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 γ-Al2O3 (SGAL) synthesized in acid medium using cationic surfactants (e.g., hexadecyltrimethylammonium 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 δ-Al2O3 at 700 °C. Controlled thermal treatment at 550 °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 m2 g−1. Among the different heating procedures tested, method B, which comprises an isothermal step at 150 °C under flowing nitrogen, led towards the most remarkable results (mesoporous γ-Al2O3 with BET area ∼ 690 m2 g−1, Dp = 2.0 nm and Vp ∼ 0.49 cm3 g−1). XRD and 27Al MAS NMR bore out a mesoporous alumina framework formed mostly by γ-Al2O3 nanocrystals with an AlVI/AlIV ratio ∼4.1, with a slightly higher share of AlV (7.4%). Different alkyltrimethylammonium bromide surfactants (alkyl ∼ C10–C18) were used for the preparation of SGAL aluminas, leading also towards meaningfully better materials under thermal treatment B (SBET ∼ 570–700 m2 g−1, Dp = 2.0–2.3, Vp = 0.45–0.50 cm3 g−1) with the only exception of the C18 alkyl chain surfactant, due to the appearance of a lamellar phase.