Solid State NMR Method for the Determination of 3D Zeolite

A description of the range of applications of these experiments in chemical systems is given in: (a) Fyfe, C. A. Solid State NMR for Chemists; CFC Pre...
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Solid State NMR Method for the Determination of 3D Zeolite Framework/Sorbate Structures: 1H/29Si CP MAS NMR Study of the High-Loaded Form of p-Xylene in ZSM-5 and Determination of the Unknown Structure of the Low-Loaded Form Colin A. Fyfe,* Anix C. Diaz, Hiltrud Grondey,† Andrew R. Lewis, and Hans Fo¨rster‡ Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, B.C., V6T 1Z1, Canada Received November 8, 2004; E-mail: [email protected]

Abstract: A general protocol is described for structure determinations of organic sorbate-zeolite complexes based on the selective, through-space, distance-dependent transfer of magnetization from protons in selectively deuterated organics to framework silicon nuclei. The method was developed using the known structure of the high-loaded ZSM-5/p-xylene complex containing p-xylene-d6 or p-xylene-d4. It was then applied to determine the unknown structure of the low-loaded ZSM-5/p-xylene complex using NMR alone. For the high-loaded complex improved data were obtained below 273 K, where slow motions and exchange processes of the p-xylene are eliminated. The general approach was validated by the exact agreement of the experimental 1H-29Si CPMAS spectra obtained at a specific contact time and the complete 24-line spectra simulated using 1/TCP vs M2 correlations from only the six clearly resolved resonances. For the low-loaded complex the 29Si resonances were assigned at 267 K, and variable contact time CP experiments were carried out between 243 and 173 K using the same specifically deuterated p-xylenes. All possible locations and orientations of the p-xylene guests were sampled, and those solutions that gave acceptable linear 1/TCP vs M2 correlations were selected. The optimum p-xylene location in this temperature range was determined to be in the channel intersection with the long molecular axis parallel to [0,1,0] (ring center fractional coordinates {-0.009, 0.250, 0.541}) with the ring plane oriented at an angle of 30 ( 3° about the crystallographic b axis. A subsequent single-crystal X-ray study confirmed this predicted structure.

Introduction

In recent years, high-resolution solid state NMR spectroscopy has emerged as a complementary technique to X-ray diffraction methods for the investigation of the structures of molecular sieve systems.1 In the case of low Si/Al ratio materials, the spectra give insight into the local silicon environments, the distribution of Si and Al over the T-sites, and the effects of chemical treatments, information not directly obtainable from diffraction measurements.2 A very direct connection between the two techniques is made in the case of highly siliceous zeolites whose 29Si MAS spectra show a series of very narrow resonances whose numbers and intensities reflect directly the numbers and occupancies of the crystallographically inequivalent T-sites in the asymmetric unit in the unit cell.3 Further, the isotropic average chemical shifts of the resonances may be correlated †

Deceased. Present address: Bruker Analytik GmbH, Silverstreifen, D-76287 Rheinstetten, Germany. ‡

(1) A description of the range of applications of these experiments in chemical systems is given in: (a) Fyfe, C. A. Solid State NMR for Chemists; CFC Press: Guelph, ON, 1984. (b) Engelhardt G.; Michel, D. High-Resolution Solid State NMR of Silicates and Zeolites; John Wiley and Sons: New York, 1987. (2) (a) Lippmaa, E.; Magi, M.; Samoson, A.; Tarmak, K.; Engelhardt, G. J. Am. Chem. Soc. 1981, 103, 4992. (b) Fyfe, C. A.; Thomas, J. M.; Klinowski, J.; Gobbi, G. C. Angew. Chem., Int. Ed. Engl. 1983, 22, 259. 10.1021/ja0432822 CCC: $30.25 © 2005 American Chemical Society

with average structural parameters from the X-ray structure of the framework, if it is known and if the resonances can be assigned to silicons at specific T-sites.4 Such correlations are most valid when the X-ray data are of high quality, preferably from single-crystal diffraction studies.5 More recently, the link between the two techniques has been further developed by the demonstration that two-dimensional COSY and INADEQUATE experiments can reliably delineate the three-dimensional bonding connectivities (SiA-O-SiB) within the framework.6 In cases where the structure is known, these experiments may be used to assign the resonances in the 29Si MAS spectrum to silicons at specific T-sites where this cannot be done from the relative intensities. Even complex cases can be solved, for example, the monoclinic form of ZSM-5 where there are 24 T-sites, all of which are of equal occupancy.7 In situations where the structures are unknown or poorly (3) (a) Fyfe, C. A.; Gobbi, G. C.; Murphy, W. J.; Ozubko, R. S.; Slack, D. A. J. Am. Chem. Soc. 1984, 106, 4435. (b) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Kennedy, G. J.; Barlow, G. E. J. Am. Chem. Soc. 1988, 110, 3373. (4) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Pasztor, C. T.; Barlow, G. E.; Bradley, S. Zeolites 1988, 8, 132. (5) Fyfe, C. A.; Feng, Y.; Grondey, H. Microporous Mater. 1993, 1, 393. (6) Fyfe, C. A.; Feng, Y.; Grondey, H.; Gies, H.; Kokotailo, G. T. J. Am. Chem. Soc. 1990, 112, 3264. (b) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T.; Gies, H. Chem. ReV. 1991, 91, 1525. (7) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T. J. Am. Chem. Soc. 1990, 112, 8812. J. AM. CHEM. SOC. 2005, 127, 7543-7558

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described, the 2D NMR data may be used to propose possible topologies or space groups for refinement of powder X-ray diffraction data.8 Much of the interest in the structures of these materials derives from a desire to understand their molecular sieving characteristics,9 which are critical to their use as catalysts, sorbents, and host frameworks. However, despite the importance of the problem, few sorbate-framework structures have been determined using diffraction techniques. Although the framework topologies can often be determined from refinements of powder X-ray data, these data are usually too limited to reliably determine the locations of organic guests within the frameworks. The microcrystalline nature of these materials in almost all cases precludes the use of single-crystal X-ray diffraction, an exception being the work of van Koningsveld and co-workers, who have determined the structures of several complexes of ZSM-5 where large enough single crystals can be obtained.10,11 In the first part of the present work we have developed a method using solid state NMR spectroscopy, specifically cross polarization (CP) MAS NMR experiments,12 as an alternative method for determining the three-dimensional structures formed between sorbed organic molecules and molecular sieve frameworks. The CP experiment is based on the heteronuclear dipolar interaction that is through-space and dependent on the internuclear distance, making it possible to obtain three-dimensional structural data. A definite advantage to using the dipolar coupling is its inverse third-power dependence on the internuclear distance, which means that relatively accurate distances can be obtained. In the second part a computer-assisted variation of the same protocol was used to determine the (unknown) structure of the low-loaded form (2-4 molecules/unit cell) with a view to testing the validity of the approach with a subsequent single-crystal structure determination. The system chosen for developing and validating the method was the orthorhombic form of zeolite ZSM-5 containing 8 molecules of p-xylene per unit cell (u.c.), as there is considerable literature data available against which to measure the viability and reliability of this approach. Most importantly, there is an excellent single-crystal X-ray diffraction derived structure by van Koningsveld and co-workers.11 In addition, the 29Si MAS NMR spectrum shows highly resolved signals, and all of them have been assigned to specific T-sites from INADEQUATE experiments.8 Previous work has reported the 2H NMR spectra of partially deuterated p-xylenes in ZSM-513a and the 1H spectra (8) (a) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T.; Mar, A. J. Phys. Chem. 1991, 95, 3747. (b) Fyfe, C. A.; Grondey, H.; Feng, Y.; Kokotailo, G. T.; Ernst, S.; Weitkamp, J. Zeolites 1992, 12, 50. (9) (a) Breck, D. W. Zeolite Molecular SieVes; John Wiley: New York, 1974. (b) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Academic Press: London, 1978. (c) Smith, J. V. Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; 1976. (d) AdVanced Zeolite Science and Applications; Jansen, J. C.; Sto¨cker, M.; Karge, H. G.; Weitkamp, J. Eds.; Stud. Surf. Sci. Catal. 85, Elsevier Science B.V.: The Netherlands, 1994. (10) (a) van Koningsveld, H. J. Mol. Catal. 1998, A134, 89. (b) van Koningsveld, H.; Jansen, J. C.; de Man, A. J. M. Acta Crystallogr. 1996, B52, 131 (c) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Acta Crystallogr. 1996, B52, 140 (d) van Koningsveld, H.; Jansen, J. C. Microporous Mater. 1996, 6, 159. (11) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1989, B45, 423. (12) (a) Hartmann, S. R.; Hahn, E. L. Phys. ReV. 1962, 128, 2042 (b) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (c) Schaefer, J.; Stejskal, E. O. J. Am. Chem. Soc. 1976, 98, 1031. (d) Stejskal, E. O. ; Schaefer, J.; Waugh, J. S. J. Magn. Reson. 1977, 28, 105. (e) Stejskal, E. O.; Memory, J. D. High-Resolution NMR in the Solid State, Fundamentals of CP MAS NMR; Oxford University Press: 1994. (f) Yannoni, C. S. Acc. Chem. Res. 1982, 15, 201. 7544 J. AM. CHEM. SOC.

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of a similar system.13b The 13C NMR spectra of the high-loaded form have been reported to show three resonances for the CH3 groups of the p-xylene molecules.14 There is only very limited structural information available from other methods concerning the structure of the low-loaded form of the p-xylene/ZSM-5 complex. The only diffraction study is the powder X-ray diffraction work of Mentzen and VigneMaeder,15 who proposed that the p-xylene was located at the channel intersection with its long axis along the straight channel, based on a comparison of simulated and experimental powder diffraction patterns. However, two different orientations of the ring gave similar agreement. There are also solid-state NMR experiments by the same group.16 There have also been attempts to predict the structure theoretically:17 Reischman et al.18 considered three possible locations for the p-xylene: the straight channel, the zigzag channel, and the channel intersection. From their calculations, they suggested that the lowest energy structure was that where the molecule was in the channel intersection with its long axis along the straight channel direction, but no further structural details were given. Pickett et al.19 found several energy minima: one in the straight channel and two in the intersection with that in the straight channel being the global energy minimum. Snurr et al.20 studied both benzene and p-xylene: the latter structure was not described in detail but the location was said to be the same as for benzene and was at the channel intersection. A recent review indicates a lack of more recent theoretical studies on the structure of this system.17b A preliminary account of some of the work presented here has been given in ref 21. Once the reliability of the method is clearly established, the longer term goal of the present work is to extend these measurements to other zeolite/sorbate systems of unknown structure where the use of single-crystal diffraction techniques is precluded by small crystallite sizes or other effects. This could improve the understanding of the size and shape selectivity properties that are central to many of the practical applications of these materials. In a broader context these structures represent an opportunity to investigate organic molecule/surface interactions in very well defined systems, as the vast majority of framework sites in these structures are part of internal surfaces that are perfectly ordered and have different shapes in different frameworks. These studies could also be reference points for the study of the interaction of organic molecules with similar but less ordered and less well characterized surfaces. Materials and Methods Sample Preparation. The highly siliceous sample of ZSM-5 was similar to those used previously for 29Si MAS NMR investigations.5-8 (13) (a) Kustanovich, I.; Fraenkel, D.; Luz, Z.; Vega, S.; Zimmermann, H. J. Phys. Chem. 1988, 92, 4134. (b) Kustanovich, I.; Vieth, H. M.; Luz, Z.; Vega, S. J. Phys. Chem. 1989, 93, 7427. (14) Reischman, P. T.; Schmitt, K. D.; Olson, D. H. J. Phys. Chem. 1988, 92, 5165. (15) Mentzen, B. F.; Vigne-Maeder, F. Mater. Res. Bull. 1987, 22, 309. (16) Lefebvre, F.; Mentzen, B. F. Mater. Res. Bull. 1994, 29, 1049. (17) (a) Cheetham, A. K.; Bull, L. M. Catal. Lett. 1992, 13, 267. (b) Fuchs, A. H.; Cheetham, A. K. J. Phys. Chem. B 2001, 105, 1947. (18) Reischman, P. T.; Schmitt, K. D.; Olson, D. H. J. Phys. Chem. 1988, 92, 5165. (19) Pickett, S. D.; Nowak, A. K.; Cheetham, A. K.; Thomas, J. M. Mol. Simul. 1989, 2, 353. (20) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 13742. (21) Fyfe, C. A.; Diaz, A. C.; Lewis, A. R.; Chezeau, J.-M.; Grondey, H.; Kokotailo, G. T. In Solid State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J. J., Ed.; ACS Symposium Series 717; 1999; p 283.

3D Zeolite Framework/Sorbate Structures Samples containing approximately 3, 6, and 8 molecules per u.c. of p-xylene-d6 and p-xylene-d4 were prepared by adding an exactly measured amount of p-xylene-d6 or p-xylene-d4 to a weighed amount of ZSM-5 in a glass vial and then sealing and heating at 353 K for 8 h. The exact loadings were checked before and after the NMR experiments by thermogravimetric analysis (TA Instruments TG51 thermogravimetric analyzer). Previous work on the p-xylene/ZSM-5 system has shown that at loadings between 2 and 4 molecules per u.c. the system is completely orthorhombic (Pnma) with 12 independent T-sites in the asymmetric unit, while between 6 and 8 molecules per u.c. it transforms completely to a second orthorhombic phase (P212121) with 24 independent T-sites.3b,c Quantitative 29Si MAS spectra of a sample with a loading of 4 molecules per u.c. showed the presence of only a small amount of the high-loaded form, but this makes an appreciable contribution to the spectrum when 1H-29Si cross polarization is used due to the polarization transfer being much more efficient in the high-loaded phase, at least at ambient temperature. To ensure that the spectral intensity came exclusively from the low-loaded form, a loading of 3.4 molecules per u.c. was used in subsequent experiments. For simplicity, this is referred to as “3 molecules” in the rest of the text. NMR Experiments. Most experiments were carried out using a Bruker MSL 400 spectrometer equipped with a standard Bruker 7 mm MAS double-tuned probe operating at 400.13 MHz for protons and 79.6 MHz for 29Si. Chemical shifts are referenced to TMS using Q8M8 as intermediate standard. Low-temperature 29Si MAS spectra were obtained using a cooling system designed to work at low temperature for long periods of time with stable spinning rates in the range 2.53.5 kHz. In this arrangement the bearing gas was also the cooling gas and a pressurized 200 L liquid nitrogen Dewar was used as the cold bearing gas reservoir. This precooled bearing gas was cooled further by passing it through a coil immersed in liquid nitrogen in a 25 L Dewar. Dry nitrogen drive gas was required to avoid disruption of the sample spinning due to ice formation. The pressurized 200 L liquid nitrogen supply was also used to provide the drive gas because the Dewar has another output from which gas can be drawn off under the control of a pressure regulator. The temperature was regulated within the probe using a Bruker VT-1000 temperature control unit in conjunction with manual maintenance of the liquid nitrogen level in the 25 L Dewar. Variable-temperature, wide-line deuterium NMR spectra were obtained using a home-built probe that accommodated both 5 mm and 10 mm horizontal solenoid coils. A quadrupolar echo sequence (90x-τ-90y-τ-acq)22 was used, and typical 90° pulse times were ca. 3 and 6 µs for the 5 and 10 mm coils, respectively. Lowtemperature 2H measurements were carried out with a flow of cold nitrogen gas and the temperature regulated using the Bruker VT-1000 temperature control unit. Inadequate Experiments. These were performed using a standard pulse sequence23 in which the last 90° pulse was replaced by a 135° pulse to provide quadrature detection in the double quantum frequency domain. The experiment was carried out at various temperatures, chosen for optimal spectral resolution and reliability of the CP parameters, using the experimental arrangement for working at low temperatures. The 29Si 90° pulse length was 8.25 µs, and the collected data were processed and plotted using the program 2D-WINNMR.24 2D Spin Diffusion Measurements. A standard 2D NOESY sequence25a was used with a cross polarization step substituted for the initial 90° pulse.25b Proton decoupling was applied during both the mixing and acquisition times, and the longest mixing time was limited to 120 ms. (22) Bloom, M.; Davis, J. M.; Valic, M. I. Can. J. Phys. 1980, 58, 1510. (23) (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. (24) 2D Win-NMR; Bruker-Franzen Analytik GmbH: Bremen, Germany, 1996. (25) (a) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R.R. J. Chem. Phys. 1979, 71, 4546. (b) Szeverenyi, N. M.; Sullivan, M. J.; Maciel, G. E. J. Magn. Reson. 1982, 47, 462.

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H/29Si CP Experiments. Variable contact time experiments on the high-loaded complexes of 6 and 8 molecules of p-xylene-d4 and p-xylene-d6 in ZSM-5 were carried out at 293 K, 273 K, and below and those on the complexes of 3 molecules of p-xylene-d6 and p-xylened4 in ZSM-5 at 293, 243, and 233 K using the experimental arrangement for long-term operation at low temperatures described above. The experiments involved variation of the contact times up to 100 ms, and typical 90° pulse times were 11 µs (1H) and 11.2 µs (29Si). The number of scans accumulated for each contact time varied between 1000 and 3000. Additional experiments at 400 MHz were carried out on the lowloaded form at even lower temperatures (down to 173 K) using a DSX spectrometer courtesy of Bruker Spectrospin (see Acknowledgments). Variable contact time experiments on the complexes of 3 molecules per u.c. of p-xylene-d6 in ZSM-5 were carried out at 213, 193, and 173 K, and for a low-loaded p-xylene-d4 sample at 213 K. Details of the fitting of the experimental data and the calculations done to locate the p-xylene molecules within the ZSM-5 framework are presented in the text.

Results and Discussion

General Comments. To determine the geometric relationship between sorbed organics and the silicon atoms of the framework, experiments such as CP,12 transferred echo double resonance (TEDOR),26 and rotational echo double resonance (REDOR)27 must be used, since these are based on the heteronuclear dipolar interaction between the nuclei that is a function of the throughspace internuclear distance. In the present work we examine the potential of the cross polarization experiment to obtain structural information on the high- and low-loaded complexes formed between p-xylene and zeolite ZSM-5. There are a number of factors that could compromise the use of the CP technique for structure determinations, and care must be taken to ensure that they are not affecting the experiments: First, the distances between the protons on the guest molecules and the silicon nuclei in the framework must be reasonably well defined. This was achieved in the present work by investigating two closely related partially deuterated systems: p-xylene-d4 (1), where the polarization source is the protons in the methyl groups, and p-xylene-d6 (2), where the polarization source is the four protons on the aromatic ring. Although these proton spin systems do not qualify as “isolated nuclei”, which is the optimum situation for obtaining unambiguous structural data, it was hoped that their isolation would be good enough to obtain useful structural information. However, because of their relative isolation, they may not qualify as truly abundant spin systems, the normal assumption in the interpretation of CP dynamics.

Second, the CP process can be affected by motions of the guest molecules, and these must be as well-defined as possible. (26) (a) Hing, A. W.; Vega, S.; Schaefer, J. J. Magn. Reson. 1992, 96, 205. (b) Hing, A. W.; Vega, S.; Schaefer, J. J. Magn. Reson. Ser. A 1993, 103, 151. (27) (a) Gullion, T., Schaefer, J. AdV. Magn. Reson. 1989, 13, 57. (b) Gullion, T.; Schaefer, J. J. Magn. Reson. 1989, 81, 196. J. AM. CHEM. SOC.

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In the present work, these were determined directly from the 2H NMR solid state spectra of the complexes. A variety of molecular motions can be taken into account in the theoretical second moment calculations as long as their nature is known. However, translational motions, particularly between inequivalent sites, may possibly invalidate the structural determination, and if these are slow, they may be difficult to detect even by 2H NMR. The cross polarization time constant (T ) values CP obtained from the fitting of the CP data can also be affected by motions; specifically, the proton relaxation time in the rotating frame T1F(Η) is very sensitive to very slow molecular motions. If T1F(Η) is comparable to or less than TCP, the normal assumption that T1F(Η) . TCP can no longer be made, and if this is not recognized, erroneous TCP values will be obtained. Last, the T1F(Η) values can be affected by the amount of sorbate present, which can be varied to a certain extent without introducing structural changes of the sorbate/framework complex, and within this stability region a loading should be selected where the relaxation behaviors, particularly in terms of T1F(Η), are most appropriate. With these factors in mind, extensive studies of both the highand low-loaded complexes were carried out. In the first part, the high-loaded complex where the structure was known is studied to develop a clear and reliable protocol that could be applied to other systems whose structures were unknown. To limit the amount of data presented, although investigations of complexes of both deuterated xylenes were carried out, in the main, only those data from the p-xylene-d6 complexes that are considered more accurate are presented in the paper and in the case of the high-loaded complex, only those data from the 8 molecules per u.c. case are shown. All the other data are presented in the Supporting Information. I: Method Development Using the High-Loaded Form of the p-Xylene/ZSM-5 Complex. The 29Si CP MAS spectra for zeolite ZSM-5 loaded with 6, 7, and 8 molecules of p-xylene are almost identical and indicate that the framework structure is the same at all loadings. As previously reported, the 8 molecule per u.c. form is orthorhombic, space group P212121, and has 24 T-sites in the asymmetric unit.11 The assignment of the different resonances to the appropriate silicons shown in Figure 1b comes from previously reported INADEQUATE experiments.7 In the spectra, the resonances corresponding to silicons 1, 3, 10, 12, 16, and 17 are reasonably well-defined, and particular attention will be paid to these in the structure determinations. Deuterium NMR Spectra. Vega and co-workers13 used the 2H NMR spectra of various deuterated p-xylene/ZSM-5 systems to investigate the motions of the sorbate molecules in detail. Although the system studied was Na+/ZSM-5 and is thus significantly different from that studied here, the theoretical spectra for the different molecular motions such as methyl group rotation and 180° ring “flips” provide excellent reference points for the present work and were used in the interpretation of the spectra described below. The 2H spectra of some of the present samples are shown in Figure 2. The spectrum of the p-xylene-d4 intercalate at 8 molecules per u.c. (Figure 2a) shows a narrow central component, broadened by the exponential multiplication used to enhance the very broad main signal. The intensity of the major broad component in the spectrum is greatly underestimated in the quadrupolar echo experiment because of its very short T2 7546 J. AM. CHEM. SOC.

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Figure 1. (a) 29Si CP MAS NMR spectrum of ZSM-5 loaded with 8 molecules of p-xylene-d6 per u.c., contact time 5 ms. (b) 29Si MAS NMR spectrum of the same sample obtained using a recycle delay of 350 s to ensure complete relaxation of the silicon nuclei. The 90° pulse length was 13 µs. Labels refer to the T-site numbering in ref 11.

Figure 2. Room-temperature 2H NMR spectra of selectively deuterated p-xylenes in ZSM-5 at a loading of 8 molecules per u.c. (a) p-Xylene-d4: 90° pulse length 2.8 µs, echo delay 12 µs, recycle delay 6 s, 5204 scans, 2500 Hz line broadening. (b) p-Xylene-d6: 90° pulse length 6 µs, echo delay 8 µs, recycle delay 1 s, 3676 scans, 200 Hz line broadening.

value, but the splitting of 130 kHz between the central singularities clearly shows that the rings are rigid at this temperature and this loading of organic molecules. In fact, the rings appear to be static up to 333 K in this experiment. The narrow central line is indicative of completely isotropic motion, but the fraction of molecules involved is small (its concentration is estimated to be 0.95 was chosen as the cutoff point for acceptable fits for both deuterated xylene data sets. These solutions were further reduced by considering the Si-H distances involving all 12 silicon atoms. Any solutions with Si-H distances shorter than 2.8 Å were rejected as reflecting repulsive interactions. Note that 2.8 Å is sufficient to also exclude collisions between the p-xylene hydrogens and the framework oxygen atoms. Despite these constraints, 106 possible solutions were found with R2 correlation values between 0.98 and 0.995 (the highest correlation value found). Because of the experimental error in the measurements of the TCP values, it would not be valid to select the location of the p-xylene molecule with the best R2 correlation value as the only criterion for best solution; in fact, the data for the p-xylened4 case provide no information on the ring orientation. One approach would be to screen these solutions to eliminate any that were not common to both complexes, but this was not done in the present study in order to see if the data from the two complexes independently led to the same solution and a different selection procedure was used. To define the expected linear relationship between 1/TCP and (∆ω2)IS, only the intensities of the six best-resolved resonances were used. However, as in the case of the known high-loaded structure, if a linear relationship is established for a particular structure, it can be used to predict the intensities of all 12 resonances in the spectrum, and these can be summed to give the total spectrum for comparison with the experimental data for any given contact time. The theoretical individual Lorentzian lines for the 12 silicon sites were calculated for a contact time of 10 ms to obtain the simulated NMR spectra for each of the 106 solutions. The total areas of the theoretical spectra (obtained by adding the 12 individual Lorentzian peak lines, assuming an average value for Io) as well as the individual Lorentzian areas for the best resolved peaks (Si 10, Si 4, and Si 3) were compared with the experimental values for both complexes. For both complexes the same three structures were found to give the best matches, further confirmation of the validity of the approach. In general, the predicted spectra calculated as described above from these three structures agree well with the two observed ones. This is again an indication of the variation in the position

3D Zeolite Framework/Sorbate Structures

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Figure 13. Plot of the experimental 1/TCP values vs the calculated second moments (for solution 58) for ZSM-5 loaded with 3 molecules of p-xylened6 per unit cell at 233 K.

Figure 12. Predicted structures of the low-loaded form of p-xylene in ZSM5, solutions 14 (green), 58 (black), and 96 (magenta). Solution 58 preserves the Pnma symmetry of the framework.

and orientation of the organic molecule that acceptably fit the data. These three structures (solutions 14, 58, and 96) are quite similar and are shown in Figure 12. For solution 58, shown in bold in the figure, the mirror plane of the p-xylene molecule coincides exactly with the crystallographic mirror plane, with the long molecular axis parallel to the y axis [1,0,0] and the plane of the aromatic ring oriented at an angle of 33° about the y axis. In solutions 14 and 96, the molecules have the aromatic rings oriented almost identically to solution 58 (with an angle of 36° along y), but they are shifted respectively by 0.1 and 0.3 Å from the mirror plane. Also in solutions 14 and 96, the long molecular axes of the p-xylene molecules are shifted respectively 9° and 12° from the [0,1,0] direction and 15° and 6° from the [0,0,1] direction. Because of the mirror plane, solutions 14 and 96 will also have mirror images, but for clarity, these are not shown in the figure. The average atomic positions for solution 96 and its mirror image are very similar to those of solution 58, as are those for solution 14’s average coordinates. The maximum differences in atomic positions between solution 58 and the average solution 96 and 14 are less than 0.5 and 0.3 Å, respectively. These structures all give acceptable fits to the observed data, and when taken together, provide an indication of the precision with which the organic molecule has been located in the framework at this temperature. The 13C CP MAS NMR spectra of the low-loaded complex of [13CH3] p-xylene in ZSM-5 acquired at 293, 253, and 233 K (not shown) show only a single resonance at 19 ppm, which indicates that the chemical environments for the methyl carbons are identical on the “NMR time scale”. Of the three best solutions, only solution 58 has the necessary local symmetry (long molecular axes of the p-xylenes perpendicular to the mirror plane in the channel intersection) to exhibit a single 13C resonance and is favored on this basis. Thus, the data from both complexes yield the same general solution for the location of p-xylene within the ZSM-5 framework. However, effects from slow motions cannot be ruled out. In the case of the high-loaded form, these caused the linear

Figure 14. Intensities of the1H/29Si CP MAS NMR signals as functions of the contact time for ZSM-5 with a loading of 3 molecules of p-xylened6 per u.c. at (a) 213 K and (b) 193 K. The lines represent the best fits calculated using eq 2 in a nonlinear least-squares fitting program with T1F(H) held constant and Io unrestrained.

relationship between 1/TCP and second moments to have a nonzero intercept, and such an effect was observed in the present work at 233 K for both complexes, as illustrated for the p-xylene-d6 case in Figure 13 (Figure SI11 for the p-xylene-d4 case) for solution 58. Similar effects are seen for the other cases. As previously noted, this could be due in part to the high degree of correlation between Io and TCP, but it could also be due to interfering motions including their effect on relaxation times. To resolve this point, additional experiments were carried out on p-xylene-d6 and p-xylene-d4 complexes at lower temperatures with the assistance of Bruker Spectrospin, Karlsruhe. The resulting contact time curves are shown in Figure 14 for the p-xylene-d6 complex. In all cases, the fits of the curves to eq 2 are improved over those at higher temperatures, and in the J. AM. CHEM. SOC.

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ARTICLES Table 5. Experimental and Calculated Parameters from the Least-Squares Fits to the Data from the 1H/29Si CP Experiments on ZSM-5 Loaded with 3 Molecules of p-Xylene-d6 per u.c. at the Temperatures Indicated Io (au)

T-site

TCP (ms)

Si 2 Si 3 Si 4 Si 8 Si 10 Si 12

213 Ka 928 ( 15 854 ( 16 367 ( 20 942 ( 39 567 ( 13 894 ( 13

21.4 ( 0.8 20.9 ( 0.9 45 ( 4 12.2 ( 0.9 36 ( 2 23.2 ( 0.8

Si 2 Si 3 Si 4 Si 8 Si 10 Si 12

193 Kb 916 ( 17 819 ( 17 609 ( 43 847 ( 39 707 ( 23 865 ( 18

32(1 28 ( 1 114 ( 11 15 ( 1 58 ( 3 31.8 ( 0.9

a Calculated with a nonlinear fitting program using eq 2 with T 1F(H) held constant at 245 ms. bCalculated with a nonlinear fitting program using eq 2 with T1F(H) held constant at 278 ms.

case of the p-xylene-d6 complex, the data sets at 213 and 193 K are identical (an almost complete overlap of the resonances of Si 2 and Si 8 precluded the use of a data set obtained at 173 K, but the other curves at this temperature are the same as those at 213 and 193 K). The parameters from these fits are given in Table 5. The same structure-fitting protocol described above for the data at 233 K was carried out with the automated program for the p-xylene-d6 CP data at 193 K. The best solution found was almost identical to solution 58, but the plane of the aromatic ring is inclined 27° about y instead of 33°. This may be due to a thermal effect, as the framework structure could be somewhat different from the X-ray structure determined at higher temperature, while the NMR data reflect the actual situation at these low temperatures. Figure 15 shows the plots of 1/TCP vs the calculated second moments for the best solution, for the complexes of ZSM-5 loaded with 3 molecules of p-xylene-d6. Data for the p-xylened4 complex at 213 K are presented in the Supporting Information (Figure SI12a). For p-xylene-d6, the intercept is zero within experimental error, suggesting that any motions now have a minimal effect on these experiments. A nonzero intercept is still found for the p-xylene-d4 data at 213 K when the location and orientation found from the 193 K p-xylene-d6 data are used to calculate the second moments, although the intercept is smaller than at 233 K. This could be due to the overestimation of TCP from the contact time fits for silicons 4 and 10, which are quite poor due to the very low signal intensities. In addition, slow motions (especially “rocking”) of the p-xylene molecules could have a larger effect on the locations of the methyl hydrogen nuclei than on the ring protons. Theoretical spectra were also calculated using various protocols to assign Io values to those silicons that are not resolved and included in the contact time curve fittings in order to assess the most appropriate method for use in future work. To this end, various possible strategies were tested for their ability to reproduce the CP curves. It was found that using the maximum Io value or the average Io value did not fit the curves very well but that using the empirical dependence between Io and the second moments found in the original fits (Table 5) worked almost as well as the original curve fitting where both TCP and 7556 J. AM. CHEM. SOC.

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Figure 15. Plots of experimental 1/TCP vs the calculated second moments for ZSM-5 loaded with 3 molecules per u.c. of p-xylene-d6 at (a) 213 K and (b) 193 K.

Io were allowed to vary. The calculated spectra for both deuterated p-xylenes for a contact time of 10 ms at the temperatures indicated are shown in Figure 16. There is excellent agreement for the p-xylene-d6 system, and the small deviation in the p-xylene-d4 case is thought to be due to the larger errors in these measurements and the more severe spectral overlap at 213 K. The final fractional coordinates of the predicted structure of the low-loaded complex of p-xylene in ZSM-5 from this work are given in Table 6, and representations of the predicted structure are shown in Figure 17. Within the uncertainties involving the use of the room-temperature framework coordinates of a related (low-loaded) system,10c the approximations in the treatment, and the fact that the data suggest a limited distribution of acceptable orientations, we consider Figure 17 to be a reliable representation of the structure. For the best solution, the p-xylene molecule is located on the mirror plane in the channel intersection (fractional coordinates of the ring center are {-0.009, 0.250, 0.541}) with the long molecular axis parallel to [0,1,0] and the plane of the aromatic ring oriented at an angle of 30 ( 3° about the crystallographic b axis. It is worthwhile to consider the implications of some more general aspects of the present work to NMR structure determinations of zeolite/sorbate complexes: First, the original intent was to check the correctness of the predicted structure by singlecrystal X-ray diffraction. After completion of the work presented here,35 we succeeded in obtaining and refining such single(35) Diaz, A. C. Ph.D. Thesis, University of British Columbia, Canada, 1998.

3D Zeolite Framework/Sorbate Structures

ARTICLES

Figure 16. (a) Experimental 29Si CP MAS NMR spectrum of ZSM-5 loaded with 3 molecules of p-xylene-d6 per u.c. at 193 K with a contact time of 10 ms. (b) 29Si CP MAS NMR spectrum calculated from the 1/TCP vs second moment correlation in Figure 15b for a contact time of 10 ms. Sum of components (black) shown in red. (c) Experimental 29Si CP MAS NMR spectrum of ZSM-5 loaded with 3 molecules of p-xylene-d4 per u.c. at a temperature of 213 K with a contact time of 10 ms. (d) 29Si CP MAS NMR spectrum calculated from the 1/TCP vs second moment correlation. Sum of components (black) shown in red. Table 6. Fractional Atomic Coordinates for the Complex of ZSM-5 Loaded with 3 Molecules per u.c. of p-Xylene-d6 a atom

x

y

z

H-1 H-2 H-3 H-4 H-5 H-6 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 Si 1 Si 2 Si 3 Si 4 Si 5 Si 6 Si 7 Si 8 Si 9 Si 10 Si 11 Si 12

-0.0998 -0.0998 0.0817 0.0817 -0.0090 -0.0090 -0.0090 -0.0624 -0.0624 -0.0090 0.0443 0.0443 -0.0090 -0.0090 0.3044 0.2811 0.1232 0.3041 0.1192 0.1828 0.4230 0.0712 0.1814 0.4217 0.2741 0.0684

0.3098 0.1901 0.1901 0.3098 0.4111 0.0888 0.3203 0.2851 0.2148 0.1796 0.2148 0.2851 0.3947 0.1052 0.0285 0.0629 0.0640 -0.1292 -0.1734 -0.1725 0.0565 0.0276 0.0573 -0.1722 -0.1724 -0.1301

0.6104 0.6104 0.4720 0.4720 0.5412 0.5412 0.5412 0.5819 0.5819 0.5412 0.5005 0.5005 0.5412 0.5412 -0.2022 0.0202 0.0191 -0.1934 0.0217 -0.3290 -0.3460 -0.1923 -0.3423 -0.3336 0.0250 -0.1904

a The p-xylene coordinates were calculated from the NMR data at 193 K for the best solution. The zeolite framework coordinates are those for the room-temperature orthorhombic structure reported in ref 10c. The coordinates of H-5 and H-6 are the centriods of the triangles formed by the hydrogen atoms in the (rotating) methyl groups.

Figure 17. (a) Predicted structure (solution 58) of the low-loaded p-xylene/ ZSM-5 complex showing the orientations of the p-xylene molecules (zigzag channel highlighted). (b) Channel intersection viewed along the straight channel, and (c) the zigzag channel. The silicon T-sites are labeled and oxygen atoms have been omitted for clarity.

crystal diffraction data.36 The structure is identical in all important aspects to that obtained in the present study. The method has thus been validated in a further critical test. A second important point is that the same structure was obtained from measurements carried out over a temperature range of about 40 °C. Although it is very important that slow motions not compromise the distance measurements, it is difficult to monitor their occurrence unless there is some symmetry-induced inequivalence involving the nuclei in the sorbate, which in general (36) Lewis, A. R. Ph.D. Thesis, University of British Columbia, Canada, 1998. J. AM. CHEM. SOC.

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will not be the case. In general, the self-consistency of structures obtained at different temperatures may be the best guide. A further implication of the present data is that some degree of slow motion can be tolerated in these distance-dependent measurements, perhaps because of the constraints imposed by the channels. Productive future research efforts will be in the development of protocols to systematize, for general application, the convergence of acceptable solutions as a function of temperature to a single limiting structure. Conclusions

In general terms, there is good agreement between the cross polarization dynamics and the known structure of the highloaded complex. Our data agree with the locations of the p-xylene molecules in the sinusoidal channels and the channel intersections. Similar results have been obtained using the REDOR pulse sequence on the same systems.35 We feel that there is a high potential for using these techniques to investigate unknown sorbate/framework structures, provided proper precautions are taken, most importantly the avoidance of any motions of the organic sorbates that could affect the results. In the present case, the presence of chemical exchange in the high-loaded form makes the problem obvious, but in general, not all motions will involve exchange and variable-temperature studies will have to be carried out to establish experimentally the temperature where the effects of such motions have been acceptably suppressed. This temperature could well be quite low. In the second part of the present study we have shown that the general protocol that we developed and tested on the (known) structure of the high-loaded form of p-xylene in ZSM-5 can be used successfully to determine an unknown structure, that of the low-loaded form. In the case of the low-loaded form

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of p-xylene in ZSM-5, completely reliable data were obtained only at 213 K and below. The same structure was determined independently from the two different specifically deuterated xylenes and from data collected over a temperature range of about 40 °C. The data also showed that although motions that interfere with the measurements must be avoided, some degree of very slow motion can be tolerated in these experiments. As long as appropriate care is taken, this general approach can be extended to other substrates and other molecular sieve frameworks. A critical test of our approach as described in the present paper will be its application to systems with unknown structures and then a check of the predicted structures by X-ray diffraction techniques. Such a study has already been carried out on the low-loaded structure of p-xylene in ZSM-5, and the predicted structure was identical to that found in a subsequent diffraction study.36 Further experiments of this type are currently in progress, as are extensions and optimization of the general approach described in the present work. Acknowledgment. C.A.F. acknowledges the support of the Natural Sciences and Engineering Research Council (NSERC) of Canada in the form of operating and equipment grants. A.C.D. thanks INTEVEP, Venezuela, for a Postgraduate Scholarship. The authors thank Bruker Spectrospin, Karlsruhe, for their help in obtaining some of the low-temperature spectra referred to in the text. Supporting Information Available: Tables and figures for complexes of 3, 6, and 8 molecules per u.c. of p-xylene in ZSM-5 not presented in the text. This material (in PDF format) is available free of charge via the Internet at http://pubs.acs.org. JA0432822