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 Gull 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. (C) 2010 Elsevier Inc. All rights reserved.