Effect of Thermal Treatment of MCM-41 on the Nature and Catalytic Properties of MCM41-supported Alkoxo-Titanium Species in Oxidation of Sulfides to Sulfoxides

Titanium tetra-isopropoxide reacts with the silanol groups of MCM41 calcined at two different temperatures, 550 and 750°C, to form well-defined surface organometallic species. For MCM41 calcined at 550°C, the amount of available silanol allows mainly the formation of the bipodal titanium complex, (≡SiO)₂Ti(O ⁱ Pr)₂, while calcination at 750°C leads to the formation of the monopodal titanium complex ≡SiO-Ti(O ⁱ Pr)₃. Ti-MCM41₍₇₅₀₎ was found to be more active and selective in the oxidation of sulfides, compared to Ti–MCM41₍₅₅₀₎, and this can be attributed to the development of a large amount of tetrahedral isolated titanium (IV) which seems to be the active site in this reaction. In the asymmetric oxidation of sulfide by H₂O₂, Ti–MCM41₍₇₅₀₎ was found also more reactive than Ti–MCM41(550), since it develops the singly bonded titanium species, ≡SiO-Ti(O ⁱ Pr)₃, which appears to be the chiral active site in this reaction.