SolidâState NMR Determination of the Zeolite ZSM-5 - American

500

J. Phys. Chem. C 2008, 112, 500-513

Solid-State NMR Determination of the Zeolite ZSM-5/ortho-Xylene Host-Guest Crystal Structure Colin A. Fyfe* and J. S. Joseph Lee Department of Chemistry, UniVersity of British Columbia, Room 300, 6174 UniVersity BouleVard, VancouVer, British Columbia, Canada V6T 1Z3 ReceiVed: June 20, 2007; In Final Form: September 21, 2007

Solid-state NMR spectroscopy has been used to successfully determine the complete three-dimensional structure of the ortho-xylene-ZSM-5 zeolite complex, using complexes of three different specifically deuterated orthoxylene molecules. The structures determined from these, at two different temperatures, using both cross polarization (CP) and CP drain experiments all agree. The ortho-xylene molecule is located at the channel intersection with one ring carbon-methyl carbon bond oriented along the straight channel while the other ring carbon-methyl carbon bond points toward the zigzag channel.

Introduction Central to the many commercial and industrial applications of zeolites1,2 as sorbents, catalysts, and for gas separations are the size and shape selectivities that they exhibit toward organic molecules. It is important to have access to a reasonable number of reliable structures of complexes of this type in order to understand the host/guest interactions that control these effects with the ultimate aim of describing them theoretically. However, relatively few such studies exist. This is primarily due to the very small crystal dimensions obtained for most zeolites that preclude the use of single-crystal X-ray diffraction techniques (an important exception is the zeolite ZSM-5, MFI topology, for which large enough crystals can be grown and a limited number of single-crystal X-ray structures of this system are available, mainly from the work of van Koningsveld and coworkers3-11). Thus, in most cases, recourse must be made to powder methods that yield much more limited data and, although it is often possible to obtain the framework structure, the locations and orientations of occluded templates or adsorbed organic molecules are much more difficult to define accurately. As an alternative to diffraction techniques, we have introduced and developed a method based on solid-state NMR spectroscopy that uses the distance-dependent through-space dipolar interactions between nuclei on the organic guests and the silicon nuclei in the host framework to solve these structures. The method has been tested on the known structure of the high-loaded form of highly siliceous zeolite ZSM-5 with p-xylene and then used to predict the structure of the low-loaded form,12 which was then confirmed by single-crystal X-ray diffraction.13 More recently, the method has been optimized computationally, the display of the results was standardized, and its robustness was demonstrated with respect to temperature and to having the exact structure of the zeolite framework.14 Several specific examples have been investigated.15,16 It should be noted that the method is limited to the study of perfectly ordered frameworks such as siliceous systems or AlPO4 materials, which have high enough resolution in their NMR spectra to assign the resonances to specific framework sites. * Corresponding author. E-mail: [email protected].

A classic example of the selectivity toward organic molecules is the behavior of zeolite ZSM-5 toward the xylene isomers, which is central to industrial xylene synthesis and isomerization and separation processes.17 Both adsorption and diffusion studies18,19 have been carried out. Furthermore, the structure of the ZSM-5 complex with p-xylene is now known from both NMR and single-crystal X-ray studies12,13 but relatively little is known about the o- and m-xylene complexes. Nagy and coworkers20 studied the 13C NMR spectra of the o-xylene/ZSM-5 system to investigate the motions of the sorbate in the framework and concluded that at a loading of 3.7 molecules per unit cell there was rotation of the methyl groups around their C3 axes but that the rings were almost static.20 The most informative work is a powder X-ray diffraction study by Nair and Tsapatsis who proposed that the organic molecules were located at the channel intersection21,22 from a Rietveld refinement of the data. The purpose of the present work was to determine the structure of the complex formed by o-xylene with MFI by the solid-state NMR method. Materials and Methods Sample Preparation. For the solid-state NMR investigation, a completely siliceous powder sample of ZSM-5 (Silicalite, crystal size 1000 21.8-40.6

315 K

ca. 5 × T1 (s) T2 (ms) T1F (1H) (ms)

50 5.5-8.3

5 52 17.5

3.5 0.2 12.4

>1000 20.8-63.1

speed 7 mm Kel-F MAS stator from Doty Scientific. The MAS spinning rates were ca. 2 kHz, and the operational frequencies for the probe were 400.13 MHz for protons and 79.495 MHz for 29Si. 29Si chemical shifts were referenced to TMS using Q8M8 as an intermediate standard. To remove the paramagnetic effects of molecular oxygen,27 N2 gas from a 200 L liquid nitrogen Dewar was used as both bearing and drive gas. Lowtemperature experiments were performed as reported previously,12 with temperature regulation of the sample by a Bruker VT-1000 temperature control unit in conjunction with a 25 L liquid nitrogen Dewar. 1H/29Si CP INADEQUATE experiments used a modified version of the standard pulse sequence28 with a 135° pulse instead of the last 90° pulse and were carried out at 273 and at 315 K. Contact times were 15 ms, and typical 90° pulse times for both 1H and 29Si were 7 µs. The total number of scans accumulated for each experiment was 384, and a total of 36 experiments were collected in the f1 dimension with an echo time of 18 ms. The 1H/29Si cross polarization (CP) experiments used the standard CP spin-lock sequence. In this work, the HartmannHahn match condition was determined directly on the sample and variable contact time CP experiments were carried out at 273 and 315 K. The typical 90° pulse time for 1H was 11 µs, recycle delays were 5 s, and the number of scans was varied between 800 and 2000. Variable contact time 29Si/1H CP drain experiments were performed at 273 and 315 K. The 29Si 90° pulse time was 11 µs, recycle delays were 250 s, and the number of scans for each contact time was 40. Deconvolution of the spectra, assignment of the silicon T-sites, fitting of the CP and CP drain curves, and the structure determinations were carried out using programs14,29 written in Mathematica version 3.0.30 Results and Discussion Characterization of the 29Si Spectra of the o-Xylene/ ZSM-5 Complex. A preliminary 29Si MAS experiment on a sample containing ca. 4 molecules per u.c. of o-xylene in ZSM-5

yielded a broad spectrum (Figure 1a). After the sample was purged with N2 gas for 12 h and N2 gas was subsequently used as both drive and bearing gas, the 29Si MAS spectrum changed drastically, giving sharply resolved peaks (Figure 1b) and illustrating the effect that paramagnetic O2 in the channels has on the relaxation mechanisms of both 1H and 29Si for this system. Upon removal of O2 by purging with N2 gas, all relaxation times (T1, T2, and T1F of both 1H and 29Si) become longer in the temperature range studied. In particular, the 29Si spin-spin relaxation T2 becomes longer (on the order of milliseconds), leading to narrower, more clearly resolved peaks. One of the drawbacks of N2 purging is that the 29Si spin-lattice relaxation time, T1, also becomes long (>200 s in the present case) so that the accurate measurements of T1 become difficult and relaxation delays are considerably longer. The increase in T1 relaxation times (Table 1) is more pronounced for 29Si, which generally has longer T1 values than 1H. One-Dimensional 29Si NMR. The space group of the ZSM-5 framework depends on the number of organic molecules per unit cell and the temperature.25 For example, the empty ZSM-5 monoclinic structure with 24 T-sites transforms completely to an orthorhombic Pnma space group with 12 T-sites at room temperature when its channel intersections are occupied with small organic sorbates in loadings of 2-4 molecules per unit cell and the 29Si NMR spectra show corresponding changes in the number of peaks. Similar effects are seen when the temperature is changed.25 29Si NMR spectra were therefore acquired to determine the range of temperatures over which the complex exhibited the orthorhombic Pnma space group. Figure 2 shows the 29Si MAS NMR spectra at temperatures from 260 to 330 K, and indicates that the orthorhombic Pnma space group is retained over all of this fairly wide temperature range. From the variable temperature NMR spectra in Figure 2, those at 273 and 315 K had the clearest peak splitting patterns and most resolved peaks and all further experiments were carried out at these two temperatures. From the peak assignments described later, at 273 K there are 10 resolved signals in the 29Si NMR spectrum 8 of which correspond to single 29Si T-sites

502 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Figure 2. 29Si MAS NMR spectra of ZSM-5 loaded with ca. four molecules of o-xylene per u.c. at the temperatures indicated. The 29Si 15° pulse length was 1.8 µs, and 128 scans were accumulated with a recycle delay of 15 s for each spectrum.

(Figure 3a) and at 315 K there are 9 resolved signals, 7 of which correspond to single 29Si sites (Figure 3b). The 29Si NMR resonances were assigned using INADEQUATE experiments.23,24,32-34 These show 22 connections at 273 K and 21 connections at 315 K (Figure 4). Because the connectivities of each silicon site are known for the ZSM-5 topology,35 each peak in the NMR spectrum can be assigned to a specific silicon T-site. This was done using a program that generates all possible assignments of the resonances and selects all those in agreement with the observed connectivities.29,36 From symmetry considerations, there are two possible assignments in each case and the final selection was arrived at by testing both in the initial structure determinations where it was found that only one gave a significant number of structure solutions. The experiments also confirmed that there were two sets of overlapping resonance peaks as indicated in Figure 3.

Fyfe and Lee Cross Polarization Experiments. The structure determination method has been described in detail previously14,29 and is only briefly reviewed here. It depends on using the throughspace, distance-dependent dipolar interactions between the 1H nuclei on the organic molecule and each of the different framework 29Si nuclei, whose resonances have now been assigned. Because the method relies on reasonably localized sources on the organic molecule for the magnetization transfer, two specifically deuterated o-xylenes were used (Figure 5a and b) and experiments were performed on complexes of each. In addition to improving the quality of the structures, this also provides an incisive check on their validity. In terms of the spin dynamics of the cross polarization experiment, the parameter of interest for the structure determinations is the cross polarization time TCP (or 1/kIS, where kIS is the rate constant for the magnetization transfer). Quantitatively, the cross polarization process is usually described using eq 1a with the assumptions that kI , kIS, and the magnetization relaxation rate constant of 29Si, kS (1/T1F(Si)) is very small compared to kIS. In order to simplify the calculations, kS is often neglected, and eq 1a becomes eq 1b. Another assumption is that the number of 1H nuclei is far greater than the number of 29Si nuclei. This second condition should usually be satisfied by the relatively large number of protons in the organic guest molecules, the high natural abundance of 1H (100.0%), and the low abundance of 29Si (4.6%). The potential shortcoming for this assumption in the present work, however, is the lower number of protons in the deuterated o-xylene/ZSM-5 complexes. Also, in the case of T1F(Si) being relatively short, the magnetization relaxation rate constant of 29Si (kS) should also be taken into account. These assumptions could possibly lead to erroneous results in the description of the behavior of the S nucleus (29Si) magnetization by eq 1b, where I0 is the theoretical CP maximum and t is the contact time.

I(t) ) I0[1 + (kS/kIS) - (kI/kIS)]-1[exp(-kI t) exp(-kIS t - kS t)] (1a) I(t) ) I0[1 - (kI/kIS)]-1[exp(-kI t) - exp(-kIS t)] (1b) In the typical CP experiment, where the interactions are between nuclei close in space, usually covalently bonded, eq 1b gives a curve where there is an exponential rise due to kIS, a maximum, and then an exponential decay due to kI. However, in some cases, the initial rising part of the curve can depend on kI while the decay is governed by kIS + kS. This phenomenon was studied by Tekely and co-workers.37,38 and is termed the

Figure 3. 1H/29Si CP MAS NMR spectra of ZSM-5 loaded with ca. four molecules of o-xylene per u.c. (a) at 273 K (b) at 315 K, contact time 4 ms, recycle delay 5 s with 1024 scans and 800 scans, respectively. The numbers above the resonances indicate their assignments to specific T-sites.

Solid-State NMR Determination

J. Phys. Chem. C, Vol. 112, No. 2, 2008 503

Figure 4. Two-dimensional 1H/29Si CP INADEQUATE spectra of the o-xylene/ZSM-5 complex at 273 K (left) and 315 K (right) together with their quantitative 29Si NMR spectra (recycle delay of 200 s). Thirty six experiments, each with 384 scans and 5 s recycle delay, were acquired in the f1 frequency dimension by incrementing t1, the echo delay during the double quantum preparation period, which was 18 ms. The sweep widths in the f2 and f1 frequency dimensions were 700 and 1400 Hz, respectively. The numbers above the resonances indicate the assigned silicon T-sites determined from the observed correlations.

spin lock conditions without the contact pulse applied on the I spins so that the decay is entirely due to kS

S0(t) ) S0 exp(-kS t)

Figure 5. The two specifically deuterated o-xylenes used (a) both methyl groups are deuterated (o-xylene-d6) (b) protons in the benzene ring are deuterated (o-xylene-d4). D7,8 and H7,8 are the simplified representations of the methyl deuteriums and methyl hydrogens (assuming methyl group rotation) placed at the centers of their equilateral triangles.

“slow CP regime”. It occurs when kI > kIS, which can easily happen in studies involving “intermolecular” interactions between two systems such as in the present work as well as the other organic/ZSM-5 systems studied earlier12,14-16 because the dipolar interactions and hence the kIS values are much smaller. The dipolar interactions could also be reduced further by motion. Furthermore, even if the system is recognized to be in the slow CP regime, when kIS ≈ kS, the latter term cannot be neglected and it is very difficult to obtain accurate values of kIS from the long contact time behavior of the conventional CP experiment. To fully address these concerns, it is important to carry out an alternative experiment, if possible, for the unambiguous measurement of the kIS values. In the present work, a 1H/29Si CP “drain” (originally termed a “double cross polarization”) experiment39-41 was used to measure the absolute kIS values. The CP drain experiment consists of two separate parts. In the first, it generates the exponential decay of the S spins under

(2)

where S0 represents the maximum signal intensity obtainable from the S spins in the absence of any loss due to relaxation processes. The exponential decay of the spin-locked S spins with a CP contact pulse on the I spins is observed in the second part of the drain experiment. In this case, the signal intensity is governed by eq 3.

Sd(t) ) S0 exp(-kS t - kIS t)

(3)

A plot of the normalized difference of these two data sets makes it possible to directly obtain the desired kIS values as it is determined by only this parameter (eq 4).

[S0(t) - Sd(t)]/S0(t) ) ∆S/S0(t) ) 1 - exp(-kIS t)

(4)

Having obtained the kIS values, the structure determination by solid-state NMR is based on the relationship between kIS and the heteronuclear second moments, (M2)IS, of the dipolar line shapes,42 shown in eq 5, where C is a constant.

kIS ) C(M2)IS/(M2)II1/2

(5)

The heteronuclear second moment of the dipolar coupling (M2)IS, M2 hereafter, is a pairwise summation of the squared values of the dipolar couplings and can be calculated for a rigid structure from the distances between the I and S spins according to van Vleck43 as in eq 6.

(∆M2)IS ) 1/5ΣDIS2 ) 1/5(γIγSµ0p/8π2)2ΣrIS-6

(6)


Fyfe and Lee

Figure 6. Variable contact time 29Si CP MAS NMR experiment on ZSM-5 loaded with ca. four molecules of o-xylene-d6 per u.c. at 273 K. The points are the experimental values of the intensities, and the solid curves are calculated according to eq 1b. The vertical axes represent the NMR signal intensities in arbitrary units, and the horizontal ones represent the range of contact times in milliseconds. The theoretical constant CP maximum I0 was 433 (au) and kI was 127 s-1 (T1F ) 7.87 ms).

Because the homonuclear second moment for the I (proton) spins is expected to be constant, we expect the proportionality of eq 7.

kIS ∝ (M2)IS ∝ (γIγS)2Σ rIS-6

(7)

For an unknown sorbate framework structure, the locations and orientations of the organic molecule can be selected by testing whether or not they yield an acceptable linear correlation between the experimental kIS and theoretical M2 values. This testing was carried out in a completely automated fashion involving several successive criteria using software described previously.14 For clarity of presentation, the results for the different experiments and systems are presented separately, and to conserve space not all data are presented as figures in the paper. Those figures and tables not in the text are given in the Supporting Information and referenced individually as such. The CP curves were first fit in terms of three variables, using single values of I0 and kI common to all data sets that gave the best fittings, and separate kIS values for each site. Subsequently, independently measured kI values (obtained by a spin-locking pulse sequence) were compared to the fitted kI values, and the two were found to be comparable to each other in magnitude. The independently determined experimental kI values were then used to fit the CP curves of some experiments. However, the fittings were found not to be as good as when kI was allowed to vary, especially at longer contact times. Because the kIS values obtained are highly correlated to I0 and the kS values are

neglected in the fitting of the CP curves, a fitted kIS value is denoted as k′IS, which indicates a “relative” CP rate constant (assuming constant I0 and kI). In CP drain experiments, no such approximations of the correlated values are needed and the obtained kIS values are absolute values, and in that way, in principle the CP drain experiments are superior to the conventional CP experiments although relaxation considerations may produce poorer experimental data in practice. CP Experiments on o-Xylene-d6/ZSM-5. Figure 6 shows the curves from a variable contact time experiment on the o-xylene-d6/MFI complex at 273 K. The general appearance of the curves, together with the low efficiency of the CP experiment compared to a fully relaxed single pulse experiment, are indicative of the system being in the slow exchange regime. The curves were fitted to eq 1b using a nonlinear least-squares fitting procedure constrained by single, common values of I0 and kI. Because of the correlation of kIS and I0 and the neglect of kS, these values are relative values only and are designated as k′IS values. As seen from the figure, silicon T-site 8 (Si8) in ZSM-5 shows the highest k′IS value, which indicates close proximity to the proton source. Alternatively, Si10 shows the weakest peak growth indicating that it is the furthest silicon atom from the source protons. Qualitatively, these relative proximities of the silicon sites to the protons rule out the o-xylene being located in the zigzag channel of ZSM-5. As an alternate analysis, an independently measured kI value (86.7 s-1) was used for the fitting of the CP curves instead of


Figure 7. Distributions of the structural parameters for the solutions determined from 1H/29Si CP data of o-xylene-d6 in the framework of ZSM-5 at (a) 273 K and (b) 315 K with linear correlations of r 2 g 0.91 (red squares), r 2 g 0.92 (blue circles), and r 2 g 0.93 (green triangles). The vertical axes refer to the numbers of solutions, and the horizontal axes show the distributions of solutions by six structural parameters for the translation (x, y, z) of the center of the o-xylene-d6 (i.e., the center of the benzene ring) in fractional coordinates and the orientation (φ, θ, ψ) of the long axis in degrees. The arrows indicate the average values of the six structural parameters with r 2 g 0.92, which are shown in Table 2.

using the one obtained from the first fitting (127 s-1) described previously. The results are presented in Figure S1 (Supporting Information) and are similar in quality to those described above. Fitting with the experimental value of kI gave different k′IS values, but the relative values are the same and both sets yielded the same final structure when carried through the complete structure analysis described below. Figure S2 shows the results from the CP experiments on the same sample at 315 K. The values are somewhat different from the results at 273 K, but the relative order of the kIS values is the same. At both temperatures, the fitted CP rate constants k′IS

J. Phys. Chem. C, Vol. 112, No. 2, 2008 505 are smaller than the kI values (both fitted and experimental), confirming that both systems are in the slow CP regime. Solving the Structure of the o-Xylene/ZSM-5 Complex. The structure determinations of the o-xylene/ZSM-5 complex were carried out using the program mentioned in the experimental section. First, the program generates all possible locations and orientations of the organic molecules in the channel system. It then searches for the locations of the o-xylene molecule in the rigid framework of ZSM-5 that are compatible with the kIS values of the resolved resonances corresponding to single Si sites according to the following criteria: (1) It must be physically reasonable (no framework atom-to-sorbate atom distances less than 2.5 Å). (2) There must be a strong linear correlation between the values of k’IS and the calculated 1H-29Si second moments for the structure as expected from eq 8 within a specified standard deviation. (3) There should also be good agreement between the experimental and calculated intensities of the complete 29Si NMR spectrum, including all overlapping peaks. For the framework coordinates, Si and O atomic positions in ZSM-5 were taken from the single-crystal XRD study of the low-loaded form of the p-dichlorobenzene/ ZSM-5 complex by van Koningsveld et al.7,11 as a representative orthorhombic structure (it has been shown that the exact framework coordinates are not essential for the success of these distance-driven calculations14,29). In order to illustrate this point, various sets of the framework coordinates for ZSM-5 were used for the structure calculation including the empty orthorhombic crystal structure at high temperature (350 K),3 and all gave the same results. The calculations were simplified using a rigid body model for o-xylene and the approximation that the protons (oxylene-d4) (or deuteriums (o-xylene-d6)) of the methyl groups could be replaced by locating them all at the center of their respective equilateral triangles. Figure S3 describes the location, that is, xyz of the center of the ring and the Euler angles with which the location and orientation of an organic molecule in the framework of ZSM-5 can be described, using x, y, and z for translation in the fractional coordinates and φ, θ, and ψ for orientation in angular degrees. To define an arbitrary long axis as used previously for p-xylene, for o-xylene an imaginary line was chosen that bisects the line between two methyl groups (assuming fast methyl group rotations) and passes through the center of the benzene ring in the molecular plane (Figure S3). To ensure that this choice did not exclude any possible solutions, other long axes such as the

Figure 8. Scatter plots of o-xylene molecules in the plane of the molecule for ZSM-5 loaded with ca. four molecules of o-xylene-d6 per u.c. as a graphical representation of solutions of acceptable o-xylene locations in the framework at r 2 g0.92. (a) 273 K and (b) 315 K. The colored dots represent the atomic locations of the hydrogen (green), carbon (red), and deuterium (blue) in o-xylene-d6.


Fyfe and Lee

TABLE 2: Average Values of the Six Structural Parameters of o-Xylene-d6 in the Framework of ZSM-5 at 273 and 315 K with r 2 g 0.92 by CP temperature

x

y

z

φ

θ

ψ

273 K 315 K

0.489(7) 0.486(7)

0.294(12) 0.292(14)

-0.033(11) -0.042(11)

37.7(56) 43.7(41)

77.4(108) 85.7(105)

17.0(135) 6.4(100)

one that crosses H1 and H4 and the one that crosses H3 and D8 were tested and were found to yield the same results in the structure determinations. Initially, all possible locations and orientations were systematically tested and those that did not involve too close contacts with the framework atoms were selected. After all of the physically reasonable locations were found, the heteronuclear second moments, M2, between the 1H nuclei in o-xylene and 29Si nuclei in the framework of ZSM-5 were calculated for each location. The cutoff value for the heteronuclear second moment calculations was set to 8 Å and the second moments for all of the single, resolved 29Si resonances were calculated. The calculated M2 values were then correlated to the experimental k′IS values in order to determine if there was an acceptable linear relationship between the M2 and k′IS values. The degree of linearity was expressed in terms of the least squared r 2 values, with a higher r 2 value indicating a better linear correlation and the particular location and orientation accepted or rejected in terms of a preselected cutoff value of r 2. If the position was still acceptable, then, at this point, the linear regression of the above correlation was used to assign the kˆ IS values of the overlapping resonances not used in the previous correlation and the kIS values obtained were used to predict the intensities of all of the peaks in the complete 29Si spectrum including all of the overlapping peaks. These were now used in a second correlation of M2 and k′IS values and a further selection made in terms of a preselected value of r 2. Initially, for the 273 K data, all of the possible locations were sampled with large steps in the translations (up to 0.03 in the fractional coordinates) and orientations (up to 7° in angle) for the o-xylene molecule. This involved, in the framework of ZSM5, ranges in translations of 0.40 e x e 0.60, 0 e y e 0.5, and -0.15 e z e 0.15 in the fractional coordinates and in orientations of 0 e φ e 180, 0 e θ e 360, and -180 e ψ e 180 in degrees for the straight channel. Considering the channel intersection is located at (0.5, 0.25, 0), these limits cover the volume of the channel and most of the channel intersection along the straight channel in the y direction taking framework symmetry into account. To confirm the absence of the organic molecules in the zigzag channel of ZSM-5, translations of 0 e x e 0.5, 0 e y e 0.5, and -0.5 e z e 0 and orientations of 0 e φ e 180, 0 e θ e 360, and -180 e ψ e 180 were tested in a separate calculation. Those ranges cover the whole zigzag channel and some part of the straight channel. From these results, approximate values of the six structural parameters were found which gave solutions with r 2 g 0.7, and the o-xylene molecule was located in the channel intersection. On the basis of this approximate location, for the 273 K data, a more detailed calculation was carried out for translations of x (step size of 0.010) between 0.42 and 0.55 in the fractional coordinate, y (step size of 0.010) between 0.1 and 0.4 in the fractional coordinate, and z (step size of 0.016 Å) between -0.12 and 0.02 in the fractional coordinate along with orientations of φ (step size of 4°) between 10 and 70°, θ (step size of 5°) between 25 and 150°, and ψ (step size of 4°) between -30 and 60°. A total of ca. 40 million possible locations and orientations were tested, and 3783 solutions were found with r 2 g 0.91. The 315 K data were tested over the same ranges and step sizes as used

TABLE 3: Fractional Atomic Coordinates for the o-Xylene-d6/ZSM-5 Complex at 273 and 315 Ka temp

atom

x

y

z

Si1 Si2 Si3 Si4 Si5 Si6 Si7 Si8 Si9 Si10 Si11 Si12

0.4228 0.3043 0.2810 0.1232 0.0712 0.1813 0.4215 0.3040 0.2740 0.1192 0.0684 0.1827

0.0566 0.0286 0.0630 0.0641 0.0277 0.0574 -0.1723 -0.1294 -0.1725 -0.1735 -0.1302 -0.1726

-0.3456 -0.2020 0.0202 0.0191 -0.1922 -0.3419 -0.3332 -0.1932 0.0250 0.0217 -0.1902 -0.3286

273 K

C1 C2 C3 C4 C5 C6 C7 C8 H1 H2 H3 H4 D7 D8

0.4390 0.4814 0.5316 0.5394 0.4970 0.4468 0.4022 0.5050 0.4068 0.4765 0.5589 0.5716 0.3929 0.5069

0.3197 0.3638 0.3385 0.2692 0.2251 0.2504 0.2036 0.1518 0.3359 0.4085 0.3669 0.2529 0.1942 0.1368

-0.0960 -0.0450 0.0175 0.0291 -0.0219 -0.0845 -0.1382 -0.0100 -0.1362 -0.0524 0.0503 0.0693 -0.1494 -0.0073

315 K

C1 C2 C3 C4 C5 C6 C7 C8 H1 H2 H3 H4 D7 D8

0.4364 0.4680 0.5179 0.5361 0.5045 0.4546 0.4213 0.5236 0.4043 0.4563 0.5382 0.5682 0.4144 0.5277

0.3015 0.3571 0.3479 0.2830 0.2274 0.2366 0.1778 0.1588 0.3074 0.3989 0.3837 0.2771 0.1658 0.1449

-0.1139 -0.0711 0.0011 0.0304 -0.0124 -0.0845 -0.1295 0.0184 -0.1603 -0.0900 0.0286 0.0768 -0.1389 0.0250

a

The o-xylene coordinates were obtained from the average solutions calculated from the NMR data at 273 and 315 K with r 2 g 0.92. The coordinates of the silicon atoms are from the low-loaded form of the p-dichlorobenzene/ZSM-5 complex.7 The coordinates of D7 and D8 are the centroids of the triangles formed by the deuterium atoms in the (rotating) methyl groups.

in the more refined calculation on the 273 K data set. A total of ca. 38 million possible locations were tested, and 7077 solutions were found with r 2 g 0.91. After the overlapped peaks were included for the final peak intensity calculation, the final number of solutions was reduced to 1581 (r 2 g 0.91) for 273 K and 1548 (r 2 g 0.91) for 315 K. Figure 7a and b shows the distributions of the structural parameters for the acceptable solutions found for 273 and 315 K with the values indicated in the captions. Figure 8 shows the spatial distribution of the acceptable solutions as the scatter plots in two dimensions and Figure S4 in three dimensions in terms of atom positions for the final solutions at 273 and 315 K with r 2 g 0.92. For 273 K, a total of 1152 solutions were found, which satisfy r 2 g 0.92. At 315 K, a total of 1017 solutions



Figure 9. Plot of the measured CP rate constants against the calculated heteronuclear second moments for the average location of o-xylene in ZSM-5 loaded with ca. four molecules of o-xylene-d6 per u.c. with r 2 g 0.92 at (a) 273 K and (b) 315 K. The solid lines are the lines of best fit, and the dashed lines represent the 95% confidence prediction intervals.

Figure 10. 29Si NMR spectra for ZSM-5 loaded with ca. four molecules of o-xylene-d4 per u.c. at (a) 273 K and (b) 315 K; the spectra, in descending order, are the experimental spectrum (contact time ) 10 ms), the predicted spectrum from the average solution of the structure calculation and the difference between the experimental and predicted spectra.

were found, which satisfy r 2 g 0.92. The black arrows above the curves in Figure 7 represent the average values of the different parameters for r 2 g 0.92 and are summarized in Table 2. From these average structural parameters, the average location of the o-xylene molecule in ZSM-5 was determined for both temperatures, and the resulting atomic coordinates are listed together with those of the framework silicon atoms of ZSM-5 in Table 3. The fractional coordinates (x, y, z) define the center of the benzene ring, and the angles (φ, θ, ψ) in degrees define the orientation of the “long axis”, which is an arbitrary line bisecting the line between the two methyl groups and passing through the center of the ring on the plane of the molecule. Figure 9a shows the degree of linear correlation between the k′IS and M2 values at 273 K for the average of the acceptable structures with r 2 g 0.92, and Figure 9b shows the corresponding data at 315 K, with r 2 g 0.92. From these linear correlations, the calculated values of k′IS were obtained and used to calculate the theoretical individual Lorentzian lines for the 12 silicons, including the overlapping peaks, which were compared to the experimental NMR spectra for one specific contact time. Figure

10a shows that the experimental NMR spectrum at 273 K is very similar to the calculated NMR spectrum for the average values of the six structural parameters with r 2 g 0.92. Even for the overlapping peaks, the differences between the experimental and calculated spectra are small given that there may well not be exact peak overlap. Figure 10b shows the corresponding spectra of the complex at 315 K, and again the differences between the experimental and calculated NMR spectra are very small. The coordinates of the average structures can now be used to construct three-dimensional pictorial representations. The final structures from these experiments are presented in Figure 11a and b, and a representation with the framework silicon atoms labeled is presented in Figure S5 for 273 K. For the average values of the structural parameters, the o-xylene ring center is located at the channel intersection between the zigzag channel and straight channel of the ZSM-5 framework (fractional coordinates of the ring center are {0.489, 0.294, -0.033}) with the arbitrary long axis of the o-xylene molecule lying along the crystallographic b axis with a deviation of 17.0 ( 13.5 ° (Figure 11a). The structure of the o-xylene-d6/ZSM-5 complex


Fyfe and Lee

Figure 11. NMR determined structures of the o-xylene-d6/ZSM-5 complex at (a) 273 K and (b) 315 K. The left figures show the location of o-xylene along the zigzag channel of ZSM-5, and those on the right show the location along the straight channel. For clarity, oxygen atoms in the framework have been omitted.

at 315 K presented in Figure 11b shows that it is almost identical to that found at 273 K, providing strong support for the solutions and indicating that the method is quite robust to temperature changes over this 42 °C range. CP Experiments on o-Xylene-d4/ZSM-5. Figures S6 and S7 show the changes in the NMR peak intensities in the variable contact time CP experiment at 273 and 315 K, respectively, for ZSM-5 loaded with ca. four molecules of o-xylene-d4 per u.c. In the 273 K data, Si6 shows the highest k′IS value and Si11 shows the lowest among the eight T-sites giving resolved, single-site signals. As mentioned previously, this is an indication of the differences in the distances between the silicon T-sites and the protons in the o-xylene molecules. To determine the relative CP constants of silicon atoms in ZSM-5 to the protons in the o-xylene-d4, the eight (273 K) and nine (315 K) peaks that belong to the single 29Si atoms were used. As the fitted kIS values and kI values indicate, at both 273 and 315 K, the o-xylene-d4/ZSM-5 system is in the slow CP regime, as was the case for the o-xylene-d6/ZSM-5 complex. Solving the Structure of the o-Xylene-d4/ZSM-5 Complex. The same procedure described above was used to determine the location of the o-xylene-d4 molecules in the framework of

ZSM-5. Initially, all of the physically possible locations were sampled exactly as for the o-xylene-d6 system. From initial calculations using large step sizes, the o-xylene molecule was again found to be at the channel intersection. For the final calculations, for 273 K, translations in x (step size of 0.010 Å) between 0.42 and 0.55, y (step size of 0.010 Å) between 0.24 and 0.36, and z (step size of 0.015 Å) between -0.15 and 0 in the fractional coordinates along with the orientations with φ (step size of 4°) between 10 and 70°, θ (step size of 4°) between 50 and 100°, and ψ (step size of 4°) between -30 and 35°, were tested. A total of ca. 7 million possible locations were tested, and 1051 acceptable solutions were found with r 2 g 0.71. For 315 K, approximately the same ranges and step sizes were used. A total of ca. 24 million possible locations were tested, and 1374 acceptable solutions found with r 2 g 0.71. Much lower r 2 values were needed for the o-xylene-d4/ZSM-5 complex to obtain reasonable numbers of solutions comparable to those for the d6 complex in order to make comparisons between the results for the two systems. The value of r 2 g 0.71 selected yielded approximately 103 acceptable solutions. There are several possible reasons for the lower quality of fit for the d4 system: First, it is not possible to specify the exact



Figure 12. Plots of the measured CP rate constants against the calculated heteronuclear second moments for the average location of o-xylene for ZSM-5 loaded with ca. four molecules of o-xylene-d4 per u.c. with r 2 g 0.72 (a) at 273 K and (b) at 315 K. The solid lines are the lines of best fit, and the dashed lines represent the 95% confidence prediction intervals.

Figure 13. NMR determined structures of the o-xylene-d4/ZSM-5 complex at (a) 273 K and (b) 315 K from the CP data. The left figures show the location of o-xylene from the zigzag channel of ZSM-5, and those on the right, the location from the straight channel. For clarity, oxygen atoms in the framework have been omitted.

locations of the protons in order to solve the structure. Hence, the three protons in the methyl groups were approximated as one “pseudoproton” at the center of their equilateral triangle, which also reduced the calculation times. There is also fast

rotation of the methyl groups about their C3 axes so the dipolar interactions between the protons in the d4 system and the silicon atoms are much weaker than those of d6 system, leading to larger errors.


Fyfe and Lee

Figure 14. Intensities of the 1H-29Si CP drain NMR signals indicated as functions of the drain contact time for ZSM-5 loaded with ca. four molecules of o-xylene-d6 per u.c. at 273 K. The points are the experimental values of the intensities, and the solid curves are calculated according to eq 4. The vertical axes represent the normalized signal difference (∆S/S0) in the CP drain data, and the horizontal ones, the range of drain contact times. For the curves corresponding to the groups of overlapped peaks, the fitted values of CP drain rate constant are the averaged values, denoted as kIS*.

TABLE 4: Average Values of the Six Structural Parameters of o-Xylene-d4 in the Framework of ZSM-5 at 273 and 315 K with r 2 g 0.72 from CP Experiments temperature

x

y

z

φ

θ

ψ

273 K 315 K

0.486(1) 0.491(6)

0.299(8) 0.290(12)

-0.048(20) -0.042(9)

40.9(64) 42.0(37)

72.0(51) 87.5(125)

5.9(51) 5.6(112)

Figure S8 shows the distributions of acceptable solutions found for the r 2 values indicated in the captions. For 273 K, a total of 928 solutions were found, which satisfy r 2 g 0.72. At 315 K, a total of 1070 solutions were found, which satisfy r 2 g 0.72. The black arrows on the curves represent the average values of the different parameters and are summarized in Table 4. From these, the average location of the o-xylene molecule in the framework was determined, giving the atomic coordinates listed in Table S1. Figure 12 shows the degree of linear correlation between the k′IS values of the resolved single-site resonances and the corresponding calculated M2 values at 273 K and at 315 K, the higher r2 value at 273 K (0.877) than at 315 K (0.774) indicating a better correlation for the 273 K data. Calculated values of k′IS for the overlapping resonances were obtained from these linear correlation graphs using the average location of o-xylene and used to calculate the complete NMR spectra for one specific contact time, which were then compared to the experimental NMR spectra. Figure S9a and b (Supporting Information) show that there is good agreement in both cases, and even for the overlapping peaks the differences between the

experimental and calculated spectra are marginal, appearing to come from errors in the deconvolutions where the resonances contributing to the overlapped peaks were assumed to have exactly the same chemical shifts. The two-dimensional scatter plots that indicate the distributions of the acceptable solutions are presented in Figure S10a and b (Supporting Information) for the experiments at 273 and 315 K, respectively. They are well-localized, indicating reliable structure determinations. Figure 13a shows the three-dimensional structure of the o-xylene-d4/ZSM-5 complex at 273 K. The o-xylene is located at the channel intersection between the zigzag channel and straight channel of the ZSM-5 framework with its long axis parallel to the b axis, similar to the previous solutions shown in Figure 11a and b. The corresponding structure at 315 K, shown in Figure 13b, shows a similar location and orientation for the o-xylene but the position is not as well-defined because its r 2 values are smaller than those at 273 K. However, it still confirms the integrity of the complex over the 42 °C temperature range. o-Xylene-d6/ZSM-5 CP Drain Experiments. As the CP studies of o-xylene/ZSM-5 indicated, the general behavior places



TABLE 5: Values of k′IS for CP and kIS from the CP Drain Plots at 273 K Si8 Si2 Si12 Si11 Si9 Si6 Si5 Si10 Si3,4 Si1,7

CP (s-1)

CP drain (s-1)

9.34(34) 7.35(13) 5.38(36) 4.99(31) 8.86(26) 5.94(13) 5.12(19) 4.85(6) 4.86(23) 7.01(23)

4.55(153) 4.39(158) 2.51(155) 3.88(140) 4.36(167) 3.93(171) 3.75(230) 3.14(137) 2.68(117) 4.34(144)

the spin dynamics of the system in the “slow CP” regime. In this situation, it is important, if possible, to confirm the results using an alternative technique to obtain absolute values for kIS. The variable contact time CP drain experiment is very appropriate for such a study, although it has disadvantages in that the relaxation delay of 29Si can be considerably longer than that of 1H, and the CP drain plots are from the differences between reference and drain spectra, significantly reducing the final signal-to-noise because absolute kIS values are obtained from a single parameter fit to the final data. Figures 14 and S11 show the CP drain plots (eq 4) for o-xylene-d6/ZSM-5 at 273 and 315 K, respectively, where each plot of the resolved 29Si T-sites of ZSM-5 is considerably poorer than those from the traditional CP experiments. However, each depends on only a single parameter, and when the kIS values from the CP drain plots are compared to those from the traditional CP experiment, both values are comparable in magnitude (Table 5 for 273 K and Table S2 for 315 K). One should note that the CP curves can be deceptive in that they are in the slow exchange regime where the rises and maxima are determined mainly by kI while the decays, which are slow and not fitted quite as well as the overall curve, have the largest contribution from kIS. In fact, the kIS values from the CP drain data, even with larger errors, yielded the structure of the complex. One of advantages of CP drain experiment is that the value of kS, which is not determined in the traditional CP experiment, is simultaneously obtained from the reference part of the CP drain data. Because the reference part of CP drain follows eq 2 for the signal decay, plotting the reference signal against contact

time should yield kS. The values of kS from the CP drain data are shown in Table S3. Although they have relatively large errors, they have comparable values to the determined kIS values that in turn are shorter than the kI values in the slow exchange regime. Solving the Structure of the o-Xylene-d6/ZSM-5 Complex. The same software used for the previous CP experiments was also used to solve the structure of o-xylene-d6 molecules in the framework of ZSM-5 from the CP drain data. Initially, all the physically possible locations were sampled as described previously. For the final calculations, for the 273 K data, translations in x (step size of 0.010 Å) between 0.42 and 0.55 in fractional coordinate, y (step size of 0.010 Å) between 0.10 and 0.40 in fractional coordinate, and z (step size of 0.016 Å) between -0.12 and 0.020 in the fractional coordinate along with the orientations with φ (step size of 3°) between 30 and 70°, θ (step size of 5°) between 25 and 150°, and ψ (step size of 3°) between -30 and 40°, were used. A total of ca. 35 million possible locations were tested in the beginning, and 458 acceptable solutions were found with r 2 g 0.71. For the 315 K data, a similar sampling regime was used. In this case, a total of ca. 33 million possible locations were tested in the beginning, and 1278 acceptable solutions were found with r 2 g 0.81. Figure S12 shows the distributions of acceptable solutions found for the r 2 values indicated in the captions. For 273 K, a total of 321 solutions were found, which satisfy r 2 g 0.72. At 315 K, a total of 917 solutions were found, which satisfy r 2 g 0.82. The black arrows on the curves represent the average values of the different parameters and are summarized in Table 6. Using these parameters, the location and orientation of the o-xylene in the framework was determined and the atomic coordinates of the atoms in o-xylene molecule calculated as listed in Table S4. Figure 15 shows the degree of linear correlation between kIS and the corresponding calculated M2 values from the average locations of the o-xylene molecule at 273 and 315 K. The lower r 2 value at 273 K (0.802) than at 315 K (0.909) indicates that the 273 K data may yield the lessreliable structure. When the two correlations are compared qualitatively in terms of their average parameters, it would appear that the better distribution of single resonance silicon T-sites over the possible range of M2 values at 315 K leads to a better correlation of the data at 315 K than that at 273 K.

Figure 15. Plot of the measured CP rate constants, kIS from the CP drain experiments against the calculated hetronuclear second moments for the average locations of o-xylene for ZSM-5 loaded with ca. four molecules of o-xylene-d6 per u.c. with (a) r 2 g 0.72 at 273 K and (b) r 2 g 0.82 at 315 K. The solid lines are the lines of best fit, and the dashed lines represent the 99% and 95% confidence prediction intervals, respectively. Each kIS value comes with its error range represented by the error bars. The 99% confidence prediction interval for the data at 273 K was chosen to obtain a number of solutions comparable to that at 315 K.


Fyfe and Lee

Figure 16. NMR determined structures of the o-xylene-d6/ZSM-5 complex at 273 and 315 K from the CP drain data. The left figures show the location of the o-xylene from the zigzag channel of ZSM-5, and the right, the location from the straight channel.

TABLE 6: Average Values of the Six Structural Parameters of o-Xylene-d6 in the Framework of ZSM-5 at 273 and 315 K with r 2 g 0.72 and r 2 g 0.82, Respectively, from the CP Drain Experiments temperature

x

y

z

φ

θ

ψ

273 K 315 K

0.485(7) 0.491(6)

0.300(9) 0.290(12)

-0.045(10) -0.042(9)

51.1(35) 42.0(37)

89.0(73) 87.5(125)

9.5(90) 5.6(112)

From the linear correlation graphs, the calculated values of kIS were obtained for the overlapping resonances and used to calculate the complete NMR spectra. The calculated NMR spectra were then compared to the experimental spectra for one specific contact time. Figures S13 and S14 (Supporting Information) show the difference spectra between the experimental and predicted NMR spectra from the average of the acceptable structures at 273 and 315 K, respectively, from the CP drain experiments. The experimental data have a high noise level as expected from the nature of the experiment. The scatter plot of the solutions in the plane of the molecule is given in Figure S15. Figure 16 presents the structures of the complex at 273 and 315 K, both of which again indicate that o-xylene is located at the channel intersection between the zigzag channel and straight channel of the ZSM-5 framework with its long axis approximately parallel to the b axis. Comments on the Structures Determined by NMR. From the NMR experiments, a total of six structures have been

determined using two different samples, at two temperatures and by two types of experiment. In general, all of the structures are in good agreement. To judge the reliability of each of them, one should consider many different factors such as the r 2 values for the linear correlations between the kIS and M2 values, the numbers of solutions, distributions of solutions, differences between the experimental and calculated NMR spectra, and so forth. Overall, the most reliable structures seem to be those of o-xylene-d6/ZSM-5 from CP experiments at the two temperatures when the parameters listed above are compared to those of the other structures. However, one should note that NMR determines the positions of the protons of the o-xylene and thus the position of the benzene ring of the o-xylene molecule in the case of o-xylene-d6/ZSM-5. The positions of the methyl groups of the o-xylene molecule may well be better reflected in the experiments on o-xylene-d4/ZSM-5 although these yielded less well-defined positions because of the inherently weaker dipolar couplings involving the protons of the methyl groups.

Solid-State NMR Determination Therefore, the results on two different samples should be viewed as complementary to each other because they probe different parts of the o-xylene molecule. In the case of CP drain structures, the structure at 315 K seems more reliable than that at 273 K because it has better fit parameters and agrees with the CP determined structures. However, the importance of the CP drain experiments extends beyond the determined structures as discussed previously, and one should view the results accordingly. Overall, all six structures are in good agreement for the position of the benzene ring in o-xylene, and the orientations of the long axes vary only very slightly. Conclusions Generally, all of the experiments that yielded reasonable numbers of solutions at high r 2 values give very consistent results for the structure of the o-xylene/ZSM-5 complex. At two different temperatures over a 42 °C temperature range, the structures of all turned out to be very similar. The o-xylene molecule is located at the intersection between the straight and zigzag channels of the ZSM-5 framework from all data sets. The arbitrary long axis of the o-xylene is almost parallel with the a axis for all data with only slight deviations, and one methyl group is oriented along the channel axis while the other points toward the zigzag channel. We feel that the NMR technique, in this case, has yielded a very reliable complete threedimensional structure of the o-xylene/ZSM-5 complex. Single-crystal XRD studies of the system failed to yield any solution because of an inability to load the o-xylene uniformly in the large crystal needed for standard X-ray experients due to the very slow diffision of the organic; at lower temperatures (80 °C), equilibrium was not established after several months and isomerization of the organic occurred at higher temperature (150 °C). However, a current analysis of a powder neutron diffraction experiment using perdeutero o-xylene and the same small crystals as the NMR study confirms the structure presented here. Acknowledgment. C.A.F. acknowledges the financial assistance of the Natural Sciences and Engineering Research Council (NSERC) of Canada in the form of Discovery and Equipment Grants. Supporting Information Available: All figures and tables referred to but not presented in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Breck, D. W. Zeolite Molecular SieVes; Academic Press: London, 1978. (2) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Wiley: New York, 1978. (3) van Koningsveld, H. Acta Crystallogr. 1990, B46, 731. (4) van Koningsveld, H.; Jansen, J. C. Microporous Mater. 1996, 6, 159. (5) van Koningsveld, H.; Jansen, J. C.; Bekkum, H. v. Acta Crystallogr. 1996, B52, 140. (6) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Zeolites 1990, 10, 235.

J. Phys. Chem. C, Vol. 112, No. 2, 2008 513 (7) van Koningsveld, H.; Jansen, J. C.; de Man, A. J. M. Acta Crystallogr. 1996, B52, 131. (8) van Koningsveld, H.; Koegler, J. H. Microporous Mater. 1997, 9, 71. (9) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1989, B45, 423. (10) van Koningsveld, H.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1987, B43, 127. (11) van Koningsveld, H.; Jansen, J. C.; De Man, A. J. M. Stud. Surf. Sci. Catal. 1995, 98, 61. (12) Fyfe, C. A.; Diaz, A. C.; Grondey, H.; Lewis, A. R.; Fo¨rster, H. J. Am. Chem. Soc. 2005, 127. 7543. (13) Lewis, A. R. Ph.D. Thesis, University of British Columbia, 1998. (14) Fyfe, C. A.; Brouwer, D. H. J. Am. Chem. Soc. 2006, 128, 11860. (15) Fyfe, C. A.; Brouwer, D. H. Can. J. Chem. 2006, 84, 345. (16) Fyfe, C. A.; Brouwer, D. H.; Tekely, P. J. Phys. Chem. A 2005, 109, 6187. (17) (a) Haag, W. O.; Olson, D. H. U.S. Patent 3856871, 1974. (b) Weisz, P. B. Pure Appl. Chem. 1980, 52, 2091. (c) Chen, N. Y.; Degnan, T. F., Jr.; Smith, C. M. Molecular Transport and Reaction in Zeolites. Design and Application of Shape SelectiVe Catalysis; VCH: New York, 1994. (d) Martens, P. M.; Tielen, M.; Jacobs, P. A.; Weitkamp, J. Zeolites 1984, 4, 98. (18) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981, 85, 2238. (19) (a) Richards, R. E.; Rees, L. V. C. Zeolites 1988, 8, 35. (b) Karge, J.; Pfeiffer, H. Zeolites 1987, 7, 90. (20) Nagy, J. B.; Derouane, E. G.; Resing, H. A.; Miller, G. R. J. Phys. Chem. 1983, 87, 833. (21) Nair, S.; Tsapatsis, M. J. Phys. Chem. B 2000, 104, 8982. (22) Nair, S.; Tsapatsis, M. J. Phys. Chem. B 2001, 105, 1276. (23) Fyfe, C. A.; Feng, Y.; Gies, H.; Grondey, H.; Kokotailo, G. T. J. Am. Chem. Soc. 1990, 112, 3264. (24) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T. J. Am. Chem. Soc. 1990, 112, 8812. (25) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Kennedy, G. J.; Barlow, G. E. J. Am. Chem. Soc. 1988, 110, 3373. (26) Werstiuk, N. H.; Kadai, T. Can. J. Chem. 1973, 51, 1485. (27) Fyfe, C. A.; Brouwer, D. H. J. Am. Chem. Soc. 2004, 126, 1306. (28) (a) Bax, A.; Freeman, R.; Frenkiel, T. A.; Levitt, M. H. J. Magn. Reson. 1981, 43, 478. (b) Mareci, T. H.; Freeman, R. J. Magn. Reson. 1982, 48, 158. (29) Brouwer, D. H. Location, Disorder, and Dynamics of Guest Species in Zeolite Frameworks Studied by Solid State NMR and X-ray Diffraction. Ph.D. Dissertation, The University of British Columbia, Vancouver, Canada, 2003. (30) Wolfram, S. Mathematica: A System for Doing Mathematics by Computer, 3.0; Wolfram Media: Champaign, IL, 1996. (31) Sheldrick, G. M. SHELX-97 Programs for Crystal Structure Analsysis, 97-2; Institute fur Anorganische Chemie der Universitat, Tammanstrasse 4: D-3400 Gottingen, Germany, 1998. (32) Fyfe, C. A.; Feng, Y.; Grondey, H.; Kokotailo, G. T.; Mar, A. J. Phys. Chem. 1991, 95, 3747. (33) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T.; Ernst, S.; Weitkamp, J. Zeolites 1992, 12, 50. (34) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T. Chem. Phys. Lett. 1990, 173, 211. (35) Fyfe, C. A.; Gies, H.; Feng, Y.; Kokotailo, G. T. Nature 1989, 341, 223. (36) Brouwer, D. H. J. Magn. Reson. 2003, 164, 10. (37) Klur, I.; Jacquinot, J.-F.; Brunet, F.; Charpentier, T.; Virlet, J.; Schneider, C.; Tekely, P. J. Phys. Chem. B 2000, 104, 10162. (38) Tekely, P.; Ge´rardy, V.; Palmas, P.; Canet, D.; Retournard, A. Solid State Nucl. Magn. Reson. 1995, 4, 361. (39) Schaefer, J.; McKay, R. A.; Stejskal, E. O. J. Magn. Reson. 1979, 34, 443. (40) Schaefer, J.; Stejskal, E. O.; Garbow, J. R.; McKay, R. A. J. Magn. Reson. 1984, 59, 150. (41) Stejskal, E. O.; Schaefer, J.; McKay, R. A. J. Magn. Reson. 1984, 57, 471. (42) Pines, A.; Gibby, G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (43) van Vleck, J. H. Phys. ReV. 1948, 74, 1168.

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