Thermosensitive Gating Effect for Selective Gas Adsorption in Ionic Metal-organic Frameworks

One of the challenges in the gas separation process is achieving exclusive admission for specific gases and thus very high selectivity, especially in the case of separation of molecules with similar size and physical chemistry properties. Among traditional adsorption mechanisms, only the steric/molecular sieving mechanism offers high selectivity by exclusively admitting certain gas molecules. However, it faces difficulty in precisely controlling the pore size of the adsorbent to match the size of gas molecules, especially for similar-sized gas pair, thereby restricting its widespread usage in gas separation.

Nevertheless, an innovative mechanism, “molecular trapdoor”, discovered in the small pore rigid zeolite with a low Si/Al ratio, offers a revolutionary approach. This mechanism relies on extra-framework cations acting as “gate-keeping” groups, regulating gas admission based on a threshold temperature (T₀). Currently, research on “molecular trapdoor” mechanism mainly focuses on small-pore zeolites. The relatively rigid framework of the zeolite is often associated with a relatively high energy barrier for gas admission, which usually causes slow adsorption kinetics making it challenging to apply them to practical gas separation processes. Thus, I have proposed extending this concept to metal-organic frameworks (MOFs) for broader applicability.

Here, for the first time, I have identified a seemingly nonporous one-dimension (1D) channel flexible MOF (denoted as Pytpy MOF) as a “molecular trapdoor” material, showing a temperature-dependent gate-opening gas admission. The extra-framework anions (i.e., nitrate) could act as “gate-keeping” groups. The guests would induce the “gate-keeping” groups to deviate from the smallest width position to the largest width position in the channel allowing for the gas admission, as supported by in depth-understanding of structure, adsorption isotherms, and density functional theory (DFT) calculations. The topological flexibility of the framework was identified as a swelling flexible structure with respect to temperature by in situ single-crystal and synchrotron X-ray diffraction, which could facilitate the gas admission and exhibit a faster adsorption rate with diffusivity of around 10^-11 m² s^-1 than the flexible “molecular trapdoor” zeolite (e.g., Na-RHO: 10^-19 m² s^-1).

Inspired by the effects of various “gate-keeping” groups observed in zeolite, I have investigated the utilization of anion size (i.e., VO₃^-, BrO₃^-), shape (i.e., F^-, Br^-, I^-, BF₄^-, ClO₄^-), and valence (i.e., Cr₂O₇^2-) on the “molecular trapdoor” mechanism in the 1D channel of Pytpy MOFs. CO₂ was initially used as a probe to identify the dominant factors (i.e., shape, size, and valence of anions) that affect this mechanism. Larger size, and more spherical shape anion was found to raise the threshold admission temperature of CO₂, as evidenced by the T₀(CO₂) = 230 K in I-Pytpy MOF and its highest “energy barrier” (ΔE_I = 0.48 eV). Subsequently, gas adsorptions experiments, including N₂, CH₄, C₃H₆, and C₃H₈ revealed that such anions could enhance selectivity for hydrocarbon gases, exemplified by the Br-Pytpy with CO₂/CH₄ selectivity of 20, N₂/CH₄ IAST selectivity of 11, and C₃H₆/C₃H₈ IAST selectivity of 23 at 233 K. This work established a guideline for fabricating 1D channel “trapdoor” MOFs with enhanced separation performance, showcasing how the choice of extra-framework anions influence gas adsorption and selectivity.

In addition to studying the above channel MOF, I identified another ionic MOF (denoted as BTR MOF) with a 3D cage rigid framework as the “trapdoor” material. Through DFT calculations, a comprehensive understanding of the gas admission mechanism was obtained. Specifically, the counterions act as the “gate-keeping” groups that tend to deviate from the aperture towards the inner cage, as supported by the presence of a secondary stable site for the “gate-keeping” groups within the inner gate. Furthermore, by modulating the anions, the threshold admission temperature and capacity of CO₂ was increased. Accordingly, a high IAST selectivity of CO₂/N₂ and CO₂/CH₄ was achieved. The increased T₀(CO₂) was attributed to the higher energy barrier caused by the difficulty motion of the “gate-keeping” groups. Additionally, D/r² constant with respect to temperature and pressure was calculated. It exhibits a dynamics-based explanation of gas admission in this “molecular trapdoor” mechanism. Based on this, a comparison of the adsorption rates between BTR MOFs and Pytpy MOFs was performed to investigate the influence of framework topological on gas admission via the “trapdoor” mechanism.

Overall, this study highlights the potential of MOFs as “molecular trapdoor” material in that counterions act as “gate-keeping” groups that can be tuned to achieve various gas separations, offering insights into designing ionic MOFs with tailored gas separation properties.