Is there any microporosity in ordered mesoporous silicas?

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Nanostructured silicas like MCM-41 and SBA-15 are crucial materials with tunable porous architectures.
• Understanding their precise pore structure, including the presence and location of micropores and mesopores, is essential for optimizing their applications.

Purpose of the Study:

• To experimentally differentiate and characterize the pore structures of MCM-41 and SBA-15.
• To determine the presence and volume of microporosity in SBA-15 and its location within the material.

Main Methods:

• Nitrogen (N2) adsorption isotherms at 77 K were performed before and after selective micropore blocking with n-nonane.
• Immersion calorimetry using liquids of varying molecular dimensions (n-hexane, 2-methylpentane, 2,2-dimethylbutane) was employed.

Main Results:

• Selective blocking with n-nonane confirmed MCM-41 is exclusively mesoporous.
• SBA-15 was shown to possess both micropores and mesopores.
• N2 adsorption on preadsorbed samples indicated micropores in SBA-15 are intrawall, comprising only 7-8% of the total pore volume.
• Calorimetric measurements estimated the size of these intrawall micropores to be less than or equal to 0.56 nm.

Conclusions:

• MCM-41 and SBA-15 exhibit distinct porous structures, with SBA-15 containing a small fraction of intrawall microporosity.
• The combined techniques of N2 adsorption and immersion calorimetry provide a robust method for characterizing complex nanoporous materials.