Non-aqueous synthesis of high surface area aluminium fluoride-a mechanistic investigation

Abstract

Fluoride can do it too! Sol–gels of metal fluorides play an important role in the formation of high surface area metal fluorides. The synthesis of amorphous high surface area metal fluorides via a recently discovered two-step synthetic route was investigated in detail, exemplified for aluminium fluoride. The first step is fluorination of aluminium alkoxide with anhydrous HF in organic solvents, which proceeds as a sol–gel process known until now only for metal oxide formation. The reaction pathway is illustrated including crystal structure determination of the intermediate aluminium alkoxide fluoride. The resulting amorphous aluminium alkoxide fluoride has to be freed in a second step of solvating alcohol and of residual alkoxidic groups. This is done by heating in a stream of a mild fluorinating agent like a HCFC or CFC or in HF to obtain high surface area and very high Lewis acidity; an inert gas such as N₂ is not sufficient. Using a variety of analytical techniques, including liquid and solid state NMR, X-ray structure analysis and XPS, the reaction pathways have been elucidated.