The use of multiple probe molecules for the study of the acid–base properties of aluminium hydroxyfluoride having the hexagonal tungsten bronze structure: FTIR and [36Cl] radiotracer studies

Abstract

The combination of several probe molecules has enabled the construction of a detailed picture of the surface of aluminium hydroxyl fluoride, AlF_2.6(OH)_0.4, which has the hexagonal tungsten bronze (HTB) structure. Using pyridine as a probe leads to features at 1628 cm⁻¹, ascribed to very strong Lewis acid sites, and at 1620–1623 cm⁻¹, which is the result of several different types of Lewis sites. This heterogeneity is indicated also from CO adsorption at 100 K; the presence of five different types of Lewis site is deduced and is suggested to arise from the hydroxylated environment. Brønsted acid sites of medium strength are indicated by adsorption of lutidine and CO. Adsorption of lutidine occurs at OH groups, which are exposed at the surface and CO reveals that these OH groups have a single environment that can be correlated with their specific location inside the bulk, assuming that the surface OH group may reflect the bulk OH periodicity. A correlation between the data obtained from CO and pyridine molecules has been established using co-adsorption experiments, which also highlight the inductive effect produced by pyridine. Adsorption of the strong Brønsted acid, anhydrous hydrogen chloride, detected by monitoring the β⁻ emission of [³⁶Cl]–HCl at the surface, indicates that surface hydroxyl groups can behave also as a Brønsted base and that H₂O–HCl interactions, either within the hexagonal channels or at the surface are possible. Finally, the formation of strongly bound H³⁶Cl as a result of the room temperature dehydrochlorination of [³⁶Cl]-labelled tert-butyl chloride provides additional evidence that HTB-AlF_2.6(OH)_0.4 can behave as a Lewis acid.