(54d) Influences of Al Density and Proximity and Framework Topology on the Dealumination Kinetics of Proton-Form Zeolites

Proton-form aluminosilicate zeolites are used as Brønsted acid catalysts and are typically subjected to oxidative regeneration protocols that lead to water exposure (~10 kPa H₂O) at high temperatures (>823 K), causing hydrothermal aging (HTA). HTA causes dealumination, in which framework Al (Al_f) are observed to be liberated from the framework to form extra-framework Al (Al_ex) species of indeterminate composition and, ultimately, form larger alumina agglomerates.^1,2 Although zeolite materials properties such as total Al content (e.g., Si/Al),³ Al distribution (e.g., among different T-sites),^4,5 and structural defects (e.g., silanol groups)⁶ have been reported to influence dealumination rates on MFI, the kinetic and mechanistic details to predict and model how zeolite material properties influence hydrothermal stability are unknown. Here, we use experiments and molecular modeling of CHA zeolites, a high-symmetry (one T-site) framework, to elucidate new mechanistic details of dealumination processes and to quantify the kinetics of dealumination with varying zeolite properties. CHA zeolites were synthesized with varying Al density (Si/Al = 12-45), and varying 6-MR paired Al site arrangement (0-40%) at fixed composition (Si/Al = 12),^7,8 and then exposed to HTA conditions (823-973 K, 8.5-90 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 titration⁹, and used to quantify dealumination rates with varying time, temperature, and H₂O pressure. The kinetically relevant step in the dealumination mechanism depends on HTA conditions, shifting from the hydrolysis of Al-(OH)-Si linkages to Al_ex agglomeration as temperature and H₂O pressure increase. We also describe analogous results on HTA of medium- and large-pore zeolites (MFI, BEA).¹⁰ This work provides new approaches to quantify dealumination kinetics of zeolites, and provide mechanistic insights into the reactions that govern dealumination under hydrothermal aging conditions.