Insights into the role of titanium sites in cyclohexanone ammoximation over titanium silicalite-1

The discovery of titanium silicalite-1(TS-1) is a milestone in the development of heterogeneous catalysts for the eco-benign and mild catalytic oxidation process. However, the interpretation of specific Ti location and its role in oxidation is severely hindered due to the limited crystal size of as-synthesized TS-1. Here, a high quality TS-1 (ca. 10 μm) was successfully fabricated by the pre-hydrolysis silica source strategy. As evidenced by powder X-ray diffraction crystallography, the preferred Ti locations were mainly found at T2, T4, and T11 in Ti-containing MFI framework. Combining multiple spectroscopy methods (i.e., FT-IR, UV–Vis, and UV Raman) and the reaction kinetics, the active center in TS-1 was demonstrated as an exposed site of hexa-coordinated titainum species. Furthermore, the powder X-ray crystallography also revealed that the ketone and oxime adsorbed in the zeolite host with intense host-guest interaction of framework oxygen atoms, and exhibiting an overwhelming spatial intimacy to the Ti species at the molecular scale. This unique phenomenon affords a plausible interpretation for the oxime reaction.