Pore size engineering in fluorinated surfactant templated mesoporous silica powders through supercritical carbon dioxide processing

Pore expansion of fluorinated surfactant templated mesoporous silica powders is demonstrated as a function of pressurized CO 2 processing conditions. Mesoporous silica powder is synthesized by sol–gel reaction induced precipitation in a base-catalyzed medium using 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl)-pyridinium chloride (HFOPC) as a template and, immediately after filtration, the precipitated material is processed in gaseous and supercritical CO 2 (88–344 bar, 45 °C) for 48 h. Characterization of the silica powders by XRD, TEM and N 2 adsorption reveals the formation of well-ordered materials with 2D hexagonal close-packed pore structure before and after CO 2 processing. An optimal aging time (time from addition of silica precursor to the sol until the filtration of the hydrolyzed sol) of 20 min prior to CO 2 processing is identified. Proper aging time results in silica powder with significant pore expansion at all processing pressures while retaining the long-range structure of the material. The pore diameter of the mesoporous material increases with increasing CO 2 pressure (from 2.60 nm (unprocessed) to 3.21 nm at 344 bar), but appears to level off above 100 bar. The pore expansion behavior is attributed to favorable CO 2 penetration in the ‘CO 2 -philic’ fluorinated tails of the surfactant template. The CO 2 expansion of base-catalyzed silica powders is significantly less than we previously observed for acid catalyzed, evaporation-driven thin film synthesis using fluorinated cationic surfactant templates. The effect of pH on self-assembly and increased silica condensation in basic conditions may inhibit pore expansion by CO 2 .