title: Thermal restructuring of silica-grafted TiCl(x) species and consequences for epoxidation catalysis.
abstract: TiCl4 grafted to dehydrated silica is an industrially applied catalyst for the epoxidation of propylene. As with many heterogeneous catalysts, the precise nature of the surface species is not yet fully known, prohibiting reliable structure-activity correlations. In this study, the speciation and restructuring of site-isolated Ti(IV) Lewis acid centers was carefully investigated by using a variety of techniques. The initially formed ≡SiOTiCl3 species were found to restructure upon heating through the transfer of Cl ligands to the silica surface, eventually leading to tripodal (≡SiO)3 TiCl species. The superior activity and stability of such tripodal species is demonstrated for catalytic olefin epoxidation under continuous flow conditions.
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zurich, Wolfgang-Pauli-Strasse 10, Switzerland.
authors.forename: Philipp
authors.initials: P
authors.lastname: Mania
authors.forename: René
authors.initials: R
authors.lastname: Verel
authors.forename: Florian
authors.initials: F
authors.lastname: Jenny
authors.forename: Ceri
authors.initials: C
authors.lastname: Hammond
authors.forename: Ive
authors.initials: I
authors.lastname: Hermans
keywords: NMR spectroscopy; epoxidation; heterogeneous catalysis; structure-activity relationships; titanium