Speciation, Structural Refinement, and Distribution of Ti Sites in Titanium Silicalite-1 From ^47/49Ti NMR...

Titanium silicalite-1 (TS-1) is industrially used for selective oxidation 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. 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-δ_iso (⁴⁹Ti) = -900 ppm; C_Q,0(⁴⁹Ti) = 7.2 MHz-which can be translated into structural parameters and a dominant first coordination environment according to an extended Czjzek model. With the implementation of a ^47/49Ti NMR crystallography protocol, benchmarked on molecular models, this signature was assigned to the presence of mononuclear Ti framework sites in pristine TS-1 structures. The observed distribution of the NMR parameters presumably originates from variations of mononuclear Ti framework sites in the first and second coordination sphere, likely due to the occupancy of several T-sites.