The very strong solid Lewis acids aluminium chlorofluoride (ACF) and bromofluoride (ABF)—Synthesis, structure, and Lewis acidity

Abstract

Aluminium chlorofluoride, ACF, (AlCl x F 3− x , x ≈ 0.05–0.25) is an amorphous solid Lewis acid with an extraordinary acid strength that was patented by DuPont in 1992. It reaches the acidity of antimony pentafluoride, SbF 5, and, in some cases, even exceeds it. In this review, an overview of known structural data of ACF and the corresponding bromine compound, ABF, is presented, together with additional unpublished data. This includes a discussion of soft chemistry synthesis, structure of bulk and surface, and Lewis acidities of molecular and solid group 13 halides. The order of Lewis acidity of gaseous halides is established to be AlF 3(g) < AlCl 3(g) < AlBr 3(g) ≈ AlI 3(g) and AlX 3(g) > GaX 3(g) > InX 3(g) (X = F, Cl, Br, I). ACF and ABF are highly distorted solids. Different spectroscopic bulk methods show that the heavy halogen, chlorine or bromine, is a necessary perturbation that generates “disorder”. Highly Lewis acidic centres have been detected by several methods on the surface of ACF and ABF, which are responsible for the high catalytic reactivity observed in many reactions. The data gained on ACF and ABF show that many published data about heterogeneously catalysed fluorine–halogen exchange reactions in presence of aluminium chloride and bromide must be treated critically. In general, ACF and ABF are the active species.