Nature and Catalytic Role of Extraframework Aluminum in Faujasite Zeolite: A Theoretical Perspective

Abstract

A comprehensive periodic DFT study complemented by ab initio thermodynamic analysis was carried out to determine the speciation of extraframework aluminum (EFAl) in faujasite zeolite. The structure and stability of a wide range of mono- bi-, tri-, and tetranuclear EFAl complexes stabilized at different locations in faujasite were investigated. The thermodynamic cycles connecting these complexes were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. Using ab initio thermodynamics analysis it was predicted that, during high-temperature zeolite activation, the EFAl species self-organize into cationic clusters with more than one Al center. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and charge-compensation environment for the large multiply charged cationic clusters. The presence of such cationic EFAl clusters inside the inaccessible...