Simulations of the non-framework cation configurations in dehydrated Na–Ca and Na–Li zeolite A

Abstract The non-framework cation positions for Na + and Ca 2+ in dehydrated zeolite 5A, a mixed sodium–calcium form, and for Na + and Li + in dehydrated sodium–lithium zeolite A have been simulated by a combination of Monte Carlo and structure optimization techniques. A framework structure model, the defined chemical composition and appropriate interatomic potentials are the initial input. The effect of varying optimization protocols is explored, with a crude dynamical simulated annealing procedure being found to be effective in yielding practical structural models. The results are in agreement with detailed structural data for zeolite 5A and analogous, but less detailed data for the 8Na + –4Li + form. However, although occupation of only six-ring window sites in zeolite 5A is found to be optimal, the lowest energy configurations have all such cations located external to the sodalite cages. Further, a model for zeolite 5A that has an ordered, interpenetrating tetrahedral arrangement of 4Ca 2+ and 4Na + in the supercage is found to be lower in energy than the disordered arrangement implied by diffraction results. Additionally, the simulations reveal atomic-level details that are not readily extractable from the averaged diffraction results, including the effect of framework symmetry and of neighboring cation configurations in determining the details of local non-framework cation sitings.