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.