Effect of the reaction conditions on the epoxidation of alkenes with hydrogen peroxide catalyzed by silica-supported titanium derivatives

Silica-supported titanium catalysts are active in the epoxidation of cyclohexene with diluted hydrogen peroxide at 80 degreesC. At low H2O2/Ti ratio the contribution of the direct mechanism of epoxidation is important, around 40% of the productive H2O2 conversion and 60% of the epoxidation reaction. However, the increase in H2O2/Ti ratio modifies these results. The contribution of the direct epoxidation to H2O2 conversion is reduced to 20-30%, whereas contribution to epoxidation is kept in the range 40-60%. Neither the silanization of the silica surface nor the substitution of the isopropoxy groups by tartaric acid improves the behavior of the solid in these conditions. However, the simultaneous variation in hydrophilic character of the surface and titanium environment increases the contribution of the direct epoxidation. In contrast, the increase in H2O2/Ti ratio reduces the epoxide hydrolysis. The catalysts lose some titanium after reaction, but in general they show higher stability than closely related solids. The activity for direct and radical contributions changes after recovering, showing the important change in nature of the catalytic sites, which are not easily regenerated by extensive washing with different solvents. In any case, with cyclooctene, an alkene that does not form radicals, the activity for direct epoxidation shows a decline in every recycling but final turnover numbers are similar in the first three runs, showing high stability of the titanium on the solid. (C) 2001 Academic Press.