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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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