Understanding Dealumination Mechanisms in Protonic and Cationic Zeolites

摘要

Zeolites can be partially hydrolyzed by steam via Al extraction which leads to the formation of a moiety with four hydroxyl groups. This irreversible dealumination process can decrease the crystallinity of zeolites and effectively modify their acidity, porosity, and catalytic activity. This paper addresses the hydrolysis mechanism of the first Al-O bond in protonic (H+) and cationic (M = Na+, K+, and Ca2+) LTA zeolites. Periodic density functional theory calculations demonstrate that cationic LTA zeolites are more kinetically stable than H -LTA by exhibiting higher reaction barriers. Analysis revealed a H2O dissociation reaction mechanism involving proton transfers and rotations in both protonic and cationic zeolites, which further leads to the Al-O bond breaking. Our results also indicate that a lower Si/Al ratio tends to favor hydrolysis reactions, while the cation identity has a small effect on the reaction barriers. A correlation between the Al-O bond length in the initial adsorbed states and the reaction activation energies is identified.