Hafnia membranes

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

Hafnia, which can be sintered up to 1850° C without transition of the monoclinic form, was used for preparing NF membranes, showing a MWCO of 420 and a pore size close to 1 nm by sintering at 450°C [36]. Various types of metal oxides such as Sn02 [37], ZnAl204 [38], which were prepared by the sol-gel process, have been proposed for the fabrication of porous membranes, although the pore size is in the UF range. 

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