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.