New Insights into the Short-Range Structures of Microporous Titanosilicates As Revealed by ^47/49Ti, ²³Na, ³⁹K, and ²⁹Si Solid-State NMR Spectroscopy

Abstract

Seven prototypical microporous titanosilicates have been studied by multinuclear solid-state NMR (SSNMR) spectroscopy, representing four typical Ti environments: square-pyramidal TiO₅ units (natisite, AM-1, ETS-4), edge-shared brookite-type TiO₆ chains (AM-4), cubane-type Ti₄O₁₆ clusters (sitinakite, GTS-1), and corner-shared TiO₆ chains (ETS-10, ETS-4). ^47/49Ti SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in Ti–O bond lengths and O–Ti–O coordination angles are reflected via <i>C</i>_Q(^47/49Ti) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric ^47/49Ti electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses ²⁹Si NMR experiments to assess phase purity and crystallinity. ²³Na NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of ³⁹K SSNMR for investigation of extra-framework counter cations is demonstrated by ETS-10, with increased spectral resolution and enhanced sensitivity to changes in local environment versus ²³Na experiments. Plane-wave DFT calculations predicted ^47/49Ti NMR parameters assisting in spectral assignments and help correlate ²³Na and ²⁹Si NMR resonances to crystallographic sites. The approach described in this work should promote further SSNMR investigations of microporous solids, such as titanosilicates, with unknown or poorly defined structures