Electronic structure of Lewis acid sites on high surface area aluminium fluorides: a combined XPS and ab initio investigation

High surface area (HS) A1F3 samples have been examined by X-ray photoelectron spectroscopy (XPS). The experimentally observed binding energy (BE) shifts were analysed by reference to core level BEs obtained from ab initio total energy calculations on a range of different, clean and hydroxylated alpha- and beta-A1F3 surfaces. Examination of the two components visible in the A1 2p emission indicates that surface A13+ sites can, depending on the local geometric structure, contribute to both a high BE peak at 77.0 eV and a low BE peak at 76.1 eV. Consequently, the areas under the peaks do not quantitatively correlate with surface area or Lewis acidity. However, a significant correlation between the number of surface A1 centres with dangling F or OH groups and the appearance of an A1 2p emission component at a BE lower than in the alpha-A1F3 bulk is predicted. The experimental F 1s emission data indicate that dangling F species are essentially absent. Examination of the O 1s emission suggests that HS A1F3 handled at room temperature under any practical laboratory conditions, including glovebox environments, probably contains intrinsically a significant amount of OH groups and adsorbed water, which results in the covering of A1F3 surfaces by dangling or bridging OH groups. These Bronsted acid species must be removed by treatment at higher temperature before HS A1F3 reagents can fully develop their Lewis acidity.