A Study of the Nature, Strength, and Accessibility of Acid Sites of H

May 24, 2008 - spectroscopy of adsorbed nitriles (isobutyronitrile and pivalonitrile) and ... The adsorption of the hindered probe pivalonitrile stron...
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J. Phys. Chem. C 2008, 112, 9023–9033

9023

A Study of the Nature, Strength, and Accessibility of Acid Sites of H-MCM-22 Zeolite Maria Bevilacqua,† Daniela Meloni,‡ Franca Sini,‡ Roberto Monaci,‡ Tania Montanari,† and Guido Busca*,† Dipartimento di Ingegneria Chimica e di Processo, UniVersita` di GenoVa, p.le J. F. Kennedy 1, I-16129 GenoVa, Italy, and Dipartimento di Scienze Chimiche, UniVersita` di Cagliari, s.s. 554, BiVio per Sestu, I-09042 Monserrato (CA), Italy ReceiVed: February 5, 2008; ReVised Manuscript ReceiVed: March 20, 2008

The acidity of H-MCM-22 zeolite has been investigated by ammonia adsorption microcalorimetry and by IR spectroscopy of adsorbed nitriles (isobutyronitrile and pivalonitrile) and of aromatics (p-, o-, and m-xylene). The most abundant “zeolitic” OHs are exposed in the supercage system, absorbing predominantly at ca. 3618-3625 cm-1. Less abundant are those exposed in the sinusoidal channel system, predominantly absorbing in the range 3600-3618 cm-1. A third OH family, responsible for a weak band at 3575 cm-1, is also observed. The adsorption of the hindered probe pivalonitrile strongly supports the conclusion that emisupercages are exposed at the 001 basal plane of the MCM-22 layered structure, where zeolitic OHs of the same nature of those exposed in the supercages indeed exist. At the external surface, weakly Brønsted acidic silanol groups (possibly two slightly different families) and strongly acidic Lewis sites are also well-evident. Monoaromatics, as well as para-disubstituted benzenes (such as p-xylene) and isopropyl-group containing molecules (such as isobutyronitrile), enter easily not only the emisupercages but also both internal channel systems of MCM-22. On the contrary, the entrance of tert-butyl-containing molecules, such as pivalonitrile, to the internal cavities is forbidden or at least strongly hindered. These data suggest that alkylation of benzene could occur both at the external emisupercages (with no diffusion limits) and in the internal cavities, without strong diffusion limits. The reactivity in the internal supercages could become significant above 200 °C when the selectivity to monoalkylation increases significantly. Alkylation of phenol with tert-butyl alcohol very likely occurs at the external surface catalyzed by Lewis acid sites. 1. Introduction Protonic zeolites are largely applied in the industry as environmentally friendly shape selective catalysts of several hydrocarbon conversion reactions.1–3 Mobil’s proprietary MCM22 zeolite4–6 is the catalyst of the EBMax ethylbenzene industrial synthesis process, consisting of the alkylation of benzene with ethylene performed in liquid phase at 160-220 °C. MCM-22 is also the component of the catalyst of the liquid-phase Mobil Rayteon process for cumene synthesis.7–10 This catalyst competes with those based on H-BEA (ENI and UOP) and on H-MOR (Dow), used for similar liquid-phase aromatic alkylation processes. MCM-22-based catalysts also have excellent behavior for the liquid-phase alkylation of phenol with alcohols such as tert-butyl alcohol.11 MCM-22 possesses a unique layered crystal structure, denoted with the IZA code MWW,12 containing two independent nonintersecting pore systems (Figure 113). One of the channel systems, denoted as “within the layers”, contains twodimensional, sinusoidal, 10-membered silicate ring channels (diameters 4.1 × 5.1 Å), which also form a cage at their intersection (6.4 × 6.9 Å). The other system, denoted as “between the layers”, consists of large supercages (12-membered rings) with dimensions 7.1 × 7.1 × 18.1 Å, which stack one above another through double prismatic, 6-membered rings and * Corresponding author. Phone: +39-010-353-6024. Fax: +39-010-3536028. E-mail: [email protected]. † Universita ` di Genova. ‡ Universita ` di Cagliari.

Figure 1. Structure of H-MWW. (Left) Relations between the supercages. (Right) View from the [001] direction.

are accessed by slightly distorted elliptical 10-membered “crossing windows” (4.0 × 5.5 Å). Eight unequivalent crystallographic positions exist for the tetrahedral atoms. What is unusual for zeolites is that one of these tetrahedral atom positions is “buried” (i.e., not at the cavity surface) and is bonded to four inaccessible oxygen atoms. In general, the synthesized MCM-22 zeolites crystallize as very thin plates with large external surface area.14 To account

10.1021/jp801072h CCC: $40.75  2008 American Chemical Society Published on Web 05/24/2008

9024 J. Phys. Chem. C, Vol. 112, No. 24, 2008 for the good performances of MWW in benzene alkylation, in spite of the quite small size of channels openings, it is assumed that a significant number of the large cavities that characterize the MWW structure are opened to the exterior at the termination of crystallites. Following this model, it is assumed that on the [001] surface of the platelet-like crystallites emisupercage pockets (or half-supercages) are present, having a free diameter of ca. 7.1 Å. Therefore, cumene and ethylbenzene formation should occur in these cavities, almost without diffusion barriers. This hypothesis is further supported by observing that MCM22 catalytic activity is significantly depressed by deactivating with 2,6-di-tert-butylpyridine, a large base that should indeed not penetrate the channels. However, spectroscopic data suggest that benzene could enter quite easily the supercages.15 Different families of acidic hydroxyl groups have been observed using IR spectroscopy,16–19 but their identification is still a matter of study. In previous investigations, differently hindered nitriles20 and aromatic molecules21 have been used as probes to detect the availability of acid sites of different zeolites such as H-FER, H-MFI, and H-MOR. Adsorption microcalorimetry is also a specific technique for the characterization of the surface acidity of solids22 and, in particular, of acid zeolites.23 In the present paper, we present the results of our reexamination of the identification of the acid sites of MCM-22 zeolite by coupling adsorption microcalorimetric data with FTIR spectroscopic results. 2. Experimental Methods 2.1. Material Preparation. MCM-22 zeolite was hydrothermally synthesized according to a slightly modified procedure given by Corma et al.24 Aerosil 200 (Degussa) and sodium aluminate (56 % Al2O3, 37.5% Na2O, Carlo Erba) were used as Si and Al sources, respectively, and hexamethyleneimine (99% HMI, Aldrich) was a template, while sodium hydroxide was used for the adjustment of pH. A typical gel composition, expressed as molar ratio of the oxides, was 2.7 Na2O/1.0 Al2O3/ 30 SiO2/15 HMI/1340 H2O. After being aged with stirring for 60 min at room temperature, the resulting gel was transferred into Teflon-lined autoclave. The gel was crystallized at 150 °C for 7 days under stirring (60 rpm). The crystalline product was washed thoroughly with deionized water until pH 150 kJ/mol, strong) )

161 µmol/g, nm (120 < Qdiff < 150 kJ/mol, medium) )122 µmol/g, and nw (70 < Qdiff < 120 kJ/mol, weak) ) 769 µmol/ g. The total concentration of the acid sites, 1052 µmol/g, is a little higher than the concentration of Al ions (947 µmol/g), suggesting that all Al ions produce acid sites either producing an unbalanced framework charge that is balanced by the proton or being active as a Lewis site. The additional acid sites may be due, for example, to silanol groups at the external surface. The amounts of sites that absorb ammonia irreversibly are nirr(P)0.2Torr) ) 450 µmol/g and nirr(final P) ) 600 µmol/g. 3.3. IR Characterization. 3.3.1. The Surface Hydroxyl Groups. Figure 4 shows the IR spectrum in the OH stretching region of a pure MCM-22 powder pressed disk after outgassing at 450 °C. The spectrum compares well with those published previously for the same zeolite with similar Si/Al ratios.15–19 In agreement with the previous literature,15–18 a deconvolution of the spectrum (Figure 4, inset) shows sharp OH stretching bands centered at 3746 and 3621 cm-1 and a broader band at 3662 cm-1. An additional weak component is also found at 3575 cm-1. The peak at 3746 cm-1 is typical for the OH stretching mode of terminal silanols, observed on silica as well as on any silica-based materials. Previous studies3,20 showed that the species responsible for the sharp OH stretching at 3746 cm-1 are located at the external surface of zeolites such as H-FER, H-MFI and H-MOR. However, the evident presence of components along the tail towards lower frequencies can be associated, in this case as well as in other cases, to silanols or other hydroxyl groups in defects of the framework of the zeolite.

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Figure 4. FT-IR spectrum of MCM-22 pressed disk outgassed at 450 °C. Inset: deconvolution of the OH stretching band system.

Our deconvolution gives rise to the appearance of two components at 3728 and 3685 cm-1 that could represent such species. The broad absorption at 3662 cm-1 falls in the region typically assigned to OHs on extraframework material, and this is the current assignment for this band also in the case of MCM22. However, the recent theoretical work of Wang et al.34 suggested a possible assignment of this band to “zeolitic” bridging OHs. The peak we observe at 3621 cm-1 is certainly associated with zeolitic bridging Si(OH)Al groups. This peak has been found to be split with components at 3628 and 3618 cm-1 by the deconvolution performed by Onida et al.35 and assigned to two species probably located at the supercages and at the 10membered ring channels, respectively. According to these authors, their environment is likely constrained because molecules such as benzene and propene interact less strongly than predicted.15 Also our deconvolution provides evidence of a lowfrequency component centered at 3602 cm-1 and indicates that the intensity of it is nearly 6 times lower than that of the band at 3621 cm-1. The small component we observe at 3575 cm-1 (3564 cm-1 in our deconvolution) has been attributed by Sastre et al.,36 as a result of calculations, to a family of Si(OH)Al species characterized by quite large Si-O(H)-Al angle (141-149°) mostly located in the supercages, while the band at 3621 cm-1 should be mostly due to species characterized by smaller Si-O(H)-Al angle (131-136°), located in different positions. Onida et al.35 attributed the weak band at 3575 cm-1 to OHs located on the hexagonal prisms. In their recent theoretical study, Zhou et al.37 attributed the three bands reported at 3628, 3618, and 3575 cm-1 to OH located at O3, O11, and O9 positions, which means in supercages, sinusoidal channels, and supercages, too. Our deconvolution allows us to calculate that the area of the component at 3564 cm-1 is four times lower than that of the component at 3621 cm-1. The spectrum reported in Figure 4 shows an additional broad absorption centered at ∼3375 cm-1, which has been also observed in the case of other zeolite samples,3 usually those quite Al-rich such as H-FER, H-CHA, and H-MOR but also in the case of H-MFI. This broad and low-frequency band may be attributed to H-bonded hydroxyl groups. In the present case, it is possible to propose an assignment of this band to hydroxyl groups located in the nonsurface oxygen positions, bonded to the buried T atom. These positions, if occupied by hydroxyl

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Figure 5. FT-IR spectra of MCM-22 pressed disk (a) outgassed at 450 °C, (b) in contact with 10 Torr of IBN vapor, (c) after outgassing 15 min at room temperature, (d) 15 min at 100 °C, and (e) 15 min at 200 °C.

Figure 6. Subtraction spectra relative to Figure 5: IBN species adsorbed on MCM-22 zeolite and desorbed after outgassing 15 min (a) at room temperature; (b) at 100 °C, and (c) at 200 °C. (d) Spectrum of species still present after outgassing at 200 °C.

groups, are not exposed at the zeolite channels and could give rise to hydrogen-bonded hydroxyl groups. 3.3.2. Adsorption of Nitriles. Differently hindered nitriles have been used recently3,20 to distinguish adsorption sites located in different cavities of medium pore zeolites. In Figure 5, the spectra of MCM-22 after activation (a) and few minutes after contact with isobutyronitrile (IBN) vapor at room temperature (b) are compared. It is evident that IBN adsorbs rapidly on MCM-22, perturbing all the sharp peaks due to free surface OHs in the region 3800-3500 cm-1. Nearly the same phenomenon occurs with acetonitrile and propionitrile (spectra not shown), which are less hindered than IBN and can penetrate more easily the zeolite cavities. As a result of IBN adsorption, together with the sharp peaks of IBN at 3000-2800 cm-1 (CH stretchings), 2300-2230 cm-1 (CN stretchings), and 1500-1200 cm-1 (CH deformations), a new strong OH stretching absorption is observed at 3405 cm-1, together with broad absorptions in the region near 3000 and 2500 cm-1, which are the two most evident components of what is usually denoted as the “ABC spectrum”.38 In Figure 6, the subtraction spectra relative to the same experiment allow us to provide evidence of the existence of several different interactions. In order to highlight the single different interactions taking place over the zeolite surface, the subtraction spectra have been obtained by subtracting from the

Acid Sites of H-MCM-22 (H-MWW) Zeolite

Figure 7. CN stretching modes of nitrile species adsorbed at the surface of MCM-22 zeolite: the spectrum of the activated sample has been subtracted. Left: (a) in contact with 10 Torr of PN vapor, (b) after outgassing 15 min at 100 °C, (c) 15 min at 200 °C, (d) spectrum of PN in CCl4 solution. Right: (a) in contact with 10 Torr of IBN vapor, (b) after outgassing 15 min at room temperature, (c) 15 min at 100 °C, (d) 15 min at 200 °C, (e) spectrum of IBN in CCl4 solution.

spectrum of a certain experiment step the spectrum of the immediately following one. The spectrum subtracted from the spectrum of the last experimental step was the spectrum of the activated sample. Spectrum a in Figure 6 is relative to the species formed by adsorption of IBN but desorbed by simple outgassing at room temperature. The positive OH stretching band at 3405 cm-1 corresponds to the negative peak at 3746 cm-1, showing that, upon this adsorption, the terminal silanol band at 3746 cm-1 is shifted (∆ν ∼ 340 cm-1) because of H-bonding with IBN. The CN stretching mode of IBN H-bonded on silanol groups is observed at 2251 cm-1, i.e., shifted upwards of a few inverse centimeters with respect to the spectrum of IBN recorded in CCl4 dilute solution (2247 cm-1; see Figure 7, right, e). Spectrum b in Figure 6 shows that the species desorbing by outgassing at 100 °C is characterized by a CN stretching at 2264 cm-1 and a very broad OH band centered near 3050 cm-1. In this case, in addition to these “positive” features, weak “negative” absorptions are observed in the range 3740-3650 cm-1. This result indicates that there are surface hydroxyl groups absorbing in this region that are a little more acidic than silanols, adsorbing IBN a little more strongly. These OHs, however, are not characterized by a sharp OH stretching band in the activated sample. In the spectrum c in Figure 6, relative to the species that disappear by outgassing at 200 °C, it is evident that the socalled ABC spectrum is associated with the perturbation of the band at 3621 cm-1, due to the zeolitic bridging OHs. The CN stretching mode of IBN H-bonded on the zeolitic OH groups is observed at 2276 cm-1, i.e., shifted upwards of ∆ν ∼ 29 cm-1 with respect to the spectrum of IBN in CCl4 dilute solution (Figure 7, right, e). This interaction is definitely a strong one, being associated with the formation of the ABC spectrum, a relevant shift up of the CN stretching of IBN and resisting outgassing at 200 °C. The ABC spectrum is due to the Fermi resonance of the OH stretching mode (strongly shifted down) with the first overtones of the OH deformation modes (in-plane δOH and out-of-plane γOH). The minima in the ABC spectrum correspond to the position of the first overtones. In fact, the minimum at 2633 cm-1 corresponds to a maximum at 1318

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9027 cm-1, which is assigned to the δOH fundamental in the H-bonded complex. In the spectrum d in Figure 6, relative to the species still present after outgassing at 200 °C, no significant features are observed in the OH stretching region. The residual features of adsorbed IBN species, with the strong CN stretching mode at 2299 cm-1, are consequently assigned to species interacting with Lewis centers. The interaction is certainly strong, in agreement with the resistance to outgassing at 200 °C of this species as well as with the strong shift up of the CN stretching mode. The strength of these sites is that expected for Al3+ exposed sites. This experiment indicates that species containing the isopropyl group, i.e. monobranched aliphatic molecules such as IBN, can enter easily the cavities of H-MCM-22 and access all free OH groups of both the external surface and the internal cavities. The critical radius of IBN can be taken as similar to that of isoalkanes (5.6 Å).39 This is definitely larger than the lower diameter of the elliptical openings of the windows to the internal cavities of MCM-22 (∼4 Å) but is comparable with the larger diameter of these openings (5-5.5 Å). Being that this molecule is relatively flat, it is not surprising that IBN can enter these cavities. This result is also consistent with the previous one that showed that IBN can penetrate the larger cavity of FER zeolite (10-membered ring channel along [001]) whose limiting dimensions are 4.2 × 5.4 Å, while its access to the smaller 8-membered ring channel along [010] (whose limiting dimensions are 3.5 × 4.8 Å) is forbidden.20 It must, however, be noted that in the region 3700-3600 cm-1 there is a significant absorption that is not perturbed by adsorption of IBN, suggesting that inaccessible OHs may exist, but they are not characterized by strong and well-evident OH bands. The weak acidity and the full accessibility of silanol groups is confirmed here for H-MWW like previously found for other zeolites. The strong acidity of zeolitic OHs absorbing at 3621 cm-1 is also confirmed, together with their accessibility to IBN. The existence of OH sites with intermediate Brønsted acidity is also found. It is not fully clear if the species absorbing at 3662 cm-1, usually identified as Al-OH groups on extraframework material, participate with these sites. The behavior of the groups responsible for the weak band at 3575 cm-1 cannot be observed too. This experiment provides also clear evidence of the existence of Lewis acid sites, which represent the most acidic sites, where adsorption of IBN is the strongest. They are certainly due to coordinatively unsaturated Al3+ cations. In Figure 8, the spectra relative to the adsorption of pivalonitrile (2,2-dimethylpropionitrile, PN) on MCM-22 are reported. In contact with PN vapor (b), we observe the full perturbation of the surface silanol groups (shifted to 3435, 3395 cm-1, the band being split possibly due to the existence of two different families of SiOH groups) showing again the full accessibility and the weak acidity of this sites. However, only a small fraction of the band at ∼ 3620 cm-1 is perturbed by PN, leaving the maximum at 3618 cm-1. In agreement with this, the ABC spectrum is much weaker with PN than with IBN. This shows that only a small part of the zeolitic OHs is accessible to PN, the predominant part of them being located in cavities not accessible to it, like the hydroxyl groups responsible for the band at 3662 cm-1 (not perturbed by PN adsorption). The analyses of the CN stretchings of PN species adsorbed on MCM-22 are compared with those of IBN species in Figure 7. Also for adsorbed PN we can see the CN stretching modes of (i) species weakly adsorbed through H-bonding on silanol groups, split at 2238, 2248 cm-1, slightly shifted up with respect to the band of the molecule in CCl4 solution (2235

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Figure 8. FT-IR spectra of MCM-22 pressed disk (a) outgassed at 450 °C and (b) in contact with 10 Torr of PN vapor.

cm-1); (ii) the band of species strongly adsorbed through H-bonding on zeolitic bridging OH groups at 2269-2272 cm-1, shifted up with respect to the free molecule; and (iii) the band of species strongly adsorbed on Lewis sites at 2295 cm-1, strongly shifted up with respect to the free molecule. The relative intensities of the CN stretchings of PN adsorbed on zeolitic bridging OHs (∼2270 cm-1) are definitely smaller with respect to those of species bonded to Lewis sites and to terminal silanols (∼2295 and ∼2250 cm-1), in contrast with what is observed for the corresponding species produced by IBN adsorption. This indicates that PN has access to only a fraction of zeolitic OHs while IBN can access to all of them. The fact that a large portion of zeolitic bridging OHs is not accessible, in the conditions of our experiment, to such a tertbutyl group containing molecule is not unexpected because of the definitely larger critical diameter of this “tridimensional” and rigid molecule (∼6.2 Å)39 with respect to the smaller dimension of the opening rings of both sinusoidal channels and superacage crossing windows of MCM-22 zeolite (∼4 Å). PN should in fact not enter the internal cavities of this zeolite, like for those of ZSM5 and FER zeolites,20 whose limiting dimensions are comparable or even larger. However, a portion of zeolitic OHs is actually perturbed by PN. This can be taken as a confirmation of the existence, at the surface of MCM-22, of emisupercages accessed by large 12membered silicate rings, where zeolitic OHs should be present. The opening of these emisupercages (∼7.1 Å) is in fact large enough to allow easily the entrance of PN and even larger than those of the main channels of H-MOR (6.5 × 7.0 Å wide) to which PN was found to have access.20 The position of the OH band that is perturbed during PN adsorption is not significantly different from that of the band observed in the activated sample as well as of the residual band still present upon adsorption. This indicates that the surface emisupercages contain bridging OHs similar to those predominant in the internal cavities. As said, PN does not perturb the band at 3662 cm-1, attributed to OHs on extraframework alumina species. Also, the sites characterized by intermediate acidity, observed using IBN, are not observed using PN. This suggests that these sites are located mostly in the internal cavities. The subtraction spectra reported in Figure 9 suggest that PN perturbs also at least a fraction of the species responsible for the weak band at 3575 cm-1. This feature is in fact observed in spectrum b as a weak negative shoulder at the side of the main

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Figure 9. Subtraction spectra relative to PN adsorption experiment. PN species adsorbed on MCM-22 zeolite and desorbed after outgassing 15 min (a) at room temperature and (b) at 100 °C. (c) Spectrum of species still present after outgassing at 100 °C.

Figure 10. FT-IR spectra of MCM-22 pressed disk (a) outgassed at 450 °C, (b) in contact with 3 Torr of pX vapor, (c) after outgassing 15 min at room temperature, and (d) after outgassing 15 min at 100 °C.

negative peak. This could indicate, in partial agreement with Onida et al.,15,17 that this feature is due to OHs located in the hexagonal prism structure, part of which are exposed in the supercages as well as also in the emisupercages. On the other hand, the data concerning PN adsorption confirm the existence of Lewis sites but indicates that they are mostly located out of the internal cavities, like terminal silanol groups. Also the existence of two different families of silanols is confirmed. 3.3.3. Adsorption of Xylenes. In Figure 10 the spectra of MCM-22 upon adsorption and desorption of p-xylene (pX) are reported. In contact with the vapor (b), the peaks at 3746, 3662, and 3621 cm-1 have fully disappeared. This shows that pX can easily reach all hydroxyl groups responsible for such bands. However, a broad absorption between 3750 and 3650 cm-1 is still present in contact with pX. This suggests that hydroxyl groups responsible for this absorption could be inaccessible to pX. A new peak arises, upon contact with pX vapor, at 3584 cm-1, together with a much broader absorption in the region 3400-3200 cm-1. Outgassing at room temperature (c) causes the peak at

Acid Sites of H-MCM-22 (H-MWW) Zeolite 3584 cm-1 to disappear almost completely, while the sharp peak of silanol groups at 3746 cm-1 is substantially restored. This behavior allows us to assign the peak at 3584 cm-1 to the OH stretching of silanol groups H-bonded to adsorbed pX. This adsorption complex disappears almost completely by brief outgassing at room temperature, showing a weak interaction, in agreement with the moderate shift of the OH stretching mode (∆νOH ∼ 190 cm-1). In parallel, it is possible to assign the broad feature at 3400-3200 cm-1, at least predominantly, to the OH stretching of zeolitic bridging Si-OH-Al groups interacting with adsorbed pX. This absorption mode, which is also associated with the disappearance of the sharp peak at 3621 cm-1, is definitely stronger than the previous one, being still fully present after outgassing at room temperature and being associated with a larger shift of the OH stretching band (∆νOH 200-400 cm-1). After outgassing at 100 °C (d), only a small part of the sharp band of zeolitic OHs is restored. Interestingly, the maximum is now observed at 3617 cm-1, while the maximum of the negative peak read in the subtraction spectrum (d - a) is at 3621 cm-1. This confirms the conclusions of Onida et al.15,17 that the peak is double and at least two different families of OH groups are responsible for the band observed at ∼3620 cm-1. We can additionally conclude that the lower frequency component is due to species from which desorption of pX is easier, while those responsible for the higher frequency component should be due to species from which the desorption of pX is more difficult. In parallel, the OH stretching associated with OHs still bonded to pX corresponds to the lowest frequency part (maximum at 3250 cm-1) of the broad absorption extending in the region 3400 - 3200 cm-1. This may indicate that actually pX is desorbed with more difficulty from the sites where the interaction is stronger (∆νOH 200-400 cm-1), as expected, indeed, diffusional effects being possibly not determinant here. The spectra reported in Figure 10 show that also the band at 3662 cm-1 is due to a species involved in quite strong interaction with pX, being resistant to outgassing at 100 °C. Additionally, a weak feature at 3730 cm-1 is observed (d - a), which also corresponds to a species interacting quite strongly with pX. These species may be associated with terminal OHs (possibly silanols) in defects of the structure. This experiment shows that the access of pX to the cavities where the free bridging OHs are supposed to be mostly located is easy. This confirms the data reported by Onida et al.15 that showed that benzene has also free access to the cavities, in agreement with the same steric hindrance of benzene and pX. In contrast with PN, these aromatic molecules are essentially planar, and their critical diameter is only slightly higher than the larger diameter of the 10-membered ring windows to the two channel systems of MCM-22. Our previous data40 show that pX and benzene also enter easily the cavities of H-MFI (whose openings are similar in size to those of MCM-22), perturbing the νOH band of the bridging OHs. In the case of H-MFI, however, the interaction between the benzene molecule and the zeolitic bridging OHs is likely a little stronger, the OH band of the H-bonded complexes being shifted split at 3230 and 3180 cm-1, i.e., at lower wavenumbers than on H-MCM22. In Figure 11 the spectra relative to the adsorption of m-xylene (mX) are reported. In contact with mX (b), the spectrum of MCM-22 in the OH stretching region is again strongly perturbed. The band of free silanol groups has fully disappeared and a strong new band is formed in parallel at 3582 cm-1, assigned to silanol groups H-bonded to mX. The slightly lower

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Figure 11. FT-IR spectra of MCM-22 pressed disk (a) outgassed at 450 °C, (b) in contact with 3 Torr of mX vapor, (c) after outgassing 15 min at room temperature, and (d) after outgassing 15 min at 100 °C.

frequency observed with mX with respect to pX may be associated with the slightly higher basicity of mX due to the meta-position of the two electron-donating methyl groups. However, as shown in the inset of Figure 11, the band at 3582 cm-1 has a clear shoulder near 3620 cm-1, as, in this case, the band of zeolitic bridging OHs has not fully disappeared. This is confirmed by the spectrum recorded after outgassing at room temperature (c). This spectrum shows the full disappearance of the band at 3582 cm-1, corresponding to the restoring of the band of free silanol groups, but also shows a weak peak with a maximum at ∼3615 cm-1. This means that a few bridging OHs, responsible for a OH stretching near 3615 cm-1, are inaccessible to mX (or the access is very slow) in these conditions, while being easily accessible to pX. Outgassing at 100 °C (d) partially but significantly restores the band at 3621 cm-1, which is completely restored by outgassing at 200 °C (e), when mX is fully desorbed. The restoring of the band of bridging OHs at 100 °C is more complete in the case of mX with respect to pX, suggesting that the interaction is weaker for mX than for pX. In Figure 12 the subtraction spectra, relative to mX adsorption experiment, are reported. The spectra (a to c) are due to the effects associated with adsorbed mX species that are desorbed by outgassing at room temperature (a), at 100 °C (b), and resisting outgassing at 100 °C (c), respectively. The subtraction spectrum (a) shows that the band at 3582 cm-1 is almost perfectly symmetrical, confirming that the shoulder evident in the spectrum in the inset of Figure 11 is due to the weak component at 3615 cm-1, which is not perturbed by mX adsorption. The spectra in Figure 12b,c show that mX desorbing at 100 °C and resisting outgassing at 100 °C is associated with the disappearance of the OH stretching band at 3621 cm-1 (observed negative in the subtraction spectrum) and that this interaction shifts the band down to 3310 cm-1, now very broad. The shift ∆νOH ∼ 310 cm-1 for mX adsorption is lower than that observed for pX adsorption (∆νOH ∼ 370 cm-1), confirming a weaker hydrogen bonding of mX with respect to pX on bridging OHs. This is in contrast with what happens for mX on silanol groups as well as with the stronger interaction of mX with respect to pX with Lewis acids such as BF3, attributed to the stronger basicity of mX arising from the meta position of the electron-donating methyl groups.41

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Figure 12. Subtraction spectra relative to Figure 10. mX species adsorbed on MCM-22 zeolite and desorbed after outgassing 15 min (a) at room temperature and (b) at 100 °C. (c) Spectrum of species still present after outgassing at 100 °C.

These results can be interpreted in relation to the relevantly higher steric hindrance of mX with respect to pX. In our previous study,21 we showed that, in the same conditions, the entrance of mX to the cavities of H-MFI zeolite is much more hindered than that of pX. It seems also reasonable to conclude that two different families of “bridging” hydroxyl groups exist in the internal cavities of MCM-22. The OH responsible for the lower frequency component (3600-3618 cm-1) are located in sites where the access of mX is hindered and the interaction of pX is weaker. The intensity of this component is also definitely lower than that of the higher frequency component (3618-3625 cm-1) whose relative OH groups are located in cavities where both mX and pX access easily and the interaction of pX is stronger. On the other hand, over this site the interaction of pX is apparently stronger than that of mX. It seems that also the OHs responsible for the band at 3662 cm-1 interact with mX more weakly and less extensively than with pX. The adsorption of o-xylene (oX) on MCM-22 (Figures 13 and 14) gives rise to results similar to those obtained with mX. Again the band due to external silanols (Figure 13b) is immediately and completely shifted down to 3584 cm-1, while the perturbation of the band at 3621 cm-1 is very slow. Experiments performed using different oX pressures (Figure 14) show that the entrance of oX, and the consequent perturbation of the internal zeolitic OHs, is easier at higher pressure. As found before for xylenes diffusion in the cavities of H-MFI,21,40 the present data, which are repeatedly obtained after different experiments, show that mX diffuses in the internal channels of the zeolite a little more slowly than oX and that both diffuse much more slowly than pX, whose entrance is apparently free. The spectra allow also the analysis of the perturbations undergone by the vibrational structure of the adsorbate molecules upon interaction. At high coverages, the spectra of the three xylenes adsorbed on MCM-22 look very similar to those of the corresponding liquids. This confirms that the predominant interaction is on the external silanol groups through a weak H-bonding. After outgassing at 100 °C, instead, the spectra of the three xylenes are all a little bit more perturbed. From Figure

Figure 13. FT-IR spectra of MCM-22 pressed disk (a) outgassed at 450 °C and (b) in contact with 3 Torr of oX vapor after 5 min and (c) 15 min.

9, 14 it is quite evident that, in these conditions, the intensities of the aromatic CH stretching components in the region 3000-3100 cm-1 are decreased significantly with respect to the methyl CH stretching components, which are in the 3000-2800 cm-1 region. Additionally, the inspection of the region 900-450 cm-1, which is available in the window of the skeletal vibrations, allows us to see that the out-of-plane deformation modes of the aromatic CH bonds are, for all the three adsorbed xylenes, shifted upwards 5-15 cm-1 with respect to the spectra of the liquids. These data confirm that the strong H-bonding interaction with the zeolitic hydroxyl groups actually occurs, involving the aromatic electron cloud. 4. Discussion The above described experiments allow us to have some further insight on the acid sites of MCM-22 zeolite. The unit cell of MCM-22 zeolite belongs to the hexagonal P6/mmm space group.42 It contains 72 tetrahedral cations in 8 crystallographic non-equivalent positions and 144 oxide atoms, in 13 crystal-

Acid Sites of H-MCM-22 (H-MWW) Zeolite

Figure 14. FT-IR spectra of oX species adsorbed on MCM-22 zeolite and desorbed after outgassing 15 min (a) at room temperature and (b) spectrum of species still present after outgassing at 100 °C.

Figure 15. Structure of H-MWW. Oxygen positions at the supercage walls (left), around the buried T atom (right up), and at walls of the sinusoidal channels (right down).

lographic nonequivalent positions (Figure 15). In Table 1 the 13 positions for oxygen atoms are reported. It is interesting to note that the T-O-T angles at O1 and at O5 are usually supposed to be 180° for symmetry reasons. Recent crystallographic determination confirmed that these angles are actually 180°. The angle at O1 affects the real size of the so-called “crossing windows”, which is just limited by the O1-O1 distance. While this size of this elliptical window is generally assumed to be 4.1 × 5.5 Å, some authors report a larger size for this window (4.0 × 5.9 Å).6,43,44 All other T-O-T bridges have angles well lower than 180°. In Table 1 it is also reported, based on the literature, toward which cavity the oxygen atoms are tilted, so determining in what cavity the proton may be active if a bridging OH is located in such position. The spectrum of the activated sample (Figure 4) shows a broad feature centred at 3375 cm-1, which can be assigned to internal H-bonded hydroxyl groups. Similar absorptions have been found in several other zeolites such H-FER,46 H-CHA,47

J. Phys. Chem. C, Vol. 112, No. 24, 2008 9031 H-MOR and H-MFI,3 in particular when they are rich in Al. It is possible that these are OHs at O4, i.e., in inaccessible positions. Our data allow us, in agreement with the previous literature,15–18 to distinguish different types of surface hydroxyl groups. Poorly acidic silanol groups absorbing at 3746 cm-1 are well-evident and are completely perturbed by nitriles and xylenes. In parallel with the previous results concerning H-FER, H-MFI, and H-MOR zeolites, we confirm here that terminal silanol groups are all exposed at the external surface of H-MWW zeolite. The adsorption of nitriles suggests that some heterogeneity of this groups may exist. This might be associated with some differences between silanols exposed on the basal planes perpendicular to [001] of the zeolite lamellae and those located at perpendicular planes limiting laterally the lamellae. In any case, the acidity of such sites is more or less the usual one for silanol groups. The data arising from adsorption of nitriles allow us to show the existence of another family of hydroxyl groups that are characterized by a broad, not well defined OH absorption in the region 3740-3650 cm-1 and are a little more acidic than silanol groups. Evidence of these groups, however, has not been obtained using PN as the probe. This suggests that they are located in the internal cavities, likely in structural defects and/ or extraframework material. PN adsorption experiments seem to indicate that the band observed at 3662 cm-1 is also likely due to species located in the internal cavities. The growth of this band with increasing Al content and also with increasing calcination time and temperature, as reported by other authors,15–18,48 fully agrees with their identification as OHs due to extraframework material. As previously found for similar species formed in H-MFI and H-MOR,3 also in this case these species should form in the cavities. In agreement with the conclusions of Onida et al.,15,17 we can distinguish three families of zeolitic bridging hydroxyl groups, two of which are characterized by a slightly different poorly resolved OH stretching frequency, 3600-3618 and 3618-3625 cm-1. Those at lower frequency cannot be reached (or are reached very slowly) by mX and oX while they are easily accessible to pX as well as to IBN. So they are in cavities where the access is more hindered. Structural data suggest that the entrance to sinusoidal channels should be more difficult than that to the supercages. In fact, the 10-membered silicate ring entrance to the sinusoidal channels (diameters 4.1 × 5.1 Å) is quoted to be smaller than the slightly distorted elliptical 10membered “crossing windows” (4.0 × 5.5 Å) allowing the access to the supercages. This suggests that the lower frequency component should be due to OHs located in the sinusoidal channel system, while the higher frequency component should be mostly due to OHs in the supercage systems. Supposing that the extintion coefficients of the OH stretchings of these two families are similar, this assignment is supported by the different intensities of the two bands. In fact, the hydroxyl groups exposed in the sinusoidal channel system are much less than those exposed in the supercage system. This assignment also agrees with the conclusions of Onida et al.15,17 and of Zhou et al.37 According to the discussion of Sastre et al.,36 species characterized by quite large Si-O(H)-Al angle (141-149°) are mostly located in the supercages, while species characterized by smaller Si-O(H)-Al angle (131-136°) are distributed among supercages and the sinusoidal channel system. It is possible that the slightly different hydroxyl group stretching frequency among

9032 J. Phys. Chem. C, Vol. 112, No. 24, 2008

Bevilacqua et al.

TABLE 1: Oxygen Positions and T-O-T Angles in the H-MCM-22 Zeolite Τ-O-T angles

oxygen positions Leonowicz et al. T1-O1-T1 T1-O2-T2 T2-O3-T3 T2-O4-T4 T4-O5-T5 T3-O6-T6 T3-O7-T3 T6-O8-T6 T6-O9-T7 T5-O10-T7 T7-O11-T8 T8-O12-T8 T8-O12-T8 a

Meier et al.

Camblor et al. ITQ145

T6-O1-T6 T1-O2-T6 T1-O3-T4 T1-O4-T7 T7-O5-T8 T4-O6-T5 T4-O7-T4 T5-O8-T5 T2-O9-T5 T2-O10-T8 T2-O11-T3 T3-O12-T3 T3-O13-T3

180 141 140 146 180 166 152 153 165 143 160 159 137

Zhoua MCM-22 180 131-135 180

144-146 ∼139

position

no. of sites

cross supercage cross supercage supercage unaccessible unaccessible supercage cross supercage sinusoidal supercage sinusoidal sinusoidal supercage sinusoidal

2 12 24 12 4 12 6 6 24 12 12 12 6

Reference 37.

systems characterized by different steric hindrance is due to a different average angle. The adsorption of PN shows that, while the predominant fraction of zeolitic OHs is located in the internal cavities, a small fraction is accessible to PN. This result, together with the discussion of Lawton et al.,6 provides evidence of the existence of the emisupercages at the external surface of the MCM-22 platelets as well as of the existence of zeolitic OHs at their internal surface. In fact, it is not surprising that these emisupercages are accessible to PN, being opened through a 12membered ring 7.1 Å wide. As first shown by Sastre et al.,36 a third much weaker absorption exists in the region typically due to the νOH mode of bridging zeolitic OHs, at 3575 cm-1. We also found it but, due to its weakness, we could not investigate its behavior upon adsorption of xylenes. We can only remark that at least part of this absorption is observed as a negative feature in the subtraction spectrum in Figure 9b. This would indicate that at least part of these is perturbed by PN, thus being located, at least in part, either in the emisupercages or at the external surface. We can tentatively assign this feature to OHs located at the O12 position, part of which are at the surface of emisupercages, the low frequency being associated with the tensioned structure. Additional information arising from the adsorption of nitriles concerns the existence of Lewis acid sites and their location mostly at the external surface of the zeolite. This is in parallel with what has been observed on other protonic zeolites such as H-FER, H-MFI, and H-MOR3. The existence of Lewis acid sites is in agreement with some very strong sites determined by the calorimetric adsorption measurements. In fact, sites adsorbing ammonia with a heat of adsorption higher than 160 kJ/mol can be assumed to be of the Lewis type, i.e. coordinatively unsaturated Al3+. It is usually assumed that Al ions in the internal zeolite framework do not act as Lewis sites because they are “shielded” by the nearby protonic centers. However, Al ions can also be located at the external surface, where the structure is relaxed. There they are active as Lewis acid sites. These data allow a more precise identification of the potentially active sites of H-MWW zeolite in catalysis. This catalyst finds commercial application in liquid-phase processes for alkylation of benzene by ethylene (Mobil EBMax process for ethylbenzene synthesis) and by propylene (Mobil Raytheon process for cumene synthesis). These processes are similar to EBOne and Q-Max processes from UOP and Eni-Polimeri

Europa processes, which use H-BEA as the catalyst. Interestingly, H-BEA has pores with definitely larger access (12membered rings, 7.6 × 6.4 Å) than H-MWW, together with a second family of smaller pores (12-membered rings, 5.5 × 5.5 Å). According to several authors, while H-BEA works, as usual for zeolites, in its internal channels, H-MWW should work at the emisupercages exposed at the lamellae surface. On the other hand, because of the flat structure of the reactant (e.g., benzene and xylene) and of the products (e.g., ethylbenzene), as well as to the critical radius of these molecules (ca. 6 Å), their diffusion in the H-BEA main channel could be even easier than their entrance to the H-MWW emisupercages. According to researchers at Exxonmobil,8 MWW zeolite is more monoalkylate selective than all other zeolites, including H-BEA. However, according to Corma et al.49 at 180 °C, MCM22 is more selective to the formation of diisopropylbenzenes (i.e. less selective for cumene synthesis) than H-BEA zeolite, while the reverse is true at 220 °C. The monoalkylation/ dialkylation ratio increases significantly (from 6 to 10) on MWW by increasing the temperature from 200 to 220 °C. The same authors also report that H-BEA is slightly more active and more selective than H-MWW in the synthesis of ethylbenzene at 240 °C. People working at ENI50 reported that, at 150 °C, H-BEA zeolite may be better than MCM-22 both in terms of activity and in terms of selectivity to monoalkylate products. In both cases, however, the Si/Al ratio as well as the morphological properties of the zeolite and of the binder must be finely tuned to optimize the performances. The better selectivity to monoalkylation on H-MWW at higher temperature suggest that, in these conditions, alkylation in the internal cavities (in particular in the supercages), where selectivity could be enhanced, could become significant. What we show here is that benzene and pX can enter quite easily not only the emisupercages but also the cavities of H-MWW. This agrees with the data concerning benzene adsorption reported by Onida et al.15,17 and those concerning pyridine adsorption reported by Meloni et al.48 The adsorption of IBN shows that also molecules with the isopropyl group (such as cumene) can go in and out the cavities easily. Thus, in principle, the alkylation reaction could also occur in the internal supercages and perhaps also in the sinusoidal channels. The improvement of selectivity at higher temperatures on MWW, in contrast with what happens for the larger pore zeolites, suggests that at the higher temperatures the reactivity in the internal cages may become significant.

Acid Sites of H-MCM-22 (H-MWW) Zeolite The data reported here indicate, in contrast, that molecules with the tert-butyl group (such as PN) are forbidden to enter the cavities. This confirms that the alkylation of phenol with tert-butyl alcohol would certainly occur at the external surface, possibly in the emisupercages. However, it is well known that this reaction can also occur through a Lewis-type catalysis. In fact, this reaction is also performed in the presence of alumina as the catalyst.51 It is possible that this reaction is catalyzed by Lewis-type centers, which have been well characterized by nitrile adsorption as well as by microcalorimetry. 5. Conclusions The data reported in this paper allow us to draw the following conclusions: At least three different families of zeolitic hydroxyl groups are observed to be active in MCM-22 zeolite catalyst. The most abundant are those exposed in the supercage system, absorbing predominantly at ca. 3618-3625 cm-1. Less abundant are those exposed in the sinusoidal channel system, predominantly absorbing in the 3600-3618 cm-1 region. The third family, responsible for a weak band at 3575 cm-1, is tentatively assigned to species located at O12, exposed in the supercages and emisupercages, too. The adsorption of the hindered probe pivalonitrile strongly supports the conclusion that emisupercages are exposed at the [001] basal plane of the MCM-22 layered structure, where zeolitic OHs of the same nature of those exposed in the supercages (bands at 3618-3625 and 3575 cm-1) indeed exist. At the external surface weakly Brønsted acidic silanol groups (possibly two slightly different families) and strongly acidic Lewis sites are also well-evident. Other Brønsted acidic sites with intermediate acidity are also observed, part of them being in the internal cavities. Monoaromatics, as well as para-disubstituted benzenes (such as p-xylene) and isopropyl-group containing molecules (such as isobutyronitrile), enter quite easily not only the emisupercages but also both internal channel systems of MCM-22. On the contrary, the entrance of tert-butyl-containing molecules in the internal cavities is forbidden or at least hindered. These data suggest that alkylation of benzene can both occur at the external emisupercages (with no diffusion limits) but also in the internal cavities, without strong diffusion limits. The activity at the internal cavities could be not negligible, in particular at high temperatures, when the H-MWW catalyst is more monoalkylated selective (above 200 °C). It seems likely that alkylation of phenol with tert-butyl alcohol should occur at the external surface or in the emisupercages. It seems possible that this reaction is catalyzed by Lewis acid sites. Supporting Information Available: XRD pattern and skeletal IR spectra of H-MCM22. This information is available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape SelectiVe Catalysis in Industrial Applications, 2nd ed.; Dekker: New York, 1996. (2) Guisnet, M.; Gilson J. P., Eds. Zeolites for Cleaner Technologies; Imperial College Press: London, 2002. (3) Busca, G. Chem. ReV. 2007, 107, 5366. (4) Rubin M. K.; Chu, P. US Patent No. 4,954,325, 1990. (5) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (6) Lawton, S. L.; Leonowicz, M. E.; Partidge, R. D.; Chu, P.; Rubin, M. K. Microporous Mesoporous Mater. 1998, 23, 109. (7) Degnan, T. F., Jr.; Morris Smith, C.; Venkat Chaya, R. Appl. Catal., A 2001, 221, 283.

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