Mechanical stability and porosity analysis of large-pore SBA-15 mesoporous molecular sieves by mercury porosimetry and organics adsorption

The mechanical stability of SBA-15 mesoporous silicas has been assessed using powder X-ray diffraction; :nitrogen, benzene, and n-heptane adsorption; and mercury porosimetry. SBA-15 was found to possess :rather low mechanical stability as compared to other mesoporous materials, viz., MCM-41 and MCM-48. These results can be explained by a simple mechanical model, which shows that SBA-15 has an unfavorable ratio between pore diameter and wall thickness. This ratio is more favorable for MCM-41 despite its,smaller wall thickness, Our study confirmed that SBA-15 contains a significant amount of micropores in addition to the hexagonal arrangement of large mesopores. The amount (represented by the additional pore volume and surface area) of these complementary pores is tunable by variation of the synthesis temperature. It was shown that SBA-15 synthesized at temperatures higher than 120 degreesC contains almost no micropores. However, the pore size distribution of these materials is broader compared to materials synthesized at a lower temperature. The observed structural properties of the different SBA-15 materials are attributed to the changes in the degree of penetration of the poly(ethylene oxide) chains of the triblock copolymer within the silica walls of SBA-15.