(320d) Tuning Crystal Habits and Acidic Properties of ?-Al2O3 through Molten-Salt Synthesis

The polymorph α-Al₂O₃ is a thermodynamically stable crystalline material among various forms of Al₂O₃ and is widely employed as a catalyst support due to its thermochemical inertness. A prototypical example is the production of ethylene oxide (EO) via ethylene epoxidation over Ag/α-Al₂O₃ [1]. Over the past 80 years, extensive research has been conducted to improve catalytic performance; however, the commercial process is still operated at only ~7 to 15% ethylene conversion to maintain a high EO selectivity [1]. One set of pathways limiting the selectivity is the catalytic decomposition of EO over α-Al₂O₃ through hydration, isomerization, and/or dimerization, forming oxygenated compounds (e.g., ethylene glycol, acetaldehyde, and 1,4-dioxane). These oxygenates undergo instantaneous combustion to CO₂ on neighboring Ag sites [2]. Since the acid sites of α-Al₂O₃ facilitate oxygenate formation, designing α-Al₂O₃ with tailored surface area while simultaneously minimizing acidity is a promising approach to enhancing catalytic performance, which necessitates the understanding of its acidic properties.

This presentation will address how the crystal habits of α-Al₂O₃ affect acidic properties, using 1-butene double-bond shift as a benchmark reaction. For this study, four distinct α-Al₂O₃ morphologies were prepared via molten-salt synthesis: hexagonal platelets, intergrown hexagonal platelets, hexagonal prisms, and stacked hexagonal prisms. Our findings reveal that the inherent surface acidity (i.e., strength and density) is highly correlated to the nature of exposed facets, as the coordination structure and density of surface hydroxyl groups are governed by crystal structure. Furthermore, at a fixed morphology, the acidic properties can be tuned by controlling the degree of dehydration through the post-heat treatment.

References

[1] Lockemeyer, J. R.; Lohr, T. L. Ethylene Oxide Catalysis Under Commercial Conditions – A Guide for Researchers. ChemCatChem 2023, 15, e202201511

[2] Miller, J. H.; Joshi, A.; Li, X.; Bhan, A. Catalytic degradation of ethylene oxide over Ag/-Al₂O₃. J. Catal. 2020, 389, 714-720.