Synthesis and characterization of PSU-1, a novel cage-like mesoporous silica

The synthesis of an ordered mesoporous silica, PSU-1 (Pennsylvania State University-1), has been successfully accomplished by co-crystallizing the reactive gels of SBA-15 and MCM-41 under microwave-hydrothermal (M-H) conditions at 373 K. The crystallized framework was characterized by means of powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), pore size entrance modification, and nitrogen adsorption. The material has a specific Brunauer-Emmett-Teller (BET) surface area of about 700 m² g^-1 and a pore volume of about 0.55 cm³ g^-1. Characterization data have demonstrated the presence of a cage-like framework for PSU-1 similar to that observed in FDU-1 and SBA-16. The cavity size for the cage-like framework of PSU-1 was found to depend on the chain length of the cationic surfactant used for MCM-41 gel formation. The PSU-1 framework with uniform pore size distribution was not obtained under conventional hydrothermal conditions suggesting that its formation depends highly on the synthesis methodology. Interestingly, the cage-like framework was found to undergo recrystallization under hydrothermal conditions to a hexagonal phase sinfilar to SBA-15. The interaction between the cationic and neutral surfactants and the resulting mixed micellar environment has been considered to explain the crystallization mechanism for the mixed gel system. Based on such an understanding and available characterization data, a hybrid crystallization mechanism of (S°H⁺X^-)(S⁺X^-I⁺) has been proposed for the formation of PSU-1.