SYNTHESIS AND STRUCTURAL STUDIES OF MICROPOROUS TITANIUM-NIOBIUM-SILICATES WITH THE STRUCTURE OF NENADKEVICHITE

A series of synthetic microporous analogues of the mineral nenadkevich ite, with Ti/Nb molar ratios ranging from 0.8 to 17.1 and a purely tit aneous sample, have been prepared. The microporous titanosilicate ETS- 4 has been found to be a precursor in the synthesis of synthetic nenad kevichite. The structure of nenadkevichite materials has been extensiv ely characterized by scanning electron microscopy, powder X-ray diffra ction, Raman spectroscopy, diffuse-reflectance ultraviolet spectroscop y (DR-UV), and Na-23, Si-29, and Nb-93 high-resolution solid-state NMR . In accord with the published structure for the mineral nenadkevichit e, DR-UV and Nb-93 solid-state NMR suggest that titanium and niobium a re in distorted octahedral coordination. Raman spectroscopy clearly sh ows the framework substitution of titanium for niobium. Tn addition, d ue to stronger dipolar interactions, the replacement of Ti-47 and Ti-4 9 (diluted quadrupolar nuclei) by Nb-93 (abundant quadrupolar nucleus) results in a significant decrease in the Si-29 spin-spin (T-2) NMR re laxation times. The Si-29 NMR spectra are strongly dependent on the ca tions present and on the state of hydration of the nenadkevichite mate rials.