Confinement effects and acid strength in zeolites

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Chemical reactivity and sorption in zeolites are influenced by confinement and Brønsted acid site (BAS) strength.
• Water presence can convert zeolite BAS into hydronium ions, altering catalytic properties.

Purpose of the Study:

• To investigate the transition of zeolite BAS to hydronium ions across varying zeolite pore sizes.
• To understand the role of water content and zeolite structure in proton solvation.

Main Methods:

• Utilized ab initio molecular dynamics simulations.
• Employed enhanced sampling techniques, including Well-Tempered Metadynamics.
• Developed and applied a novel set of collective variables for accurate analysis.

Main Results:

• At low water content (1-2 water/BAS), protons are shared between zeolite and water.
• At higher water content (>2 water/BAS), protons become solvated within adjacent water clusters.
• Proton solvation is favored by entropy, which increases with water concentration and is independent of pore size.

Conclusions:

• Water content dictates the proton's state, transitioning from shared to solvated forms.
• Enthalpy dominates the free energy of complexes at low water loadings, influenced by BAS strength and site accessibility.
• Entropy drives proton solvation at higher water concentrations, irrespective of zeolite pore dimensions.