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 CO2 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 CO2 (88–344 bar, 45 °C) for 48 h. Characterization of the silica powders by XRD, TEM and N2 adsorption reveals the formation of well-ordered materials with 2D hexagonal close-packed pore structure before and after CO2 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 CO2 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 CO2 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 CO2 penetration in the ‘CO2-philic’ fluorinated tails of the surfactant template. The CO2 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 CO2.