Effect of dehydration on porous structure of monolithic aerogels based on fibrous aluminum oxyhydroxide

Aluminum oxide monolithic aerogels (PMAOs) with specific surfaces of up to 400 m²/g and densities of up to 0.004 g/cm³ were obtained by the direct oxidation of metallic aluminum by water vapors. Studies of the obtained aerogels using the methods of scanning and transmission electron microscopy showed that a freshly prepared aerogel represents the amorphous structure from alumina fibrous with a diameter of about 5 nm. Parameters of a porous aerogel structure were determined from the isotherms of adsorption of nitrogen vapors at 77 K. It is shown that the specific surface area decreases by 50% upon the dehydration of aerogel by temperature pretreatment at 900°C until the phase transition temperature is reached. At the same time, heat treatment at temperatures above 1200°C results in a phase transition at which significant contraction of structure occurs and the specific surface area decreases to 20 m²/g. The number of primary adsorption centers was obtained from comparative plots of water vapor adsorption. It was found that preliminary dehydration only slightly affects the number of primary centers of water adsorption.