Surface modification of fluorinated aluminas: Application of solid state NMR spectroscopy to the study of acidity and surface structure

Solid-state NMR methods have been used to determine the mechanism for the fluorination reaction of CHClF2 on alumina. The initial stages of fluorination lead to the formation of terminal FAl groups (i.e., fluorine bound through a single bond to an aluminum atom), which is consistent with a recent XPS report [O. Boese, W.E.S. Unger, E. Kemnitz, S.L.M. Schroeder, Phys. Chem. Chem. Phys. 4 (2002) 2824]. 27Al NMR indicates that five-coordinate aluminum sites are initially consumed during the fluorination reaction, suggesting that the five-coordinate aluminum species are predominately localized near the surface of the high surface area (∼3–5 nm) alumina particles. The sorption of basic phosphines as probe molecules for acid sites, coupled with 31P NMR, was used to follow the changes in acidity after fluorination of the surface. The untreated alumina samples do not contain Brønsted acid sites, partial fluorination of the surface leading to their creation. In contrast, the Lewis acid sites, initially present on the untreated alumina, decrease in concentration on fluorination. Continued fluorination of the surface eventually leads to the destruction of the Brønsted acid sites, some Lewis acid sites remaining but in lower concentrations than on the initial untreated alumina.