Design of Well-Defined Mesoporous Silicas via Surfactant Templating Method Enhanced by the Use of Swelling Agents

Surfactant-templated ordered mesoporous materials continue to attract tremendous attention as these materials are characterized by reproducibility and predictability of their synthesis as well as their wide range of potential applications, which serve as future opportunities for additional advancement. The main purpose of this dissertation is to advance the understanding how to control the structural features and properties in the synthesis of well-defined porous materials via surfactant templating method, while keeping in mind that the uniformity of pore size and structural ordering are essential characteristics for these well-defined materials. The work was primarily focused on the issue of the unit-cell size and pore size adjustment in the large-pore domain (that is, for pore diameters above 12 nm) for two-dimensional hexagonal silica structures with cylindrical pores (referred to as SBA-15 silicas). The use of common poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, surfactants, commercially known as Pluronics® in combination with appropriate hydrophobic micelle swelling agents was pursued. The main hypothesis was that it is possible to judiciously select surfactant/swelling agent pairs to achieve optimal structural adjustment capabilities. Moreover, it was hypothesized that different surfactant/swelling agent pairs may work most effectively in certain temperature intervals. The choice of Pluronic tri-block copolymer as main templating agent, the selection of micelle expanders, the adjustment of initial synthesis temperature (including room temperature conditions), and the adjustment of the amount of silica precursor (tetraethylorthosilicate) can systematically affect the structure of porous silica materials formed via the surfactant-micelle-templating synthesis approach. The advancement in pore size tailoring discussed in this dissertation focuses on the above aspects. In particular, considerations based on the extent of solubilization of organic compounds in Pluronic surfactants paved the way to the identification of new excellent swelling agent for the synthesis of large-pore SBA-15. Ordered SBA-15 silica with d100 interplanar spacing of up to about 30 nm has been successfully synthesized. Modifications in the synthesis approach in terms of shortening the duration of the synthesis to as little as six hours and eliminating the need for temperature control to carry out the SBA-15 synthesis were found effective for the synthesis around room temperature. Highly ordered face-center-cubic silica materials have also been synthesized using surfactants with moderate content of the hydrophilic PPO domains challenging our current understanding of viable selections of surfactants for the synthesis of materials with spherical mesopores. These materials exhibited increased mesopore volumes when compared to the silicas obtained via the traditional synthesis of materials with spherical pores involving block copolymers with a high fraction of the hydrophilic PEO domains. Overall, the dissertation demonstrates that the synthesis mixture composition and synthesis conditions can be predictively selected to achieve particular structural properties (such as unit-cell size) or to observe formation of material under particular conditions. Moreover, some additional opportunities emerge as we explore the predictability of the synthesis pathways