Abstract
Acidities of silica-alumina systems are studied by the CNDO/2-FA method. A hypothetical atom is suggested for saturating the covalence of the outermost oxygen atoms of the models. It turns out that the hydrogen charge Q H can serve as a rough measure of Brønsted acidity of silica-alumina systems, whereas the LUMO energy as a measure of Lewis acidity can only be used for comparing the generally accepted Lewis sites of the same kind of atom in the same valence state and with the same coordinate number. Using the Q H data, the strong Brønsted site is assigned to the structure AlOHSi, and the moderate one is assigned to the SiOH in connection with a four-coordinate aluminum. The Q H data do not support the view that Si-OH in connection with a three-coordinate aluminum or AlOH is the strong Brønsted site.
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