Barely porous organic
cages (POCs) successfully separate hydrogen
isotopes (H2/D2) at temperatures below 100 K.
Identifying the mechanisms that control the separation process is
key to the design of next-generation hydrogen separation materials.
Here, ab initio molecular dynamics (AIMD) simulations
are used to elucidate the mechanisms that control D2 and
H2 separation in barely POCs with varying functionalization.
The temperature and pore size dependence were identified, including
the selective capture of D2 in three different CC3 structures
(RCC3, CC3-S, and 6ET-RCC3). The temperature versus capture trend was reversed for the 6ET-RCC3 structure, identifying
that the D2 and H2 escape mechanisms are unique
in highly functionalized systems. Analysis of calculated isotope velocities
identified effective pore sizes that extend beyond the pore opening
distances, resulting in increased capture in minimally functionalized
CC3-S and RCC3. In a highly functionalized POC, 6ET-RCC3, higher velocities
of the H isotopes were calculated moving through the restricted pore
compared to the rest of the system, identifying a unique molecular
behavior in the barely nanoporous pore openings. By using AIMD, mechanisms
of H2 and D2 separation were identified, allowing
for the targeted design of future novel materials for hydrogen isotope
separation.