Unraveling the Single-Site Origin of Strong Brønsted Acidity in Fluorinated γ-Al₂O₃

Precise control of the surface structure and acidity of γ-Al₂O₃ materials is critical for advancing their applications as catalysts and catalyst supports, yet remains challenging owing to the structural complexity and dynamic evolution of aluminum hydroxyl moieties upon modification. Herein, we unveil the identity and evolution of the surface sites of fluorine- and chlorine-modified γ-Al₂O₃ using state-of-the-art solid-state NMR, combining high-field (up to 18.8 T), ultrafast MAS (up to 60 kHz), and multinuclear multidimensional correlation (¹H-²⁷Al, ¹⁹F-²⁷Al, and ¹⁹F-³¹P, etc.) techniques, complemented by trimethylphosphine (TMP) probe adsorption. Notably, we unambiguously identify the intrinsic strong Brønsted acid site (BAS), present exclusively on fluorinated Al₂O₃, as a surface bridging hydroxyl bound to a stable, monofluoride-incorporated tetracoordinated aluminum center, denoted F₁-Al_IV-μ₂-OH. This strong BAS shows a previously unrecognized structural resemblance to bridging acid sites in fluorinated zeolites, as corroborated by the convergence of ¹H, ¹⁹F and ²⁷Al NMR signatures, and exhibits exceptional stability toward air and moisture. The superior catalytic performance of F-Al₂O₃ in the octadecene conversion reaction further validates this acidity-performance relationship. Overall, this work helps to resolve the long-standing debates on the nature of surface acidity in alumina-based materials, establishes a structural benchmark for the targeted design of highly efficient fluorinated catalysts, and opens new avenues for precise acid-site engineering in heterogeneous catalysis.