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

Abstract

This paper describes a spectroscopic study of the heterogenisation of a novel liquid-phase epoxidation catalyst, a titanium(IV) silsesquioxane complex. Its immobilisation is performed exclusively via a straightforward adsorption 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 heterogeneous liquid-phase epoxidation catalysts are obtained. These highly active catalytic materials were extensively characterised using diffuse reflectance UV-Vis, XPS and Raman spectroscopy. With these techniques the strong adsorption of the intact catalytic complex within an all-silica MCM-41 host is demonstrated. A spectroscopic investigation on the ancillary cyclohexyl ligands of the silsesquioxane complex reveals strong interactions upon adsorption inside the MCM-41 pore. The interaction of these cyclohexyl ligands with the MCM-41 pore wall, as shown by Raman spectroscopy, reveals a constrained configuration of these ligands. Moreover, a hardly affected cyclopentadienyl ligand on the titanium site upon adsorption suggests a preferable orientation 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 agreement with the high catalytic activity found for these heterogeneous catalysts.