Effect of acid concentration on pore size in polymer-templated mesoporous alumina

Abstract

Ordered mesoporous alumina and alumina-titania composites can be prepared in a facile and reproducible manner by mixing metal precursors, acid, solvent and polymer template in one-pot and performing evaporation induced self assembly followed by removal of the template. Structural characteristics, particularly pore size, of these materials can be tailored by controlling the acid concentration in the synthesis mixture. The optimal range for alumina materials with a large range of pore widths (4.0–10.5 nm), high surface areas (up to 467 m² g⁻¹) and significant pore volumes (up to 0.74 cm³ g⁻¹) was found to be within the range of 1.2 and 3.0 moles of nitric acid per mole of metal. For well ordered, thermally stable titania-alumina samples the range over which nitric acid concentration produces high surface area (431 m² g⁻¹), large pore volume (0.79 cm³ g⁻¹) and significant pore widths (up to 15.5 nm) is 1.2 to 3.0 moles of nitric acid to metal. The effect of acid concentration on the pore width is likely due to the combination of a salt-like effect from the increased concentration of H⁺ and NO₃⁻ ions, the effect of increased water concentration in the synthesis mixture and the direct relationship between the metal alkoxide hydrolysis rate and acid concentration.