Impact of Ligand in Nickel-Based Metal-Organic Frameworks on Selectively Adsorbing 85Krypton from N2-enriched Radioactive Off-Gas

Removing radioactive 85Kr effectively during spent fuel reprocessing is still a significant challenge due to the low absorption capacity and poor 85Kr/N2 selectivity of the currently available adsorbents. Herein, three types of nickel-based metal-organic frameworks (MOFs) were synthesized, including Ni-FA, Ni(ina)2, and JUC-86, to investigate the impact of ligands on selective 85Kr adsorption via experimental and quantum calculations, aiming to explore the critical structural properties that enhance Kr adsorption capabilities. Analysis of the experimental characterization reveals that the interaction between Kr and MOFs was influenced by the properties of ligands, independent of the specific surface area. Quantum computing results indicate that aromatic structured ligands can form multiple Kr···π interactions with Kr, thereby enhancing the affinity between the framework and Kr, and ligands containing N-heterocycles can further augment electron polarization between Kr and the framework, providing a unique affinity for Kr through van der Waals forces. JUC-86 features both aromatic and nitrogen-heterocyclic structures, exhibiting the highest ligand polarity, which enables it to achieve a Kr uptake of 2.71 mmol/g, surpassing those of Ni(ina)2 (1.62 mmol/g), Ni-FA (0.76 mmol/g), and all previously reported adsorbents. More importantly, the JUC-86 demonstrated the best Henry constant of 8.72 mmol/g/bar for Kr, and the selectivity of Kr/N2 was calculated to be 9.03, showing an excellent Kr affinity that indicates potential for capturing trace 85Kr from N2-rich off-gas streams. These findings revealed the key structural characteristics for 85Kr adsorption of three Ni-MOFs and indicated that higher ligand polarity and extra functional groups are key factors in improving 85Kr affinity, aside from pore size.

Exploring Host-Guest Interactions in the α-Zn3(HCOO)6 Metal-Organic Framework

Metal-organic frameworks (MOFs) are promising gas adsorbents. Knowledge of the behavior of gas molecules adsorbed inside MOFs is crucial for advancing MOFs as gas capture materials. However, their behavior is not always well understood. In this work, carbon dioxide (CO₂) adsorption in the microporous α-Zn₃(HCOO)₆ MOF was investigated. The behavior of the CO₂ molecules inside the MOF was comprehensively studied by a combination of single-crystal X-ray diffraction (SCXRD) and multinuclear solid-state magnetic resonance spectroscopy. The locations of CO₂ molecules adsorbed inside the channels of the framework were accurately determined using SCXRD, and the framework hydrogens from the formate linkers were found to act as adsorption sites. ⁶⁷Zn solid-state NMR (SSNMR) results suggest that CO₂ adsorption does not significantly affect the metal center environment. Variable-temperature ¹³C SSNMR experiments were performed to quantitatively examine guest dynamics. The results indicate that CO₂ molecules adsorbed inside the MOF channel undergo two types of anisotropic motions: a localized rotation (or wobbling) upon the adsorption site and a twofold hopping between adjacent sites located along the MOF channel. Interestingly, ¹³C SSNMR spectroscopy targeting adsorbed CO₂ reveals negative thermal expansion (NTE) of the framework as the temperature rose past ca. 293 K. A comparative study shows that carbon monoxide (CO) adsorption does not induce framework shrinkage at high temperatures, suggesting that the NTE effect is guest-specific.

Ultrafast water sensing and thermal imaging by a metal-organic framework with switchable luminescence

A convenient, fast and selective water analysis method is highly desirable in industrial and detection processes. Here a robust microporous Zn-MOF (metal-organic framework, Zn(hpi2cf)(DMF)(H2O)) is assembled from a dual-emissive H2hpi2cf (5-(2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1H-imidazol-1-yl)isophthalic acid) ligand that exhibits characteristic excited state intramolecular proton transfer (ESIPT). This Zn-MOF contains amphipathic micropores (<3 Å) and undergoes extremely facile single-crystal-to-single-crystal transformation driven by reversible removal/uptake of coordinating water molecules simply stimulated by dry gas blowing or gentle heating at 70 °C, manifesting an excellent example of dynamic reversible coordination behaviour. The interconversion between the hydrated and dehydrated phases can turn the ligand ESIPT process on or off, resulting in sensitive two-colour photoluminescence switching over cycles. Therefore, this Zn-MOF represents an excellent PL water-sensing material, showing a fast (on the order of seconds) and highly selective response to water on a molecular level. Furthermore, paper or in situ grown ZnO-based sensing films have been fabricated and applied in humidity sensing (RH<1%), detection of traces of water (<0.05% v/v) in various organic solvents, thermal imaging and as a thermometer.

Rational synthesis of Ni3(HCOO)6/CNT ellipsoids with enhanced lithium storage performance: inspired by the time evolution of the growth process of a nickel formate framework

[Ni₃(HCOO)₆]/CNT composites exhibited significantly enhanced electrochemical performance in comparison with the pristine [Ni₃(HCOO)₆].

“Explosive” synthesis of metal-formate frameworks for methane capture: an experimental and computational study

We present an ultrafast “explosive” synthesis of nickel-formate frameworks, which show prominent performance for methane capture from nitrogen, proved using experiments and simulations.

Highly efficient adsorbent design using a Cu-BTC/CuO/carbon fiber paper composite for high CH4/N2 selectivity

Cu-BTC/CuO/CFP, which was obtained via atomic layer deposition, has higher selectivity for CH₄/N₂, temperature uniformity, and lower pressure drop compared to Cu-BTC.

Converting 3D rigid metal–organic frameworks (MOFs) to 2D flexible networks via ligand exchange for enhanced CO2/N2 and CH4/N2 separation

Ligand exchange converts a 3D rigid MOF into a unique 2D flexible network achieving enhanced gas separation.