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Rationally Fabricating Three-Dimensional Covalent Organic Frameworks for Propyne/Propylene Separation
Abstract
Efficient propyne/propylene separation to obtain polymer-grade propylene is a crucial and challenging process in industrial production, but it has not yet been realized in the covalent organic framework (COF) field. Addressing this challenge, we synthesize two three-dimensional COF adsorbents via a [8 + 4] construction approach based on an octatopic aldehyde monomer. Upon using the continuous rotation electron diffraction technique and structural simulation, both COFs are successfully determined as rare flu topology. Various characterization techniques prove that both COFs exhibit high crystallinity, high porosity, and good stability. Attributed to their interconnected micropores and nonpolar pore environment, these COFs can efficiently remove trace amounts of propyne from the propyne/propylene (1/99, and 0.1/99.9, v/v) mixture to obtain high-purity propylene (>99.99%), validated by dynamic breakthrough experiments. This work paves a new avenue for propyne/propylene separation using COFs as highly efficient adsorbents.
... VOCs是空气污染的主要成分之一, 包括苯、甲 图 6 (网络版彩色)COF树脂用于碘吸附示意图. (a) COF-OH-X及iCOF-AB-X结构示意图 [42] ; (b) COF TGDM结构示意图及其吸附位点 [43] Figure 6 (Color online) Schematic illustrations of adsorption of iodine by COF resin. (a) Structures of COF-OH-X and iCOF-AB-X [42] ; (b) structure of COF TGDM and its adsorption sites [43] [49] ; (b) structure of BTT-TAPT-COF and its mechanism of adsorption in different pH environment [50] 评 述 气体分子的高选择性筛分实现了丙烷/丙烯气体的分 离. 同年, 张振杰课题组 [55] 开发了2种新型亚胺基三维 COF材料--NKCOF-36和NKCOF-37, 其对丙烷丙烯 气体具有很好的分离纯化效果, 可以得到聚合级的高 纯丙烯. 2023年, 该课题组 [23] 通过熔融聚合法, 利用廉 价的2,3,5,6-四甲基吡嗪和对苯二甲醛反应合成了乙烯 基NKCOF-62, 利用COF材料从C 2 H 6 /C 2 H 4 /C 2 H 2 三组分 气体中分离得到了聚合级高纯乙烯. ...
... The current process for the removal of alkynes from alkenes is partial catalytic hydrogenation. [2] The conversion of alkynes is achieved with noble metal catalysts under high temperature and high pressure; however, this method exhibits poor alkene selectivity and is economically demanding. Additionally, the use of alkynes as important raw chemicals is desirable. ...
Zeolitic octahedral metal oxide is a newly synthesized all‐inorganic zeolitic material and has been used for adsorption, separation, and catalysis. Herein, a new zeolitic octahedral metal oxide was synthesized and characterized. The porous framework was established through the assembly of [P2Mo13O50] clusters with PO4 linkers. Guest molecules occupied the framework, which could be removed through heat treatment, thereby opening the micropores. The pore characteristics were controlled by the cations within the micropore, enabling the adjustment of the interactions with alkynes and alkenes. This resulted in good separation performance of ethylene/acetylene and propylene/propyne even under high temperature and humidity conditions. The high stability of the material enabled the efficient recovery and reuse without discernible loss in the separation performance. Due to the relatively weak interaction between the adsorbed alkyne and the framework, the adsorbent facilitated the recovery of a highly pure alkyne. This feature enhances the practical applicability of the material in various industrial processes.
... Of note, such a framework possesses interconnected structures in which the neatly arranged [Ga-S] chains were connected by the [Mg-S] framework via covalent bonds. Such interconnected structure endowed [Mg/Ga-S] ∞ structural framework with strong chemical stability, which has also been demonstrated in the reported covalent organic frameworks 38 . Additionally, the stable structural framework can be used as a template to accommodate a series of A atoms (A=Li, Na, Ca, Sr, Ba, and even La) while spatially confining these atoms into atomic-scale channels via coordination configurations. ...
Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical properties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tetrahedra single/double chains to construct an octahedral crystal field, we synthesized the stable crystalline La(II)-chalcogenide, LaMg6Ga6S16. The nature of the divalent La²⁺ cations can be identified by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and electron paramagnetic resonance, while the stability is confirmed by the differential thermal scanning, in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La²⁺(5d¹), LaMg6Ga6S16 displays an ultrabroad-band green emission at 500 nm, which is the inaugural instance of La(II)-based compounds demonstrating luminescent properties. Furthermore, as LaMg6Ga6S16 crystallizes in the non-centrosymmetric space group, P−6, it is the second-harmonic generation (SHG) active, possessing a comparable SHG response with classical AgGaS2. In consideration of its wider band gap (Eg = 3.0 eV) and higher laser-induced damage threshold (5×AgGaS2), LaMg6Ga6S16 is also a promising nonlinear optical material.
... The network-like chemistry of covalent organic frameworks enables top-down design at the molecular level, leading to alignments guided by geometric structures [2][3][4]. The advantages of such structural diversity and control over functions are perfectly reflected in COFs, which are widely used in fields such as gas storage [5,6], separation [7][8][9], catalysis [10][11][12][13], optoelectronics [14][15][16][17], sensing [18][19][20] and biological fields [6,21]. It is worth noting that the unique physical and chemical properties of COF materials are closely related to their crystal structures [22,23]. ...
Over the past two decades, covalent organic frameworks (COFs) have become the most widely studied porous crystalline materials. Their specific physical and chemical properties are determined by the arrangement of atoms (crystal structure). Therefore, the determination of their structure is arguably the most important characterization step for COFs. Although single-crystal X-ray diffraction is the most widely used method for structure determination, confirmation of the structure of COFs is limited to lattice fringes in transmission electron microscopy (TEM) because of their small crystal size (nanocrystals) or poor crystal quality. At present, many two-dimensional COFs (2D-COFs) have clear powder X-ray diffraction (PXRD) patterns, but specific lattice fringes are not available for all 2D-COFs. This severely hinders the development of the COF field. Here, we discovered the lattice shrinkage behavior of COFs under electron beam irradiation by comparing the lattice fringes of 2D-COFs under different conditions. By comparing the lattice fringes of a 1,3,5-tris-(4-aminophenyl)triazine-1,3,5-tris-(4-formylphenyl)triazine covalent organic framework (TAPT-TFPT COF) at room temperature and under liquid nitrogen freezing conditions, we found that the lattice fringes are in good agreement with the PXRD and the theoretical values of the COF (2.213 nm) under freezing conditions. However, the lattice fringe spacing is only 1.656 nm at room temperature. The discovery not only provides new insights into the TEM characterization of COFs, but also further expands the range of crystalline COF materials.
... It is worth mentioning that for such COFs, new topologies are discovered mainly by adopting linkers with different con gurations and simply changing building blocks with the same connective polyhedron can hardly produce new topologies. For instance, all of the reported [8(+2)]-c COFs are in bcu or its derived nets [21][22][23][24] , and all of the reported [8+4]-c COFs are in u, scu or tty topologies [25][26][27][28] . ...
Developing three-dimensional (3D) covalent organic frameworks (COFs) has paramount significance across numerous applications. However, the conventional design approach that relies on regular building blocks significantly restricts the structural diversity of COFs. In this study, we successfully designed and synthesized two 3D COFs, named JUC-643 and JUC-644, employing a novel strategy based on irregular 8-connected (8-c) building blocks. By using a continuous rotation electron diffraction technique combined with powder X-ray diffraction patterns, their structures were solved and revealed a unique linkage with double helical structure, a phenomenon previously unreported in COFs. In order to precisely describe the topology, these structures should be deconstructed into the unprecedented [4+3(+2)]-c nets instead of the traditional [8(+2)]-c or [6(+2)]-c net. Furthermore, one of the materials (JUC-644) has demonstrated exceptional adsorption capability towards C3H8 and n-C4H10 (11.28 and 10.45 mmol g-1 at 298 K and 1 bar respectively), surpassing the adsorption performance of all known porous materials, and breakthrough experiments have also highlighted the remarkable C3H8/C2H6 and n-C4H10/C2H6 selectivity. This pioneering concept of incorporating irregular building blocks in 3D COFs introduces a promising avenue for designing intricate architectures while enhancing their potential application in the recovery of C2H6 from natural gas liquids.
... Such interconnected structure endowed [Mg/Ga-S] ∞ structural framework with strong chemical stability, which has also been demonstrated in the reported covalent organic frameworks. 37 Additionally, the stable structural framework can be used as a template to accommodate a series of A atoms (A = Li, Na, Ca, Sr, Ba, and even La) while spatially con ning these atoms into atomic-scale channels via coordination con gurations. In particular, the coordination bond lengths of A-S are in the range of 2.934-3.138 ...
Divalent lanthanide inorganic compounds can exhibit unique electronic configurations and physicochemical proper-ties, yet their synthesis remains a great challenge because of the weak chemical stability. To the best of our knowledge, although several lanthanide monoxides epitaxial thin films have been reported, there is no chemically stable crystalline divalent lanthanide chalcogenide synthesized up to now. Herein, by using octahedra coupling tet-rahedra single/double chains to construct a novel octahedral crystal field, we synthesized the first stable crystalline La(II)-chalcogenide, LaMg6Ga6S16. The nature of the divalent La2+ cations can be identified by X-ray photoelectron spectroscopy, while the stability is confirmed by the in-situ variable-temperature powder X-ray diffraction and a series of solid-state reactions. Owing to the particular electronic characteristics of La2+(5d1), LaMg6Ga6S16 displays a broad-band green emission at 500 nm under the excitation of 360 nm. Clearly, this is the first time for La(II)-based com-pounds to exhibit luminescent properties.
... The network-like chemistry of covalent organic frameworks enables top-down design at the molecular level, leading to alignments guided by geometric structures [1][2][3]. The advantages of such structural diversity and control over functions are perfectly reflected in COFs, which are widely used in fields such as gas storage [4,5], separation [6,7], catalysis [8][9][10], optoelectronics [11][12][13][14], sensing [15][16][17] and biological fields [5,18]. It is worth noting that the unique 2 physical and chemical properties of COF materials are closely related to their crystal structures [19,20]. ...
Over the past two decades, covalent organic frameworks (COFs) have become the most widely studied porous crystalline materials. Its specific physical and chemical properties are determined by the arrangement of atoms (crystal structure). Therefore, the determination of its structure is arguably the most important characterization step for COFs. Although single crystal X-ray diffraction (SCXRD) is the most widely used method for structure determination, the size of COF crystals is too small, or their crystal quality is so poor that confirmation of their structure is limited to lattice fringes by transmission electron microscopy (TEM). At present, many 2D-COFs have clear PXRD patterns, but not all 2D-COFs can obtain specific lattice fringes. This severely hinders the development of the COF field. Here, we discover the lattice shrinkage behavior of COFs under electron beam irradiation by comparing the lattice fringes of 2D-COFs under different conditions. By comparing the lattice fringes of TAPT-TFPT COF at room temperature and under liquid nitrogen freezing conditions, we found that the lattice fringes are in good agreement with the Powder X-ray diffraction (PXRD) and theoretical values of COF (2.213 nm) under freezing conditions, however, the lattice fringe spacing is only 1.656 nm at room temperature. The discovery not only provides new insights into the TEM characterization of COFs, but also further expands the range of crystalline COF materials.
Zeolitic octahedral metal oxide is a newly synthesized all‐inorganic zeolitic material and has been used for adsorption, separation, and catalysis. Herein, a new zeolitic octahedral metal oxide was synthesized and characterized. The porous framework was established through the assembly of [P 2 Mo 13 O 50 ] clusters with PO 4 linkers. Guest molecules occupied the framework, which could be removed through heat treatment, thereby opening the micropores. The pore characteristics were controlled by the cations within the micropore, enabling the adjustment of the interactions with alkynes and alkenes. This resulted in good separation performance of ethylene/acetylene and propylene/propyne even under high temperature and humidity conditions. The high stability of the material enabled the efficient recovery and reuse without discernible loss in the separation performance. Due to the relatively weak interaction between the adsorbed alkyne and the framework, the adsorbent facilitated the recovery of a highly pure alkyne. This feature enhances the practical applicability of the material in various industrial processes.
3D covalent organic frameworks (3D COFs) constitute a new type of crystalline materials that consist of a range of porous structures with numerous applications in the fields of adsorption, separation, and catalysis. However, because of the complexity of the three‐periodic net structure, it is desirable to develop a thorough structural comprehension, along with a means to precisely determine the actual structure. Indeed, such advancements would considerably contribute to the rational design and application of 3D COFs. In this review, the reported topologies of 3D COFs are introduced and categorized according to the configurations of their building blocks, and a comprehensive overview of diffraction‐based structural determination methods is provided. The current challenges and future prospects for these materials will also be discussed.
Discovery of new topology covalent organic frameworks (COFs) is a mainstay in reticular chemistry and materials research because it not only serves as a stepwise guide to the designed construction of covalent‐organic architectures but also helps to comprehend function from structural design point‐of‐view. Proceeding on this track, the first 3D COF, TUS‐38, with the topology is constructed by reticulating a planar triangular 3‐c node of D3h symmetry with a tetragonal prism 8‐c node of D2h symmetry via [3 + 8] reversible imine condensation reaction. TUS‐38 represents a twofold interpenetrated multidirectional pore network with a high degree of crystallinity and structural integrity. Interestingly, stemming from the nitrogen‐rich s‐triazine rings with electron‐deficient character and ─C ═ N─ linkages composing the TUS‐38 framework that benefit to the charge–transfer and hence dipole–dipole electrostatic interactions between the framework and iodine in addition to exclusive topological characteristics of the exotic the net, TUS‐38 achieves an exemplary capacity for iodine vapor uptake reaching 6.3 g g⁻¹. Recyclability studies evidence that TUS‐38 can be reused at least five times retaining 95% of its initial adsorption capacity; while density functional theory (DFT) calculations have heightened the understanding of the interactions between iodine molecules and the framework.
Artificial photosynthesis has been regarded as a promising solution toward solar energy conversion, generating storable and transportable chemical fuels such as hydrogen (H2) and hydrogen peroxide (H2O2). However, the design of robust catalytic sites not only affects the activity, but also identify the atomic‐level correlation between active sites and natural photosynthesis performance. Herein, a synthesis method of single‐atomic Iron (Fe) active sites anchored on novel covalent organic framework (COF) for the production of H2O2 under visible light irradiation. When benzyl alcohol is the most sacrificial agent, the state‐of‐the‐art Fe‐based COF exhibits an excellent H2O2 generation rate of 4130 µmol g⁻¹ h⁻¹, over 5.3 times higher than that of pristine COF, achieving an apparent quantum yield of 6.4% at 420 nm. The enhanced photocatalytic performance is ascribed to the synergistic effect of atomically dispersed Fe sites and COF hosts, reducing the reaction energy barrier for the formation of *OOH intermediates and optimizing the adsorption of O2 and thus promoting two‐electron oxygen reduction reaction (ORR). This work establishes an atomic‐level engineering approach to build atomically dispersed Fe active sites on COF photocatalysts and provides in‐depth insight upon the ORR mechanism for promising artificial photosynthesis of H2O2.
Due to the high surface area and uniform porosity of covalent organic frameworks (COFs), they exhibit superior properties in capturing and detecting even trace amounts of gases in the air. However, the COFs materials that possess dual detected functionality are still less reported. Here, an imine‐based COF containing thiophene as a donor and triazine as an acceptor to form spatial‐distribution‐defined D‐A structures was prepared. D‐A system between thiophene and triazine facilitates the charge transfer process during the protonation process of the imine and the triazine units. The obtained COF exhibits simultaneous sensing ability toward both acidic and alkaline vapors with obvious colorimetric sensing functionality.
Redox‐active tetrathiafulvalene (TTF)‐based covalent organic frameworks (COFs) exhibit distinctive electrochemical and photoelectrical properties, but their prevalent two‐dimensional (2D) structure with densely packed TTF moieties limits the accessibility of redox center and constrains their potential applications. To overcome this challenge, an 8‐connected TTF linker (TTF‐8CHO) is designed as a new building block for the construction of three‐dimensional (3D) COFs. This approach led to the successful synthesis of a 3D COF with the bcu topology, designated as TTF‐8CHO‐COF. In comparison to its 2D counterpart employing a 4‐connected TTF linker, the 3D COF design enhances access to redox sites, facilitating controlled oxidation by I2 or Au³⁺ to tune physical properties. When irradiated with a 0.7 W cm⁻² 808 nm laser, the oxidized 3D COF samples (IX−${\mathrm{I}}_{\mathrm{X}}^{-}$@TTF‐8CHO‐COF and Au NPs@TTF‐8CHO‐COF) demonstrated rapid temperature increases of 239.3 and 146.1 °C, respectively, which surpassed those of pristine 3D COF (65.6 °C) and the 2D COF counterpart (6.4 °C increment after I2 treatment). Furthermore, the oxidation of the 3D COF heightened its photoelectrical responsiveness under 808 nm laser irradiation. This augmentation in photothermal and photoelectrical response can be attributed to the higher concentration of TTF·+ radicals generated through the oxidation of well‐exposed TTF moieties.
Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.
Noted for their unique structural and functional
attributes including especially high surface areas, open interconnected
nanochannels, and rich active sites, three-dimensional (3D) covalent
organic frameworks (COFs) have flourished as frontier materials for
energy, environmental, and biomedical research. However, the narrow
options of 3D organic building blocks, poor reversibility in covalent
linkage formation, and highly complicated crystal structure analysis
raise several challenges to the construction of 3D COFs for which the
number of 3D COF structures is still restricted to a few. Here, we
report on the design and synthesis strategy to develop two 3D COFs,
namely, TUS-440 and TUS-441, with 2- and 3-fold interpenetrated pts topology, respectively, by combining a Td-symmetric
tetrahedral vertex with two C2-symmetric rectangular linkers. The resulting COFs demonstrate well-defined crystalline porous
structures and Brunauer−Emmett−Teller surface areas of 1320 and 1016 m2 g−1
, respectively. In vitro drug delivery studies
substantiate these COFs as efficient nanocarriers with good loading and sustained release of anticancer drugs (cytarabine and 5-
fluorouracil), showing great potential for increasing therapeutic efficacy and reducing the dosing frequency.
Compared with two-dimensional (2D) covalent organic frameworks (COFs), three-dimensional (3D) COFs have significant advantages, such as larger specific surface areas, complex interconnected structures, and fully accessible active sites, which have...
Developing three-dimensional (3D) covalent organic frameworks (COFs) has paramount significance across numerous applications. However, the conventional design approach that relies on regular building blocks significantly restricts the structural diversity of COFs. In this study, we successfully designed and synthesized two 3D COFs, named JUC-643 and JUC-644, employing a novel strategy based on irregular 8-connected (8-c) building blocks. By using a continuous rotation electron diffraction technique combined with powder X-ray diffraction patterns, their structures were solved and revealed a unique linkage with double helical structure, a phenomenon previously unreported in COFs. In order to precisely describe the topology, these structures should be deconstructed into the unprecedented [4+3(+2)]-c nets instead of the traditional [8(+2)]-c or [6(+2)]-c net. Furthermore, one of the materials (JUC-644) has demonstrated exceptional adsorption capability towards C3H8 and n-C4H10 (11.28 and 10.45 mmol g-1 at 298 K and 1 bar respectively), surpassing the adsorption performance of all known porous materials, and breakthrough experiments have also highlighted the remarkable C3H8/C2H6 and n-C4H10/C2H6 selectivity. This pioneering concept of incorporating irregular building blocks in 3D COFs introduces a promising avenue for designing intricate architectures while enhancing their potential application in the recovery of C2H6 from natural gas liquids.
CO2 concentration in the atmosphere has increased by about 40% since the 1960s. Among various technologies available for carbon capture, adsorption and membrane processes have been receiving tremendous attention due to their potential to capture CO2 at low costs. The kernel for such processes is the sorbent and membrane materials, and tremendous progress has been made in designing and fabricating novel porous materials for carbon capture. Covalent organic frameworks (COFs), a class of porous crystalline materials, are promising sorbents for CO2 capture due to their high surface area, low density, controllable pore size and structure, and preferable stabilities. However, the absence of synergistic developments between materials and engineering processes hinders achieving the qualitative leap for net-zero emissions. Considering the lack of a timely review on the combination of state-of-the-art COFs and engineering processes, in this Tutorial Review, we emphasize the developments of COFs for meeting the challenges of carbon capture and disclose the strategies of fabricating COFs for realizing industrial implementation. Moreover, this review presents a detailed and basic description of the engineering processes and industrial status of carbon capture. It highlights the importance of machine learning in integrating simulations of molecular and engineering levels. We aim to stimulate both academia and industry communities for joined efforts in bringing COFs to practical carbon capture.
Covalent organic frameworks (COFs) provide a unique platform with tunable structures allowing precise control of pore sizes, shapes and functions. The key to synthesizing COFs with desired structures is to precisely control the conformation and geometry of building blocks as well as the growth direction of COFs. To achieve this, steric effects are noteworthy that may have a significant impact on the assembly of COFs. Specifically, the introduction of sterically demanding substituents or bulky groups into monomers of COFs will lead to intramolecular conformational changes and intermolecular repulsions, which induce structural changes in COFs, including changes in torsion angles, interlayer distances, stacking modes and topologies of 2D COFs, and changes in spatial nodes, interpenetration and topologies of 3D COFs. This review will help to understand the impacts of steric effects on the structures of COFs and to take them into extensive consideration in the design and synthesis of COFs with novel functionalities and structural attributes.
Three-dimensional covalent organic frameworks (3D COFs), with interconnected pores and exposed functional groups, provide new opportunities for the design of advanced functional materials through postsynthetic modification. Herein, we demonstrate the successful postsynthetic annulation of 3D COFs to construct efficient CO2 reduction photocatalysts. Two 3D COFs, NJU-318 and NJU-319Fe, were initially constructed by connecting hexaphenyl-triphenylene units with pyrene- or Fe-porphyrin-based linkers. Subsequently, the hexaphenyl-triphenylene moieties within the COFs were postsynthetically transformed into π-conjugated hexabenzo-trinaphthylene (pNJU-318 and pNJU-319Fe) to enhance visible light absorption and CO2 photoreduction activity. The optimized photocatalyst, pNJU-319Fe, shows a CO yield of 688 μmol g-1, representing a 2.5-fold increase compared to that of unmodified NJU-319Fe. Notably, the direct synthesis of hexabenzo-trinaphthylene-based COF catalysts was unsuccessful due to the low solubility of conjugated linkers. This study not only provides an effective method to construct photocatalysts but also highlights the unlimited tunability of 3D COFs through structural design and postsynthetic modification.
The polypyrene polymer with an extended π-conjugated skeleton is attractive for perfluorinated electron specialty gas (F-gas) capture as the high electronegativity of fluorine atoms makes F-gases strongly electronegative gases. Herein, a polypyrene porous organic framework (termed as Ppy-POF) with an extended π-conjugated structure and excellent acid resistance was constructed. Systematic studies have shown that the abundant π-conjugated structures and gradient electric field distribution in Ppy-POF can endow it exceptional adsorption selectivity for high polarizable F-gases and xenon (Xe), which has been collaboratively confirmed by single-component gas adsorption experiments, time-dependent adsorption rate tests, dynamic breakthrough experiments, etc. Electrostatic potential distribution and charge density difference based on Grand Canonical Monte Carlo simulations and density functional theory calculations demonstrate that the selective adsorption of F-gases and Xe in Ppy-POF is attributed to the strong charge-transfer effect and polarization effect between Ppy-POF and gases. These results manifest that the POF with an extended π-conjugated structure and gradient electric field distribution has great potential in efficiently capturing electron specialty gases.
Noted for their unique structural and functional attributes including especially high surface areas, open interconnected nanochannels and rich active sites, three-dimensional (3D) covalent organic frameworks (COFs) have emerged as frontier materials for energy, environmental and biomedical research. However, the limited options of 3D organic building blocks, poor reversibility in covalent linkage formation and highly complicated crystal structure analysis raise several challenges to the construction of new 3D COFs for which the number of 3D COF structures is still restricted to a few. In this contribution, we report the designed synthesis of two new 3D COFs, namely TUS-440 and TUS-441, with two- and three-fold interpenetrated pts topology, respectively, by combining a Td-symmetric tetrahedral vertex with two C2-symmetric rectangular linkers. The resulting COFs demonstrate well-defined crystalline porous structures and Brunauer−Emmett−Teller surface areas of 1320 and 940 m2 g-1. In vitro drug delivery studies substantiate these COFs as efficient nanocarriers with good loading and sustained release of anticancer drugs (cytarabine and 5-fluorouracil), showing great potential for increasing therapeutic efficacy and reducing dosing frequency.
Lithium-sulfur batteries (LSBs) have been considered as a promising candidate for next-generation energy storage devices, which however still suffer from the shuttle effect of the intermediate lithium polysulfides (LiPSs). Covalent-organic frameworks (COFs) have exhibited great potential as sulfur hosts for LSBs to solve such a problem. Herein, a pentiptycene-based D2h symmetrical octatopic polyaldehyde, 6,13-dimethoxy-2,3,9,10,18,19,24,25-octa(4'-formylphenyl)pentiptycene (DMOPTP), was prepared and utilized as a building block toward preparing COFs. Condensation of DMOPTP with 4-connected tetrakis(4-aminophenyl)methane affords an expanded [8 + 4] connected network 3D-flu-COF, with a flu topology. The non-interpenetrated nature of the flu topology endows 3D-flu-COF with a high Brunauer-Emmett-Teller surface area of 2860 m2 g-1, large octahedral cavities, and cross-linked tunnels in the framework, enabling a high loading capacity of sulfur (∼70 wt %), strong LiPS adsorption capability, and facile ion diffusion. Remarkably, when used as a sulfur host for LSBs, 3D-flu-COF delivers a high capacity of 1249 mA h g-1 at 0.2 C (1.0 C = 1675 mA g-1), outstanding rate capability (764 mA h g-1 at 5.0 C), and excellent stability, representing one of the best results among the thus far reported COF-based sulfur host materials for LSBs and being competitive with the state-of-the-art inorganic host materials.
Metal covalent organic frameworks (MCOFs) as a bridge between covalent organic frameworks (COFs) and metal organic frameworks (MOFs) have recently attracted much attention. Though the COFs with metal ions in...
Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.
Three metal covalent organic frameworks (MCOFs), namely RuCOF‐ETTA, RuCOF‐TPB and RuCOF‐ETTBA, were synthesized by incorporating the photosensitive RuII tris(2,2′‐bipyridine) unit into the skeleton. Interestingly, each RuCOF contains three isostructural covalent organic frameworks that interlock together with the RuII centers serving as point of registry. The covalently linked network coupling with uniformly distributed RuII units allowed the RuCOFs to exhibit superior chemical stability, strong light‐harvesting ability, and high photocatalytic activity toward hydrogen evolution (20 308 μmol g⁻¹ h⁻¹). This work illustrates the potential of developing versatile MCOFs‐based photocatalysts from functionalized metal complex building unit and further enriches the MCOFs family.
Covalent organic frameworks (COFs) have attracted extensive interest due to their unique structures and various applications. However, structural diversities are still limited, which greatly restricts the development of COF materials. Herein, we report two unusual cubic (8-connected) building units and their derived 3D imine-linked COFs with bcu nets, JUC-588 and JUC-589. Owing to these unique building blocks with different sizes, the obtained COFs can be tuned to be microporous or mesoporous structures with high surface areas (2728 m² g⁻¹ for JUC-588 and 2482 m² g⁻¹ for JUC-589) and promising thermal and chemical stabilities. Furthermore, the high selectivity of CO2/N2 and CO2/CH4, excellent H2 uptakes, and efficient dye adsorption are observed. This research thus provides a general strategy for constructing stable 3D COF architectures with adjustable pores via improving the valency of rigid building blocks.
The separation of C2H2/CO2 is not only industrially important for acetylene purification but also great scientific challenge due to their very similar molecular size and physical properties. To address this difficulty, herein, we present an ultramicroporous hydrogen‐bonded organic framework (HOF‐FJU‐1) from tetracyano bicarbazole to separate C2H2 from CO2 by taking advantage of differences in their electrostatic potential distribution. This material possesses a suitable pore environment and electrostatic potential distribution fitting well to C2H2, thus showing extra strong affinity to C2H2 (46.73 kJ mol⁻¹) and the highest IAST selectivity of 6675 for C2H2/CO2 separation among the adsorbents reported. The single crystal X‐ray diffraction reveals that the suitable pore environment in HOF‐FJU‐1 provides multiple C−H⋅⋅⋅π and hydrogen‐bonded interactions N⋅⋅⋅H−C with C2H2 molecules. Dynamic breakthrough experiments demonstrate its outstanding separation performance to C2H2/CO2 mixtures.
The structural diversity of three‐dimensional (3D) covalent organic frameworks (COFs) are limited as there are only a few choices of building units with multiple symmetrically distributed connection sites. To date, 4 and 6‐connected stereoscopic nodes with Td, D3h, D3d and C3 symmetries have been mostly reported, delivering limited 3D topologies. We propose an efficient approach to expand the 3D COF repertoire by introducing a high‐valency quadrangular prism (D4h) stereoscopic node with a connectivity of eight, based on which two isoreticular 3D imine‐linked COFs can be created. Low‐dose electron microscopy allows the direct visualization of their 2‐fold interpenetrated bcu networks. These 3D COFs are endowed with unique pore architectures and strong molecular binding sites, and exhibit excellent performance in separating C2H2/CO2 and C2H2/CH4 gas pairs. The introduction of high‐valency stereoscopic nodes would lead to an outburst of new topologies for 3D COFs.
The practical applications of crystalline porous materials toward industrial separations require shaping them into robust monoliths (e.g., foams). The current methods are often complicated, environmentally unfriendly, and unscalable, significantly hampering their practical application. Herein, we report a scalable, facile ‘one-step thermomolding’ method to fab-ricate a class of hierarchically olefin-linked COF foams via melt polymerization. The obtained pure COF foams possess higher crystallinity and porosity than literature, good moldability, and processability, greatly benefiting their practical applications (e.g., efficient oil-water separation with 99% removal efficiency and >100 cycle reusability). Moreover, a COF exhibiting the smallest (0.58 nm) eclipsed stacking pore among all 2-dimensional COFs with neutral skeletons is first fabricated via this protocol, demonstrating good C2H2 purification performance and recyclability, surpassing the current benchmark materials. We believe that this facile fabrication strategy can promote access to a broad diversity of new COF foams and enlighten further avenues for greener routes to large-scale synthesis and usage of COFs.
Propyne/propylene (C3H4/C3H6) separation is an important but challenging industrial process to produce polymer‐grade C3H6 and recover high‐purity C3H4. Herein, we report an ultrastable TiF6²⁻ anion cross‐linked metal–organic framework (ZNU‐2) with precisely controlled pore size, shape and functionality for benchmark C3H4 storage (3.9/7.7 mmol g⁻¹ at 0.01/1.0 bar and 298 K) and record high C3H4/C3H6 (10/90) separation potential (31.0 mol kg⁻¹). The remarkable C3H4/C3H6 (1/99, 10/90, 50/50) separation performance was fully demonstrated by simulated and experimental breakthroughs under various conditions with excellent recyclability and high productivity (42 mol kg⁻¹) of polymer‐grade C3H6 from a 1/99 C3H4/C3H6 mixture. A modelling study revealed that the symmetrical spatial distribution of six TiF6²⁻ on the icosahedral cage surface provides two distinct binding sites for C3H4 adsorption: one serves as a tailored single C3H4 molecule trap and the other boosts C3H4 accommodation by cooperative host–guest and guest–guest interactions.
It is highly challenging to construct three-dimensional (3D) crystalline covalent organic frameworks (COFs) with flexible building blocks and further explore their tunable or adaptive characteristics due to crystallization and structure determination difficulties. Herein, we constructed three crystalline isostructural 3D-OC-COFs based on a newly synthesized flexible organic cage ( 6NH2-OC·4HCl) through a novel “in-situ acid-base neutralization strategy”. Interestingly, network conformations could be fine-tuned by rationally introducing different substituents into the starting dialdehydes during the assembly process, resulting in one expanded structure ( 3D-OC-COF-H) and two different contracted structures ( 3D-OC-COF-OH and 3D-OC-COF-Cl), which mainly ascribed to the hinge-like motions of organic cage building blocks. By virtue of the unique pore environments and different functional groups, we further employed these COFs for CO2 capture, of which 3D-OC-COF-OH exhibited superior CO2/CH4 separation performance. This study not only demonstrates a new strategy for constructing 3D cage-based COFs, but also identifies a novel factor that affects flexible building blocks for realizing structural diversity.
Selective separation of propyne/propadiene mixture to obtain pure propadiene (allene), an essential feedstock for organic synthesis, remains an unsolved challenge in the petrochemical industry, thanks mainly to their similar physicochemical properties. We herein introduce a convenient and energy-efficient physisorptive approach to achieve propyne/propadiene separation using microporous metal-organic frameworks (MOFs). Specifically, HKUST-1, one of the most widely studied high surface area MOFs that is available commercially, is found to exhibit benchmark performance (propadiene production up to 69.6 cm ³ /g, purity > 99.5%) as verified by dynamic breakthrough experiments. Experimental and modeling studies provide insight into the performance of HKUST-1 and indicate that it can be attributed to a synergy between thermodynamics and kinetics that arises from abundant open metal sites and cage-based molecular traps in HKUST-1.
Self‐assembly of N,N,N′,N′‐tetrakis(4‐carboxyphenyl)‐1,4‐phenylenediamine with the help of different solvents provides isostructural hydrogen‐bonded organic frameworks (HOF‐30). Single‐crystal X‐ray diffraction (SCXRD) analysis reveals HOF‐30 possesses 3D ten‐fold interpenetrated dia nets connected by two kinds of hydrogen bonds, namely solvent‐bridged carboxyl dimers and carboxyl⋅⋅⋅carboxyl dimers. Degassing treatment for HOF‐30 yields HOF‐30a with 3D ten‐fold interpenetrated dia nets but linked with sole carboxyl⋅⋅⋅carboxyl dimers. Reversible hydrogen‐bond‐to‐hydrogen‐bond transformation between solvent‐bridged carboxyl dimers in HOF‐30 and carboxyl⋅⋅⋅carboxyl dimers in HOF‐30a has been unveiled by single‐crystal and powder X‐ray diffraction. In addition, HOF‐30a enables the selective adsorption of propyne over propylene according to single‐component sorption and breakthrough experiments. The preferred propyne location in HOF has also been identified by SCXRD test.
For the separation of ethane from ethylene, it remains challenging to target both high C2H6 adsorption and selectivity in a C2H6‐selective material. Herein, we report a reversible solid‐state transformation in a labile hydrogen‐bonded organic framework to generate a new rod‐packing desolvated framework (ZJU‐HOF‐1) with suitable cavity spaces and functional surfaces to optimally interact with C2H6. ZJU‐HOF‐1 thus exhibits simultaneously high C2H6 uptake (88 cm³ g⁻¹ at 0.5 bar and 298 K) and C2H6/C2H4 selectivity (2.25), which are significantly higher than those of most top‐performing materials. Theoretical calculations revealed that the cage‐like cavities and functional sites synergistically “match” better with C2H6 to provide stronger multipoint interactions with C2H6 than C2H4. In combination with its high stability and ultralow water uptake, this material can efficiently capture C2H6 from 50/50 C2H6/C2H4 mixtures in ambient conditions under 60 % RH, providing a record polymer‐grade C2H4 productivity of 0.98 mmol g⁻¹.
Propylene is one of the world’s most important basic olefin raw material used in the production of a vast array of polymers and other chemicals. The need for high purity grade of propylene is essential and traditionally achieved by the very energy-intensive cryogenic separation. In this study, a pillared inorganic anion SIF62− was used as a highly selective C3H4 due to the square grid pyrazine-based structure. Single gas adsorption revealed a very high C3H4 uptake value (3.32, 3.12, 2.97 and 2.43 mmol·g−1 at 300, 320, 340 and 360 K, respectively). The values for propylene for the same temperatures were 2.73, 2.64, 2.31 and 1.84 mmol·g−1, respectively. Experimental results were obtained for the two gases fitted using Langmuir and Toth models. The former had a varied degree of representation of the system with a better presentation of the adsorption of the propylene compared to the propyne system. The Toth model regression offered a better fit of the experimental data over the entire range of pressures. The representation and fitting of the models are important to estimate the energy in the form of the isosteric heats of adsorption (Qst), which were found to be 45 and 30 kJ·Kmol−1 for propyne and propylene, respectively. A Higher Qst value reveals strong interactions between the solid and the gas. The dynamic breakthrough for binary mixtures of C3H4/C3H6 (30:70 v/v)) were established. Heavier propylene molecules were eluted first from the column compared to the lighter propyne. Vacuum swing adsorption was best suited for the application of strongly bound materials in adsorbents. A six-step cycle was used for the recovery of high purity C3H4 and C3H6. The VSA system was tested with respect to changing blowdown time and purge time as well as energy requirements. It was found that the increase in purge time had an appositive effect on C3H6 recovery but reduced productivity and recovery. Accordingly, under the experimental conditions used in this study for VSA, the purge time of 600 s was considered a suitable trade-off time for purging. Recovery up to 99%, purity of 98.5% were achieved at a purge time of 600 s. Maximum achieved purity and recovery were 97.4% and 98.5% at 100 s blowdown time. Energy and power consumption varied between 63–70 kWh/ton at the range of purge and blowdown time used. The VSA offers a trade-off and cost-effective technology for the recovery and separation of olefins and paraffin at low pressure and high purity.
C3 hydrocarbons (HCs), especially propylene and propane, are high‐volume products of the chemical industry as they are utilized for the production of fuels, polymers, and chemical commodities. Demand for C3 HCs as chemical building blocks is increasing but obtaining them in sufficient purity (>99.95%) for polymer and chemical processes requires economically and energetically costly methods such as cryogenic distillation. Adsorptive separations using porous coordination networks (PCNs) could offer an energy‐efficient alternative to current technologies for C3 HC purification because of the lower energy footprint of sorbent separations for recycling versus alternatives such as distillation, solvent extraction, and chemical transformation. In this review, we address how the structural modularity of porous PCNs makes them amenable to crystal engineering that in turn enables control over pore size, shape, and chemistry. We detail how control over pore structure has enabled PCN sorbents to offer benchmark performance for C3 separations thanks to several distinct mechanisms, each of which is highlighted. We also discuss the major challenges and opportunities that remain to be addressed before the commercial development of PCNs as advanced sorbents for C3 separation becomes viable.
Covalent organic frameworks, cross-linked crystalline polymers constructed from rigid organic precursors connected by covalent interactions, have emerged as a promising class of nanoporous materials owing to their highly desirable combination of attributes, including facile chemical tunability, structural diversity, and excellent stability. Despite the distinct advantages offered by three-dimensional covalent organic frameworks, research efforts have predominantly focused on the more synthetically-accessible, two-dimensional variants. Here we present an overview of synthetic approaches to yield three-dimensional covalent organic frameworks, identify synthetic obstacles that have hindered progress in the field and recently-employed methods to address them, and propose alternative techniques to circumvent these synthetic challenges.
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. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
A trace amount of acetylene (C2H2, 1%) removal from ethylene/acetylene mixture is crucial for the production of polymer-grade ethylene (C2H4), which is a highly challenging task and currently is mainly realized via highly energy-intensive technologies. Herein, we first report the highly efficient trapping of C2H2 from C2H2/C2H4 mixtures using two hexafluorogermanate (GeF62–, GeFSIX) anion-functionalized hybrid ultramicroporous materials, GeFSIX-2-Cu-i and GeFSIX-14-Cu-i (also termed ZU-32 and ZU-33), as novel adsorbents. These GeFSIX materials exhibit high thermal stability and tunable pore structures and have a high density of electronegative GeF62– anions decorated on the pore surface. ZU-32 with an aperture size of 4.5 Å exhibits preferential binding ability for C2H2 molecules and thus offers an excellent separation selectivity of 67 for the C2H2/C2H4 (1/99) mixture. ZU-33 with 4,4-azopyridine (azpy, 9.0 Å) as an organic linker exhibits a contracted pore window size with pyridine ring tilting of ca. 30°, which could efficiently block C2H4 molecules but still permit C2H2 molecules trapping into the pore channels. The strong hydrogen-bonding interactions between GeF62– and C2H2 and cooperative van der Waals (vdW) interactions between organic linkers and C2H2 enable ZU-33 to exhibit a record high C2H2 volumetric uptake of 61.5 cm³ cm–3 at 0.01 bar and 298 K with separation selectivity over 1100 for C2H2/C2H4 (1/99) mixture. The actual separation performances of GeFSIX materials are evaluated by conducting experimental breakthrough tests. A total of 99.9999% C2H4 can be obtained from a C2H2/C2H4 (1/99) mixture by one-step column adsorption using GeFSIX materials as adsorbents with a C2H4 productivity up to 1662.1 mL g–1. The binding sites of GeFSIX materials for C2H2 molecules were investigated by first-principles density functional theory (DFT) calculations. This work not only indicates that GeFSIX materials are lead candidates for the separation of C2H2/C2H4 mixture but also are attractive adsorbents for the separation of other gas mixtures containing acidic or polar gaseous components.
Propyne/propylene (C3H4/C3H6) separation is a critical process for the production of polymer-grade C3H6. However, optimization of the structure of porous materials for the highly efficient removal of C3H4 from C3H6 remains challenging due to their similar structures and ultralow C3H4 concentration. Here, it is first reported that hybrid ultramicroporous materials with pillared inorganic anions (SiF6²⁻ = SIFSIX, NbOF5²⁻ = NbOFFIVE) can serve as highly selective C3H4 traps for the removal of trace C3H4 from C3H6. Especially, it is revealed that the pyrazine-based ultramicroporous material with square grid structure for which the pore shape and functional site disposition can be varied in 0.1–0.5 Å scale to match both the shape and interacting sites of guest molecule is an interesting single-molecule trap for C3H4 molecule. The pyrazine-based single-molecule trap enables extremely high C3H4 uptake under ultralow concentration (2.65 mmol g⁻¹ at 3000 ppm, one C3H4 per unit cell) and record selectivity over C3H6 at 298 K (>250). The single-molecule binding mode for C3H4 within ultramicroporous material is validated by X-ray diffraction experiments and modeling studies. The breakthrough experiments confirm that anion-pillared ultramicroporous materials set new benchmarks for the removal of ultralow concentration C3H4 (1000 ppm on SIFSIX-3-Ni, and 10 000 ppm on SIFSIX-2-Cu-i) from C3H6.
Most olefins (e.g., ethylene and propylene) will continue to be produced through steam cracking (SC) of hydrocarbons in the coming decade. In an uncertain commodity market, the chemical industry is investing very little in alternative technologies and feedstocks because of their current lack of economic viability, despite decreasing crude oil reserves and the recognition of global warming. In this perspective, some of the most promising alternatives are compared with the conventional SC process, and the major bottlenecks of each of the competing processes are highlighted. These technologies emerge especially from the abundance of cheap propane, ethane, and methane from shale gas and stranded gas. From an economic point of view, methane is an interesting starting material, if chemicals can be produced from it. The huge availability of crude oil and the expected substantial decline in the demand for fuels imply that the future for proven technologies such as Fischer-Tropsch synthesis (FTS) or methanol to gasoline is not bright. The abundance of cheap ethane and the large availability of crude oil, on the other hand, have caused the SC industry to shift to these two extremes, making room for the on-purpose production of light olefins, such as by the catalytic dehydrogenation of propane.
Covalent organic frameworks (COFs), formed by reversible condensation of rigid organic building blocks, are crystalline and porous materials of great potential for catalysis and organic electronics. Particularly with a view of organic electronics, achieving a maximum degree of crystallinity and large domain sizes while allowing for a tightly π-stacked topology would be highly desirable. We present a design concept that uses the 3D geometry of the building blocks to generate a lattice of uniquely defined docking sites for the attachment of consecutive layers, thus allowing us to achieve a greatly improved degree of order within a given average number of attachment and detachment cycles during COF growth. Synchronization of the molecular geometry across several hundred nanometers promotes the growth of highly crystalline frameworks with unprecedented domain sizes. Spectroscopic data indicate considerable delocalization of excitations along the π-stacked columns and the feasibility of donor-acceptor excitations across the imine bonds. The frameworks developed in this study can serve as a blueprint for the design of a broad range of tailor-made 2D COFs with extended π-conjugated building blocks for applications in photocatalysis and optoelectronics.
Since the intrinsic topological network determines their pore characteristics and functional applications, it is important to construct 3D COFs with target topology from predesigned functional building blocks. However, when starting from precursors with same connectivity but different bulky groups, the structure and topology of 3D COFs may change, which will greatly increase the complexity of the crystal structure determination. Therefore, it is essential to understand how to control the steric hindrance effects and synthesize 3D COFs with target topology. Herein, we report the topology control of 3D COFs by adjusting steric hindrance effects of functional building blocks. Starting from a quadrilateral building block with sterically hindered phenyl groups, we have been able to achieve the target pts topology instead of the unprecedented ljh network that we reported recently by elongating the tetrahedral building block. This result clearly shows that it is promising to precisely control the topology of 3D COFs by judiciously selecting building blocks with steric hindrance and suitable dimensions.
Reticular chemistry allows the control of crystalline frameworks at atomic precision according to the predesigned topological structures. However, only a limited number of topological structures of three-dimensional (3D) covalent organic frameworks (COFs) have been established. In this work, we developed a series of 3D COFs with an unprecedented she topology, which were constructed with D3d- and D4h-symmetric building blocks. The resulting COFs crystallize in a space group of Im3̅m, in which each D3d unit connects with six D4h units to form a noninterpenetrated network with a uniform pore size of 2.0 nm. In addition, these COFs exhibited high crystallinity, excellent porosity, and good chemical and thermal stability. The crystalline structures, composition, and physicochemical properties of these networks were unambiguously characterized. Notably, the inbuilt porphyrin units render these COFs as efficient catalysts for photoredox C-C bond forming and photocatalytic carbon dioxide reduction reactions. Thus, this work constitutes a new approach for the construction of 3D she-net COFs and also enhances the structural diversity and complexity of COFs.
Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 × 10-5 S cm-1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer-Emmett-Teller (BET) surface area of 2340 and 1602 m2 g-1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium-sulfur batteries (LSBs) with high capacities (1035-1155 mA h g-1 at 0.2 C, 1 C = 1675 mA g-1), excellent rate capabilities (713-757 mA h g-1 at 5.0 C), and superior cycling stability (71-83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.
The exploration of highly crystalline three-dimensional (3D) covalent organic frameworks (COFs) with new topologies remains challenging. In this work, we rationally designed and synthesized two highly crystalline 3D COFs, constructed by an octatopic linker and porphyrin-based tetratopic linkers through an [8 + 4] approach. The COF structures were successfully determined as non-interpenetrated scu topology using the continuous rotation electron diffraction (cRED) technique and structural simulation. The scu network was further verified by both high-resolution transmission electron microscopy (TEM) and pore size distribution based on N2 sorption isotherms. Due to the exposed catalytic porphyrin sites, good photoelectric activity, and high structure robustness, these COFs can serve as highly efficient heterogeneous photocatalysts for various reactions, including oxidative amine coupling and cycloaddition reactions between tertiary aniline and maleimide, with a broad substrate scope (22 examples). This work enriches the topological varieties of 3D COFs and provides a class of highly efficient photocatalysts.
Rational design of covalent organic frameworks (COFs) to broaden their diversity is highly desirable but challenging due to the limited, expensive, and complex building blocks, especially compared with other easily available porous materials. In this work, we fabricated two novel bioinspired COFs, namely, NUS-71 and NUS-72, using reticular chemistry with ellagic acid and triboronic acid-based building blocks. Both COFs with AB stacking mode exhibit high acetylene (C2H2) adsorption capacity and excellent separation performance for C2H2/CO2 mixtures, which is significant but rarely explored using COFs. The impressive affinities for C2H2 appear to be related to the sandwich structure formed by C2H2 and the host framework via multiple host-guest interactions. This work not only represents a new avenue for the construction of low-cost COFs but also expands the variety of the COF family using natural biochemicals as building blocks for broad application.
The connectivity of building units for 3D covalent organic frameworks (COFs) has long been primarily 4 and 6, which have severely curtailed the structural diversity of 3D COFs. Here we demonstrate the successful design and synthesis of a porphyrin based, 8-connected building block with cubic configuration, which could be further reticulated into an unprecedented interpenetrated pcb topology by imine condensation with linear amine monomers. This study presents the first case of high-connectivity building units bearing 8-connected cubic nodes, thus greatly enriching the topological possibilities of 3D COFs.
3D covalent organic frameworks (COFs) represent a unique class of crystalline porous materials that allow the precise integration of organic molecular building blocks into 3D networks through the formation of covalent bonds. Considering their numerous open sites and hierarchical pore structures, 3D COFs have shown interesting potential in many areas, especially gas adsorption and catalysis. However, the chemistry of 3D COFs has been restrained largely due to their limited structural diversity and complicated structural determination. In this review, we summarize the key strategies and techniques used to diversify and determine the structure of 3D COFs. Finally, the remaining challenges and prospects concerning the structural design and determination of 3D COFs are presented.
Highly efficient removal of trace propyne (C3H4) (propyne <1000 ppm) from propylene (C3H6) is an essential and challenging industrial process due to the high molecular similarity of C3H4 and C3H6. Herein, we created a new ultramicroporous metal-organic framework (NKMOF-11) with exceptional water stability, superior C3H4 bind affinity, and ultrahigh uptake capacity of C3H4 at ultra-low pressure (0.1 mbar). Modelling studies unveiled that the excellent performance of NKMOF-11 can be attributed to the suitable pore aperture and unique binding sites for C3H4 through strong hydrogen bonding and π–π interactions. Attributed to the preferred adsorption of C3H4, NKMOF-11 possessed ultrahigh selectivities towards C3H4/C3H6 mixtures (1/99 and 1/999 (v/v)) at room temperature. The simulation and experimental breakthrough results further revealed NKMOF-11 possesses excellent separation performance towards C3H4 and C3H6 binary mixtures (1/99 and 1/999) and set a new record for the productivity of polymer-grade C3H6 (>99.996%) among all reported materials. This study paves a new avenue for the design of adsorbent materials with both high selectivity and high productivity for C3H4/C3H6 binary mixture.
Purification of propylene from the propyne (C3H4)/propylene (C3H6) mixture is a significant and challenging process in the chemical industry. Nowadays, the removal of propyne from propylene mainly relies on the energy-intensive hydrogenation catalyzed by noble metals. We herein report three gallate-based MOFs, namely M(II)-gallate (M = Ni, Mg, Co), which provide excellent performance in terms of removing propyne from propyne/propylene mixture (1/99, v/v). The C3H4 uptake capacities of Mg-, Co-, Ni-gallate can reach 3.75, 3.21 and 2.65 mmol/g, while the C3H6 uptake capacities are only 1.50, 1.49, and 0.9 mmol/g, respectively, at ambient conditions. Particularly, the productivity of 99.9999% pure C3H6 in Co-gallate and Mg-gallate was 1580 and 1420 mL/g, respectively, outperforming the state-of-the-art material USTA-200 (1400 mL/g). The adsorption mechanism was further investigated by using the first-principle DFT-D calculations, revealing the excellent C3H4/C3H6 separation ability of M-gallate originates from stronger supramolecular interactions and C-H…O interactions between the hydrogen atoms from C3H4 and oxygen atoms from M-gallate frameworks. Besides, the M-gallate materials also show excellent regeneration ability. Thus, this work demonstrates that the family of M-gallate materials shows industrially promising porous material for propylene purification by adsorption process.
Adsorptive separation of acetylene (C2H2) from carbon dioxide (CO2) promises a practical way to produce high-purity C2H2 required for industrial applications. However, challenges exist in the pore environment engineering of porous materials to recognize two molecules due to their similar molecular sizes and physical properties. Herein, we report a strategy to optimize pore environments of multivariate metal−organic frameworks (MOFs) for efficient C2H2/CO2 separation by tuning metal components, functionalized linkers and terminal ligands. The optimized material UPC-200(Al)-F-BIM, constructed from Al3+ clusters, fluorine-functionalized organic linkers, and benzimidazole terminal ligands, demonstrated the highest separation efficiency (C2H2/CO2 uptake ratio of 2.6), and highest C2H2 productivity among UPC-200 systems. Experimental and computational studies revealed the contribution of small pore size and polar functional groups on the C2H2/CO2 selectivity and indicated the practical C2H2/CO2 separation of UPC-200(Al)-F-BIM.
A novel fluorinated biphenyldicarboxylate ligand of 3,3',5,5'-tetrafluorobiphenyl-4,4'-dicarboxylic acid (H2-TFBPDC) and its terbium metal-organic framework, {[Tb2(TFBPDC)3(H2O)]·4.5DMF·0.5H2O}
n
(denoted as JXNU-6), were synthesized. JXNU-6 exhibits a three-dimensional (3D) framework built from one-dimensional (1D) terbium carboxylate helical chains bridged by TFBPDC2- linkers. The 3D framework of JXNU-6 features 1D fluorine-lined channels. The gas adsorption experiments show that the activated JXNU-6 (JXNU-6a) displays distinct adsorption behavior for propyne (C3H4) and propylene (C3H6) gases. The effective removal of a trace amount of C3H4 from C3H6 was achieved by JXNU-6a under ambient conditions, which is demonstrated by the column-breakthrough experiments. The modeling studies show that the preferential binding sites for C3H4 are the exposed F atoms on the pore surface in 1D channels. The strong C-H···F hydrogen bonds between C3H4 molecules and F atoms of TFBPDC2- ligands dominate the host-guest interactions, which mainly account for the excellent C3H4/C3H6 separation performance of JXNU-6a. This work provides a strategy for specific recognition toward C3H4 over C3H6 through the C-H···F hydrogen bond associated with the fluorinated organic ligand.
Covalent organic frameworks (COFs), as an emerging class of crystalline porous polymers connected by dynamic covalent bonds, have been well studied over the past decade. Recently, three dimensional (3D) COFs have attracted extensive interest for the synthesis and applications of novel COFs. The principal reason for this rising trend is based on their unique porous features and excellent performances compared to previously reported two dimensional (2D) frameworks with the layered AA-stacking mode. This critical review describes the current state-of-the-art development of 3D COFs in the design principles, synthetic methods, functionalization strategies, and potential applications. Some major challenges associated with future perspectives are further discussed, inspiring the development of 3D COFs.
Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with highly tuneable structures and functionalities. COFs have been proposed as ideal materials for applications in the energy-intensive field of molecular separation due to their notable intrinsic features such as low density, exceptional stability, high surface area, and readily adjustable pore size and chemical environment. This review attempts to highlight the key advancements made in the synthesis of COFs for diverse separation applications such as water treatment or the separation of gas mixtures and organic molecules, including chiral and isomeric compounds. Methods proposed for the fabrication of COF-based columns and continuous membranes for practical applications are also discussed in detail. Finally, a perspective regarding the remaining challenges and future directions for COF research in the field of separation has also been presented.
Hydrogen-bonded frameworks (HOFs) with permanent microporosity are still hard to obtain, due to the inherent nature of easy-to-collapse of HOF skeleton on solvent removal. On the other hand, tuning the electronic structure of HOFs by incorporating redox-active units for expanding the functionality of this class of material presents a waiting-for unveiled and highly desired field. In this work, the redox-active imide unit was utilized to construct HOF. The resultant HOF (namely ECUT-HOF-30) enables robust microporous structure of ca. 4.0 Å, performing effective separation of C2H2/CO2 via the unique molecular recognition from imide oxygen atoms. Whilst exciting optoelectronic active properties such as photochromism and electrochromism were observed in this HOF. The proof-of-concept results open up a gate for fundamental design of advanced HOFs with redox-active unit and versatility.
Capturing carbon dioxide (CO2) from flue gases with porous materials has been considered as a viable alternative technology to replace the traditional liquid amine adsorbents. A large number of microporous metal-organic frameworks (MOFs) have been developed as CO2-capturing materials. However, it is challenging to target materials with both extremely high CO2 capture capacity and gas selectivity (so-called trade-off) along with moderate regeneration energy. Herein, we developed a novel porous material, [Cu(dpt)2(SiF6)]n (termed as UTSA-120; dpt = 3,6-Di(4-pyridyl)-1,2,4,5-tetrazine), which is isoreticular to the net of SIFSIX-2-Cu-i. This material exhibits simultaneously high CO2 capture capacity (3.56 mmol g-1 at 0.15 bar and 296 K) and CO2/N2 selectivity (~ 600), both of which are superior to SIFSIX-2-Cu-i and most of other MOFs reported. Neutron powder diffraction experiments reveal that the exceptional CO2 capture capacity at low-pressure region and the moderate heat of CO2 adsorption can be attributed to the suitable pore size and dual functionalities (SiF62- and tetrazine), which not only interact with CO2 molecules but also enable the dense packing of CO2 molecules within the framework. Simulated and actual breakthrough experiments demonstrate that UTSA-120a can efficiently capture CO2 gas from the CO2/N2 (15/85, v/v) and CO2/CH4 (50/50) gas mixtures under ambient conditions.
The separation of acetylene from ethylene is a crucial process in the petrochemical industry, as even small acetylene impurities can lead to premature termination of ethylene polymerization. Herein, we present the synthesis of a robust, crystalline naphthalene diimide porous aromatic framework via imidization of linear naphthalene-1,4,5,8-tetracarboxylic dianhydride and triangular tris(4-aminophenyl) amine. The resulting material, PAF-110, exhibits impressive thermal and long-term structural stability, as indicated by thermogravimetric analysis and powder X-ray diffraction characterization. Gas adsorption characterization reveals that PAF-110 has a capacity for acetylene that is more than twice its ethylene capacity at 273 K and 1 bar, and it exhibits a moderate acetylene selectivity of 3.9 at 298 K and 1 bar. Complementary computational investigation of each guest binding in PAF-110 suggests that this affinity and selectivity for acetylene arises from its stronger electrostatic interaction with the carbonyl oxygen atoms of the framework. To the best of our knowledge, PAF-110 is the first crystalline porous organic material to exhibit selective adsorption of acetylene over ethylene, and its properties may provide insight into the further optimized design of porous organic materials for this key gas separation.
A preference for ethane
Industrial production of ethylene requires its separation from ethane in a cryogenic process that consumes large amounts of energy. An alternative would be differential sorption in microporous materials. Most of these materials bind ethylene more strongly that ethane, but adsorption of ethane would be more efficient. Li et al. found that a metal-organic framework containing iron-peroxo sites bound ethane more strongly than ethylene and could be used to separate the gases at ambient conditions.
Science , this issue p. 443
Separation of propylene/propyne (C3H4/C3H6) is more difficult and challenging than that of acetylene/ethylene (C2H2/C2H4) because of their closer molecular sizes and has not been well explored. Herein, we carry out a comprehensive screening of a series of metal‐organic frameworks with broad types of structures, pore sizes and functionalities, and identify UTSA‐200 as the best separating material for the deep removal of trace C3H4 from C3H4/C3H6 mixtures. Gas sorption isotherms reveal that UTSA‐200 exhibits by far the highest reported C3H4 adsorption capacity (95 cm3 cm−3 under 0.01 bar and 298 K) and the record C3H4/C3H6 selectivity, mainly attributed to the suitable dynamic pore size to efficiently block the larger C3H6 molecule while the strong binding sites and pore flexibility to capture smaller C3H4 molecules, as elucidated by detailed structural and neutron powder diffraction studies. This material thus provides a record purification capacity for the removal of trace C3H4 from a 1/99 (or 0.1/99.9) C3H4/C3H6 mixture to produce 99.9999% pure C3H6 with a productivity of 62.0 (or 142.8) mmol g‐1, as revealed by experimental breakthrough curves.
The implement of one‐step removal of multi‐component gases based on single material will significantly improve the efficiency of separation processes but still remains challenging, due to the difficulty to precisely fabricate the porous materials with multiple binding sites that are tailored for the different guest molecules. Here, we report a novel niobium oxide‐fluoride anion‐pillared interpenetrated material ZU‐62 (also termed as NbOFFIVE‐2‐Cu‐i, NbOFFIVE = NbOF52‐) featuring asymmetric O/F node coordination for the simultaneous removal of trace propyne and propadiene from the propylene, industrial processes relevant for the production of polymer‐grade propylene. The narrow‐distributed nanospace (aperture of Site I: 6.75 Å, Site II: 6.94 Å, and Site III: 7.20 Å) derived from the special coordination geometry within ZU‐62 customized the corresponding energy favorable binding sites for the propyne and propadiene that enable the recorded propadiene uptake (1.74 mmol g‐1) as well as the excellent propyne uptake (1.87 mmol g‐1) under ultra‐low pressure (5000 ppm). The multisite classified capture mechanism was revealed by modeling studies.
Despite tremendous efforts, precise control in the synthesis of porous materials with ideal nanocages for desired gas separation applications still remains a challenge. Microporous metal–organic frameworks (MOFs) have provided the rich chemistry to enable us precise control and design of structures, pore cavities, and functionalities at the molecular level. Here, we propose and design a microporous MOF (termed as ZJUT-1, ZJUT = Zhejiang University of Technology) with a fine-tuned nanocage, exhibiting the desired size, sharp, and functionalities that are suitable for trapping a single propyne (C3H4) molecule. Adsorption and computational studies indicate that such optimized nanocages can not only reduce the uptake of propylene (C3H6), but also strengthen the C3H4–host interactions through multiple hydrogen-bonding between SiF62-/-NH2 and C3H4 molecule. This material thus shows remarkably different C3H4 and C3H6 adsorption capacities, with the largest uptake ratio of 3.06 at 1 bar and 298 K, affording a very high selectivity (up to 70) for C3H4/C3H6 (1/99) separation. The actual breakthrough experiments demonstrate that ZJUT-1 can efficiently remove trace amount of C3H4 from the important raw C3H4/C3H6 mixtures under ambient conditions with 0.19 mmol g−1 C3H4 uptake capacity to produce 99.9995% pure C3H6.
A novel covalent-triazine framework (CTF-PO71) is designed and prepared from an organic pigment molecule for high-performance gas separation. The functional sites with different electrostatic potentials on the pore surface of CTF-PO71 demonstrate a strong interaction between C2H2 and CTF-PO71 to achieve preferential adsorption of C2H2 over C2H4, thus enabling effective capture of a trace amount of C2H2 from the gas mixture. This is the first organic porous polymer that is capable of separating C2H2 and C2H4. The commercial availability and the low cost of the pigment as well as the high stability of the resultant framework endow CTF-PO71 with a significant potential for practical applications.
The removal of trace amounts of propyne from propylene is critical for the production of polymer-grade propylene. We herein report the first example of metal–organic frameworks of flexible–robust nature for the efficient separation of propyne/propylene mixtures. The strong binding affinity and suitable pore confinement for propyne account for its high uptake capacity and selectivity, as evidenced by neutron powder diffraction studies and density functional theory calculations. The purity of the obtained propylene is over 99.9998%, as demonstrated by experimental breakthrough curves for a 1/99 propyne/propylene mixture.
Selective separation of acetylene (C2H2) from carbon dioxide (CO2) or ethylene (C2H4) needs specific porous materials whose pores can realize sieving effects while pore surfaces can differentiate their recognitions for these molecules of similar molecular sizes and physical properties. We report a microporous material [Zn(dps)2(SiF6)] (UTSA-300, dps = 4,4′-dipyridylsulfide) with two-dimensional channels of about 3.3 Å, well-matched for the molecular sizes of C2H2. After activation, the network was transformed to its closed-pore phase, UTSA-300a, with dispersed 0D cavities, accompanied by conformation change of the pyridyl ligand and rotation of SiF62– pillars. Strong C–H···F and π–π stacking interactions are found in closed-pore UTSA-300a, resulting in shrinkage of the structure. Interestingly, UTSA-300a takes up quite a large amounts of acetylene (76.4 cm³ g–1), while showing complete C2H4 and CO2 exclusion from C2H2 under ambient conditions. Neutron powder diffraction and molecular modeling studies clearly reveal that a C2H2 molecule primarily binds to two hexafluorosilicate F atoms in a head-on orientation, breaking the original intranetwork hydrogen bond and subsequently expanding to open-pore structure. Crystal structures, gas sorption isotherms, molecular modeling, experimental breakthrough experiment, and selectivity calculation comprehensively demonstrated this unique metal–organic framework material for highly selective C2H2/CO2 and C2H2/C2H4 separation.
A framework for molecular assembly
Covalent molecular frameworks are crystalline microporous materials assembled from organic molecules through strong covalent bonds in a process termed reticular synthesis. Diercks and Yaghi review developments in this area, noting the parallels between framework assembly and the covalent assembly of atoms into molecules, as described just over a century ago by Lewis. Emerging challenges include functionalization of existing frameworks and the creation of flexible materials through the design of woven structures.
Science , this issue p. eaal1585
Separating one organic from another
Separating closely related organic molecules is a challenge (see the Perspective by Lin).The separation of acetylene from ethylene is needed in high-purity polymer production. Cui et al. developed a copper-based metal-organic framework with hexafluorosilicate and organic linkers designed to have a high affinity for acetylene. These materials, which capture four acetylene molecules in each pore, successfully separated acetylene from mixtures with ethylene. Propane and propylene are both important feedstock chemicals. Their physical and chemical similarity, however, requires energy-intense processes to separate them. Cadiau et al. designed a fluorinated porous metal-organic framework material that selectively adsorbed propylene, with the complete exclusion of propane.
Science , this issue pp. 141 and 137 ; see also p. 121
A particular palladium sulphide phase (Pd4S) supported on carbon nanofibers is shown to be one of the most selective alkyne hydrogenation catalysts reported to date. Propyne and ethyne (in the absence of the corresponding alkene) can be converted in the gas phase to the corresponding alkene with 96% and 83% selectivity at 100% alkyne conversion. A bulk phase PdS powder (pre-reduced at 523 K) also demonstrated excellent performance (79% ethene selectivity). Other bulk phase metal sulphides (Ni2S3 and CuS) were tested and whilst the nickel analogue was found to be active/selective the performance was poorer than observed with either supported or unsupported Pd sulphide. Exceptional alkene selectivity extends to mixed alkyne/alkene feeds using the Pd4S/CNF catalyst – 86% and 95% alkene selectivity for C3 and C2 mixes, respectively. This report opens up exciting possibilities for using metal sulphides as highly selective hydrogenation catalysts.