Influence of the thermal treatment upon the textural properties of sol-gel mesoporous γ-alumina synthesized with cationic surfactants

The influence of the thermal treatment over the textural properties of sol-gel mesoporous gamma-Al2O3 (SCAL) synthesized in acid medium using cationic surfactants (e.g., hexaclecyltrimethylammonium chloride/bromide) as templates was investigated. BET surface areas steadily decreased (at most 20-25%) on increasing either time or temperature of the conventional calcination, with a observed phase change to delta-Al2O3 at 700 degrees C. Controlled thermal treatment at 550 degrees C allowed for a more ordered condensation between aluminium oxo/hydroxo layers reducing grain growth, that disorders the mesostructured framework and decreases BET surface areas to 250-350 m(2) g(-1). Among the different heating procedures tested, method B, which comprises an isothermal step at 150 degrees C under flowing nitrogen, led towards the most remarkable results (mesoporous gamma-Al2O3 with BET area similar to 690 m(2) g(-1), D-p = 2.0 nm and V-p similar to 0.49 cm(3) g(-1)). XRD and Al-27 MAS NMR bore out a mesoporous alumina framework formed mostly by gamma-Al2O3 nanocrystals with an Al-VI/Al-IV ratio similar to 4.1, with a slightly higher share of Al-V (7.4%). Different alkyltrimethylammonium bromide surfactants (alkyl similar to C-10-C-18) were used for the preparation of SGAL aluminas, leading also towards meaningfully better materials under thermal treatment B (S-BET similar to 570-700 m(2) g(-1), D-p = 2.0-2.3, V-p = 0.45-0.50 cm(3) g(-1)) with the only exception of the C-18 alkyl chain surfactant, due to the appearance of a lamellar phase. (C) 2009 Elsevier Inc. All rights reserved.