J. Rocha et al., SYNTHESIS AND STRUCTURAL STUDIES OF MICROPOROUS TITANIUM-NIOBIUM-SILICATES WITH THE STRUCTURE OF NENADKEVICHITE, Journal of physical chemistry, 100(36), 1996, pp. 14978-14983
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.