Evolution of the texture and structure of SiO2-Al2O3 xerogels and aerogels as a function of the Si to Al molar ratio

SiO2-Al2O3 aerogels and xerogels with a Si to Al molar ratio r(Si/Al) varying from 0.25 to 20, were made by sol-gel process in acidic conditions at pH 2 and respectively dried by the CO2 supercritical method and by solvent evaporation. The Al precursor was also chelated with ethylacetoacetate, which made it possible to study the structure and texture of such gels in conditions favorable to the formation of mixed Al-O-Si bonds. Nitrogen adsorption isotherms according to the Brunauer, Emmett and Teller method (BET), Al-27 magic angle spinning nuclear magnetic resonance (Al-27 MAS-NMR), Fourier Transformed Infrared Spectroscopy (FTIR) and Infrared absorbance spectra after Temperature Programmed Desorption (TPD) of pyridine, showed that the Si-O infrared asymmetric stretching vibration and the Bronsted acidity relative to the Lewis acidity, depended on the ratio of Al-IV to Si atoms.