Evolution of the Texture and Structure of SiO_2-Al_2O_3 Xerogels and Aerogels as a Function of the Si to Al Molar Ratio

摘要

SiO_2-Al_2O_3 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 CO_2 supercritical method and by solvent evaporation. The Al precursror 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), ~27 Al magic angle spinning nuclear magnetic resonance (~27Al MAS-NMR), Fourier Transformed Infrared Spectroscopy (FTIR0 and Infrared absorpance 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.