Synthesis and Structural Studies of Microporous Titanium−Niobium−Silicates with the Structure of Nenadkevichite

A series of synthetic microporous analogues of the mineral nenadkevichite, with Ti/Nb molar ratios ranging from 0.8 to 17.1 and a purely titaneous sample, have been prepared. The microporous titanosilicate ETS-4 has been found to be a precursor in the synthesis of synthetic nenadkevichite. The structure of nenadkevichite materials has been extensively characterized by scanning electron microscopy, powder X-ray diffraction, Raman spectroscopy, diffuse-reflectance ultraviolet spectroscopy (DR-UV), and 23Na, 29Si, and 93Nb high-resolution solid-state NMR. In accord with the published structure for the mineral nenadkevichite, DR-UV and 93Nb solid-state NMR suggest that titanium and niobium are in distorted octahedral coordination. Raman spectroscopy clearly shows the framework substitution of titanium for niobium. In addition, due to stronger dipolar interactions, the replacement of 47Ti and 49Ti (diluted quadrupolar nuclei) by 93Nb (abundant quadrupolar nucleus) results in a significant decrease in the 29Si spin−spin (T 2) NMR relaxation times. The 29Si NMR spectra are strongly dependent on the cations present and on the state of hydration of the nenadkevichite materials.