Mesostructure Control Using a Titania-Coated Silica Nanosphere Framework with Extremely High Thermal Stability

We report on a new synthetic method for mesoporous titania/silica (MTS), which is framed by titania-coated silica nanospheres. The MTS has extremely high thermal stability (stable up to 1000 °C), a high surface area (690−480 m 2 /g), a high pore volume (0.65−2.16 cm 3 /g), and a narrow pore size distribution. The anatase−rutile transformation of the MTS was observed at 1400 °C, which is the highest transformation temperature ever reported. We deposited titania thin film with a thickness of about 1.5 nm on surface of independent silica nanospheres (5 nm in particle diameter) via the sol−gel reaction of titanium isopropoxide in as-prepared sol of a silica nanosphere−citric acid nanocomposite. After eliminating the citric acid by calcination at 500 °C, we successfully synthesized the MTS, and by varying the concentration of the citric acid, we could precisely control the pore diameter from 3.7 nm up to 25 nm. Despite the calcination at 1000 °C, the pore diameter of the MTS was maintained with narrow pore size distribution, high surface area, and high pore volume. In addition, we successfully prepared MTS membranes that were supported on porous stainless steel. These MTS membranes have a high gas permeance and stability against water vapor. Moreover, by using MTS with various pore diameters and pore volumes, the permeance and pore diameter of the MTS membrane were easily controlled.