Access to Local Structures of HS-AlF3 and Its Precursor Determined by High-Resolution Solid-State NMR

High-surface aluminum fluoride (HS-AlF3) is all interesting catalytic material and possesses high surface areas compared to crystalline aluminum fluorides and strong Lewis acidity. HS-AlF3 and its precursor aluminum isopropoxide fluoride are X-ray amorphous, which makes precise structural elucidations very difficult, which is, in turn, the prerequisite for understanding its extraordinary properties. For this reason Al-27 NMR using magic angle sample spinning (MAS) and multiple quantum NMR (MQMAS) has been carried out at different magnetic field strengths up to 19.9 T in order to gain maximum information about the various AlF6-O-x(x) (x = 0, 1, 2, ...) octahedra present in the precursor and HS-AlF3 Structure. The Al-27 NMR spectra are almost featureless irrespective of the applied B-0 fields because of distributions of isotropic chemical shifts and second order quadrupolar effects. Nevertheless, it was possible to verify the presence of four different octahedra in the materials following the results of earlier investigations of the sol-gel of these HS-AlF3 samples. Different oxygen-containing AlF5O and AlF4O2 are found in the precursor structure, whereas AlF6 sites determine the HS-AlF3 structure besides residual AlF5O and AlF4O2 octahedra after postfluorination of the precursor.