Entropic and energetic selectivity in air separation with microporous materials

Materials such as zeolites, carbon molecular sieves, and polymers are used increasingly in the separation of air, based on the difference in diffusion rate between oxygen and nitrogen through the material. The design of impro ved materials requires knowledge of the molecular-level phenomena responsib le for the separation, particularly relative roles of energetic and entropi c (confinement) effects. This issue is difficult to resolve experimentally, as evidenced by the wide range in reported literature values reviewed here . A complementary approach is taken based on a combination of molecular mod eling, statistical mechanics, and transition-state theory. Selectivities fo r molecular models of oxygen and nitrogen in microporous structures are cal culated rising a Monte Carlo technique and resolved into entropic and energ etic components for a range of pore window sizes. Atomic-level flexibility (vibration) is considered as well. The calculated entropic selectivities ar e significantly lower than reported theoretical results, but still consiste nt with experimental data. The energetic selectivity is very sensitive to t he window dimensions and flexibility, but the entropic contribution is much less affected. This also contradicts some previous assumptions in the lite rature.