Dynamics of exchange at gas-zeolite interfaces I: Pure component n-butane and isobutane

We present the results of Molecular Dynamics and Monte Carlo simulations of n-butane and isobutane in silicalite. We begin with a comparison of the bu lk adsorption and diffusion properties for two different parameterizations of the interaction potential between the hydrocarbon species, both of which have been shown to reproduce experimental gas-liquid coexistence curves. W e examine diffusion as a function of the loading of the zeolite, as well as the temperature dependence of the diffusion constant at loading and for in finite dilution. Both force fields give accurate descriptions of bulk prope rties, We continue with simulations in which interfaces are formed between single component gases and the zeolite. After reaching equilibrium, we exam ine the dynamics of exchange between the bulk gas and the zeolite. In parti cular, we examine the average time spent in the adsorption layer by molecul es as they enter the zeolite from the gas in an attempt to probe the micros copic origins of the surface barrier. The microscopic barrier is found to b e insignificant for experimental systems. Finally, we calculate the permeab ility of the zeolite for n-butane and isobutane as a function of pressure. Our results underestimate the experimental results by an order of magnitude , indicating a strong effect from the surface barrier in these simulations, Our simulations are performed for a number of different gas temperatures a nd pressures, covering a wide range of state points.