More environmental and energy-related problems could benefit from separation processes based on nanoporous membranes, such as zeolite membranes. As more challenges in materials synthesis are overcome, membranes are an interesting, energy-efficient alternative to distillation, absorption and adsorption processes. One example is the separation of CO₂ and N₂ in flue gases from power plants, in order to concentrate and sequester CO₂.

Zeolites are microporous, crystalline aluminosilicates. A commonly used zeolite is ZSM-5, which contains a regular array of straight channels in the y-direction, and zigzag channels in the x-z plane. The pore diameter is around 5.5Å. These very narrow pore channels, with different interactions between the pore surface and CO₂ and N₂, lead to different adsorption and diffusion rates, which allows the membrane to separate CO₂ and N₂. Combined with its high thermal stability, ZSM-5 is an attractive option for CO₂/N₂ separations, also at high temperatures.

Molecular simulations are useful to assess zeolites and other nanoporous materials for their capability to separate components, especially when experiments are expensive or difficult to carry out, and the importance of parasitic effects (such as those caused by grain boundaries and other heterogeneities) on membrane performance is hard to evaluate. In addition, computer simulations provide interesting mechanistic insight, and clues to the design of new membranes with optimized properties.

The duration of fully atomistic molecular dynamics simulations on macroscopic, heterogeneous systems as a complete Al-containing zeolite membrane is prohibitive, even on supercomputers. Therefore, to study transport through a ZSM-5 membrane we combine short-time molecular dynamics with Grand Canonical Monte-Carlo simulations of adsorption, coarse-grained kinetic Monte-Carlo simulations of diffusion, and a new net-transport mean-field theory. We calculate macroscopic diffusivities, membrane permeability and selectivity, and study their dependence on the geometric and chemical membrane structure. Results are compared with experimental measurements. This multiscale simulation approach is easily extended to other zeolites and other gas mixtures.