Unraveling the Single-Site Origin of Strong Brønsted Acidity in Fluorinated γ-Al 2 O 3

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-AlIV-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.