Transmission Electron Microscopy Study of the Porous Structure of Aluminas Synthesized by Non-Ionic Surfactant Templating Route

Organized mesoporous and microporous aluminas have been synthesized by a sol-gel route in non-aqueous media using poly(oxyethylene) block polymers as directing agents. Gel compositions adjusted to obtain direct micelles (in 1,4-dioxane solution) and reverse micelles (in cyclohexane solution) were tested. The influence of synthesis conditions, such as temperature, solvent, use of additives or chemical modification of the precursor, on the alumina porous structure has been analysed by transmission electron microscopy (TEM). Mesoporous aluminas with a pore structure originated by the cross-linking of corrugated oxide-hydroxide platelets of nanometric size were obtained using aluminum sec -butoxide as precursor. The addition of ammonium fluoride or amines to the synthesis gel, as well as higher calcination temperatures, caused a decrease in the surface area due to the condensation of the crystallites into a dense porous structure composed of small polycrystalline agglomerates. Microporous aluminas with higher density and a disordered but isotropic porous structure were obtained by chemical modification of the alkoxide precursor with chelating agents such as ethyl acetoacetate or triethanolamine.