Effect of preparation conditions on the phase transformation of mesoporous alumina

A post-hydrolysis method is proposed for the easy-and-fast preparation of mesoporous gamma-alumina (MA) at room temperature using an alkyl carboxylate as a chemical template. Water was used as an initiating solvent in the hydrolysis of aluminum alkoxide in the terminal reaction procedure. Hydrolysis and condensation reactions were simultaneously induced. The initial phase of as-made MA was bayerite and/or (pseudo)boehmite, and was transformed into active alumina at 250 degrees C, which remained thermally stable at temperatures up to 600 degrees C. The phase transformation of alumina was analyzed with DTA, XRD and Al-27 MAS NMR. Based on an Al-27 MAS NMR analysis, calcined alumina prepared via a hydrothermal method contained a high ratio of Al-VI to Al-IV (similar to 4.8), which is not fully transformed into active alumina, and aluminum hydroxide (AlOOH or Al(OH)(3)) was detected. These results suggest that high temperature is required to obtain an active form of alumina, when the hydrothermal method is used. Another factor is the ratio of Al-VI to Al-IV in the MA, which is also dependent on the pH of the reactant. Pore size, surface area, and pore structure could be controlled by adjusting the molar ratio of water to aluminum precursor, and the preparation temperature. The pore size of MA was adjustable from 2 to 7 nm, and the MA indicated a large surface area of 300 similar to 500 m(2)/g. (c) 2005 Elsevier B.V. All rights reserved.