Statistical thermodynamics of Si,Al ordering in aluminosilicate faujasites

The Si,Al distribution on the zeolitic framework of faujasite is analyzed by a Monte Carlo method, by following a ‘‘simulated-annealing’’ procedure. Equilibrium atom distributions are obtained at the formation temperature of synthetic X and Y zeolites (${\mathit{T}}_{\mathit{F}}$∼350 K). The heat capacity ${\mathit{c}}_{\mathit{v}}$ is calculated from the energy fluctuations in the canonical ensemble, for framework compositions in the range of 48–96 Al atoms per cell (Al atomic fraction from 0.25 to 0.5). ${\mathit{c}}_{\mathit{v}}$ presents two peaks as a function of Al loading, which correspond to two order-disorder phase transitions in the Si,Al distribution. The configurational entropy has been evaluated from the simulation results by thermodynamic integration along a reversible path. The configurational free energy shows characteristic variations for the three structural phases obtained, which are discussed in terms of different arrangements of Si and Al atoms over the building blocks of the zeolitic framework.