Quantum Effects on the Diffusivity of Hydrogen Isotopes in Zeolites

Abstract

The molecular sieving of H2 and its isotopes produced by nuclear plants is a long-standing research where some adsorption processes are well identified. However, some mechanisms governing the diffusion of the proton and its isotopes within a faujasite NaX at low temperatures (40–100 K) are still troublesome to characterize. Notwithstanding, an understanding of the processes governing adsorption diffusion within narrow pores is essential for the development of recycling procedures of gases produced by nuclear plants. At cryogenic temperatures, it is well known that quantum effects are revealed and the heterogeneity of the guest structure plays an important role in the adsorption process. Here, we focus on the consequences of these two factors on molecular sieving and transport coefficients based on molecular dynamics including the Feynman–Hibbs quantum approximation. Our results show at temperatures below 77 K that H2 has a lower adsorption capacity and a lower diffusion coefficient than those of D2 and T2. Here, we give an original explanation of this diffusion inversion in terms of the activation energies. We show that this energy is greater for H2 by 30 and 50% than that for D2 and T2, respectively. Moreover, experimentally, it has been shown that the pore heterogeneity of the faujasite NaX leads to an increase of the self-diffusion coefficient with loading. For explaining this unexpected behavior, several authors have proposed a scenario based on either privileged adsorption sites or the residence time of molecules near the guest pore walls, followed by isotropic jumps. Here, we report an original analysis based on the mean square displacements, revealing the presence of slow and fast molecular mobility regimes. The ensemble of our results provide useful physical information about the development of recycling procedures of gases produced by nuclear plants.

New force field for GCMC simulations of D2/H2 quantum sieving in pure silica zeolites

“…Concerning the zeolites, several molecular simulations of adsorption and diffusion of hydrogen and deuterium are described in the literature [22][23][24][25][26][27] . They have been performed with Monte Carlo or Molecular Dynamics methods using different force fields including the Feynman-Hibbs variational approach to take into account quantum effects.…”

Elucidation of Diffusivity of Hydrogen Isotopes in Flexible MOFs by Quasi‐Elastic Neutron Scattering

“…To date, however, only a few MOFs have been utilized to investigate the application of diffusivity to gas separation either experimentally (e.g., pulsed-field-gradient (PFG) NMR and quasi-elastic neutron scattering (QENS)) or through molecular dynamic simulations. [8][9][10][11] Stallmach et al used PFG NMR to study the diffusion of hydrocarbons, such as methane, ethane, n-hexane, and benzene, in MOF-5 and revealed for the first time the diffusivity correlation between the surface area and the pore size. [8] Similarly, Pusch et al explored CO 2 and CH 4 self-diffusion in ZIF-8 at elevated pressure and found that the former diffuses slightly faster than the latter.…”

Incorporation of trivalent cations in NaX zeolite nanocrystals for the adsorption of O2 in the presence of CO2

“…23 In the present study, we use a primitive rhombohedral cell of FAU containing around 165 atoms to reduce computational effort. [24][25][26][27] It can be seen in Figure 1 that the primitive cell of faujasite contains two supercages and eight hexagonal windows connecting the sodalite with the supercage. 25,28 Please do not adjust margins Please do not adjust margins Interaction energies have been calculated according to the following equation:…”