Abstract
The problem concerning the nature of Bronsted acidity of amorphous silica-aluminas has been discussed for years but still is not completely solved. Therefore, highly dehydroxylated powdered samples of amorphous silica-alumina M-46 ''Houdry'' with Al/sub 2/O/sub 3/ content of 12 wt% were used. The samples were heated under vacuum at 1070 K and pressure of 10/sup -2/ N m/sup -2/ for 2 h. Acidic hydroxyls were regenerated by adsorption of small amounts of H/sub 2/O (D/sub 2/O) at room temperature. Infrared spectra of OH (OD) groups were measured in diffuse scattered light in the range 2000-4000 cm/sup -1/ using a double-beam ratio recording Perkin-Elmer 580B spectrophotometer and special ir cells supplied with CaF/sub 2/ windows. Results show an obvious similarity between acidic OH groups of amorphous silica-alumina and zeolites. Their amount in silica-alumina is however much lower. For M-46 it is of the order of 5 x 10/sup 19/ OH/g or only about 1% of the total amount of surface silanol groups. In conclusion, it should be noted that after adsorption of D/sub 2/O the acidic OH groups also can be detected by ir spectroscopy for samples pretreated at lower temperatures, for instance at 770 K. For these samples, however, themore » corresponding bands are not so pronounced because the major part of acidic groups likely interacts with neighboring silanol groups via H bonding.« less
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