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Chem. Mater. 2004, 16, 600-603
Solid-State NMR Studies of the Fluoride-Containing Zeolite SSZ-44 Richard J. Darton,† Darren H. Brouwer,‡ Colin A. Fyfe,‡ Luis A. Villaescusa,†,§ and Russell E. Morris*,† School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife KY16 9ST, U.K., Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver BC, V6T 1Z1, Canada, and Departamento De Quı´mica, Universidad Polite´ cnica De Valencia, Camino De Vera S/N, 46071 Valencia, Spain Received October 8, 2003. Revised Manuscript Received December 16, 2003
Magic-angle spinning NMR has been used to probe the local structure of as-made zeolite SFF containing occluded fluoride and N,N-diethyl-2,6-dimethylpiperidinium. The Si-F bond distance has been measured using two methods: variable contact time cross polarization studies (1.74 Å) and spinning sideband intensity fitting at relatively slow spinning speeds (1.79 Å). Both measurements yield results that are significantly shorter than the internuclear Si-F distance as measured by single-crystal X-ray diffraction (∼1.9 Å), which is based on the average positions of the atoms due to disorder and hence does not reflect the true bond distance.
Introduction The use of fluoride ions in zeolite syntheses has produced some exciting results.1 Among these are the preparations of larger crystals2 that are relatively defect-free on removal of the organic guest molecules and fluoride ions by calcination.3 Two main functions of the fluoride ions in the synthesis of zeolites have been proposed: first, as a mineralizer which improves the solubility of silicate species at near neutral pHs, and second as a catalyst for the formation of Si-O-Si bonds through condensation reactions.3,4 More recently it has been shown by NMR and X-ray diffraction that the fluoride ions that balance the charge of the organic structure-directing agents (SDA), which are occluded into the zeolites during their synthesis, can be covalently incorporated into the framework. In addition, fluoride has also been shown to interact strongly with the SDAs leading to noncentrosymmetric ordering of the organic even when the zeolite framework itself is centrosymmetric.5,6 Previous work on the crystal structures of fluoride-containing zeolites has shown that the fluoride ion usually occupies small cages consisting of four-membered rings and it is thought that fluoride energetically stabilizes these cages.3,7 Nuclear magnetic resonance spectroscopy was the first technique used to study the presence of fluoride in * To whom correspondence should be addressed. Phone: +44 1334 463 818. Fax: +44 1334 463 808. E-mail:
[email protected]. † University of St. Andrews. ‡ University of British Columbia. § Universidad Polite ´ cnica De Valencia. (1) Flanigen, E. M.; Patton, R. L. U.S. Patent 4,073,865, 1978. (2) Kuperman, A. S.; Oliver, S.; Ozin, G. A.; Garces, J. M.; Olken, M. M. Nature, 1993, 365, 239. (3) Camblor, M. A.; Villaescusa, L. A.; Diaz-Cabanas, M. J. Top. Catal. 1999, 9, 59. (4) Barrett, P. A.; Diaz-Cabanas, M. J.; Camblor, M. A.; Jones, R. H. Faraday Trans. 1998, 94, 2475. (5) Bull, I.; Villaescusa, L. A.; Teat, S. J.; Camblor, M. A.; Wright, P. A.; Lightfoot, P.; Morris, R. E. J. Am. Chem. Soc. 2000, 122, 7128. (6) Villaescusa, L. A.; Wheatley, P. S.; Bull, I.; Lightfoot, P.; Morris, R. E. J. Am. Chem. Soc. 2001, 123, 8797.
zeolites.8-10 Koller et al. showed that the five-coordinate species [SiO4/2F]- could be detected using 29Si solid-state NMR spectroscopy, as the silicon nuclei in these units give resonances at about -145 to -150 ppm in the absence of fluoride motion.8 Silicon-fluorine J-coupling splits the peaks for these five-coordinate species into an unequal doublet, which is caused by the combination of the J-coupling, dipolar coupling, and the chemical shift anisotropy. If the fluoride ions are mobile between two Si sites (i.e., dynamic disorder) then a broad averaged resonance centered on around -125 ppm is produced, as seen in [F,-TPA]-MFI by Fyfe and coworkers.10 X-ray crystallography has since been used to confirm the location of fluoride in a number of different zeolites.4,5,11-13 However, in a previous publication we showed that the local structure of the five-coordinate [SiO4/2F]- unit in STF as determined by XRD did not reflect the true local structure because it was an average between [SiO4/2F]- and SiO4/2 units.14 This variation from the true local structure is caused by incomplete occupancy of the fluoride ions in the form of either static or dynamic disorder and often leads to the reporting of incorrect Si-F bond distances of between 1.84 and 1.99 Å. However, using solid-state NMR techniques (7) Koller, H.; Wolker, A.; Villaescusa, L. A.; Diaz-Cabanas, M. J.; Valencia, S.; Camblor, M. A. J. Am. Chem. Soc. 1999, 121, 3368. (8) Koller, H.; Wolker, A.; Eckert, H.; Panz, C.; Behrens, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 2823. (9) Fyfe, C. A.; Brouwer, D. H.; Lewis, A. R.; Chezeau, J. M. J. Am. Chem. Soc. 2001, 123, 6882. (10) Fyfe, C. A.; Lewis, A. R.; Chezeau, J. M.; Grondey, H. J. Am. Chem. Soc. 1997, 119, 12210. (11) van de Goor, G.; Freyhardt, C. C.; Behrens, P. Z. Anorg. Allg. Chem. 1995, 621, 311. (12) Camblor, M. A.; Diaz-Cabanas, M.-J.; Perez-Pariente, J.; Teat, S. J.; Clegg, W.; Shannon, I. J.; Lightfoot, P.; Wright, P. A.; Morris, R. E. Angew. Chem., Int. Ed. 1998, 37, 2122. (13) Attfield, M. P.; Weigel, S. J.; Taulelle, F.; Cheetham, A. K. J. Mater. Chem. 2000, 10, 2109. (14) Fyfe, C. A.; Brouwer, D. H.; Lewis, A. R.; Villaescusa, L. A.; Morris, R. E. J. Am. Chem. Soc 2002, 124, 7770.
10.1021/cm034976x CCC: $27.50 © 2004 American Chemical Society Published on Web 01/24/2004
NMR of Fluoride-Containing Zeolite SSZ-44
(including REDOR, variable contact time cross polarization studies, and spinning sideband intensity fitting)14 we measured the Si-F bond distance in STF as 1.74 Å which agreed well with predictions made by Attfield and co-workers, who used density functional theory simulations on fluoride ions in SOD and FER frameworks to predict that the [SiO4/2F]- units are very close to trigonal bipyramidal with Si-F bond distances of between 1.71 and 1.76 Å.15 This information from the solid-state NMR measurements was then used to re refine the structure and show that this model was indeed consistent with the single-crystal XRD structure. A similar situation was found for F-MFI where the disorder is dynamic rather than static in nature.16 The as-synthesized SFF framework is closely related to the STF framework, with both consisting of a basic building unit made up of two fused [415262] cages.17 We are particularly interested in these materials, as it is possible, by controlling the temperature and amount of solvent present, to prepare a series of SFF/STF intergrowth samples with regions of SFF structure varying from 0% all the way to 100%.18 This may be an especially important method for controlling other zeolite intergrowth structures. However, despite the fact that it is the water content in the synthesis that seems to control the phase composition and intergrowth nature of the material, the fluoride does seem to be a necessary component. It is important then that we ascertain the role of the fluoride ions in such a synthesis. The first step in such studies is to accurately model the location and local structure of the fluoride ion. Given the difficulties in obtaining the true local structure of the fluoride from single-crystal X-ray diffraction data as described above, solid-state NMR is the most important technique for such studies. The aim of this work is to investigate the structure of SFF and to accurately determine the Si-F distance in the five-coordinate [SiO4/2F]- unit by solid-state NMR using a sample of as-synthesized purely siliceous SFF zeolite. Experimental Section The sample of fluoride containing pure silica SFF was synthesized using the SDA N,N-diethyl-2,6-dimethylpiperidinium (DECDMP, 1).19 Synthesis of the single crystals of SFF was as follows. Tetraethyl orthosilicate was hydrolyzed by the addition of an aqueous solution of the hydroxide form of DECDMP. The ethanol produced by the reaction and the excess water were allowed to evaporate before HF (48%) was added and the gel was stirred by hand. The final molar ratios in the synthesis were 0.5:0.5:1:4 DECDMP/F/SiO2/H2O. The reaction mixture was then sealed in a PTFE-lined autoclave and heated to 170 °C for 7 days. The resultant small white crystals of [F, DECDMP]-SFF were recovered by filtration and dried in air. Microcrystal X-ray Diffraction. The single crystals of [F, DECDMP]-SFF were too small (15 × 8 × 5 µm) for the collection of data using our in-house diffractometers, so microcrystal XRD was carried out at the high-flux single(15) Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708. (16) Fyfe, C. A.; Brouwer, D. H.; Lewis, A. R.; Chezeau, J.-M.; J. Am. Chem. Soc. 2001, 123, 6882. (17) Wagner, P.; Zones, S. I.; Davis, M. E.; Medrud, R. C. Angew. Chem., Int. Ed. 1999, 38, 1269. (18) Villaescusa, L. A.; Zhou, W.; Morris, R. E. Unpublished results. (19) Villaescusa, L. A.; Puche, M.; Camblor, M. A. International Symposium on Zeolites and Microporous Crystals, ZMPC 2000, Sendai, Japan, August 2000, Book of Extended Abstracts, 2-P-015, 2000.
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Figure 1. SFF Framework showing fluoride ions (green) but without SDA molecules. crystal diffraction station 9.8 at the CCLRC Daresbury Laboratory Synchrotron Radiation Source, Cheshire, UK.5 The data were collected at low temperature (150 K) using a Bruker AXS SMART CCD area-detector diffractometer with X-rays of wavelength 0.689 Å horizontally focused by a silicon (111) monochromator and vertically focused using a bent palladiumcoated zerodur mirror. The refinements of the data were carried out by least-squares methods using the programs SHELX-9720 and WINGX.21 However, the quality of the crystals was poor and the results from the refinement were not of sufficient standard to yield any more than the gross structural features of the material. Solid-State NMR. The solid-state NMR experiments were performed on a Bruker Avance-400 spectrometer operating at frequencies of 400.13 MHz for 1H, 376.434 MHz for 19F, and 79.495 MHz for 29Si. All the experiments were carried out at ambient temperature using a Bruker 4-mm MAS probe, which is capable of spinning the sample up to 15 kHz. The 29Si chemical shifts were referenced to tetramethylsilane (TMS) with Q8M8 (the cubic octamer Si8O12[OSi(CH3)3]8) or octadecasil as a secondary reference. The 19F chemical shifts were referenced to CFCl3 with octadecasil again as a secondary reference. The pulse lengths and cross polarization matching conditions were first determined on octadecasil and then more accurately on the SFF sample. 1 Hf29Si CP MAS spectra were collected with a 1H 90° pulse length of 3.6 µs and a recycle delay of 3 s, and with a contact time of 3 ms and an acquisition time of about 50 ms. The matching condition for 29Si was set at the +1 spinning sideband condition at both 1.9 and 15 kHz spin rates. The 19Ff29Si CP MAS experiments were all run with a 15-s (∼T1) recycle delay and no decoupling, and the samples were spun at 12 kHz. The matching condition was set to the +1 spinning sideband with a 19F 90° pulse of 5.1 µs and a 29Si pulse of 4.1 µs. A recycle delay of 15 s was used to obtain the best S/N. Calculations. The simulation and fitting of the CP curves were performed using programs written for Mathematica, and the simulation and fitting of the spinning sideband manifolds were performed using the SIMPSON simulation program.22
Results and Discussion The basic framework of siliceous SFF consists of onedimensional pores with 10-membered ring windows all built up from a basic building unit consisting of two [415262] linked cages (Figure 1). The single-crystal XRD (20) Sheldrick, G. M. SHELX-97, Programs for Crystal Structure Analsysis v.97-2; Institute fur Anorganische Chemie der Universitat of Gottingen: Gottingen, Germany, 1998. (21) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (22) Bak, M.; Rasmussen, J. T.; Nielson, N. C. J. Magn. Reson. 2000, 147, 296.
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Figure 2. (a) Deconvolution (least-squares refinement) of the 1Hf29Si CP MAS spectrum (12 kHz spin rate) of [F,DECDMP]SFF with 16 peaks of equal area. (b) Comparison of the 29Si{1H} CP MAS spectrum (upper) and 19Ff29Si CP MAS spectrum (lower) collected at a spinning rate of 12 kHz. The 19Ff29Si contact time was 1 ms.
data confirmed the framework topology but was of poor quality, and certainly not high enough to enable the refinement of the disordered DECDMP+ cations. The XRD study did show that the fluoride ions were disordered over two identical sites within the [415262] cages, giving the structure shown in Figure 1 where there are eight T-sites in the asymmetric unit due to the pseudo symmetry from the disorder (and not the 16 T-sites from the true local symmetry). This structure gave a Si-F bond distance of 1.89(2) Å, which because the fluoride ion is disordered, is greater than the true bond distance. Fast-Spinning 29Si spectra. The fast-spinning (12 kHz) 1Hf29Si CP MAS spectrum of [F,DECDMP]-SFF is shown in Figure 2a. The peaks are not as well resolved as those in our previous work on STF,14 but it is still possible to deconvolute the spectrum into sixteen peaks of equal intensity, which is twice as many as predicted from the average XRD structure. This is because the XRD results indicate the spatially averaged symmetry, whereas NMR probes the real local symmetry of the fused cage pair. This indicates the disorder is static in nature and implies the diffraction experiment result is a pseudosymmetry. The spectrum is expected to be close to quantitative as there are many 1H nuclei well distributed throughout the structure, and each silicon site is on the surface of the SDA-containing channel. The J-coupled doublet centered at -147 ppm is the five-coordinate silicon with a coupling of 160 Hz and this indicates unambiguously the presence of a Si-F covalent bond with no exchange between different sites (Figure 2). Measurement of Si-F distance by MAS NMR. A series of 19Ff29Si CP MAS9 experiments at variable contact times were acquired for the [F,DECDMP]-SFF sample at 12 kHz spinning rate. The peak area for the five-coordinate silicon species was plotted as a function of contact time, and the oscillations were fitted according to the equations given in ref 9 (Figure 3). This gives a value for the effective dipolar coupling constant (if we assume the anisotropy in J (∆J) ≈ 0) of 4.30 ( 0.15 kHz, corresponding to a Si-F internuclear distance of 1.72 ( 0.02 Å. This distance is shorter that that from the XRD data and is in agreement with the distance determined for [F,DECDMP]-STF. The 19F/29Si dipolar coupling constant was also obtained by fitting the spinning sideband profiles in the slow-spinning (1. 9 kHz) 1Hf29Si CP MAS spectrum.
Figure 3. Experimental and simulated 19Ff29Si CP curves for the five-coordinate Si of [F,DECDMP]-SFF at the +1 spinning sideband matching condition with a 12 kHz spin rate (80 scans per spectrum, 15 s recycle time). The data were fitted with a dipolar coupling of 4.30 ( 0.15 kHz, corresponding to a F-Si internuclear distance of 1.74 ( 0.02 Å. The solid line is the best fit and the dashed lines represent estimates of the error limits.
Figure 4. Slow spinning (1.9 kHz) 1Hf29Si CP MAS spectra of [F,DECDMP]-SFF. (a) 1Hf29Si CP MAS spectrum with 3 ms contact time (48 000 scans, 3 s recycle time). (b) Intensities of the isotropic and spinning sidebands of five-coordinate peaks extracted by subtraction of the Q4 silicon resonances and the background from the experimental 1Hf29Si CP MAS spectrum; intensity scaled by a factor of 4 relative to spectrum (a). (c) Simulated effective chemical shift anisotropy (CSA) patterns of the doublets of the five-coordinate peak: (x) δiso ) -148.1 ppm, δ′aniso ) 2.0 kHz, h ) 0; (o) δiso ) -146.0 ppm, δ′aniso ) 9.8 kHz, h ) 0.
This can be done because the peaks of the doublet for the five-coordinate species no longer have equal intensities (Figure 4), an observation previously made by Koller
NMR of Fluoride-Containing Zeolite SSZ-44
et al.7 The difference in intensity is due to interplay between the chemical shift anisotropy (CSA), dipolar coupling, and the J-coupling tensors as fully explained in ref 14. The spinning sideband manifolds can be simulated well assuming an axially symmetric CSA tensor (η ) 0) with effective shielding anisotropy values of 9.8 kHz and 2.0 kHz. Using the equation D ) (µ0γSiγFh)/(16π3rF-Si3) where γI is the gyromagnetic ratio for nucleus I and rF-Si is the silicon fluoride bond distance, the effective dipolar coupling constant is calculated to be 3.9 kHz which corresponds to an internuclear distance of 1.79 Å if it is assumed that the anisotropy in J (∆J) ≈ 0. The error in this measurement is probably larger than that from the variable contact time CP measurement above because of the difficulties involved in fitting the sidebands accurately, but it is shorter than the distance measured by XRD and close to that obtained for the zeolite STF.14 Conclusions MAS NMR has enabled us to accurately measure the Si-F internuclear distance in [F,DECDMP]-SFF, which
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agrees in magnitude with that of our previous study.14 Without the use of MAS NMR to probe the local structure, the data from XRD alone (at least at the resolution at which it is normally collected) could never be used without some doubts about its accuracy. A higher-resolution XRD experiment and the knowledge of the true local structure around the five-coordinate [SiO4/2]- unit from the MAS NMR should enable the full elucidation of the structure of [F-DECDMP]-SFF by diffraction. However, this will require the preparation of higher-quality single crystals, which may or may not be possible in this system. Further work to address this point is in progress. Acknowledgment. We thank the EPSRC for funding. R.E.M. thanks the Royal Society for provision of a research fellowship and L.A.V. thanks the Spanish Ministry of Sciences and technology for additional funding through the “programa Ramo´n y Cajal”. CM034976X
