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 adsorptiondesorption 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 SiO 2 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.

Synthesis of Monolithic Microhoneycombs Equipped with Unique Hierarchical Pore Systems Using Ice Templating

“…Therefore, the sizes of the channels can be controlled by adjusting the freezing rate; a higher freezing rate leads to smaller channels 15) . From the same reason, the sizes of the channels tend to become small when the hydrophilicity of the hydrogel is high.…”

“…In the case of silica, monolithic microhoneycombs having BET surface areas over 1000 m can be easily synthesized. Moreover, like typical silica gels, such micropores can be converted to mesopores by aging the microhoneycombs in a controlled atmosphere 15) . As shown in Fig.…”

Hierarchically structured meso-macroporous aluminosilicates with high tetrahedral aluminium content in acid catalysed esterification of fatty acids

“…These strategies can be summarised into two categories. One concerns the use of templates such as polymeric beads and silica opals [14][15][16], soft sacrificial template [17], foam [18,19], emulsions [20,21], ice crystals [22], natural structures [23] such as bacterial threads [24] with the combination of amphiphilic molecules or zeolite crystal seeds [25][26][27][28]. Other one is related to post-synthesis treatments [29].…”

Synthesis, Characterization and Application of Nanostructured Porous Metal Oxides

“…6 For the preparation of macroporous TMOs, it is well known that the major technique is the "hard template" method; 7 including templating using ordered colloidal crystal, emulsion, polymer with rod-coil structure, ice, water bubbler and even bacteria. [8][9][10][11] So far, the most applied and successful template would be the colloidal particles, typically silica or polymer, such as polystyrene spheres. 2 Depending on the choice of the inorganic sources and template removal method, different single metal oxides can be readily obtained.…”