For anisotropic nanoporous materials, guest diffusion was often reflected by a diffusion tensor rather than a scalar diffusion coefficient. Moreover, the resulting diffusion anisotropy could notably differ for different guest molecules. As a particular class of such systems, an array of two types of channels, mutually intersecting each other was considered, where the rates of diffusion in the different directions depended upon the nature of the guest molecules. The simultaneous adsorption of two types of guest molecules was considered, as in technical applications of porous materials such as catalysis. A case study was presented in which atomistic molecular dynamics and coarse-grained dynamic Monte Carlo simulations were compared and shown to yield qualitatively similar results for non-steady-state diffusion. The two techniques were complementary. Molecular dynamics simulations were able to predict the details of molecular propagation over distances of a few unit cells, whereas the evolution of sorption profiles over distances comparable with entire crystallites could be studied with diffusion quantum Monte Carlo simulations. Consideration of these longer length and time scales was necessary for applications of such systems in chemical separations and heterogeneous catalysis.

Guest-Specific Diffusion Anisotropy in Nanoporous Materials: Molecular Dynamics and Dynamic Monte Carlo Simulations. P.Bräuer, A.Brzank, L.A.Clark, R.Q.Snurr, J.Kärger: Adsorption, 2006, 12[5-6], 417-22