Structural characterization of Ti centres 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 centre in titanium silicalite and the reaction intermediates in the ammoximation of cyclohexanone are concisely reviewed. The Ti is in tetrahedral coordination in vacuo and expands its coordination sphere upon interaction with adsorbates. In the presence of H2O and H2O/H2O2 solutions 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 important role in the ammoximation reaction. The geometries of the peroxidic species (open or bridged) are discussed also on the basis of ab initio calculations.