Densification of 2-2.5 nm nanocrystals assemblies in the three-level hierarchical structure of gamma-Al2O3 aerogel significantly increases the surface acidity, as determined using indicator titration, NH3-TPD, and FTIR of adsorbed pyridine. Thermal treatment at 1073 K and insertion of additional alumina inside the aerogel pores eliminated the slit micropores with shrinkage of nanocrystals assemblies (N-2-adsorption. HRTEM, SEM) increasing the contact interface by a factor of 2. It caused a fivefold increase in alumina surface acidity and the strength of Lewis acid sites with no measurable dehydration (weight loss, TGA). This was attributed to the formation of additional low-coordinated aluminum ions with higher charge in the areas with atomic disorder of high-angle grain boundaries, detected by Al-27 MAS NMR, HRTEM, XPS, and XRD. Densification of nanocrystalline alumina aerogel yielded a higher catalytic activity in dehydration of isopropanol: it was 10-fold more active compared with commercial gamma-alumina on the catalyst weight basis. (C) 2011 Elsevier Inc. All rights reserved.