Resolving Transformative Reactions in Zeolites under Catalytic Hydrothermal Conditions

The nanoscale structuring of zeolites and related porous solids is vitally important to enhance adsorption/desorption, accessibility of active sites, mass transfer, and catalytic conversion. Specifically, the chemical behavior of zeolites imparts significant roles on the reaction rates, selectivity, and product yields, which are collectively affected during catalytic reactions. To understand variations in surface reactivity and the associated intermediate species, here we examine the impact of postsynthesis ion-exchange, calcination, and hydrothermal reactions on the surface and bulk Al site distributions in Faujasite (FAU)-type Y and X zeolites ion-exchanged with H+, Na+, and K+ counterions. Ex situ X-ray diffraction and electron microscopy analysis reveal substantial changes in the long-range order, and porosity measurements and analysis suggest that these transformative reactions strongly depend on the nature of cations and reaction conditions. These results are corroborated by the analysis of 1D 27Al and 2D 27Al–1H solid-state NMR spectra acquired at a high-field (28.2 T, 1H and 27Al Larmor frequencies are 1200 and 313 MHz, respectively), which allowed the local structures of distinct framework/extra-framework Al species, Lewis and Brønsted-acid sites (BASs), to be resolved with unprecedented sensitivity and resolution. The through-space proximities between the AlIV, AlV, and AlVI species, SiOH defects and AlOH groups, and BAS are observed, providing structural insights into the framework/extra-framework interface. Notably, hydrothermal reactions lead to the formation of framework-associated AlIV species in H+-FAU zeolites at miniscule concentrations (∼10 wt %) that are expected to influence the overall catalytic activity. These findings deepen our understandings of reactive surfaces and interfaces of solid supports under catalytic reactions, providing atomic-scale insights into structure–stability–activity relationships.