Confinement Effects on Adsorption and Diffusion of Hexane in Nanoporous MCM-41 with Different Pore Sizes:A Molecular Dynamics Study

摘要

We report the results of molecular dynamics(MD)simulations using a new semi-empirical intermolecular interaction potential on the adsorption and diffusion of hexane in siliceous MCM-41 at 300 K.The potential function is tuned to give an adsorption energy of-9.1 kcal/mol,reproducing the experimental value for a corresponding pore size.We investigated MCM-41 models with four different pore sizes and studied loadings from one molecule of hexane up to a loading corresponding to the density of liquid hexane.As a result of confinement in MCM-41,the free energy of adsorption of hexane increases when the pore sizes decrease;for example,the adsorption energy increases from-9.1 to 13.7 kcal/mol for the largest to the smallest pore size for a loading of one molecule.Also,the adsorption energy increases by 3-4 kcal/mol for all pore sizes when the loading is increased from one hexane molecule to the density of liquid hexane.The calculated self-diffusion coefficients of hexane in MCM-41 with a pore diameter of 27 A are in the order of 1 X 10~(-2)cm~2/s,depending on the loading,which is in reasonable agreement with available experimental data.The self-diffusion coefficients decrease with increasing loadings and when the pore sizes decrease.The average distance between the centers of the mass of hexane molecules in the smallest pores is only marginally less than in the larger pores and in the liquid phase.For low loadings the hexane molecules lie parallel to the pore channel for every pore size.When the loading is increased,they build up concentric rings.These rings of hexane molecules are less well separated from each other in the larger models,and thus their structure more resembles the liquid phase.