Intrinsic D2/H2 Selectivity of NaX Zeolite: Interplay between Adsorption and Kinetic Factors

We present the first calculations of intrinsic D-2 over H-2 (D-2/H-2) selectivity in well-defined crystalline nanoporous material, namely, NaX zeolite. Feynman path integrals with realistic force field (Kowalczyk, P.; Gauden, P. A.; Terzyk, A. P.; Pantatosald, E.; Papadopoulos, G. K. J. Chem. Theory Comput. 2013, 9, 2922-2929) are used to calculate zero- and finite-pressure adsorption D-2/H-2 selectivity on NaX at 77 K. The kinetic selectivity is computed by classical molecular dynamics with Feynman Hibbs quantum effective potentials. We found that within Henry's law region the intrinsic D-2/H-2 selectivity of NaX is only similar to 1.22-1.31. On the contrary, the theoretical and experimental adsorption D-2/H-2 selectivities on NaX are 1.49 and 1.6, respectively. A reduction of adsorption selectivity by approximately 13-19% is explained by faster self-diffusion of H-2 than D-2 molecules in NaX crystal (i.e., normal isotope kinetic effect on self-diffusion). Our results clearly demonstrate the interplay between adsorption and kinetic factors that may have important implications for separation of H-2/D-2 mixtures using permselective nanoporous membranes.