Refinement of pore size at sub-angstrom precision in robust metal-organic frameworks for separation of xylenes

Highly Efficient Separation of Intermediate-Size m-Xylene from Xylenes via a Length-Matched Metal-Organic Framework with Optimal Oxygen Sites Distribution

Xylene separation is crucial but challenging, especially for the preferential separation of the intermediate-size m-xylene from xylene mixtures. Herein, exploiting the differences in molecular length and alkyl distribution among xylenes, we present a length-matched metal-organic framework, formulated as Al(OH)[O2C-C4H2O-CO2], featuring an effective pore size corresponding to m-xylene molecular length combined with multiple negative O hydrogen bond donors distribution, can serve as a molecular trap for efficient preferential separation of the intermediate-size m-xylene. Benchmark separation performance was achieved for separating m-xylene from a ternary mixture of m-xylene/o-xylene/p-xylene, with simultaneous record-high m-xylene uptake (1.3 mmol g-1) and m-xylene/p-xylene selectivity (5.3) in the liquid-phase competitive adsorption. Both vapor- and liquid-phase fixed-bed tests confirmed its practical separation capability with benchmark dynamic m-xylene/p-xylene and m-xylene/o-xylene selectivities, as well as excellent regenerability. The selective and strong m-xylene binding affinity among xylene molecules was further elucidated by simulations, validating the effectiveness of such a pore environment for the separation of intermediate-size molecules.

Energy-Efficient Separation of Xylene Isomers through Stacked 1-Aminopyrene Polymer Composite

Developing high-performance adsorbents for energy-efficient separation of xylene isomers has important research and application value. Identification and separation of xylene isomers (PX, MX, and OX) at room temperature based on the different relative positions of two methyl groups on the benzene ring is an unprecedented attempt. Herein, 1-aminopyrene polymer (PAP) is designed and electro-synthesized composited with a supporting electrolyte as an adsorbent for the separation of xylene isomers using a multi-stage dispersed liquid-solid adsorption process at room temperature. Each -NH-pyrene-NH- unit can form a force field to identify and discriminate xylene isomer through π-π interaction combined with -CH3···-NH- interactions of different strength, thereby achieving separation of PX, MX, and OX isomers by amplifying the slight differences in the adsorption capacity and diffusion rates. The density functional theory (DFT) simulations provide a more quantitative analysis of the adsorbate-adsorbent interactions to illustrate PX > MX > OX order of adsorption selectivity. This study may offer an alternative strategy for the precise designing of energy-efficient and adsorption-based separation materials.

Selective Separation of C8 Aromatics by an Interpenetrating Metal-Organic Framework Material

O-xylene (OX) is an important chemical raw material, but it is often produced in mixtures with other C8 aromatics. Similar physicochemical properties of the C8 isomers make their separation and purification very difficult and energy intensive. There is an unmet need for an adsorbent that would be effective for the separation of OX from the other C8 isomers. This work reports a three-dimensional interpenetrated metal-organic framework, SYUCT-110, that interacts with each of the single-component C8 isomers to form. The selectivity of C8 aromatic hydrocarbons was determined through liquid-phase batch uptake experiments. The results revealed that the selectivity order was OX > PX > MX > ethylbenzene (EB). The selectivity values were found to be 2.63, 1.58, 5.51, 3.71, 1.86, and 3.02 for OX/MX, OX/PX, OX/EB, PX/MX, MX/EB, and PX/EB, respectively. The adsorption capacity of OX was 71 mg/g. Grand Canonical Monte Carlo simulations were used to study the C8 adsorption sites, revealing that π···π interactions are the main reason for the observed adsorption selectivity. The adsorption energy calculation results also verified the selectivity of SYUCT-110 for the synthesis of OX.

Electron beam and thermal stabilities of MFM-300(M) metal-organic frameworks

This work reports the thermal and electron beam stabilities of a series of isostructural metal-organic frameworks (MOFs) of type MFM-300(M) (M = Al, Ga, In, Cr). MFM-300(Cr) was most stable under the electron beam, having an unusually high critical electron fluence of 1111 e- Å-2 while the Group 13 element MOFs were found to be less stable. Within Group 13, MFM-300(Al) had the highest critical electron fluence of 330 e- Å-2, compared to 189 e- Å-2 and 147 e- Å-2 for the Ga and In MOFs, respectively. For all four MOFs, electron beam-induced structural degradation was independent of crystal size and was highly anisotropic, although both the length and width of the channels decreased during electron beam irradiation. Notably, MFM-300(Cr) was found to retain crystallinity while shrinking up to 10%. Thermal stability was studied using in situ synchrotron X-ray diffraction at elevated temperature, which revealed critical temperatures for crystal degradation to be 605, 570, 490 and 480 °C for Al, Cr, Ga, and In, respectively. The pore channel diameters contracted by ≈0.5% on desorption of solvent species, but thermal degradation at higher temperatures was isotropic. The observed electron stabilities were found to scale with the relative inertness of the cations and correlate well to the measured lifetime of the materials when used as photocatalysts.

Unexpected Molecular Sieving of Xylene Isomer Using Tethered Ligand in Polymer-Metal-Organic Frameworks (polyMOFs)

Promising advances in adsorption technology can lead to energy-efficient solutions in industrial sectors. This work presents precise molecular sieving of xylene isomers in the polymer-metal-oragnic framework (polyMOF), a hybrid porous material derived from the parent isoreticular MOF-1 (IRMOF-1). PolyMOFs are synthesized by polymeric ligands bridged by evenly spaced alkyl chains, showing reduced pore sizes and enhanced stabilities compared to its parent material due to tethered polymer bridge within the pores while maintaining the original rigid crystal lattice. However, the exact configuration of the ligands within the crystals remain unclear, posing hurdles to predicting the adsorption performances of the polyMOFs. This work reveals that the unique pore structure of polyIRMOF-1-7a can discriminate xylene isomers with sub-angstrom size differences, leading to highly selective adsorption of p-xylene over other isomers and alkylbenzenes in complex liquid mixtures (αpX/OM = 15 and αpX/OME = 9). The structural details of the polyIRMOF-1-7a are elucidated through computational studies, suggesting a plausible configuration of alkyl chains within the polyMOF crystal, which enable a record-high p-xylene selectivity and stability in liquid hydrocarbon. With this unprecedented molecular selectivity in MOFs, "polymer-MOF" hybridization is expected to meet rigorous requirements for high-standard molecular sieving through precisely tunable and highly stable pores.

Gate-opening Induced by C8 Aromatics in a Double Diamondoid Coordination Network

Coordination networks (CNs) that undergo guest-induced structural transformations are of topical interest thanks to their potential utility in separations and storage applications. Herein, we report a double diamondoid (ddi) topology CN, [Ni2(bimpz)2(bdc)2(H2O)] n or X-ddi-2-Ni (H2bdc = 1,4-benzenedicarboxylic acid, bimpz = 3,6-bis(imidazol-1-yl)pyridazine), that undergoes structural transformations induced by C8 isomers, i.e., xylenes (o-xylene, OX; m-xylene, MX; p-xylene, PX) and ethylbenzene (EB). X-ddi-2-Ni was characterized by single-crystal to single-crystal transformations from a nonporous phase, X-ddi-2-Ni-β, to isostructural C8-loaded phases, namely X-ddi-2-Ni-OX, X-ddi-2-Ni-MX, X-ddi-2-Ni-PX and X-ddi-2-Ni-EB. X-ddi-2-Ni accommodates two C8 isomers per Ni unit, resulting in relatively high uptake (ca. 50 wt %), but with low selectivity toward C8 isomers as found using nuclear magnetic resonance (NMR) and gas chromatography (GC). In addition, a narrow range of gate-opening pressures for each isomer was determined from dynamic vapor sorption, consistent with the nonadaptable nature of the C8-loaded phase determined crystallographically, also supported by modeling.

Exceptional capture of methane at low pressure by an iron-based metal-organic framework

The selective capture of methane (CH4) at low concentrations and its separation from N2 are extremely challenging owing to the weak host-guest interactions between CH4 molecules and any sorbent material. Here, we report the exceptional adsorption of CH4 at low pressure and the efficient separation of CH4/N2 by MFM-300(Fe). MFM-300(Fe) shows a very high uptake for CH4 of 0.85 mmol g-1 at 1 mbar and 298 K and a record CH4/N2 selectivity of 45 for porous solids, representing a new benchmark for CH4 capture and CH4/N2 separation. The excellent separation of CH4/N2 by MFM-300(Fe) has been confirmed by dynamic breakthrough experiments. In situ neutron powder diffraction, and solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modelling, reveal a unique and strong binding of CH4 molecules involving Fe-OH⋯CH4 and C⋯phenyl ring interactions within the pores of MFM-300(Fe), thus promoting the exceptional adsorption of CH4 at low pressure.

Confinement of p-Xylene in the Pores of a Bilanthanide Metal-Organic Framework for Highly Selective Recognition

The rapid and accurate sensing of p-xylene, an essential raw material with a multi-billion-dollar market, in xylene mixture is of great significance in industry; however, the highly similar molecular structures, energy levels, and spectral characteristics of xylene isomers make the selective recognition extremely challenging. Metal-organic frameworks (MOFs) exhibiting tailorable pores and potential binding sites provide prospects for xylene sensing but a comprehensive understanding of the pore effect is still elusive, primarily due to the intricacies involved in the sensing process. Herein, we reported a robust bilanthanide MOF NKU-999-EuTb with precisely engineered pores to accommodate p-xylene, of which the binding sites were confirmed by single crystal X-ray diffraction and dynamic magnetic susceptibilities. NKU-999-EuTb exhibits high-performance in selective recognition for p-xylene towards its isomers. Through a systematical study, it was revealed that absorbing p-xylene into the pores governs the sensing performance. This work provides insights for developing advanced sensing materials for complex isomers.

Molecular Dynamics Simulation of the Adsorption and Diffusion of C8 Aromatic Isomers in MIL-47(V)

The separation of C8 aromatic isomers (oX: o-xylene, pX: p-xylene, mX: m-xylene, and EB: ethylbenzene) remains an enormous challenge in industrial production due to their similar molecular structures and physical properties. Porous materials with suitable pore structures and selective recognition sites to discriminate the slight structural differences of isomers are imminently needed. In this paper, MIL-47(V) with a three-dimensional (3D) grid structure of 10.5 × 10.5 Å2 and a one-dimensional (1D) diamond channel was selected as the adsorbent. However, the mechanism of the adsorption and separation of C8 aromatic isomers in porous materials still needs to be understood. Given the importance of C8 aromatic isomers' confinement in MIL-47(V) for adsorption and diffusion applications, it is important to understand C8 aromatic isomers' behavior in MIL-47(V). Here, we demonstrated from a simulation perspective that metal-organic frameworks MIL-47(V) with one-dimensional (1D) diamond channels can identify C8 aromatic isomers. Molecular dynamics (MD) simulations have shown that organic ligands with guest response sites of MIL-47(V) can effectively distinguish between C8 aromatic isomers by adaptation to the shape of a specific isomer. MIL-47(V) has high adsorption and an excellent separation sequence between C8 aromatic isomers: oX > pX ≈ mX > EB. Significant differences exist in π-π superposition interactions between C8 aromatic isomers and between C8 aromatic isomers and the skeletons. This phenomenon is mainly caused by the unique pore structure and guest response characteristics of MIL-47(V). This work is identified as a supplementary instruction to experimental research and is expected to provide profound insights into research on developing C8 aromatic isomers' adsorption and separation and theoretical support.

Switching Adsorbent Layered Material that Enables Stepwise Capture of C8 Aromatics via Single-Crystal-to-Single-Crystal Transformations

Separation of the C8 aromatic isomers, xylenes (PX, MX, and OX) and ethylbenzene (EB), is important to the petrochemical industry. Whereas physisorptive separation is an energy-efficient alternative to current processes, such as distillation, physisorbents do not generally exhibit strong C8 selectivity. Herein, we report the mixed-linker square lattice (sql) coordination network [Zn2(sba)2(bis)]n·mDMF (sql-4,5-Zn, H2sba or 4 = 4,4'-sulfonyldibenzoic acid, bis or 5 = trans-4,4'-bis(1-imidazolyl)stilbene) and its C8 sorption properties. sql-4,5-Zn was found to exhibit high uptake capacity for liquid C8 aromatics (∼20.2 wt %), and to the best of our knowledge, it is the first sorbent to exhibit selectivity for PX, EB, and MX over OX for binary, ternary, and quaternary mixtures from gas chromatography. Single-crystal structures of narrow-pore, intermediate-pore, and large-pore phases provided insight into the phase transformations, which were enabled by flexibility of the linker ligands and changes in the square grid geometry and interlayer distances. This work adds to the library of two-dimensional coordination networks that exhibit high uptake, thanks to clay-like expansion, and strong selectivity, thanks to shape-selective binding sites, for C8 isomers.

Separating Xylene Isomers with a Calcium Metal-Organic Framework

The purification of p-xylene (pX) from its xylene isomers represents a challenging but important industrial process. Herein, we report the efficient separation of pX from its ortho- and meta- isomers by a microporous calcium-based metal-organic framework material (HIAM-203) with a flexible skeleton. At 30 °C, all three isomers are accommodated but the adsorption kinetics of o-xylene (oX) and m-xylene (mX) are substantially slower than that of pX, and at an elevated temperature of 120 °C, oX and mX are fully excluded while pX can be adsorbed. Multicomponent column breakthrough measurements and vapor-phase/liquid-phase adsorption experiments have demonstrated the capability of HIAM-203 for efficient separation of xylene isomers. Ab initio calculations have provided useful information for understanding the adsorption mechanism.

Analysis and Refinement of Host-Guest Interactions in Metal-Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) are a class of hybrid porous materials characterized by their periodic assembly using metal ions and organic ligands through coordination bonds. Their high crystallinity, extensive surface area, and adjustable pore sizes make them promising candidates for a wide array of applications. These include gas adsorption and separation, substrate binding, and catalysis, of relevance to tackling pressing global issues such as climate change, energy challenges, and pollution. In comparison to traditional porous materials such as zeolites and activated carbons, the design flexibility of organic ligands in MOFs, coupled with their orderly arrangement with associated metal centers, allows for the precise engineering of uniform pore environments. This unique feature enables a rich variety of interactions between the MOF host and adsorbed gas molecules, which are fundamental to understanding the observed uptake capacity and selectivity for target gas molecules and thus the overall performance of the material.In this Account, a data set for three-dimensional MOFs has been constructed based upon the structural analysis of host-guest interactions using the largest experimental database, the Cambridge Structural Database (CSD). A full screening was performed on structures with guest molecules of H2, C2H2, CO2, and SO2, and the relationship between the primary binding site, the isosteric heats of adsorption (Qst), and the adsorption uptake was extracted and established. We review the methodologies to refine host-guest interactions based primarily on our studies on the host-guest chemistry of MOFs. The methods include ligand functionalization, variation of metal centers, formation of defects, addition of single atom sites, and control of pore size and structure. In situ structural and dynamic investigations using diffraction and spectroscopic techniques are powerful tools to visualize the details of host-guest interactions upon the above modifications, affording key insights into functional performance at a molecular level. Finally, we give an outlook of future research priorities in the study of host-guest chemistry in MOF materials. We hope this Account will encourage the rational development and improvement of future MOF-based sorbents for applications in challenging gas adsorption, separations, and catalysis.

pH-Regulated Refinement of Pore Size in Carbon Spheres for Size-Sieving of Gaseous C8 , C6 and C3 Hydrocarbon Pairs

Selective separation of industrial important C8 , C6 and C3 hydrocarbon pairs by physisorbents can greatly reduce the energy intensity related to the currently used cryogenic distillation techniques. The achievement of size-sieving based on carbonaceous materials is desirable, but commonly hindered by the random structure of carbons often with a broad pore size distribution. Herein, a pH-regulated pre-condensation strategy was introduced to control the carbon pore architecture by the sp2 /sp3 hybridization of precursor. The lower pH value during pre-condensation of glucose facilitates the growth of aromatic nanodomains, rearrangement of stacked layers and a concomitant transition from sp3 -C to sp2 -C. The subsequent pyrolysis endows the pore size manipulated from 6.8 to 4.8 Å and narrowly distributed over a range of 0.2 Å. The refined pores enable effective size-sieving of C8 , C6 and C3 hydrocarbon pairs with high separation factor of 1.9 and 4.9 for C8 xylene (X) isomers para-X/meta-X and para-X/ortho-X, respectively, 5.1 for C6 alkane isomers n-hexane/3-methylpentane, and 22.0 for C3 H6 /C3 H8 . The excellent separation performance based-on size exclusion effect is validated by static adsorption isotherms and dynamic breakthrough experiments. This synthesis strategy provides a means of exploring advanced carbonaceous materials with controlled hybridized structure and pore sizes for challenging separation needs.

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

The fine-tuning of metal-organic framework (MOF) pore structures is of critical importance in developing energy-efficient xenon/krypton (Xe/Kr) separation techniques. Capitalizing on reticular chemistry, we constructed a robust Y-based MOF (NU-1801) that is isoreticular to NPF-500 with a shortened organic ligand and a larger metal radius while maintaining the 4,8-connected flu topology, giving rise to a narrowed pore structure for the efficient separation of a Xe/Kr mixture. At 298 K and 1 bar, NU-1801 possessed a moderate Xe uptake of 2.79 mmol/g but exhibited a high Xe/Kr selectivity of 8.2 and an exceptional Xe/Kr uptake ratio of about 400%. NU-1801 could efficiently separate a Xe/Kr mixture (20:80, v/v), as validated by breakthrough experiments, due to the outstanding discrimination in van der Waals interactions of Xe and Kr toward the framework confirmed by grand canonical Monte Carlo simulations. This work highlights the importance of reticular chemistry in designing structure-specific MOFs for gas separation.

Simultaneous Recognition and Separation of Organic Isomers Via Cooperative Control of Pore-Inside and Pore-Outside Interactions

Despite the desirability of organic isomer recognition and separation, current strategies are expensive and complicated. Here, a simple strategy for simultaneously recognizing and separating organic isomers using pillararene-based charge-transfer cocrystals through the cooperative control of pore-inside and pore-outside intermolecular interactions is presented. This strategy is illustrated using 1-bromobutane (1-BBU), which is often produced as an isomeric mixture with 2-bromobutane (2-BBU). According to its structure, perethylated pillar[5]arene (EtP5) and 3,5-dinitrobenzonitrile (DNB) are strategically chosen as a donor and an acceptor. As a result, their cocrystal exhibited stronger pore-inside interactions and much weaker pore-outside interactions with 1-BBU than with 2-BBU. Consequently, nearly 100% 1-BBU selectivity is achieved in two-component mixtures, even in those containing trace 1-BBU (1%), whereas free EtP5 only achieved 89.80% selectivity. The preference for linear bromoalkanes is retained in 1-bromopentane/3-bromopentane and 1-bromohexane/2-bromohexane mixtures, demonstrating the generality of this strategy. Selective adsorption of linear bromoalkanes induced a naked-eye-detectable color change from red to white. Moreover, the cocrystal are used over multiple cycles without losing selectivity.

Aliphatic-Bridged Early Lanthanide Metal–Organic Frameworks: Topological Polymorphism and Excitation-Dependent Luminescence

Six new three-dimensional metal–organic frameworks based on early lanthanide(III) cations and trans-1,4-cyclohexanedicarboxylic acid (H2chdc) were obtained. Their crystal structures were determined by single-crystal X-ray diffraction analysis. The structure of [La2(H2O)4(chdc)3]·2DMF·H2O (1; DMF = N,N-dimethylformamide) contains one-dimensional infinite La(III)-carboxylate chains interconnected by cyclohexane moieties to form a highly porous polymeric lattice with 30% solvent accessible volume. Compounds [Ln2(phen)2(chdc)3]·0.75DMF (2Ln; Ln3+ = Ce3+, Pr3+, Nd3+ and Sm3+; phen = 1,10-phenanthroline) are based on binuclear carboxylate building blocks, which are decorated by chelate phenanthroline ligands and interconnected by cyclohexane moieties to form more dense isostructural coordination frameworks with primitive cubic pcu topology. Compound [Nd2(phen)2(chdc)3]·2DMF·0.67H2O (3) is based on secondary building units similar to 2Ln and contains a coordination lattice isomeric to 2Ln with a rare hexagonal helical snz topology. Thermal stability and luminescent properties were investigated. For 2Sm, a strong and nonmonotonous dependence of the luminescence color on the variation of excitation wavelength was revealed, changing its emission from pinkish red at λex = 340 nm to white at λex = 400 nm, and then to yellow at lower excitation energies. Such nonlinear behavior was rationalized in terms of the contribution of several different luminescence mechanisms. Thus, 2Sm is a rather rare example of a highly tunable monometallic lanthanide-based luminophore with possible applications in light-emitting devices and optical data processing.

Tunable Confined Aliphatic Pore Environment in Robust Metal-Organic Frameworks for Efficient Separation of Gases with a Similar Structure

The fine-tuning of the pore structure of metal-organic frameworks (MOFs) is of critical importance to developing energy-efficient processes for the challenging separation of structurally similar molecules. Herein, we demonstrate a strategy to realize a quasi-three-dimensional refinement of the pore structure that utilizes the tunability of ring size and number in polycycloalkane-dicarboxylate ligands. Two hydrolytically stable MOFs with a confined aliphatic pore environment, ZUL-C1 and ZUL-C2, were, for the first time, synthesized and applied in separating low-concentration C2-C3 hydrocarbons from natural gas and ultralow-concentration Xe from used nuclear fuel (UNF) off-gas. Validated by X-ray diffraction and modeling, an expansion of the polycycloalkane moiety enables sub-angstrom contraction in specific directions and forms a pore surface with more alkyl sites, which affords stronger trapping of guest molecules with relatively higher polarizability. The resultant material exhibits record C₂H₆/CH₄ and C₃H₈/CH₄ selectivities coupled with a benchmark low-pressure C₂H₆ capacity in alkane mixture separation and also a benchmark Xe capacity at extremely diluted feed concentration and record Kr productivity for the Xe/Kr (20:80, v/v) mixture in Xe/Kr separation.

Discrimination of xylene isomers in a stacked coordination polymer

The separation and purification of xylene isomers is an industrially important but challenging process. Developing highly efficient adsorbents is crucial for the implementation of simulated moving bed technology for industrial separation of these isomers. Herein, we report a stacked one-dimensional coordination polymer {[Mn(dhbq)(H 2 O) 2 ], H 2 dhbq = 2,5-dihydroxy-1,4-benzoquinone} that exhibits an ideal molecular recognition and sieving of xylene isomers. Its distinct temperature-adsorbate–dependent adsorption behavior enables full separation of p -, m -, and o -xylene isomers in both vapor and liquid phases. The delicate stimuli-responsive swelling of the structure imparts this porous material with exceptionally high flexibility and stability, well-balanced adsorption capacity, high selectivity, and fast kinetics at conditions mimicking industrial settings. This study may offer an alternative approach for energy-efficient and adsorption-based industrial xylene separation and purification processes.

VOC Mixture Sensing with a MOF Film Sensor Array: Detection and Discrimination of Xylene Isomers and Their Ternary Blends

Detection and recognition of volatile organic compounds (VOCs) are crucial in many applications. While pure VOCs can be detected by various sensors, the discrimination of VOCs in mixtures, especially of similar molecules, is hindered by cross-sensitivities. Isomer identification in mixtures is even harder. Metal-organic frameworks (MOFs) with their well-defined, nanoporous, and versatile structures have the potential to improve the VOC sensing performance by tailoring the adsorption affinities. Here, we detect and identify ternary xylene isomer mixtures by using an array of six gravimetric, quartz crystal microbalance (QCM)-based sensors coated with selected MOF films with different isomer affinities. We use classical molecular simulations to provide insights into the sensing mechanism. In addition to the attractive interaction between the analytes and the MOF film, the isomer discrimination is caused by the rigid crystalline framework sterically controlling the access of the isomers to different adsorption sites in the MOFs. The sensor array has a very low limit of detection of 1 ppm for each pure isomer and allows the isomer discrimination in mixtures. At 100 ppm, 16 different ternary o-p-m-xylene mixtures were identified with high classification accuracy (96.5%). This work shows the unprecedented performance of MOF-sensor arrays, also referred to as MOF-electronic nose (MOF-e-nose), for sensing VOC mixtures. Based on the study, guidelines for detecting and discriminating complex mixtures of volatile molecules are also provided.

Homogeneously Mixing Different Metal-Organic Framework Structures in Single Nanocrystals through Forming Solid Solutions

Pore engineering plays a significant role in the applications of porous materials, especially in the area of separation and catalysis. Here, we demonstrated a metal-organic framework (MOF) solid solution (MOSS) strategy to homogeneously and controllably mix NU-1000 and NU-901 structures inside single MOF nanocrystals. The key for the homogeneous mixing and forming of MOSS was the bidentate modulator, which was designed to have a slightly longer distance between two carboxylate groups than the original tetratopic ligand. All of the MOSS nanocrystals showed a uniform pore size distribution with a well-tuned ratio of mesopores to micropores. Because of the appropriate pore ratio, MOSS nanocrystals can balance the thermodynamic interactions and kinetic diffusion of the substrates, thus showing exceedingly higher separation abilities and a unique elution sequence. Our work proposes a rational strategy to design mixed-porous MOFs with controlled pore ratios and provides a new direction to design homogeneously mixed MOFs with a high separation ability and unique separation selectivity.

An Adaptive Hydrogen-Bonded Organic Framework for the Exclusive Recognition of p-Xylene

Separation of xylene isomers is one of the most important but most challenging and energy-intensive separation processes in the petrochemical industry. Here, we report an adaptive hydrogen-bonded organic framework (HOF-29) constructed from a porphyrin based organic building block 4,4',4'',4'''-(porphyrin-5,10,15,20-tetrayl) tetrabenzonitrile (PTTBN), exhibiting the exclusive molecular recognition of p-xylene (pX) over its isomers of o-xylene (oX) and m-xylene (mX), as clearly demonstrated in the single crystal structure transformation and ¹ H NMR studies. Single crystal structure studies show that single-crystal-to-single-crystal transformation from the as-synthesized HOF-29 to the pX exclusively included HOF-29⊃pX is triggered by the encapsulation of pX molecules, accompanied by sliding of the 2D layers and local distortion of the ligand, which provides multiple C-H⋅⋅⋅π interactions.

Fine pore engineering in a series of isoreticular metal-organic frameworks for efficient C2H2/CO2 separation

The separation of C₂H₂/CO₂ is not only industrially important for acetylene purification but also scientifically challenging owing to their high similarities in physical properties and molecular sizes. Ultramicroporous metal-organic frameworks (MOFs) can exhibit a pore confinement effect to differentiate gas molecules of similar size. Herein, we report the fine-tuning of pore sizes in sub-nanometer scale on a series of isoreticular MOFs that can realize highly efficient C₂H₂/CO₂ separation. The subtle structural differences lead to remarkable adsorption performances enhancement. Among four MOF analogs, by integrating appropriate pore size and specific binding sites, [Cu(dps)₂(SiF₆)] (SIFSIX-dps-Cu, SIFSIX = SiF₆^2-, dps = 4.4'-dipyridylsulfide, also termed as NCU-100) exhibits the highest C₂H₂ uptake capacity and C₂H₂/CO₂ selectivity. At room temperature, the pore space of SIFSIX-dps-Cu significantly inhibits CO₂ molecules but takes up a large amount of C₂H₂ (4.57 mmol g^-1), resulting in a high IAST selectivity of 1787 for C₂H₂/CO₂ separation. The multiple host-guest interactions for C₂H₂ in both inter- and intralayer cavities are further revealed by dispersion-corrected density functional theory and grand canonical Monte Carlo simulations. Dynamic breakthrough experiments show a clean C₂H₂/CO₂ separation with a high C₂H₂ working capacity of 2.48 mmol g^-1.

A new boron cluster anion pillared metal organic framework with ligand inclusion and its selective acetylene capture properties

A novel microporous boron cluster pillared metal–organic framework BSF-10 was synthesized with ligand inclusion for efficient C2H2/CO2 and C2H2/C2H4 adsorption separation.

Selective perrhenate/pertechnetate removal by a MOF-based molecular trap

Rational design of anion-exchange materials for selective elimination of radioactive anionic contaminants poses a great challenge. Rather than relying on a size-compatible effect, combination of nano-sieve pore, hydrophobic cationic cavity,...

Xylene Recognition in Flexible Porous Coordination Polymer by Guest-Dependent Structural Transition

Xylene isomers are crucial chemical intermediates in great demand worldwide; the almost identical physicochemical properties render their current separation approach energy consuming. In this study, we utilized the soft porous coordination polymer (PCP)'s isomer-specific structural transformation, realizing o-xylene (oX) recognition/separation from the binary and ternary isomer mixtures. This PCP has a flexible structure that contains flexible aromatic pendant groups, which both work as recognition sites and induce structural flexibility of the global framework. The PCP exhibits guest-triggered "breathing"-type structural changes, which are accompanied by the rearrangement of the intraframework π-π interaction. By rebuilding π-π stacking with isomer species, the PCP discriminated oX from the other isomers by its specific guest-loading configuration and separated oX from the isomer mixture via selective adsorption. The xylene-selective property of the PCP is dependent on the solvent; in diluted hexane solution, the PCP favors p-xylene (pX) uptake. The separation results combined with crystallographic analyses revealed the effect of the isomer selectivity of the PCP on xylene isomer separation via structural transition and demonstrated its potential as a versatile selective adsorptive medium for challenging separations.

Separation and Purification of Hydrocarbons with Porous Materials

This Minireview focuses on the developments of the adsorptive separation of methane/nitrogen, ethene/ethane, propene/propane mixtures as well as on the separation of C8 aromatics (i.e. xylene isomers) with a wide variety of materials, including carbonaceous materials, zeolites, metal-organic frameworks, and porous organic frameworks. Some recent important developments for these adsorptive separations are also highlighted. The advantages and disadvantages of each material category are discussed and guidelines for the design of improved materials are proposed. Furthermore, challenges and future developments of each material type and separation processes are discussed.

Thermal decarboxylation for the generation of hierarchical porosity in isostructural metal-organic frameworks containing open metal sites

The effect of metal-cluster redox identity on the thermal decarboxylation of a series of isostructural metal-organic frameworks (MOFs) with tetracarboxylate-based ligands and trinuclear μ3-oxo clusters was investigated. The PCN-250 series of MOFs can consist of various metal combinations (Fe3, Fe/Ni, Fe/Mn, Fe/Co, Fe/Zn, Al3, In3, and Sc3). The Fe-based system can undergo a thermally induced reductive decarboxylation, producing a mixed valence cluster with decarboxylated ligand fragments subsequently eliminated to form uniform mesopores. We have extended the analysis to alternative monometallic and bimetallic PCN-250 systems to observe the cluster's effect on the decarboxylation process. Our results suggest that the propensity to undergo decarboxylation is directly related to the cluster redox accessibility, with poorly reducible metals, such as Al, In, and Sc, unable to thermally reduce at the readily accessible temperatures of the Fe-containing system. In contrast, the mixed-metal variants are all reducible. We report improvements in gas adsorption behavior, significantly the uniform increase in the heat of adsorption going from the microporous to hierarchically induced decarboxylated samples. This, along with Fe oxidation state changes from 57Fe Mössbauer spectroscopy, suggests that reduction occurs at the clusters and is essential for mesopore formation. These results provide insight into the thermal behavior of redox-active MOFs and suggest a potential future avenue for generating mesoporosity using controlled cluster redox chemistry.

An updated structural model of the A domain of the Pseudomonas putida XylR regulator poses an atypical interplay with aromatic effectors

A revised model of the aromatic binding A domain of the σ⁵⁴ -dependent regulator XylR of Pseudomonas putida mt-2 was produced based on the known 3D structures of homologous regulators PoxR, MopR and DmpR. The resulting frame was instrumental for mapping a number of mutations known to alter effector specificity, which were then reinterpreted under a dependable spatial reference. Some of these changes involved the predicted aromatic binding pocket but others occurred in distant locations, including dimerization interfaces and putative zinc binding site. The effector pocket was buried within the protein structure and accessible from the outside only through a narrow tunnel. Yet, several loop regions of the A domain could provide the flexibility required for widening such a tunnel for passage of aromatic ligands. The model was experimentally validated by treating the cells in vivo and the purified protein in vitro with benzyl bromide, which reacts with accessible nucleophilic residues on the protein surface. Structural and proteomic analyses confirmed the predicted in/out distribution of residues but also supported two additional possible scenarios of interaction of the A domain with aromatic effectors: a dynamic interaction of the fully structured yet flexible protein with the aromatic partner and/or inducer-assisted folding of the A domain.

Regulating metal-organic frameworks as stationary phases and absorbents for analytical separations

Metal-organic frameworks (MOFs) are highly ordered framework systems composed of metal centers and organic linkers formed through coordination bonds. The diversity of metal elements and easily modified organic ligands, together with controllable synthetic approaches, gives rise to the designability of various MOF structures and topologies and the capability of MOFs to be functionalized. Their structural diversity provides MOFs with many unique properties, such as permanent porosity, flexible structures, thermostability, and high adsorption capacity, leading to great practicability in technical applications. In this review, we concentrate on the applications of MOFs in the field of gas chromatography, high-performance liquid chromatography, and the enrichment of biomolecules, based on rational arrangements in the structures and functions of MOFs. Moreover, we emphasize the importance of structural and chemical regulations for the improvement of separation efficiency.

Multiple Functions of Gas Separation and Vapor Adsorption in a New MOF with Open Tubular Channels

Separation or purification is one of the difficult problems in the petrochemical industry. To help solve the difficulty of separation or purification for C₂H₂/CO₂ and C₂H_n/CH₄ in the chemical industry, we synthesized a new metal-organic framework (MOF), [Ni(dpip)]·2.5DMF·H₂O (1), by a bipyridyl-substituted isophthalic acid ligand. The MOF includes two types of one-dimensional (1D) tubular channels with different sizes and porous environments. The unique tubular channels lead to not only remarkable gas sorption capacity of C₂H₄, C₂H₂, and CO₂, but also good selectivity for C₂H₂/CH₄, C₂H₂/CH₄, CO₂/CH₄, and C₂H₂/CO₂, as demonstrated by single-component sorption isotherm results, ideal adsorbed solution theory calculations, and dynamic breakthrough curves. Grand canonical Monte Carlo (GCMC) simulation reveals preferential adsorption sites in the MOF for CO₂, C₂H₂, and C₂H₄. The MOF also exhibits an obvious size-selective absorption effect on vapor molecules.