M. Crocker et al., DO TITANYL GROUPS EXIST IN TITANIUM SILICATES - AN EXPERIMENTAL-STUDY, Colloids and surfaces. A, Physicochemical and engineering aspects, 139(3), 1998, pp. 351-368
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.