Design Elements for Enhanced Hydrogen Isotope Separations in Barely Porous Organic Cages

Abstract

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.

Porous organic cages for gas separations

“…In 2022, Vogel et al used ab initio molecular dynamics (AIMD) simulations to explain how the internal functionalized POCs could manage D 2 and H 2 separation, leading to the targeted design of novel porous adsorbents for hydrogen isotope separation. 128…”

Recent advances in the applications of porous organic cages

“…Based on the cocrystal structured POC work mentioned above, Vogel et al 57 investigated the temperature-dependent structure-property relationships existing between RCC3, CC3-S, 6ET-RCC3, and H 2 /D 2 by using ab initio molecular dynamic simulations. The RCC3 and CC3-S exhibited decreasing pore apertures as the temperature increased, whereas the 6ET-RCC3 displayed the opposite effects.…”

Porous Liquids: Computational Design for Targeted Gas Adsorption

“…Second, current DFT simulations do not consider the temperature and vibrational effects that can alter the stability of gas binding sites. Even at low temperatures, variation in pore window sizes can alter the ability of a gas molecule to diffuse into a pore as we have seen in the evaluation of H 2 and D 2 diffusion into and out of RCC3, CC3-S, and 6ET-RCC3 POCs . The functionalization of the porous host and the solvent molecule can interfere with the binding geometries of weakly bound gas molecules.…”

“…Even at low temperatures, variation in pore window sizes can alter the ability of a gas molecule to diffuse into a pore as we have seen in the evaluation of H 2 and D 2 diffusion into and out of RCC3, CC3-S, and 6ET-RCC3 POCs. 79 The functionalization of the porous host and the solvent molecule can interfere with the binding geometries of weakly bound gas molecules. Additionally, the relatively weak physisorption of the gas molecules in PLs, 30,31 combined with their low energy regeneration procedures, 23,54 indicate that this may be of specific concern in these materials.…”