Effect of Lanthanum Ions on the Brønsted Acidity of Faujasite and Implications for Hydrothermal Stability

Abstract

Lanthanum stabilizes faujasite in fluid catalytic cracking and allows for longer catalyst material lifetimes in petroleum refining. Despite its extensive use for decades, a thorough, mechanistic understanding of how lanthanum increases the hydrothermal stability of faujasite remains out of reach. Dealumination, the main contributor to loss of zeolitic hydrothermal stability, occurs by reactions that involve water molecules. Brønsted acid sites give desired catalytic properties to zeolites; however, they reduce hydrothermal stability because they attract water molecules to aluminum tetrahedra, promoting dealumination. In this work, the energetically favored binding sites of water molecules and lanthanum ions in hydrogen-exchanged faujasite with a Si/Al ratio of five were determined with density functional theory calculations and ab initio molecular dynamics simulations. Also, the Brønsted acid strength of faujasite was quantified, using constrained ab initio molecular dynamics simulations, with and without lanthanum ions bound to faujasite. Results indicate that water and lanthanum ions are energetically favored to bind in the same sites in faujasite, and that lanthanum ions increase the acidity of Brønsted acid sites adjacent to, and not adjacent to, bound lanthanum ions in faujasite. Implications toward hydrothermal stability are discussed, because lanthanum ions will compete with water molecules for binding sites, as well as change the protonation state, and therefore hydrophilicity, of aluminum tetrahedra in faujasite.

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