(358f) Modeling the Brønsted Acidity of Lanthanum Exchanged Faujasite

Lanthanum stabilizes faujasite in fluid catalytic cracking, which increases catalyst material lifetimes. Despite its extensive use for decades, many unknowns remain on how lanthanum increases faujasite hydrothermal stability. The hydrothermal stability of zeolites decreases by dealumination, which occurs via reactions that involve water molecules. Brønsted acid sites give desired catalytic properties to zeolites, however they reduce hydrothermal stability by attracting water molecules to aluminum tetrahedra and promoting dealumination. The energetically favored binding sites of water molecules and lanthanum ions in hydrogen exchanged faujasite were determined with density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. Potentials of mean force of proton transfer were obtained with constrained AIMD simulations to quantify the Brønsted acidity of faujasite, with and without lanthanum ions. 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 sites adjacent to, and not adjacent to, bound lanthanum ions in faujasite. Implications toward hydrothermal stability are discussed, since lanthanum ions change the protonation state, and therefore hydrophilicity, of aluminum tetrahedra in faujasite.