High-field 17 O NMR studies of the SiOAl bond in solids

Transcription

1 18 September 1998 Chemical Physics Letters High-field O NMR studies of the SiOAl bond in solids U.-T. Pingel a, J.-P. Amoureux b, T. Anupold c, F. Bauer a, H. Ernst a, C. Fernandez b, D. Freude a,), A. Samoson c a UniÕersitat Leipzig, Linnestraße 5, D Leipzig, Germany b UniÕersite des Sciences et Technologies de Lille, F VilleneuÕe d Ascq Cedex, France c Institute of Chemical Physics and Biophysics, Akadeemia Tee 23, Tallinn, Estonia Received 29 June 1998 Abstract Oxygen- multiple-quantum magic-angle spinning and double rotation nuclear magnetic resonance studies were performed in the field of.6 T. A correlation between the isotropic value of the chemical shift and the SiOAl bond angle in zeolites was found. The correlation may be used for the determination of bond angles in SiOAl sites without long-range order. It is shown that multiple-quantum and double rotation techniques are complementary tools for getting highly-resolved O NMR spectra of solids. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Solid-state NMR techniques have strongly developed during the last two decades and are routinely used for the determination of the coordination geometry in crystalline and amorphous materials. Several correlations between bond angles and values of the chemical shift were found, e.g. the 27 Al chemical shift as a function of the AlOAl bond angle in 29 aluminates by Weller et al. wx 1 or the Si chemical shift as a function of the SiOSi bond angle in different forms of the zeolite Na-ZSM-5 by Fyfe et al. wx 2. The chemical shift of the O NMR signal should be most sensitive to the TOT angle ŽTsSi, Al.. However, there are two large disadvantages of the O NMR spectroscopy, which must be con- ) Corresponding author. freude@physik.uni-leipzig.de quered: The low O natural abundance of 0.037% can be overcome by a post-synthesis isotope exchange with water or oxygen gas enriched in O. The strong anisotropic quadrupole broadening in the spectra of the spin Is5r2 nuclei can be removed by double rotation of the sample Ž DOR., which has been introduced by Samoson et al. wx 3 or by the multiple quantum magic-angle spinning Ž MQ MAS. NMR spectroscopy of the quadrupole nuclei, which has been introduced by Frydman and Harwood wx 4. It has been shown by Bull et al. wx 5 and Amoureux et al. wx 6 that a very high external magnetic field is favourable for O DOR and MQ MAS NMR, respectively. The present study is the first attempt to present values of the O chemical shift as a function of the AlOSi bond angle in zeolites by means of MQ MAS and DOR NMR in the very high field of.6 T. The aim is the determination of AlOSi angles in those materials, which are not accessible for X-ray structure determination r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S

2 346 ( ) U.-T. Pingel et al.rchemical Physics Letters Experimental Zeolites were donated by Wilhelm Schwieger Ž Na-ZSM-5. and by the Zeosorb ŽNa-LSX, a lowsilicate X-type, and Na-A. and treated in a reactor with H O Ž25% or 43% O enriched. 2 at 2508C for some hours in a nitrogen stream. The framework SirAl ratios obtained by 29 Si MAS NMR spectroscopy are 19 for the zeolite Na-ZSM-5 and exactly 1 for the zeolites Na-LSX and Na-A. O 3Q MAS experiment and O DOR experiments were performed in the external magnetic field of.6 T Ž Bruker DMX 750. at MHz and in the field of 11.7 T Ž Bruker MSL 500. at 67.8 MHz. DOR probes home made in Tallin and Bruker MAS probes were used for the MQ MAS experiments. The O NMR scales are referenced to H 2 O. Triple-quantum experiments were performed by means of two strong pulses and an additional weak z-filter pulse wx 7. A total ring down delay of 7 ms after the z-filter pulse and a repetition time of 200 ms were used. A 24-phase cycle was applied twice with an additional 308 phase shift in between, in order to acquire the real and imaginary parts for the second Fourier transform. The non-selective nutation frequency of 100 khz was determined for a H 2 O sample at the Larmor frequency of MHz. For this rf power, the widths of the first and second pulse were adjusted for maximum signal to 3.4 and 1.2 ms, respectively. The selective pr2 pulse length of the z-filter was adjusted to 50 ms. No spinning sidebands appear in the multiplequantum dimension, because t1 was increased in steps of the reciprocal spinning frequency. The time domain consisted of 64 and 1024 steps in t1 and t 2, respectively. 480 scans were accumulated for each t 1 step. The programs XfShear and PULSAR were used for the shearing of the 2D spectra and the simulation of the anisotropic lineshape, respectively wx 8. DOR experiments were performed typically in a synchronized manner wx 9 with outer rotor speed at 1400 Hz and inner rotor speed at Hz. 3. Results and discussion With the Larmor frequency n L, the quadrupole coupling constant Cqcc and the asymmetry parameter h, we have in the case of the nuclear spin Is5r2 and triple or single quantum transitions for MQ Fig. 1. O NMR spectra of the hydrated zeolite Na-A Ž SirAls1.. The sheared 3Q MAS spectrum is presented with anisotropic projection on the top and isotropic projection on the side on the left-hand side. Anisotropic slices of the 2D spectrum for three isotropic values are given on the right-hand side above. The spectrum below is an ordinary MAS spectrum with a fit, which uses the results of the 2D spectrum. For peaks 1 3, cf. Table 1.

3 ( ) U.-T. Pingel et al.rchemical Physics Letters MAS or DOR, respectively, the following equations for the isotropic value of the total shift: =10 Cqcc 1q 3 h d3q MAS isosdcs isoq, n L =10 Cqcc 1q 3 h d DOR sdcs iso y n L The isotropic quadrupole shift for the triple quantum transition is of opposite sign Ž factor y10r. to that for the single quantum transition. The parame- 2 1 ters d and C Ž 1q h 2. CS iso qcc 3 can be obtained from two DOR NMR experiments at two different Larmor frequencies. For the MQ MAS technique all parameters can be obtained from one spectrum. Fig. 1 shows the O 3Q MAS NMR and MAS NMR spectra of the zeolite Na-A, whereas the DOR NMR spectra of the same sample are given in Fig. 3. Three signals, one peak with shoulder and another well-resolved peak can be found in the DOR spectrum in the field of.6 T. This corresponds to three different SiOAl bond angles, which can be deterw10 x, cf. Table 1. Two signals without shoulders are re- mined from the X-ray data of the zeolite Na-A solved in the isotropic projection of the 3Q MAS spectrum. But three slices from the 2D spectrum, cf. Fig. 1, were taken, since the existence of three sites was proved by the DOR spectrum. The deconvolution of the MAS spectrum uses the quadrupole parameters obtained by a simulation of the three anisotropic slices of the MQ MAS spectrum and gives finally the real intensities of three signals. Intensities were obtained also in a direct fit of the center line of the DOR spectrum under the assumption of equal envelope line shapes for the spinning sidebands of all species. The intensities ratio of ca. 1:1:2 for sites O-1, O-2, O-3, respectively, is in good agreement with the relative occurrence of the SiOAl bond angles in the X-ray data. This allows for site 3 the assignment of the isotropic chemical shift to the SiOAl angle. For sites 1 and 2 the assignment is based on the assumption that the chemical shift decreases with increasing bond angle. The spectra of zeolite Na-LSX are shown in Fig. 2Fig. 4. Only three signals are resolved in the isotropic projection of the 2D spectrum or the DOR spectrum, whereas angles for four different oxygen sites can be calculated from the X-ray data of the hydrated zeolite Na-X w11 x. Therefore, the peak in the middle position is fitted by two signals and four slices are used for the determination of the quadrupole Table 1 SiOAl angles determined by XRD as reported in Ref. w10x for the A-type and w11x for the LSX-type, and the parameter obtained from the various O NMR spectra Zeolite Na-A Na-LSX Oxygen site: O-1 O-2 O-3 O-1 O-2 O-3 O-4 SiOAl angle Ž Q d Ž ppm. 3Q MAS iso MAS d CS iso Ž ppm MAS C qcc Ž MHz MAS h MAS intensity Ž % DOR d Ž ppm. DOR 11.7 T DOR d DOR 11.7 T Ž % DOR d DOR.6 T Ž ppm DOR d intensity Ž %. DOR.6 T DOR d Ž ppm ( CS iso 1 DOR C 1q h 2 Ž MHz qcc 3 The peak numbers denote the different SiOAl sites. From the X-ray structure determination it follows that in the X-type zeolite all oxygen sites have the same occurrence, whereas in the zeolite Na-A the population ratio O-1:O-2:O-3 is 1:1:2. DOR intensities for the Na-LSX at.6 T were omitted, since a interference was superimposed to the signal.

4 348 ( ) U.-T. Pingel et al.rchemical Physics Letters Fig. 2. O NMR spectra of the hydrated zeolite Na-LSX SirAls1. The sheared 3Q MAS spectrum is presented with anisotropic projection on the top and isotropic projection on the side on the left-hand side. Anisotropic slices of the 2D spectrum for four isotropic values are given on the right-hand side above. The spectrum below is an ordinary MAS spectrum with a fit, which uses the results of the 2D spectrum. For peaks 1 4, cf. Table 1. The signal at 0 ppm is due to zeolite water. Fig. 3. O DOR spectra of the zeolite Na-A Ž left. and Na-ZSM-5 Ž right. at the external fields of 11.7 T Ž top. and.6 T Ž bottom.. Asterisks denote the outer rotor spinning sidebands of the DOR spectra.

5 ( ) U.-T. Pingel et al.rchemical Physics Letters with the highest value of the isotropic chemical shift at 50.3 ppm corresponds to atoms O1, which is located in the hexagonal prism in bridging position. The relatively high intensity of the corresponding signal might be explained by an isotope effect, the insertion of O at the oxygen bridging positions is favored; however, no isotope effect has been observed for the zeolite Na-A. The data obtained from the O 3Q MAS NMR spectra of the zeolites Na-A Ž SirAls 1. and LSX Ž SirAls 1. yield a correlation between the isotropic value of the chemical shift dcs iso and the SiOAl bond angle f. It holds Fig. 4. O MAS Ž top. and DOR Ž bottom. spectra of the zeolite Na-LSX at the external field of 11.7 T. Asterisks denote the spinning sidebands of the DOR spectra. parameters. The deconvolution of the MAS spectrum gives intensities, cf. Table 1, which are not in agreement with the ratio 1:1:1:1, as expected from the X-ray data. For the assignment of the signals we use again the assumption that the chemical shift decreases with increasing bond angle. Then, the signal dcs iso Ž ppm. sy0.71f q 143.7, with a correlation coefficient r s 0.93, cf. Table 1 and Fig. 5. This equation may be used to obtain the for SiOAl bond angles from the experimentally measured values of d in materials where X-ray structure determinations are not possible: in materials such as glasses and also zeolites with SirAl)1 where X-ray diffraction cannot distinguish between Al and Si sites. For example, Na-ZSM-5 Ž SirAls 19. has a reported isotropic chemical shift of the SiOAl site of Fig. 5. Correlation of O chemical shift d with the SiOAl bond angle f. The points represent the experimentally obtained values, whereas the best fit is given by the straight line.

6 350 ( ) U.-T. Pingel et al.rchemical Physics Letters wx dcs iso s 30" 1 ppm 6. The O 3Q MAS NMR spectrum of the zeolite Na-ZSM-5 wx 6 gives a better resolution than the O DOR NMR spectra measured at 11.7 or.6 T, which are given in Fig. 3. The single SiOAl signal at 30 ppm with a fwhm linewidth of 6 ppm corresponds to angles in the range 160.1" 168. SiOSi angles of the 48 oxygen sites in the zeolite Na-ZSM-5 can be obtained from X-ray data w12 x. A range of 154.1"6.78 is obtained, if the angle distribution is approximated by a Gaussian distribution. It results that no significant difference can be obtained for the SiOAl and SiOSi angles in the zeolite Na-ZSM-5. Dirken et al. w13x determined an isotropic chemical shift of 33"1 ppm using O 3Q MAS NMR for the SiOAl sites in an aluminium silicate glass. The equation above gives a mean angle of for the SiOAl site in the glass. However, more O MQ MAS NMR and O DOR NMR data of crystalline materials are necessary, in order to make a reliable conclusion about the chemical shift, bond angle correlation. Acknowledgements We are grateful to Professor Dr. D. Michel and Dr. Larry Beck for advice. This work was supported by the Deutsche Forschungsgemeinschaft under the projects Fr 902r6-1, Mi 390r5-2 and SFB 294. AS and TA are grateful to the Estonian Science Foundation for supporting development of DOR technology. References wx 1 M.T. Weller, M.E. Brenchley, D.C. Apperley, N.A. Davis, Solid State NMR 3 Ž wx 2 C.A. Fyfe, Y. Feng, H. Grondey, Microporous Mater. 1 Ž wx 3 A. Samoson, E. Lippmaa, A. Pines, Mol. Phys. 65 Ž wx 4 L. Frydman, J.S. Harwood, J. Am. Chem. Soc. 1 Ž wx 5 L.M. Bull, A.K. Cheetham, T. Anupold, A. Reinhold, A. Samoson, J. Sauer, B. Bussemer, Y. Lee, S. Gann, J. Shore, A. Pines, R. Dupree, J. Am. Chem. Soc. 120 Ž wx 6 J.-P. Amoureux, F. Bauer, H. Ernst, C. Fernandez, D. Freude, D. Michel, U.-T. Pingel, Chem. Phys. Lett. 285 Ž wx 7 J.-P. Amoureux, C. Fernandez, S. Steuernagel, J. Magn. Reson. A 123 Ž wx 8 J.-P. Amoureux, C. Fernandez, Y. Dumazy, J. Chim. Phys. 2 Ž wx 9 A. Samoson, E. Lippmaa, J. Magn. Reson. 84 Ž w10x V. Gramlich, W.M. Meier, Z. Kristallogr. 133 Ž w11x H. Toufar, Zeosorb, personal communication. w12x H. van Konigsveld, J.C. Jansen, H. van Bekkum, Zeolites 10 Ž w13x P.J. Dirken, S.C. Kohn, M.E. Smith, E.R.H. van Eck, Chem. Phys. Lett. 266 Ž