17O NMR studies of low silicate zeolites

Multiple-quantum magic-angle spinning and double-rotation NMR techniques were applied in the high field of 17.6 T to the study of oxygen-17-enriched zeolites A and LSX with the ratio Si/Al = 1. A monotonic correlation between the isotropic value of the chemical shift and the Si-O-Al bond angle alpha (taken from X-ray data) could be found. Hydration of the zeolites causes a downfield O-17 NMR chemical shift of about 8 ppm with respect to the dehydrated zeolites. Ion exchange of the hydrated zeolites generates stronger chemical shift effects. The increase of the basicity of the oxygen framework of the zeolite LSX is reflected by a downfield shift of approx. 10 ppm going from the lithium to the cesium form, and the substitution of sodium by thallium in the zeolite A causes a shift of 34 ppm for the O3 signal. O-17 DOR NMR spectra are superior to O-17 3QMAS NMR spectra, featuring a resolution increase by a factor of 2 and are about equal with respect to the sensitivity. The residual linewidths of the signals in the O-17 DOR and O-17 5QMAS NMR spectra can be explained by a distribution of the Si-O-Al angles in the zeolites. (C) 2001 Academic Press.