Crystallographic location of active sites in ZSM-5 determined by quantitative STEM

Abstract

In aluminosilicate zeolites, the atomic-scale insights into catalytic performance are tied to Brønsted acid sites (BASs), the primary active sites generated by the substitution of aluminum (Al) for silicon (Si) in the tetrahedral framework, with a proton (H⁺) compensating for the resultant charge imbalance. The profound influence of Al distribution on BAS density, spatial arrangement, and acidity is well established. Yet, the precise atomic positions of these Al atoms remain poorly resolved. Using silver (Ag) as a molecular probe, this study combines synchrotron X-ray diffraction (SXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to reveal the specific locations of Al atoms in ZSM-5, a prototypical zeolite catalyst. Statistical analysis of HAADF-STEM images unambiguously identifies the crystallographic adsorption sites of Ag at T4, T6, and T8, linking their distribution directly to the predominant framework Al sites, which correlates perfectly with the predominant Al sites identified by our previous work. By mapping these Al sites, we establish an atomic-scale model for single atom catalysis within the zeolite framework. This work develops methodologies further to elucidate the structure-activity relationship of industrially relevant zeolite catalysts, providing the foundational knowledge for rationally designing zeolite catalysts with optimised active sites and enhanced performance. Graphical abstract: [Figure not available: see fulltext.]