Aluminas, particularly γ -Al₂O₃, are widely used in modern energy and environmental industries, serving as catalysts or catalyst supports. Halogen-modified alumina is often employed to achieve superior catalytic performance. In particular, introducing fluorine or chlorine can modify surface acidity.
However, the atomic-scale origin of this change has remained controversial for decades due to the complexity of halogenated alumina and the challenges associated with its spectroscopic characterization.
A research team led by Profs. HOU Guangjin and JI Yi from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) made a breakthrough. They have identified the strong Brønsted acid site (BAS) in fluorinated γ -Al₂O₃ using state-of-the-art solid-state nuclear magnetic resonance (NMR) techniques.
The study was published in Journal of the American Chemical Society .
The team employed advanced solid-state NMR techniques, combining high magnetic fields (up to 18.8 T), ultrafast magic-angle spinning (MAS, up to 60 kHz), and multinuclear multidimensional correlation experiments (¹H–²⁷Al, ¹⁹F–²⁷Al, ¹⁹F–³¹P, etc.), along with trimethylphosphine (TMP) probe adsorption.
These sites were unambiguously identified as bridging hydroxyl groups bound to a stable, monofluoride-incorporated tetracoordinated aluminum center, denoted as F 1 −Al IV − μ 2 −OH. Researchers found these sites are present only on fluorinated γ- Al₂O₃ while absent on chlorinated analogues.
Notably, the ¹H, ¹⁹F, and ²⁷Al NMR signatures of these sites converge with those of bridging acid sites in fluorinated zeolites, uncovering an unrecognized structural similarity between the two classes of materials.
Further experiments found that these strong Brønsted acid sites exhibit exceptional robustness, remaining intact after exposure to air, moisture, and even liquid water washing. Catalytic tests using 1-octadecene conversion demonstrated that the presence of these sites enhances catalytic activity and aromatization performance, directly linking the atomic-scale structure to macroscopic catalytic behavior.
"Our work resolves the long-standing controversy regarding the nature of strong acidity in halogenated aluminas and establishes a structural benchmark for the targeted design of fluorinated catalysts," said Prof. HOU.
Journal of the American Chemical Society
Computational simulation/modeling
Not applicable
Unraveling the Single-Site Origin of Strong Brønsted Acidity in Fluorinated γ-Al2O3
20-Mar-2026