LONG-RANGE VERSUS SHORT-RANGE SI, AL ORDERING IN ZEOLITE-X AND ZEOLITE-Y

A Monte Carlo method is employed to study the Si, Al distribution in zeolites X and Y. The simulation includes long-range Coulomb interactions and oxygen polarization effects. Both contributions to the lattice energy are found to be necessary for a realistic description of the tetrahedral cation ordering. The equilibrium Si, Al distributions show short-range order for all analyzed compositions (1 < Si/Al < 3), in good agreement with Si-28 NMR results. Long-range ordering is only found in the X region (Si/Al < 1.4). Two order-disorder transitions are found at framework compositions for which earlier X-ray diffraction analyzes detected discontinuities in the lattice parameter as a function of Al loading.