Reticular Chemistry in Pore Engineering of a Y-Based Metal-Organic Framework for Xenon/Krypton Separation

Purification of Xe and SF6 through Adaptive Contractions in a Flexible Metal-Organic Framework

Overcoming the trade-off effects between adsorption capacity, adsorption selectivity, and adsorption enthalpy of an adsorbent is very important but remains a huge challenge. Here, we report a flexible metal-organic framework (FJI-H36); it can selectively adsorb Xe from Xe/Kr mixtures with high adsorption capacity but very low adsorption enthalpy. Structural analyses show that such excellent adsorption performances come from the adaptive contraction of the flexible framework; pore shrinkage can enhance the interactions between adsorbed Xe and the framework and offset some of the adsorption heats. For SF6/N2 mixtures, FJI-H36 can also enhance the adsorption performance of SF6 through adaptive contraction, resulting in both high adsorption selectivity and low adsorption enthalpy. This not only provides a new adsorbent for the purification of Xe/Kr/SF6 but also offers a potential solution to overcome the trade-offs among adsorption capacity, adsorption selectivity, and adsorption enthalpy of a specific adsorbent.

Unraveling metal effects on CO2 uptake in pyrene-based metal-organic frameworks

Pyrene-based metal-organic frameworks (MOFs) have tremendous potential for various applications. With infinite structural possibilities, the MOF community often relies on simulations to identify the most promising candidates for given applications. Among thousands of reported structures, many exhibit limited reproducibility - in either synthesis, performance, or both - owing to the sensitivity of synthetic conditions. Geometric distortions that may arise in the functional groups of pyrene-based ligands during synthesis and/or activation cannot easily be predicted. This sometimes leads to discrepancies between in silico and experimental results. Here, we investigate a series of pyrene-based MOFs for carbon capture. These structures share the same ligand (1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy)) but have different metals (M-TBAPy, M = Al, Ga, In, and Sc). The ligands stack parallel in their orthorhombic crystal structure, creating a promising binding site for CO2. As predicted, the metal is shown to affect the pyrene stacking distance and, therefore, the CO2 uptake. Here, we investigate the metal's intrinsic effects on the MOFs' crystal structure. Crystallographic analysis shows the emergence of additional phases, which thus impacts the overall adsorption characteristics of the MOFs. Considering these additional phases improves the prediction of adsorption isotherms, enhancing our understanding of pyrene-based MOFs for carbon capture.

A Microporous Hydrogen-Bonded Organic Framework with Open Pyrene Sites Isolated by Hydrogen-Bonded Helical Chains for Efficient Separation of Xenon and Krypton

Achieving efficient xenon/krypton (Xe/Kr) separation in emerging hydrogen-bonded organic frameworks (HOFs) is highly challenging because of the lack of gas-binding sites on their pore surfaces. Herein, we report the first microporous HOF (HOF-FJU-168) based on hydrogen-bonded helical chains, which prevent self-aggregation of the pyrene core, thereby preserving open pyrene sites on the pore surfaces. Its activated form, HOF-FJU-168a is capable of separating Xe/Kr under ambient conditions while achieving an excellent balance between adsorption capacity and selectivity. At 296 K and 1 bar, the Xe adsorption capacity of HOF-FJU-168a reached 78.31 cm3/g, with an Xe/Kr IAST selectivity of 22.0; both values surpass those of currently known top-performing HOFs. Breakthrough experiments confirmed its superior separation performance with a separation factor of 8.6 and a yield of high-purity Kr (>99.5 %) of 184 mL/g. Furthermore HOF-FJU-168 exhibits excellent thermal and chemical stability, as well as renewability. Single-crystal X-ray diffraction and molecular modeling revealed that the unique electrostatic surface potential around the open pyrene sites creates a micro-electric field, exerting a stronger polarizing effect on Xe than on Kr, thereby enhancing host-Xe interactions.

Ultramicroporous Metal-Organic Framework Featuring Multiple Polar Sites for Efficient Xenon Capture and Xe/Kr Separation

Efficient adsorption separation of xenon/krypton (Xe/Kr) mixtures is an important technological challenge due to their similar sizes and shapes. Herein, we report an ultramicroporous metal-organic framework (MOF), ZJU-Bao-302a, with pore sizes close to the kinetic diameter of Xe and pore surfaces lined with a high density of polar sites, including methyl groups, amines, and uncoordinated oxygen atoms. The synergistic effect of these polar sites enables ZJU-Bao-302a to exhibit a high Xe uptake of 2.77 mmol g-1 and a balanced Xe/Kr selectivity of 14.6 under ambient conditions. Dynamic breakthrough experiments demonstrate the material's capability to efficiently separate Xe/Kr mixtures (20/80) as well as capture Xe at ultralow concentrations (400 ppmv) from nuclear reprocessing exhausts, achieving a superior dynamic Xe capacity of 24.2 mmol kg-1. Density functional theory calculations reveal that the localized polar groups/atoms in ZJU-Bao-302a provide more effective recognition sites for Xe than Kr, enhancing the thermodynamic selectivity. This study highlights the importance of integrating tailored pore sizes and dense polar sites in metal-organic frameworks for developing high-performance Xe/Kr separation adsorbents.

Effective Strategy toward Obtaining Reliable Breakthrough Curves of Solid Adsorbents

Metal-organic frameworks (MOFs) have demonstrated their versatility in a wide range of applications, including chemical separation, gas capture, and storage. In industrial adsorption processes, MOFs are integral to the creation of selective gas adsorption fixed beds. In this context, the assessment of their separation performance under relevant conditions often relies on breakthrough experiments. One aspect frequently overlooked in these experiments is the shaping of MOF powders, which can significantly impact the accuracy of breakthrough results. In this study, we present an approach for immobilizing MOF particles on the surface of glass beads (GBs) utilizing trimethylolpropane triglycidyl ether (TMPTGE) as a binder, leading to the creation of MOF@GB materials. We successfully synthesized five targeted MOF composites, namely, SIFSIX-3-Ni@GB, CALF-20@GB, UiO-66@GB, HKUST-1@GB, and MOF-808@GB, each possessing distinct pore sizes and structural topologies. Characterization studies employing powder X-ray diffraction and adsorption isotherm analyses demonstrated that MOFs@GB retained their crystallinity and 73-90% of the Brunauer-Emmett-Teller area of their parent MOFs. Dynamic breakthrough experiments revealed that, in comparison to their parent MOFs, MOF@GB configurations enhanced the accuracy of breakthrough measurements by mitigating pressure buildup and minimizing reductions in the gas flow rate. This work underscores the significance of meticulous experimental design, specifically in shaping MOF powders, to optimize the efficacy of breakthrough experiments. Our proposed strategy aims to provide a versatile platform for MOF powder processing, thereby facilitating more reliable breakthrough experiments.

Polypyrene Porous Organic Framework for Efficiently Capturing Electron Specialty Gases

The polypyrene polymer with an extended π-conjugated skeleton is attractive for perfluorinated electron specialty gas (F-gas) capture as the high electronegativity of fluorine atoms makes F-gases strongly electronegative gases. Herein, a polypyrene porous organic framework (termed as Ppy-POF) with an extended π-conjugated structure and excellent acid resistance was constructed. Systematic studies have shown that the abundant π-conjugated structures and gradient electric field distribution in Ppy-POF can endow it exceptional adsorption selectivity for high polarizable F-gases and xenon (Xe), which has been collaboratively confirmed by single-component gas adsorption experiments, time-dependent adsorption rate tests, dynamic breakthrough experiments, etc. Electrostatic potential distribution and charge density difference based on Grand Canonical Monte Carlo simulations and density functional theory calculations demonstrate that the selective adsorption of F-gases and Xe in Ppy-POF is attributed to the strong charge-transfer effect and polarization effect between Ppy-POF and gases. These results manifest that the POF with an extended π-conjugated structure and gradient electric field distribution has great potential in efficiently capturing electron specialty gases.