Titania-silica mixed oxides. III. Epoxidation of {alpha}-isophorone with hydroperoxides
- Swiss Federal Inst. of Technology, Zuerich (Switzerland)
The epoxidation of an {alpha}-keto olefin, {alpha}-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 CO{sub 2}. The morphology of the catalysts was studied by HRTEM. The influence of the following reaction parameters was investigated: temperature, the chemical nature of hydroperoxide and solvent, and the ratio of olefin to catalyst or peroxide. The highest reaction rate and selectivity were obtained in alkylaromatic solvents of low polarity. Polar solvents compete for the active coordination sites and lower the reaction rate. All tested aerogels provided high epoxide selectivity (98-100%) at 333 K, using t-butyl hydroperoxide as oxidant in an olefin: peroxide molar ratio of 5:1. No epoxide formation was observed with H{sub 2}O{sub 2}. A considerable leaching of titanium in the aqueous oxidizing medium (evidenced by FTIR and ICP-AES analysis) is attributed to the hydrolysis of the Ti-O-Si bonds. The aerogels containing 20 or 2 wt% TiO{sub 2} show the highest rate, related to the amount of catalyst or TiO{sub 2}, respectively. There is a positive correlation between oxidation rate and Ti-O-Si connectivity, characteristic of Ti-dispersion in the silica matrix. The suppressed reactivity of a-isophorone compared to cyclohexene is attributed to the electron-withdrawing effect of the carbonyl group, whereas the influence of the 2-methyl group (steric hindrance and electron release) is of secondary importance. The best titania-silica aerogel proved to be superior to TiO{sub 2}-on-SiO{sub 2}, TS-1, and titania-silica xerogels. 50 refs., 11 figs., 4 tabs.
- OSTI ID:
- 224037
- Journal Information:
- Journal of Catalysis, Vol. 157, Issue 2; Other Information: PBD: Dec 1995
- Country of Publication:
- United States
- Language:
- English
Similar Records
Homogeneity of titania-silica mixed oxides: On UV-DRS studies as a function of titania content
Titania-silica mixed oxides: I. Influence of sol-gel and drying conditions on structural properties