Currently, there is no theoretical justification for the Robeson bound for sorption driven processes such as CO 2 /N 2 separation. Using DMFT, we demonstrate that for what is commonly thought as an equilibrium sorptiondriven separation, diffusion still plays a significant role in the separation mechanism. Using very simple models of structural elements to isolate various scenarios, we are arriving at the following conclusions, some of them deliberately more speculative than the others to prompt further theoretical and computation studies: