Flexibility-Driven Mechanisms in Nanoporous Materials: Breathing, Gating, and Thermal Expansion for High-Temperature Hydrogen Isotope Separation

Abstract

The demand for deuterium (D2), the stable isotope of hydrogen (H2), has been exponentially increasing due to its wide range of applications across various fields. In particular, deuterium is being utilized in the semiconductor industry as a substitute for H2 to extend the lifespan of semiconductors. As the demand for D2 continues to rise, the importance of efficient hydrogen isotope separation technologies becomes increasingly critical. However, H2 and D2 have similar identical size, shape, and thermodynamic properties. Therefore, hydrogen isotope separation is one of the difficult challenges. Traditional methods like cryogenic distillation dominate the field but are highly energy consuming and operationally inefficient. Therefore, hydrogen isotopes can be efficiently separated using nanoporous materials via the mechanism of kinetic quantum sieving (KQS). The KQS effect allows the de Broglie wavelength difference of the hydrogen isotopes to make a diffusion difference, allowing them to be separated. As nanoporous materials with pore sizes optimized for the KQS effect can more efficiently separate hydrogen isotopes, it is important to control the pore size of porous materials. However, it is challenging to precisely control the pore size of nanoporous materials. Furthermore, KQS-based systems can realize efficient isotope separation at low temperatures (below 77 K). Therefore, raising the operating temperature of KQS is essential to enhance its efficiency in isotope separation processes. To overcome the challenges of low separation operating temperatures and pore control in the KQS effect, flexible nanoporous materials have been introduced. The flexibility of these porous materials can influence pore sizes, enabling the formation of pore size suitable for the KQS effect. Moreover, the breathing and gating effects enable adsorption and separation of hydrogen isotopes at high temperatures. In this study, we investigated high-temperature hydrogen isotope separation by using MIL-53(Al) with a breathing effect, porous carbon material (SS-600) with a gating effect triggered temperature, and Cu-ZIF-gis with lattice- driven gating. Advanced cryogenic thermal desorption spectroscopy (AC-TDS) was used to investigate the hydrogen isotope separation performance of flexible nanoporous materials. In addition, neutron scattering experiment was measured to investigate the dynamics of hydrogen isotopes in flexible structures. MIL-53(Al) is well known to exhibit a breathing effect, where its pore structure transitions in response to external stimuli such as pressure, temperature, and guest molecules. Quasi- elastic neutron scattering (QENS) measurements were performed to observe the diffusion behavior of hydrogen isotopes in MIL-53(Al) with breathing effect. When the pores of MIL- 53(Al) were closed, diffusion differences in hydrogen isotope were observed at low temperatures, indicating a behavior similar to that of materials with rigid structures. However, when the pores were fully open due to the breathing effect, D2 diffused faster than H2 into pores, with the diffusion difference between isotopes increasing as the temperature increases. Owing to this unique inverse trend, a new strategy is suggested for achieving higher operating temperatures for efficient isotope separation utilizing a flexible metal–organic framework system. Bio-derived nanoporous carbon, as another flexible porous material, enables efficient separation of hydrogen isotopes through temperature-responsive gating. The distinctive characteristics of this material, such as its suitable pore sizes for KQS that lead to strong diffusion limitation, as well as its capacity to operate at higher temperatures, overcome the limitations of existing crystalline porous materials. It is remarkable that this activated carbon derived from biological sources, even without any strong binding sites, can release hydrogen isotope at a higher temperature of 180 K in comparison to MOF-74(Ni) with open metal sites but releases mostly at 90-100 K. The separation performance was also demonstrated to reach up to 120 K. This finding suggests that the thermal pore size modulation of inexpensive porous carbon can significantly increase the operating temperature for precise separation of hydrogen isotopes. Lastly, as another nanoporous material with gating effect, Cu-ZIF-gis was studied for hydrogen isotope separation. Cu-ZIF-gis, a Cu-based zeolitic imidazolate framework has 1D channels with pore size of 2.4 Å, smaller than the kinetic diameter of H2 (2.98 Å). However, Cu-ZIF-gis leverages a new strategy named "Lattice-driven gating", which uses lattice expansion to can be adsorb hydrogen isotopes and control the aperture at elevated temperatures up to 180 K. Despite the lack of strong binding sites, it achieves effective isotope uptake via KQS above 120 K. Through QENS analysis, we observe substantial differences in the molecular mobility of H2 and D2 above 150 K. This method is compatible with existing LNG cryo-infrastructure, marking a significant advancement in sustainable isotope separation technologies.