Separating hydrogen isotopologues via kinetic quantum sieving: Understanding important pore characteristics for an efficient separation

The potential performance of porous membranes in separating hydrogen isotopologues has been explored employing model systems and quanto-mechanical calculations including both zero-point energy and a numerically exact description of tunneling effects along the reaction coordinate. Membranes have been modeled as cylindrically pierced impenetrable wall, whereas diatomic molecules are described as dumbells composed of hard-sphere atoms. With the relative energetics of diatomics confined into cylindrical pores suggesting that differences in the adiabatic energy profiles between isotopologues for pore radii lower than 2.1 & Aring; should favor transport of heavier species, we investigated the selectivity for the latter process when membranes are 2.0 & Aring; thick. Chosen a pore radius, the results suggest that non-interacting pores represent the best compromise between selectivity and permeance, the addition of attraction between the membrane walls and molecular projectiles improving permeance while markedly depressing selectivity. A repulsive interaction with the pore inner surface, instead, reduced both properties. Finally, sieving molecules through a double membrane layer was found to marginally impact on the separation properties, which could be improved, at best, by 25% with a careful selection of the inter-membrane distance. Our results appear useful for the process of designing more effective sieving systems to separate di-deuterium molecules from its lighter counterparts.