Reactivity of fluorinated γ-alumina and β-aluminium(III) fluoride surfaces towards hydrogen halides and tert-butyl chloride

The Lewis acids beta-aluminium(III) fluoride and gamma-alumina, fluorinated at room temperature with sulfur tetrafluoride, both interact with hydrogen fluoride and chloride, as demonstrated by radiotracer measurements using [F-18] and [Cl-36]. The different behaviour of HCl towards the two surfaces is rationalised by considering plausible surface sites and, in the case of beta-AlF3 the role of residual water. Both materials promote dehydrochlorination of tert-butyl chloride. beta-Aluminium(III) fluoride also has some catalytic activity in Friedel-Crafts alkylation whereas oligomerisation of Bu'Cl dominates on fluorinated gamma-alumina. The different behaviour appears to be due to the presence of both Lewis and Bronsted surface acidity on gamma-alumina that has been fluorinated under static conditions. A description for this surface is proposed.