Intrinsic D2/H2 selectivity of NaX Zeolite: Interplay between adsorption and kinetic factors

We present the first calculations of intrinsic D2 over H2 (D2/H2) 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.; Pantatosaki, E.; Papadopoulos, G. K. J. Chem. Theory Comput. 2013, 9, 2922-2929) are used to calculate zero- and finite-pressure adsorption D2/H2 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 D2/H2 selectivity of NaX is only ∼1.22-1.31. On the contrary, the theoretical and experimental adsorption D2/H2 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 H2 than D2 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 H2/D2 mixtures using permselective nanoporous membranes.