Titania-silica mixed oxides: Influence of reaction additives on the epoxidation of (E)-2-hexen-1-ol

The epoxidation of the allylic alcohol (E)-2-hexen-1-ol has been investigated using an amorphous titania-silica aerogel with a TiO2 content of 20 wt%. The catalyst was prepared by the alkoxide-sol-gel route and the solvent was removed by semicontinuous extraction with supercritical CO2 at 313 K. The epoxidation was carried out with tert.-butylhydroperoxide as oxygen source. Epoxide ring opening by the acid-catalyzed attack of (E)-2-hexen-1-ol and formation of high-molecular-weight by-products were observed as the main side reactions under optimized conditions the competitive oxidation of the alcoholic function was negligible. The rate acid selectivity of the epoxidation reaction could significantly be improved by careful drying of the aerogel in situ at 473 K, by azeotropic distillation of toluene from the reaction mixture, and/or by addition of solid bases (NaHCO3, zeolite 4A) to the reaction mixture. High epoxide selectivities up to 90-98% were obtained at 50-90% peroxide conversion within 5-60 min, using a catalyst/hexenol ratio of 3.3 wt%. The activity and selectivity of the aerogel is comparable to that of TS-1. The positive influence of solid bases was found to be due to their minor solubility in the weakly polar medium and to the neutralization of the acidic sites on the aerogel. Contrary to earlier observations, bases can increase or decrease the rate, depending on the water content of the system. Zeolite 4A acts as a base and a drying agent. (C) 1998 Academic Press.