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

Sol-get mesostructured gamma-Al(2)O(3) (SGAL) with pore walls largely formed by gamma-Al(2)O(3) 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 m(2) g(-1), pore diameters within the 4-15 nm range and pore volumes larger than 0.40 cm(3) 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 (D(p) similar to 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 degrees 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 m(2) g(-1). Under special conditions, ordered mesostructured alumina with S(BET) similar to 626 m(2) g(-1), D(p) similar to 2.8 nm and narrower pore walls (similar to 3.5 nm) largely formed by gamma-Al(2)O(3) nanocrystals was prepared. (c) 2005 Elsevier Inc. All rights reserved.