Microwave synthesis of an aluminum fluoride hydrate with cationic vacancies: structure, thermal stability, and acidic properties
Résumé
Aluminum fluoride hydrate was synthesized by a microwave hydrothermal process. The structure derives from the ReO3 type structure, that is, the high temperature structure of α-AlF3. The cubic symmetry adopted by this compound arises from water molecules, stabilized as ligand surrounding Al3+ cations which induces cationic vacancies as revealed by Rietveld refinement. The following chemical formula Al0.82◻0.18F2.46(H2O)0.54 is supported by chemical analysis and TGA measurements. This represents the first example of aluminum vacancy compound in the Al-based fluorides chemistry. High field 27Al NMR spectroscopy enabled identification and quantification of the following species: AlF6 and AlF6−x(H2O)x with x = 1, 2, 3, and showed that vacancies are mainly surrounded by water molecules but also by a low content of fluoride ions as also evidenced by 19F NMR. The hydrogen bonding network, which takes place in the vicinity of the cationic vacancies, was characterized by FTIR and 1H NMR spectroscopies. A 2:1 complex X···H−O−H···X where X is a proton acceptor, which is related to a shift of the νsym(OH) and νasym(OH), was detected. This complex appears to be stable up to 773 K. The phase transition into the rhombohedral form occurs at around 573 K, but at such a temperature a large amount of water molecules associated to cationic vacancies still remained thanks to the 2:1 complex. The acidic properties have been revealed by pyridine and CO probe molecules. At 573 K, the solid exhibits both strong Lewis and Brønsted acidities with an equivalent number of sites providing bifunctionality. The strong acidic behavior highlights the effect of water molecules/cationic vacancies on the surface structure. Whereas the Lewis strength acidity progressively decreases with dehydration, Brønsted acidity remains strong even at 773 K.