ENERGETICS OF CATION ORDERING IN THE FAUJASITE FRAMEWORK - MONTE-CARLO SIMULATIONS

The energetics of Si,Al ordering in zeolites X and Y is studied by the Monte Carlo method. The microscopic model for the interatomic potential includes long-range Coulomb interactions and oxygen polarization energies. A comparison of Al-O-Si-O-Al group concentrations obtained by simulation and by analysis of earlier Si-29 NMR spectra yields an effective charge difference of 0.32e between Si and Al in this structure. The lattice stabilization due to Si,Al ordering is calculated to go from 560 to 1533 kJ/mol in the range 48-96 Al atoms per unit cell. The main contribution to this lattice stabilization is caused by the avoidance of Al-Al pairs. The average stabilization energy per Al-Al pair varies between 17 and 24 kJ/mol, depending on the framework composition. The simulations performed at the formation temperature of synthetic X and Y zeolites (T(F) approximately 350 K) yield equilibrium Si,Al distributions with lattice energies clearly above the absolute minimum corresponding to each composition (especially for ratios Si/Al < 1.3), as a consequence of thermal disorder at that temperature.