Desolvation-Degree-Induced Structural Dynamics in a Rigid Cerium-Organic Framework Exhibiting Tandem Purification of Ethylene from Acetylene and Ethane

Due to the industrial requirements for high production and high quality of ethylene, efficient purification of ethylene from acetylene and ethane is of prime importance but challenging. Dynamic metal-organic frameworks (MOFs) have demonstrated intriguing structural dynamics and diverse applications recently. Among them, although a few flexible ones have exhibited interesting ethylene purification capability, rigid ones were yet barely investigated for such purpose. In this regard, a cerium(III)-based MOF was solvothermally synthesized, which is rigid and assembled from rod molecular building blocks associated with coordinative N,N-dimethylformamide (DMF) molecules. After liberating different degrees of DMF ligands via heating under vacuum or acetone exchange, both partially desolvated compounds of Ce-MOF-1 and Ce-MOF-2 were concertedly isolated in a fashion of single-crystal to single-crystal transformation. Although both newly generated materials crystallize in the same space group, they exhibit dissimilar unit cell parameters and slightly distinct ultramicropore sizes and pore microenvironments, thanks to the discrepancy in the desolvation degree. Consequently, Ce-MOF-1 and Ce-MOF-2 individually demonstrate C₂H₂- and C₂H₆-selective adsorption behavior, resulting in the potential tandem separation of C₂H₄ from C₂H₂ and C₂H₆ mixtures. The above results were successfully supported by not only single gas adsorption isotherms but also grand canonical Monte Carlo (GCMC) calculation studies and dynamic breakthrough experiments. The present work may pave the way for rigid MOFs aiming at advancing applications via solid-state structural dynamics.

A microporous chromium-organic framework fabricated via solvent-assisted metal metathesis for C2H2/CO2 separation

Removal of CO₂ or C₂H₄ from C₂H₂ is still a challenging task due to their similar physical-chemical properties. Here, a bifunctional ligand decorated with amino and sulfoxide groups, 5',5''''-sulfonylbis (2'-amino-[1,1':3',1''-terphenyl]-4,4''-dicarboxylic acid) (H₄L), was employed to construct a new microporous iron-organic framework (Fe-MOF) with the formula [(Fe₃O)(L)_1.5(H₂O)₃]_n. This MOF can serve as a parent structure to obtain the isostructural Cr-MOF by solvent-assisted metal metathesis. Furthermore, the gas adsorption and separation performance of these two MOFs were systematically investigated. Compared to Fe-MOF, Cr-MOF shows a moderately higher CO₂, C₂H₂ and C₂H₄ uptake capacity. Additionally, Cr-MOF can selectively adsorb C₂H₂ over CO₂ and C₂H₄. The separation potential towards C₂H₂/C₂H₄ and C₂H₂/CO₂ was further established via IAST calculations of mixture adsorption equilibrium. IAST selectivity values of Cr-MOF are 3.4 for C₂H₂/C₂H₄ and 6.9 for C₂H₂/CO₂ at 298 K and initial pressure, indicating its potential C₂H₂ separation ability.

Identifying the Gate-Opening Mechanism in the Flexible Metal-Organic Framework UTSA-300

Atomic-level understanding of the gate-opening phenomenon in flexible porous materials is an important step toward learning how to control, design, and engineer them for applications such as the separation of gases from complex mixtures. Here, we report such mechanistic insight through an in-depth study of the pressure-induced gate-opening phenomenon in our earlier reported metal-organic framework (MOF) Zn(dps)₂(SiF₆) (dps = 4,4'-dipyridylsulfide), also called UTSA-300, using isotherm and calorimetry measurements, in situ infrared spectroscopy, and ab initio simulations. UTSA-300 is shown to selectively adsorb acetylene (C₂H₂) over ethylene (C₂H₄) and ethane (C₂H₆) and undergoes an abrupt gate-opening phenomenon, making this framework a highly selective gas separator of this complex mixture. The selective adsorption is confirmed by pressure-dependent in situ infrared spectroscopy, which, for the first time, shows the presence of multiple C₂H₂ species with varying strengths of bonding. A rare energetic feature at the gate-opening condition of the flexible MOF is observed in our differential heat energies, directly measured by calorimetry, showcasing the importance of this tool in adsorption property exploration of flexible frameworks and offering an energetic benchmark for further energy-based fundamental studies. Based on the agreement of this feature with ab initio-based adsorption energies of C₂H₂ in the closed-pore structure UTSA-300a ("a" refers to the activated form), this feature is assigned to the weakening of the H-bond C-H···F formed between C₂H₂ and fluorine of the MOF. Our analysis identifies the weakening of this H-bond, the expansion of the closed-pore MOF upon successive C₂H₂ coadsorption until its volume is close to that of the open-pore MOF, and the spontaneous gate opening to energetically favor C₂H₂ adsorption in the open-pore structure as crucial steps in the gate-opening mechanism in this system.

Gating effect for gas adsorption in microporous materials-mechanisms and applications

In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. cesium, potassium, etc.). These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing.

The effects of continuous- and stop-flow gas streams on adsorptive removal of benzene vapor using type - II covalent organic polymers

Various materials have been investigated for the adsorptive removal of volatile organic compounds (VOCs, such as benzene). However, most materials proposed for the adsorptive removal of gaseous benzene (and other VOCs) perform relatively poorly (e.g., an impractically low-service 10% breakthrough volume [BTV10] at < 100 ppm). The adsorbent uptake rate (mg g^-1 min^-1) can also be assessed as a function of the gas-stream flow rate (or space velocity). The main aim of this study is to explore the effect of two different gas-stream supply modes - stopped flow (at a fixed stream flow rate of 330 mL atm min^-1) vs. continuous flow (a variable-stream flow rate of 100, 200, or 330 mL atm min^-1) on the adsorption metrics of gaseous benzene on 5 mg of two types of - II covalent organic polymers (COPs: CBAP-1 [DETA], CD; or CBAP-1 [EDA], CE). The sorbent tube outlet stream was sampled by two respective sampling methods (i.e., a large-volume injector [LVI] for stopped flow vs. syringe injection [SI] for continuous flow) for sample quantitation by gas chromatography flame-ionization detection (GC-FID). The observed BTV10 values in the two sampling modes were similar when tested using 10 ppm benzene, irrespective of sorbents: 56/60 (CD) vs. 620/624 L atm g^-1 (CE). BTV10 values increased systematically with decreasing stream-flow rates to reflect the importance of space velocity in adsorptive removal of benzene. The overall assessment of adsorption performance between stopped flow (LVI) and continuous flow (SI) revealed that the performance of the adsorbent is independent of flow mode (e.g., when performance was compared at flow rate of 330 mL min^-1).

Two urea-functionalized metal–organic frameworks based on a pillared-layer strategy for gas adsorption and separation

Two pcu type Cu-MOFs based on urea-functionalized ligands have been synthesized by a pillared-layer strategy. Compound 1 shows good adsorption and separation behaviors of CO₂, C₂H₆, and C₃H₈ over CH₄, compound 2 exhibits a gate-opening behavior for N₂ adsorption.

A new three-dimensional cobalt(II) coordination polymer based on V-shaped 3,4'-oxydibenzoate: synthesis, crystal structure and magnetic properties

A new coordination polymer (CP), namely poly[(μ-4,4'-bipyridine)(μ₃-3,4'-oxydibenzoato)cobalt(II)], [Co(C₁₄H₈O₅)(C₁₀H₈N₂)]_n or [Co(3,4'-obb)(4,4'-bipy)]_n (1), was prepared by the self-assembly of Co(NO₃)₂·6H₂O with the rarely used 3,4'-oxydibenzoic acid (3,4'-obbH₂) ligand and 4,4'-bipyridine (4,4'-bipy) under solvothermal conditions, and has been structurally characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography and powder X-ray diffraction (PXRD). Single-crystal X-ray diffraction reveals that each Co^II ion is six-coordinated by four O atoms from three 3,4'-obb^2- ligands, of which two function as monodentate ligands and the other as a bidentate ligand, and by two N atoms from bridging 4,4'-bipy ligands, thereby forming a distorted octahedral CoN₂O₄ coordination geometry. Adjacent crystallographically equivalent Co^II ions are bridged by the O atoms of 3,4'-obb^2- ligands, affording an eight-membered Co₂O₄C₂ ring which is further extended into a two-dimensional [Co(3,4'-obb)]_n sheet along the ab plane via 3,4'-obb^2- functioning as a bidentate bridging ligand. The planes are interlinked into a three-dimensional [Co(3,4'-obb)(4,4'-bipy)]_n network by 4,4'-bipy ligands acting as pillars along the c axis. Magnetic investigations on CP 1 disclose an antiferromagnetic coupling within the dimeric Co₂ unit and a metamagnetic behaviour at low temperature resulting from intermolecular π-π interactions between the parallel 4,4'-bipy ligands.

Fabrication of hierarchically porous MIL-88-NH2(Fe): a highly efficient catalyst for the chemical fixation of CO2 under ambient pressure

A hierarchically micro- and mesoporous MIL-88-NH₂ metal organic framework was prepared through an easy template directed methodology.

A novel metal–organic framework based on hexanuclear Co(ii) clusters as an anode material for lithium-ion batteries

A new metal–organic framework, [Co(L)]·1.5H₂O (1), has been successfully synthesized, where its electrochemical application has been investigated.