Quantum Sieving for Isotopic Separations of Gases Using Porous Materials30 Years of Progress

This paper reviews theoretical and experimental efforts to establish microporous materials that can be used to separate isotopologues of molecular gases such as H2 and D2. Emphasis is placed on the use of simplified models to highlight the quantum phenomena that make these separations possible. In equilibrium adsorption, differences in zero-point energy in the adsorbed states of molecules favor binding of heavier isotopologues (e.g., D2 relative to H2). Experimental data showing this effect was reported as early as 1933, but the theoretical work of Beenakker et al. in 1995 [Beenakker, J. J. M. Chem. Phys. Lett. 1995, 232, 379–382.] spurred modern efforts to develop materials with strong so-called quantum sieving. In porous materials where the transition state for molecular diffusion is more strongly confined than the energy minima associated with equilibrium adsorption differences in zero-point energies between these two sites can lead to isotopic differences in molecular diffusivities. This effect favors the diffusion of heavier species (e.g., D2) relative to lighter species (e.g., H2). This so-called kinetic quantum sieving has been observed experimentally in porous carbons and in porous organic cage materials. We show that quantum tunneling, which favors hopping of lighter species across energy barriers, diminishes the strength of kinetic quantum sieving but that it appears to make only a small contribution to the net molecular diffusivities in many porous materials of practical interest.