Neck Size of Ordered Cage-Type Mesoporous Silica FDU-12 and Origin of Gradual Desorption

To evaluate the neck size of ordered cage-type mesoporous silica FDU-12 and also to elucidate the origin of the gradual desorption always observed for the ordered silicas with cage-like pores of large necks, we measured successive adsorption of water at 283 K and nitrogen or argon at 77 K on five kinds of FDU-12 samples. For all the materials examined here, the amount of nitrogen or argon condensed inside the large cavities decreased rapidly over a relatively small range of water filling with an increase in water filling. Eventually, it reached a small value at water filling, well before the onset of pore condensation of water inside the large cavities. This clearly indicates that most of the cavities are isolated from bulk nitrogen or argon gas with water frozen in the necks. With the assumption that the necks in cage-like pores are cylindrical in shape, we assessed the pore size distribution of the necks available for desorption from the equilibrium relative pressure of water, in which the amount of nitrogen or argon condensed in the large cavities drops considerably. The neck size in the materials is increased with an increase of the hydrothermal treatment temperature and time. For a sample hydrothermally treated at 373 K for 7 days, the hysteresis loop of argon at 77 K gradually closed above the corresponding lower limit of the adsorption hysteresis, as is often observed in cage-like pores of large necks. However, the wide distribution of the capillary evaporation pressure did not correspond directly to the pore size distribution of the necks in the material.