By Brad Chmelka
Advances in solid-state NMR characterization capabilities enable the compositions, structures, and properties of heterogeneous catalysts to be measured, understood, and correlated in ways that have not previously been possible. Examples include nanoporous aluminosilicate zeolites and supported metal clusters, whose industrially important reaction properties have been challenging to understand and control. This has been due in part to their non-stoichiometric compositions and heterogeneous surfaces, which result in complicated distributions of structural order and disorder that have important influences on their adsorption and reaction behaviors. These challenges can nevertheless be overcome by using powerful solid-state NMR spectroscopy techniques, notably dynamic-nuclear-polarization (DNP)-enhanced, two-dimensional (2D), or in situ NMR methods. In combination with complementary scattering and bulk property measurements, they enable heterogeneous catalysts to be probed over multiple length scales to obtain and correlate insights on local bonding environments and interactions with macroscopic adsorption and reaction properties. Recent results will be presented on measuring and understanding at an atomic level the compositions and structures of zeolite catalysts, in particular the influences that heteroatom distributions, exchangeable cations, and diverse adsorption sites have on catalyst reactivities. The analyses provide guidance for the rational design and syntheses of zeolite catalysts for applications in hydrocarbon conversion, automotive pollution mitigation, and separations. Our work in these areas is supported by and conducted with diverse industrial partners, whose crucial roles will be highlighted.