Ordered Macroporous Silica by Ice Templating

Abstract

Ordered macroporous silica, a silica gel microhoneycomb (SMH), has been prepared through a method which uses micrometer-sized ice crystals as a template. Template ice crystals, which have a continuous rod shape, a polygonal cross section, and ordered diameters, were grown inside precursor silica hydrogels under a condition where the pseudo-steady-state growth of them continues. Besides their ordered macroporosity, micro-/mesopores develop inside the honeycomb walls through the freeze-drying of SMHs soaked in tert-butyl alcohol. SMHs have straight and polygonal macroporous voids, which are created and retained through the formation and removal of the ice crystals. Micromorphology including macropore size and wall thickness, micro-/mesoporosity inside the honeycomb walls, and thermal stability of SMHs were investigated in detail through scanning electron microscopy observation, nitrogen adsorption−desorption measurements, and thermogravimetric analysis. It was found that the macropore size of the SMHs can be controlled by changing the immersion rate into a cold bath and the freezing temperature without changing the micro-/mesoporosity of their honeycomb walls. It was also found that the thickness of the honeycomb walls was affected by the SiO2 concentration and the macropore size. On the other hand, the porosity of the honeycomb walls could be controlled to be microporous as well as mesoporous by hydrothermal treatment of as-prepared SMHs in basic aqueous solutions. Moreover, it was found that SMHs with developed mesopores showed a higher stability against heat treatment.