Pore Structures and Acid-Base Properties of γ‑Al₂O₃ Synthesized Using Different Precipitation and Drying Conditions

Although the alkali precipitation method is a significant industrial synthesis method for metal oxides, the effects of each process step on the physical and chemical properties of metal oxides have not been systematically investigated. This study performed a systematic investigation of the effects of precipitation conditions, such as the precipitant type and drying procedure, on the physical and chemical properties of γ-Al₂O₃. The γ-Al₂O₃ samples obtained by calcination of precursors synthesized via precipitation using NH₄OH as the precipitant exhibited higher amounts of surface hydroxyl groups than those synthesized using NaOH solution, because of the residual Na⁺ cations derived from the precipitant. Furthermore, in the case of γ-Al₂O₃ obtained via the precipitation method using NH₄OH solution, replacing the water present between the precipitate particles with various low-surface-tension solvents, such as n-hexane, produced γ-Al₂O₃ with large meso/macropore volumes. The surface tension of the solvent had negligible effects on the surface area of the resulting γ-Al₂O₃. The γ-Al₂O₃ product with large meso/macropore volumes exhibited excellent catalytic activity for the Knoevenagel condensation reaction between benzaldehyde and malononitrile, although there was no correlation between the surface area and catalytic activity. These tests revealed the influence of the γ-Al₂O₃ pore structure on catalytic reactions, which is highly meaningful for clarifying the role of hierarchical porosity. In addition, the study findings suggest that appropriate precipitation conditions and drying procedures can tune the acid-base properties and meso/macropore structures of γ-Al₂O₃ for use as a high-performance catalyst.