H-Bond Formation and Proton Transfer in H-MCM-22 Zeolite as

Whereas the H-bonding tendency is in the order H−ZSM-5 ≥ H−MCM-22 > H−MOR, protonation of propene at 300 K is slower with H−MCM-22 than with ...
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1684

J. Phys. Chem. B 2002, 106, 1684-1690

H-Bond Formation and Proton Transfer in H-MCM-22 Zeolite as Compared to H-ZSM-5 and H-MOR: An FTIR Study Barbara Onida,† Francesco Geobaldo,† Flaviano Testa,‡ Rosario Aiello,‡ and Edoardo Garrone*,† Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy, and Dipartimento di Ingegneria Chimica e dei Materiali, UniVersita` degli Studi della Calabria, 87030 ArcaVacata di Rende (CS), Italy ReceiVed: June 11, 2001; In Final Form: NoVember 1, 2001

Brønsted acidity of two samples of H-MCM-22 with different Al content is studied both in terms of the ability to transfer the proton to ammonia, ethene, and propene and in terms of the tendency to engage in H bonding with organic molecules (acetone, benzene, toluene, ethene, propene). Comparison is made with similar results for H-ZSM-5 and H-Mordenite zeolites. Whereas the H-bonding tendency is in the order H-ZSM-5 g H-MCM-22 > H-MOR, protonation of propene at 300 K is slower with H-MCM-22 than with H-ZSM-5 and H-mordenite, and ethene oligomerization does not take place on the H-MCM-22 sample with the lowest Al content. Steric constraints seem to exist at the proton sites of MCM-22, which may explain the lower ability to transfer the proton with respect to H-ZSM-5 and H-mordenite.

Introduction zeolite1

containing two pore MCM-22 is a relatively new systems, one constituted by two-dimensional sinusoidal channels composed of 10 MR (member ring) openings, the other by large supercages with dimensions 7.1 × 7.1 × 18.1 Å defined by 12-member rings. The supercages stack one above the other through double prismatic six-member rings and are accessed by slightly distorted elliptical 10-member-ring windows. The two pore systems are independent, and molecules cannot migrate from one to the other within the same crystal.2 The protonic form of H-MCM-22 is an active catalyst for many reactions requiring acidic sites, such as catalytic cracking, olefin isomerization, conversion of paraffins to olefins and aromatics, and alkylation of paraffins with light olefins.3,4 Recently, dealuminated MCM-22 has been shown to be an excellent catalyst for the selective formation of p-xylene in toluene disproportionation.4 Its catalytic properties are intermediate between 10-MR and 12-MR zeolites in various reactions.5-9 12 MR pockets exist at the external surface of MCM-22 crystals, which may play a role in certain catalytic processes, imparting unique characteristics to the external surface of MCM-22.10 Available information on the Brønsted acidity of MCM-22 comes from catalytic and spectroscopic studies. NMR data11,12 show that at least three different tetrahedral aluminum sites exist in MCM-22. This suggests that three different types of Si(OH)Al species should be present. In the IR spectrum, the O-H stretching mode region of MCM-22 shows two peaks related to acidic hydroxyl species, a prominent one at 3624 cm-1 and another rather weak one at 3580 cm-1.13,14 In a previous paper,13 we have assigned the latter, related to species not readily accessible to probe molecules, to hydroxyl species in the hexagonal prisms; the * To whom correspondence should be addressed. E-mail: garrone@ athena.polito.it. Tel: +39-011-5644661. Fax: +39-011-5644699. † Politecnico di Torino. ‡ Universita ` degli Studi della Calabria.

former has been interpreted as being due to two components at 3628 and 3618 cm-1, respectively, related to species sitting at the supercages and at the 10 member rings. The Brønsted acidity of these two latter species has been studied in the same paper13 as the tendency to H bond with weak basic probes (CO, N2). Comparison was made on this basis with H-ZSM-5 and H-mordenite, because these zeolites contain 10 MR and 12 MR channels, respectively. The values of the shift of the stretching frequency, ∆νOH, as a consequence of H bonding indicate that the acidity of the three zeolites is close to each other. Nevertheless, a sequence in acidity can be established on this basis as H-MOR < H-MCM-22 < H-ZSM-5. This is at variance with the higher νOH value for Si(OH)Al species in H-MCM-22 (3624 cm-1 compared with 3612-3609 cm-1 in both H-ZSM-5 and H-mordenite14-16), which might suggest a lower acidity of Brønsted sites in MCM-22. Sastre and Lewis have proposed a model for AFI and CHA structures, according to which, within the same zeolite, the electric field at the proton sites correlates with the OH stretching frequency.17 It is proposed that the smaller the norm of the electric field at proton sites is, the easier the removal of the proton (i.e., the higher its acidity) will be. Thus, according to these authors, the mere OH stretching frequency can therefore be considered a measure of acidity. Sastre et al. have recently extended the same model to MCM-2218 and proposed that two types of hydroxyls characterized by different acidity are possibly present on MCM-22. Although Brønsted acidity is the ability to transfer the proton and H-bonding properties may be only indirectly related to it, it has been proposed by Paukshtis,19 and widely used in the past,20 that the deprotonation energy of a protonic zeolite could be evaluated from H-bonding interactions.19 On this basis, one is led to assume that the tendency to transfer the proton of H-MCM-22 is intermediate between that of H-MOR and that of H-ZSM-5. In the present work, we study the acidity of the protonic form of MCM-22 as the tendency to transfer a proton to both ammonia and light olefins (propene and ethene). The ability to

10.1021/jp0122068 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/24/2002

H-Bond Formation and Proton Transfer in Zeolite

J. Phys. Chem. B, Vol. 106, No. 7, 2002 1685

form oligomers of light olefins is compared again to that of H-ZSM-5 [15] and of H-mordenite.16 In this study, we have also gathered evidence on the ability to form H bonds with the same olefins. In addition, the study of the H-bonding tendency of the Brønsted site in MCM-22 is extended to benzene, toluene, and acetone. This has allowed us to draw a fairly comprehensive picture of the H-bonding ability of acidic hydroxyls in MCM22 and to compare the different measures of acidity (H-bonding tendency and proton transfer) for MCM-22, H-ZSM-5, and H-mordenite. The effect of two different Si/Al ratios was also examined. Experimental Section Two samples of MCM-22 zeolite were considered, having a ratio Si/Al equal to 14 and 20, hereafter referred to as sample A and sample B, respectively. They were synthesized according to the procedure described in refs 21 and 22 in a hydroxyl medium. The template was removed by first heating in a vacuum up to 773 K and maintaining the sample at the same temperature for 2 h and then treating in 250 mbar of O2 at the same temperature for an additional 2 h. To remove some residual Na+ cations, the calcined samples were allowed to stand in a 1 M aqueous solution of NH4NO3 at about 320 K for 48 h and then calcined again. Aluminum content has been evaluated both by energy-dispersive spectrometry (EDS) analysis on the powder and by atomic absorption analysis after dissolving the zeolite samples in HF/H2SO4 water solution at 60 °C. For IR measurements, thin self-supporting wafers were prepared and activated under dynamic vacuum (10-4 mbar) for 2 h at 773 K. Spectra (128 scans) were collected on a Bruker FTIR Equinox 55 equipped with a MCT cryodetector working with 2 cm-1 resolution. Partial deuteration of sample A was carried out by using C6D6 as a deuterating agent as follows. On the sample outgassed at 773 K, C6D6 was adsorbed at 300 K, and the sample was maintained in contact with the ambient vapor pressure of C6D6 for 12 h (renewing C6D6 two times) and then outgassed at 423 K for 30 min. On the basis of the IR intensities of Si(OH)Al and Si(OD)Al species, a fraction of 80% has been exchanged. All interactions have been studied at about 300 K; only the reaction of propene has been followed both at ambient temperature and at temperatures in the range of 77-300 K.23 Gases were from Messer, and liquids were from Merck; all reagents were high purity. Results and Discussion OH Species. Figure 1 reports the spectra of both sample A (Si/Al ) 14) and sample B (Si/Al ) 20) in the region of the hydroxyl stretch after activation. Curve 1, which refers to sample A, has been already reported and interpreted.13 The following is observed: (i) a rather intense sharp band at 3747 cm-1 due to isolated silanols at the exterior of the zeolite microcrystals; (ii) a shoulder at about 3730 cm-1 due to interacting SiOH species in structural defects; (iii) a weak absorption at 3670 cm-1 related to AlOH species partially anchored to the zeolitic framework, a product of incipient dealumination; (iv) a band at 3624 cm-1 (in which two components have been single out13) due to Si(OH)Al species located at 10 MR channels and supercages; (v) a shoulder at about 3580 cm-1 attributed to Si(OH)Al species located in a strongly constrained environment, probably the hexagonal prisms capping the supercages.13 Curve 2 (describing sample B, which is poorer in Al, that is, with fewer Brønsted sites) has been given the intensity of spectrum 1 as it concerns the peak due Si(OH)Al species by

Figure 1. Comparison of the IR spectra in the OH stretching region of sample A (H-MCM-2 Si/Al ) 14, curve 1) and sample B (HMCM-22 Si/Al ) 20, curve 2) after outgassing at 773 K.

multiplying by the appropriate factor (about 1.5) to ease the comparison in this crucial spectral region. The same features are seen as in spectrum 1. Microcrystals of sample B seem bigger because the band due to external isolated SiOH species is less intense. The component at 3730 cm-1 has about the same intensity, that is, the two samples have a comparable number of defects. The band due to the Brønsted sites appears slightly narrower, that is, the corresponding sites seem better defined, and the band is marginally shifted to lower frequency (3622 cm-1), as seen with the help of the vertical line in Figure 1. Figure 2 concerns the interaction of sample A with ammonia. Because the goal was to single out possible differences in acidity between hydroxyl species, the experiment was carried out as follows. The sample was first saturated at room temperature with ammonia at 10 mbar (which brought about the conversion of acidic hydroxyls into NH4+ species) and then evacuated at 423 K to eliminate weakly bound species, like NH3 molecules H bonded to silanols. The thermal stability of NH4+ species was studied monitoring the appearance of the corresponding hydroxyl species as a function of the temperature of outgassing (section a), to which corresponds the decrease of the deformation band of ammonium ion (section b). On the whole, the experiment described in Figure 2 is a sort of spectroscopic temperature-programmed desorption (TPD). Reappearance of acidic hydroxyls takes place in a narrow temperature interval (473573 K). The first species are seen at 3627 cm-1, and then the band grows shifting to 3624 cm-1; simultaneously, a weak band appears at about 3580 cm-1. The results confirm the presence of two components in the envelope centered at 3624 cm-1, hypothesized in our previous work:13 the less acidic species are those less thermally stable, that is, those at higher frequency. In contrast with what was proposed by Sastre et al.,18 the 3580 cm-1 band is not related to an OH species more acidic than the others because it appears at the same temperatures as the main peak. H Bonding. Figure 3 illustrates the room-temperature interaction of benzene with hydroxyls of sample A. Section a of the figure refers to a sample in the H form and section b to the same sample in the D form. An H-bonding interaction takes place: the band at 3624 cm-1 decreases, and a broad absorption is formed at about 3330 cm-1. The shift observed is 294 cm-1. The decrease of the band related to Si(OH)Al species is somewhat obscured by the parallel interaction of SiOH species, the stretching mode of which is shifted from 3745 to about 3615 cm-1. Considering the D sample avoids this overlap because the use of C6D6 as the deuterating agent converts a minor

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Figure 2. IR spectra concerning the reappearance of acidic hydroxyl species after contact with 10 mbar of NH3 (a), and outgassing at 443 K (curve 1), 473 K (curve 2), 503 K (curve 3), 523 K (curve 4), and 573 K (curve 5) and NH4+ bending region (b).

fraction of SiOH species into the SiOD form. Figure 3b shows that the Si(OD)Al band at 2672 cm-1 is shifted by 194 cm-1. It also shows that the OD counterpart of the 3580 cm-1 (found at 2638 cm-1) marked by an asterisk is not affected by C6H6 adsorption. This confirms previous descriptions of this species as being located in a constrained environment (hexagonal prisms), at variance with the model proposed by Sastre et al.18 The same experiment carried out with sample B (figure not reported) yielded similar features, though the shift observed was slightly larger (∼310 cm-1). In the case of the MCM-22 samples, interaction of benzene gives rise to a remarkably low ∆νOH value (294 and 310 cm-1), not only lower than that observed for H-ZSM-5 (360 cm-1)24-26 and H-MOR24,27 but even lower than that caused by CO on the same H-MCM-22 systems (320-330 cm-1), in contrast with the fact that benzene is known to be a stronger basic probe than CO. Similar results are obtained with adsorption of toluene (spectra not reported), which shows a higher ∆νOH value for H-ZSM-5 (400 cm-1, similar to the value observed by Lercher et al.28) than for H-MCM-22 (360 cm-1). The IR spectra concerning the interaction of acetone with sample A (not reported) are complex because of the presence

Onida et al.

Figure 3. IR spectra concerning the room-temperature adsorption of benzene on sample A in H form (a) and D form (b): (curve 1) sample outgassed at 773 K; (curve 2) after adsorption of C6H6.

TABLE 1: Strength of the H Bonding between Several Weak Bases and the Brønsted Site in Different Zeolitic Systems, as Measured by the Shifts Undergone by the OH Stretching Mode (cm-1)a ∆νOH (cm-1) probe

H-ZSM-5b (20)c

H-MORb (9)c

sample A (14)c

N2d COd C6H6 toluene (CH3)2CO C2H4 C3H6

125 330 360 400 990 389 539

109 300 350

120 320 294 360 960 384 484e

946 360 510e

sample B (20)c 130 330 310 390 ∼500

a Bold values are indicative of steric hindrance. b Data for H-ZSM-5 and H-MOR are from ref 24. c Numbers in parentheses represent the Si/Al ratio of the system. d Adsorption at a nominal temperature of 77 K. e Adsorption at temperature about 200 K.

of Fermi resonance;29 this notwithstanding, it is possible to evaluate the shift suffered by the OH stretching mode of Si(OH)Al species to be 980 cm-1. Table 1 gathers the shifts observed on the two H-MCM-22 samples, as compared with the same data concerning H-ZSM-5 and H-MOR zeolites

H-Bond Formation and Proton Transfer in Zeolite

Figure 4. Difference IR spectra related to increasing contact time of sample A with propene (initial pressure ) 30 mbar, curve 1) at 300 K. Spectra were recorded every 4 min. Broken curve was recorded after 30 min. Section a shows the O-H and C-H stretching region; Section b shows the C-H bending region (arrows show the behavior of bands with increasing contact time).

taken from ref 24. As it concerns the low-temperature data for the adsorptions of CO and N2 on H-ZSM-5 and H-mordenite, several sources are available, as reported by Kno¨zinger and Huber,30 who noticed that the values provided by Zecchina and co-workers24 are slightly higher than the others. We ascribe this to an instrumental effect, and we have chosen to report in Table 1 the data by Zecchina and co-workers, because the lowtemperature cell used in their work and in ours13 is the same. Proton Transfer. The other molecules in Table 1 (ethene and propene) are able both to engage in H bonding and to accept a proton. Spectra related to increasing contact time of sample B with propene at 300 K are shown in Figure 4. The spectrum recorded before adsorption of propene has been subtracted. Upon contact with 30 mbar of propene (curve 1, section a), the Si(OH)Al bands dramatically decrease, and a broad band due to Si(OH)Al species H bonded to propene molecules appears at 3150-3100 cm-1.15,16 At 3080-3060 cm-1, the absorption due to the stretching modes of unsaturated CH groups in molecular propene is seen.24 The more intense bands at 3000-2800 cm-1 are due to saturated CH groups of oligomeric species that are being formed. In section b of Figure 4, the CdC stretching mode of propene involved in H bonding with Si(OH)Al species is observed at 1633 cm-1, together with the same mode of the physisorbed molecule at 1645 cm-1. CH bending modes are observed at 1460-1370 cm-1, mixed with those related to oligomeric products at about 1465, 1380, and 1365 cm -1. With contact time, the band due to H-bonded species decreases because protonation of the olefin molecules becomes the

J. Phys. Chem. B, Vol. 106, No. 7, 2002 1687 predominant phenomenon,15,16 as also shown by the increase in the intensity of saturated CH stretching modes. As oligomerization proceeds, the 3200 cm-1 band decreases further and two bands are seen to grow at about 3700 and 3500 cm-1. These last two bands are particularly informative because they are due to SiOH and Si(OH)Al species, respectively, engaged in H-bonding with saturated hydrocarbons.15,16 This means that saturated alkyl chains substitute the propene double bond in H bonding both with Si(OH)Al species and with external silanols. The shifts are more limited than those with the double bond because the basic properties of the single C-C bond are less marked. The shift in the O-H stretch of SiOH species is about one-third of that concerning the Si(OH)Al species. The shift observed for Si(OH)Al species is about 124 cm-1, which is close to that reported for the interaction with alkanes of six-seven C atoms.15,28 In the CH bending mode region (Figure 4b), the CdC stretching mode of propene decreases because the proton transfer from the Brønsted site to one unsaturated carbon atom occurs, whereas the CH bending mode of saturated oligomers increases at 1465 cm-1. The growing doublet of bands at 1380 and 1365 cm-1 (δsCH3) is due to the formation of >C(CH3)2 groups31 and is therefore indicative of branching. An additional band ascribed to allylic cationic species of the type CdC-C(+) or CdC-CdC-C(+) arising from dehydrogenation of the oligomers increases at 1540 cm-1.32 To characterize better the H-bonded intermediate, adsorption of propene on sample A has been carried out at a temperature low enough to prevent any proton transfer to observe only the IR absorptions due to the H-bonded complex (spectra not reported). Propene has been dosed in a low-temperature cell at a nominal temperature of 77 K, and then the system has been allowed to thaw. A temperature intermediate between ambient and the liquid nitrogen boiling point (reckoned to be about 200 K) was high enough to ensure a sizable vapor pressure of propene and still low enough to prevent H transfer. The band due to the Si(OH)Al species engaged in H bonding was clearly seen at about 3140 cm-1, corresponding to a ∆νOH value of 484 cm-1. Figure 5 illustrates the room-temperature contact of ethylene with sample A. The starting curve shows the formation of H-bonded adducts. With time, bands due to saturated oligomers appear, and features similar to the propene experiment are seen, that is, the interaction of both SiOH and Si(OH)Al groups with the growing oligomers. As oligomerization proceeds, the band due to silanols engaged in H bonding with alkylic chains shifts from 3704 to 3699 cm-1, probably because interaction occurs with longer and longer chains.28 In contrast, with sample B, no oligomerization of ethylene has been observed, and the interaction was limited to H bonding. Acidity as the Tendency to Hydrogen Bond. Table 1 summarizes the available evidence on the bathochromic shifts suffered by the acidic Si(OH)Al species of samples A and B when interacting with the basic molecules used in the present work, together with the literature data concerning the two zeolites utilized for comparison. Figure 6 illustrates the same data in the form of the so-called Bellamy-Hallam-Williams plot (section a). The shift for H-ZSM-5 has been assumed to be the independent variable, and the data for the other two zeolites (H-MOR and sample A) have been considered as dependent variables. As noted by Zecchina and co-workers,24 the data for N2, CO, ethene, propene, and acetone do line up in the case of H-MOR. We calculated the corresponding slope, R ) 0.939 ( 0.004, that is, slightly less than one (the slope for

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Onida et al.

Figure 5. IR spectra related to increasing contact time of sample B with ethylene (initial pressure ) 30 mbar) at 300 K: (a) O-H and C-H stretching region; (b) C-H bending region. Curve 1 is the first spectrum recorded after dosing ethylene. Other curves refer to spectra recorded after 30, 40, and 50 min (arrows show the behavior of bands with increasing contact time).

H-ZSM-5), indicating a smaller tendency to form H-bonded species. As it concerns sample A, the corresponding data also line up precisely, with the notable exceptions of propene and benzene. Leaving aside these two data, the slope R is calculated to be 0.977 ( 0.003. Data for sample B have not been reported in Figure 6a to avoid overcrowding the figure. Data in Table 1 allow the calculation that R has the value 1.00 ( 0.003, that is, H-MCM-22 with a Si/Al ratio of 20 is as acidic as the reference H-ZSM-5. Figure 6b (lower plot) shows the R value for the different zeolites versus the aluminum content, expressed as Al/Si ratio. This plot gives evidence of the decrease in the strength of H bonding when the aluminum content increases and seems to suggest that the relative tendency to engage in H bonding of Brønsted sites of zeolites depends linearly on the Al/Si content. Data reported by Zecchina and co-workers in ref 24 allow calculation of a value of R for HY of about 0.89. Assuming Al/Si ) 0.33 for this zeolite,24 the corresponding point (not reported in the diagram) lines up with the others, giving support to the above considerations. An increase in acidity due to a decrease in Al content is expected. It is usually assumed that, within the same structure, such an effect smears out for Si/Al > 6.33 For the four zeolites examined, the dilution effect does not fade away. A quantitative description of the change in acidity of zeolites as a function of Si/Al ration is provided by the Sanderson electronegativity scale.33 The net charge on the acidic hydrogen atom, taken from ref 33, also depends linearly on Si/Al (Figure 6b, upper plot).

Figure 6. Bellamy-Hallam-Williams plot for H-MCM-22, H-MOR, and H-ZSM-5 (a): abscissa, ∆νSi(OH)Al value for H-ZSM-5; ordinate, the same for the other two zeolites. The slope R of the straight lines passing through the origin are reported in section b versus Al content (expressed as Si/Al, lower plot). The upper plot describes the charge on the acidic hydroxyl as computed according to the Sanderson electronegativity model33 as a function of Al/Si.

The reason that propene and benzene do not line up in the BHW plot, so constituting exceptions, is probably steric hindrance. Suppose the shape of the site is such that it allows the >O-H‚‚‚B adduct to assume the conformation that optimizes the binding energy with the smaller molecules but not with the larger ones, for example, propene and benzene (Figure 7 illustrates a schematic model for this); a weaker H bond with the Si(OH)Al would result because H bonding is markedly sensitive to the geometry. The weak interaction of propene with the acidic sites of H-MCM-22 is also witnessed by the CdC stretching frequency of the interacting molecules, shifted by 14 cm-1 with respect to the free molecules, whereas for both H-ZSM-5 and H-MOR the observed shifts are higher (19 and 15 cm-1, respectively).15,16 Instead, the CdC stretching frequency of ethene adsorbed on H-MCM-22, H-ZSM-5, and

H-Bond Formation and Proton Transfer in Zeolite

Figure 7. Pictorial scheme of hindered and free H-bond interactions at the Brønsted site.

H-MOR is shifted by the same value (11 cm-1) in all three cases with respect to the unperturbed molecule, in agreement with the similar ∆νOH observed for the three zeolites (Table 1). It is worth noting that acetone and propene have the same number of C atoms and similar size but the former does not apparently show any hindrance in H bonding. This seems to mean that the molecular size is not the only determining factor, but more important is the geometry of the complex. For both propene and benzene, the molecule must be perpendicular to the OH group to optimize the binding energy because the interaction takes place with the double bond and the aromatic ring, respectively. Thus, bulky groups at the unsaturated C atom, as in propene, give rise to steric hindrance for the formation of a H bond. In the case of acetone, instead, the interaction occurs with the oxygen atom along the CdO direction and no hindrance is caused by the methyl groups on the C atoms. Comparison of Scales of Acidity. As it concerns acidity defined as the ability to engage in H bonding, the result of the above discussion is that the four zeolites under consideration are in the order sample B ) H-ZSM-5 > sample A > H-MOR. As it concerns acidity as the ability to undergo proton transfer, we note the following. In Figure 4, after 1 min of contact of C3H6 with sample B at 300 K, the absorptions due to H-bonded complexes are still present in the spectrum, and vestiges are seen even after 30 min (broken curve). This is remarkable because the same intermediate is very unstable with H-ZSM-5 [15] and H-MOR,16 and it can be observed, under circumstances comparable to the present ones, only by means of time-resolved measurements in the first 30 s and the first second of the reaction, respectively. With sample B, proton transfer to ethene does not take place at all. All of this indicates that the ease of proton transfer is in the order H-MOR > H-ZSM-5 > sample A > sample B. The two criteria for acidity are clearly in contrast. The key factor could be the Al content, which, as discussed above, is in the order H-MOR (Si/Al ) 9) > sample A (Si/Al ) 14) > H-ZSM-5 (Si/Al ) 20) ) sample B (Si/Al ) 20). A low content in Al increases the tendency of OH species toward H bonding, whereas it depresses the ability to undergo proton transfer. The reason for this latter phenomenon probably lies16 in the ionic character of the framework, which is higher when the Al content is higher, which may decrease the energy barrier for proton migration. This argument, however, cannot hold alone for the present case of H-MCM-22, because the Al content is higher than in H-ZSM-5. The reason that proton transfer to propene and ethene at 300 K is slower in H-MCM22 than in H-ZSM-5 and H-mordenite is probably to be found in different steric constraints at the acidic sites. Stabilization of the transition state in olefin protonation may determine the proton donor ability.34 Such stabilization may

J. Phys. Chem. B, Vol. 106, No. 7, 2002 1689 depend on flexibility34 and local steric constraints of the zeolite structure, as suggested by a computational study of the protonation of isobutene in chabazite.35,36 Steric hindrance at the proton site has been observed in the present case for both benzene and propene. Though not directly observed for Hbonded ethene, it could affect the transition state of ethene protonation, the geometric conformation of which is surely different from that of H-bonded complex. This may explain the lower ability of H-MCM-22 to transfer protons to both ethene and propene with respect to H-MOR and H-ZSM-5. Nature of the Oligomerization Products. Branched species are formed from the very beginning of the chain growth. The same result has been obtained for H-mordenite,16 whereas no branching has been observed for oligomerization of propene in H-ZSM-5.15 In the present case, the intensity ratio, I(νasym,CH3)/ I(νasym,CH2) (2960 and 2934 cm-1, respectively), keeps constant at about 1:3 along the experiment. This is at variance with what is observed both in H-ZSM-5 and in H-mordenite, in which the intensity ratio I(νasym,CH3)/I(νasym,CH2) was always less than one.15,16 Because νasym,CH2 is intrinsically less intense than νasym,CH3, this means that oligomers formed in MCM-22 have a higher degree of branching compared with H-ZSM-5 and H-MOR. A similar evidence has been observed in SAPO-40,37 in which 12 MR channels are present. This is due to a larger available space in which the chains grow, that is, the large supercages of MCM-22 or even the external surface. Oligomerization of ethene on sample B gives rise to species much more branched than those obtained in H-ZSM-515 for the same reason. Similarly to what is observed for H-ZSM-5, alkylic chains seem to be slightly more branched than those obtained with propene, as indicated by the intensity ratio I(νasym,CH3)/I(νasym,CH2) of 1.4. With both propene and ethene, formation of allylic cationic species is observed, which are coke precursors. Acidic Sites. Evidence reported in the present paper has shown that the 3580 cm-1 band is not intrinsically more acidic than the other Si(OH)Al species. Also, the ammonia experiment has brought further support to the presence of two (at least) Si(OH)Al species. The interaction of the oligomers with external SiOH species is evidence that at least a fraction of the oligomerization process takes place at the external surfaces of the zeolite crystals. External acidity has been shown to be important in the isomerization of toluene to p-xylene7 and has been proposed to be partially responsible for the formation of cumene by alkylation of benzene.38 The steric hindrance met by some relatively bulky molecules indicates that the sites for H-bonding interaction have restricted surroundings. The location of sterically hindered OH species is not straightforward, one reason being that although the 3624 cm-1 is made up of two components, these behave essentially in the same manner both with propene and benzene, indicating that, even in the case in which their geometrical environments were different, such differences would not affect significantly their H bonding with “bulky” molecules. One possibility is that the two types of hydroxyls might usually sit at different locations (supercage and 10 MR channels) but they may transform readily the latter into the former upon interaction, as tunneling of the proton from one side to the other of the silica wall seems rather easy. Conclusions The presence of three types of Si(OH)Al species in H-MCM22 hypothesized in a previous work13 has been confirmed, these

1690 J. Phys. Chem. B, Vol. 106, No. 7, 2002 absorbing at 3628, 3618, and 3580 cm-1. The latter is not readily accessible to C6D6 and probably sits at the hexagonal prisms; its frequency is lower because of some interaction with the walls and not because of any intrinsically higher acidity. The other two species are probably located at the supercages and at 10 MR channels. Their environment seems to be constrained because molecules such as benzene and propene interact less strongly than predicted. Their acidity has been assessed both as a tendency to engage in H bonding and as the ability to transfer the proton. Comparison with similar data for H-ZSM-5 and H-MOR shows that the former increases with decreasing Al content. The latter is counter-related with the Al content, but steric constraints are probably to be invoked to explain the reduced reactivity in promoting olefin oligomerization. In both reactions with propene and reactions with ethene, branched oligomers are formed, and the degree of branching is higher than that observed in H-ZSM-5 and H-mordenite because of the larger available space for the growing chains in H-MCM-22. Oligomeric growth on the external surface of the crystals also occurs. Acknowledgment. Helpful discussions with R. Inzitari and L. Borello are gratefully acknowledged. We thank the Italian Ministry of Education MURST for financial support. References and Notes (1) Rubin, M. K.; Chu, P. U.S. Patent 4,954,325, 1990. (2) Leonowitz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (3) Absil, R. P. L.; Bowes, E.; Green, G. J.; Marler, D. O.; Shihabi, D. S.; Socha, R. F. U.S. Patent 5,085,762, 1992. Del Rossi, J. K.; Huss, A., Jr. U.S. Patent 5,107,047, 1992. Dessau, R. M.; Partridge, R. D. U.S. Patent 4,962,250, 1990. Huss, A., Jr.; Kirker, G. W.; Keville, K. M.; Thomson, R. T. U.S. Patent 4,992,615, 1991. (4) Ravishankar, R.; Bhattarcharya, D.; Jacob, N.; Sivasanker, S. Microporous Mater. 1995, 4, 83. (5) Corma, A.; Corell, C.; Llopis, F.; Martine´z, A.; Pe`rez-Pariente, J. Appl. Catal. A 1994, 115, 121. (6) Corma, A.; Davis, M.; Forne´s, V.; Gonza`les-Alfaro, V.; Lobo, R.; Orchille`s, A. V. J. Catal. 1997, 167, 438. (7) Kumar, N.; Lindsfors, L.-E. Appl. Catal. A 1996, 1478, 175. (8) Wu, P.; Komatsu, T.; Yashima, T. Microporous Mesoporous Mater. 1998, 22, 343. (9) Me´riaudeau, P.; Tuan, V. A.; Nghiem, V. T.; Lefevbre, F.; Ha, V. T. J. Catal. 1999, 185, 378.

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