Molecular Motion of Hydrogen-Bonded CH3CN in H-MFI: A 1H, 2H

zeolite H-MFI have been explored using 1H, 2H, and 13C nuclear magnetic resonance (NMR) ... narrowing of NMR proton dipolar line shapes19 and from 13C...
0 downloads 0 Views 108KB Size
Download PDF
5124

J. Phys. Chem. B 2000, 104, 5124-5131

Molecular Motion of Hydrogen-Bonded CH3CN in H-MFI: A 1H, 2H, and Nuclear Magnetic Resonance Study

13C

Multinuclear

B. H. Suits,† J. Sˇ epa,‡ R. J. Gorte,‡ and David White*,§ Department of Physics, Michigan Technological UniVersity, Houghton, Michigan 49931, Department of Chemical Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: January 14, 2000; In Final Form: March 2, 2000

The dynamics associated with the adsorption complex formed by CH3CN at Brønsted sites in the high-silica zeolite H-MFI have been explored using 1H, 2H, and 13C nuclear magnetic resonance (NMR) spectra measured as a function of temperature between 78 and 400 K. A simple NMR line-shape theory, based on rapid, smallangle reorientations of the CH3CN molecular axis with a temperature-dependent amplitude, has been used to account for the data. An anisotropic, angular motion is observed with a small amplitude at low temperatures increasing to approximately (35° from its average position at room temperature. This motion is primarily constrained to a plane in the zeolite, but a distribution in amplitudes for different types of sites is required to fully account for the data. At higher temperatures, the powder line shapes are completely motionally narrowed, presumably due to exchange between physisorbed molecules and those bound to the sites.

Introduction Among the most industrially important applications of zeolites is their use as acid catalysts. Zeolites are active for acidcatalyzed reactions because of the Brønsted-acid sites, which are generated as a result of charge-balancing protons associated with framework aluminum (Al) substitution in the structure.1 For high-silica zeolites, like H-MFI, the Brønsted-acid site concentration is approximately equal to the framework Al concentration. Furthermore, well-defined adsorption complexes, in a concentration of one per site, have been identified for various alcohols,2-4 amines,5 nitriles,6 pyridines,7 thiols,8 ketones,9,10 CO,11 diethyl ether,6 and metal halides.12,13 Differences in rates and selectivities for a given reaction in various structures are often attributed to acidity differences; however, factors other than the intrinsic proton affinity of a given zeolite structure will contribute to its catalytic properties. The primary process for stabilization of an adsorption complex involves formation of a hydrogen bond, as in the case of acetone or acetonitrile,14,15 or proton transfer, as in the case of amines and pyridine. However, additional interactions of the reactant molecule with the framework-cavity environment, sometimes referred to as “matrix effects”,16 can also assist activation of an adsorbed molecule. These matrix effects include not only the local bonding of the reactant molecule with the Brønsted proton but additional effects arising from host-guest electrostatic and van der Waals interactions and from the long-range influence of the extended solid (embedding effects).17 A detailed understanding of the mechanisms for chemical reaction requires knowledge about the dynamics of adsorbed molecules in the environment of the zeolite cavity. Despite a wealth of experimental data on the local bonding and structure for the static adsorption complex, there is relatively little information on these dynamics. Because of the time scales for * Corresponding author. † Department of Physics. ‡ Department of Chemical Engineering. § Department of Chemistry.

dynamical processes in the gas and condensed phases, they are usually studied by optical techniques that are difficult to apply to zeolites. However, although the time scale for these events is much shorter than that probed by nuclear magnetic resonance (NMR) spectroscopy, it is possible to obtain dynamically averaged information from NMR measurements that, with the aid of appropriate theoretical simulations, can be used to understand the molecule-framework interactions. For example, the molecular motion of organic molecules in various zeolites has been probed using 2H NMR line shapes, as described in recent reviews by van Santen et al.16 and Shantz and Lobo.18 We have also shown that similar information on the spatially isolated adsorption complex can be obtained from motional narrowing of NMR proton dipolar line shapes19 and from 13C chemical-shielding tensors.20-22 In this paper, we examine the molecular motion of the stoichiometric adsorption complex formed by acetonitrile at the Brønsted sites in H-MFI from 78 to 400 K. Acetonitrile was chosen because it provides an interesting model of more reactive molecules, such as simple aldehydes, ketones, and alcohols. Each of these molecules forms hydrogen bonds with the Brønsted sites that are similar in nature and strength.6,23 Therefore, the dynamical processes that occur in the complexes that lead to acid-catalyzed, aldol condensation of acetone,24 the unimolecular rearrangement of allyl alcohol,25 and the dehydration of 2-methyl-2-propanol26 may well be similar to those observed with acetonitrile. Unlike the other molecules, acetonitrile does not react with other products and remains hydrogen bonded to the Brønsted sites to well above 400 K.22 Between 78 and 400 K, at loadings of less than one molecule per acid site, acetonitrile is localized at the Brønsted site in the time scale of the NMR proton-dipolar, deuterium-quadrupolar, and 13C-chemical-shielding anisotropy interactions.22 Because nearly free rotation of the methyl group about the C3 symmetry axis occurs at all temperatures,19 the choice of this asymmetrictop molecule limits the dynamical processes that affect the NMR signal, in the absence of site exchange, to the libration of the molecular axis in the potential that confines and isolates the

10.1021/jp000216n CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000

Molecular Motion of Hydrogen-Bonded CH3CN molecule. The molecular motion with increasing temperature therefore results from thermal excitation of low-frequency, molecule-framework reorientational modes in a “breathing” framework. From earlier 1H NMR studies, we concluded that motional narrowing of the anisotropic dipolar tensor in the vicinity of room temperature could be accounted for by site exchange involving very short correlation times.19 However, a hopping model cannot account for the observed line shape of the powder patterns for the 13C-chemical-shielding anisotropy.22 In this paper, we report NMR line shapes for the 1:1 adsorption complex of acetonitrile using 1H NMR of CH3CN in D-MFI, 2H NMR of CD3CN in H-MFI, and 13C NMR of CH313CN in H-MFI, along with a theory to account for the motional narrowing of the proton-dipolar, the deuteriumquadrupolar, and the 13C chemical-shielding tensors. To compute NMR line shapes, simple analytical models are employed describing the statistics of the molecular motion associated with thermal excitations of the adsorption complex. The line-shape calculations could also be performed using results from “firstprinciple” molecular dynamics calculations,27,28 such as those recently carried out for the methanol adsorption complex in a number of zeolites including H-MFI.29 However, the computational expense of calculations necessary to define the statistics for the large unit cell of MFI (288 atoms) is presently prohibitive. In a future study, we will present results for the acetonitrile complex in CHA (36 atoms per unit cell) that permit us to compare the analytical models with results from firstprinciple simulations. Experimental Section The MFI used in these studies was obtained from Chemie Uetikon AG, Zeocat-Pentasil-PZ2/54N. X-ray diffraction of the MFI showed that it was highly crystalline, and atomic adsorption spectroscopy gave a bulk concentration of 630 µmol/g of Al (Si/Al2 of 53). The pore volume, measured by n-hexane uptake at room temperature and 10 Torr, was 0.174 cm3/g, which can be compared with the ideal pore volume of 0.19 cm3/g for the MFI structure. The sample was placed in the hydrogen form by repeated ammonium ion exchange, followed by calcinations. After this treatment, the Brønsted-acid site concentration was determined to be 500 µmol/g from the amount of isopropylamine that decomposed to propene and ammonia between 575 and 650 K in simultaneous temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) measurements.30 To obtain the deuterated form of the sample, H-MFI was repeatedly exposed to D2O vapor and evacuated, after which the sample was gradually heated to 750 K in vacuo until the pressure fell below 10-6 Torr. Using NMR as the analytical tool, it was found that the proton background of the deuterated samples was always 2T1 (T1 is the spin-lattice relaxation time) values at all temperatures to avoid significant saturation effects. Because the quantity of adsorbed molecules in most samples was ∼35 µmol, the number of scans required to obtain a reasonable signal-tonoise was large. For the 1H NMR spectra, several hundred scans were required at 78 K and several thousand in the vicinity of room temperature. For the 2H NMR spectra, the number of scans that were required was a factor of 10 higher, whereas 13C NMR spectra required an intermediate number of scans. Experimental Results Three different samples were used for studying the temperature dependence of the powder line shapes of the 1:1 acetonitrile adsorption complex. For the 1H dipolar studies, natural abundance CH3CN was adsorbed on a fully deuterated sample, D-MFI. For both the 13C chemical-shielding and 2H-quadrupolar studies, the protonated form of the zeolite, H-MFI, was used together with the isotopically enriched adsorbates CH313CN for the 13C studies and CD3CN for the 2H studies. In Figure 1, the dipolar-enhanced (20% of the central line) 1H NMR powder patterns are shown at 293 K for various loadings. When the concentration of CH3CN is less than one molecule per Brønsted site, the 1H line shapes are independent of loading, as shown in Figures 1a-c. This insensitivity to loading below a 1:1 coverage at room temperature has also been observed in 13C studies of the chemical-shielding anisotropies of adsorbed CH313CN and CH313COCH3 in H-MFI.9,20 However, for loadings in excess of one per Brønsted site, most of the dipolar part of the line shape collapses into the center line due to rapid exchange between chemisorbed and physisorbed

5126 J. Phys. Chem. B, Vol. 104, No. 21, 2000

Suits et al.

Figure 1. Methyl proton spectra of CH3CN at 295 K as a function of loading in D-MFI: (a) 0.48 molecules per Brønsted site; (b) 0.70 molecules per Brønsted site; (c) 0.90 molecules per Brønsted site; (d) 1.38 molecules per Brønsted site.

species, as shown in Figure 1d. This change in the line shape was even more pronounced in the 13C-shielding anisotropy, where a very small excess of adsorbate beyond the available acid sites leads to extreme motional narrowing that is characteristic of site exchange.9,20 Although less dramatic, similar observations have been reported in 2H-quadrupolar line shapes as a function of loading for other adsorbates in other acidic zeolites.18,37-39 The 1H-dipolar, 13C-chemical-shielding, and 2H-quadrupolar powder patterns for an acetonitrile concentration of 0.7 ( 0.02 per Brønsted site, at a magnetic field of 3.5 T, are shown in Figures 2 through 5 for temperatures ranging between 78 and 400 K. The line shapes did not change when measured at a field of 8.4 T at room temperature and lower. The figures illustrate the changes in the powder patterns due to thermal excitation of the molecular reorientations for three nuclei of the hydrogen-bonded acetonitrile. Figure 2 shows the entire 1H NMR line shapes for CH3CN in D-MFI, whereas the spectra in Figure 3 contain only 20% of the central line to display more clearly the temperature-sensitive, dipolar powder patterns. At 78 K, the dipolar splitting, measured from the separation of the sharp features in the spectrum, is 29.9 kHz. This value is only 0.03 kHz less than the “rigid-lattice” value for fixed-axis CH3CN, in which the methyl protons rotate or tunnel freely about the C3 symmetry axis perpendicular to the plane of rotation of the protons.19,36 With increasing temperature between 78 and 300 K, this splitting decreases monotonically while the sharp discontinuities of the powder pattern broaden due to motional narrowing. The line-shape modeling in the next section will show that the line-shape changes can be attributed to librations of the molecular axis of the complexed acetonitrile molecule. However, at higher temperatures, the disappearance of the dipolar pattern and the narrowing of the central line suggest that the character of the motion has changed. In fact, the line

Figure 2. Methyl proton spectra of 1:1 acetonitrile stoichiometric adsorption in D-MFI as a function of temperature (0.7 molecules per Brønsted site).

shape becomes essentially that of a Lorentzian at 400 K. This temperature is close to the peak desorption temperature observed for TPD of acetonitrile in a vacuum;6 therefore, physisorbed species must be present at equilibrium for rapid site exchange with complexed molecules at the acid sites. The changes in the quadrupolar powder patterns for CD3CN in H-MFI with temperature are shown in Figure 4. At 78 K, the sharp features of the spectrum correspond to that of an axially symmetric powder pattern with an average quadrupolarcoupling constant of 39 kHz. This result, like the methyl-proton pattern already discussed, is typical of a CD3 group reorienting rapidly about its fixed, C3-symmetry axis, with a splitting nearly equal to that of the “rigid lattice”. For the neat solid at 78 K, where the barriers to molecular reorientation are considerably higher, we measured a quadrupolar splitting of 40 kHz. In Figure 6, the changes in the proton-dipolar and deuterium-quadrupolar splittings with temperature are compared assuming that one observes a splitting corresponding to 99% of that exhibited by the “rigid lattice” for both cases at 78 K. Within experimental error, the trends appear to be the same for both nuclei.

Molecular Motion of Hydrogen-Bonded CH3CN

J. Phys. Chem. B, Vol. 104, No. 21, 2000 5127

Figure 3. The data of Figure 2 scaled to more clearly show the dipolar broadened portion of the spectrum.

Furthermore, the 1H and 2H powder patterns suggest that there is a different character to the molecular motion above 300 K compared with that at the lower temperatures. At 340 K, a center line begins to appear; and, as with CH3CN, the powder pattern reduces to that of a Lorentzian with full width at half maximum (fwhm) of ∼3.5 kHz at 385 K. The proton-decoupled, axially symmetric, 13C NMR spectra are shown with increasing temperature in Figure 5. At 78 K, a “rigid-lattice” powder pattern is observed due to the chemicalshielding tensor and includes features due to the 13C-14N dipole interactions. The spectral features due to this tensor permit the determination of the orientation of the principal components of the 13C-chemical shielding relative to the 14N electric-field gradients and has already been discussed in detail.20 Between 78 K and room temperature, the changes in the shape of the powder patterns are minimal, except for a broadening and decrease of the anisotropy that can be attributed to reorientations of the molecular axis of the complexed acetonitrile. The disappearance of the structure due to the 13C-14N dipole coupling, most evident at 295 K (Figure 5), is likely due to the fast T1 relaxation of 14N at the higher temperatures. Above 300 K, there is again considerable motional narrowing evidenced

Figure 4. Methyl deuterium quadrupole powder patterns of the 1:1 CD3CN stoichiometric adsorption complex in H-MFI as a function of temperature (0.7 molecules per Brønsted site).

by the fact that the line shape is unaffected by proton decoupling, becoming a narrow Lorentzian at 400 K (fwhm of ∼0.8 kHz) centered about the isotropic chemical shift.22 Line Shape Modeling and Discussion Because the bending and stretching frequencies of the acetonitrile molecule are large compared with kBT at room temperature and below, we can ignore these motions and treat the molecule as a linear, rigid, molecular axis, defined by the C-C-N bond of the adsorption complex. The only degrees of freedom that determine the orientation of the adsorbed molecule relative to the zeolite crystallographic axes (the fixed laboratory frame) are the rapid rotations of the methyl group about the molecular axis36 and the libration of the molecular axis in the framework. We will further assume the frequencies of these motions are much greater than the spin interactions considered here and that they are uncoupled from one another. Hence, the

5128 J. Phys. Chem. B, Vol. 104, No. 21, 2000

Suits et al. We consider changes in the 1H NMR spectra of CH3CN, the NMR spectra of CH313CN, and the 2H spectra of CD3CN due to molecular motion of the adsorbed molecule within the zeolite. Within the rigid-molecule approximation, the underlying molecular motion must be the same for all three cases. The NMR shifts for 1H are principally due to the H-H nuclear dipolar interactions. As long as the three 1H nuclei remain equivalent, they give rise to a pseudo-quadrupolar spectrum, which is axial along the molecular axis arising from total nuclear spin 3/2, combined with single-line spectra from the total spin 1/2 cases.36 In the case of 13C NMR spectra, the shifts are due to the combination of an axial chemical shift (ν⊥ ) 104 ppm relative to the isotropic shift of liquid TMS at 295 K, or 4 kHz at 3.5 T) and the 13C-14N nuclear-dipole interaction (0 and ( 1.4 kHz). Both of those interactions are axial along the molecular axis.41 The shifts in the 2H NMR spectra are predominately due to the time-average, electric-quadrupole interaction in the presence of a rapidly rotating CD3 group. Due to the rapid rotation, the electric-quadrupole interaction can be described by an average Hamiltonian that is axial along the molecular axis.42 Initially, we considered a hopping model where the molecule was assumed to undergo a large change in orientation with a correlation time, τc. Reasonably good agreement could be obtained for the spectral line shapes using such a model; however, spectra for the three different nuclei could not be simultaneously modeled, even approximately, using the same value of τc at any given temperature. Furthermore, additional measurements for 13C and 2H were performed at room temperature as a function of applied field (up to 8.4 T) and no change in line shape was observed, demonstrating that we are in the slow- or fast-motion limit. Therefore, we have discarded the large-angle hopping model as an explanation for our data. Instead, we examined smaller-angle motions, assuming that the time scale for this motion is rapid compared with the inverse of the NMR spectral line width, but that the amplitude of the motion can change with temperature. This model appropriately describes vibrational and/or librational motion of the molecule with a frequency typical of lattice vibrations (∼1012 Hz). In what follows, we assume the rotational motion of the methyl group is uncorrelated with this additional angular motion. In the reference frame of a stationary molecule, where the z-axis is along the molecular axis, the spectra for all three nuclei in the absence of motion can be written as the result of a sum over all orientations of the magnetic field of the signal, S, which is (or would be) observed for each orientation, 13C

Figure 5. Nitrile 13C-chemical-shielding powder patterns of the 1:1 13CH CN stoichiometric adsorption complex in H-MFI as a function 3 of temperature (0.7 molecules per Brønsted site; ppm scale is relative to liquid TMS at 295 K).

n

S(θ,φ) )

∑δ(ω - ωo - ωk) k)1

(1)

ωk ) ωko(1 - 3cos2 θ)

Figure 6. Quadrupolar (O) and dipolar (X) splittings of methyl deuterium and proton powder patterns, respectively, as a function of temperature.

spin interactions can then be treated using an average Hamiltonian.40

Here, δ(ω) is the Dirac delta function, θ and φ are the usual polar coordinates describing the orientation of the applied magnetic field within the molecular frame, and ωo is the center frequency, which includes isotropic shifts. We are not concerned with isotropic shifts in this model. The integer, n, is the number of discrete NMR lines present for a given orientation of the magnetic field. Values of n and ωko for CH3CN and the three different nuclei discussed here are shown in Table 1. To calculate the line shapes in the presence of rapid molecular reorientation, we take the z-axis to be along the time-averaged orientation of the molecular axis. We use a classical model where the instantaneous frequency is given by ωko (1 - 3 cos2θ′), where θ′ is the angle between the magnetic field and the instantaneous orientation of the molecular axis. The motion

Molecular Motion of Hydrogen-Bonded CH3CN

J. Phys. Chem. B, Vol. 104, No. 21, 2000 5129

TABLE 1: Parameters Used in Line shape Simulations (eqs 1-4) nucleus 1H 2H 13

C

n

ωko

5 2 3

( 15 kHz, 0,0,0 ( 20 kHz 4.0, 4.0 ( 1.4 kHz

is assumed to always be very rapid compared with ωk, so the observed frequency, ω j k, will then be the time average of the instantaneous frequencies. The instantaneous orientation of the molecule is described using polar angles (R, ξ) corresponding to a rotation about the y-axis by an angle R, followed by a rotation about the z-axis by an angle ξ. Then, p(R, ξ) is the probability (per unit solid angle) that the molecule will be found at (R, ξ). Recognizing that θ′ is now a function of (R, ξ), the average frequency shift is then the average over all angles of ωko(1 - 3 cos2θ′)p(R, ξ). As a starting point, we consider two limiting cases. In the first case, we assume that, although there may be a distribution associated with the tip angle R, all possible values of ξ occur with equal probability. Because the average is performed over the surface of a sphere, we refer to this as the 2-d model. For the second case, there is a distribution in R (with an average of zero), but ξ is fixed. Because we will ultimately perform an average over all orientations of the magnetic field, the particular value chosen for ξ is arbitrary. Hence, one can view this latter case as motion where the molecular axis is restricted to, for example, the x-z plane. Because the average here is over the circumference of a circle, we refer to this as the 1-d model. Further computation requires knowledge of p(R, ξ). To see what happens, we take as a starting point a very simple square distribution where the probability is zero except when -Ro < R < Ro. For the 2-d model, all solid angles in this range occur with equal probabilities. For the 1-d model, all possible values of R in this range occur with equal probability. It is then straightforward to obtain

ω jk )

ωko [cos Ro(1 + cos Ro)](1 - 3 cos2 θ) 2

Figure 7. Simulation of methyl proton spectra of acetonitrile adsorption complex as a function of temperature using the 1-d model.

(2)

for the 2-d case, and

3 ω j k ) ωko 1 - (1 + f(Ro))cos2 θ 2

[

]

(1 - f(Ro)) sin2 θ cos2 φ) (3) for the 1-d case, where f(Ro) ) sin(2Ro)/2Ro. In both cases, the line shape is then computed numerically by summing the contributions to the line shape for all orientations (θ, φ). Ultimately the line shape depends on the maximum amplitude of the motions, Ro. The expression for the 2-d case (eq 2) clearly shows that only the overall amplitude of the shifts will change with changes in Ro, and the resulting shape of the spectrum (aside from a scale factor on the frequency axis and the effects of other broadening) will not change. This prediction is not what is observed experimentally. Calculated line shapes for several values of Ro (given in radians) based on this simplified 1-d model (eq 3) are shown in Figures 7-9. Line broadening has been added to smooth the calculated results. Despite the approximations made, this simplified 1-d model using a single adjustable parameter qualitatively reproduces the line shapes for all of the observed spectra at room temperature and below, both with respect to the splittings and broadening. The 1-d model corresponds to a highly anisotropic motion. At this point, we note that 2H NMR spectra very similar to those shown here have been obtained theoretically and experi-

Figure 8. Simulation of methyl deuterium quadrupolar powder patterns of acetonitrile adsorption complex as a function of temperature using the 1-d model.

mentally using a large-angle, hopping model.42,43 Clearly, additional information beyond a comparison of line-shape calculations and data needs to be considered in each case to determine whether one is dealing with large-angle hopping or smaller-amplitude, rapid motion. In the present case, the need to simultaneously account for the changes that occur in the powder patterns of three different nuclei eliminate a hopping model. If a Gaussian distribution with a corresponding root-meansquare (rms) amplitude Ro, for cases where Ro , π, is used in place of the square distribution of the 1-d model, the same

5130 J. Phys. Chem. B, Vol. 104, No. 21, 2000

Suits et al.

Figure 9. Simulation of 13C-chemical-shielding powder patterns of the acetonitrile adsorption complex as a function of temperature using the 1-d model.

expression is obtained for ω j k, except that f(Ro) ) exp(-2(Ro2)). Therefore, a Gaussian distribution produces the same family of line shapes as does the square distribution. For Ro < 1, the line shape obtained using the square distribution with a maximum amplitude Ro is the same as the line shape obtained using the Gaussian distribution with an rms amplitude of Ro/x3. In what follows, values of R0 quoted are for the square distribution. We have also examined a less restrictive case between the 1-d and 2-d models. Again, taking the average molecular orientation to be along the z-axis, the instantaneous orientation of the molecule is described by starting along the z-axis, rotating by an angle R about the y-axis as before, and then rotating about the x-axis by an angle β. Using the square distribution where all combinations of R and β are in the range -Ro < R < Ro and -βo < β < βo and occur with equal probability, one obtains

( [

ω jk ) ωko

cos2 θ (1 + f(Ro) + f(βo) + f(Ro)f(βo)) 3 1 - + sin2 θ sin2 φ(1 + f(Ro) - f(βo) - f(Ro)f(βo)) 4 + 2 sin2 θ cos2 φ(1 - f(Ro))

])

(4) where f is defined in eq 3. In the limit βo ) 0, this situation is identical to the 1-d model; when Ro) βo, the resulting line shapes are similar to those of the 2-d model already described. As indicated earlier, a reasonably good fit could be made for all the nuclei at the lower temperatures using the 1-d model and a single value of R0 at each temperature (see Figures 7-9). This result leads to the conclusion that, near room temperature, R0 is ∼0.6-0.7 rad. However, at room temperature and above, the quality of the fits deteriorates. In particular, the almost flat spectrum observed for 2H at 333 K could not be simulated with any of the models. Recognizing that there are 12 inequivalent tetrahedral sites in MFI, giving rise to even more potential hydroxyl sites, we considered the possibility that there is a distribution of R0 values large enough to affect the line shapes. It was found that the spectral shapes above room temperature

Figure 10. Simulation of methyl deutrium quadrupolar powder patterns of acetonitrile adsorption complex as a function of temperature using the intermediate model (eq 4) plus a distribution of amplitudes, R0. A Gaussian distribution of amplitudes with mean R0 and with an rms deviation of R0/3 and with β0 fixed at 0.3 rad was used for the simulations shown.

could be reproduced very accurately if a distribution of R0 values were assumed. However, because introducing a distribution of R0 values also affects the spectral splittings, it was also necessary to introduce some 2-d character to the motion using eq 4. We did not attempt to fit all the spectra completely using all the independent parameters. Instead, a search was performed to find a parametrization that allowed one variable to describe the temperature variations in the spectra. The following spectral features were particularly useful to consider: the temperature variation of the peak-to-peak distance for the 2H spectra and for the dipolar portion of the 1H spectra (see Figure 6), the fwhm height and the relative size of the dip in the center of the 2H spectra, and the general shape of the wings on the 2H spectra. For the models described here it was found that the peak-topeak distance for the 2H spectrum is largely determined by R0; the fwhm for the 2H spectra is mostly determined by β0; and the size of the dip and shape of the wings for the 2H spectra for the larger values of R0 are mostly determined by the width of the distribution. It was found that using a Gaussian distribution for R0 (truncated at zero to avoid negative values), with rms width of ∼1/3 R0 , and fixing β0 ) R0 (where  ) 1 for R0 < 0.3 and  ) 0.3/R0 otherwise) produces a set of spectra that closely matches all the spectral features as a function of temperature when the single variable R0 is varied. Figure 10 illustrates this result for the 2H NMR simulations, where the spectra are labeled with the mean value of R0. Because the 1H NMR powder patterns are essentially the same as those for 2H, except for the additional center line (see Figure 6), one finds an equally good fit for the 1H spectra. It is not unreasonable to expect that there are several types of sites in MFI and that the average amplitude of molecular fluctuations can differ by about (30% for the various sites. The amplitude of the fluctuations at higher temperatures is assumed to be determined primarily by interactions of the molecule with the siliceous walls of zeolite cavity. The NMR measurements of CH3CN on CHA, a material for which all T

Molecular Motion of Hydrogen-Bonded CH3CN sites are similar, indicate that a distribution of R0 is not needed in that case.45 Therefore, an interpretation that assumes a distribution of Brønsted sites in MFI, with different barriers to molecular reorientations, is reasonable. Because interactions of the molecule with the siliceous framework make up only a fraction of the total interaction with the site, the energetics associated with adsorption at various sites may well be indistinguishable in calorimetric measurements, as has been observed.44 At the highest temperatures shown, the spectra include a motionally narrowed component at the center, on top of a broad spectrum that resembles the room-temperature spectrum. This component is observed most clearly in the 1H data at 335 K (Figure 3) and the 2H spectrum at 340 K (Figure 4) where a narrower center line is beginning to emerge. Indeed, the 1H NMR spectrum closely resembles the high-loading case (Figure 1d). The situation is less clear, though present, for the 13C spectrum at 345 K (Figure 5). At even higher temperatures, the spectra for the three different nuclei become single narrow lines. Therefore, at the higher temperatures, we observe the superposition of two types of spectra. One type corresponds to molecules that remain bound to the site on the time scale of the NMR measurement (i.e. milliseconds) and the second corresponding to molecules undergoing (nearly) isotropic reorientation. As shown in Figure 1d, having even a small fraction of the molecules be physisorbed (in that case because there are more molecules than there are binding sites) can initiate a rapid exchange process that causes the larger fraction of the molecules to exhibit a narrow NMR spectrum. We presume that a similar situation exists at higher temperatures when the loading is less than one per site. At any given instant, only a small fraction of the molecules may be unassociated with a site; however, through the exchange process, these physisorbed molecules still cause the narrowing of the NMR contribution from all molecules in their vicinity. As the temperature is raised, this narrowing is more likely to occur. At present there are, unfortunately, too many unknowns to attempt a calculation to describe this phenomenon in more detail. Conclusion The dynamics associated with CH3CN molecules adsorbed at Brønsted sites in MFI have been determined based on 1H, 2H, and 13C NMR spectra measured as a function of temperature and a simple NMR line-shape theory based on rapid, smallangle reorientation of the molecule with a temperature-dependent amplitude. An anisotropic angular motion is observed, with small amplitude at low temperatures increasing to approximately (35° from its average position at room temperature. To fully account for the spectra, a distribution of amplitudes from site to site is required. At higher temperatures, the spectra can be described as the superposition of a broad component, resembling that due to the motion at room temperature, and a more completely motionally narrowed component, with the fraction of the motionally narrowed component increasing with temperature. The completely motionally narrowed component is presumed to arise from the presence of physisorbed molecules. Acknowledgment. This work was supported, in part, by the National Science Foundation, Grant #CTS9713023. The help of Dr. Lichang Yang in preparing some of the samples is gratefully acknowledged. References and Notes (1) Farneth, W. E.; Gorte, R. J. Chem. ReV. 1995, 95, 615. (2) Ison, A.; Gorte, R. J. J. Catal. 1984, 89, 150. (3) Grady, M. C.; Gorte, R. J. J. Phys. Chem. 1985, 89, 1305.

J. Phys. Chem. B, Vol. 104, No. 21, 2000 5131 (4) Aronson, M. T.; Gorte, R. J.; Farneth, W. E. J. Catal. 1986, 98, 434. (5) Parrillo, D. J.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J. Appl. Catal. 1990, 67, 107. (6) Lee, C.; Gorte, R. J.; Farneth, W. E. J. Phys. Chem. B 1997, 101, 3811. (7) Lee, C.; Parrillo, D. J.; Gorte, R. J.; Farneth, W. E. J. Am. Chem. Soc. 1996, 118, 3262. (8) Pereira, C.; Gorte, R. J. “Proceedings of the 9th International Zeolite Conference”, Volume 2; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworths: Markham, ON, Canada, 1993; p 243. (9) Biaglow, A. I.; Gorte, R. J.; White, D. J. Phys. Chem. 1993, 97, 7135. (10) Sˇ epa, J.; Lee, C.; Gorte, R. J.; White, D.; Kassab, H.; Evleth, E.; Allavena, M. J. Phys. Chem. 1996, 100, 18515. (11) Savitz, S.; Myers, A. L.; Gorte, R. J. J. Phys. Chem. B 1999, 103, 3687. (12) Fletcher, W. P.; Gilbert, C. S.; Biaglow, A. I. Catal. Lett. 1997, 47, 135. (13) Fletcher, W. P.; Biaglow, A. I. Catal. Lett. 1998, 54, 217. (14) Pelmenschikov, A. G.; van Santen, R. A.; Ja¨nchen, J.; Meijer, E. J. Phys. Chem. 1993, 97, 11071. (15) Kubelkova, L.; Kotrla, J.; Florian, J. J. Phys. Chem. 1995, 99, 10285. (16) Koller, H.; Engelhardt, G.; van Santen, R. A. Top. Catal. 1999, 9, 163. (17) van Santen, R. A.; Kramer, G. S. Chem. ReV. 1995, 95, 637. (18) Shantz, D. F.; Lobo, R. F. J. Phys. Chem. B 1998, 102, 2339. (19) Sˇ epa, J.; Gorte, R. J.; Suits, B. H.; White, D. Chem. Phys. Lett. 1998, 289, 281. (20) Sˇ epa, J.; Gorte, R. J.; White, D.; Kassab, E.; Allavena, M. Chem. Phys. Lett. 1996, 262, 321. (21) Gorte, R. J.; White, D. Microporous Mater. 2000, 35-36, 447. (22) Sˇ epa, J.; Gorte, R. J.; White, D.; Suits B. H.; Swaminathan, U. S. “Proceedings of the 12th International Zeolite Conference”; Treacy, M. M. J., Marcus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society, 1998; p 2287. (23) Gorte, R. J.; White, D. Top. Catal. 1997, 4, 57. (24) Biaglow, A. I.; Sˇ epa, J.; Gorte, R. J.; White, D. J. Catal. 1995, 151, 373. (25) Biaglow, A. I.; Sˇ epa, J.; Gorte, R. J.; White, D. J. Catal. 1995, 154, 208. (26) Aronson, M. T.; Gorte, R. J.; Farneth, W. E.; White, D. J. Am. Chem. Soc. 1989, 111, 840. (27) (a) Nusterer, E.; Blo¨chl, P. E.; Schwartz, K. Angew. Chem. 1996, 108, 187; (b) Schwartz, K.; Nusterer, E.; Blo¨chl, P. E. Catal. Today 1999, 50, 501. (28) Shah, R.; Gale, J. D.; Payne, M. C. J. Phys. Chem. 1996, 100, 11688. (29) Stich, I.; Gale, J. D.; Terakura, K.; Payne, M. C. J. Am. Chem. Soc. 1999, 121, 3292. (30) Kofke, T. J. G.; Gorte, R. J.; Farneth, W. E. J. Catal. 1988, 114, 34. (31) Biaglow, A. I. Ph.D. Thesis, University of Pennsylvania, Philadelphia, PA, 1993. (32) Biaglow, A. I.; Gorte, R. J.; Kokotailo, G. T.; White, D. J. Catal. 1994, 148, 779. (33) Carduner, K. R.; Villa, M.; White, D. ReV. Sci. Instrum. 1984, 55, 68. (34) Tecmag, Houston, TX 77081. (35) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (36) Murphy, M. D.; Semack, M. G.; White, D. J. Magn. Reson. 1987, 72, 143. (37) Kustanovitch, I.; Luz, Z.; Vega, S.; Vega, A. J. J. Phys. Chem. 1990, 94, 3138, and references therein. (38) Hunger, M.; Horvath, T. J. Am. Chem. Soc. 1996, 118, 12302. (39) Hunger, M.; Horvath, T. Ber. Bungsen-Ges. Phys. Chem. 1995, 99, 1316. (40) See, for example, Mehring, M. High-Resolution NMR in Solids, 2nd ed.; Springer-Verlag: Heidelberg, 1983. (41) Suits, B. H.; Sˇ epa, J.; White, D. J. Magn. Reson. 1996, A120, 88. (42) Wittebort, R. J.; Olenjniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1987, 86, 5411. (43) Zaborowski, E.; Vega, S. Mol. Phys. 1993, 78, 703. (44) Parrillo, D. J.; Gorte, R. J. J. Phys. Chem. 1993, 97, 8786. (45) Trout, B.; Suits, B. H.; Gorte, R. J.; White, D., manuscript in preparation.