Unequivocal evidence of the presence of titanols in Ti-MCM-48 mesoporous materials. A combined diffuse reflectance UV-Vis-Nir and ²⁹Si-MAS-NMR study

Mesoporous Ti-MCM-48 materials were synthesised with titanium loading varying from 1 to 3 wt% TiO₂. Post-synthesis silylation treatment of the silica surface led to the transformation of silanols, (SiO)₃SiOH, into (SiO)₃SiOSi(CH₃)₃ groups, which were identified by means of ²⁹Si-MAS-NMR and diffuse reflectance (DR) Nir spectroscopy. DR UV-Vis spectroscopy was performed to clarify the nature of Ti(IV) sites present in these samples. The UV-Vis spectra show a band at 210 nm with a shoulder at 230 nm, typically assigned to oxygen to tetrahedral Ti(IV) (LMCT) electronic transitions. Interestingly, the component at 230 nm, which was assigned to an electronic transition that involves titanium sites linked to OH groups (named as titanols), strongly decreased upon the silylation treatment. This is a clear evidence that, beside silanols, also titanols were silylated, leading to (SiO)₃TiOSi(CH₃)₃ sites. This fact can be taken as an unequivocal assignment of the shoulder at 230 nm due to oxygen to tetrahedral Ti(IV) electronic transition related to titanol species.