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

An EPR study of Ti3+ in titanosilicate molecular sieves, TS-1, TiMCM-41, ET S-10 and ETS-4 is reported. Ti4+ is reduced to Ti3+ by dry hydrogen above 6 73 K. Ti ions in TS-I and TiMCM-41 are located in tetragonally elongated Td and those of ETS-10 and ETS-4 in a tetragonally compressed O-h geometric p ositions. Reduction at 873 K revealed the presence of two non-equivalent Ti 3+ 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 Ti 3+ with O-2 or H2O2 results in a diamagnetic titanium(IV) hydroperoxo speci es. Under the same conditions, an octahedrally coordinated Ti3+ forms a par amagnetic titanium(nr) superoxo species. The higher catalytic activity of T S-1 and TiMCM-41 in selective oxidation reactions is probably a consequence of the formation of the hydroperoxy species on their surface during the ca talytic reaction. The presence of Pt in the vicinity of Ti enables the use of H-2 and O-2 (instead of H2O2) to generate the active hydroperoxy site. T he absence of formation of titanium hydroperoxy species in ETS-4 and ETS-10 is the cause of their inactivity in selective oxidation reactions. (C) 200 0 Elsevier Science B.V. All rights reserved.