(670f) Quantifying the Kinetics of Framework Dealumination during Hydrothermal Aging of Proton-Form CHA Zeolites

Aluminosilicate zeolites are used as Brønsted acid catalysts in their proton forms, and typical oxidative regeneration protocols lead to exposure to water (~10 kPa H₂O) at high temperatures (>823 K), which causes hydrothermal aging (HTA). This process is characterized by the hydrolysis of framework Al-(OH)-Si connectivities and the removal of framework Al (Al_f) atoms to form extraframework Al (Al_ex) species. Prior work has proposed that Al density (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 proton-form MFI zeolites. However, the kinetics and mechanisms of framework dealumination and their dependence on material properties, including Al density, local Al arrangement (e.g., isolated versus paired configurations), and structural defects (e.g., silanol groups), are difficult to determine using low symmetry frameworks such as MFI. Here, we investigate high-symmetry (e.g., single T-site) CHA zeolite samples synthesized with varying Al density (Si/Al = 12-45), 6-MR paired Al site arrangement (2-28%) at fixed composition (Si/Al = 12), and crystallization media (hydroxide versus fluoride) at fixed composition (Si/Al = 45) using previously reported methods.^5,6 These samples were exposed to HTA conditions (923 K, 10 kPa H₂O) for varying durations of time. The number of Al_f sites after HTA treatments was determined by quantifying the number of H⁺ sites by NH₃ gas titration⁷, and used to determine kinetic models for Al_f removal as a function of time and material properties. Dealumination kinetics show the strongest dependence on bulk Al density, with rates that increase with decreasing Al-Al inter-site distance, supporting models for the irreversible loss of Al upon reaction and aggregation of multiple Al_ex species. These results provide new approaches to quantify the kinetics of dealumination in zeolites, and provide fundamental insights into the reactions and mechanisms that govern dealumination under hydrothermal aging conditions.