(627c) Measuring & Understanding the Atomic-Scale Environments and Interactions of Heteroatom Sites in Zeolite Catalysts

Heteroatom-containing zeolites, such as chabazite (SSZ-13), faujasite (zeolite Y), ZSM-5, and zeolite Beta, are of technological interest because they exhibit high surface areas, tunable pore dimensions, and controllable catalytic activities for hydrocarbon conversion and reduction of nitrogen oxides. The local environments and distributions of heteroatom (e.g., B, Al) sites in zeolite frameworks strongly influence the macroscopic adsorption properties and catalytic activities. Measuring the atomic-scale heteroatom environments that influence catalytic activities and selectivities is often challenging, in part due to the heterogeneous environments around heteroatom sites and structural disorder introduced during post-synthesis treatments such as calcination. Here, detailed information on the long-range structural order and atomic environments in aluminosilicate zeolites are enabled by complementary transmission electron microscopy (TEM), X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (NMR) spectroscopy. Notably, recently-developed dynamic-nuclear-polarization (DNP)-enhanced NMR techniques provide significantly enhanced signal sensitivity (ca. x100) that allows the detection and analysis of distinct heteroatom species, which are challenging to characterize using scattering techniques. The analyses establish the types, distributions, and relative proximities of different framework silicon and aluminum environments, which correspond to framework acid and cation-exchange sites. These results yield new insights on calcined zeolites and elucidate the effects of post-synthesis treatments on the types and distributions of catalytically-active sites, which are correlated with the catalytic properties of zeolite catalysts that are important for technological applications.