FLUCTUATIONS IN ZEOLITE APERTURE DIMENSIONS SIMULATED BY CRYSTAL DYNAMICS

The lattice dynamics of zeolite frameworks have often been suggested to influence the performance of zeolite materials in catalytic and sorptive applications. In a direct study of the influence of the framework dynamics on pore structure, constant volume crystal dynamics methods are used to simulate variations in the aperture dimensions with temperature of the six representative zeolite structure types SOD (sodalite; 6-ring-six oxygen atoms define the aperture), RHO (rho; 8-ring), TON (theta-1; 10-ring), MFI (silicalite; 10-ring), LTL (Linde type L; 12-ring), and *BEA (beta; 12-ring). The framework flexibilities are explicitly modeled by a crystal mechanics force field with parameters taken from quantitative interpretations of Raman and infrared spectroscopic data. These simulations reveal substantial motion of the framework atoms about their equilibrium positions. The variations in the fluctuations of the effective aperture sizes with temperature 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 in the pore window motion. The definition, extent, and period of the motion depend on the framework connectivity. It is most pronounced in the SOD and RHO frameworks, previously known from experiment to be most susceptible to static framework distortion. The change in cross-sectional area of the 12-ring window in the LTL framework with time is also periodic, a direct demonstration of pore-mouth breathing motion.