It is shown by 27Al MAS-NMR that, besides tetrahedral framework Al (~60 ppm), extraframework pentacoordinated or tetrahedrally distorted Al (~30-40 ppm), and octahedral extraframework aluminum (~0 ppm), tetrahedral Al in amorphous silica-alumina is formed during the dealumination of Y zeolite, either by steam or by SiCl4 treatment. This silica-alumina show characteristic hydroxyl groups at 3600-3610 cm-1, with acid strength higher than that of zeolite framework hydroxyls. By mild acid or basic treatments it is possible to change the proportion of the different types of aluminum that affect the cracking of gas-oil. It has been found that in highly steam-dealuminated HY zeolites a radical, cracking mechanism increases the formation of C1 and C2 on HY zeolites with less than 5 Al per unit cell.
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