Heptane adsorption in silicalite-1: Molecular dynamics simulation

Abstract

Molecular dynamics (MD) simulations have been used to study the adsorption process of n-heptane molecules in silicalite-1 at 300 K. MD simulated results were compared to experimental neutron diffraction (ND) and experimental self-diffusion coefficients. The analysis of MD data indicated a packing of the adsorbed molecules around 4 mol./u.c., which is not the consequence of an enthalpic effect but of an entropic effect. The role of the n-heptane chain flexibility ( cis– trans conformation) in relation with the silicalite-1 channel type (straight versus sinusoidal) was outlined and enabled to understand the mobility change arising at 4 mol./u.c., according to previous experimental results. The MD simulation also allowed to identify adsorption sites, three in the straight channels and three in the sinusoidal channels and to characterize their position, energy and occupation. Site position but only relative occupation data were in good agreement with neutron diffraction data. The assumption of a “commensurate freezing” to explain the step isotherm is discussed in the light of the MD simulation and ND refinement results.