Study of the stability of barium-exchangrd A-typezeolite and molecular dynamics calculation on the collapse of zeolite framework

Abstract

Part A: From the geometry shown by x-ray crystallographic studies, potential energy functions suitable for energetic calculations in Ba-exchanged zeolite A, ($Ba_{x/2}Na_{12-x}-A$, 1 ≤ × ≤ 12), have been obtained by using the method of constraints. The influence of the water content and degree of Ba exchange on the stability of the framework has been investigated energetically using these potential functions. The stability of the framework decreases as the Ba content increases and as the water content decreases. Part B: Collapse of A-type zeolite framework, especially melting phenomena, with increasing temperature was investigated by using the method of molecular dynamics. As the temperature increases, the zeolite framework becomes unstable due to the increased thermal motions and finally collapses to an amorphous liquidlike phase at 2100 K. Diffusion of zeolite framework atoms as well as the changes in the peak maxima of radial distribution functions and the distribution of(Si,Al)T-O-T(Si,Al)bond angles upon temperature changes are discussed in connection with the structural collapse. Part C: Molecular dynamics calculations (MD) for A-type zeolite were carried out at several temperatures and the radial distribution functions (rdf) were obtained at those temperatures. Using the rdf of the 625 K structures for A-type zeolite and cristobalite-type structure selected as the standard structures, the relative proportions of them by rdf fitting at several temperatures were obtained. According to our findings, A-type zeolite starts to collapse around 900 K and at 1800 K, nearly fifty percent of A-type zeolite transforms into the cristobalite-type structure. This transformation is maximized at 2400 K and the cristobalite-type structure also starts to collapse above this temperature. Though the temperatures where characteristic structural changes are found were higher than the experimental results, rdf fitting method was successfully applied to the prediction of the ...