Tailoring penta-coordinated aluminium species on silica–alumina via flame spray pyrolysis for enhanced arene benzylation

Abstract

Flame spray pyrolysis (FSP) was employed to tailor the coordination environment of aluminum species on amorphous silica–alumina (ASA) for enhanced solid acidity. By regulating the oxygen flow, the formation of penta-coordinated Al (Al^V) species was promoted, especially the surface-distorted Al^Vb type. Advanced solid-state NMR and XAS analyses confirmed that Al^V species incorporate into the silica network and interact with nearby silanol groups to form strong Brønsted acid sites (BASs) through pseudo-bridging structures. The resulting ASA catalysts exhibited increased acid strength, with TMPO-³¹P NMR shifts up to 67 ppm, comparable to zeolite-like acidity. These structural features correlate with significantly improved catalytic activity and selectivity in arene benzylation. This study establishes a clear synthesis–structure–acidity–function relationship and offers a rational strategy for designing high-performance ASA-based solid acids.