(269e) Atomic-Level Insights into Molecular Adsorption and Reaction Properties at Distinct Heteroatom Sites in Aluminosilicate Zeolite Catalysts
Heteroatom-containing zeolites are of technological interest because they exhibit high surface areas, highly uniform sub-nanometer pore dimensions, and high catalytic activities for diverse applications, including hydrocarbon conversion (e.g., fluidized catalytic cracking, FCC, and methanol-to-olefin, MTO, processes). Heteroatoms, such as B or Al, in zeolite frameworks are associated with molecular adsorption and reaction sites, and therefore the types and distributions of heteroatom environments strongly influence the macroscopic reaction properties of zeolite catalysts. However, measurements of local heteroatom environments, their distributions in zeolite frameworks, and their interactions with adsorbed molecular guest species, such as structure-directing organic molecules, reactants, or catalytic reaction products, have been extremely challenging. This has been due, in part, to non-stoichiometric zeolite framework compositions and distributions of both framework and extra-framework species that lack long range order, which are often exacerbated by post-synthesis treatments, such as calcination, steaming, or exposure to reaction conditions. Nevertheless, detailed atomic-level insights on heteroatom environments and their interactions with adsorbed molecular guest species are provided by solid-state nuclear magnetic resonance (NMR) spectroscopy, correlated with complementary transmission electron microscopy, X-ray diffraction, and catalytic reaction analyses. Notably, two-dimensional dynamic-nuclear-polarization (DNP)-enhanced NMR techniques provide substantially increased NMR signal sensitivity (ca. 100x) and resolution, which enables detection and correlation of specific heteroatom environments with the adsorbed molecules with which they interact. Such measurements have, until now, been infeasible by scattering or other spectroscopic techniques. Results will be presented for methanol, toluene, and carbonaceous deactivation products adsorbed on aluminosilicate zeolites Y and SSZ-13, which are associated with technologically important cracking or methanol-to-olefin reactions. These analyses identify heteroatom sites in the framework and at exterior surfaces that have distinct adsorption and reaction behaviors, providing useful new insights for the design of highly effective and stable catalysts.