ARTICLE IN PRESS. Solid State Nuclear Magnetic Resonance

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1 Solid State Nuclear Magnetic Resonance 35 (2009) Contents lists available at ScienceDirect Solid State Nuclear Magnetic Resonance journal homepage: 17 O DOR and other solid-state NMR studies concerning the basic properties of zeolites LSX Denis Schneider a, Helge Toufar b, Ago Samoson c, Dieter Freude d, a Bruker BioSpin GmbH, Silberstreifen, Rheinstetten, Germany b Süd-Chemie Zeolites GmbH, ChemiePark, Bitterfeld-Wolfen, Germany c National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn 12618, Estonia d Institut für Experimentelle Physik I, Universität Leipzig, Linnéstr. 5, Leipzig, Germany article info Article history: Received 12 November 2008 Received in revised form 10 February 2009 Available online 26 February 2009 Keywords: NMR DOR MAS Oxygen-17 Zeolites LSX abstract We demonstrate complementary 1 H, 17 O, 27 Al and 29 Si measurements for basic low-silica-x zeolites, which were unloaded and pyrrole and formic acid-loaded. It was found that the acid base-system is not stabile, if the loading exceeds one pyrrole molecule or two formic acid molecules per supercage. 17 O DOR NMR spectra exhibit at least four lines, which are broadened by a distribution of chemical shifts in a similar extend as the 29 Si MAS NMR spectra are broadened by distribution of Si O Al angles. A strong cation influence upon 17 O shifts was observed. But there was no strong influence of the acid molecules on the mean value of the 17 O shift of the spectra. & 2009 Elsevier Inc. All rights reserved. 1. Introduction Various techniques of the nuclear magnetic resonance are well-established in the investigation of the interaction of molecules with solid surfaces in porous catalysts [1,2]. Acidic properties of hydrogen forms of zeolites are reflected by the proton mobility and can be monitored by 1 H MAS NMR spectroscopy of the framework. The question arises whether the isotropic value of the 17 O NMR chemical shift can reveal those moleculeframework interactions which are denoted as basicity of the zeolite framework [3,4]. An increasing number of 17 O NMR investigations were performed in the last decade [5,6]. The 17 O-enrichement of the sample, an external magnetic field above 14 T and the application of experimental procedures for averaging second-order quadrupolar broadenings are necessary, in order to obtain highly resolved 17 O NMR spectra. Pure siliceous crystalline material like siliceous faujasite [7] and siliceous ferrierite [8] has an excellent shortrange order yielding highly resolved spectra, but no basic properties of the framework. Also crystalline alumosilicates with the silicon-to-aluminum ratio of one, like zeolite Na A and sodalite, exhibit a good short-range order and show well-resolved 17 O NMR spectra [9]. But only the faujasite type zeolite can be synthesized with Si/Al ¼ 1 and can be used as a basic catalyst. Corresponding author. Fax: address: freude@uni-leipzig.de (D. Freude). After publishing the first 17 O NMR results concerning the basicity of the zeolite LSX [10] we became aware that the chemical stability of an ion exchanged and 17 O enriched zeolite LSX during activation (dehydration under vacuum) and loading (with acid molecules) is a crucial point [11]. We improved the ion exchange procedure for the zeolites, changed the 17 O enrichment method and limited strongly the quantity of zeolites and acid molecules [11]. For all samples of the present study it was proved by 1 H, 27 Al and 29 Si MAS NMR spectroscopy that there is no significant change in the sample during 17 O enrichment of the zeolites, their activation, loading with acid molecules and NMR measurements. The main point of this study is how acid molecules influence the basic oxygen framework of the zeolitic catalyst. We apply DOR for measuring the 17 O NMR spectra in the present study, since it completely averages out second-order quadrupolar broadenings and gives a higher resolution than 3QMAS NMR spectroscopy [10]. 2. Experimental 2.1. Zeolites Na 70% K 30% LSX from Tricat Zeolites GmbH is the basic low-silica-x zeolite denoted as Na K LSX. The synthesis of lowsilica-x zeolite with atomic Si/Al ratio close to one has to be performed in the simultaneous presence of Na and K ions [12]. Sodium is required as a template for the formation of the /$ - see front matter & 2009 Elsevier Inc. All rights reserved. doi: /j.ssnmr

2 88 D. Schneider et al. / Solid State Nuclear Magnetic Resonance 35 (2009) secondary building blocks of faujasite type zeolites, specifically for the formation of the truncated sodalite cages; potassium does template the arrangement of these units to cubic faujasite structures. In the presence of sodium only, the sodalite cages will arrange to sodalite (SOD) or LTA structures, depending on the overall alkalinity of the batch, in the presence of potassium only zeolite structures not featuring the sodalite cage like redingtonite (EDI) are formed. Typically, the material obtained from the synthesis has a cation distribution of about 70% Na and 30% K. This material can be converted to other cationic forms by conventional ion exchange. For larger cations, specifically for large alkaline cations like rubidium and caesium, this method is limited to relative low exchange rateso70%, resulting in materials whose basic properties are still dominated by the remaining sodium and potassium ions. A significantly higher degree of exchange, 490% for Rb and 470% for Cs, can be achieved by applying the reaction of the NH 4 -form of the zeolite with alkaline hydroxides [13]. Then a small fraction of cation on sites I (see Fig. 5) remains as NH 4 and causes an intrinsic instability of the material due to the potential formation of Brønstedt acidic sites after deammonization at higher temperatures or interaction with other acids. But we could improve the stability of the zeolite by reaction of the residual NH 4 -sites with a stoichiometric amount of KOH, resulting in a zeolite with 90% Rb and 10% K-cations. Zeolite Na 70% K 30% LSX (Na K LSX) represents the zeolite with the relatively weak basic property, whereas Rb 90% K 10% LSX (Rb K LSX) corresponds to the zeolite with relatively strong basic property due to the larger cations. All cation exchanged forms have Si/Al ¼ as determined by 29 Si MAS NMR. The zeolite was dehydrated at 100 1C under vacuum before 17 O-enrichement. Then 300 ml 20% enriched H 2 O per gram zeolite were added and the zeolite was kept in a fused glass ampoule for one day at 80 1C. After this procedure we measured the 27 Al and 29 Si MAS NMR spectra again. Cation exchanges with only Li, Na, K, or Cs show a broadening of the narrow (about 1.8 ppm) line in the 29 Si MAS NMR spectrum or an appearance of an additional signal at about 0 ppm in the 27 Al MAS NMR spectrum. But such 29 Si broadening and 0-ppm-signal for 27 Al were not observed for zeolites Na K LSX and Rb K LSX. A 17 O-enrichement of about 15% was determined by a quantitative 17 O MAS NMR comparison with a reference sample Sample preparation Acid molecules under study are pyrrole (C 4 H 5 N) and formic acid (CH 2 O 2 ). Other acids like acetic or hydrochloric acid could not be used, since the zeolite framework was not stable after loading these acids. Samples were first dehydrated under vacuum at 400 1C for one day, then loaded at room temperature and finally fused in a glass tube, in order to avoid desorption or water loading. Before the begin of the DOR experiments, we used a glove bag, which was flushed with dried nitrogen for some hours, in order to brake the glass tube and transfer the sample from the broken glass ampoule into the rotor. The glove bag was also used for taking the samples after the 17 O DOR experiment from the DOR rotor into the 4-mm-MAS-rotor, in order to check the sample again by means of 1 H, 27 Al and 29 Si MAS NMR spectroscopy. Some samples were fused within a small tube with 3 mm outer diameter and put into the 4-mm-MAS-rotor, in order to find the maximum loading of the zeolite by 1 H MAS NMR spectroscopy. We obtained a maximum loading of one pyrrole and two formic acid molecules per cavity (1/8 unit cell). 1 H MAS NMR spectroscopy shows that the molecules undergo a chemical reaction for higher loadings, which were used in previous studies [10]. It should be mentioned here that fused sample tubes cannot be used in DOR experiments NMR measurements 1 H MAS NMR measurements were performed on an Avance- 400-spectrometer at n L ¼ 400 MHz and n rot ¼ 10 khz in a 4-mm- MAS-rotor by means of a Hahn echo after one rotation period. Single-pulse excitation was used for the 27 Al MAS NMR spectra at n L ¼ 104 MHz and n rot ¼ 8 khz. 27 Al MAS NMR spectra exhibit, in addition to the signal of the framework aluminum at about 60 ppm, only for damaged zeolites a strong signal at about 0 ppm, which can be denoted to extra-framework aluminum atoms. 29 Si MAS NMR measurements were carried out at n L ¼ 79.4 MHz and n rot ¼ 4 khz in a 4-mm-MAS-rotor with single-pulse excitation. The corresponding spectra show a significant line broadening, if the zeolite has been damaged. 17 O DOR experiments were mainly performed in the external magnetic field of 17.6 T (Bruker AVANCE 750 with wide-bore magnet) at MHz and a nutation frequency of 12.5 khz. The latter corresponds to a selective (central transition) p/2-duration of 6.7 ms. A comparison of DOR spectra of hydrated zeolites Rb LSX, which were obtained at different external fields, has shown that the resolution increases from 14.1 to 17.6 T. But a further increase to 21.1 T (AVANCE 900 Munich) did not yield an improved resolution. Narrow-bore DOR probes were constructed at the Institute of Chemical Physics in Tallinn. The recent design has a maximum outer rotor speed of 2.2 khz. For the present study we used mainly an older design with an outer rotor speed of 1.2 khz (maximum is 1.5 khz) and an inner rotor speed of about 5 khz. Air bearings for inner and outer rotor and a computerassisted pneumatic 4-channel-unit allow DOR operation for a long time (days). The pulse program syncdor suppresses odd spinning side bands [14]. Pulse duration of 6.7 ms was used, in order to obtain the maximum excitation of the central transition. The repetition time was 0.6 s. The 17 O NMR signal of pure water was used as chemical shift reference. 3. Results and discussion Liquid NMR spectra of the acids dissolved in CDCl 3, which are not shown here, exhibit for pyrrole two narrow signals of CH groups at 6.37 and 6.87 ppm and the broad signal (fwhm ¼ 0.5 ppm) of the NH group at 8.19 ppm and for formic acid the narrow signal of the CH group at 8.04 ppm and the broad signal of the OH group at 10.8 ppm [11]. The signals of the acid protons of both molecules are very sensitive with respect to the environment. The signals of the ring protons of pyrrole in zeolites LSX lie in the range ppm. The narrow signal of the formic acid is at 8.04 ppm, if the pure liquid is dissolved in CDCl 3. But it appears at 3.8 and 4.8 ppm, if formic acid is loaded on zeolite Na K LSX and Rb K LSX, respectively [11]. The area under the signal is denoted as NMR intensity. For pyrrole we expect the intensity ratios 1:2:2 for the acid hydrogen atom and the two kinds of ring hydrogen. This ratio is reflected in spectra of the fused sample and in samples which were refilled into the MAS rotor by means of the glove bag [11]. But the spectrum (a) in Fig. 1 exhibit an additional signal at 4.3 ppm, which is caused by a little amount of water penetrated into the samples. It can be seen in spectra (a) and (b) that the acidic hydrogen of the molecules with chemical shifts above 8 ppm is stable during our experiments. The acid signals decrease and finally disappear with increasing loadings higher than one pyrrole or two formic acid molecules per cavity. This can be explained by chemical reactions of the molecules. Spectra of the

3 D. Schneider et al. / Solid State Nuclear Magnetic Resonance 35 (2009) Table 1 Chemical shifts and line widths of 29 Si MAS NMR signals, mean Si O Al angles and widths of the distribution of mean angles for some samples. d (ppm) fwhm (ppm) a (deg) Da (deg) Li LSX, non-enriched, hydrated Na LSX, non-enriched, hydrated K LSX, non-enriched, hydrated Rb LSX, non-enriched, hydrated Cs LSX, non-enriched, hydrated Na K LSX, dehydrated Rb K LSX, dehydrated Na K LSX with formic acid Na K LSX with pyrrole Rb K LSX with formic acid Rb K LSX with pyrrole Samples were enriched in 17 O, if not noted else. 150 Cs samples under study do not change, if they were obtained within one day. But changes occur, if samples were kept for some days. We could prove the concentration of molecules by intensity comparison of the molecule signals with a very weak signal of about 0.2 silanol groups (zeolite framework defects, not noticeable in Fig. 1) per cavity at about 1.2 ppm. It shows that two times sample transfer in the glove bag does not strongly decrease the concentration of adsorbed molecules. The position of the silanol signal depends on the susceptibility of the sample. We learned from the fact that the signal has a chemical shift of 1.2 ppm for both zeolites Na LSX and Rb K LSX that different cations have a small influence on susceptibility shifts in our study. Therefore, we exclude susceptibility effects also for the discussion of 17 O NMR results. 29 Si MAS NMR spectroscopy is also an important auxiliary aid, if 17 O NMR is in the focus of the study. The isotropic chemical shift of the 29 Si NMR signal depends on the four Si O Si angles a i, which occur between neighboring tetrahedral. The dependence is best described in terms of the mean (i ¼ 1, 2, 3, 4) value cos a=ðcos a 1Þ. Radeglia and Engelhardt [15] proposed an equation depending on the number n of Al atoms in the second coordination sphere. With n ¼ 4wehave dð 29 SiÞ=ppm ¼ 223:9 cos a=ðcos a 1Þþ12:8. (1) Freude et al. [10] obtained from 17 O 3QMAS NMR spectra of sodium forms of the zeolites A and LSX the equation dð 17 OÞ=ppm ¼ 214 cos a=ðcos a 1Þþ136. (2) 17 O shifts due to changes of a and line broadenings due to a distribution of Si O Al angles are reflected in the 29 Si MAS NMR spectra, as can be seen from Eqs. (1) and (2). Table 1 contains 29 Si NMR shifts and line widths of several samples. Mean Si O Al angles and the widths of angle distributions were obtained by means of Eq. (1). The mean angles increase with increasing ionic radius [16]. Fig. 2 shows the values for the hydrated zeolites LSX. The distribution of mean angles increases also with the ionic radius and upon dehydration. It should be mentioned that the values Da in Table 1 characterize the fwhm for the distribution of the mean Si O Al angle, which is an average over four angles. Doubled values for the distribution widths of one Si O Al angle 4 Fig H MAS NMR spectra of the zeolites LSX acquired after the DOR experiments: (a) Na K LSX with pyrrole and (b) Rb K LSX with formic acid. 0 4 averaged Si-O-Al angle / Li + Na are expected, if the four angles are independently distributed. Thus, the residual linewidths of the 29 Si MAS NMR yield distribution widths (fwhm) of the (non-averaged) Si O Al angles of 8.61 and 7.41 for the dehydrated zeolites Na K LSX and Rb K LSX, respectively. Fig. 4 shows 17 O DOR NMR spectra. The zeolite LSX has the faujasite structure which is denoted as structure type FAU [17]. Each silicon or aluminum atom is surrounded by four oxygen atoms denoted from O1 to O4. Therefore, we expect four 17 O signals of identical intensity. But well-resolved four signals with nearly identical intensity could be observed in only few cases. Fig. 4 shows as an example two spectra of a hydrated zeolite Rb LSX, which was provided by Dr. V. Bosáček. The DOR spectra consist of four lines. Fig. 4 demonstrates also a special feature of the spectra of quadrupolar nuclei, namely that the displayed shift is not the isotropic chemical shift d CS iso. A quadrupole shift is superimposed to the chemical shift. We have for DOR spectra of I ¼ 5/2 nuclei [18] d DOR =ppm ¼ d CS iso =ppm ionic radius / Å P 2 Q, (3) n 2 L where the quadrupole parameter P Q is defined as P Q ¼ C qcc (1+Z 2 /3) 1/2. C qcc denotes the quadrupole coupling constant and Z is the asymmetry parameter. In a previous study [10] we obtained from all 17 O multiple-quantum MAS NMR spectra of hydrated, dehydrated andloadedzeoliteslsxthevalueofp Q ¼ MHz. Inserting this K + Rb+ Fig. 2. Mean Si O Al angles obtained from the 29 Si NMR shift as a function of the ionic radius for hydrated zeolites LSX containing one predominant (X90%) cation.

4 90 D. Schneider et al. / Solid State Nuclear Magnetic Resonance 35 (2009) value into Eq. (3), we have for n L ¼ and 122 MHz quadrupole shifts of 6.7 and 4.7 ppm, respectively. Therefore, a value of 6.7 ppm has to be added to the shift values in Figs. 3 6 and in Table 2, in order to obtain the intrinsic isotropic chemical shift. The 122-MHzspectrum in Fig. 4 is shifted by 2 ppm with respect to the MHzspectrum due to the different Larmor frequency. We performed a deconvolution of the center bands of the spectra in Fig. 3 into four signals corresponding to the four oxygen sites in faujasite type zeolites, see Fig. 5. Results are presented in Fig. 6 and Table 2. More or less strong deviations from the expected equal intensity of the four sites can be observed. An explanation by an isotopic effect for the 17 O-enrichement of the four oxygen sites is hardly believable. It is more likely that the deconvolution into four signals is not sufficient, since the different occupation of cation sites complicates the situation in the zeolite LSX. Table 2 denotes the oxygen sites from S1 to S4. For dehydrated zeolite Na K LSX corresponds this numbering to the well-established numbering from O1 to O4, which was introduced by Olson [19] for the hydrated zeolite Na-X. Fig. 5 shows the oxygen positions and the cation positions in a dehydrated Na LSX, which were taken from Porcher et al. [20]. Unfortunately, besides Fig O DOR NMR spectra of the hydrated zeolite Rb LSX, which were measured at n L ¼ 122 MHz (on the top) and n L ¼ MHz (on the bottom). Na-K-LSX, dehydr. III III Na-K-LSX, pyrrole III b I O1 III a I Na-K-LSX, formic O3 II O4 O2 II Rb-K-LSX, dehydr. Rb-K-LSX, pyrrole Rb-K-LSX, formic Fig O DOR NMR spectra of zeolites Na K LSX and Rb K LSX which were dehydrated and then loaded with pyrrole or formic acid. Asterisks denote spinning sidebands in a distance of the twofold outer rotor frequency from the center band. 0 Fig. 5. Faujasite structure, oxygen sites after Olson [19] and cation sites in the dehydrated zeolite LSX after Porcher et al. [20]. these X-ray data, very few data about a K-X [21] and a Cs-X [22] exist and data about the LSX zeolite exchanged with other cations or loaded with molecules other than water cannot be found in the literature. Therefore, we have no justification to denote the four lines in the spectrum to the well-defined positions O1 O4. A theoretical consideration of Liu et al. [23] found a systematic influence of T O T angle on the chemical shifts of the 29 Si, 27 Al and 17 O nuclei, but less definite for 17 O. Thus we use S1 S4 instead of O1 O4, in order to denote the signals with decreasing values of the chemical shift. All cation positions I, II, see Fig. 5, and additional supercage sites are occupied in a hydrated zeolite [19]. In this case only four oxygen positions can describe the lattice of the zeolite. For the dehydrated Na LSX we have much sites and the occupation probability becomes low. The distribution of the site occupation is completely unclear for a zeolite loaded with pyrrole or formic acid. It is clear that the deconvolution into four signals is a very rough approximation.

5 D. Schneider et al. / Solid State Nuclear Magnetic Resonance 35 (2009) dehydrated Rb-K-LSX dehydrated Na-K-LSX pyrrole loaded Rb-K-LSX Fig. 6. Deconvolution of some 17 O DOR NMR spectra. Spectrum widths are about 35 ppm. Shift values can be taken from Fig. 4 or Table 2. Table 2 DOR shifts and relative intensities obtained from the deconvolution of the spectra. S1 S2 S3 S4 Average (ppm) Na K LSX, dehydrated 36.6 ppm 25.9% 29.4 ppm 27.8% 26.6 ppm 23.7% 18.7 ppm 22.6% 28.2 Na K LSX,formic acid 36.6 ppm 15.3% 30.0 ppm 33.2% 27.4 ppm 31.7% 19.3 ppm 19.8% 28.1 Na K LSX, pyrrole 38.2 ppm 23.1% 30.8 ppm 32.8% 28.0 ppm 28.1% 19.7 ppm 15.9% 29.9 Rb K LSX, dehydrated 49.7 ppm 34.7% 41.1 ppm 30.3% 38.0 ppm 22.8% 29.1 ppm 12.2% 42.0 Rb K LSX, formic acid 47.0 ppm 24.7% 40.0 ppm 31.9% 36.3 ppm 26.3% 30.4 ppm 17.1% 39.1 Rb K LSX,pyrrole 46.5 ppm 41.7% 43.4 ppm 19.0% 38.9 ppm 18.2% 35.1 ppm 21.1% 42.1 Note that a value of 6.7 ppm has to be added, in order to obtain the isotropic chemical shift. The last column contains the averaged shifts of the four sites. It corresponds to the center of gravity of the center band. Deconvolutions were performed using linewidths of about 10 ppm, see Fig. 6. It corresponds for the dehydrated zeolites to the expected twofold of the broadening of the 29 Si MAS NMR spectra, see Table 1. We can conclude that the DOR spectra do not show any quadrupole broadening for the dehydrated zeolites. But the loaded zeolites exhibit the same broadening of the single 17 O signals, whereas 29 Si signals are narrowed in comparison to the dehydrated and unloaded sample. This can be explained by a stronger distribution of the chemical shift upon loading with a small amount of molecules. Two influences on the 17 O shift were found [10] in addition to the dependence on the Si O Al angle. The dehydration of the zeolite causes a shift of 4 ppm and larger cations increase the shift up to 34 ppm [10]. In the present study we have an increase of 13.8 ppm going from the dehydrated Na K LSX to the Rb K LSX, see Table 2. Now we come back to the main question of this study, whether the 17 O NMR shift reflects interactions of acid molecules with the basic framework of the zeolite. We look at the average shifts in Table 2, which are independent of the assumptions for the deconvolution, and consider the values of the unloaded zeolites and the zeolites loaded with the weak acid pyrrole or the stronger formic acid. A shift of about 5 ppm can be observed for the position S4, if the zeolite Rb K LSX is loaded with pyrrole. This could be a hint that the ring molecule pyrrole influences predominantly one site, in opposite to the stronger formic acid. But we cannot find significant changes in the shift values neither for the less basic zeolite Na K LSX nor for the stronger basic zeolite Rb K LSX aside from this one finding. In conclusion, we could not obtain reliable reflection of the base acid interaction in our 17 O NMR spectra. It could be verified by the comparison with the 29 Si NMR spectra that the resolution of the DOR spectra is limited by the distribution of isotropic values of the chemical shift. It means that the resolution cannot be improved. Some effects derogated the study. Zeolites with only one kind of cations like Li LSX or Cs LSX could not be used, since they were not sufficiently stable. A set of organic acids could not be applied, since already acetic acid, the next after formic acid, is not stable within the zeolite LSX for a sufficiently long time. In addition, the loading with formic acid was limited to two molecules per supercage, i.e. one molecule per 24 framework oxygen atoms. But we measure a time averaged influence of the chemical shift and, therefore, the effect is reduced by a factor of 24 at such a small loading. Another point is that in the dehydrated zeolite LSX exist more than 34 positions for 12 cations per supercage. This causes an unclear distribution of occupations and chemical shift distributions. Last not least we should notice that the interpretation of 17 O NMR shifts is not straightforward. Comparing the Na K LSX and the Rb K LSX we have an increased electron density for the larger cations. The chemical shift difference can be explained by a paramagnetic shift. But we are not sure about the 17 O shift direction under the influence of an acid molecule, since also diamagnetic shifts occur in the 17 O NMR spectroscopy. 4. Conclusions 17 O DOR NMR spectroscopy yields spectra of low-silica-x zeolites, which are not broadened by second-order quadrupole interaction. The resolution of DOR is superior to 3QMAS, since the residual line broadening depends only on the distribution of chemical shifts in a similar extend as 29 Si MAS NMR spectra are broadened by the distribution of Si O Al angles. Topic of the present study is an NMR investigation how acid molecules influence the basic oxygen framework of a zeolitic catalyst. Except the shift of one signal component upon pyrrole loading in zeolite Rb K LSX, we could not find a reliable reflection of the base acid interaction in our 17 O NMR spectra. The stability of the acid-loaded zeolites was monitored by 1 H MAS NMR spectroscopy, which shows clearly the signal of the acid hydrogen atom. Chemical reactions did not occur within a few days, if the maximum loadings were one pyrrole molecule or two formic acid molecules per supercage. Other acids like acetic or hydrochloric acid could not be used, since the zeolite framework was not stable after loading these acids. This could be seen by 27 Al and 29 Si MAS NMR spectroscopy. Also the cation exchange of the zeolite LSX was proved to be a crucial point of this study. Exchange products with one predominant (X95%) cation were not stable after 17 O-enrichment and acid loading. For the basic version of the zeolite we obtained the most stable result by Rb-exchange of the Na K-form and a

6 92 D. Schneider et al. / Solid State Nuclear Magnetic Resonance 35 (2009) weak K-re-exchange. A conservative 17 O-enrichement procedure at 80 1C was necessary, in order to avoid structure defects in the zeolite. Acknowledgments We acknowledge advice from Connie Dee Cuizon Sato. This work was supported by the Deutsche Forschungsgemeinschaft under the projects Fr 902/16 and AVANCE 750 and by the Max-Buchner-Stiftung. We are also grateful to Prof. Dr. H.C.H. Kessler and Dr. R. Haessner for providing measurements on the AVANCE 900. Reference [1] D. Freude, J. Kärger, NMR techniques, in: F. Schüth, K. Sing, J. Weitkamp (Eds.), Handbook of Porous Materials, Wiley-VCH, Chichester, 2002, pp [2] M. Hunger, Applications of in situ spectroscopy in zeolite catalysis, Microporous Mesoporous Mater. 82 (2005) [3] D. Barthomeuf, Si, Al ordering and basicity clusters in faujasites, J. Phys. Chem. B 109 (2005) [4] J. Weitkamp, L. Puppe, Catalysis and Zeolites, Springer, Berlin, Heidelberg, [5] S.E. Ashbrook, M.E. Smith, Solid state O-17 NMR an introduction to the background principles and applications to inorganic materials, Chem. Soc. Rev. 35 (2006) [6] J.E. Readman, C.P. Grey, M. Ziliox, L.M. Bull, A. Samoson, Comparison of the O- 17 NMR spectra of zeolites LTA and LSX, Solid State Nucl. Magn. Reson. 26 (2004) [7] L. Bull, A. Cheetham, T. Anupold, A. Reinhold, A. Samoson, J. Sauer, B. Bussemer, Y. Lee, S. Gann, J. Shore, A. Pines, R. Dupree, A high-resolution 17O NMR study of siliceous zeolite faujasite, J. Am. Chem. Soc. 120 (1998) [8] L.M. Bull, B. Bussemer, T. Anupold, A. Reinhold, A. Samoson, J. Sauer, A.K. Cheetham, R. Dupree, A high-resolution O-17 and Si-29 NMR study of zeolite siliceous ferrierite and ab initio calculations of NMR parameters, J. Am. Chem. Soc. 122 (2000) [9] T. Loeser, D. Freude, G.T.P. Mabande, W. Schwieger, O-17 NMR studies of sodalites, Chem. Phys. Lett. 370 (2003) [10] D. Freude, T. Loeser, D. Michel, U. Pingel, D. Prochnow, O-17 NMR studies of low silicate zeolites, Solid State Nucl. Magn. Reson. 20 (2001) [11] D. Schneider, NMR-Untersuchungen an 17 O-Kernen in porösen Festkörpern, Abteilung Grenzflächenphysik, Thesis, Universtität Leipzig, Leipzig, [12] F. Wolf, H. Fürtig, E. Lemnitz, DD 43221, [13] H. Toufar, et al., US , [14] Y. Wu, B.Q. Sun, A. Pines, A. Samoson, E. Lippmaa, NMR experiments with a new double rotor, J. Magn. Reson. 89 (1990) [15] R. Radeglia, G. Engelhardt, Correlation of Si-O-T (T ¼ Si or Al) angles and Si-29 NMR chemical shifts in silicates and aluminosilicates. Interpretation by semiempirical quantum-chemical considerations, Chem. Phys. Lett. 114 (1985) [16] D.R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, [17] C. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Amsterdam, [18] D. Freude, Quadrupole nuclei in solid-state NMR, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000, pp [19] D.H. Olson, A reinvestigation of the crystal structure of the zeolite hydrated NaX, J. Phys. Chem. 74 (1970) [20] F. Porcher, M. Souhassou, Y. Dusausoy, C. Lecomte, The crystal structure of a low-silica dehydrated NaX zeolite, Eur. J. Miner. 11 (1999) [21] S.B. Jang, Y. Kim, Chemistry and crystallographic studies of metal-ion exchanged zeolite-x.1. The crystal-structure of fully dehydrated and fully K+-exchanged zeolite-x, K-92-X, Bull. Korean Chem. Soc. 16 (1995) [22] T. Sun, K. Seff, N.H. Heo, V.P. Petranovskii, A cationic cesium continuum in zeolite-x, J. Phys. Chem. 98 (1994) [23] Y. Liu, H. Nekvasil, J. Tossell, Explaining the effects of T-O-T bond angles on NMR chemical shifts in aluminosilicates: a natural bonding orbital (NBO) and natural chemical shielding (NCS) analysis, J. Phys. Chem. A 109 (2005)