Silica-supported titanium derivatives as catalysts for the epoxidation of alkenes with hydrogen peroxide: A new way to tuneable catalytic activity through ligand exchange

Titanium tetraisopropoxide supported on silica [Si–Ti(OiPr)] is an efficient catalyst for the epoxidation of alkenes, cyclooctene, and cyclohexene with 30% hydrogen peroxide at 80°C. In the case of cyclohexene, allylic oxidation is also produced through a radical mechanism and the epoxide undergoes hydrolysis to the trans-diol due to the acidity of the catalyst and the presence of water. The catalyst can be recycled with very low titanium leaching and has a slightly lower catalytic activity after two cycles. These best results are obtained when an alkene/H2O2 ratio=20 is used. Lowering this ratio up to 5 has a detrimental effect on the final yield but does not affect the epoxidation/allylic selectivity. Further decrease to alkene/H2O2=1 leads to very low activity and selectivity. This catalyst bears isopropoxy groups that can be substituted by nonfunctionalized and functionalized diols. Such a process modifies the activity and selectivity of the catalyst, showing that it is possible to modulate the performance of this kind of titanium catalyst by changing the environment of the catalytic sites. The substitution with ethylene glycol improves the performance of the recovered catalyst. The catalyst prepared by treating the original Si–Ti(OiPr) with tartaric acid [Si–Ti(TA)] gives rise to high overall oxidation yields, with 97% selectivity in H2O2 and an epoxidation/allylic oxidation ratio around 65/35. Moreover, it is very stable during at least three cycles. End-capping of the silica surface has a positive effect on the epoxide hydrolysis but not on the epoxidation/allylic oxidation selectivity, in contrast with previously described results. The lower functionalization of the silylated solids noticeably increases the turnover numbers. Filtration experiments in the reaction conditions have shown that Si–Ti(OiPr) and Si(ec)–Ti(TA) are truly heterogeneous catalysts, with no contribution of the leached species. However, Si–Ti(TA) leads to leaching of active species, although an important part of them remains on the solid after three filtrations, and the contribution to the activity of homogeneous and heterogeneous species is nearly the same.This work was made possible by the generous financial support of the CICYT (Project MAT96-1053).Peer reviewe