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

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 CQ(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.