Entropy effects in adsorption and diffusion of alkane isomers in mordenite: an investigation using CBMC and MD simulations

Abstract

Configurational-bias Monte Carlo (CBMC) simulations were carried out to determine the sorption isotherms for hexane isomers (n-hexane, 2-methylpentane and 2,2-dimethylbutane) and butane isomers (n-butane and iso-butane) in MOR zeolite. The hexane and butane isomers adsorb exclusively within the 12-ring channels. For a given carbon number, the sorption strength, and capacity, increases with the degree of branching. This is due to the increased packing efficiency within the one-dimensional 12-ring channels when the degree of branching increases. Molecular dynamics (MD) simulations were performed to determine both the Maxwell-Stefan (M-S) diffusivities D_i, and the self-diffusivities, D_i,self, of pure components and mixtures, for a range of molecular loadings. Diffusion was found to be uni-dimensional, and no single file diffusion behavior is observed. Entropy effects cause a reversal in the hierarchy of diffusivity values at high loadings. For example, at low loadings the more compact 22DMB diffuses slower than the slimmer nC6 molecule, whereas at high loadings the reverse is true. The diffusion in mixtures of alkane isomers could be predicted very well by the Maxwell-Stefan diffusion formulation, using only pure component adsorption and diffusion data. In these predictions the Reed-Ehrlich model is used to describe the loading dependence of the pure component M-S diffusivities D_i. Furthermore, correlation effects are described by self-exchange coefficients D_ii, obtained from fitting the pure-component diffusion data. The special features of adsorption and diffusion in one dimensional zeolite topologies are emphasized by comparison with simulations in FAU and MFI zeolites.