The Lewis acids β-aluminium() fluoride and γ-alumina, fluorinated at room temperature with sulfur tetrafluoride, both interact with hydrogen fluoride and chloride, as demonstrated by radiotracer measurements using [ 18 F] and [ 36 Cl]. The different behaviour of HCl towards the two surfaces is rationalised by considering plausible surface sites and, in the case of β-AlF 3 , the role of residual water. Both materials promote dehydrochlorination of tert-butyl chloride. β-Aluminium() fluoride also has some catalytic activity in Friedel-Crafts alkylation whereas oligomerisation of Bu t Cl dominates on fluorinated γ-alumina. The different behaviour appears to be due to the presence of both Lewis and Brønsted surface acidity on γ-alumina that has been fluorinated under static conditions. A description for this surface is proposed.

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Pellets of alumina-based aerogels, doped with chromium or chromium and titanium, have been prepared according to a proposed preparation procedure from preformed and partly dried aqueous gels precipitated from metal chloride solutions. After supercritical drying, low density and high surface area (>300 m2 g-') aerogel pellets were obtained, consisting of ca. 5 nm boehmite crystals. Fluorination of such aerogels with CHF, resulted in high conversion to fluoride with the formation of mainly WAIF, and some P/y-AIF,. Substantial crystal growth was observed with uniform pellet shrinkage and a large decrease in surface area. In contrast to fluorinated materials prepared from commercial y-A1203 by fluorination with HF, fluorinated aerogels have a much more expanded and heterogeneous structure with a broad distribution of size/shape of particles and pores. Fluorinated aerogels show catalytic activity in trichloroethene hydrofluorination to CF,CH,CI.By the fluorination of alumina-based aerogels, very porous, high surface area and catalytically active fluorides can be prepared. Such highly dispersed materials are believed to be very useful in studying heterogeneous catalytic reactions of technically important fluoro-derivatives of lower halogenated hydrocarbons.