Sol-gel synthesis of mesostructured γ-alumina templated by cationic surfactants

Sol-gel mesostructured γ-Al2O3 (SGAL) with pore walls largely formed by γ-Al2O3 nanocrystals has been synthesized by a new three step (hydrolysis, condensation and calcination) sol-gel procedure in acid medium with cationic surfactants (hexadecyltrimethylammonium chloride/bromide) and in the absence of hydrolysis retarding agents. SGAL aluminas showed BET surface areas larger than 300 m2 g−1, pore diameters within the 4-15 nm range and pore volumes larger than 0.40 cm3 g−1. Hydrogen chloride, water and surfactant contents as well as the thermal treatment deeply influence the final properties of the obtained aluminas. Hydrogen chloride/aluminium isopropoxide molar ratios larger than 1 are required to prepare uniform mesostructured aluminas of wormhole-like appearance with a narrow mesopore size distribution (Dp ∼ 4.7-6.0 nm). A pore size decrease, accompanied by narrower mesopore size distributions has been observed when diminishing both the water/aluminium isopropoxide and template/aluminium isopropoxide molar ratios. Controlled two step thermal treatment (firstly heating under flowing nitrogen and subsequently under air flow at 550 °C for 5 h) allows to reduce the coalescence of the aluminium moieties forming the pore walls, by increasing the BET surface area values above 500 m2 g−1. Under special conditions, ordered mesostructured alumina with SBET ∼ 626 m2 g−1, Dp ∼ 2.8 nm and narrower pore walls (∼3.5 nm) largely formed by γ-Al2O3 nanocrystals was prepared.