Atomic locations and adsorbate interactions of Al single and pair sites in H-ZSM-5 zeolite

The distribution of substitutional aluminum (Al) atoms in zeolites affects molecular adsorbate geometry, catalytic activity, and shape and size selectivity. Accurately determining Al positions has been challenging. We used synchrotron resonant soft x-ray diffraction (RSXRD) at multiple energies near the Al K-edge combined with molecular adsorption techniques to precisely locate "single Al" and "Al pairs" in a commercial H-ZSM-5 zeolite. This analysis depicts three distinct Al tetrahedral (T) sites: T8, T6, and T4. A combined suite of characterizations, including ammonia temperature-dependent desorption, neutron powder diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculations, reveal isolated ammonia adsorption on T8 as "single Al" in the straight channel and bridged ammonia adsorption on T6 and T4 as an "Al pair" (Al_T6-O-Si_T5-O-Al_T4) in the straight-sinusoidal intersection.