A Combined Solid-State NMR and Diffraction Study of the Structures and Acidity of Fluorinated Aluminas: Implications for Catalysis

We have investigated the structures of a series of pristine and Zn 2+ -impregnated aluminas following fluorination with HF, by using both solid-state NMR and X-ray powder diffraction methods. In the absence of any cation impregnation, α-AlF 3 and small amounts of β-AlF 3 are formed at a temperature of 400 °C with an HF/N 2 ratio of 1/1. Even higher fractions of β-AlF 3 are formed from the impregnated phases for HF levels of 30% and higher, along with a significant concentration of α-AlF 3 . In contrast, fluorination of Zn 2+ -impregnated γ-Al 2 O 3 in dilute HF feeds resulted in formation of the aluminum (hydroxy)fluoride pyrochlore phase, AlF 2 OH, in significant quantities. Thus, even very low levels of transition metals (<2 wt %) appear to play an important role in controlling the phase formed during the fluorination reaction. The formation of the metastable β-AlF 3 and pyrochlore phases, which both contain three- and six-membered Al rings, is ascribed to the presence of high concentrations of protons, either in the starting material or in the fluorinating agent. The pyrochlore phase is stabilized in the presence of Zn, presumably due to the vacant A site in this structure, which can accommodate Zn 2+ ions more readily than the vacancies in the β-AlF 3 structure. Basic probe molecules (dimethylphenylphosphine), in combination with 31 P NMR, were used to study and quantify the surface acidic sites that are formed as a result of fluorination. On the basis of our work, we suggest that the Zn 2+ ions are not directly implicated as catalytically active centers. The presence of Zn causes an associated increase in the concentration of coordinatively unsaturated aluminum sites, which can then act as the catalytically active centers for F/Cl exchange reactions.