Synthesis, characterization and catalytic properties of sol-gel derived mixed oxides

Alumina and 1:1 mixed oxides of Al₂O₃-ZrO₂, Al₂O₃-TiO₂, SiO₂-TiO₂ and ZrO₂-TiO₂ were synthesized via a sol-gel process by the so-called neutral amine route, followed by calcination at 600 °C. The mixed oxides were characterized before calcination by X-ray diffraction, revealing ordered hexagonal (Al₂O₃, Al₂O₃-ZrO₂, ZrO₂-TiO₂) and lamellar (SiO₂-TiO₂, Al₂O₃-TiO₂) structures. After calcination, all samples exhibited amorphous structures. Surface areas and pore characteristics of all materials were determined by nitrogen adsorption isotherms. The calcined oxides are active catalysts in the epoxidation of cyclooctene with tert-butyl hydroperoxide as oxidant. Epoxide yields from 14 to 45% were found for reaction times of 24 h. The titanium oxide containing catalysts are the most active and selective ones. On the other hand, the poisoning of acidic centers yields a decreasing activity while increasing selectivity.