Structure and properties of amorphous silico-aluminas. Part 5.—Nature and properties of silico-alumina surfaces

Abstract

In order to find to what extent the surface properties of amorphous silico-aluminas and aluminas are related to the atomic organization, the radial electron distribution and X-ray fluorescence spectroscopy were used to determine the kind of structure defects. For high-surface area solids (>100m²/g) a noticeable fraction of the cations and oxygen anions are in the first or the two first surface layers so that the averaged structure parameters obtained from these methods integrate the bulk and the surface organization. Assuming that the defects in the bulk and on the surface are comparable, it should be possible to correlate surface properties, such as the acid properties, with the nature of these defects. Moreover, if some quantitative agreements appear between e.g., the total number of defects and the number of surface acid sites of some sorts, the defects concentrations on the surface phase might be related to the total number of defects. The two above assumptions seem to be acceptable. The three main structure defects appear to be (i) the aluminium cations substituting silicon cations in tetrahedral coordination; (ii) aluminium cations in a perturbed tetrahedral arrangement and (iii) silicon in a perturbed tetrahedral arrangement. In these perturbed tetrahedral arrangements, one at least among the four oxygens might be displaced, exposing partially the cations to the outside. In aluminas, defects of type (ii) are the most probable; they are related to the Lewis acidity. In silico-aluminas, throughout the range of composition, 0<R= Al₂O₃/(Al₂O₃+SiO₂)⩽1, defects of type (i) are at the origin of the Brönsted acid sites whilst for R >0.5, defects of type (iii) could provide a new source of Lewis acidity. These observations are supported by the variations in the catalytic cracking properties. Defects of type (ii) are also probable in the composition range 0.3<R>0.6. Also infra-red spectroscopic features of adsorbed NH₃ seem to confirm these deductions.