STRUCTURAL CHARACTERIZATION OF TI CENTERS IN TI-SILICALITE AND REACTION-MECHANISMS IN CYCLOHEXANONE AMMOXIMATION

The main results obtained by means of many physical methods (IR, Raman , UV-Vis and XAFS spectroscopies) concerning the structure of the Ti c entre in titanium silicalite and the reaction intermediates in the amm oximation of cyclohexanone are concisely reviewed. The Ti is in tetrah edral coordination in vacuo and expands its coordination sphere upon i nteraction with adsorbates. In the presence of H2O and H2O/H2O2 soluti ons one of the SiOTi bridges is hydrolyzed with formation of (=SiO)(3) L(2)TiOH (L = H2O) and (=SiO)(3)L(2)TiOOH species, respectively. When NH3 is dosed on (=SiO)(3)L(2)TiOOH structures (=SiO)(3)L(2)TiOO(-)NH(4 )(+) species are formed. These species are thought to play an importan t role in the ammoximation reaction. The geometries of the peroxidic s pecies (open or bridged) are discussed also on the basis of ab initio calculations.