The Origin of Porosity in Hydrous Zirconia Gels. III. Sodium Hydroxide as a Precipitant

Abstract

When used as precipitants for the preparation of hydrous zirconia gels, aqueous NaOH solutions generate materials whose porosities are strongly dependent on their outgassing temperatures. Microporous solids exhibiting high apparent BET-nitrogen surface areas, and possessing considerable proportions of primary (narrow) micropores, are obtained at lower outgassing temperatures (<250°C). Thermal treatment above this temperature leads to the generation of mesoporous materials exhibiting little or no micropore volume.

It is suggested that the ready removal of loosely-bound or absorbed water from the gel at lowertemperatures, to generate an essentially microporous structure, is catalysed by the presence of NH₃ ligands. Decreasing ammonia concentration (as in the sequence NH₄HCO₃, NaHCO₃, NaOH). coupled with increasing precipitant basicity leads to a higher proportion of strongly bound water (hydroxo ligands) in the gel. This can only be removed by outgassing at higher temperatures to yield a material which is almost exclusively mesoporous.