Effects of Pressure, Thermal Treatment, and O₂ Loading in MCM41, MSU-H, and MSU-F Mesoporous Silica Systems Probed by Raman Spectroscopy

We present a Raman study of the effects induced by pressure, thermal treatments, and O₂ loading in MCM41, MSU-H, and MSU-F representative mesoporous silica. We compared the starting powders with the mechanically pressed tablets produced applying pressures of ∼0.2 and ∼0.45 GPa. The spectra of the three untreated tablets evidence that the main value of the Si-O-Si angle decreases and that in the MCM41 and the MSU-H Si-O-Si hydrolysis occurs, whereas such a process is absent or much less efficient in the MSU-F. Despite their different networks, the three powders tend to crystallize in cristobalite when treatments are at 1000 °C. The MCM41 and MSU-H tablets exhibit behavior similar to their starting powders, whereas the MSU-F tablets tend to form the tridymite crystalline phase. Such a finding could be related by the differences in the Si-O-Si hydrolysis, occurring during the tablets production. Finally, we inserted O₂ molecules within the interstices of the SiO₂ wall only in MSU-F. By monitoring the O₂ content as a function of the delay time from the loading end, we provide evidence that the O₂ remains in the interstices for a sufficiently large time to modify the material properties by inserting small molecules and not only through the surface functionalization.