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 Zn2+-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-AlF3 and small amounts of β-AlF3 are formed at a temperature of 400 °C with an HF/N2 ratio of 1/1. Even higher fractions of β-AlF3 are formed from the impregnated phases for HF levels of 30% and higher, along with a significant concentration of α-AlF3. In contrast, fluorination of Zn2+-impregnated γ-Al2O3 in dilute HF feeds resulted in formation of the aluminum (hydroxy)fluoride pyrochlore phase, AlF2OH, in significant quantities. Thus, even very low levels of transition metals (