Flame made titania/silica epoxidation catalysts

Flame aerosol technology has been used to prepare titania/silica mixed oxid e nanoparticles. A stream of argon containing evaporated hexamethyldisiloxa ne and titanium-tetraisopropoxide was brought into a methane-oxygen diffusi on flame. Upon condensation, nanoparticles of titania/silica with high tita nia dispersion were formed. The powders were characterized using transmissi on electron microscopy, nitrogen adsorption, pore size analysis, laser-assi sted ion-coupled plasma mass spectroscopy, UV-VIS spectroscopy, and diffuse reflectance IR spectroscopy. The powders consisted of agglomerates of prim ary 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 l ow oxygen-to-methane ratios were used. The catalytic materials were tested for the epoxidation of 2-cyclohexenol by tert.-butylhydroperoxide at 90 deg reesC. The activity and selectivity were investigated as a function of flam e 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%, wher eas olefin selectivity was 83% at 80% conversion. The best catalytic perfor mance was obtained with nanoparticles containing 1 to 3.2 wt% titania. The rate and selectivity achieved with the flame made catalysts were independen t of the oxygen-to-methane ratio in the flame at high oxygen flow rate, whe reas at low oxygen flow rate the activity and selectivity decreased conside rably. (C) 2001 Academic Press.