible to determine whether the LF band is also involved or not, because it is superimposed to the new band formed near 3535 cm-1 upon CO adsorption. Upon outgassing, the new bands

Infrared Spectroscopy of Heterogeneous Catalysts

Figure 7. FT-IR spectra of sample B (H-Y) after activation at 773 K (a), in contact with CO 20 Torr at 130 K (b), and after outgassing at 140 K (c), 160 K (d), and 170 K (e). Top: unsubtracted spectra.

progressively disappear, while the high frequency components are progressively recovered. The spectrum obtained after outgassing at 173 K shows the new band at 3535 cm-1 fully disappeared while the second new band now located at 3292 cm-1 still appeared. Additionally, it shows almost perfectly recovered all features of the spectrum of the activated sample except for the HF band which is still evidently less intense than before. This also is evident in the subtraction spectrum, that shows the component near 3620 cm-1 as the main negative feature. This allows us to attribute the band shifting from 3325 to 3292 cm-1 to the perturbation of supercage OH groups responsible for the HF band, while the component formed at 3535 cm-1 is likely mostly due to the perturbation of the OHs on extraframework species (EF). This would confirm the significant Brønsted acidity of EF material (∆ν ∼ 150 cm-1). It seems likely that at least most of the sodalite cage OHs do not interact with CO as on sample A. The formation of the νOH bands at 3535 and 3325-3292 cm-1 corresponds to the formation of a νCO band shifting from 2165 to 2178 cm-1, confirming the assignment of both bands to CO H-bonded on acidic OHs, while the strong component observed only at high CO coverages at 2141 cm-1 is due to liquid-like CO. An extremely weak feature at 2228 cm-1 is the only possible trace of Lewis acid sites in this experiment. Due to the complexity of the spectrum in the νOH region, the study of the adsorption of PN on sample B does not allow us to have clear conclusions on the involvement of supercage and sodalite components in the interaction. In Figure 8, the subtraction spectra obtained on samples A and B after PN adsorption followed by outgassing at 423 K are reported and compared. The negative peaks corresponding to the perturbation of free hydroxy groups show that external silanols, EF hydroxy groups, and HF supercage OHs do participate in the interaction, while in this case the participation of sodalite cage OHs is not evident. The positive features confirm the much decreased concentration of terminal silanols on sample B with respect to sample A, with the decreased intensity of the νOH band at 3390 cm-1. The perturbation of the νOH band of hydroxy groups of extraframework species seems to be associated with the formation of a main νOH band, very broad, centered near 3100 cm-1, confirming their medium-high acidity, while the species responsible for strong quasi-symmetrical H-bonding interaction are the supercage OHs responsible for the HF band. The amount of Lewis acidity seems to be only slightly enhanced on sample

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Figure 8. Subtracted FT-IR spectra (activated sample spectra subtracted) of PN adsorption on sample A (USY) after outgassing at 423 K and on sample B (H-Y) after outgassing at 423 and 473 K.

B with respect to sample A. It seems likely that EF material tends to hinder the entrance of PN in the cavities in the case of sample B, so avoiding interaction with sodalite cavity sites as well as, likely, with part of sites also in the main supercage. Only in the case of sample B, we observe at relatively high temperature (250 °C) upon outgassing an important decomposition of PN, giving rise to the formation of new strong bands at 3300-3150 and 2110 cm-1, which could be tentatively assigned to adsorbed HCN46 (νCH and νCN, respectively) produced by PN decomposition/elimination. In agreement with this, the CH stretching and bending regions of the spectrum are also modified, and are now consistent with the spectrum of polyisobutene formed upon adsorption of isobutene over several solid acids47 including protonic zeolites.48 Thus pivalonitrile decomposes into HCN and isobutene, which later polymerizes. This may be an indication of the particular reactivity of EF material in HY, possibly with the synergy between Brønsted and Lewis sites.31 The spectrum of our REY sample is reported, after activation, in Figure 2. The spectrum shows two main bands centered at 3629 and 3532 cm-1, with shoulders at 3610 and 3550 cm-1. Additional bands are found weakly split at 3690, 3676, and 3744 cm-1, the last very weak. The spectrum is quite similar to that of other REY zeolite samples.49–51 In particular, the weak band at 3744 cm-1 is assigned to silanol groups likely mainly located at the external surface of the zeolite, while the split band at 3690 and 3676 cm-1 is usually assigned to extraframework material, which may include OH groups on RE oxide species. The couple of bands at 3629 and 3532 cm-1 (with the corresponding shoulders at 3610 and 3550 cm-1) is typical of bridging Si-OH-Al Brønsted acidic sites of FAU zeolites (HF and LF, respectively), although the lower frequency component, being very sensitive to the nature of the rare earth cation, certainly involves an interaction with such cations. The adsorption of CO (Figure 9) provides again evidence of the strong acidity of supercage OHs, which are shifted to near 3330 cm-1, with the corresponding formation of a CO band at 2176 cm-1. In any case, it seems that the Brønsted acid strength of the most acidic sites of sample C (REY) is smaller than that of most of the supercage sites of sample A (USY). The spectra obtained after adsorption of PN on sample C (Figure 10) at r.t. give rise to an at least partial perturbation of all surface hydroxy group families. A partial perturbation of the band at 3532 cm-1 is observed in the presence of the PN vapor (Figures 10b and 11b) but is restored even by outgassing at r.t. (Figure 10c and 11c). Following outgassing at 373 K

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Montanari et al. after contact with PN (thus the maximum for the OHs which are perturbed by PN) is measured at 3529 cm-1 (Figure 11, inset), suggesting that the more shifted down the LF OH stretching the more accessible are the hydroxy groups. Conclusions

Figure 9. FT-IR spectra of sample C (REY) after activation at 773 K (full line) and after contact with CO 20 Torr at 130 K and outgassing at 180 K (dashed line). Below: the corresponding subtraction.

Figure 10. FT-IR spectra of sample C (REY) after activation at 773 K (a), after contact with PN vapor (b), and after outgassing at r.t. (c), at 373 K (d), and at 473 K (e).

Figure 11. Expansion of Figure 10. FT-IR spectra of sample C (REY) after activation at 773 K (a), after contact with PN vapor (b), and after outgassing at r.t. (c), at 373 K (d), and at 473 K (e).

shows that the strongest interaction is that of the OHs absorbing at 3629 cm-1, which resists such an outgassing and gived rise to a very pronounced A,B,C pattern. Actually also Lewis bonded species resist this outgassing (CN stretching at 2295 cm-1). This confirms that the interaction of PN with OH responsible for the HF band at 3629, 3610 cm-1 does not undergo substantial hindering. On the contrary, the hindered nitrile is able to interact only weakly with OHs located in the sodalite cage, responsible for the LF band at 3532, 3550 cm-1. An analysis of the shape of the LF band does not allow a clear splitting to be revealed, in spite of its asymmetry. In any case, the position of the maximum of the peak in contact with PN is measured at 3533 cm-1, while the minimum in the subtraction spectrum measured

The comparison of the experiments described here allows us to propose the following conclusions. The shift down of νCO of adsorbed carbon monoxide (∆ν ) 410 or 450 cm-1) indicates that stronger Brønsted sites may exist (or stronger OH · · · CO interactions may be established) in the cavities of USY than on REY and HY. According to our previous studies, these sites, located on supercages (thus on O1 or O4 positions), may give rise to stronger interactions also than those observed on H-MOR, H-MFI, and H-FER zeolites. This is in agreement with the data reported by Navarro et al.27 In any case, also the predominant normal OH groups on USY appear to establish stronger OH · · · CO interactions than those of HY and REY (∆ν ∼ 350 cm-1 with respect to 300-330 cm-1). The same Brønsted site acidity strength scale is obtained if νCN of H-bonded pivalonitrile is considered (shifted up to 2277 cm-1 on USY), taking into account that such a molecule, much more hindered, is not able to enter all the cavities or to reach all the existing sites. In particular, this may be the case of our B sample (HY with much EF material) when indeed it seems likely that only part of the HF band is perturbed by adsorption of PN. USY also displays the strongest Lewis acid sites, as deduced by the position of νCN of pivalonitrile adsorbed on Lewis sites (shifted up to 2297 cm-1 on USY). The strength of Lewis sites of USY resembles those of the strongest sites of silica-alumina and pure alumina. Sodalite cage OHs, responsible for the LF νOH band, appear to be reached by PN in the case of USY, giving rise, apparently, to strong interaction which is only destroyed above 200 °C, confirming previous data concerning acetonitrile.41 This may be due to the ability of the sCtN moiety of nitriles to penetrate the hexagonal six-ring window separating the supercage from the sodalite cage, the rest of the molecule being still located in the supercage. The different behavior of the nitriles from CO (which do not interact significantly with sodalite cavity OHs) may be due to its stronger basicity. Working with USY and pivalonitrile, we were unable to distinguish for their accessibility two components in the LF band, as previously done by Romero Sarria et al.29,30 using trimethylamine and low Si/Al ratio HY free from EF materials. These authors provided evidence of the existence, in their zeolite sample, of OH groups in the hexagonal prisms. In fact, the access of trimethylamine in these cavities is likely even more difficult than that of PN or totally forbidden. On the other hand, maybe this position is fully unoccupied in our high Si/Al ratio sample. The amount of both Brønsted and Lewis acid sites is far higher on REY than on USY, in agreement with its much lower Si/Al ratio and the presence of RE cations. Also, on REY, a much higher Lewis site to Brønsted site ratio has been observed (ratio between Lewis to Brønsted bonded PN CN stretching band). However, the strength of Lewis sites (measured using both CO and PN) of REY is lower than that on USY, being likely mostly associated to rare earth cations, whose Lewis acidity is indeed expected to be lower than that of Al ions due to their larger size.

Infrared Spectroscopy of Heterogeneous Catalysts In the case of REY, partial and weak interaction of LF sodalite cage hydroxy groups with PN is observed. Additionally, few hydroxy groups, mostly of the EF nature or interacting with EF materials (absorption near 3680 and 3600 cm-1), appear to be unaccessible to PN on REY. Both of these phenomena could be indicative of some steric hindrance effects on the penetration of PN, arising from the presence of RE cations and extraframework species on REY. In fact, rare earth ions such as La and Ce tend to occupy mostly I, I′, II, and V positions35 (see Figure 1), thus hindering the access to sodalite cages but also the diffusion among different supercages. Additionally, the occupation of I and I′ cationic position should result in the at least partial disappearance of OHs in the hexagonal prisms. In fact, we do not observe in REY absorption at 3500 cm-1 assigned by Romero Sarria et al.29,30 to OHs in the hexagonal prisms. The weakness of the band due to silanol groups (νOH near 3745 cm-1) in the spectra of both sample B (EF rich HY) and C (REY), unusual for acid zeolites, suggests that in both cases part of the cations and EF species are located at the external surface of the zeolite, perturbing or exchanging the silanol groups. The data on sample B (EF rich HY) confirm the mediumstrong Brønsted acidity of the hydroxy groups on EF material and the high reactivity of these species, possibly with the synergy between their Brønsted and Lewis sites. In fact, only over this material we observe the decomposition of PN producing polyisobutene and HCN at 523 K. References and Notes (1) Buswell, A. M.; Krebs, K.; Rodebush, W. H. J. Am. Chem. Soc. 1937, 59, 2603–2605. (2) Terenin, A.; Roev, L. Spectrochim. Acta 1959, 11, 946–957. (3) Mapes, J. E.; Eischens, R. P. J. Phys. Chem. 1954, 58, 1059–1062. (4) Sheppard, N.; Yates, D. J. C. Proc. R. Soc. 1956, A238, 69. (5) Thibault-Starzyk, F.; Seguin, E.; Thomas, S.; Daturi, M.; Arnolds, H.; King, D. A. Science 2009, 324, 1048–1051. (6) Jobson, E.; Baiker, A.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1 1990, 86, 1131–1137. (7) Eigenmann, F.; Maciejewski, M.; Baiker, A. Thermochim. Acta 2006, 440, 81–92. (8) Roedel, E.; Urakawa, A. Baiker A. Catal. Today, doi: 10.1016/ j.cattod.2009.11.013. (9) Bu¨rgi, T.; Wirz, R.; Baiker, A. J. Phys. Chem. B 2003, 107, 6774– 6781. (10) Bu¨rgi, T.; Baiker, A. AdV. Catal. 2006, 50, 227–283. (11) Zecchina, A.; Otero Area´n, C. Chem. Soc. ReV. 1996, 25, 187– 197. (12) Kno¨zinger, H.; Huber, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2047–2059. (13) Hadjiivanov, K. I.; Vayssilov, G. N. AdV. Catal. 2002, 47, 307– 511. (14) Nesterenko, N. S.; Thibault-Starzyk, F.; Montouillout, V.; Yuschenko, V. V.; Fernandez, C.; Gilson, J.-P.; Fajula, F.; Ivanova, I. I. Microporous Mesoporous Mater. 2004, 71, 157–166. (15) Montanari, T.; Bevilacqua, M.; Busca, G. Appl. Catal. A 2006, 307, 21–29. (16) Armaroli, T.; Trombetta, M.; Gutie`rrez Alejandre, A.; Ramirez Solis, J.; Busca, G. Phys. Chem. Chem. Phys. 2000, 2, 3341–3348. (17) Trombetta, M.; Busca, G.; Storaro, L.; Lenarda, M.; Casagrande, M.; Zambon, A. Phys. Chem. Chem. Phys. 2000, 2, 3529–3537.

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