Variables in the synthesis of unusually high pore volume aluminas

Abstract

This paper summarizes our successful efforts to obtain high pore volume aluminas (> 5.0 cm³ g⁻¹ ). The pore volumes of these aerogels, prepared by the hydrolysis of aluminium isopropoxide (AIP) in alcoholic media, are very sensitive to the amounts of water and alcohol employed in the synthesis. An unexpectedly large ratio of water to alkoxide yields the highest pore volumes. By suitable selection of initial levels of aluminium alkoxide, methanol and water, one obtains average pore volumes of 8.6 cm³ g⁻¹. A two-step formulation of AIP, methanol, and water followed by hypercritical solvent removal to form the alumina aerogel yields the highest pore volumes. Other alternative approaches also increase the yield of aerogel produced within the autoclave. Pore volumes of about 7 cm³ g⁻¹ result if the premix is concentrated twofold after formulation and before autoclave processing. Lower pore volumes result if the reactant charge is increased directly by simply increasing the concentration of AIR It appears that the reduction in pore volume results in part from the higher isopropanol content in the final reaction mixture (isopropanol is a co-product from AIP hydrolysis).