Effect of Ti Speciation on Catalytic Performance of TS-1 in the Hydrogen Peroxide to Propylene Oxide Reaction

Hydrogen peroxide to propylene oxide (HPPO) reaction is an attractive process exploiting titanium silicalite-1 (TS-1) as a catalyst in combination with aqueous hydrogen peroxide as an oxidizing agent. Beyond the industrial interest, TS-1 represents one of the most widely characterized catalysts due to its unique properties. However, a unified description on the speciation of the different Ti species and their correlation to catalytic performances is missing in the literature. This work aims to exploit spectroscopic techniques (namely, diffuse reflectance UV-vis, Raman, FT-IR, and Ti K-edge XANES) in a qualitative and quantitative way to thoroughly characterize Ti sites in a selected set of industrially relevant TS-1 samples, each one owning a peculiar Ti speciation. The outcomes of this study have been then related to the activity of each catalyst in HPPO reaction, showing its linear correlation with the content of perfect Ti sites (i.e., isomorphously substituting Si in the zeolitic framework). Other Ti species, such as amorphous TiOx and bulk titania, are instead not involved in the peroxide conversion (neither in a detrimental way).