Stabilization of Silica Mesophases and Post-synthesis Hydrothermal Treatment

3 Stabilization of Silica Mesophases and Post-synthesis Hydrothermal Treatment 

Zeolites such as Y, Beta, and ZSM-5 are widely used commercial catalysts, but their applications are strongly limited by their small pore sizes. One solution is to use ordered mesoporous materials such as MCM-41 and SBA-15. These materials exhibit good catalytic properties for the catalytic conversion of bulky reactants. Although the mesoporous silica materials made from normal synthesis processes, such as MCM-41 and MCM-48, have high thermal stability, their hydrothermal stability is poor. Calcined samples can be destroyed with moisture or water, even at room temperature. Most calcined samples became amorphous in cold water within a few minutes. The main reason is the hydrolysis reaction of the amorphous silica wall (Si—O Si bonds broken). 

The relatively low catalytical activities of mesoporous materials such as MCM-41 and MCM-48, as compared with zeolites, can be typically attributed to the low acidity or low oxidation ability of catalytically active species, which is strongly related to the amorphous nature of the mesoporous walls. Therefore, increasing acidity, oxidation ability, and hydrothermal stability are great goals for rational syntheses of ordered mesoporous materials. 

Through studies over the last few decades, several methods have been developed for improvement of the stabilities of mesoporous silica materials. These strategies were shown to increase either the wall thickness or the degree of polymerization of the silica wall. Incorporation of metal oxides, such as zirconia, alumina, and vanadium oxides, was also demonstrated to be an effective method to improve the hydrothermal stability. 

The main routes for improving hydrothermal stability of mesoporous silica materials will be discussed in this section. 

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