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

High surface area (HS) AlF3 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 α- and β-AlF3 surfaces. Examination of the two components visible in the Al 2p emission indicates that surface Al3+ 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 Al centres with dangling F or OH groups and the appearance of an Al 2p emission component at a BE lower than in the α-AlF3 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 AlF3 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 AlF3 surfaces by dangling or bridging OH groups. These Brønsted acid species must be removed by treatment at higher temperature before HS AlF3 reagents can fully develop their Lewis acidity.