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, alpha-AlF3 and small amounts of beta-AlF3 are formed at a temperature of 400 degreesC with an HF/N-2 ratio of 1/1. Even higher fractions of beta-AlF3 are formed from the impregnated phases for HF levels of 30% and higher, along with a significant concentration of alpha-AlF3. In contrast, fluorination of Zn2+-impregnated gamma-Al2O3 in dilute HF feeds resulted in formation of the aluminum (hydroxy)fluoride pyrochlore phase, AlF2-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 beta-AlF3 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 Zn2+ ions more readily than the vacancies in the beta-AlF3 structure. Basic probe molecules (dimethylphenylphosphine), in combination with P-31 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 Zn2+ 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.