Manipulation of mesoporous structure and crystallinity in spontaneously self-assembled hierarchical metal oxides

Spontaneous self-assembly starting from liquid metal alkoxide precursors offers a convenient template-free route to hierarchical meso/macroporous metal oxide materials with unique macroporous structure. Tailoring of the macropore morphology in these materials has been the focus of recent studies. However, the freshly prepared solids contained amorphous walls and, in many cases, poor structure at the micro/meso scale. While being closely associated to one another, the formation of pores at different scales is governed by independent mechanisms. Examined is the extent of manipulation and control of the structure at meso scale by means of hydrothermal aging in alumina, titania and zirconia materials that possess macroporous structure. The influence of this aging on the textural properties and crystalline phase composition as well as on the previously formed macropore structure is demonstrated. The results indicate that hierarchical materials with well-defined mesopores and macropores with crystalline walls can be successfully synthesized for all three metal oxides by a selective combination of spontaneous self-assembly, hydrothermal aging and thermal treatment. Aging results in the formation of crystalline phases for aluminum hydroxides/oxyhydroxides and titania, while the aged zirconia materials were amorphous. Hydrothermal treatment of the titania and zirconia materials produced higher surface area and mesopores with larger apertures while preserving the macropore structure. However, for aluminum hydroxides/oxyhydroxides, there was a relatively small increase in surface area, which was also accompanied by deterioration of the macropore structure.