Pore Expansion in Fluorinated Surfactant Templated Silica Powders by Supercritical Carbon Dioxide Processing

The effect of supercritical CO₂ processing of mesoporous silica particles synthesized using a perfluoroalkylpyridinium chloride surfactant template is reported here. Surfactant templated nanoporous silica has generated much interest because of its broad range of potential application in separation, adsorption, catalysis, biomimetics, and sensing. This work extends our recent demonstration of the CO₂ processing of acid-catalyzed silica thin films to base-catalyzed powders. Pore diameter, wall thickness, and surface area analysis has been performed using nitrogen adsorption, x-ray diffraction (XRD), and transition electron microscopy (TEM). Templated silica processed in supercritical CO₂ (34-139 bar, 45 °C) retained a well-ordered, 2D hexagonal structural framework while demonstrating pore diameter expansion relative to unprocessed silica. The aging time of the sol prior to supercritical CO₂ exposure (which determines the degree of silica condensation) was varied (20 min to 24 hr) to assess its effect on pore structure. Shorter aging periods (less than 2 hrs) resulted in increased pore expansion, surface area, and pore order over the range of CO₂ processing conditions. The observed pore expansion (as great as 3.11 nm at 139 bar and 45 °C) indicates significant CO₂ – fluorosurfactant interactions when compared to previously reported results of supercritical CO₂ processing of hydrocarbon surfactant templated materials.