Abstract
Faujasite is one of the most industrially employed zeolite material, mainly for catalytic applications. Its properties are dependent on its pore network, which can be tuned by dealumination upon steaming. Previous theoretical studies on the subject based on the density functional theory identified plausible dealumination mechanisms for simple bulk structures of faujasite, but to gain a more complete understanding of dealumination, and to be able to compare to experimental situations, theoretical studies must be performed on more realistic and complex systems. In this article, we identify the intermediates and transition states for the first Al–O bond hydrolysis initiating dealumination in the bulk of faujasite for various Si/Al ratios, in the presence of silanol nests defects, and at the external surface of faujasite. We find that Al–O bond breaking is made easier from a kinetic perspective on some sites of the external surface of the material, considering the water adsorbed state as the reference. Moreover, the presence of defects and a decrease of the Si/Al ratio also lower the free energy barriers necessary to dealuminate. These trends are less clear when considering apparent free energy barrier, when the water desorbed state has been taken as a reference. Subtle local effects enter into play explaining the barrier rankings, including a participation of the hydrogen bond network strengthening at the outermost surface of the zeolite, which significantly stabilizes the highest free energy transition state in some cases. These hydrogen bonds make Brønsted–Evans–Polanyi relationships blurrier at the surface than at the bulk sites, suggesting that local effects on the stabilization of the transition states need to be explicitly considered.
Dr. Thomas Jarrin, Dr. Theodorus de Bruin, and Dr. Céline Chizallet
https://doi.org/10.1021/acscatal.3c05517
ACS Catalysis
2024, 14, 3, 1639–1652
