NMR studies of the structure and basic properties of zeolites

DOR and MQMAS NMR techniques were applied in the high field of 17.6 T to the study of several oxygen-17 enriched zeolites with the ratio Si/Al = 1. The tendency that the isotropic chemical shift of the O NMR decreases with increasing bond angle (or s-character of the oxygen hybrid orbital) was confirmed by this study. A quite linear correlation exists for the oxygen sites in the zeolites A and LSX, if the data are limited to the sodium form of the hydrated zeolites. A fit of seven signals of the hydrated sodium zeolites A and LSX, which could be assigned by means of the asymmetry parameter of the quadrupol coupling constant, gives the equation δ /ppm = −214ρ + 136 with a correlation coefficient of 0.92, if the chemical shift is plotted against the s-character of the oxygen hybrid orbital ρ = cosα/(cosα−1), where α denotes the Si-O-Al angle. The influence of cations on the isotropic chemical shift was studied in case of the Li-, Na-, K-, Rband Cs-form of the zeolite LSX. Ion exchange of the hydrated zeolites (from the Na form to K-, Rb-, Csand Tl forms) generates strong chemical shift effects. The increase of the basicity of the oxygen framework of the zeolite LSX is reflected by a downfield shift of ca. 10 ppm going from the lithium to the cesium form. The substitution of sodium by thallium in the zeolite A causes a shift of 34 ppm for the O3 signal.