Molecular simulation and adsorption studies of n-hexane in ZSM-11 zeolites

The effects of packing of n-hexane molecules in the channels of a ZSM-11 zeolite are studied by means of adsorption microcalorimetric and volumetric experiments and molecular simulation. We find that the packing density within the zeolite channels is significantly higher than the bulk liquid density. This effect, somewhat characteristic of the n-hexane/ZSM-11 adsorbate/adsorbent system, was first found by Guil et al. (1998) [13] and it is here confirmed by new adsorption measurements carried out at various temperatures and on pure silica ZSM-11, and very especially by means of extensive grand-canonical Monte Carlo simulations. The analysis of simulation snapshots, angular distribution functions and intermolecular correlations, in conjunction with the calorimetric measurements, indicate that the excess of packing is not connected to any form of phase transition, but results from the optimum size matching of the n-hexane molecules within the zeolite channels. As a result, this feature is mostly temperature independent, being induced by steric effects. Computer simulations performed for n-butane and n-octane on ZSM-11 indicate that this effect is to be expected for other linear alkanes as well.

n-Hexane molecules tend to adopt mostly linear conformations inside the ZSM-11 zeolite channels, with no particular preference for channel intersections. The ratio between the width of the zeolite channel and the height of the linear alkane is close to one, by which the channels at full load have very little available volume. As a consequence the effective density of n-hexane inside the channels is higher than that of the bulk liquid.Figure optionsDownload as PowerPoint slideResearch highlights
► n-Hexane molecules pack inside the channels of ZSM-11 zeolites at higher densities than in the bulk fluid and this effect is essentially temperature independent.
► This feature, confirmed by molecular simulation and volumetric and microcalorimetric adsorption experiments, is due to the appropriate matching of the channel diameter and the molecular width in mostly all-trans conformations.
► This seems to be a common feature with other linear alkanes and the related ZSM-5 zeolite.