Molecular-Sieving Separation of Methanol/Benzene Azeotrope by a Flexible Metal-Organic Framework.

Separation of methanol/benzene azeotrope mixtures is very challenging not only by the conventional distillation technique but also by adsorbents. In this work, we design and synthesize a flexible Ca-based metal-organic framework MAF-58 consisting of cheap raw materials. MAF-58 shows selective methanol-induced pore-opening flexibility. Although the opened pores are large enough to accommodate benzene molecules, MAF-58 shows methanol/benzene molecular sieving with ultrahigh experimental selectivity, giving 5.1 mmol g-1 high-purity (99.99%+) methanol and 2.0 mmol g-1 high-purity (99.97%+) benzene in a single adsorption/desorption cycle. Computational simulations reveal that the preferentially adsorbed, coordinated methanol molecules act as the gating component to selectively block the diffusion of benzene, offering a new gating adsorption mechanism.

Reverse Separation of Carbon Dioxide and Acetylene in Two Isostructural Copper Pyridine-Carboxylate Frameworks.

Separating acetylene from carbon dioxide is important but highly challenging due to their similar molecular shapes and physical properties. Adsorptive separation of carbon dioxide from acetylene can directly produce pure acetylene but is hardly realized because of relatively polarizable acetylene binds more strongly. Here, we reverse the CO2 and C2H2 separation by adjusting the pore structures in two isoreticular ultramicroporous metal-organic frameworks (MOFs). Under ambient conditions, copper isonicotinate (Cu(ina)2), with relatively large pore channels shows C2H2-selective adsorption with a C2H2/CO2 selectivity of 3.4, whereas its smaller-pore analogue, copper quinoline-5-carboxylate (Cu(Qc)2) shows an inverse CO2/C2H2 selectivity of 5.6. Cu(Qc)2 shows compact pore space that well matches the optimal orientation of CO2 but is not compatible for C2H2. Neutron powder diffraction experiments confirmed that CO2 molecules adopt preferential orientation along the pore channels during adsorption binding, whereas C2H2 molecules bind in an opposite fashion with distorted configurations due to their opposite quadrupole moments. Dynamic breakthrough experiments have validated the separation performance of Cu(Qc)2 for CO2/C2H2 separation.

Observation of Interpenetrated Topology Isomerism for Covalent Organic Frameworks with Atom-Resolution Single Crystal Structures.

Rational control and understanding of isomerism are of significance but still remain a great challenge in reticular frameworks, in particular, for covalent organic frameworks (COFs) due to the complicated synthesis and energy factors. Herein, reaction of 3,3',5,5'-tetra(4-formylphenyl)-2,2',6,6'-tetramethoxy-1,1'-biphenyl (TFTB) with 3,3',5,5'-tetrakis(4-aminophenyl)bimesityl (TAPB) under different reaction conditions affords single crystals of two 3D COF isomers, namely, USTB-20-dia and USTB-20-qtz. Their structures with resolutions up to 0.9-1.1 Å have been directly solved by three-dimensional electron diffraction (3D ED) and synchrotron single crystal X-ray diffraction, respectively. USTB-20-dia and USTB-20-qtz show rare 2 × 2-fold interpenetrated dia-b nets and 3-fold interpenetrated qtz-b frameworks. Comparative studies of the crystal structures of these COFs and theoretical simulation results indicate the crucial role of the flexible molecular configurations of building blocks in the present interpenetrated topology isomerism. This work not only presents the rare COF isomers but also gains an understanding of the formation of framework isomerism from both single crystal structures and theoretical simulation perspectives.

Pore Modulation of Hydrogen-Bonded Organic Frameworks for Efficient Separation of Propylene.

Developing hydrogen-bonded organic frameworks (HOFs) that combine functional sites, size control, and storage capability for targeting gas molecule capture is a novel and challenging venture. However, there is a lack of effective strategies to tune the hydrogen-bonded network to achieve high-performance HOFs. Here, a series of HOFs termed as HOF-ZSTU-M (M=1, 2, and 3) with different pore structures are obtained by introducing structure-directing agents (SDAs) into the hydrogen-bonding network of tetrakis (4-carboxyphenyl) porphyrin (TCPP). These HOFs have distinct space configurations with pore channels ranging from discrete to continuous multi-dimensional. Single-crystal X-ray diffraction (SCXRD) analysis reveals a rare diversity of hydrogen-bonding models dominated by SDAs. HOF-ZSTU-2, which forms a strong layered hydrogen-bonding network with ammonium (NH4 + ) through multiple carboxyl groups, has a suitable 1D "pearl-chain" channel for the selective capture of propylene (C3 H6 ). At 298 K and 1 bar, the C3 H6 storage density of HOF-ZSTU-2 reaches 0.6 kg L-1 , representing one of the best C3 H6 storage materials, while offering a propylene/propane (C3 H6 /C3 H8 ) selectivity of 12.2. Theoretical calculations and in situ SCXRD provide a detailed analysis of the binding strength of C3 H6 at different locations in the pearl-chain channel. Dynamic breakthrough tests confirm that HOF-ZSTU-2 can effectively separate C3 H6 from multi-mixtures.

Achieving High Performance Metal-Organic Framework Materials through Pore Engineering.

Achieving high performance functional materials has been a long-term goal for scientists and engineers that can significantly promote science and technology development and thus benefit our society and human beings. As well-known porous materials, metal-organic frameworks (MOFs) are crystalline open frameworks made up of molecular building blocks linked by strong coordination bonds, affording pore space for storing and trapping guest molecules. In terms of porosity, MOFs outperform traditional porous materials including zeolites and activated carbon, showing exceptional porosity with internal surface area up to thousands of square meters per gram of sample and with periodic pore sizes ranging from sub-nanometer to nanometers. Numerous MOFs have been synthesized with potential applications ranging from storing gaseous fuels to separating intractable industrial gas mixtures, sensing physical and chemical stimulus, and transmitting protons for conduction. Compared to traditional porous materials, MOFs are distinguished for their exceptional capability for pore adjustment and interior modification through pore engineering, which have made them a preeminent platform for exploring functional materials with high performance.Rational combinations of rigid building units of different geometry and multibranched organic linkers have provided MOFs with diverse pore structures, ranging from spherical to cylindrical, slit, and tubular ones isolating or interconnecting in different directions, which can be optimized for high-capacity gas storage. Based on the isoreticular principle and building blocks approach in MOF chemistry, the pore adjustment of porous materials can be performed with exquisite precision, making them suitable to address industrially important gas separation. The large pore cavities in MOFs are readily available for encapsulation of different functional guest species, resulting in novel MOF composite materials with various functions.In this Account, we summarize our recent research progress on pore engineering to achieve high-performance MOF materials. We have been able to tune and optimize pore structures, immobilize specific functional sites, and incorporate guest species into target MOF materials for hydrogen storage, methane storage, light-hydrocarbon purification, and proton conduction, especially for various industrially important gas separations including acetylene removal and ethylene and propylene purification. By engineering the porosity and pore chemistry that endows MOFs with multiple functionalities, our research endeavors have brought about the customization of high-performance MOF materials for corresponding application scenarios.

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.

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.

Electrostatically Driven Selective Adsorption of Carbon Dioxide over Acetylene in an Ultramicroporous Material.

Separating acetylene from carbon dioxide is important but highly challenging owing to their similar physical properties and molecular dimensions. Herein, we report highly efficient electrostatically driven CO2 /C2 H2 separation in an ultramicroporous cadmium nitroprusside (Cd-NP) with compact pore space and complementary electrostatic potential well fitting for CO2 , thus enabling molecular quadrupole moment recognition of CO2 over C2 H2 . This material shows a high CO2 /C2 H2 uptake ratio of 6.0 as well as remarkable CO2 /C2 H2 selectivity of 85 under ambient conditions with modest CO2 heat of adsorption. Neutron powder diffraction experiments and molecular simulations revealed that the electrostatic potential compatibility between pore structure and CO2 allows it to be trapped in a head-on orientation towards the Cd center, whereas the diffusion of C2 H2 is electrostatically forbidden. Dynamic breakthrough experiments have validated the separation performance of this compound for CO2 /C2 H2 separation.

A Molecular Compound for Highly Selective Purification of Ethylene.

Purification of C2 H4 from an C2 H4 /C2 H6 mixture is one of the most challenging separation processes, which is achieved mainly through energy-intensive, cryogenic distillation in industry. Sustainable, non-distillation methods are highly desired as alternatives. We discovered that the fluorinated bis(pyrazolyl)borate ligand supported copper(I) complex {[(CF3 )2 Bp]Cu}3 has features very desirable in an olefin-paraffin separation material. It binds ethylene exclusively over ethane generating [(CF3 )2 Bp]Cu(C2 H4 ). This molecular compound exhibits extremely high and record ideal adsorbed solution theory (IAST) C2 H4 /C2 H6 gas separation selectivity, affording high purity (>99.5 %) ethylene that can be readily desorbed from separation columns. In-situ PXRD provides a "live" picture of the reversible conversion between [(CF3 )2 Bp]Cu(C2 H4 ) and the ethylene-free sorbent in the solid-state, driven by the presence or removal of C2 H4 . Molecular structures of trinuclear {[(CF3 )2 Bp]Cu}3 and mononuclear [(CF3 )2 Bp]Cu(C2 H4 ) are also presented.

A Microporous Hydrogen-Bonded Organic Framework for the Efficient Capture and Purification of Propylene.

Adsorptive separation of propylene/propane (C3 H6 /C3 H8 ) mixture is desired for its potential energy saving on replacing currently deployed and energy-intensive cryogenic distillation. Realizing efficient C3 H6 /C3 H8 separation in the emerging hydrogen-bonded organic frameworks (HOFs) is very challenging owing to the lack of functional sites for preferential gas binding. By virtue of crystal engineering, we herein report a functionalized HOF (HOF-16) with free -COOH sites for the efficient separation of C3 H6 /C3 H8 mixtures. Under ambient conditions, HOF-16 shows a significant C3 H6 /C3 H8 uptake difference (by 76 %) and selectivity (5.4) in contrast to other carboxylic acid-based HOFs. Modeling studies indicate that free -COOH groups together with the suitable pore confinement facilitate the recognition and high-density packing of gas molecules. The separation performance of HOF-16 was validated by breakthrough experiments. HOF-16 is stable towards strong acidity and water.

Hydrogen-Bonded Organic Frameworks as a Tunable Platform for Functional Materials.

As a novel class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metal-organic building blocks through intermolecular hydrogen-bonding interactions, have attracted more and more attention. Over the past decade, a number of porous HOFs have been constructed through judicious selection of H-bonding motifs, which are further enforced by other weak intermolecular interactions such as π-π stacking and van der Waals forces and framework interpenetration. Since the H-bonds are weaker than coordinate and covalent bonds used for the construction of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), HOFs have some unique features such as mild synthesis condition, solution processability, easy healing, and regeneration. These features enable HOFs to be a tunable platform for the construction of functional materials. Here, we review the H-bonding motifs used for constructing porous HOFs and highlight some of their applications, including gas separation and storage, chiral separation and structure determination, fluorescent sensing, heterogeneous catalysis, biological applications, proton conduction, photoluminescent materials, and membrane-based applications.

Optimizing Pore Space for Flexible-Robust Metal-Organic Framework to Boost Trace Acetylene Removal.

Isoreticular principle has been employed to realize a flexible-robust metal-organic framework (MOF) with extended pore structure for the adsorptive removal of trace acetylene from ethylene under ambient conditions. The substitution from zinc(II) to copper(II) of high coordination distortion leads to elongated Cu-F bonds that expand the closed pore cavities in the prototypical MOF from 3.5 × 3.9 × 4.1 to 3.6 × 4.3 × 4.2 Å3. The optimal cavity size together with strong binding sites thus endows the new Cu analogue to possess open pore space accessible for trace C2H2 within a substantial low-pressure range while excluding C2H4 molecules, as validated by gas isotherms and single-crystal structure of its partially C2H2-loading phase. In contrast to the Zn prototype, at 298 K and 1.0 bar, the guest-free Cu analogue shows significant C2H2 uptake increase with a total capacity of 4.57 mmol g-1, and gains an over two orders of magnitude jump in IAST selectivity for C2H2/C2H4 (1/99, v/v). These results are higher than the benchmark MOFs for molecular sieving of C2H2/C2H4, leading a high C2H4 productivity of 14.9 mmol g-1. Crystallography studies, molecular modeling, selectivity evaluation, and breakthrough experiments have comprehensively demonstrated this flexible-robust MOF as an efficient adsorbent for C2H2/C2H4 separation.

An Ultramicroporous Metal-Organic Framework for High Sieving Separation of Propylene from Propane.

Highly selective adsorptive separation of olefin/paraffin through porous materials can produce high purity olefins in a much more energy-efficient way than the traditional cryogenic distillation. Here we report an ultramicroporous cobalt gallate metal-organic framework (Co-gallate) for the highly selective sieving separation of propylene/propane at ambient conditions. This material possesses optimal pore structure for the exact confinement of propylene molecules while excluding the slightly large propane molecules, as clearly demonstrated in the neutron diffraction crystal structure of Co-gallate⊃0.38C3D6. Its high separation performance has been confirmed by the gas sorption isotherms and column breakthrough experiments to produce the high purity of propylene (97.7%) with a high dynamic separation productivity of 36.4 cm3 cm-3 under ambient conditions. The gas adsorption measurement, pore size distribution, and crystallographic and modeling studies comprehensively support the high sieving C3H6/C3H8 separation in this MOF material. It is stable under different environments, providing its potential for the industrial propylene purification.

Isoreticular Microporous Metal-Organic Frameworks for Carbon Dioxide Capture.

The isoreticular principle has been applied to construct two copper metal-organic framework (MOF) analogues with different porosities for the adsorptive capture of CO2 from N2 and CH4 at 1 atm and 298 K. By using a 4-substituted isophthalate linker with a bulky nitro group, the microporous MOF [Cu(BDC-NO2)(DMF)] (UTSA-93 or CuBDC-NO2; H2BDC-NO2 = 4-nitroisophthalic acid and DMF = N,N'-dimethylformamide) has been synthesized with mot topology, showing a compact pore structure with a size of 6.0 × 7.0 Å2 in contrast to that of 6.9 × 8.5 Å2 in the prototypical MOF with a bromo group. The optimized pore structure allows the nitro-functionalized MOF to capture CO2 with a higher capacity of about 2.40 mmol g-1 under ambient conditions, in contrast to 1.08 mmol g-1 in the bromo-functionalized analogue. The adsorption selectivity of CuBDC-NO2-a for a CO2/N2 (15:85) mixture (28) under ambient conditions is also higher than that of the bromo-substituted prototype (25) and comparable with those of several MOF materials. Moreover, dynamic breakthrough experiments of the nitro-functionalized MOF have been performed to illustrate its separation potential toward a CO2/N2 mixture.

Multifunctional porous hydrogen-bonded organic framework materials.

Hydrogen-bonded organic frameworks (HOFs) represent an interesting type of polymeric porous materials that can be self-assembled through H-bonding between organic linkers. To realize permanent porosity in HOFs, stable and robust open frameworks can be constructed by judicious selection of rigid molecular building blocks and hydrogen-bonded units with strong H-bonding interactions, in which the framework stability might be further enhanced through framework interpenetration and other types of weak intermolecular interactions such as ππ interactions. Owing to the reversible and flexible nature of H-bonding connections, HOFs show high crystallinity, solution processability, easy healing and purification. These unique advantages enable HOFs to be used as a highly versatile platform for exploring multifunctional porous materials. Here, the bright potential of HOF materials as multifunctional materials is highlighted in some of the most important applications for gas storage and separation, molecular recognition, electric and optical materials, chemical sensing, catalysis, and biomedicine.

Mixed Metal-Organic Framework with Multiple Binding Sites for Efficient C2 H2 /CO2 Separation.

The separation of C2 H2 /CO2 is particularly challenging owing to their similarities in physical properties and molecular sizes. Reported here is a mixed metal-organic framework (M'MOF), [Fe(pyz)Ni(CN)4 ] (FeNi-M'MOF, pyz=pyrazine), with multiple functional sites and compact one-dimensional channels of about 4.0 Šfor C2 H2 /CO2 separation. This MOF shows not only a remarkable volumetric C2 H2 uptake of 133 cm3 cm-3 , but also an excellent C2 H2 /CO2 selectivity of 24 under ambient conditions, resulting in the second highest C2 H2 -capture amount of 4.54 mol L-1 , thus outperforming most previous benchmark materials. The separation performance of this material is driven by π-π stacking and multiple intermolecular interactions between C2 H2 molecules and the binding sites of FeNi-M'MOF. This material can be facilely synthesized at room temperature and is water stable, highlighting FeNi-M'MOF as a promising material for C2 H2 /CO2 separation.

Tuning Gate-Opening of a Flexible Metal-Organic Framework for Ternary Gas Sieving Separation.

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C2 H4 ), acetylene (C2 H2 ), and carbon dioxide (CO2 ) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C2 H2 and CO2 through gate-opening and only negligible amount of C2 H4 . Breakthrough experiments demonstrated that this material can simultaneously capture C2 H2 and CO2 , yielding polymer-grade (>99.99 %) C2 H4 from single breakthrough separation.

Pore Space Partition within a Metal-Organic Framework for Highly Efficient C2H2/CO2 Separation.

The pore space partition (PSP) approach has been employed to realize a novel porous MOF (FJU-90) with dual functionalities for the challenging C2H2/CO2 separation under ambient conditions. By virtue of a triangular ligand (Tripp = 2,4,6-tris(4-pyridyl)pyridine), the cylindrical channels in the original FJU-88 have been partitioned into uniformly interconnected pore cavities, leading to the dramatically reduced pore apertures from 12.0 × 9.4 to 5.4 × 5.1 Å2. Narrowing down the pore sizes, the resulting activated FJU-90a takes up a very large amount of C2H2 (180 cm3 g-1) but much less of CO2 (103 cm3 g-1) at 298 K and 1 bar, demonstrating it to be the best porous MOF material for this C2H2/CO2 (50%:50%) separation in terms of the C2H2 gravimetric productivity. IAST calculations, molecular modeling studies, and simulated and experimental breakthrough experiments comprehensively demonstrate that the pore space partition strategy is a very powerful approach to constructing MOFs with dual functionality for challenging gas separation.

Our journey of developing multifunctional metal-organic frameworks.

Metal-organic frameworks (MOFs) are organic-inorganic hybrid solids constructed from the coordination interaction of metal ions/clusters with organic linkers, which currently represent one of the most rapidly expanding platforms for new functional materials. Based on well-established approaches, involving tuning the pore sizes, incorporation of functional sites and post-synthetic modification, the pore structures of MOFs can be readily controlled for multifunctional applications. In this brief review, we summarize and highlight our research progresses during our journey on developing functional MOFs for various applications including gas storage, gas separations, luminescent sensing, proton conduction, and molecular recognitions.

Molecular sieving of ethylene from ethane using a rigid metal-organic framework.

There are great challenges in developing efficient adsorbents to replace the currently used and energy-intensive cryogenic distillation processes for olefin/paraffin separation, owing to the similar physical properties of the two molecules. Here we report an ultramicroporous metal-organic framework [Ca(C4O4)(H2O)], synthesized from calcium nitrate and squaric acid, that possesses rigid one-dimensional channels. These apertures are of a similar size to ethylene molecules, but owing to the size, shape and rigidity of the pores, act as molecular sieves to prevent the transport of ethane. The efficiency of this molecular sieve for the separation of ethylene/ethane mixtures is validated by breakthrough experiments with high ethylene productivity under ambient conditions. This material can be easily synthesized at the kilogram scale using an environmentally friendly method and is water-stable, which is important for potential industrial implementation. The strategy of using highly rigid metal-organic frameworks with well defined and rigid pores could also be extended to other porous materials for chemical separation processes.