Pore size engineering in mesoporous silicas using supercritical CO ₂

In this paper we investigate the use of supercritical carbon dioxide (sc-CO₂) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO₂₀PPO₆₉PEO₂₀), P85 (PEO₂₆PPO₃₉PEO₂₆), and F127 (PEO ₁₀₆PPO₇₀PEO₁₀₆), as a function of CO ₂ pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.