Effect of the surface energy barrier on sorbate diffusion in AlPO4-5

The importance of pore exit effects on the diffusion of molecules in AlPO4-5 pores is evaluated using two molecular modeling techniques. In the first approach, a dual control volume grand canonical molecular dynamics technique is used to obtain molecular fluxes of methane out of the truncated crystal as a function of temperature and sorbate loading. The simulation results indicate the presence of a low-temperature surface barrier for diffusion, which retards the flux of methane relative to its apparent flux in the intracrystalline regions of the material. This pore exit barrier tends to diminish as temperature and loading increase. An explanation based on clustering phenomena is proposed to explain the latter. Next, a simple activated transport model is proposed to predict the relative importance of the surface barrier on the transport of sorbates in AlPO4-5. The potential of mean force for a single sorbate molecule along the pore axis of a truncated crystal provides the required activation energy barriers for the model. The model correctly predicts the reduction in the importance of exit effects with an increase in the temperature. It is also observed that exit effects become more important as the ratio of the size of the sorbate molecule to the pore size approaches unity. In particular, exit effects are significant in micrometer-thick AlPO4-5 crystals in the case of large molecules such as SnBr4 and CCl4 at room temperature.