Xenon in sodium Y zeolite. 2. Arrhenius relation, mechanism, and barrier height distribution for cage-to-cage diffusion

Abstract

Various geometrical and energetic distribution functions and other properties connected with the cage-to-cage diffusion of xenon in sodium Y zeolite have been obtained from long molecular dynamics calculations. Analysis of diffusion pathways reveals two interesting mechanisms-surface-mediated and centralized modes for cageto-cage diffusion. The surface-mediated mode of diffusion exhibits a small positive barrier, while the centralized diffusion exhibits a negative barrier for the sorbate to diffuse across the 12-ring window. In both modes, however, the sorbate has to be activated from the adsorption site to enable it to gain mobility. The centralized diffusion additionally requires the sorbate to be free of the influence of the surface of the cage as well. The overall rate for cage-to-cage diffusion shows an Arrhenius temperature dependence with Ea = 3 kJ/mol. It is found that the decay in the dynamical correction factor occurs on a time scale comparable to the cage residence time. The distributions of barrier heights have been calculated. Functions reflecting the distribution of the sorbate-zeolite interaction at the window and the variations of the distance between the sorbate and the centers of the parent and daughter cages are presented.

Diffusion of Sorbates in Zeolites Y and A: Novel Dependence on Sorbate Size and Strength of Sorbate-Zeolite Interaction

“…This has been obtained from the molecular dynamics simulation by accumulating the data for about 4 ps before, during, and after every intercage migration. 19 This gives the local energetic barrier for diffusion across the window. The variation of U, is shown for two values of y (see Figure 11).…”

Deviation from Arrhenius behaviour of rate of intercage diffusion in Zeolite Y1

“…The rate of cage visits is obtained from the rate of intercage diffusion by eliminating all the diffusion events from cage i to j which are either followed or preceded by diffusion from cage j to i. More details are discussed in the earlier work on zeolite Y [1]. The diffusion coefficient is obtained from the long-time slope of the mean square displacement versus time plot [7].…”

“…A number of results on zeolite Y obtained from molecular dynamics (MD) simulations have been analysed by calculating three important properties, namely, the rate of intercage diffusion, k c , the rate of cage visits, k v and the diffusion coefficient, D [1][2][3]. Calculations on xenon sorbed in zeolite NaY indicate that the slope obtained from the Arrhenius plot of the rate of intercage diffusion leads to an activation energy of about 3.02 kJ mol −1 [1]. In zeolite NaCaA k c shows a peculiar behaviour: below 500 K, the slope of the Arrhenius plot of k c suggests a negative activation energy of −0.95 kJ mol −1 .…”

Simulating the Behavior of Organic Molecules in Zeolites

“…Zeolitic sorbents are used in several industrially important processes, such as hydrocarbon and air separations. The most important industrial area of catalytic application is in fluid catalytic cracking (FCC) (77,72) whereby crude oil is broken down to lower molecular weight fragments used in gasoline or as feedstock for the petrochemical industry. Here an acidic form of Y zeolite which has same framework structure as the mineral faujasite is employed as the primary acidic catalyst.…”