Precise Identification of the Atomic-Scale Spatial Distribution of Al Sites within Zeolite
Zeolites are extensively employed in industrial catalysis and adsorption separation due to their thermal stability, ordered porosity, and tunable acidity. However, the atomic-level spatial distribution of aluminum sites—the active centers—remains unresolved by conventional X-ray diffraction (XRD) because Al and Si possess similar outer-shell electron densities, precluding distinct identification. Even zeolites with identical Si/Al ratios exhibit divergent catalytic behavior, underscoring the need for precise Al site mapping to establish structure-activity relationships. Existing methods, such as integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) with molecular probes (pyridine, p-xylene), provide only lateral observations and lack macroscopic context. Cobalt exchange quantifies Al pairs and single sites but fails to deliver accurate atomic-scale positioning. This work highlights a synchrotron resonant soft XRD (RSXRD) technique, reported in Science (Volume 387), that resolves exact Al positions in a commercial ZSM-5 zeolite containing 12 T atoms. Through Rietveld refinement at multiple energies near the Al K-edge, combined with molecular adsorption, neutron powder diffraction, solid-state NMR, and density functional theory (DFT), the study identifies a 'Single Al' site at T8 and 'Al pairs' at T4 and T6. Probe molecules (trimethylphosphorus oxide, acetone, pyridinium) predominantly adsorb at T8 (straight channel) and T6 (cross-channel), while T4 exhibits poor accessibility. This integrated approach achieves atomic-level characterization of Al distribution, offering a robust methodology for elucidating zeolite active-site architecture.