# Investigation of the Dynamic Behaviour of H2 and D2 in a Kinetic Quantum Sieving System

## TL;DR

Researchers studied how hydrogen isotopes H2 and D2 behave in a special porous material, finding that D2 moves more slowly and can be separated from H2 at low temperatures.

## Contribution

The study demonstrates a novel method for hydrogen isotope separation using kinetic quantum effects in porous organic cages.

## Key findings

- D2 has a diffusion coefficient about one-sixth that of H2 in the porous organic cages due to nuclear quantum effects.
- At 77 K, D2 has longer residence times in the pores compared to H2.
- Hydrogen isotope separation is feasible using temperature or pressure swings at liquid nitrogen temperatures.

## Abstract

Porous organic cages (POCs) are nanoporous materials
composed of
discrete molecular units that have uniformly distributed functional
pores. The intrinsic porosity of these structures can be tuned accurately
at the nanoscale by altering the size of the porous molecules, particularly
to an optimal size of 3.6 Å, to harness the kinetic quantum sieving
effect. Previous research on POCs for isotope separation has predominantly
centered on differences in the quantities of adsorbed isotopes. However,
nuclear quantum effects also contribute significantly to the dynamics
of the sorption process, offering additional opportunities for separating
H2 and D2 at practical operational temperatures.
In this study, our investigations into H2 and D2 sorption on POC samples revealed a higher uptake of D2 compared to that of H2 under identical conditions. We
employed quasi-elastic neutron scattering to study the diffusion processes
of D2 and H2 in the POCs across various temperature
and pressure ranges. Additionally, neutron Compton scattering was
utilized to measure the values of the nuclear zero-point energy of
individual isotopic species in D2 and H2. The
results indicate that the diffusion coefficient of D2 is
approximately one-sixth that of H2 in the POC due to the
nuclear quantum effect. Furthermore, the results reveal that at 77
K, D2 has longer residence times compared to H2 when moving from pore to pore. Consequently, using the kinetic difference
of H2 and D2 in a porous POC system enables
hydrogen isotope separation using a temperature or pressure swing
system at around liquid nitrogen temperatures.