Non-equilibrium molecular dynamic simulations were used to study the kinetics of adsorption of n-butane molecules in a silicalite membrane. This simple well-known process was chosen so as to demonstrate that the process was characterized by two stages, both non-isothermal. In the first stage the large chemical driving force leads to a rapid uptake of n-butane in all the membrane and a simultaneous increase in the membrane temperature, explained by the large enthalpy of adsorption, ΔH = -61.6kJ/mol butane. A diffusion coefficient for transport across the external surface layer was calculated from the relaxation time; a value of 3.4 x 10-9m2/s was found. During the adsorption, a significant thermal driving force develops across the external surface of the membrane, which leads to an energy flux out of the membrane during the second stage. In this stage a thermal conductivity of 3.4 x 10-4W/Km was calculated from the corresponding relaxation time for the surface, confirming that the thermal conduction was the rate-limiting step. The aim of this paper was to demonstrate that a thermal driving force must be taken into account in addition to a chemical driving force in the description of transport in nano-porous materials.

Thermal Effects during Adsorption of n-Butane on a Silicalite-1 Membrane: a Non-Equilibrium Molecular Dynamics Study. I.Inzoli, J.M.Simon, S.Kjelstrup, D.Bedeaux: Journal of Colloid and Interface Science, 2007, 313[2], 563-73