Understanding Dealumination Mechanisms in Protonic and Cationic Zeolites

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.