On the nature of the Lewis acid sites of aluminum-modified silica. A theoretical and experimental study

The structures and relative Lewis acidities of the catalytic sites of four aluminum-modified silicas have been studied by means of theoretical calcula tions on model compounds, as well as by spectroscopic and catalytic experim ents. Theoretical calculations indicate that the silica treated with Et2AlC l has aluminum sites that are more acidic than those of AlCl3. The thermic treatment of the aluminum-modified silicas leads to the appearance of new, very strong acidic sites, as shown by infrared and reactivity experiments. A possible structure for these sites is proposed on the basis of the theore tical calculations.