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The influence of different types of post-synthetic treatment (exposure at calcination, at boiling treatment in neutral, acidic and alkaline aqueous solutions, treatment in hydrofluoric acid) on the state of the silanol surface of the mesostructured mesoporous silicate materials МСМ-41 and SBA-15 has been studied. The material alteration has been monitored by X-ray diffraction, IR-spectroscopy, thermal analysis and nitrogen adsorption. It has been shown that post-synthetic treatment changes the concentration of silanol groups on the material surface. The limit value has been established by experiment: about 3–4 ОН/nm 2 for MCM-41 and 5–6 ОН/nm 2 for SBA-15, with its excess resulting in the material destruction. The distribution of the re-formed Si–OH groups over the surface is of irregular focal nature. In spite of the destruction, the specific characteristics of the material (inner surface, volume, and lattice parameter) remain the same. The treatment type and medium content influence the material destruction rate, but not the limit density of the silanol covering. Based on the molecular model of MCM-41 and percolation theory an interpretation has been proposed due to the mechanism of fragmentary material decay. Fragments of the surface with the concentration of the ОН-groups exceeding the limit eliminate from material because the concentration of ОН-groups is correlated with concentrations of the broken silaxane bonds (Si–O–Si), which responsible for the surface connectivity. |
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