Flame Made Titania/Silica Epoxidation Catalysts

Abstract

Flame aerosol technology has been used to prepare titania/silica mixed oxide nanoparticles. A stream of argon containing evaporated hexamethyldisiloxane and titanium-tetraisopropoxide was brought into a methane–oxygen diffusion flame. Upon condensation, nanoparticles of titania/silica with high titania dispersion were formed. The powders were characterized using transmission electron microscopy, nitrogen adsorption, pore size analysis, laser-assisted ion-coupled plasma mass spectroscopy, UV–VIS spectroscopy, and diffuse reflectance IR spectroscopy. The powders consisted of agglomerates of primary particles from 10 to 20 nm in diameter at high oxygen-to-methane ratios in the flame. The morphology changed to larger, spherical particles when low oxygen-to-methane ratios were used. The catalytic materials were tested for the epoxidation of 2-cyclohexenol by tert.-butylhydroperoxide at 90°C. The activity and selectivity were investigated as a function of flame parameters used for catalyst preparation. The flame made catalysts showed better selectivity for the epoxidation than corresponding aerogels, but at lower activity. Epoxidation selectivity referred to TBHP reached 93%, whereas olefin selectivity was 83% at 80% conversion. The best catalytic performance was obtained with nanoparticles containing 1 to 3.2 wt% titania. The rate and selectivity achieved with the flame made catalysts were independent of the oxygen-to-methane ratio in the flame at high oxygen flow rate, whereas at low oxygen flow rate the activity and selectivity decreased considerably.