(714d) Framework Structures and Heteroatom Distributions in the Zeolite Catalyst SSZ-70

Aluminosilicate zeolites exhibit high surface areas, uniform nanopore dimensions, and high catalytic activities for numerous reactions, including transformations of hydrocarbons (e.g., olefin epoxidation). Although zeolites often have similar bulk compositions, their diverse framework architectures, heteroatom (e.g., B, Al) contents and distributions, and associated exchangeable cations yield drastically different adsorption and reaction properties. A longstanding challenge has been to determine the local environments and distributions of non-stoichiometric heteroatoms in zeolite catalysts, particularly those having structures with complicated order and disorder over different length scales. An industrially relevant example is the zeolite SSZ-70, which is synthesized via semi-crystalline layered intermediates that provide sub-nanometer-size galleries that improve molecular diffusion, but which tend to exhibit turbostratic disorder. Here, synchrotron X-ray powder diffraction and transmission electron microscopy analyses establish the framework structures and long-range order of SSZ-70, which is comprised of irregularly-stacked MWW-type zeolitic layers. Furthermore, sensitivity enhanced solid-state NMR techniques enable acquisition of two-dimensional (2D) ²⁹Si{²⁹Si} and ²⁷Al{²⁹Si} through-bond-mediated correlation spectra, notably at natural isotopic abundance ²⁹Si (4.7%), which directly establish the covalent ²⁹Si-O-²⁹Si and ²⁷Al-O-²⁹Si connectivities of nearest-neighbor tetrahedral (T) sites in zeolites. The analyses enable identification and quantification of different types of interlayer silanols in SSZ-70, which are promising targets for functionalization. Furthermore, 2D ²⁷Al{²⁹Si} correlation spectra of aluminosilicate SSZ-70 detect and resolve signals from distinct framework ²⁷Al sites, establishing the siting and occupancies of Al heteroatoms at specific T sites within the zeolite framework, which was previously inaccessible by other means. The majority (94%) of the heteroatom sites in Al-SSZ-70 are at the surfaces of the interlayer channels, which accounts for the high catalytic activity of materials prepared by its delamination. The approach is expected to be generalizable to other semi-ordered zeolite catalysts, elucidating the origins of their macroscopic properties and providing new criteria for design of future heterogeneous catalysts.