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

Abstract

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.; Pantatosaki, 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 ∼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.

Exploiting Diffusion Barrier and Chemical Affinity of Metal–Organic Frameworks for Efficient Hydrogen Isotope Separation

“…As a promising system for D 2 /H 2 separation, nanoporous materials have attracted significant research interest, as they can directly capture heavier isotope gas molecules via kinetic quantum sieving (KQS) − or chemical affinity quantum sieving (CAQS). − Heavier isotopes, having a shorter de Broglie wavelength at cryogenic temperatures, experience a lower diffusion barrier and can diffuse faster than lighter isotopes in a confined space, resulting in enrichment of the product with heavier isotope materials using the KQS effect . Hirscher et al enforced KQS effects by decorating the internal surface of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) with Cl – and pyridyl groups, respectively, to optimize pore aperture, leading to significantly enhanced isotope selectivity ( S D2/H2 ) of 7.5 at 60 K and 9.7 at 22 K, respectively. , However, due to the weak binding energies of hydrogen isotopes on the internal surface of porous materials, this approach effectively operates at only very low cryogenic temperatures.…”

Computationally-Guided Synthetic Control over Pore Size in Isostructural Porous Organic Cages

“…Traditional molecular sieving is impractical for the separation of isotopes, but kinetic quantum sieving is possible at low temperatures in materials with sufficiently narrow pore diameters (PD) of less than 0.7 nm. 21 Zeolites, 22 porous carbons, 23 and metal–organic frameworks 24 have been shown to have selectivity for D 2 over H 2 ; achieving D 2 /H 2 selectivity with a solution-processable porous molecular material could lead to new isotope separation membranes. We therefore targeted POCs with smaller pores than CC3 α, but with retention of the same 3-D diamondoid pore topology.…”

Kinetic and Chemical Affinity Quantum Sieving Effects of ZIF-67 for H2 and D2 and Gas Chromatographic Separation of Hydrogen Isotopes on Columns Packed with ZIF-67@NH2-γ-Al2O3

“…For the diffusion of gas molecules in porous materials, when the number of molecules is small, the diffusion of mixed gas molecules will affect each other weakly. When the load number in MOF materials is large, the self-diffusion coefficients of H 2 and D 2 are very similar and relatively low due to frequent intermolecular collisions, so the kinetic effect in the dense adsorption phase can be ignored . For the diffusion of H 2 and D 2 in ZIF-67, to avoid too low diffusion coefficients of hydrogen and deuterium, the number of molecules of H 2 and D 2 in the research system was taken to be less than 30 due to their saturated adsorption capacities, and the diffusion coefficient of pure H 2 or D 2 in ZIF-67 was studied.…”