~(17)O NMR studies of the structure and basic properties of zeolites

摘要

DOR and MQMAS NMR techniques were applied in the high field of 17.6 T to the study ofrnseveral oxygen-17 enriched zeolites with the ratio Si/Al = 1. The tendency that the isotropic chemicalrnshift of the ~(17)O NMR decreases with increasing bond angle (or s-character of the oxygen hybridrnorbital) was confirmed by this study. A quite linear correlation exists for the oxygen sites in thernzeolites A and LSX, if the data are limited to the sodium form of the hydrated zeolites. A fit of sevenrnsignals of the hydrated sodium zeolites A and LSX, which could be assigned by means of thernasymmetry parameter of the quadrupol coupling constant, gives the equation d /ppm = -214r + 136rnwith a correlation coefficient of 0.92, if the chemical shift is plotted against the s-character of thernoxygen hybrid orbital r = cosa/(cosa-1), where a denotes the Si-O-Al angle. The influence ofrncations on the isotropic chemical shift was studied in case of the Li-, Na-, K-, Rb- and Cs-form ofrnthe zeolite LSX. Ion exchange of the hydrated zeolites (from the Na form to K-, Rb-, Cs- and Tlrnforms) generates strong chemical shift effects. The increase of the basicity of the oxygen frameworkrnof the zeolite LSX is reflected by a downfield shift of ca. 10 ppm going from the lithium to the cesiumrnform. The substitution of sodium by thallium in the zeolite A causes a shift of 34 ppm for the O3rnsignal.