Is Kinetic Molecular Sieving of Hydrogen Isotopes Feasible?

Abstract

We investigate the possibility of kinetic molecular sieving of hydrogen isotopes by studying their dynamical properties in the one-dimensional channels of microporous aluminophosphate AlPO 4-25 at low temperatures. We use transition state theory as well as molecular dynamics simulations, using an effective quantum potential obtained via the Feynman−Hibbs path integral formalism. With the help of the free energy profile and barrier determined using the Widom particle insertion method, we demonstrate that transition state theory offers an effective and convenient method to determine self-diffusion coefficients, showing excellent agreement with those obtained from molecular dynamics simulations. Quantum-effect-induced kinetic molecular sieving, in which the heavier isotope (deuterium) diffuses faster than the lighter hydrogen, is observed at low temperature, consistent with our simulation results for narrow window zeolite-ρ, and experimental evidence with 3 Å carbon molecular sieve in the recent literature. The free energy profile provides insight into this remarkable counterintuitive behavior, showing that at sufficiently low temperature the free energy barrier for diffusion is smaller for deuterium than hydrogen and exhibits inverse temperature dependence. These findings suggest low-temperature kinetic molecular sieving of hydrogen isotopes as an attractive route for their separation.