Influence of the sol-gel acidic synthesis conditions on the porous texture and acidity of SiO2-Al2O3 catalysts with a low Al proportion

The relationship between the porous texture, the coordination number of Al atoms, the type of surface acidity, and the sol-gel acidic synthesis conditions at two pH values (pH approximate to 2 and pH approximate to 0), was studied in SiO2-Al2O3 aerogels made with a molar Si to Al ratio r(Si/Al) = 100. The influences of chelation of the Al precursor and of the rate of addition of this Al precursor to the Si precursor sol were also investigated and found to have minor effects. Infrared absorbance after pyridine adsorption, Al-27 nuclear magnetic resonance (NMR), and nitrogen adsorption isotherms by the Brunauer, Emmett, and Teller (BET) method showed that the structure of the gel network considerably changed depending on the pH, with most Al atoms having a coordination number of TV at pH approximate to 2 while they had a coordination number of VI at pH approximate to 0. This result was also found to correlate very well with an enhancement of Bronsted acidity at pH approximate to 2 by comparison with Lewis acidity, and also of the specific surface area.