Effect of precursor nature on the behavior of titanium-polysiloxane homogeneous catalysts in primary alkene epoxidation

This work describes the synthesis, some structural characteristics and the performance of a series of titanium-containing polysiloxanes as homogeneous catalysts. These polysiloxanes were synthesized by mixing a titanium precursor, such as tetraisopropoxytitanium or tetrakis(2-ethylhexyloxy) titanium, with a silicon precursor of the trichloro(phenyl)silane, triethoxy(phenyl)silane or trimethoxy(phenyl)silane type in toluene solvent and then slowly hydrolyzing with water. Depending on the precursors, the polysiloxanes exhibited different physical properties; some of them were solids, while others were viscous liquids or even fluid liquids. These properties are related to the molecular weight of the polysiloxane, which is determined by the nature of the precursors. All the Ti-containing polysiloxanes were tested in the epoxidation reaction of oct-1-ene using EBHP (ethylbenzene hydroperoxide) as an oxidant. The reaction was selective to epoxide since in all cases the only product detected from the oct-1-ene was 1,2-epoxyoctane. The conversion profile of EBHP was found to depend not only on the amount of titanium incorporated into the polymeric structure but also on the symmetry and chemical environment of TiIV sites. It was observed that polysiloxanes containing tetrahedrally coordinated TiIV resulted in active catalysts whereas those in which titanium appeared in an octahedral environment were found to be inactive in alkene epoxidation.

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