Synthesis of Mesoporous Materials: Liquid-Crystal Templating versus Intercalation of Layered Silicates

A comparison of the synthetic conditions required to form both MCM-41 and mesoporous materials obtained from layered silicates suggests that the mechanisms of formation of these two materials are different. X-ray diffraction patterns, transmission electron micrographs, sorption capacity measurements, and synthesis data are all consistent with MCM-41 formed by a silicate anion initiated liquid-crystal templating mechanism. Retention of the silicate layer throughout the synthetic process suggests that layered silicate derived materials are formed by the intercalation of the silicate layers. Although both materials are formed in the presence of a cationic surfactant, the surfactant functions in different roles. For MCM-41, the surfactant molecules form a micelle/liquid-crystal phase that serve as templates around which the silica condenses. The surfactant molecules in the layered silicate system function to swell and separate the layers. Rheological data of the surfactant solutions used in both synthesis systems also support the different formation mechanism pathways. At the surfactant concentrations typically used for MCM-41 formation, rheological data suggest the presence of micellar structure, whereas no solution structure was detected in the lower surfactant concentrations used in the layered silicate system. Although the intercalated silicate based materials can be synthesized with pores in a similar size range as MCM-41 products, the pore size distribution is broader. Also, MCM-41 materials have 5 times the total pore volume and hydrocarbon sorption capacity compared to the layered silicate derived materials.