Infrared Study of the Surface Properties of HTB-Type Al−, Cr−, Fe−Hydroxyfluorides

The surface properties of iron, chromium, and aluminum fluorides in their hexagonal tungsten bronze (HTB) form have been investigated by infrared spectroscopy. The presence of hydroxyls is clearly observed. H/D exchange experiments with different deuterated probe molecules having various molecular sizes show that they are localized inside of the channels of the structure. The ν(OH) bands in the infrared spectra of these materials are downward shifted compared to those of corresponding metal oxides, whereas the δ(OH) in-plane bending mode presents an unusual high wavenumber. These spectral features are compared to those observed on zeolites, for which hydroxyl environment and bridged conformation are similar. HTB compounds exhibit both strong Lewis and Brønsted acid sites. The use of two basic probes with a different size (pyridine and ammonia) allows one to localize and to quantify these two kinds of acidity. Brønsted acidity is related to the presence of hydroxy groups into the microporous channels and to chemisorbed HF, whereas Lewis acidity is due to defect sites both on the outer surface of crystallites and in the channels. The strength of acid sites is unambiguously found stronger than that reported for Al, Cr, and Fe oxides. This result is discussed taking into account the electronegativity of fluorine but also the bridged conformation of OH groups, as in the case of zeolites.