Preparation and characterisation of mesoporous, high-surface-area zirconium(IV) oxide

Abstract

Mesoporous, high-surface-area zirconium(IV) oxide may be prepared by incorporation of cationic quaternary ammonium surfactants in the hydrous oxide and subsequent calcination of the inorganic/organic intermediate. The surfactants are incorporated by cation exchange at a pH above the isoelectric point of the hydrous oxide. Powder X-ray diffraction (XRD) patterns of the materials exhibit broad, low-angle reflections indicating some ordering of the structure on a scale of 2.5 nm and above. At room temperature there is no dependence of the scale of ordering on the chain length of the surfactant. However, after calcination to 723 K and above, the ordering is a linear function of the chain length. A scaffolding, rather than a templating, mechanism is invoked to explain this ordering. Calcination of the materials to 723–973 K results in the formation of a mesoporous zirconium(IV) oxide. Materials prepared using n-alkyltrimethylammonium hydroxides of chain lengths C₁₄ and C₁₆ exhibit an increase in the pore size distribution with the chain length of the surfactant. BET surface areas of the calcined materials are 240–360 m² g^–1 depending upon the chain length of the incorporated surfactant. These values compare favourably with those of calcined ZrO₂ aerogels. Calcination of the materials to above 1023 K results in crystallisation to tetragonal zirconium(IV) oxide. Crystallisation is complete by 1323 K. It is suggested that the high surface: volume ratio of the zirconium(IV) oxide results in surface-energy stabilisation of the tetragonal as opposed to the monoclinic phase.