Quantum Effect Induced Reverse Kinetic Molecular Sieving in Microporous Materials

Abstract

We report kinetic molecular sieving of hydrogen and deuterium in zeolite rho at low temperatures, using atomistic molecular dynamics simulations incorporating quantum effects via the Feynman-Hibbs approach. We find that diffusivities of confined molecules decrease when quantum effects are considered, in contrast with bulk fluids which show an increase. Indeed, at low temperatures, a reverse kinetic sieving effect is demonstrated in which the heavier isotope, deuterium, diffuses faster than hydrogen. At 65 K, the flux selectivity is as high as 46, indicating a good potential for isotope separation.

Quantum effect induced kinetic molecular sieving of hydrogen and deuterium in microporous materials

“…For small values of the de Broglie wavelength, (1) may be approximated as (Kumar and Bhatia 2005;Kumar et al 2006)…”

“…However, all these studies have been confined to equilibrium adsorption and the importance of quantum effects on the dynamics is poorly understood. Recently, we (Kumar and Bhatia 2005) have investigated the Fig. 1 2 × 2 × 2 Unit cells of zeolite Rho II structure.…”

Adsorption dynamics of hydrogen and deuterium in a carbon molecular sieve bed at 77K

“…3, the CMS can give rise to more serious diffusion restrictions. At low temperatures and small pore dimensions on the order of de Broglie wavelengths, kinetic quantum effects on pore diffusion are significant [13,18,19,21,22,48,49]. When the pore size of the adsorbents was small, the successive adsorption stage, after initial surface diffusion and pore diffusion, was governed by quantum sieving stemming from different zero-point energies, and the translational and rotational confinements of the hydrogen isotope molecules.…”

“…Extremely high D 2 separation selectivity, on the order of 10 2 for a H 2 -D 2 mixture gas, was predicted by quantum simulation using single wall carbon nanotubes [16,17]. Kinetic molecular sieving of H 2 and D 2 in zeolite RHO was demonstrated by using atomistic molecular dynamics simulations, and the flux selectivity was as high as 46 at 65 K [18]. However, it was only up to 22.66 at 30 K resulting from Monte Carlo and molecular dynamics simulations [19].…”

“…In addition, structural nanomaterials with uniform pore size distribution were commonly discussed in most experimental and theoretical studies [6,9,[12][13][14][17][18][19][21][22][23][24][25][26]. However, few of hydrogen isotope separation studies were found for adsorbents with wide pore size distribution even though many pelletized adsorbents used in industries have a wide pore size distribution.…”