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J. Phys. Chem. B 2002, 106, 10518-10522
ARTICLES IR Evidence that Secondary Interactions May Hamper H-Bonding at Protonic Sites in Zeolites Barbara Onida, Barbara Bonelli, Luisa Borello, Sonia Fiorilli, Francesco Geobaldo, and Edoardo Garrone* Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24. 10129 Torino, Italy ReceiVed: June 10, 2002
When a molecule, engaged in H-bonding with an acidic proton in zeolites, is large enough to interact with the surrounding walls, the IR spectroscopic measure of the acidity is affected, but information may be gained on the geometry of the site. Examples concerning MCM-22, ZSM-5, SAPO-40, and Theta-1 are discussed.
Introduction A classical way to evaluate the relative acidity of different molecules in solution by IR means is the use of BellamyHallam-William (BHW) plots,1,2 where shifts in frequency caused by H-bonding of two acidic groups to a set of mildly basic molecules are plotted one against the other: a straight line is observed, the slope of which is the measure sought. H-bond formation has been commonly used to measure the acidity of hydroxyl species in solids,3-13 often with a procedure similar to BHW plots. In this case, with sufficiently large molecules, the possibility arises that secondary interactions may occur with the surroundings of the acidic hydroxyl species, so hampering the H bonding itself, and affecting the measure of acidity. This aspect is dealt with in the present paper by scrutinizing a set of data concerning different molecules and solids. Data reported concern the interaction of the isolated SiOH species in severely dehydrated Aerosil and the Brønsted site Si(OH)Al in a few zeolitic systems: ZSM-5, MCM-22, Theta, and SAPO-40. Data for silica and ZSM-5 are from the literature.3,6-9 Also considered is the mesoporous silica MCM41, exhibiting the same isolated hydroxyl species as Aerosil, to check whether any steric hindrance is introduced by mesoporosity. The set of weakly basic B molecules employed are N2, CO, ethylene, benzene, propene, toluene, and 1,3,5-trimethylbenzene (TMB), the strength of which as bases is in the order listed.9 Stronger bases have not been considered, because in this case Fermi resonance between the stretching and other modes of the acidic hydroxyl species is observed, causing the appearance of Evans windows which somehow disturb the evaluation of the shift.9 Ethene and propene can both engage in H bonding and act as proton acceptors. At RT, proton transfer to ethene is slow with all zeolites considered, so that the H-bonded π-complex * To whom correspondence should be addressed. E-mail: garrone@ athena.polito.it. Phone: +39-011-5644661. Fax: +39-011-5644699.
TABLE 1: Shifts ∆νOH of the OH Stretching Mode Observed with Basic Probes ∆ν(OH) system
N2
CO
C2H4
C3H6
C6H6
C7H8
TMB
Aerosila MCM-41 SAPO-40 ZSM-5 MCM-22 Theta-1 cross section (nm)
40 40 115 125b 130 120 0.31
90 90 290 330b 330 316 0.34
104a
152a 152 503 540c 500 450 0.65
120a 120 330 360b 310 285 0.72
147
167
a
360 390c 390 320 0.48
380 400 370 0.82
0.88
Reference 8. b References 4-7. c References 10 and 21.
Figure 1. MCM-22 and MCM-41 in contact with CO at a nominal temperature of 77 K (curve 1 and 3, respectively) and benzene at room temperature (curves 2 and 4, respectively).
can be readily observed. Proton transfer to propene is faster, but only with ZSM-5 does the process require fast time-resolved experiments.10
10.1021/jp026274l CCC: $22.00 © 2002 American Chemical Society Published on Web 09/19/2002
Secondary Interactions May Hamper H Bonding
J. Phys. Chem. B, Vol. 106, No. 41, 2002 10519
Figure 2. Bellamy-Hallam-William plots referred to SAPO-40, ZSM-5, MCM-22, and Theta-1, taking as reference the ∆ν values for the isolated silanol of Aerosil.
Experimental Section
Results and Discussion
SiO2 and ZSM-5 (Si/Al ) 25) were from Degussa and Zeolist, respectively. SAPO-40 (Si/Al ) 0.12) and MCM-22 (Si/Al ) 22) samples were prepared according to the literature.14,15 MCM41 was prepared according to ref 16, treated in flowing air up to 823 K, and maintained at the same temperature for 6 h, to remove the template. Theta-1 (Si/Al ) 20) was prepared at the University of Calabria.17 For FT-IR measurements, the powders were pressed into thin, self-supporting wafers; spectra were collected, at a resolution of 2 cm-1, in the 4000-500 cm-1 range, on a Bruker FTIR Equinox 55 spectrometer, equipped with a MCT cryodetector (128 scan). Pretreatments were carried out using a vacuum frame in an IR cell equipped with KBr windows. Wafers were outgassed at 773 K. Adsorption of CO and N2 was carried out at the nominal temperature of 77 K. The software “CS Chem3D Ultra” (Cambridge Soft. Corporation) has been used to measure bulkiness of the probes.
Table 1 reports all data as shifts (∆ν) with respect to the unperturbed O-H value. Some irregularities are evident. With silica (both Aerosil and MCM-41), the benzene shift (120 cm-1) is larger than that of ethene (104 cm-1), whereas with zeolites, it is definitely smaller. With MCM-22 and Theta-1, the benzene shift is even lower than that observed with CO. With most zeolites (Y, ZSM-5, mordenite, beta), as well as Aerosil and MCM-41, the opposite takes place: Figure 1 illustrates this fact by comparing the CO and benzene shifts with MCM-41 and MCM-22. We ascribe these irregularities to the presence of walls surrounding the acidic site, with which relatively big molecules such as benzene (at variance with ethene and CO) may interact, not allowing the H-bonded adduct to assume the optimal conformation. H bonding is strongly sensitive to the geometry and is influenced by even small perturbations of the B‚‚‚H
10520 J. Phys. Chem. B, Vol. 106, No. 41, 2002
Onida et al.
Figure 3. Correlation between extent of hampering and largest transverse molecular diameter of probe molecule for SAPO-40, ZSM-5, MCM-22, and Theta-1.
distance or the O-H‚‚‚B angle.1,2 This explanation is in line with what was reported by Su and Barthomeuf, on the interaction of C-H groups of benzene with basic oxygens of the framework in faujasites,13 and with the very recent computational work by Kemner et al.,18 showing that van der Waals interactions with walls orient ferrocene molecules in the cavities of NaY. The secondary interactions with the surrounding walls, preventing the optimal conformation from the point of view of the H-bonded adduct, may be regarded as a steric hindrance at the proton site, the extent of which may be studied by means of BHW plots. Such secondary interactions with the immediate surrounding of the Brønsted site have little to do (and have not to be mistaken) with the possible interactions that a molecule diffusing along the zeolite channels may encounter with the pore walls. Also, the hampering to the H-bonding by secondary interactions studied here is different from the inaccessibility of
some protonic species in zeolites (faujasites and mordenite, for example) which has been dealt with by many authors.11,12 For this reason, no reference is made to the different structures of considered zeolites, in terms of pore dimensions. Because of the nonporous nature of the solid, the isolated SiOH species in Aerosil may be reasonably assumed to be nonhampered: for this reason, in all BHW plots SiOH values have been adopted as independent variables. With MCM-41, exactly the same data as with Aerosil are observed, so proving that the silanol species is the same in the two samples, and that mesoporosity does not cause any hampering to the acidic center. Figure 2 shows the BHW plot for zeolitic systems: the broken straight lines have been drawn through the origin discarding the points deviating from linearity and applying the least-squares method. From the slopes, the known scale of acidity is obtained: SiO2(Aerosil) ≡ SiO2 (MCM-41) < SAPO-40
