MCM-41 heterogenised titanium silsesquioxane epoxidation catalysts: a spectroscopic investigation of the adsorption characteristics

This paper describes a spectroscopic study of the heterogenisation of a nov el liquid-phase epoxidation catalyst, a titanium(IV) silsesquioxane complex . Its immobilisation is performed exclusively via a straightforward adsorpt ion of the homogeneous catalyst, i.e. the metal complex, in the pores of an MCM-41 host material. Applying all-silica MCM-41 hosts, stable and heterog eneous liquid-phase epoxidation catalysts are obtained. These highly active catalytic materials were extensively characterised using diffuse reflectan ce UV-Vis, XPS and Raman spectroscopy. With these techniques the strong ads orption of the intact catalytic complex within an all-silica MCM-41 host is demonstrated. A spectroscopic investigation on the ancillary cyclohexyl li gands of the silsesquioxane complex reveals strong interactions upon adsorp tion inside the MCM-41 pore. The interaction of these cyclohexyl ligands wi th the MCM-41 pore wall, as shown by Raman spectroscopy, reveals a constrai ned configuration of these ligands. Moreover, a hardly affected cyclopentad ienyl ligand on the titanium site upon adsorption suggests a preferable ori entation in which the catalytic active titanium site is pointing inside the MCM-41 pore. As such, the accessibility of the titanium site for substrate and oxidant in heterogeneous epoxidation catalysis is guaranteed, in agree ment with the high catalytic activity found for these heterogeneous catalys ts.