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

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 H 2 and D 2 at practical operational temperatures. In this study, our investigations into H 2 and D 2 sorption on POC samples revealed a higher uptake of D 2 compared to that of H 2 under identical conditions. We employed quasi-elastic neutron scattering to study the diffusion processes of D 2 and H 2 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 D 2 and H 2. The results indicate that the diffusion coefficient of D 2 is approximately one-sixth that of H 2 in the POC due to the nuclear quantum effect. Furthermore, the results reveal that at 77 K, D 2 has longer residence times compared to H 2 when moving from pore to pore. Consequently, using the kinetic difference of H 2 and D 2 in a porous POC system enables hydrogen isotope separation using a temperature or pressure swing system at around liquid nitrogen temperatures.