Leveraging Diffusion Kinetics to Reverse Propane/Propylene Adsorption in Zeolitic Imidazolate Framework-8

The separation challenge posed by propylene/propane mixtures arises from their nearly identical molecular sizes and physicochemical properties. Metal-organic frameworks (MOFs) have demonstrated potential in addressing this challenge through the precision tailoring of pore sizes and surface chemistry. However, introducing modifications at the molecular level remains a considerable hurdle. This work presents an approach to reversibly tune the propylene/propane adsorption preference in zeolitic imidazolate framework-8 (ZIF-8) by manipulating the particle size and gas flow rate. Systematically increasing the ZIF-8 crystals from 9 to 224 μm restricts propane diffusion, thereby reversing its preferential adsorption over propylene. Furthermore, raising the gas flow rate of mixed propylene/propane shifts the rate-determining breakthrough step from thermodynamic equilibrium to kinetics, again reversing the adsorption preference in a particular ZIF-8 sample. We propose "dynamic selectivity (Sd(t))" as a concept that incorporates both thermodynamic and kinetic factors to elucidate these unexpected findings. Moreover, the driving force equation, grounded on the concept of Sd(t), has improved the precision and stability of the computational simulation for fixed-bed adsorption processes. This work underscores the potential of diffusion-based modulation, implemented through manageable external changes, as a viable strategy to optimize separation performance in porous adsorbent materials.

Prediction of Xe/Kr Separation in Metal-Organic Frameworks by a Precursor-Based Neural Network Synergistic with a Polarizable Adsorbate Model

Adsorption and separation of Xe/Kr are significant for making high-density nuclear energy environmentally friendly and for meeting the requirements of the gas industry. Enhancing the accuracy of the adsorbate model for describing the adsorption behaviors of Xe and Kr in MOFs and the efficiency of the model for predicting the separation potential (SP) value of Xe/Kr separation in MOFs helps in searching for promising MOFs for Xe/Kr adsorption and separation within a short time and at a low cost. In this work, polarizable and transferable models for mimic Xe and Kr adsorption behaviors in MOFs were constructed. Using these models, SP values of 38 MOFs at various temperatures and pressures were calculated. An optimal neural network model called BPNN-SP was designed to predict SP value based on physical parameters of metal center (electronegativity and radius) and organic linker (three-dimensional size and polarizability) combined with temperature and pressure. The regression coefficient value of the BPNN-SP model for each data set is higher than 0.995. MAE, MBE, and RMSE of BPNN-SP are only 0.331, -0.002, and 0.505 mmol/g, respectively. Finally, BPNN-SP was validated by experiment data from six MOFs. The transferable adsorbate model combined with the BPNN-SP model would highly improve the efficiency for designing MOFs with high performance for Xe/Kr adsorption and separation.

ResponZIF Structures: Zeolitic Imidazolate Frameworks as Stimuli-Responsive Materials

Zeolitic imidazolate frameworks (ZIFs) have long been recognized as a prominent subset of the metal-organic framework (MOF) family, in part because of their ease of synthesis and good thermal and chemical stability, alongside attractive properties for diverse potential applications. Prototypical ZIFs like ZIF-8 have become embodiments of the significant promise held by porous coordination polymers as next-generation designer materials. At the same time, their intriguing property of experiencing significant structural changes upon the application of external stimuli such as temperature, mechanical pressure, guest adsorption, or electromagnetic fields, among others, has placed this family of MOFs squarely under the umbrella of stimuli-responsive materials. In this review, we provide an overview of the current understanding of the triggered structural and electronic responses observed in ZIFs (linker and bond dynamics, crystalline and amorphous phase changes, luminescence, etc.). We then describe the state-of-the-art experimental and computational methodology capable of shedding light on these complex phenomena, followed by a comprehensive summary of the stimuli-responsive nature of four prototypical ZIFs: ZIF-8, ZIF-7, ZIF-4, and ZIF-zni. We further expose the relevant challenges for the characterization and fundamental understanding of responsive ZIFs, including how to take advantage of their flexible properties for new application avenues.

Preferential SOx Adsorption in Mg-MOF-74 from a Humid Acid Gas Stream

The preferential adsorption of SO_x versus water in Mg-MOF-74 from a humid SO_x gas stream has been investigated via materials studies and nuclear magnetic resonance (NMR). Mg-MOF-74 has been synthesized and subsequently loaded simultaneously with water vapor and SO_x (62-96 ppm) in an adsorption chamber at room temperature over a time period of 4 days with a sample taken every 24 h. Each sample was analyzed by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA)-mass spectrometry, and scanning electron microscopy-energy-dispersive spectroscopy. The metal-organic framework (MOF) showed retained crystallinity and peak intensity in PXRD, and after 2 days, it showed no obvious degradation to the structure. Use of multiple techniques, including TGA, identified 10% by weight of SO_x species, specifically H₂S and SO₂, within the MOF. ¹H solid-state NMR shows a substantial reduction of H₂O when SO_x is present, which is consistent with SO_x preferentially binding to the oxophilic metal site of the framework. After 14 weeks aging, the sulfur remains present in the three-dimensional MOF, with only half being desorbed after 23 weeks in air.

Effect of Larger Pore Size on the Sorption Properties of Isoreticular Metal-Organic Frameworks with High Number of Open Metal Sites

Four isostructural CPO‐54‐M metal‐organic frameworks based on the larger organic linker 1,5‐dihydroxynaphthalene‐2,6‐dicarboxylic acid and divalent cations (M=Mn, Mg, Ni, Co) are shown to be isoreticular to the CPO‐27 (MOF‐74) materials. Desolvated CPO‐54‐Mn contains a very high concentration of open metal sites, which has a pronounced effect on the gas adsorption of N₂, H₂, CO₂ and CO. Initial isosteric heats of adsorption are significantly higher than for MOFs without open metal sites and are slightly higher than for CPO‐27. The plateau of high heat of adsorption decreases earlier in CPO‐54‐Mn as a function of loading per mole than in CPO‐27‐Mn. Cluster and periodic density functional theory based calculations of the adsorbate structures and energetics show that the larger adsorption energy at low loadings, when only open metal sites are occupied, is mainly due to larger contribution of dispersive interactions for the materials with the larger, more electron rich bridging ligand.

Cobalt-Exchanged Poly(Heptazine Imides) as Transition Metal-Nx Electrocatalysts for the Oxygen Evolution Reaction

Poly(heptazine imides) hosting cobalt ions as countercations are presented as promising electrocatalysts for the oxygen evolution reaction (OER). A facile mixed-salt melt-assisted condensation is developed to prepare such cobalt poly(heptazine imides) (PHI-Co). The Co ions can be introduced in well-controlled amounts using this method, and are shown to be atomically dispersed within the imide-linked heptazine matrix. When applied to electrocatalytic OER, PHI-Co shows a remarkable activity with an overpotential of 324 mV and Tafel slope of 44 mV dec^-1 in 1 m KOH.

Squeezing the box: isoreticular contraction of pyrene-based linker in a Zr-based metal–organic framework for Xe/Kr separation

A new Zr-based metal–organic framework with ftw topology, NU-1106, was synthesized using 1,3,6,8-pyrene tetracarboxylate and studied for its Xe/Kr separation capabilities.

Thermodynamic evidence of a transition in ZIF-8 upon CH4 sorption

We present the results of an experimental study of methane sorption in ZIF-8. We measured isotherms at five different temperatures between 87 K and 107 K. We have observed three sub-steps in each of the isotherms. The intermediate sub-step had not been observed experimentally in previous studies of this system. This newly determined experimental feature suggests that a transition is taking place in the sorbed system (this newly observed sub-step occurs over a loading interval where published computer simulation results for CH4 in ZIF-8 had identified a structural transition occurring in the sorbent). We have studied the kinetics of adsorption for this system (we measure the time required for the system to reach equilibrium after gas is added to the experimental cell as a function of sorbent loading). We observed a sharp peak in the equilibration time at high loadings, below saturation. We have explored the isosteric heat of adsorption, and its dependence on sorbent loading, for this system. We found a broad peak in the isosteric heat at loadings corresponding to the intermediate isotherm sub-step. Previously reported computer simulations for the isosteric heat dependence on loading for CH4 in ZIF-8 are in good agreement with our experimental results for this quantity.

Construction of functional coordination polymers derived from designed flexible bis(4-carboxybenzyl)amine

A series of coordination polymers (CPs) have been constructed from bis(4-carboxybenzyl)amine and a series of bis(imidazole) linkers. Luminescence sensing measurements indicate that two zinc-based CPs both show highly selective and sensitive sensing for acetone and Cr₂O₇²⁻/CrO₄²⁻.

Noble gas supported boron-pentagonal clusters B5Ngn3+: exploring the structures and bonding

A novel type of trivalent BNg five-membered cational species B₅Ng_n³⁺(Ng = He~Rn, n = 1~5) has been found and investigated theoretically using the B3LYP and MP2 methods with the def2-QZVPPD and def2-TZVPPD basis sets. The geometry, harmonic vibrational frequencies, bond energies, charge distribution, bond nature, aromaticity, and energy decomposition analysis of these structures were reported. The calculated B-Ng bond energy is quite large (the averaged bond energy is in the range of 209.2~585.76 kJ mol^-1) for heavy rare gases and increases with the Ng atomic number. The analyses of the molecular wavefunction show that in the BNg compounds of heavy Ng atoms Ar~Rn, the B-Ng bonds are of typical covalent character. Nuclear independent chemical shifts display that both B₅³⁺ and B₅Ng_n³⁺(n=1~5) have obvious aromaticity. Energy decomposition analysis shows that these BNg compounds are mainly stabilized by the σ-donation from the Ng valence p orbital to the B₅³⁺ LUMO. These findings offer valuable clues toward the design and synthesis of new stable Ng-containing compounds.

Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal-Organic Frameworks

Purification of the C8 alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene remains among the most challenging industrial separations, due to the similar shapes, boiling points, and polarities of these molecules. Herein, we report the evaluation of the metal-organic frameworks Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) and Co2( m-dobdc) ( m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co2(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Multicomponent liquid-phase adsorption measurements further demonstrate that Co2(dobdc) maintains this selectivity over a wide range of concentrations. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation through the extent of interaction between each C8 guest molecule with two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, counter to the presumed rigidity of the M2(dobdc) structure, Co2(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.

Low temperature synthesis study of metal-organic framework CPO-27: investigating metal, solvent and base effects down to -78 °C

CPO-27-M (M = Co, Mg, Ni, Zn) metal-organic frameworks have been successfully synthesized at temperatures down to -78 °C in a range of solvent systems and their crystallinity and morphology analyzed by powder X-ray diffraction and scanning electron microscopy. CPO-27-Mg and -Zn could be synthesized at lower temperatures using MeOH-NaOH as the solvent with CPO-27-Zn showing the most crystalline material at -78 °C. CPO-27-Zn afforded the most crystalline samples of all studies in MeOH-TEA. However, in MeOH a non-porous monomeric [Zn(H₂dhtp)(H₂O)₂] complex was formed when no base was present. In THF with base (NaOH, TEA) the reaction produced crystalline MOFs in a controlled and stable manner at low temperatures, whilst the reagents were insoluble in THF at low temperature when no base was present. SEM was used to analyze the morphologies of the products.

Reticular Chemistry and the Discovery of a New Family of Rare Earth (4, 8)-Connected Metal-Organic Frameworks with csq Topology Based on RE4(μ3-O)2(COO)8 Clusters

In recent years, the design and discovery of new metal-organic framework (MOF) platforms with distinct structural features and tunable chemical composition has remarkably enhanced by applying reticular chemistry rules and the molecular building block (MBB) approach. We targeted the synthesis of new rare earth (RE)-MOF platforms based on a rectangular-shaped 4-c linker, acting as a rigid organic MBB. Accordingly, we designed and synthesized the organic ligand 1,2,4,5-tetrakis(4-carboxyphenyl)-3,6-dimethyl-benzene (H₄L), in which the two methyl groups attached to the central phenyl ring lock the four peripheral carboxyphenyl groups to an orthogonal/vertical position. We report here a new family of RE-MOFs featuring the novel inorganic building unit, RE₄(μ₃-O)₂ (RE: Y³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Yb³⁺), with planar D_2h symmetry. The rigid 4-c linker, H₄L, directs the in situ assembly of the unique 8-c RE₄(μ₃-O)₂(COO)₈ cluster, resulting in the formation of the first (4, 8)-c RE-MOFs with csq topology, RE-csq-MOF-1. The structures of the yttrium (Y-csq-MOF-1) and holmium (Ho-csq-MOF-1) analogues were determined by single-crystal X-ray diffraction analysis. Y-csq-MOF-1 was successfully activated and tested for Xe/Kr separation. The results show that Y-csq-MOF-1 has high isosteric heat of adsorption for Xe (33.8 kJ mol^-1), with high Xe/Kr selectivity (IAST 12.1, Henry 12.9) and good Xe uptake (1.94 mmol g^-1 at 298 K and 1 bar), placing this MOF among the top-performing adsorbents for Xe/Kr separation.

Structural characterization of framework-gas interactions in the metal-organic framework Co2(dobdc) by in situ single-crystal X-ray diffraction

The crystallographic characterization of framework-guest interactions in metal-organic frameworks allows the location of guest binding sites and provides meaningful information on the nature of these interactions, enabling the correlation of structure with adsorption behavior. Here, techniques developed for in situ single-crystal X-ray diffraction experiments on porous crystals have enabled the direct observation of CO, CH4, N2, O2, Ar, and P4 adsorption in Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate), a metal-organic framework bearing coordinatively unsaturated cobalt(ii) sites. All these molecules exhibit such weak interactions with the high-spin cobalt(ii) sites in the framework that no analogous molecular structures exist, demonstrating the utility of metal-organic frameworks as crystalline matrices for the isolation and structural determination of unstable species. Notably, the Co-CH4 and Co-Ar interactions observed in Co2(dobdc) represent, to the best of our knowledge, the first single-crystal structure determination of a metal-CH4 interaction and the first crystallographically characterized metal-Ar interaction. Analysis of low-pressure gas adsorption isotherms confirms that these gases exhibit mainly physisorptive interactions with the cobalt(ii) sites in Co2(dobdc), with differential enthalpies of adsorption as weak as -17(1) kJ mol-1 (for Ar). Moreover, the structures of Co2(dobdc)·3.8N2, Co2(dobdc)·5.9O2, and Co2(dobdc)·2.0Ar reveal the location of secondary (N2, O2, and Ar) and tertiary (O2) binding sites in Co2(dobdc), while high-pressure CO2, CO, CH4, N2, and Ar adsorption isotherms show that these binding sites become more relevant at elevated pressures.

Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics

The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of Zn^II centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N₂ O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.

Selective adsorption of chlorinated volatile organic compound vapours by microcrystalline 1D coordination polymers

Microcrystalline 1D coordination polymers 1–3Pwd are able to adsorb vapours of chlorinated volatile organic compounds (Cl-VOCs), displaying interesting selectivity patterns, as demonstrated by ¹H-NMR and X-ray diffraction analyses.

On the importance of a precise crystal structure for simulating gas adsorption in nanoporous materials

We show that simulation of gas adsorption in nanoporous sorbents may be highly sensitive to accurate crystallographic coordinates, even for frameworks anticipated to have low flexibility.

Understanding the Adsorption Mechanism of Xe and Kr in a Metal-Organic Framework from X-ray Structural Analysis and First-Principles Calculations

Enhancement of adsorption capacity and separation of radioactive Xe/Kr at room temperature and above is a challenging problem. Here, we report a detailed structural refinement and analysis of the synchrotron X-ray powder diffraction data of Ni-DODBC metal organic framework with in situ Xe and Kr adsorption at room temperature and above. Our results reveal that Xe and Kr adsorb at the open metal sites, with adsorption geometries well reproduced by DFT calculations. The measured temperature-dependent adsorption capacity of Xe is substantially larger than that for Kr, indicating the selectivity of Xe over Kr and is consistent with the more negative adsorption energy (dominated by van der Waals dispersion interactions) predicted from DFT. Our results reveal critical structural and energetic information about host-guest interactions that dictate the selective adsorption mechanism of these two inert gases, providing guidance for the design and synthesis of new MOF materials for the separation of environmentally hazardous gases from nuclear reprocessing applications.

Formation of a quasi-solid structure by intercalated noble gas atoms in pores of CuI-MFU-4l metal–organic framework

Ten crystallographically different positions for Xe and eight positions for Kr form a quasi-solid structures within the large-pore metal–organic framework Cu^I-MFU-4l.