Direct synthesis of functional mesoporous silica by neutral pH nonionic surfactant assembly: Factors affecting framework structure and composition

The preparation of a wide range of organically functionalized wormhole-motif and hexagonal mesoporous MSU-X silicas was achieved by a one-step synthesis process involving the simultaneous addition of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) to solutions of structure-directing nonionic surfactant micelles, followed by fluoride-mediated hydrolysis/cross-linking and surfactant extraction. The effect of various synthesis parameters, including relative reagent concentration (MPTMS/TEOS ratio), temperature, and surfactant type, on the structure and composition of the mesostructures was investigated. Generally, higher MPTMS/TEOS ratios resulted in materials with higher functional group loadings, while increasing temperature also produced more highly functionalized materials. Although increasing synthesis temperature produced materials with greater pore diameters and lattice spacings, an increased organosilane content in the mesostructures produced materials with diminished pore diameters and lattice spacings. Thus, MSU-X materials with fine-tuned composition and pore dimensions were produced by systematically varying these synthesis parameters. We propose that the amphiphilic character of the nonionic surfactants is affected both by temperature and by the addition of the comparatively hydrophobic organosilane constituent in the micelle, thus forming mesostructures with corresponding compositional and structural features.