Microscopic Observation of Kinetic Molecular Sieving of Hydrogen Isotopes in a Nanoporous Material

We report quasielastic neutron scattering studies of ${\mathrm{H}}_2\mathrm{\text{−}}{\mathrm{D}}_2$ diffusion in a carbon molecular sieve, demonstrating remarkable quantum effects, with the heavier isotope diffusing faster below 100 K, confirming our recent predictions. Our transition state theory and molecular dynamics calculations show that while it is critical for this effect to have narrow windows of size comparable to the de Broglie wavelength, high flux requires that the energy barrier be reduced through small cages. Such materials will enable novel processes for kinetic molecular sieving of hydrogen isotopes.

Figure 1

Comparison between experimental and calculated QENS spectra obtained for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ in Takeda 3 Å CMS, at different temperatures: (a) 30 K, (b) 50 K, (c) 77 K, (d) 120 K (loading: $0.5\mathrm{mmol}/\mathrm{g}$, $Q=0.486{\AA }^{-1}$). These spectra correspond to the grouping of 10 detectors ranging from 0.454 to $0.525{\AA }^{-1}$.

Figure 2

Diffusion coefficients for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ in Takeda 3 Å CMS, as a function of temperature (loading: $0.5\mathrm{mmol}/\mathrm{g}$), showing typical error bars. Inset (a) depicts the pore size distribution of the CMS obtained [13] from the interpretation of experimental high pressure ${\mathrm{CO}}_2$ adsorption data. Inset (b) depicts experimental and molecular dynamics simulation results for self-diffusivity of ${\mathrm{H}}_2$ in zeolite rho [7].

Figure 3

Linear fit of Eq. (2) to experimental data of self-diffusivity of hydrogen isotopes in CMS Takeda 3 Å. The inset depicts the value of $F_2(r_{*b})$ for CMS Takeda 3 Å ($F_2^{\mathrm{C}}$). Here $r_x^y$ is the radius of cage ($x=b$) and window or pore mouth ($x=*$) in CMS Takeda 3 Å ($y=\mathrm{C}$).

Figure 4

(a) 3D view of periodic slitlike carbon model containing 240 carbon atoms. Diffusion path (bending arrows) shows movement of fluid molecules. (b) A top view of one pore wall of the carbon model with carbon atoms removed [red spheres in (a)] to create cages, showing the diffusion path of an adsorbed molecule through neighboring cages (broken circle), and through the dividing surface (vertical dashed line).