Enhanced Lewis acidity by aliovalent cation doping in metal fluorides

A model regarding the generation of acidity in binary metal fluorides has been proposed and its validity has been examined for several binary fluoride systems with the general compositions MF3/M′F3 and MF2/M′F3. In accordance with this hypothesis, the binary systems (CrF3/AlF3, CrF3/FeF3 and AlF3/VF3) do not show acidities larger than the sum of the acidities of the component fluorides. The hypothesis predicts the generation of Lewis acidity when MF2 is the major component (host) and generation of Brønsted acidity when MF3 acts as the host for the MF2/M′F3. The experimental results (surface acidity and catalytic activity) confirmed the predictions made from this hypothesis for binary combinations MgF2/ M′F3 (M′=Cr, Al, Fe, V). The application of this model is discussed in terms of other parameters: ionic radii and the fluoride affinity of the metal fluorides involved.