J. Phys. Chem. 1994, 98, 930-933
930
Acidic Hydroxyl Groups in Zeolites X and Y: A Correlation between Infrared and Solid-state NMR Spectra Barbara Gil,? Ewa Broclawik,$Jerzy Datka,’*+and Jacek Klinowski’*o Department of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakbw, Poland, Institute of Catalysis, Polish Academy of Sciences, Niezapominajek 1 , 30-239 Krakbw, Poland, and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1 E W,U.K. Received: July 12, 1993; In Final Form: October 1 1 , 1993”
Five signals from Si(nA1) units, where n = 0-4,are found in 29Si magic-angle-spinning (MAS) NMR spectra of zeolites X and Y (synthetic faujasites) with different Si/Al ratios. Since only Si(4Al), Si(3Al), Si(2Al), and Si(lA1) units a r e associated with bridging OH groups, there can be four possible kinds of such groups: A13Si-OH-AISi3, A12SiSi-OH-AISi3, AlSizSi-OH-AlSi3, and Si3Si-OH-AlSi3. Four kinds of hydroxyls of different acid strengths are indeed found by IR spectroscopy, and their relative populations depend on the Si/A1 ratio as do the relative intensities of 29Si M A S N M R signals. The four kinds of hydroxyls have been correlated with Si(nA1) signals as observed by N M R . The distribution of acid sites of different strengths can therefore be deduced from 29SiM A S NMR spectra, which may be of considerable practical importance in heterogeneous catalysis.
A
Introduction The study of acidic surface sites capable of donating protons to adsorbed molecules is one of the most important in heterogeneous catalysis.’ It is vital to know the structure, concentration, strength, and accessibility of the Brernsted and Lewis acid sites and the details of their interaction with the adsorbed organics. The Brernsted acidity of zeolites arises from the presence in their hydrogen forms (usually prepared by the heat treatment of the ammonium form) of Si-OH-A1 groupings (“bridging hydroxyl groups”). Much attention has been devoted to the determination of their number and structure. Powder diffraction methods have elucidated the frameworkstructures of many zeolites but not the precise conformation of the Si-OH-A1 grouping. N M R has provided much relevant i n f ~ r m a t i o n . ~First, , ~ different kinds of hydroxyl groups can be directly and quantitatively observed by IH magic-angle-spinning (MAS) NMR2. Second, the fact that the second moment of the proton resonance signal from rigid polycrystalline solids is very sensitive to the distance between nuclei coupled via the dipolar interaction enables protonaluminum distances to be determined.3v4 More detailed information about hydroxyl groups in zeolites is provided by infrared (IR) ~pectroscopy.s-~ Quantum-chemical calculations*-l* indicate that the acid strength of bridging hydroxyls depends on the geometry of the bridge (bond lengths and angles) and on the number of nearby A1 atoms. In zeolites in which all hydroxyls have the same bridge geometry and where there is the same number of nearby A1 atoms (protonic forms of zeolites A and X with Si/Al = l ) , bridging hydroxyls are “homogeneous”, i.e., have the same acid strength.sJ3 On the other hand, in zeolites containing hydroxyls with different bridge geometries, such as ZSM-5,6s7J4 and in materials with different numbers of A1 in the vicinity of the bridge (zeolite Y with Si/Al = 2.6),13J5 OH groups with different acid strengths are simultaneously present. Framework Si in zeolites is tetrahedrally coordinated, and thus there are five different possible environments of a silicon atom, denoted as Si(nA1) where n (14) signifies the number of aluminum atoms connected, via oxygens, to a silicon. Each type of Si(nA1) building block corresponds to a definite range of 29Sichemical Jagiellonian University. t Polish Academy of Sciences. 1 University of Cambridge.
*Abstract published in Advance ACS Absrracrs, December 15, 1993.
0022-3654/94/2098-0930%04.50/0
1.35
1.87
1.07
Figure 1. IR spectra of hydroxylgroupsin (A) zeolites X and (B) zeolites Y with different Si/A1 ratios.
shift.I6J7 When, as is the case with zeolites X and Y,the 29Si MAS N M R spectrum contains more than one peak and is correctly assigned in terms of Si(nA1) units, the Si/Al ratio in the zeolitic framework may be calculated from the spectrum.17J8 29Si MAS N M R is also of considerable assistance in determining the ordering of Si and A1 atoms in the framework.17J8 We shall demonstrate that IR spectra of OH groups in zeolites correlate well with 29Si MAS N M R spectra. This may provide information on the geometry of the bridging hydroxyls and the number of nearby A1 atoms. In MAS N M R spectra of zeolites A and X (%/A1 = l ) , only the Si(4Al) signal is present,l6l9 so only A138i- -0H-AISi3 hydroxyls may exist. By contrast, zeolite Y @/A1 = 2.5) gives separate 29Si signals from Si(3A1), Si(2A1), Si( lAl), and Si(OA1) units, the aluminum content being too low for the number of Si(4AI) units to be significant. As Si(OA1) units cannot create bridging OH groups, there are three 0 1994 American Chemical Society
Acidic Hydroxyl Groups in Zeolites
The Journal of Physical Chemistry, Vol. 98, No. 3, I994 931
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Figure 3. (A) IR spectra of OH groups in zeolites X with different Si/AI ratios interacting with benzene and chlorobenzene (normalized to the same band area); (B)-(E) second derivatives of the spectra. possible kinds of hydroxyls: AlzSiSi- -OH- -A& AlSiZSi-OHAISi3, and Si3Si-OH-AISi3. Three kinds of bridging hydroxyls of different acid strengths are indeed detected by I R spectroscopy in zeolite Na,H-Y (Si/Al = 2.56).13J5 We have studied a series of zeolites X and Y with Si/Al ratios from 1.06 to 7.02. As was explained above, the intensity ratio of Si(nA1) signals in 2% MAS N M R depends on the Si/Al ratio. The question arises whether the relative populations of Al3Si-OH-AlSi3, AlzSiSi-OH-AISi3, AlSizSi-OH-AISi3, and Si3Si-OH-AlSi3 hydroxyls depend on the Si/Al ratio in a similar
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Figure 4. (A) IR spectra of OH groups in zeolites Y with different Si/Al ratios interacting with benzene and chlorobenzene (normalized to the same band area); (B)-(E) second derivatives of the spectra. fashion. We have studied the properties of zeolitic O H groups by recording the IR spectra of hydroxyls hydrogen bonded to benzene and chlorobenzene molecules. In general, the magnitude of the shift of the IR bands after adsorption of a donor molecule is proportional to the acid strength of the OH group.20 The frequency shift accompanying hydrogen bonding increases with the acid strength of the OH group involved.z1J2
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Experimental Section Zeolite Na-X with Si/Al = 1.06 was kindly supplied by Dr. H. Karge. Samples with Si/Al ratios of 1.19, 1.35, 1.67, 1.87, 2.00, 2.39, 2.56, and 2.75 are the same as those used in ref 23. Zeolites Y with Si/Al= 4.14,5.03,5.89, and 7.03 were obtained by dealumination with S i c 4 and were used in ref 24. All samples were converted into Na,NH4 forms by ion exchange with an aqueous solution of CH3COONH4 at 330 K. The degree of ammonium exchange of the samples is given in Table 1, We note that, ~ for steric reasons, zeolites X can only be partially exchanged with the ammonium ions. For I R studies zeolites were pressed into thin wafers (4-8 mg/ cmz) and activated in situ in an IR cell. The activation temperature depended on the Si/Al ratio and was 570 K for zeolites Na,NH4-X with Si/Al = 1.06 and 1.19; 670 K for Na,NH4-X @/A1 = 1.35, 1.67, and 1.87); and 720 K for Na,NH4-Y (Si/Al > 2.00). For simplicity, the Na,H-X and Na,H-Y forms examined will henceforth be referred to as zeolites X and Y. Benzene and chlorobenzene used for IR experiments were of analytical grade. Small portions of benzene and chlorobenzene were sorbed until practically all Si-OIH-A1 groups were hydrogen bonded (which corresponds to the disappearance of the 3650-3660-cm-l IR band). Results and Discussion The IR spectra of OH groups in our samples are shown in Figure 1. Only one Si-OIH-A1 band (at 3652-3660 cm-1) is
:
932
Gil et al.
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
3ENZENE
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Figure 5. 29SiMAS NMR spectra of zeolites X and Y with different Si/AI ratios and IR spectra of Si-OH-AI groups interacting with benzene and chlorobenzene (band fit). The NMR signals from Si(OA1) units which do not generate bridging hydroxyls are marked in black.
present in the spectra of zeolites X (Si/AI < 2.00). Two distinct hydroxyl bands; Si-OIH-Al(363 1-3647 cm-l) and Si-03H-Al (ca. 3550 cm-l), are found for zeolites Y (Si/Al = 2.00-7.02). A third band, at ca. 3600 cm-l, can be seen in the highly dealuminated zeolite Y (Si/Al = 7.02). The spectra shown in Figure 1 have been normalized to the same integrated intensity of the Si-OlH-A1 band (363 1-3660 cm-1). The IR frequency of this band strongly depends on the Si/AI ratio and shifts from 3660 to 3631 cm-’ as the ratio increases from 1.06 to 7.02. Figure 2 shows the spectra recorded before and after sorption of benzene or chlorobenzene on activated samples at room temperature, as well as the difference spectra. The spectra of Si-OlH-A1 groups hydroben bonded to benzene and chlorobenzene (normalized to the same band area) are given in Figures 3A (zeolites X) and 4A (zeolites Y). The spectra were smoothed using the spline method,25and the second derivative was calculated (Figures 3 and 4B-E). A comparison of the second derivative diagrams suggests that the bands of hydrogen-bonded hydroxyl groups are composed of several submaxima. The frequencies of these were estimated from the positions of minima in the second derivative plots: 3440, 3350, 3380, 3330, 3270 cm-l (benzene sorption) and 3470, 3420, 3460, 3290 cm-1 (chlorobenzene sorption). These values were taken as input data for the band fit procedure described by Phita and Jones.26~2~The results are given in Figures 5 and 6, and the submaxima frequencies are listed in Table 1. A comparison of the results in Figure 5 and Table 1 indicates that there are four kinds of Si-OIH-A1 groups of various acid strengths. We shall refer to them as O H ( l ) , OH(2), OH(3), and OH(4) (the numbering is unrelated to the four inequivalent oxygen atoms in the framework). In zeolite X only 2-3 groups with the lowest acid strength can be seen. Three submaxima corresponding to the highest acid strength, OH(2), OH(3), and OH(4), are detected in zeolite Y (the submaximum with the lowest Av overlaps with the Si-03H-Al band at ca. 3550 cm-l). The IR frequencies of the submaxima corresponding to hydroxyls OH( 1)-OH(4) interacting with benzene or chloroben-
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Figure 6. (A, left) Relative populations (normalized to 100%)of OH( l), OH(2), OH(3), and OH(4) groups in IR spectra of NaH-faujasites with different Si/AI ratios. (B, right) Relativeintensities (normalized to 100%) of Si(nAl) signals in 29SiMAS NMR spectra of faujasites with different Si/AI ratios. Only Si( 1AI), Si(2Al), Si(3AI), and Si(4AI) building blocks, which can give rise to bridging hydroxyls, are taken into account.
zene are practically independent of the Si/AI ratio. A small shift (10-20 cm-l) in the case of OH(2) and OH(3) on going from zeolite X to zeolite Y may be due to a partial overlap of these submaxima and Si-03H-AI (at ca. 3550 cm-l). The conclusion that four kinds of Si-OlH-A1 groups with different acid strength exist in zeolites X and Y agrees well with 29SiMAS NMR spectra given in Figure 5. Up to five Si(nA1) signals, where n = 0-4,may be present. As Si(OA1) cannot create bridging hydroxyls, the four signals from Si(lAl), Si(2A1), Si(3A1), and Si(4AI) correspond to Si3Si-OH-AISi3, AlSi2Si-OH-AISi3, Al2SiSi-OH-AISi3, and A13Si-OH-AISi3 groupings, respectively, and those species can be assigned to OH(4)-OH( 1) groups monitored by IRspectroscopy. It is reasonable to assume that the most acidic hydroxyl groups, Le., OH(4) groups which show the highest Av, correspond to the Si3Si-OH-AISi3 cluster in which aluminum is surrounded by the largest number
The Journal of Physical Chemistry, Vol. 98, No. 3, 1994 933
Acidic Hydroxyl Groups in Zeolites TABLE 1: Submaxima Frequencies in IR Spectra of Zeolites Chlorobenzene
X and Y with Different SUA1 Ratios upon Sorption of Benzene and
IR frequencies of submaxima (cm-l) Si/A1 exchange OH(1) ratio degree 3481 1.06 28 3474 1.19 42 3474 1.35 43 347 1 1.67 42 1.87 46 3475 2.00 100 2.39 81 2.56 81 2.15 68 4.15 85 5.03 87 5.89 57 7.02 44 H IR interpretation A13Si-O-AISi3 NMR - .-.__ interpretation Si(4AI)
benzene sorption
3425 3423 3421 3410 3410 3408 3408 3415 3412 3415 H A12SiSi-O-AISij Si(3A1)
Si(2A1)
of Si atoms, and thus to the Si( 1Al) N M R signal. Similarly, the less acidic OH( 1) groups correspond to the Al3Si-OH-AISi3 cluster and thus to the Si(4Al) signal. Our aim was to see whether the relative populations of OH(1)-OH(4) groups as determined by I R spectroscopy depend on the %/A1 ratio in the same way as the relative intensities of Si(nA1) signals in 29SiMAS N M R spectra. Before such scrutiny is undertaken, two assumptions need to be made. First, as the IR spectra were recorded on dehydrated H-forms of the zeolites and the N M R spectra on hydrated Na-forms, we assume that the Si(nA1) species distribution does not change upon dehydration and ion exchange. We believe this assumption to be fully justified, since such treatment is not expected to cause the migration of Si or A1 throughout thezeoliticframeworkortovary thedistribution ofSi(nA1) units. The second assumption concerns the distribution of protons between Si(nA1) units. As we could not use zeolite H-X with a high aluminum content (because of its inherent instability), we assume that the probability of protonation of all Si(nA1) units is the same. Both these assumptions are used in the interpretation of the results from I R and NMR. Results given in Figures 5 and 6 show that the population of OH(1)OH(4) groups (determined by IR) does depend on the %/A1 ratio in the same way as the population of Si(nA1) units. In zeolite X with Si/Al = 1.03 there is only one Si(4Al) signal and only one kind of bridging hydroxyls: Al3Si-OH-AISi3, Le., OH(1). As the Si/Al ratio increases, the N M R signals from Si(3A1), Si(2Al), and Si(lA1) as well as the IR bands from OH(2), OH(3), and OH(4) appear and grow in intensity. In the two highly dealuminated samples (Si/Al = 5.89 and 7.02), the Si(OAl) and Si(lA1) signals and IR bands of OH(4) groups dominate. The relative intensities of Si(nA1) signals in 29SiN M R spectra and of OH(l)-OH(4) submaxima of O H bands are given in Figure 6. Both contributions of Si(nA1) signals and of OH( 1)OH(4) submaxima show thesamedependenceon theSi/Al ratio. Si(4Al) is the only signal and OH( 1) are the only hydroxyls in zeolite X with Si/Al 1. The contribution of both species decreases monotonically with the Si/Al ratio. The contribution of Si(3Al) and as well as that of corresponding OH(2) hydroxyls increases up to Si/Al = 1.8 and then decreases. The contribution of Si(2Al) and of the corresponding OH(3) hydroxyls increases up to Si/AI = 2.5 and then decreases. The contribution from Si(A1) and from the most acidic OH(4) hydroxyl increases monotonically with the Si/Al ratio. The results given in Figure 6 indicate a very good agreement between 29Si MAS N M R information on the environment of Si atoms and I R information on the acid strength of bridging
-
chlorobenzene sorption
OH01 0 ~ 4 ) OH( 1 ) 3343 3347 3439 3357 3436 3360 3441 3372 3319 3369 3318 3368 3317 3368 3314 3366 3302 3305 3367 3364 3301 3383 3312 H H H AISi$3i-O-AISi, Si,Si-O-AlSi3 A13Si-O-AISi3 Si(lA1)
OW) 3410 3393 3385 3373 3373 3372 3371 3372 3373 3372 3373 H AIzSiSi-O-AlSi,
Si(4A1)
Si(3AI)
ow31 -
-
3326 3325 3324 3324 3322 3322 3322 3321 H AlSi&-O-AISi, Si(2AI)
OW41
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3277 3216 3273 3275 3269 3269 3268 3267 H si3Si-O-AlSi~ Si(lA1)
hydroxyls. The contributions of Si(nA1) signals and of OH( 1)OH(4) groups shows the same dependence on the Si/Al ratio. Not only are the shapes of lines given in Figure 6 the same, but the absolute contributions of Si(nA1) and O H species are also very close. Such good agreement indicates that the assignment of the bridging hydroxyls is correct. This means that one may predict the existence or otherwise of various kinds of acidic hydroxyls in zeolites by examining the 29Si MAS N M R spectra. This may have a considerable importance for the study of the chemical properties of zeolitic catalysts. Acknowledgment. The sample of zeolite Na-X was kindly supplied by Dr. H. Karge of the Fritz Haber Institute, Berlin. We are grateful to Dr. M. Kawalek and Mr. T. Abramowicz for performing ion exchange and chemical analysis. References and Notes (1) Jacobs, P. A. Carboniogenic Actiuiry of Zeolites; Elsevier: Amsterdam, 1977. (2) Freude, D.;Hunger, M.;Pfeifer, H. Zeit. Phys. Chem. NF 1987,152, 429. (3) Freude, D.; Klinowski, J.; Hamdan, H. Chem. Phys. Lerr. 1988,149, 355. (4) Freude, D.; Klinowski,J. J. ChemSoc.,Chem. Commun. 1988,141 1 . (5) Datka, J. Zeolites 1991, 11, 739. (6) Datka, J.; Boczar, M.; Rymarowicz, P. J . Catal. 1988, 114, 368. (7) Datka, J.; Boczar, M.; Zeolites 1991, 1 1 , 397. (8) Beran, S. J . Mol. Caral. 1984, 23, 31. (9) Beran, S. Z. Phys. Chem. 1983, 137, 89. (10) Schrader, K.-P.; Sauer, J.; Leslie, M.; Catlow, C. R. A. Zeolites 1992, 12, 20. (1 1) Kazansky,V.B.StructureandReactivityof ModifiedZeolites;Jacobs, P. A., Jaeger, N. I., Jird, P., Kazansky, V. B., Schulz-Ekloff, G., Eds.; Elsevier: Amsterdam, 1984; p 61. (12) Zhidomirov, G. M.; Kazansky, V. B. Adu. Caral. 1986, 34, 131. (13) Datka, J.; Gil, B. J. Caral., in press. (14) Datka, J.; Boczar, M. React. Kin. Catal. Lerr., in press. (1 5) Datka, J.; Boczar, M.; Gil, B. hngmuir, in press. (16) Lippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A.-R. J . Am. Chem. Soc. 1980, 102,4889. (17) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; MHgi, M. Z . Anorg. Allg. Chem. 1981, 482,49. (18) Klinowski, J.; Fyfe, C. A.; Gobbi, G. C. J. Chem. Soc., Faraday Trans. I1985,81, 3003. (19) Klinowski, J. Chem. Rev. 1991, 91, 1459. (20) Hair, M. L.; Hertl, W. J . Phys. Chem. 1970, 74, 91. (21) Cogeshall, N. D. J. Phys. Chem. 1950, 18,978. (22) Datka, J. J. Chem. Soc., Faraday Trans. I1981, 77, 511. (23) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S. J . Chem. Soc., Faraday Trans. II1982, 78, 1025. (24) Klinowski, J.; Fyfe, C. A.; Gobbi, G. C. J . Chem. Soc., Faraday Trans. I1985, 81, 3003.(25) Schulz, H. M. Spline Analysis; Prentice-Hall: Englewood Cliffs, NJ; 1973. (26) Phita, J.; Jones, R. N. Can J . Chem. 1966, 44, 3031. (27) Phita, J.; Jones, R. N. Can J. Chem. 1967, 45, 2347.
