Redox and catalytic chemistry of Ti in titanosilicate molecular sieves: an EPR investigation

Abstract

An EPR study of Ti³⁺ in titanosilicate molecular sieves, TS-1, TiMCM-41, ETS-10 and ETS-4 is reported. Ti⁴⁺ is reduced to Ti³⁺ by dry hydrogen above 673 K. Ti ions in TS-1 and TiMCM-41 are located in tetragonally elongated T_d and those of ETS-10 and ETS-4 in a tetragonally compressed O_h geometric positions. Reduction at 873 K revealed the presence of two non-equivalent Ti³⁺ sites in TS-1 and TiMCM-41. Ti⁴⁺ ions in a tetrahedral geometry are more difficult to reduce than in an octahedral symmetry. The effects of cation exchange and Pt impregnation, on the geometry and reducibility of titanium in ETS-10, are also examined. Interaction of a tetrahedrally coordinated Ti³⁺ with O₂ or H₂O₂ results in a diamagnetic titanium(IV) hydroperoxo species. Under the same conditions, an octahedrally coordinated Ti³⁺ forms a paramagnetic titanium(IV) superoxo species. The higher catalytic activity of TS-1 and TiMCM-41 in selective oxidation reactions is probably a consequence of the formation of the hydroperoxy species on their surface during the catalytic reaction. The presence of Pt in the vicinity of Ti enables the use of H₂ and O₂ (instead of H₂O₂) to generate the active hydroperoxy site. The absence of formation of titanium hydroperoxy species in ETS-4 and ETS-10 is the cause of their inactivity in selective oxidation reactions.