Control of surface acidity and catalytic activity of γ-Al ₂O₃ by adjusting the nanocrystalline contact interface

Densification of 2-2.5 nm nanocrystals assemblies in the three-level hierarchical structure of γ-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 ²⁷Al 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 γ-alumina on the catalyst weight basis.