Speciation, Structural Refinement, and Distribution of Ti Sites in Titanium Silicalite‐1 from 47/49Ti NMR Crystallography at 28.2 Tesla

Titanium zeotypes such as titanium silicalite-1 (TS-1) are industrially used for selective oxidations with hydrogen peroxide, e.g. Hydrogen Peroxide to Propylene Oxide (HPPO) and ammoximation processes. Despite being discovered over 40 years ago, TS-1 is seeing an increase in use in the chemical industry. The catalyst structure, especially the Ti speciation and local environment in the zeolitic framework as well as the nuclearity of the Ti sites in the pristine zeolite material are still debated. The lack of molecular insight into this industrial relevant catalyst hampers the rational development of Ti-zeotype materials for more selective and sustainable oxidation processes, as well as potential new applications. Here, we address this issue by applying high field (28.2 T) 47/49Ti NMR and 17O NMR spectroscopy, for an array of TS-1 catalysts. For extra-framework-free TS-1 catalysts the Ti sites are associated with a distinct NMR signature – δiso (49Ti) = -900 ppm; CQ,0(49Ti) = 7.2 MHz; ηQ,0 = 0.2 – which can be translated into structural parameters and a dominant first coordination environment according to an extended Czjzek model. The dominant Ti environment and its associated distribution was found to be independent of Ti weight loading, hinting towards similar Ti site across these samples. Combining high-field 47/49Ti NMR measurements of a hierarchical TS-1 in the hydrated and dehydrated state enabled to resolve an additional 47/49Ti NMR signatures, linked to extra-framework TiO2. Further, we develop a 47/49Ti NMR crystallography protocol aided by 17O NMR spectroscopy, benchmarked on a library of molecular models. This method is applied to identify the Ti sites present in both classical and hierarchical TS-1s, their distortion and location. Overall, this highlights the presence of mononuclear Ti framework sites in pristine Ti-zeotype structures. The observed distribution of the NMR parameters presumably originates from variations of mononuclear Ti framework in the first and second coordination sphere, likely due to the occupancy of several T-sites.