The molecular sieving of isotopes has for long been considered impossible, as they are of essentially identical size and shape; however, in recent years this perspective has been modified with respect to hydrogen and its isotopes. It is now recognized that, because of their low mass, quantum effects can lead to sufficiently large differences in their de Broglie wavelengths at cryogenic conditions to make their molecular sieving possible in nanoporous materials having pores of molecular dimension. Whilst the majority of the attention has focused on their adsorption equilibrium, our recent theoretical work, based on molecular dynamics simulations employing the Feynman-Hibbs path integral formulation, has shown even more remarkable effects on diffusion, with the heavier isotope deuterium diffusing faster than hydrogen at sufficiently low temperatures in microporous zeolite rho as well as alumino-phosphate AlPO4-25. Here we will report the first microscopic experimental verification of this interesting effect, as well as our extensive theoretical studies employing both the Feynman-Hibbs and more accurate path integral approaches.
Microscopic Quasi-Elastic Neutron Scattering experiments have been conducted to determine the diffusivities of hydrogen and deuterium over a wide temperature range in a microporous carbon having sufficiently narrow nanopores. These measurements clearly demonstrate cross-over of the diffusivities at about 100 K, with deuterium showing larger broadening of the energy spectrum and diffusing faster below this temperature. This value is in remarkable accord with that from the simulations with other materials discussed above, despite the approximation inherent to the Feynman-Hibbs approach. The molecular mechanism for this phenomenon is also examined by simulation of the self-diffusivity of H2 and D2 in various carbon models using equilibrium molecular dynamics incorporating quantum effects. Equilibrium calculations using path integral Monte Carlo simulations have also been conducted for this carbon, and together with our molecular dynamics simulations provide the necessary ingredients for analyzing the feasibility of kinetic molecular sieving in this carbon. The results of these studies will be reported.