Proceedings: 14 th International Zeolite Conference 25  30 April 2004 Editors: E. van Steen et al Cape Town, South Africa ISBN: 0-958-46636-X Produced by: Document Transformation Technologies HAS 17 O NMR BEEN ESTABLISHED AS A SPECTROSCOPIC TOOL FOR ZEOLITE CHARACTERIZATION? Ernst, H., Freude D., Kanellopoulos, J., Loeser, T., Prochnow, D. and Schneider, D. Universitt Leipzig, Abteilung Grenzflchenphysik, LinnØstr. 5, 04103 Leipzig, Germany. E-mail: freude@uni-leipzig.de ABSTRACT Several zeolites of type A, X, and sodalite with Si/Al = 1, the zeolites Y, ZSM-5 and the aluminum phosphate AlPO 4 -14 isotopically enriched in 17 O were analyzed by means of 17 O NMR in the field of 17.6 T using several solid-state NMR techniques for quadrupole nuclei. 17 O DOR NMR spectra are superior to 17 O 3QMAS NMR spectra, featuring a resolution increase by a factor of two and are about equal with respect to the sensitivity. The residual linewidths of the signals in the 17 O DOR and 17 O 5QMAS NMR spectra can be explained by a distribution of the Si-O-Al angles in the zeolites. 1 H, 27 Al, and 29 Si MAS NMR investigations were performed in addition. It was confirmed that in principle the isotropic 17 O chemical shift δ ( 17 O) decreases with increasing T-O-T bond angle α for the zeolites under study. For some zeolites a linear correlation was found, e. g. A and X, for some other, e.g. sodalite and AlPO 4 -14, it could not be found. The sensitivity of the 17 O NMR shift with respect to ion exchange and the adsorption of molecules is discussed including the question how the 17 O NMR shift reflects the basicity of the oxygen framework. Cation-exchanged alumosilicates LSX of Si/Al-ratio 1 were loaded with small molecules (e.g. pyrrol, formic acid, methyl iodide) for the investigation of the latter problem. Keywords: zeolites, NMR, MAS, DOR, MQMAS, oxygen-17 INTRODUCTION Oxygen is the most abundant element in the zeolites, but 16 O has no nuclear spin. The 17 O isotope with nuclear spin I = 5/2 has the disadvantage of a very low natural abundance (0.037%) and the high costs of isotopic enrichment. However, the applications of solid-state 17 O NMR spectroscopy should be extended, in order to obtain an additional tool for structural probing of the oxygen framework of zeolitic materials and the basic properties of the porous catalysts as well. New NMR techniques, e.g., dynamic angle spinning (DAS) [1, 2], double rotation (DOR) [3], multiple-quantum excitation in combination with magic-angle spinning (MQMAS) [4] and satellite transition in combination with magic-angle spinning (STMAS) [5] have recently been developed for quadrupole nuclei with half-integer spins. In addition, the perturbing effect of the electric quadrupole interaction is reduced at the higher magnetic fields, which are now available. Numerous 17 O NMR investigations applying these techniques were performed, in order to correlate the obtained NMR parameters of resolved oxygen signals with structure data obtained by diffraction methods: Grandinetti et al. [6] investigated the SiO 2 polymorph coesite, Mueller et al. [7] measured the 17 O signals of diopside, forsterite, clinoenstatite, wollastonite and larnite by DAS NMR, Bull et al. [8, 9] investigated the silicon-rich zeolites Sil-Y and ferrierite by 17 O DOR NMR. We investigated several hydrated zeolites by 17 O 3Q MAS and DOR NMR and demonstrated the reasonably good resolution in the field of 17.6 T, whereas we obtained non-sufficient resolution in the lower field of 11.7 T [10, 11]. Pingel et al. [11] proposed a correlation between Si-O-Al angles and the 17 O chemical shift, whereas Bull et al. [8, 9] claimed that no simple correlation appears to exist between the zeolite bond angles and the 17 O NMR parameters. Freude et al. [12] argued that the influence of the adsorbed molecules and the various cations cannot be described by a linear correlation. Loeser et al. [13] found no such correlation for sodalites. Also Profeta et al. [14] claimed that no simple correlation between the chemical shift, C qcc NMR parameters, and Si-O-Si angle exists. Another topic is, how the isotropic value of the 17 O NMR chemical shift can monitor molecule-framework interactions and basic properties of the zeolite framework. Both topics, characterization of zeolite structure and oxygen-framework/molecule interaction as well, will be discussed in the present contribution. 1173