Abstract
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.