ZEOLITE STRUCTURAL PROBLEMS FROM A COMPUTATIONAL PERSPECTIVE

ABSTRACT Computer modeling techniques can be a substantial aid in zeolite structure solutions or refinements, and a means of extracting structural insight from the averaged structural results that diffraction experiments provide. Comparison against databases of hypothetical zeolite structure types, model building or editing subject to space group and translational symmetry constraints, and simulated annealing are three effective computational routes to framework structure solution. Simulated annealing, a Monte Carlo based optimization procedure, is used to adjust an initially random arrangement of the required number of T-atoms in the unit cell (T= tetrahedral species, Si, AI etc.) so as to best match prescribed geometrical and diffraction pattern matching constraints. Simulated annealing has also been exploited in studying aluminum distributions in zeolite frameworks, based on comparison against measured 29 Si nmr data. The effects of finite temperature on zeolite structures have been simulated by crystal dynamics, using force field parameters taken from interpretations of zeolite vibrational spectra. Fluctuations in the effective aperture sizes with temperature are found to depend on the framework connectivity, consistent with experimental observation. The frequency spectra of the O–O distances across the apertures reveal generally well-defined periodicities to the pore window motion; a breathing motion is manifested in the LTL-framework as a change in cross-sectional area of the 12-ring window with time during the dynamics simulations.