Sorbate properties and cage-to-cage diffusion of argon in NaCaA: a molecular dynamics study

Abstract

Detailed molecular dynamics simulations of argon in zeolite NaCaA are reported. Thermodynamic, structural, and dynamical properties of the sorbate as a function of temperature have been obtained. The properties calculated include various site-site radial distribution functions, different energy distribution functions, selfdiffusion coefficients, the power spectra, and properties relating to cage-to-cage diffusion. The results suggest that sorbate is delocalized above 300 K. Both modes of cage-to-cage diffusion-the surface-mediated and centralized diffusion-are associated with negative barrier heights. Surprisingly, rate of cage-to-cage diffusion is associated with negative and positive activation energies below and above 500 K. The observed differences in the behavior of the rate of cage-to-cage diffusion between Xe-NaY and Ar-NaCaA systems and the nature of the potential energy surface are discussed. Presence of sorbatezeolite interactions results in significant enhancement in the rate of cage-to-cage diffusion and rate of cage visits. It is shown that properties dependent on the long-time behavior such as the diffusion coefficient and the rate of cages visited exhibit the expected Arrhenius dependence on temperature.

Temperature and Size Effects on Diffusion in Carbon Nanotubes

“…As the levitation effect is explained for these zeolite systems in terms of window-size/guest-size ratio and for CNT systems in terms of CNT-size/guest-size ratio, it is consequential that the temperature effects in CNTs can be compared to temperature effects on events in the window region. In ref 26, a minimum in the Arrhenius plot for the cage-to-cage crossing rate was found. This observation was explained by different distances to the zeolite surface.…”

Conformation of confined alkanes: n-butane in zeolite Y

“…Earlier simulations of ours on methane and xenon in zeolite NaY and also argon in zeolite NaCaA shows that with increase in loading the population near the cage centre shows an increase. [9][10][11] One normally expects that at higher loadings, the adsorbates prefer to occupy regions near the cage centre due to crowding at the periphery of the cage. The present result is contrary to this and seems to arise from the fact that the additional n-butane molecules prefer to accommodate themselves near the inner surface probably due to stronger sorbate-sorbate and sorbatezeolite interactions.…”

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

“…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 . Above 500 K the activation energy was found to be positive with E a = 1.38 kJ mol −1 [3]. These surprising results were interpreted in terms of the underlying potential energy surface and the surfacemediated (s.-m.) and centralized diffusion (c.d.)…”

“…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].…”