Do titanyl groups exist in titanium silicates? An experimental study

Abstract

Two synthetic routes have been investigated, aimed at the preparation of silica-supported titanyl (>TiO) and titanol (>Ti(OH) 2) groups, the latter corresponding to the hydrated form of the titanyl group. In the first synthetic route, the titanyl complex TiOCl 2(NMe 3) 2 was reacted with an aerosil, and the resulting material thermally treated to remove residual Cl and NMe 3 ligands. In an alternative route, silica (aerosil and silica-gel) was reacted with Ti(CH 2Ph) 4 to afford mainly anchored >Ti(CH 2Ph) 2 moieties, which were subsequently hydrolysed. Characterization of the resulting materials using a combination of surface analytical techniques revealed that in all cases at least two titania phases were obtained, corresponding to isolated tetrahedral Ti sites, and an amorphous form of TiO 2 containing six-coordinate titanium. For the syntheses based on Ti(CH 2Ph) 4, UV-vis and XPS data indicated that the relative proportion of the two phases formed was dependent on the support employed, aerosil affording predominantly (≡SiO) 2Ti(OH) 2 sites. No evidence was found for the presence of three-coordinate titanyl species, >TiO, even when the aerosil-supported >Ti(OH) 2 sites were calcined at 500°C. It is, therefore, concluded that titanyl groups are unlikely to be present in significant concentrations in titanium silicates. When tested in the epoxidation of 1-octene with tert-butyl hydroperoxide, the model systems were found to display epoxidation activity comparable with that of a wide-pore Ti–zeolite, Ti-MCM-41. The observed turnover frequency was found to increase with increasing dispersion of the titania, consistent with the notion that isolated, Lewis acidic Ti(IV) centres are the most active sites for epoxidation catalysis.