Structure - Functionality Relationships of Grafted Ti-MCM41 Silicas. Spectroscopic and Catalytic Studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210-230 nm in diffuse reflectance (DR) W-Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at lambda greater than or equal to 250 nm in the DR UV-Vis and emit very weakly in the 500-600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. less than or equal to 0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.