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 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 calcnlated based on the predicted distributions. The statistically weighted average of the free energy differences, of Na+/NH4+ exchange in zeolite is compared that in an aqueous solution of the cations. The equilibrium isotherm is predicted and found to agree well with experimental data.