DO TITANYL GROUPS EXIST IN TITANIUM SILICATES - AN EXPERIMENTAL-STUDY

Two synthetic routes have been investigated, aimed at the preparation of silica-supported titanyl (>Ti=O) and titanol (>Ti(OH)(2)) groups, t he latter corresponding to the hydrated form of the titanyl group. In the first synthetic route, the titanyl complex TiOCl2(NMe3)(2) was rea cted with an aerosil, and the resulting material thermally treated to remove residual Cl and NMe3 ligands. In an alternative route, silica ( aerosil and silica-gel) was reacted with Ti(CH2Ph)(4) to afford mainly anchored >Ti(CH2Ph)(4) moieties, which were subsequently hydrolysed. Characterization of the resulting materials using a combination of sur face analytical techniques revealed that in all cases at least two tit ania phases were obtained, corresponding to isolated tetrahedral Ti si tes, and an amorphous form of TiO2 containing six-coordinate titanium. For the syntheses based on Ti(CH2Ph)(4), UV-vis and XPS data indicate d that the relative proportion of the two phases formed was dependent on the support employed, aerosil affording predominantly (=SiO)(2)Ti(O H)(2) sites. No evidence was found for the presence of three-coordinat e titanyl species, >Ti=O, even when the aerosil-supported >Ti(OH)(2) s ites were calcined at 500 degrees C. It is, therefore, concluded that titanyl groups are unlikely to be present in significant concentration s in titanium silicates. When tested in the epoxidation of 1-octene wi th tert-butyl hydroperoxide, the model systems were found to display e poxidation activity comparable with that of a wide-pore Ti-zeolite, Ti -MCM-41. The observed turnover frequency was found to increase with in creasing dispersion of the titania, consistent with the notion that is olated, Lewis acidic Ti(IV) centres are the most active sites for epox idation catalysis. (C) 1998 Elsevier Science B.V. All rights reserved.