Aging of MCM41, MSU-H and MSU-F mesoporous systems investigated through the Raman spectroscopy

Here we report an experimental investigation, based on the Raman spectroscopy, on the aging of some mesoporous silica based systems. In details, we studied the aging in air of the MCM41, the MSU-H and the MSU-F materials by acquiring the Raman spectra of as received and of mechanically pressed, at 0.2 and 0.45 GPa, powders. Our data evidenced that the starting powders of the MCM41 and of the MSU-H undergo structural modification when they are exposed to the ambient atmosphere, such modification consisting in the decrease of the D2 Raman band (originated by the three member rings). At variance the powders of the MSU-F appear to be stable. Furthermore, by pressing the starting powders to produce tablets we evidenced in all the materials a pressure related variation of the Si-O-Si angle distribution. Simultaneously, in the MCM41 and in the MSU-H, we detected the decrease of the D2 band and the Si-O-Si hydrolysis, these latter two processes being absent or much less efficient in the MSU-F. Monitoring the Raman spectra of the tablets as a function of the time we found that the MSU-F material is stable, whereas the MCM41 and the MSU-H change. In particular, we measured a clear decrease of the D2 band and a small, but systematic, increase of the SiOH groups, this finding suggesting that probably a part of the Si-O-Si hydrolysis is still acting.