Comments on the²⁷Al NMR Visibility of Aluminas

Abstract

The influence of surface area, paramagnetic impurities, and spinning speed on the 27 Al MAS NMR visibility in a number of γ- and η-Al 2 O 3 samples was investigated. It is shown that magic angle spinning (MAS) under typical conditions (∼10 kHz spinning speed at a 9.4 T field) is insufficient to resolve all aluminium sites in the samples. Since, in general, the number of strongly distorted sites in aluminas increases with the surface area, the 27 Al MAS NMR signal intensities at 10−15 kHz spinning decrease. Differences between γ- and η-Al 2 O 3 intensities are explained by their structural differences, causing weaker distortions in η- than in γ-Al 2 O 3 . At spinning speeds up to 29 kHz all aluminium is accounted for in the MAS spectra. Paramagnetic iron impurities up to 1440 ppm do not have a significant influence on the 27 Al MAS NMR visibility of the investigated samples.