Enhancement of chemoselectivity in epoxidation reactions over TS-1 catalysts by alkali and alkaline metal ions

Alkali metal ions, when present during the synthesis of TS-1, lead to inactive oxidation catalysts. However, when added, in small amounts, to the reaction medium during the epoxidation of allyl alcohol or allyl chloride by H₂O₂ over TS-1, they increase the selectivity for the epoxide. To probe this phenomenon in detail, the influence of pH and alkali and alkaline earth ions on the structure and catalytic activities of the oxo-Ti species generated in H₂O₂. TS-1 and TiMCM-41 systems have been investigated using EPR and diffuse reflectance UV-Vis spectroscopies. In acidic and neutral pH, two types of superoxo-Ti species (Ti(O₂√⁻)), A and B, are observed over TS-1. In the presence of alkali and alkaline metal ions or at high pH, Ti³⁺ ions and a new type of Ti(O₂√⁻) species, A′ are observed. Only the B-type species, however, is observed on TiMCM-41. The A-type are more reactive than the B-type. Epoxide selectivities approaching 100% can be achieved by a proper control of the reaction medium.