Anomalous diffusion of linear and branched pentanes within zeolite NaY

Abstract

Molecular dynamics studies of n-pentane and iso-pentane in zeolite NaY are reported. Radial distribution of the pentane isomers within the supercage, angle distribution of the end-to-end vector with the radial vector, distribution of end-to-end distance in the two isomers, average guest-zeolite interaction energy, properties during intercage migration event, diffusivity, autocorrelation function for the centre of mass and angular velocities are reported. Results suggest that iso-pentane has a higher diffusivity and lower activation energy than n-pentane. This surprising result is attributed to the need for change from trans to gauche during intercage migration in the case of n-pentane. This is suggested by the increase in gauche population in n-pentane near the window while no such increase is observed in iso-pentane. Another reason for the higher diffusivity of iso-pentane appears to be due to the similarity of the cross-sectional diameter of iso-pentane with the diameter of the 12-ring window.