Prediction of Na⁺/NH₄⁺ exchange in faujasite zeolite by molecular dynamics simulation and thermodynamic integration method

Molecular dynamics simulations and thermodynamic integration method are used to calculate free energy differences when replacing Na⁺ with NH₄⁺ in hydrated faujasite (Y-type) zeolite and in an aqueous solution of these cations. Force field parameters are optimized by quantum mechanics density functional theory calculations and then validated by calculating cation distributions in dehydrated zeolite and differences in hydration free energies of the cations. Using force field and Monte Carlo simulations, we predict cation distributions at different sites of hydrated zeolite with different mole fractions of NH₄⁺. The free energy changes upon replacement of Na⁺ with NH₄ ⁺ at different sites are calculated based on the predicted distributions. The statistically weighted average of the free energy differences of Na⁺/NH₄⁺ exchange in zeolite is compared with that in an aqueous solution of the cations. The equilibrium isotherm is predicted and found to agree well with experimental data.