Synthesis and swelling of large pore diameter mesoporous silica spheres

Abstract

A synthetic protocol has been developed for the production of spherical SBA-15 type mesoporous silica particles with macroscopic particle diameters between 1 and 5 µm and mesopore diameters between 6 and 11 nm. Controlled expansion of the mesopores within silica spheres was achieved using supercritical carbon dioxide (sc-CO₂). Our method uses the tunable density of sc-CO₂ to induce the controlled swelling of the triblock copolymer surfactant templating mixtures of P123 (PEO₂₀PPO₆₉PEO₂₀) and cetyltrimethylammonium bromide (CTAB). At pressures of approximately 482 bar, pore diameters of up to 109 Å can be achieved in the P123–CTAB mixtures whilst pore ordering is still retained. This represents a pore expansion of approximately 28%, similar to that previously reported by us using a similar swelling technique (Hanrahan et al., Chem. Mater., 2004, 16, 424; Langmuir, 2005, 21, 4163). It is suspected that the spheres are formed via an emulsion templating technique with the CTAB controlling the emulsion droplet size and the P123 acting as the structure directing agent. Scanning electron microscopy (SEM) and multi angle laser light scattering (MALLS) confirmed the presence of spherical particles and particle size respectively. Transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and nitrogen absorption techniques confirmed that the sc-CO₂ treatment did not effect the hexagonal ordering of the silica. Turbidity measurements were utilized to study the growth of the spheres during the silica condensation reaction.