New Insights into the Short-Range Structures of Microporous Titanosilicates As Revealed by Ti-47/49, Na-23, K-39, and Si-29 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). Ti-47/49 SSNMR spectra at 21.1 T are related to the coordination, crystal symmetry, and local environment of Ti. Distortions in TiO bond lengths and OTiO coordination angles are reflected via C-Q(Ti-47/49) values that range from 8 to 16 MHz. Several titanosilicates feature axially symmetric Ti-47/49 electric field gradient (EFG) tensors that permit facile spectral assignment and detection of deviations in local symmetry. This study uses Si-29 NMR experiments to assess phase purity and crystallinity. Na-23 NMR is used to probe the location and mobility of the sodium ions in the framework. The potential of K-39 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 Na-23 experiments. Plane-wave DFT calculations predicted Ti-47/49 NMR parameters assisting in spectral assignments and help correlate Na-23 and Si-29 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.