A comparative study of surface acidity in the amorphous, high surface area solids, aluminium fluoride, magnesium fluoride and magnesium fluoride containing iron(III) or aluminium(III) fluorides

Abstract

The behaviour of the sol–gel prepared, amorphous solids, high surface area (HS) aluminium fluoride and magnesium fluoride in promoting room temperature dehydrochlorination of tert-butyl chloride (Bu t Cl), in their catalytic activity for the dismutation of chlorodifluoromethane and in the temperature programmed desorption of ammonia is similar, indicating that, unexpectedly, both solids exhibit significant surface Lewis acidity. Using a similar approach, it has been demonstrated that surface Lewis acidity in HS-MgF 2 is enhanced by the incorporation of amorphous iron(III) fluoride and probably also by amorphous aluminium(III) fluoride. A second, unexpected feature is the substantial retention of anhydrous hydrogen chloride by all the solids, which is observed by the use of chlorine-36 labelling, when they are exposed at room temperature either to Bu t Cl or to HCl directly. The detailed behaviour of H 36Cl towards HS-AlF 3 depends on the fluorinating agent, dichlorodifluoromethane or anhydrous hydrogen fluoride, which is used in the second stage of HS-AlF 3 synthesis. This observation and the pattern of the results obtained overall lead to the proposal that strongly adsorbed HCl behaves as an unconventional Lewis base towards these solids.