Structural Insights into Aluminum Chlorofluoride (ACF)

The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlClxF3-x, x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, 19F MAS NMR, and 27Al SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of (alpha)-AlF3·3H2O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF3 prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF6 octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AlF3. Terminal fluorine atoms were detected in ACF by 19F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl3 phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.