J. Phys. Chem. 1995, 99, 5485-5491
5485
Deuterium NMR Characterization of Brflnsted Acid Sites and Silanol Species in Zeolites J. M. Kobe, T. J. Gluszak,' J. A. Dumesic, and T. W. Root* Department of Chemical Engineering, University of Wisconsin, 141 5 Johnson Drive, Madison, Wisconsin 53706 Received: August 30, 1994; In Final Form: December 6, 1994@
Solid-state deuterium NMR has been used to characterize Bransted acid sites and nonacidic silanol species in D-Y, D-mordenite, and D-ZSM-5 zeolites. Bransted acid deuterons may be static, with quadrupole coupling constants (QCC) near 240 kHz and asymmetries (11) of 0. These acidic deuterons may also be motionally averaged to give QCC = 120 kHz and 11 = 1, consistent with jumping between two oxygen sp3 orbitals. These results indicate that the acid deuteron does not remain fixed in the A1-0-Si plane. Isolated silanol species display a narrow, axially symmetric powder pattem, consistent with an Si-0-D bond angle of 116". Most of the silanol species are more densely packed, and the variety of hydrogen-bonded positions possible results in a Gaussian peak about 56 kHz wide. Deuterium NMR shows promise as a technique for characterizing both acidic and nonacidic hydroxyl groups in zeolites.
Introduction Various techniques have been used to study the acid strength of zeolites, including titration with indicators,'-3 microcalorimetry'-8 and temperature-programmed desorption of bases:-'2 spectroscopic characterization of adsorbed base^,'^-'^ and analysis of acid-catalyzed reaction kinetic^.'^-*^ However, direct investigation of the acid sites usually relies on spectroscopic techniques. For example, infrared spectroscopy is able to distinguish between nonacidic silanol species and several types of acid sites in Y zeolite^.'^^^^^^^ Another technique, 'H MAS or CRAMPS NMR, is able to detect a variety of proton sites in zeolite^.'^^^^-^^ Different peaks are seen for nonacidic silanol groups, several types of acid sites, and residual ammonium ions in the zeolite. These techniques have provided useful information about the acid sites and their changes with sample preparation; however, much of this information has been qualitative, and it has proven difficult to relate the spectral peak positions to site characteristics in a predictive fashion. We explore here a new technique, deuterium NMR, that offers the promise of further quantitative characterization of Bransted acid sites. Recently, wide-line (nonspinning) deuterium NMR has been applied to zeolites as a probe of Bransted acid ~ i t e s . ~In' ,our ~~ previous a broad doublet was observed for acidic deuterons, and a narrower Gaussian peak was seen for surface silanol sites in a Y zeolite. This present paper reports results of a systematic study of the *H quadrupolar NMR spectra of several zeolites and an interpretation of the spectra observed.
Theoretical Background Using deuterons instead of protons for ion exchange in zeolites allows the use of the nuclear quadrupole interaction to obtain information about the 0-D bond. This effect can be large in comparison with the usual chemical shift information in an NMR spectrum. For example, a typical 2H chemical shift corresponds to a frequency range of 250 Hz, while a quadrupole coupling for a deuteron can be on the order of 250 000 Hz. The deuterium nucleus, with nuclear spin I = 1, has a quadrupole moment that interacts with the electric field gradient
' Current address:
General Mills, Inc., Minneapolis, MN 55427.
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, February 15, 1995.
(EFG) at the site of the deuteron. The EFG is described by a diagonalized second-order tensor containing the second derivatives of the electric potential V(i.e., V, = (a2V/ax,ax,). Because the sum of the three diagonal terms V, Vrv,and Vz, must equal zero, the EFG at the site of the deuteron can be described using two parameters: the quadrupolar coupling constant QCC = VzzeQ/h (where eQ is the electric quadrupole moment of deuterium) to characterize the magnitude of the EFG, and the asymmetry 7 = ( ( V , - V,,)/V,J to indicate deviations from axial symmetry. Because the components of the EFG are ordered so that lVzzlL lVJ)l L IV,l, 7 can have values between 0 and 1; when 11 = 0, the deuteron is in an axially symmetric environment. In general, quadrupole spectra can be quite complex; however, in the case of 2H NMR, interpretation of a spectrum and the EFG generating it is fairly simple. The electron density on the deuteron resides in the spherically symmetric 1s orbital and does not contribute to the EFG at the deuteron. In addition, the deuteron is typically bonded to a single atom (e.g., oxygen), and all other charges are significantly farther away. Therefore, at least 80-90% of the EFG at the deuteron results from the atom to which it is b ~ n d e d . In ~ ~practice, . ~ ~ *H NMR spectra show two strong influences, either from changes in the EFG due to changes in the D-X bond or from averaging of the EFG due to motion between several different sites or orientations of the D-X bond axis. This latter effect is often observed with polymers,35 where the different anisotropic motions of the polymer produce characteristic partial averaging of the spectrum seen for a static C-D bond. A difficulty with 2H NMR is that the spectra are broad and must be excited with short, high-power pulses. Furthermore, the large widths of the spectra require many acquisitions to achieve an acceptable signal-to-noise ratio. These problems are complicated because deuterium has a small gyromagnetic ratio and generates a signal that is weak compared to common NMR nuclei. Furthermore, because deuterium is found only at 0.0156% natural abundance, samples to be studied must be enriched in deuterium. However, because deuterium is an I = 1 nucleus, the spectra can be easier to interpret than typical I3C static spectra, as the line shapes are intrinsically symmetric around the observation frequency.
0022-3654/95/2099-5485$09.00/0 0 1995 American Chemical Society
5486 J. Phys. Chem., Vol. 99,No. 15, 1995
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Experimental Methods Na-Y zeolites with SUA1 ratios of 2.4 and 9 were obtained from Dr. W. S . Millman of the University of WisconsinMilwaukee. Nb-ZSM-5 (SUA1 = 34) was provided by Professor R. Gorte of the University of Pennsylvania, and H-ZSMJ (SUA1 = 13) was obtained from Dr. T. Degnan of Mobil Corp. N&-mordenite (SUA1 = 13) was provided by Dr. B. Meyers of Amoco Corp. In this paper, the two Y zeolites will be referenced as Y-2.4 and Y-9, based on the SUA1 ratio; likewise, the two ZSM-5 catalysts will be designated as ZSM5-13 and ZSM-5-34. The iron contents of the Y-2.4, Y-9, mordenite, and ZSM-5-34 catalysts were less than 0.045,0.095, 0.01 1, and 0.15 wt %, respectively (Galbraith Laboratories, Knoxville, TN). To exchange the sodium cations with ammonium ions, the Y zeolites were twice exchanged in a 1 N solution of NH4NO3 and rinsed four times in deionized water. Deuterium labeling was accomplished by stirring the ammonium-exchanged zeolite in a 1 N solution of ND4N03 (Aldrich, 99.9% D) in D20 (Aldrich, 99.9% D) for 2 days, followed by two washings with D20;the catalysts were then heated in vacuum at 723 K for 4 h. For the H-ZSM-5-13, deuterium exchange was accomplished by exposing the zeolite at 373 K to 4.6 Torr of D20 vapor for 5 h; it was then heated in vacuum at 673 K for 4 h. NMR samples were packed in 10 mm sample tubes prior to drying, giving typical sample masses of about 0.40 g. This corresponds to sample sizes ranging from about 2000 pmol of acid sites for the Y-2.4 sample to 190 pmol of sites for the ZSM-5-34 sample. NMR spectra were collected on a Chemagnetics CMC-300A spectrometer operating at 45.97 MHz for deuterium. All spectra were obtained from a wideline probe using the quadrupolar echo sequence, 9O,-z-9O,-z-acquire, with pulse lengths of 2.5 ps and echo delays (z) of 100 ps, except as noted. Spin-lattice relaxation times were measured using saturation recovery, and spin-spin relaxation times were determined from spin echoes obtained over a range of echo delays. Depending on the 2H content in the sample, spectra were accumulated from 500- 1100 scans at typical recycle delays of 50 s, except for the D-ZSM-5-13 spectrum, for which 7800 scans were acquired. Spectra were fit to theoretical powder patterns using a nonlinear, least-squares, tensor-fitting routine. Uncertainties in parameters obtained are f 5 kHz for the broadest features, and proportionally less for narrower peaks.
Results Figures 1-4 show spectra for deuterium exchanged into Y-9, mordenite, ZSM-5-13, and ZSM-5-34. As reported earlier,32 the spectrum for D-Y-9 zeolite (see Figure 1) can be modeled as the combination of a broad pattem with QCC = 236 kHz and asymmetry 7 = 0 that accounts for 55% of the spectral area and a Gaussian shape with full width at half maximum (fwhm) of 62 kHz that accounts for 45% of the area. The first spectral component is assigned to bridging hydroxyl groups. This type of pattern, which has singularities or “peaks” located at frequencies ~ V and Q has shoulders extending to f 2 v Q , is referred to as a Pake doublet (see Figure 6) and has QCC = 8/3VQ. The spectrum of D-mordenite (see Figure 2) reflects a variety of different sites. The broadest contribution to this spectrum has singularities at f 9 0 kHz, similar to the location of the “peaks” of the Pake doublet seen for the D-Y zeolite; however, these singularities are features of a spectrum that is described by an asymmetric EFG with 7 = 1 and a QCC of 125 kHz. This feature accounts for 67% of the spectral intensity. A narrow Pake doublet is also present with maxima at f 2 4 kHz
Frequency (kHz) Figure 1. NMR spectrum (1024 scans) of D-Y-9 zeolite. The composite fit spectrum has contributions from a broad Pake doublet and a Gaussian peak
I
200
100
0
-100
-200
Frequency (kHz) Figure 2. NMR spectrum (872 scans) of D-mordenite zeolite (SUA1 = 13). The composite fit spectrum has contributions from a broad 7 = 1 peak, a n m o w Pake doublet, and a sharp Lorentzian peak.
containing 28% of the spectral area that is fitted to a pattem with QCC = 65 kHz and 7 = 0. Finally, a sharp singularity in the center of the spectrum, in excess of that expected for the powder pattem with 7 = 1, is fitted to a Lorentzian line with fwhm of 4 kHz that contains 5% of the area. The spectrum of D-ZSMJ-13 (see Figure 3) contains several components present in different amounts for other zeolites. The singularities in the pattem at approximately f 9 0 kHz can be fit by a combination of the Pake doublet seen for the Y zeolite, an 7 = 1 pattern similar to that seen for mordenite, and a Gaussian peak. The 7 = 0 peak has a QCC of 220 kHz and contains approximately 21% of the spectral intensity; the 7 = 1 peak has a QCC of 114 kHz and accounts for 70% of the spectral area. The Gaussian peak has a fwhm of 52 kHz and contains the remaining 9% of the spectrum. The spectrum of D-ZSM-5-34 is shown in Figure 4. Both this spectrum and that of D-ZSM-5-13 display similar features.
Deuterium NMR of Zeolites
200
100
J. Phys. Chem., Vol. 99,No. 15, 1995 5487
0
-100
-200
Frequency (kHz)
Frequency (kHz) Figure 3. Symmetrized NMR spectrum (7800 acquisitions) of D-ZSM5-13 zeolite. This spectrum is fit to a broad 7 = 1 peak, a broad Pake doublet, and a Gaussian peak. This spectrum was collected with a pulse length of 3.0 p s and an echo delay of 120 p s .
200
100
0
-100
2
-200
Frequency (kHz) Figure 4. Symmetrized NMR spectrum (1,088 scans) of D-ZSM-534 zeolite. The composite fit spectrum has contributions from a broad Pake doublet, a broad 7 = 1 peak, and a Gaussian peak.
For the D-ZSM-5-34, the r = 0 peak has a QCC of 225 kHz and comprises 19% of the spectrum, while the 7 = 1 peak has a QCC of 113 kHz and contains 66% of the spectral area. The Gaussian peak, while having a similar width to the Gaussian peak of the D-ZSM-5-13 (fwhm = 54 kHz), has a much larger area, accounting for 15% of the spectrum. Because of problems caused by nonideal excitation, both ZSM-5 spectra were asymmetric in form. Therefore, these spectra were symmetrized to facilitate deconvolution, which also increased the uncertainty of the fits of the ZSM-5 spectra compared to those of the Y-9 and mordenite. Because of the symmetrization, some features are present in the spectrum that were not observed in the original spectrum. For example, some broadening is observed around 35 kHz that might be interpreted as the shoulders of a narrow Pake doublet. However, as these features are not apparent from
Figure 5. NMR spectra of D-Y-2.4 zeolite: (a) after drying at 373 K for 1 day (512 scans) and (b) after drying at 373 K for 7 days (1400 scans, symmetrized).
the unsymmetrized spectrum, they must be treated as artifacts of the spectral symmetrization and not as true species. Figure 5 shows spectra of two samples of Y-2.4 and displays the effect of different sample preparations. Spectrum 5a is of a D-Y zeolite with a final drying step of 24 h at 373 K. Visible in the spectrum are the broad Pake doublet and the Gaussian peak, as seen for the more siliceous Y-9 zeolite above. In contrast, the sample dried for 7 days at 373 K (Figure 5b) produces a spectrum with the same broad Pake doublet but with additional detail in the central features. This region shows singularities at f 2 6 kHz in addition to a reduced contribution by the Gaussian peak, indicating that some interconversion of the species responsible for these spectra may have occurred. The TI data were fitted to two models, because these data were not well-described by a single-exponential recovery. The first model was a double exponential of the form M ( t ) = MOh ( l - exp(-t/T,,t)) f (1 - x)(1 - exp(-t/T1,2))], based on the assumption that two deuterium species relax at different rates. The second model was a stretched exponential of the form M(t) = Mo[l - exp(-(t/T~)”~)],which is indicative of a distribution for the values of TI caused by the control of relaxation by a small number of randomly distributed paramagnetic centers in the sample.36 This spin-diffusion process has been seen for protons in Y zeolite with iron impuritie~.~’The T2 data were best fit to a double-exponential model, with the exception of the ZSM-5-34, which fit best to single-exponential relaxation. The relaxation times calculated for all five zeolites are listed in Table 1. It has been shown that deuterons can exhibit orientation-dependent, inhomogeneous relaxation when quadrupolar effects dominate the r e l a ~ a t i o nand , ~ ~the samples examined here did display frequency-dependent nonuniform relaxation. However, precise analysis of the deuteron relaxation is not the focus of this work, and the exponential expressions stated above are sufficient t o fit the overall relaxation of deuterons in the zeolites studied.
Discussion The spectral components from the various species present in these zeolites can be categorized using the prototypical spectra shown in Figure 6. An a peak designates a broad Pake doublet with QCC of approximately 240 kHz and q = 0. Such spectra have intensity between the frequencies f2YQ = +3/4QCC =
5488 J. Phys. Chem., Vol. 99, No. 15, 1995
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TABLE 1: Deuteron Relaxation Times Y-9
Y-2.4
TI (double-exponential)
24% 76%
TI (half-power exponential)
TZ(double-exponential)
87%
13%
0.84s 24s 53 s 79ys 5600ys
20% 80% 65% 35%
0.68 s 13s 26 s 480ys 81OOys
mordenite 41% 59% 88% 12%
0.97 s 12s 6.5 s 730ys 212ms
IVo o,Si/ o
200
(4
100 0 -100 -200 Frequency (kHz)
Figure 6. Prototypical wide-line deuterium NMR spectra. The a peak is a Pake doublet ( q = 0), the a' peak is an 7 = 1 pattem, and the p peak is a motionally reduced Pake doublet. The y peak is a Gaussian line shape, and the 6 pattem is a Lorentzian line.
f 1 8 0 kHz, with maxima at f3/8QCC f 9 0 kHz. The a' peak designates a powder pattern with asymmetry 11 = 1, having singularities at ~ V =Q 90 kHz. The /3 peak is the narrow Pake doublet with a QCC of about 67 kHz (maxima at f3/8QCC G 25 kHz) and 11 = 0. The y peak is a Gaussian component with full width at half-maximum of about 60 kHz. The 6 peak is a sharp Lorentzian with fwhm less than 5 kHz. All of the model and observed 2H NMR spectra are centered close to 0 Hz, so chemical shift differences are negligible, as expected. By considering the motion and EFGs characteristic of models for each species, we suggest that these spectral components may be due to deuterons associated with Br#nsted acid sites (a and a'), surface or defect silanol groups (/? and y), and residual ammonium ions (6). The assignment of the 6 component to ammonium ions is clear as these ions are typically tumbling rapidly, thereby averaging the EFG of the N-D bond to a small value;39the residual line width is indicative of the residual EFG at the cation site in the zeolite. The other site assignments are addressed individually below. Silanol Species. The ,8 peak is attributed to isolated silanol species present at defects or on the surface of zeolite crystals. In terminal silanol groups, it is expected that the 0 - D bond orientation may rotate freely about the Si-0 bond axis, describing a cone in space, as shown in Figure 7a. This motion produces an axially symmetric environment with an effective quadrupole coupling constant QCC' that is reduced from the coupling of a static 0-D bond, QCCo, by the relation
QCC' = QCCo((3cos2 0 - 1)/2))
(1)
where 0 is the angle between the 0 - D bond orientation and the rotation axis. If we approximate the nonacidic silanol O-D bond as being comparable with the 0-D bond in D20 (QCC
ZSM-5- 13
40% 60% 78% 22%
0
ZSM-5-34
1.5 s 27s 16 s 240~s 2100~s
/;\di0
100%
/:, Qi 6
0
I
1.5 s 24s 5.3 s 770ys
58% 42%
Q
/ii, 0 6
0
'
(d)
Figure 7. Possible orientations of deuterons in zeolites: (a) a rotating silanol, (b) an H-bonded silanol cluster, (c) an sp2 Bransted site, and (d) an sp3 Bronsted site with the deuteron jumping between the two oxygen lone pairs.
= 320 kHz40), then the observed magnitude IQCC'l of 67 kHz can be used in eq 1 to determine the value of the Si-0-D angle. Although all known values of static coupling constants are positive,34we do not know the sign of the observed QCC '; therefore, there are two solutions (133 If 2" and 116 f 2") for the Si-0-D angle. Quantum chemical calculations and models for terminal silanol site^^',^^ predict bond angles of 115 f 2", so the NMR-derived value of 116" is reasonable. The y peak is attributed to hydrogen-bonded silanol clusters, either silanol nests at defects, such as silicon vacancies inside the zeolite crystal, or high-density terminal silanol groups on the exterior surface of the crystal (see Figure 7b). This broad Gaussian peak is also seen3*for deuterated hydroxyl groups on the surface of amorphous silica. Hydrogen-bonding between silanol species produces an asymmetry in the 2H spectrum and a decrease in the QCC?3 At high local hydroxyl density, several H-bonded orientations will be available for each group, and there will be additional motional averaging of the observed EFG as the different hydrogen-bonded groups exchange with each other. The net effect of a range of different hydrogen bonding interactions with various neighbors and motional averaging between these positions could produce a Gaussian peak, as seen for the y species on either silica or zeolites. This identification of the p and y peaks as being produced by silanol groups, at low and high local concentration, respectively, is consistent with the two spectra of Y-2.4 zeolite shown in Figure 5. The more extensively dried sample contains fewer H-bonded clusters and more isolated silanol species. This treatment is reflected in the conversion of the y peak to the p peak, as shown in Figure 5b. In addition, these spectral components exchange with protonated or deuterated base molecules more slowly than the broad a or a' components seen in zeolite s p e ~ t r a , ~consistent *.~ with the assignment of the y and p components to the less-acidic silanol species. Ernst et al. also assign the y peak to defect silanol groups, but they assign the p peak to defect AlOD groups.45 However, that assignment
J. Phys. Chem., Vol. 99, No. 15, 1995 5489
Deuterium NMR of Zeolites
TABLE 2: Quadrupole Coupling Constants for Various 0 - D Soeciee suecies occ (kHz) ref D20m" HDOm" alumina hydroxyls"' zeolites rho"' [3] mordenite"' [13] Xmas[ 1.41 Ymas[2.4,2.6] Y"'[2.4, 91 ZSM-5"' [13,34] ZSM-5"as [22]
320 319 280
40 46 55
25 1 250 236 236,224 236 220-229,225 208
31 45 45 45
QCC values obtained from (wl) wide-line 2H NMR, (mas) magicangle spinning 2H NMR, and (mw) microwave spectroscopy. The values in brackets are the silicon-to-aluminum ratios of the zeolites.
cannot explain the conversion, described above, of the y peak to a ,4 peak upon dehydroxylation of Y zeolite and silica. Br~nsted Acid Sites. The a and a' components are attributed to Bronsted acid sites, due to their large EFGs and their facile protoddeuteron transfer with base^.^^,^ The a and a' peaks appear to be related to each other, because the outer singularities of the a' peak match well with the inner singularities of the a peak. This relation suggests that the two species may be related through the motion of the deuteron. If a deuteron is experiencing any form of motion, its quadrupole moment will interact with the averaged EFG that the deuteron observes. Accordingly, the observed QCC (notated as (QCC)) may not have the same value as the QCC of the static deuteron. Because all observed EFGs of static, non-hydrogen-bonded deuterons have asymmetries less than 0.243and because a static 0-D bond would be expected to have an asymmetry near zero, then the a' peak, with an asymmetry near unity, must be caused by deuteron motion. If an r,~= 1 peak is generated by motional averaging of the EFGs of several orientations, then the actual QCC of the static deuteron will be twice the value of the observed (QCC). Thus, doubling the observed (QCC) values obtained for a' peaks allows comparison of 0-D bond QCCs, as indicated in Table 2. The QCC values in Table 2 range from 320 to 208 kHz, decreasing progressively from nonacidic hydroxyl groups in water through weakly acidic hydroxyl species on alumina, to the acidic hydroxyl groups of zeolites. The QCC correlates with acid strength, with the smaller coupling constants indicating weaker 0 - D bonds a.id more readily dissociated deuterons. The different zeolite structures all yield QCCs of 237 f 14 kHz. The values obtained are in good agreement with the QCC of zeolite e measured by Vega and Luz3I using wide-line *H NMR and also correspond well to the QCCs measured by Emst et a145using magic-angle spinning (MAS) *HNMR for X and Y zeolites. There is a discrepancy for Z S M J between the value of the QCC reported by Emst (208 kHz) and our value (225 kHz), although their MAS spectra and our wide-line spectra do appear to be comparable. The net EFG at a site is the sum of the interactions with the charges of all neighboring nuclei as well as with the electrons in the environment. For deuterons, the nuclear contribution is larger than the electronic contribution, since the electron density of the oxygen atom extends past the deuteron position and partially cancels itself. Therefore, the resulting coupling constants are positive. Since these two terms are nearly equal, typically differing by 10-20%, small changes in either term will produce a magnified change in the difference. A decrease in QCC can be interpreted as a decrease in the contribution of
the oxygen nucleus andor an increase in the electronic contribution of the oxygen. The decrease in QCC from 320 kHz to 240 kJ3z from nonacidic water hydroxyls to acidic zeolite hydroxyls may be correlated with changes in the 0 - D bond. For example, if the electron density near the oxygen atom is the same as that in water and the cause of change in the QCC is an increase in 0-D bond length, then this change would indicate an increase from 0.95746to 1.04 A. This estimated 0 - D bond length is substantially larger than those obtained computationally (0.9600.973 A)41,42,47,48 and would necessarily produce a significant charge redistribution. Altematively, if the bond length remains near 0.957 A, then the weakening of the bond may be accompanied by transfer of electron density from the deuteron to the oxygen as the deuteron becomes more acidic. Electrons in the deuteron 1s orbital produce no EFG at the deuteron, so this transfer of electron density away from the deuteron would increase the electronic contribution to the EFG and decrease the QCC. Simplistically, this change in QCC could be viewed as indicating a shift of about 0.2 electrons from the deuterium atom to the oxygen atom. The resulting 0-D bond would be more polar, and the acid deuteron would be more readily polarized. Acidic Deuteron Motion. The shapes of the a and a' peaks attributed to Bransted acid sites must be reconciled with the geometry of the site. The conventional picture of the bridging AI-OD-Si species is that the four atoms are coplanar (Figure 7c) in a static or rigid position. The Pake doublet of the a peak indicates a static axially symmetric environment, consistent with this geometry or any other static 0 - D bond orientation. The a' peak, however, cannot be produced by a single, static 0-D bond. Rather, in polymer application^,^^ this type of pattem is seen to be produced by a variety of exchange or motional averaging models. The simplest model that produces this spectrum with r] = 1 is two-site exchange between sites of equal populations at an orientation angle of 109.5". This angle is the tetrahedral angle of an sp3 atom. Indeed, oxygen usually has sp3 hybridization, and this model would produce the observed NMR spectrum if the deuteron were hopping between two sp3 lobes. Such a geometry and the motion described are shown in Figure 7d. This geometry can produce both the a and a' peaks at different jump rates. At jumping rates that are slow compared to the QCC (Le., v,,,dQCC < 10-3),49 the spectrum would portray a static deuteron and produce an a peak. When jumping is rapid (Le., v,,,dQCC > lo3), the motionally averaged a' peak would result. The failure of the conventional picture of a Brdnsted acid site as a planar species with an sp2-hybridizedoxygen to produce the observed 2H NMR spectra is cause for examination of the supporting evidence for this picture. The planar A1-OH-Si species was suggested by Olson and Dempsey as a possible geometry for Bransted acid sites. This geometry is mentioned in X-ray diffraction50 and neutron diffraction studies5' of faujasite. However, exchange between the two sp3 sites could result in an apparently good fit of the diffraction data to an average position. Furthermore, proton sites are difficult to determine via diffraction techniques in zeolites with higher Si/ A1 ratios because of the correspondingly lower number of sites containing protons. Consideration of different tricoordinated oxygen specie$ indicates that sp2 hybridization is seen in complexes with metals where there is significant d-p n bonding, as might also be expected between the oxygen lone pair and the empty Si 3d levels. While this interaction has been used to explain the large Si-0-Si bond angles, the observed angles near 140°50are actually larger than would be expected
5490 J. Phys. Chem., Vol. 99, No. 15, 1995 for either sp2oxygen (120") or sp3 oxygen (log"), and distortions due to the zeolite lattice structure must be present regardless of the oxygen hybridization. In summary, while many factors are consistent with an sp2 planar structure, none of them offer independent confirmation of this structure. The major disagreement identified in this paper with the standard geometry is that it cannot produce the a' peak seen here with 2H NMR. It should be noted that the a' peak can be produced from a variety of motional models. From the indicated asymmetry (7 = 1) and the retention of the spectral singularities at the same frequencies as the singularities in the a peak, it can be shown that any applicable model must include only 0-D orientations that lie in a single plane. This restriction allows us to eliminate models involving jumps between large numbers of oxygen atoms in the zeolite lattice. The model proposed here, with the deuteron jumping between lobes on an sp3 oxygen, has the virtue of being simple, consistent with the data, and independent of the actual lattice structure of the zeolite. Thus, the model is applicable to both Z S M J and mordenite without requiring longrange orientational similarities between bridging hydroxyl sites in these two different structures. Considerations for an sp3 trigonal pyramid structure provide strong support for this arrangement. First, the sp2 geometry is seen only for strongly n-bonded systems, and oxygens with only u bonds tend to form sp3hybrids instead. This trigonal structure is observed for hydronium (H@+) and oxonium (R@) ions,52,53the oxygen analogs of amines. This sp3 structure allows the simple jump motion described above, which is analogous to the well-known umbrella inversion for ammonia and amines. Recent density functional theory computations on the structure of acidic sites in ZSM-5 indicate that the bridging hydroxyl species is not planar.54 The geometry suggested places the acid proton 10" out of the plane; while this value is not consistent with the value we predict (Le., 55"), the density functional studies represent independent identification of the inadequacy of the planar Bransted geometry using an approach quite different from the 2H NMR experiments described here. One test of this motional model is to examine whether the jump energetics are reasonable for the proposed structure. Motional averaging typically produces the static spectrum for jump rates small compared to the strength of the interaction being averaged, and a fully averaged spectrum for jump rates that are fast. Here, the quadrupolar interaction is on the order of 240 kHz. We will assume that jump rates have an Arrhenius form, k = YO exp(-E,/RT), with a preexponential factor YO = loi3s-l. The Y zeolite spectrum contains only the a peak and thus has a jump rate slower than 2.4 x lo2 SKI,corresponding to an activation energy greater than 14.3 kcaymole. For mordenite, with only the a' peak, the jump rate is faster than 2.4 x lo8 s-l, giving an activation energy less than 6.2 kcal/ mol. If one views this motion as an umbrella inversion mode, then there is good agreement with reported activation energies seen for amine inversion5*of 4-10 kcal/mol. Indeed, the sp2 planar structure commonly advocated for the Bronsted acid site is seen to correspond to the transition state between the two equilibrium positions of a deuteron on the sp3 oxygen. This inversion through a planar transition state with a lone pair in a p orbital provides a lower-energy pathway for umbrella inversion than detaching one substituent and moving it to the sp3 lone pair. Other jump models may also be consistent with the asymptotic a and a' spectra seen here at low and high jump rates and with the energetics estimated above. These various models will also have different predictions for the instantaneous hydrogen location in the zeolite structure and for its availability to reactant
Kobe et al. molecules in the zeolite cavities and channels. Typically, different models with the same limiting behavior differ in the spectra produced in the intermediate-exchange range. Work is currently underway using temperature to control jump frequency and to obtain spectra at intermediate jump rates so that the validity of the jump model proposed here and the activation energies of the deuteron motion can be determined.
Summary Deuterium NMR spectra reveal different hydroxyl sites in zeolites. Terminal hydroxyl groups at surfaces or internal defects yield narrow spectra, as seen in either /3 or y peaks depending on whether they are at low density and rotating freely or at high density and hydrogen bonding with each other, respectively. These spectral components offer the opportunity to characterize zeolite framework defects and monitor nonacidic deuterons in a zeolite. Two broader peaks (a and a') are identified with the Bransted acid sites. The decrease in QCC in zeolites by 25% from the value observed for D20 indicates the weakening of the 0 - D bond strength and the increased acidity of these sites. In addition, the range of spectra observed exhibiting these quadrupole couplings can be explained by a model involving motional averaging. Accordingly, Bransted acid deuterons have mobility over a limited range of orientations, consistent with a simple model in which the bridging hydroxyl groups are pyramidal rather than planar and the oxygen is more near sp3 than sp2hybridization. This break from the commonly accepted view of Bransted sites as planar bridging hydroxyls may have significant consequences for our understanding of acid site accessibility and reactivity.
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