Titanium Defective Sites in TS-1: Structural Insights by Combining Spectroscopy and Simulation

Ti silicates, and in particular, titanium silicalite-1 (TS-1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H2O2 as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS-1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.