Catalytic sites in silica-supported titanium catalysts: silsesquioxane complexes as models

The number of Sisingle bondOsingle bondTi bonds in silica-supported titanium catalysts is an important parameter that affects activity and selectivity. Although comparisons between different catalysts can be made by infrared spectroscopy, estimation of the contribution of the different species is a difficult task. UV spectroscopy has been used to characterize three different titanium–silsesquioxane complexes. The wavelength in solution is highly dependent on the concentration, and broad bands are obtained by DRUV, at wavelengths that correlate with the number of Sisingle bondOsingle bondTi bonds. The deconvolution of the DRUV spectra of silica-supported titanium catalysts allows identification of the three types of tetrahedral (pentacoordinated) species, together with octahedral species. These results correlate well with the known silanol densities of the different silica supports, and the catalytic activity and selectivity for epoxidation are clearly favored by the reduction in octahedral titanium and the increase in the number of tripodal species.