Morphology and dispersion control of titania-silica monolith with macro-meso pore system

Macroporous gels with bicontinuous morphology in micrometer range were prepared in a titania-silica system containing 5 and 7.6 mass % titania using tetraethoxysilane and four kinds of Ti precursors, two titanium alkoxides, titanium chloride and titanium sulfate, under coexistence of poly(ethylene glycol) (PEG) with an average molecular weight of 20,000. In all the systems with different Ti precursors, the addition of PEG induced phase separation, and the macroporous morphology was formed when the transitional structure of phase separation was frozen-in by sol-gel transition of inorganic components. However, we can see large differences in phase separation tendency and Ti dispersion in silica network depending on the Ti precursors used. When titanium alkoxides were added into pure silica sol-gel system, phase separation tendency largely decreased, so that low temperature reaction was necessary for macropore formation. When we used titanium salts, on the other hand, phase separation tendency does not change much from pure silica system. The difference has been tentatively attributed to the difference in the mixing level of Ti in silica network. Although titania tended to aggregate when titanium alkoxides were used as precursors, Ti could be well dispersed in silica gel matrix when acetylacetone was added in the alkoxide system or when titanium salts were used as Ti precursors.