5962
J. Phys. Chem. 1993,97, 5962-5964
Inhomogeneity of Bronsted Acid Sites in H-Mordenite Vladimir L. Zbolobenko,+Marina A. Makarova, and John Dwyer’ Chemistry Department, UMIST, PO Box 88. Manchester M60 1 QD, UK Received: November 12, 1992; In Final Form: February 18, I993
Bronsted acid sites in H-mordenite have been studied using adsorption-desorption of probe molecules (ammonia, benzene, cyclohexane) and monitored by FTIR spectroscopy. Both the second derivative of the spectrum and NH3 thermodesorption show that the asymmetrical stretch band of bridged OH groups a t 3607 cm-’ consists of two components, one a t a higher frequency of 3612 cm-l (lower effective acidity) and another a t a lower frequency of 3585 cm-I (higher effective acidity) with the intensity ratio of 2:l. The low-frequency OH groups are inaccessible to bulky molecules such as benzene and cyclohexane, which suggests that they are located in the small channels.
Introduction
Experimental Section
Bridged hydroxyls =Si(OH)Al= provide the basis for Bronsted acidity in zeolites and consequently have been widely investigated. Infrared spectroscopy is most widely used to study Bronsted sites, and an excellent review of early work is given by Ward.’ In general, hydroxyl stretches in the region 3600-3650 cm-I are assigned to bridged hydroxyls in the larger pores of zeolites (10 or 12 rings) and lower frequencies are associated with hydroxyl vibration into smaller pores (6 or 8 rings).2 Within a given pore system, frequencies can depend upon local composition and structure.3 In faujasitic zeolites and related materials (EMT, SAPO-37), the hydroxyls vibrating in larger supercages and those vibrating in the smaller (0) cages are well resolved in the infrared spectrum (high-frequency (HF) and lowfrequency (LF) bands, respectively). On the other hand, resolution of the band associated with hydroxyls in a single type of pore, for example, the larger pores of FAU, has met with limited success. There have been attempts to deconvolute numerically both the H F and L F bands in faujasite (FAU)4and to deconvolute numerically bands perturbed by sorbates in H-ZSM-5,5but these methods are dependent upon curve-fitting procedures, which can always account for quite minor asymmetry in peak shapes. An attempt to deconvolute the bands for silanols in H-ZSM-5 and for the HF band in FAU using line-narrowing procedures does suggest heterogeneity for the silanol hydroxyls, but for the HF FAU band the position is less clear. In some compositions, there is evidence for a t least two types of hydroxyl, but in other compositions this could not be established.6 The structure of mordenite is known to consist of two types of channels. One is composed of 12 rings (6.5 X 7.0 %.) and the other, which comprises the side pockets to the main channels, is composed of 8 rings (2.6 X 5.7 A).’ Consequently, there should be at least two bands associated with the hydroxyl region of H-mordenite (H-MOR), one corresponding to vibration into the larger pores and the other, at lower frequency, vibrating into the smaller pores. Nevertheless, although the literature concerning H-MOR is very extensive, we are not aware of any work demonstrating this. Consequently, in the present paper we reexamine the hydroxyl region of H-MOR with the purpose of providing more conclusive evidence for two types of hydroxyl within the structure.
The following materials were used for the present study: H-MOR (Laporte, Si/Al = 7.7), Dz (Messer Griesheim, 99.7+%), NH3 (Aldrich, 99.99+%), benzene, and cyclohexane (Aldrich, 99.9%). FTIR studies were carried out using a Cygnus-100 Mattson FTIR spectrometer and a special I R cell having a thermostated zone which facilitated a high-temperature treatment of samples in-situ.* The cell was also connected to a calibratedvolume fitted with a pressure gauge, for ammonia adsorption, and had a liquidinjecting system (silicon septum/microsyringe). The mordenite sample was pressed into self-supporting disks ( m 10 mg, p = 7-9 mg/cm2), placed into the cell, heated at 1 deg/min to 400 O C under vacuum, and then held a t 400 OC overnight (vacuum of Torr). All the spectra were collected at room temperature using 100 scans and a resolution of 2 cm-I. The wavenumber interval was 4000-1300 cm-I. Deuterium exchange was carried out at 300 OC and 500 Torr for 30 min (twice). For a stepwise adsorption or desorption of ammonia, the tablet was moved to the heated zone and then, after treatment, returned toitsinitial position to record a spectrum. This procedure was repeated for each step. Benzene and cyclohexane were sorbed at room temperature.
Present address: Center for Catalysis and Surface Science, Northwestern University, 2137 Sheridan Road, Evanston, IL 60201. +
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Results and Discussion TheOHregionofthespectrumfor theinitialsampleofH-MOR is shown in Figure l a (curve 1). It consists of an intense band at 3607 cm-I from Bronsted acid groups, a much weaker band a t 3745 cm-I from terminal OH groups, and a hardly observed band at -3680 cm-I presumably from AI-OH units. The appearance of the spectrum is similar to that reported previously for mordenite samples with a Si/AI ratio close to 5.9-i However, thestructureoftheintenselineat 3607 cm-I has, toourknowledge, never been discussed in any detail in the literature. More accurate consideration demonstrates that its shape is complex and asymmetrical. This can clearly be seen from the second derivative of the spectrum, which reveals the presence of two peaks at 3612 and 3584 cm-I which are hidden inside the broad initial line (Figure 1, curve 2). Deuterium exchange shifts the spectral region of interest to the right into a range of frequencies where the shape of the spectrum is not affected by traces of adsorbed water, the presence of which cannot always be excluded a priori (Figure Ib, curve 2). The parameters of the spectrum in this case are 2662 cm-I
0022-3654/93/2097-5962%04.00/0 0 1993 American Chemical Society
Brbnsted Acid Sites in H-Mordenite
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Figure 1. Hydroxyl region of the IR spectrum of H-MOR (curve 1 ) and its second derivative (curve 2, extended by 50); (a, left) initial sample; (b, right) after deuterium exchange.
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Figure 3. Comparison of the shapes of the spectra during ammonia adsorption and thermodesorption: (1) curve 5 from Figure 2a, (2) curve 4 from Figure 2b, (3) their difference. A b
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Figure 4. Deconvolution of the IR band from Briinsted OH groups in H-MOR: ( 1 ) HF component, (2) LF component, (3) their sum (e.g., simulated spectrum), (4) experimental spectrum.
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Figure 2. (a, top) Hydroxyl region of H-MOR during the process of ammonia adsorption of 150 OC: (1) initial sample, then after addition of N H j in an amount of (pmollg) (2) 230, (3) 460, (4) 700, (5) 930, (6) 1160, (7) 1400. (b, bottom) Hydroxyl region of H-MOR after ammonia thermodesorptionat ("C) (1) 150, (2) 200, (3) 250, (4) 300, (5) 350, (6) 400, (7) 450.
for Brbnsted OD, 2759 cm-1 for terminal Si-OD, and -2715 cm-' for AI-OD groups. Nevertheless, the structure of the band at 2662 cm-l remains the same as in the OH region. It consists of two overlapped contributory bands, at 2664 and at 2646 cm-I, which are visible from the second derivative of the spectrum (Figure lb, curve 2). Thus, it can be concluded that real inhomogeneity of Brbnsted OH groups can be observed in the IR spectrum of H/D-MOR. To distinguish these two types of Bransted acid sites more clearly, we used stepwise adsorption-thermodesorption of ammonia. Figure 2 presents results on the ammonia poisoning of the acid OH groups at 150 OC. Step-by-stepaddition of ammonia proportionally decreases the intensity of the band at 3607 cm-I, but its shape remains the same, which suggests that ammonia
molecules, at this temperature, do not have any preferential sites for adsorption. However, the ammonia thermodesorption reveals quite a different picture (Figure 2b). The band that appears and grows after heating and pumping at different temperatures has a maximum at 3612 cm-I, and its shape is narrower than that of the original line. Only at higher temperatures of desorption (450 "C),when the intensity of the band is almost restored, does it widen and achieve its initial shape. Thus, these results indicate that the HFand LF hydroxylgroups differ significantlyin their properties to retain ammonia, the LF ones providing the stronger sorption sites. This is in good agreement with results of microcalorimetric measurements of ammonia sorption into H-MOR (Si/AI = 12.6), which are accounted for by postulating two types of Brbnsted site, one of which holds the ammonia more str0ng1y.I~ Furthermore, if we now choose a pair of these bands with similar heights-a wide band from the adsorption set, containing both H F and LF components (Figure 2a), and a narrow band from the desorption set (Figure 2b) which is a pure H F component-the subtraction of one band from another will create an imageof the LFcomponent with the intensity corresponding to the fraction of OH groups free from ammonia. An illustration of this approach is given in Figure 3, where two spectra of OH groups, one during NH3 adsorption and the other during NH, thermodesorption, are overlaid, and their difference clearly generates a LF band at 3585 cm-I. The results of deconvoluting the original band arising from Brbnsted acid sites in H-MOR (at 3607 cm-1) according to this subtraction are presented in Figure 4. The LF component is slightly wider than the H F one (44 versus 36 cm-I), and the ratio of their intensities is approximately 2:l. In order to understand better the nature of the OH groups producing the H F and LF components, adsorption of benzene
5964 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 P. 0
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Zholobenko et al.
in the observed stronger binding. Additionally, the stronger binding of ammonia to the low-frequency sites might reflect enhanced acidity for these sites, but it is not possible, on the basis of the current results, to separate effects due to sorption and intrinsic acidity. In any event, since the retention of ammonia at elevated temperatures is often taken to be a measure of acid site strength, it is reasonable to associate the sites in mordenite showing stronger binding of ammonia (LF sites) with higher effective acidity. The results of this article are in a good agreement with the workof Jacobset al.,2whopostulated that theIR bandsof hyroxyls accommodated in 6- and 8-ring cavities undergo bathochromic shifts to lower frequencies in comparison with bands for hydroxyls in larger cages. This is attributred to perturbation of OH groups in smaller pores by framework oxygens, perturbation increasing with decreasing radius of the rings. The value of the shift (27 cm-I) in the case of H-MOR is close to the shifts proposed for other 8-ring zeolites: 33 cm-I for erionite, 29 cm-I for Rho, and 42 cm-I for stilbite.2 However, to our knowledge, this is the first clear evidence for unambiguous decomposition of the unresolved I R band belonging to hydroxyls in both the 12-and 8-ring channels of H-MOR. Conclusions
3600
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and cyclohexane at room temperature was examined. These are both weak bases and should perturb Brdnsted acid sites. On the other hand, however, their dimensions ( u = 5.85 A for C6H6 and 6.OA for CsHI2l4)should prevent them from penetrating into the 8-ring channel^.^^-^^ During the early stages of adsorption, a gradual increase in the quantity of probe molecules injected into the IR cell leads to a proportional decrease in the band at 3607 cm-I. However, even a large excess of these bases is not able to perturb all the O H groups. The extra amount of the base above that necessary for saturationdoesnotcauseany changesin spectra. The spectra of H-MOR after adsorption of excess base are shown in Figure 5. The shape of the OH band left unperturbed is clearly distorted by overlap with the wide band from the hydroxyls that are hydrogen-bonded to benzene (band a t -3250 cm-I, Figure Sa) or to cyclohexane (at -3500 cm-I, Figure 5b). Nevertheless, the position of the band (3588 cm-I), its half-width (-46 cm-I), and its relative intensity in comparison with that of the initial band allow us to identify this OH band clearly as the LF component. It should be noted that exactly the same spectroscopic behavior of H-MOR during benzene adsorption has been previously observed by Karge,'6 and even the slight shift of the resulting OH band toward lower frequencies has been pointed out. However, no explanation for this fact has been provided. Thus, the HF component of the OH band at 3607 cm-I is generated by OH groups in 12-ring channels and the L F component by OH groups in the 8-ring channels. According to the results described above, the hydroxyl vibrating in the larger pores shows weaker interaction with ammonia than the hydroxyl vibrating in the smaller cages. This is consistent with the siting of the L F band in the smaller pores, which should facilitate multicoordination of NH4+ to surface oxygen, resulting
Two types of BrBnsted acid sites exist in H-mordenite. One consists of OH groups vibrating into the large channels ( H F I R band at 3612 cm-I), with lower effective acidity, and the other type is associated with hydroxyls in the smaller channels (LF IR band at 3585 cm-I), with higher effective acidity. The ratio of the intensities of the bands associated with these two types of site is 2:l. Normally these two bands ( H F and LF) are overlapped, giving one asymmetrical band at 3607 cm-l which is widely reported. However, ammonia treatment and benzene or cyclohexane adsorption make unambiguous decomposition possible. Acknowledgment. We thank the EEC for financial support (BRITE EURAM 4633) for M.A.M. and the SERC for an equipment grant (GR/D/99768). We also thank K. M. AlGhefaili for assistance with the experimental work and for discussion. Additionally, we thank one of the referees for a helpful comment, which we will address in a subsequent paper. References and Notes (1) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph Series 171; Washington, D.C., 1976; p 118. (2) Jacobs, P. J.; Mortier, W. J. Zeolites 1982, 2, 226. (3) Dwyer, J. In Guidelinesfor Mastering the Properties of Molecular Sieues; Barthomeuf, D., et al., Ed.; NATO AS1 Series B, 221; Plenum: New York, 1990; p 241. (4) Dombrowsky, D.; Hoffman, J.; Fruwert, J.; Stock, T. J . Chem. SOC., Farad. Trans. 1985, 81, 2257. (5) Datka, J.; Boczar, M.; Rymarowicz, P. J. Catal. 1988, 114, 368. (6) Dwyer, J.; Dewing, J.; Thompson, N. E.; O'Malley, P. J.; Karim, K. J . Chem. SOC.,Chem. Commun. 1989, 13, 843. (7) Meier, W. M.; Olson, D. H . Atlas of Zeolite Structure Types; Butterworth-Heinemann: London, 1992; p 144. (8) Thompson, N. E. Ph.D. Thesis, UMIST, 1991. (9) Ha, B. H.; Barthomeuf, D. J. Chem. SOC.,Faraday Trans. I 1979, 75, 2366. (10) Karge, H. G. Z . Phys. Chem. Neue Folge 1980, 122, 103. (11) Bankos, I.; Valyon, J.; Kapustin, G. I.; Kallo, D.; Klyachko, A. L.; Brueva, T. R. Zeolites 1988, 8, 189. (12) Lavalley, J. C.; Maache, M.; Saussey, J. Proc. SPIE-Int. Soc. Opt. Eng. 1990, 1341, 244. (13) Klyachko, A. L.; Kapustin, G. I.; Brueva, T. R.; Rubinstein, A. M. Zeolites 1987, 7, 119. (14) Breck, D. W. Zeolite Molecular Sieues; Wiley: New York, 1974; p 636. (15) Barrer, R. M.; Peterson, D. L. Proc. R. SOC.1964, 280A, 466. (16) Karge, H. G. In Mol. Sieues-2, Int. Coni, 4th; Katzer, J. R., Ed.; ACS Symp.,Ser. 40; Washington, D.C., 1977; p 584. (17) Mishin, I. V.; Plakhotnik, V. A.; Kapustin, G. I.; Klyachko, A. L.; Slink'in, A. A. Kinet. Katal. 1983, 24, 1448.
