EFFECTS OF CRYSTALLINITY AND MORPHOLOGY ON THE SURFACE-PROPERTIES OF ALUMINA

Five samples of alumina (one amorphous and four γ-aluminas prepared from different boehmites) were compared using physical methods (X-ray diffraction, transmission electron microscopy, 27Al NMR and Fourier transform infrared spectrometry) and the butene isomerization test. All the γ-alumina samples preferentially present the (110) face, whereas the ratio of the (001) and (111) faces depends on the precursor. Isomerization tests and IR experiments with different probe molecules show very similar acidities for the crystallized samples, which is discussed taking account the models of Knözinger and Ratnasamy and Tsyganenko et al. The lower Lewis acidity of the amorphous sample is related to the smaller number of AlIV sites shown by NMR experiments. The crystallinity effect is also evidenced by an IR study of the hydroxyl groups, the amorphous sample showing a specific band at 3787 cm-1. © 1990.