Comments on the 27Al NMR visibility of aluminas

The influence of surface area, paramagnetic impurities, and spinning speed on the Al-27 MAS NMR visibility in a number of gamma- and eta-Al2O3 samples was investigated. It is shown that magic angle spinning (MAS) under typical conditions (similar to 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 Al-27 MAS NMR signal intensities at 10-15 kHz spinning decrease. Differences between gamma- and eta-Al2O3 intensities are explained by their structural differences, causing weaker distortions in eta- than in gamma-Al2O3. At spinning speeds up to 29 kHz all aluminium is accounted for in the MAS spectre. paramagnetic iron impurities up to 1440 ppm do not have a significant influence on the Al-27 MAS NMR visibility of the investigated samples.