COMMENTS ON THE AL-27 NMR VISIBILITY OF ALUMINAS

The influence of surface area, paramagnetic impurities, and spinning s peed on the Al-27 MAS NMR visibility in a number of gamma- and eta-Al2 O3 samples was investigated. It is shown that magic angle spinning (MA S) under typical conditions (similar to 10 kHz spinning speed at a 9.4 T field) is insufficient to resolve all aluminium sites in the sample s. Since, in general, the number of strongly distorted sites in alumin as increases with the surface area, the Al-27 MAS NMR signal intensiti es at 10-15 kHz spinning decrease. Differences between gamma- and eta- Al2O3 intensities are explained by their structural differences, causi ng weaker distortions in eta- than in gamma-Al2O3. At spinning speeds up to 29 kHz all aluminium is accounted for in the MAS spectre. parama gnetic iron impurities up to 1440 ppm do not have a significant influe nce on the Al-27 MAS NMR visibility of the investigated samples.