The Identity in Atomic Structure and Performance of Active Sites in Heterogeneous and Homogeneous, Titanium−Silica Epoxidation Catalysts

The nature of the Ti(IV) active site in heterogeneous titano−silica epoxidation catalysts is shown to be same as that of a Ti(IV)-centered site bound to soluble homogeneous silsesquioxane catalysts. A range of techniques, encompassing pre- and near-edge X-ray absorption spectroscopy and molecular dynamics calculations, provides quantitative information pertaining to the four-coordinated site that is tripodally anchored to silica. The catalytic performance of a series of engineered active sites in [( c -C 5 H 9 ) 7 Si 7 O 12 TiOXPh 3 ], where X = Si, Ge, or Sn, shows that the order of enhancement is Ge > Sn > Si. This is compared with previously published 7 performances of engineered active sites in solid titano−silicate catalysts. The key importance of accessibility to the four-coordinated Ti(IV) site is highlighted here.