The catalytic fluorination of hydrocarbons facilitates the large-scale production of
chlorofluorocarbons for a wide range of applications including aerosol propellants, refrigerants
and solvents. Lewis acid catalysts, such as Swarts catalysts based on antimony
pentafluoride, are commonly used. Recently, a sol-gel based synthesis method has been
developed which yields very high surface area aluminium fluoride (HS-AlF3) that has a
Lewis acidity comparable to that of the Swarts catalysts. This makes HS-AlF3 a promising
candidate for use in several Lewis acid catalysed reactions. Despite the importance of
the surface in the catalytic process little is known about the detailed atomic scale structure
of AlF3 surfaces.
Surface thermodynamics calculations, based on hybrid-exchange density functional
theory, are employed to predict the composition and structure of AlF3 surfaces. The
surfaces of AlF3 expose under coordinated Al ions that are potential Lewis acid sites.
Under standard atmospheric conditions the AlF3 surfaces are shown to adsorb water above
the under coordinated Al ions. Theoretical characterisation of the under coordinated Al
ions shows that the most reactive type of site is not exposed on crystalline α-AlF3 samples,
however, it is predicted to occur in small quantities on β crystallites. It is speculated that
such sites occur in higher quantities on the high surface area materials. This result may
explain the different reactivity of α-, β- and HS-AlF3. Our detailed understanding of
AlF3 surfaces allows us to propose a reaction centre and mechanism for the dismutation
of CCl2F2 on β-AlF3.
Aluminium chloride is extensively used as a catalyst in Friedal-Crafts reactions. It
is therefore, commonly assumed that pure crystalline AlCl3 is strongly Lewis acidic. Ab
initio surface thermodynamics calculations are used to study the surfaces of crystalline
AlCl3 and show that it is chemically inert.