Meso-macroporous γ-alumina by template-free sol-gel synthesis: The effect of the solvent and acid catalyst on the microstructure and textural properties

Template-free sol-gel method was employed under non-acidic conditions to prepare meso-macroporous nanocrystalline γ-Al₂O₃. Aluminum tri-isopropoxide and various solvents including 2-propanol, 1-butanol, 2-butanol, tert-butanol, and toluene were employed. The effect of the solvent and the presence of an acid catalyst on the textural properties of the calcined alumina were studied. The prepared alumina was characterized by powder XRD, N₂ adsorption, SEM and TEM. The textural properties were found to depend on the solvent and the presence or absence of the acid catalyst. Starting with aluminum isopropoxide in alcoholic solvents, where colloidal solutions were initially obtained, resulted in meso-macroporous γ-alumina with relatively high surface areas, 350-500m²/g, and large total pore volumes, 1.4-2.0cc/g. Alumina obtained in 1-butanol exhibited the highest surface areas, ∼500m²/g, and very narrow distribution of the observed mesopores, between 4 and 10nm. On the other hand, starting with solutions where the precursor was completely soluble, lower surface areas and lower total pore volumes were obtained with negligible macroporosity. Similar results were also obtained in the presence of the acid catalyst. The unique properties of alumina prepared from aluminum isopropoxide in alcoholic solvents in the absence of an acid catalyst were referred to the presence of sol particles of the precursor in the starting solution which acted as templates after hydrolysis in the gel network. The effect of the acid catalyst was referred to its role in enhancing the precursor solubility besides its role in increasing the rate of hydrolysis on the account of the rate of condensation.