TITANIA-SILICA MIXED OXIDES .3. EPOXIDATION OF ALPHA-ISOPHORONE WITH HYDROPEROXIDES

The epoxidation of an cy-keto olefin, cx-isophorone, has been studied using high-surface-area, amorphous titania-silica aerogels. The mixed oxides were prepared by the alkoxide-sol-gel route and the solvent was removed by semicontinuous extraction with supercritical CO2. The morp hology of the catalysts was studied by HRTEM. The influence of the fol lowing reaction parameters was investigated: temperature, the chemical nature of hydroperoxide and solvent, and the ratio of olefin to catal yst or peroxide. The highest reaction rate and selectivity were obtain ed in alkylaromatic solvents of low polarity. Polar solvents compete f or the active coordination sites and lower the reaction rate. All test ed aerogels provided high epoxide selectivity (98-100%) at 333 K, usin g t-butyl hydroperoxide as oxidant in an olefin:peroxide molar ratio o f 5:1. No epoxide formation was observed with H2O2. A considerable lea ching of titanium in the aqueous oxidizing medium (evidenced by FTIR a nd ICP-AES analysis) is attributed to the hydrolysis of the Ti-O-Si bo nds. The aerogels containing 20 or 2 wt% TiO2 show the highest rate, r elated to the amount of catalyst or TiO2, respectively. There is a pos itive correlation between oxidation rate and Ti-O-Si connectivity, cha racteristic of Ti-dispersion in the silica matrix. The suppressed reac tivity of Lu-isophorone compared to cyclohexene is attributed to the e lectron-withdrawing effect of the carbonyl group, whereas the influenc e of the 2-methyl group (steric hindrance and electron release) is of secondary importance. The best titania-silica aerogel proved to be sup erior to TiO2-on-SiO2, TS-1, and titania-silica xerogels. (C) 1995 aca demic Press,Inc.