TETRANEOPENTYLTITANIUM DERIVED SILICA-SUPPORTED COMPLEXES - PART-2 - INFLUENCE OF THE HYDROPHOBIC CHARACTER OF SILICA ON THE CATALYTIC PROPERTIES

Titanium containing catalysts were synthesized using hydrophobic silic as as support; they were tested for the epoxidation of cyclohexene wit h aqueous H2O2. It is shown that, despite the low number of surface si lanols and the presence of surface hydrophobic methyl-silane entities, tetraneopentyltitanium TiNp4 reacts with the surface silanols, with f ormation of the complex =SiOTiNp3, 1, chemically bonded to the surface . Hydrolysis of this surface complex 1 leads to the formation of =SiOT i(OH)(3); contrary to what was observed with hydrophilic silica suppor ts, the anchoring bond =SiO-Ti is maintained. Calcination of 1 decompo ses the silica surface modifying methyl groups, SiMe3 to Si(OH)(3); th e chemical environment of Ti is then no longer hydrophobic in characte r; titanium remains very well dispersed. The two types of solids catal yze the epoxidation of cyclohexene with aqueous H2O2, with an epoxidat ion yield and selectivity significantly higher than those obtained wit h the analogs supported on hydrophilic silicas. A zirconium containing catalyst synthesized in a similar manner from ZrNp4 and a hydrophobic silica shows also enhanced selectivity or epoxide formation; interest ingly; the rate of the successive decomposition of the epoxide is lowe red. These properties are explained by the achievement of a better iso lation of the Ti and Zr active sites on the hydrophobic silicas. (C) 1 998 Academic Press.