Quantifying the kinetics of framework dealumination during hydrothermal aging of proton-form CHA zeolites

Aluminosilicate zeolites are used as Brønsted acid catalysts in their H-forms and for nitrogen oxides (NO_x) reduction in their Cu-exchanged forms, and are typically exposed to oxidative regeneration protocols at high temperatures to remove carbonaceous deposits. Yet, exposure to water-containing environments (~10 kPa H₂O) at high temperatures (>823 K) causes hydrothermal aging, characterized by the hydrolysis of framework Al-(OH)-Si connectivities, among others, and eventually the removal of framework Al (Al_f) atoms to form extraframework Al (Al_ex) species in dealumination events. Empirical observations indicate that, in their Cu-form, small-pore (eight-membered ring; 8-MR) cage-window zeolite topologies such as chabazite (CHA) show improved resistance to dealumination and framework degradation compared to medium- and large-pore zeolites after exposure to hydrothermal aging conditions. However, the kinetics and mechanisms of framework dealumination, and their dependence on material properties including the Al site density, local arrangement, and crystallite size, remain unclear. Here, we synthesized H-form CHA zeolite samples with varying Al content (Si/Al = 12-22), 6-MR paired Al site arrangement (2-28%) and crystallite size (0.18-1.1 mm) at fixed composition (Si/Al = 12-15) using previously reported methods, and exposed them to hydrothermal aging conditions (923 K, 10 kPa H₂O) for varying amounts of time. The number of Al_f sites after each treatment was determined by quantifying the number of H⁺ sites by NH₃ TPD, and used to determine kinetic models for Al_f removal as a function of time and material properties (active site density, distribution, crystallite size). Kinetic values measured were compared across samples to provide insight into the influence of zeolite properties on dealumination kinetics of CHA zeolites. Altogether, these results provide new approaches to quantify the kinetics of dealumination on zeolites of varying material properties, to gain fundamental insights into the reactions and mechanisms that govern dealumination under hydrothermal aging conditions.