(689e) 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. Carbonaceous deposits accumulate within zeolite pores requiring oxidative regeneration protocols at high temperatures and generating water during combustion reactions. 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. It has been proposed that the total Al content (e.g., Si/Al),¹ Al distribution (e.g., among different T-sites)^2,3 and structural defects (e.g., silanol groups)⁴ influence dealumination kinetics (723-873 K, 5-35 kPa H₂O) on H-form MFI zeolites. However, the kinetics and mechanisms of framework dealumination, and their dependence on material properties, including Al density, local arrangement, and crystallite size, are difficult to determine in low symmetry frameworks such as MFI. 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^5,6 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 TPD⁷, 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 the 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.