Water-generated dangling linkers in a metal-organic framework.

As a highly promising candidate for hydrogen storage, crucial to vehicles powered by fuel cells, metal–organic frameworks (MOFs) have attracted the attention of chemists in recent decades. H2 uptak...

In petroleum refining industries, the fracturing process allows for the cracking of long-chain hydrocarbons into a mixture of small olefin and paraffin molecules that are then separated via the energetically and monetarily demanding cryogenic distillation process. In an attempt to mitigate both energetic and capital consumptions, selective sorption of light hydrocarbons by tunable sorbents, such as metal-organic frameworks (MOFs), appears to be the most promising alternative for a more efficient gas separation process. MOFs are novel porous materials assembled from inorganic bricks connected by organic linkers. From a crystal engineering stand point, MOFs are advantageous in creating a range of microporous (0.2–2.0 nm) to mesoporous (>50 nm) void cavities, presenting unique opportunities for the functionalization of both the organic linkers and the void. Of significant importance is the MOF-74-M family (M = metal), characterized by a high density of open metal sites, that is not fully coordinated metal centers. This family of MOF is also known as CPO-27-M. MOF-74 have demonstrated more separation potential than other known MOFs and zeolites. Density functional theory (DFT), as implemented within a linear combination of atomic orbital (LCAO) approach, has been used to investigate the selective sorption of C1-C4hydrocarbons in MOF-74-Mg/Zn. The study was first implemented by adopting a molecular cluster approach, and later by applying periodic boundary conditions (PBC). While both modellistic approaches agree in showing significant differences in binding energies between olefins and paraffins adsorbed at the MOFs’ open metal sites, results reported at the molecular cluster level show underestimation when compared to those obtained at the PBC level. The use of PBC models allow for the correcting of binding energies for basis set superposition error (BSSE), molecular lateral interaction (LI), zero-point energy (ZPE), and thermal energy (TE) contributions. As such, results obtained at the PBC level are directly comparable to experimental calorimetric values (i.e., heat of adsorptions). This work discusses, for the first time, the origin of the fictitious agreement between binding energies obtained with molecular clusters and experimental heats of adsorption, identifying its origin as due to compensation of errors. Spectroscopy studies based on the intensities and frequency shifts with respect to the molecules in the gas phase are presented as a further investigation of the interaction of the small hydrocarbons (C1-C2) with the open metal sites in MOF-74-Mg. In an attempt to provide a more comprehensive description of the behavior of the hydrocarbon molecules, results from diffusion mechanism studies

Metal–Organic Frameworks (MOFs) with open metal sites (OMS) interact strongly with a range of polar gases/vapors. However, under ambient conditions, their selective adsorption is generally impaired due to a high OMS affinity to water. This led previously to the privilege selection of hydrophobic MOFs for the selective capture/detection of volatile organic compounds (VOCs). Herein, we show that this paradigm is challenged by metal(III) polycarboxylates MOFs, bearing a high concentration of OMS, as MIL‐100(Fe), enabling the selective capture of polar VOCs even in the presence of water. With experimental and computational tools, including single‐component gravimetric and dynamic mixture adsorption measurements, in situ infrared (IR) spectroscopy and Density Functional Theory calculations we reveal that this adsorption mechanism involves a direct coordination of the VOC on the OMS, associated with an interaction energy that exceeds that of water. Hence, MOFs with OMS are demonstrated to be of interest for air purification purposes.

Imparting Superhydrophobicity to Porphyrinic Coordination Frameworks Using Organotin

Numerous open metal sites and well-developed micropores are the two most significant characteristics that should be imparted to design metal–organic frameworks (MOFs) as effective catalysts. However, the construction of the best MOF catalyst with both these characteristics is challenging because the creation of numerous open metal sites generally triggers some structural collapse of the MOF. Herein, we report the construction of well-structured but defected MOFs through the growth of defected MOFs, where some of the original organic linkers were replaced with analog organic linkers, on the surface of a crystalline MOF template (MOF-on-MOF growth). Additional open metal sites within the MOF-74 structure were generated by replacing some of the 2,5-dihydroxy-1,4-bezenedicarboxylic acid presenting in MOF-74 with 1,4-benzenedicarboxylic acid due to the missing hydroxyl groups. And the resulting additional open metal sites within the MOF-74 structure resulted in enhanced catalytic activity for the cyanosilylation of aldehydes. However, the collapse of some of the well-developed MOF-74 structure was also followed by structural defects. Whereas, the growth of defected MOF-74 (D-MOF-74) on the well-crystallized MOF-74 template led to the production of relatively well-crystallized D-MOF-74. Core–shell type MOF-74@D-MOF-74 having abundant open metal sites with a preserved crystallinity exhibited the efficient catalytic cyanosilylation of several aldehydes. Additionally, MOF-74@D-MOF-74 displayed excellent recyclability during the consecutive catalytic cycles.

The vastness of materials space, particularly that which is concerned with metal-organic frameworks (MOFs), creates the critical problem of performing efficient identification of promising materials for specific applications. Although high-throughput computational approaches, including the use of machine learning, have been useful in rapid screening and rational design of MOFs, they tend to neglect descriptors related to their synthesis. One way to improve the efficiency of MOF discovery is to data-mine published MOF papers to extract the materials informatics knowledge contained within journal articles. Here, by adapting the chemistry-aware natural language processing tool, ChemDataExtractor (CDE), we generated an open-source database of MOFs focused on their synthetic properties: the DigiMOF database. Using the CDE web scraping package alongside the Cambridge Structural Database (CSD) MOF subset, we automatically downloaded 43,281 unique MOF journal articles, extracted 15,501 unique MOF materials, and text-mined over 52,680 associated properties including the synthesis method, solvent, organic linker, metal precursor, and topology. Additionally, we developed an alternative data extraction technique to obtain and transform the chemical names assigned to each CSD entry in order to determine linker types for each structure in the CSD MOF subset. This data enabled us to match MOFs to a list of known linkers provided by Tokyo Chemical Industry UK Ltd. (TCI) and analyze the cost of these important chemicals. This centralized, structured database reveals the MOF synthetic data embedded within thousands of MOF publications and contains further topology, metal type, accessible surface area, largest cavity diameter, pore limiting diameter, open metal sites, and density calculations for all 3D MOFs in the CSD MOF subset. The DigiMOF database and associated software are publicly available for other researchers to rapidly search for MOFs with specific properties, conduct further analysis of alternative MOF production pathways, and create additional parsers to search for additional desirable properties.

Towards modeling spatiotemporal processes in metal–organic frameworks

Metal-organic frameworks (MOFs), consisting of inorganic metal centres coordinated with organic linkers, are a group of hybrid materials emerging recently and have attracted immense attention because of their exceptionally high microporosity, tuneable pore size and uniformly-structured cavities. The controlled synthesis of crystals with defined morphology is critical for their performance in terms of adsorption, gas separation, and catalysis etc. Some instructive attempts have been conducted successfully to manipulate the morphology of MOFs by adjusting reaction parameters, changing solvents and using templates or additives. Among them, the organic additives with their high efficiency and repeatability have been frequently introduced. However, organic additives tend to be difficult to be removed from MOFs micropore structures and have adverse impacts on the crystallinity due to introducing defects. Therefore, there is an increasing interest in fashioning the morphology of MOFs by inorganic coordination strategy.This project focuses on controlled synthesis of MOFs by using inorganic additives or precursors and investigates their morphology-dependent applications in terms of adsorption and catalysis. The studies include the morphology control of copper-based MOFs and the study of morphology transformation mechanism, as well as exploring the morphology dependent performance on gas adsorption and catalysis. It aims to establish guidelines for the controlled synthesis of MOFs via inorganic strategies.In the first part of the experimental chapter, we used Mg/Al layered double hydroxide (LDH) nanosheets as modulators to tune the growth orientation of HKUST-1 without either pore blockage or crystallinity degradation. Through the introduction of LDH during the hydrothermal process, HKUST-1 crystal shape transfers from octahedron with fully exposed {111} facets to tetrakaidecahedron with both {100} and {111} facets. The exposure of {100} facet with large pore size facilitates the activation of MOF, thereby providing more open metal sites. The tetrakaidecahedral HKUST-1 exhibits high acetylene uptake of 275 cm3 (STP)/g at 298 K and 1 atm, which is the highest value ever reported to the best of our knowledge. Time-dependent experiment revealed that the morphology control of LDH was due to: a) competitive coordination between aluminium ions released from LDH and copper ions with organic ligands; b) fast deprotonation of ligands induced by the hydroxyl function group on the LDH. In order to further reveal the mechanism of aluminium ion on the morphology control of copper-based MOFs, we used aluminium nitrate as an inorganic competitive coordinator to control the growth orientation of copper-based MOFs, such as HKUST-1, MOF-14 and Cu-MOF-74. Through monitoring the reactant composition, we find that the competitive coordination induced by the added aluminium nitrate mainly affects the crystal growth stage rather than the nucleation stage. The kinetic study further reveals that Al3+ competes with Cu2+ to coordinate with ligands, restraining the growth rate of certain facet and resulting in the orientated growth of copper-based MOFs. Hydroxylation of toluene was utilized as a model reaction to investigate the facet-catalytic activity for as-synthesized HKUST-1. The selectivity of the cresol increases with the morphology transformation of HKUST-1 from octahedron to cube.In the third part of the experimental chapter, we used metal oxide to accelerate the deprotonation of ligands. A universal method for controlled-synthesis of urchin-like superstructure MOFs by matching the dissolution of metal source and the crystallization of MOFs was developed through using nanosized metal oxide as precursors. Cu-MOF74, Co-BTC and Mn-BTC with urchin-like superstructures (US) were synthesized successfully by using proposed strategy. Cu-MOF-74 was selected as an example to investigate the mechanism of the formation of superstructure MOFs. Taking advantage of their unique structure, the US-MOFs are used as useful catalysts for selective catalytic reduction of NO.

The high moisture content of lignite restricts its extensive and efficient use. Furthermore, the reabsorption of lignite is also a factor that affects lignite spontaneous combustion. Therefore, it is of great importance to study the process and mechanism of water molecule desorption and adsorption on lignite and coke (25–950°C) to achieve the clean and efficient utilization of lignite and environmental protection. Proton nuclear magnetic resonance (1H-NMR), thermogravimetric analysis, and other techniques were used in this study to explore the water molecule absorption and desorption processes of lignite pyrolysis at different temperatures (25–950°C) and the special contributions of ether bonds to water molecule adsorption. A mechanism of lignite water molecule adsorption was proposed. The results showed that ether bonds played a special role in the water molecule adsorption by pyrolyzed lignite. The ether bond content was greater in the coal samples at 300 and 950°C, which changed the trend of lignite water molecule absorption and the distribution of water (T2) detected in the 1H-NMR experiments and delayed the escape of water molecules during moisture desorption. The total amount of adsorbed water decreased first and then increased in the coal samples as the pyrolysis temperature increased. However, the maximum adsorption interactions of each coal sample increased first and then decreased. This was the result of the interactions between the pores and the oxygen-containing functional groups. Based on the above analysis, water molecule adsorption mechanism models of lignite and coke were constructed. This study offers a new approach for investigating the water molecule adsorption and adsorption mechanisms of lignite and coke.

This final technical report summarizes work exploring strategies to generate second generation metal organic frameworks (MOFs). These strategies were (a) the formation of interpenetrated frameworks and (b) the generation of coordinatively unsaturated metal centers (open metal sites). In the first phase of the project the effectiveness of these strategies was evaluated experimentally by measuring the saturation hydrogen uptake at high pressure and low temperature of 14 MOFs. The results of these studies demonstrated that surface area is the most useful parameter that correlates with ultimate hydrogen capacity. The strategy of interpenetration has so far failed to produce MOFs with high surface areas and therefore high saturation capacities for hydrogen have not been achieved. The incorporation of coordinatively unsaturated metal centers, however, is a promising strategy that allows higher heats of H2 adsorption to be realized without compromising surface area. Based on these initial findings, research efforts in phase two have concentrated on the discovery of new ultrahigh surface area materials with metal centers capable of supporting coordinative unsaturation without structural collapse. One approach has been the synthesis of new organic linkers that have more exposed edges, which is a factor that contributes to increasing surface area, at least when considering subunits of graphene sheets. Another strategy has been to synthesize MOFs with reduced symmetry linkers in order to generate structure types that are less likely to interpenetrate. Successful implementation of these strategies has resulted in the synthesis of 7 new compounds one of which is the highest surface area Cu based MOF reported to date.

Metal-organic frameworks (MOFs) have been largely investigated worldwide for their use in the capture of radioactive iodine due to its potential release during nuclear accident events and reprocessing of nuclear fuel. The present work deals with the capture of gaseous I2 under a continuous flow and its subsequent transformation into I3- within the porous structures of three distinct, yet structurally related, terephthalate-based MOFs: MIL-125(Ti), MIL-125(Ti)_NH2, and CAU-1(Al)_NH2. The synthesized materials exhibited specific surface areas (SSAs) with similar order of magnitude: 1207, 1099, and 1110 m2 g-1 for MIL-125(Ti), MIL-125(Ti)_NH2, and CAU-1(Al)_NH2, respectively. Because of that, it was possible to evaluate the influence of other variables over the iodine uptake capacity─such as band gap energies, functional groups, and charge transfer complexes (CTC). After 72 h of contact with the I2 gas flow, MIL-125(Ti)_NH2 was able to trap 11.0 mol mol-1 of I2, followed by MIL-125(Ti) (8.7 mol mol-1), and by CAU-1(Al)_NH2 (4.2 mol mol-1). The enhanced ability to retain I2 in the MIL-125(Ti)_NH2 was associated with a combined effect between its amino group (which has a great affinity toward iodine), its smaller band gap (2.5 eV against 2.6 and 3.8 eV for CAU-1(Al)_NH2 and MIL-125(Ti), respectively), and its efficient charge separation. In fact, the presence of a linker-to-metal charge transfer (LMCT) mechanism in MIL-125(Ti) compounds separates the photogenerated electrons and holes into the two distinct moieties of the MOF: the organic linker (which stabilizes the holes) and the oxy/hydroxy inorganic cluster (which stabilizes the electrons). This effect was observed using EPR spectroscopy, whereas the reduction of the Ti4+ cations into the paramagnetic Ti3+ species was evidenced after irradiation of the pristine Ti-based MOFs with UV light (<420 nm). In contrast, because CAU-1(Al)_NH2 exhibits a purely linker-based transition (LBT)─with no EPR signals related to Al paramagnetic species─it tends to exhibit faster recombination of the photogenerated charge carriers as, in this case, both electrons and holes are located over the organic linker. Furthermore, the transformation of the gaseous I2 into In- [n = 5, 7, 9, ...] intermediates and then into I3- species was evaluated using Raman spectroscopy by following the evolution of their respective bands at about 198, 180, and 113 cm-1. This conversion─which is favored by an effective charge separation and smaller band gaps─increases the I2 uptake capacity of the compounds by creating specific adsorption sites for these anionic species. In fact, because the -NH2 groups act as an antenna to stabilize the photogenerated holes, both In- and I3- are adsorbed into the organic linker via an electrostatic interaction with these positively charged entities. Finally, changes regarding the EPR spectra before and after the iodine loading were considered to propose a mechanism for the electron transfer from the MOFs structure to the I2 molecules considering their different characteristics.

Mechanism of CO2 adsorption on Mg/DOBDC with elevated CO2 loading

This critical review covers the application of computer simulations, including quantum calculations (ab initio and DFT), grand canonical Monte-Carlo simulations, and molecular dynamics simulations, to the burgeoning area of the hydrogen storage by metal-organic frameworks and covalent-organic frameworks. This review begins with an overview of the theoretical methods obtained from previous studies. Then strategies for the improvement of hydrogen storage in the porous materials are discussed in detail. The strategies include appropriate pore size, impregnation, catenation, open metal sites in metal oxide parts and within organic linker parts, doping of alkali elements onto organic linkers, substitution of metal oxide with lighter metals, functionalized organic linkers, and hydrogen spillover (186 references).

State of the art in visible-light photocatalysis of aqueous pollutants using metal-organic frameworks

Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Fluorous Metal–Organic Frameworks with Unique Cage-in-Cage Structures Featuring Fluorophilic Pore Surfaces for Efficient C2H2/CO2 Separation Xing-Ping Fu†, Yu-Ling Wang†, Xue-Feng Zhang, Zhenjie Zhang, Chun-Ting He and Qing-Yan Liu Xing-Ping Fu† College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 Department of Ecological and Resources Engineering, Fujian Key Laboratory of Eco-Industrial Green Technology, Wuyi University, Wuyishan, Fujian 354300 , Yu-Ling Wang† College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 , Xue-Feng Zhang College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 , Zhenjie Zhang College of Chemistry and State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071 , Chun-Ting He College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 and Qing-Yan Liu *Corresponding author: E-mail Address: [email protected] College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 https://doi.org/10.31635/ccschem.021.202101575 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The similarities in molecular size and physical properties of acetylene (C2H2) and carbon dioxide (CO2) produce a formidable challenge for their separation. Herein, we report two isoreticular fluorinated metal–organic frameworks (MOFs), labeled as JXNU-11(Fe2M) (M = Ni and Co), featuring unique octahedral cages encapsulated by cuboctahedral cages. JXNU-11(Fe2Ni) shows a record high C2H2-capture amount of 4.8 mmol g−1 and a long C2H2/CO2 breakthrough interval time of 55 min g−1 in an actual breakthrough experiment based on the equimolar C2H2/CO2 mixture under ambient conditions, indicating a high-performance material for C2H2 capture and C2H2/CO2 separation. Computational simulations revealed that the nanosized double-shell cages decorated with abundant fluorine and carboxylate oxygen atoms afford the optimized pore spaces for preferentially trapping C2H2, which account for the remarkable C2H2-capture capacity and the highly efficient C2H2/CO2 separation for JXNU-11(Fe2M). This work not only develops a design strategy for building the cage-in-cage structures in MOFs, but also provides universal guidance on designing porous MOFs with highly polarized and fluorophilic pore cages to capture C2H2. Download figure Download PowerPoint Introduction Acetylene (C2H2), the simplest alkyne, is an essential chemical for many common chemical products, such as acetaldehyde, acetic acid, plastic, and synthetic rubber,1 and electronic materials in the electronics industry.2 Additionally, C2H2 is an important flammable gas widely utilized for welding and metal cutting. The production of C2H2 from the oxidative coupling of natural gas is a commonly used preparation method for C2H2 gas in the petrochemical industry.3 Such a C2H2 preparation process leads to a small amount of carbon dioxide (CO2) in the C2H2 product. The presence of CO2 in C2H2 starting material has a seriously negative effect on subsequent preparation processes for the down-stream chemical products. Thus separation of C2H2/CO2 mixtures is vital in the petrochemical industry. However, the similar molecular sizes, as well as the same kinetic diameter (3.3 Å) of the linear-shaped C2H2 and CO2 molecules4–9 ( Supporting Information Table S1), cause the C2H2/CO2 separation to be a formidable task. Organic solvent extraction and cryogenic distillation are the current technologies for purifying C2H2 gas but are energy-intensive and environmentally unfriendly. Therefore, it is important to develop a highly energy-efficient and environment-friendly technology for achieving C2H2/CO2 separation. Metal–organic frameworks (MOFs) with porous and crystalline structures are fascinating porous solid materials.5–14 MOFs show highly promising potential in gaseous mixture separation due to their designable frameworks' structures and tunable pore environments.15–19 Recently, many MOFs have been utilized for the separation of C2H2/CO2 mixtures,20–29 and a few of them exhibit high-performance C2H2/CO2 separation.30–34 In the column breakthrough separation of binary gases, the breakthrough time and the captured amount for the late eluted adsorbate in the column are the important parameters for evaluating the separation efficiency of an adsorbent. The long breakthrough time for the late eluted gas generally results in a large captured amount for the late eluted gas, leading to a high separation efficiency of the adsorbent. However, in practice breakthrough experiments based on the equimolar C2H2/CO2 mixture, C2H2 breakthrough time for MOFs is generally shorter than 50 min g−1 under ambient conditions.35–40 The top-performing MOFs are SNNU-45 and ZJU-74a with the C2H2 breakthrough times of 113 and 81 min g−1,41,42 respectively. In contrast, the C2H2-captured amounts for MOFs in the packed columns are commonly less than 3 mmol g−1. SNNU-45 and ZJU-74a have the highest C2H2-capture amounts of 3.5 and 3.64 mmol g−1, respectively. It has been well demonstrated that the fluorine atoms of the inorganic anions, including MF62− (M = Si4+ and Ti4+) and NbOF52,− in the hybrid MOFs are the preferential bind sites for the acidic hydrogen atoms of the alkyne.43–47 The multiple C–H···F interactions between the C2H2 molecule and inorganic MF62− or NbOF52− groups of MOFs have resulted in an efficient C2H2/CO2 separation.47–49 Thus the fluorinated MOFs are a highly appealing platform for C2H2/CO2 separation. Unfortunately, due to the synthetic inaccessibility of the suitable fluorinated organic ligands, the fluorinated MOFs have thus been largely unexplored.50–54 Herein, we present two isostructural fluorous MOFs [termed as JXNU-11(Fe2M), M represents the divalent metal ions of Ni2+ and Co2+] with a fluorinated organic linker, featuring unique cage-in-cage structures. Remarkably, JXNU-11(Fe2Ni) shows a record-high C2H2-capture amount and a long C2H2 breakthrough time in the practice breakthrough experiment, resulting in high-performance C2H2/CO2 separation. The double-shell cages possessing highly electronegative F and O atoms in JXNU-11(Fe2M) are suitable pore cages to match the size of C2H2. The C2H2 molecules are trapped in the highly fluorophilic and polarized double-shell cages through strong host–guest interactions, which are responsible for the highly efficient C2H2/CO2 separation by JXNU-11(Fe2M). Experimental Section Synthesis of JXNU-11(Fe2Ni) A mixture of FeCl3·6H2O (8.10 mg, 0.03 mmol), Ni(NO3)3·6H2O (4.36 mg, 0.015 mmol), 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate acid (4.71 mg, 0.015 mmol), 1,3,5-tris(4-carboxyphenyl)benzene (6.58 mg, 0.015 mmol), N,N-dimethylformamide (DMF) (2 mL), and CH3COOH (0.19 mL) was capped in a 20 mL vial and heated at 100 °C for 2 days. After cooling to room temperature, brown triangle-shaped crystals were obtained. Elemental analysis for {[Fe2Ni(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]·6DMF·2.5H2O}n (C68H77F4O24.5N6Fe2Ni: 1616.74). Calcd/found: H, 4.80/4.71; C, 50.51/50.41; N, 5.19/5.02. IR data (KBr, cm−1): 3427 (m), 1662 (s), 1615 (s), 1394 (s), 1253 (w), 1185 (w), 1147 (w), 1098 (w), 1035 (s), 858 (s), 807 (w), 781 (s), 683 (w), 663 (w), 589 (m), 498 (m), 443 (s). Synthesis of JXNU-11(Fe2Co) A mixture of FeCl3·6H2O (8.10 mg, 0.03 mmol), Co(NO3)3·6H2O (4.37 mg, 0.015 mmol), 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate acid (4.71 mg, 0.015 mmol), 1,3,5-tris(4-carboxyphenyl)benzene (6.58 mg, 0.015 mmol), DMF (2 mL), and CH3COOH (0.17 mL) was capped in a 20 mL vial and heated at 100 °C for 2 days. After cooling to room temperature, brown triangle-shaped crystals were obtained. Elemental analysis for {[Fe2Co(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]·4DMF·H2O}n (C62H60F4O21N4Fe2Co: 1443.77). Calcd/found: H, 4.18/4.21; C, 51.57/51.46; N, 3.88/3.92. IR data (KBr, cm−1): 3420 (m), 1655 (s), 1612 (s), 1394 (s), 1254 (w), 1185 (w), 1146 (w), 1098 (w), 1034 (s), 859 (s), 808 (w), 781 (s), 706 (w), 663 (w), 589 (m), 500 (m), 443 (s). Methods X-ray single-crystal diffraction experiments were carried out with a Rigaku Oxford SuperNova diffractometer, and powder X-ray diffraction patterns were recorded on a Rigaku DMax 2500 powder diffractometer. Gas sorption–desorption isotherms were measured on a Micromeritics ASAP 2020 HD88 adsorption analyzer. Breakthrough experiments for separation of C2H2/CO2 (v/v, 50/50) were carried out in a fixed bed with a gas chromatograph detection system. The detailed experimental methods are provided in Supporting Information. Results and Discussion Compounds JXNU-11(Fe2M) constructed from the oxygen-centered heterometallic trimeric [Fe2M(μ3-O)(COO)6] (M = Ni2+ and Co2+) clusters (Figure 1a) were prepared. Compound JXNU-11(Fe2M) formulated as [Fe2M(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]n is based on the linear 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate (TFBPDC2−) and the triangular 1,3,5-tris(4-carboxyphenyl)benzene (BTB3−) ligands (Figure 1b) and was characterized by single-crystal X-ray diffraction ( Supporting Information Table S2). The phase purity of the bulk samples was confirmed by powder X-ray diffraction ( Supporting Information Figure S2). The metal concentrations in both compounds were determined by inductively coupled plasma atomic emission spectroscopy ( Supporting Information Table S3). The two compounds are isostructural and crystallize in a trigonal R3m space group. The trimeric [Fe2M(μ3-O)] core is coordinated by two TFBPDC2−, four BTB3−, and three water ligands ( Supporting Information Figure S1). Each [Fe2M(μ3-O)(COO)6] cluster is linked by four BTB3− ligands and two TFBPDC2− ligands to generate a three-dimensional (3D) framework. Exploration of the structure indicates the 3D framework contains two kinds of cages: a small octahedral cage formed from six [Fe2M(μ3-O)(COO)6] clusters, three TFBPDC2−, and four BTB3− ligands with a diameter of ∼12 Å (Figure 1c), and a large cage composed of 12 [Fe2M(μ3-O)(COO)6] clusters, six TFBPDC2−, and ten BTB3− ligands with a diameter of ∼26 Å, which can be described as a cuboctahedron (Figure 1d). Thus the 3D framework has close packing of the octahedral and cuboctahedral cages (Figure 1e). Such large voids in the cages in a single 3D framework lead to the generation of a twofold interpenetrated network (Figure 1f). It is remarkable that each octahedral cage is encapsulated by a large cuboctahedral cage from the interpenetrating network (Figure 1g). Such a unique cage-in-cage structure in a MOF is distinctive. The double-shell nested cages are reminiscent of the interesting Russian-doll-like cage. In addition, because of the presence of the highly polar C–F and C–O bonds and dense F atoms on the pore surfaces for these cages, double-shell nested cages exhibit highly polarized and fluorophilic character. As shown in Figure 1g, part of the voids in the large cuboctahedral cage are occupied by the small octahedral cage. The remaining pores of the total 3D framework are filled by the disordered guest solvent molecules, which occupy 64.1% and 65.6% of the volumes of unit cell for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively. JXNU-11(Fe2M) materials exhibit excellent chemical stability after exposure to air for a long time or immersion in water with pH values ranging from 3 to 11 for 24 h ( Supporting Information Figure S3). The solvent-free JXNU-11(Fe2M) was obtained through solvent-exchange and heat under vacuum ( Supporting Information Figure S4). Figure 1 | Structures of JXNU-11(Fe2M). (a) [Fe2M(μ3-O)(COO)6] unit and (b) TFBPDC2− and BTB3− ligands. (c) 3D framework of JXNU-11(Fe2M). (d) Octahedral and (e) cuboctahedral cages. (f) Twofold interpenetrating frameworks of JXNU-11(Fe2M). (g) Cage-in-cage structure in JXNU-11(Fe2M). The fluorine atoms in (b–d) are represented as green balls. Download figure Download PowerPoint The permanent porosity of JXNU-11(Fe2M) was confirmed by N2 adsorption isotherms at 77 K. The N2 sorption isotherms for both compounds have typical type I sorption behavior with the saturation sorption amounts of 561 and 556 cm3 g−1 (Figure 2a). The pore volumes for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) from the experimental N2 data are 0.87 and 0.86 cm3 g−1, respectively, which are close to the corresponding crystal structure calculated values of 0.87 and 0.89 cm3 g−1. Additionally, the pore sizes are mainly around 6.8 and 11 Å (Figure 2a, inset), in good agreement with the obtained values from the crystal structures. The Brunauer–Emmett–Teller surface areas for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) were calculated to be 2208 m2 g−1 (Langmuir surface area of 2418 m2 g−1) and 2122 m2 g−1 (Langmuir surface area of 2392 m2 g−1) ( Supporting Information Figure S5), respectively. Figure 2 | Adsorption data of JXNU-11(Fe2M). (a) N2 adsorption–desorption isotherms at 77 K and pore size distribution for JXNU-11(Fe2M). C2H2 and CO2 adsorption–desorption isotherms of JXNU-11(Fe2M) at 273 K (b) and 298 K (c). Download figure Download PowerPoint The C2H2 and CO2 sorption isotherms of JXNU-11(Fe2M) (Figures 2b and 2c) were collected to evaluate the gas separation potentials for JXNU-11(Fe2M). JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) exhibit high C2H2 adsorption capacities of 191 and 180 cm3 g−1 at 273 K and 1 bar, respectively, which are higher than the best-performance MOFs, including FJU-6-TATB (160 cm3 g−1),55 ATU-Cu (134 cm3 g−1),33 ZJU-74a (107 cm3 g−1),42 JXNU-10(Y) (94.9 cm3 g−1),27 CuI@UIO-66-(COOH)2 (71 cm3 g−1),56 and MOF-OH (68.7 cm3 g−1).57 At 298 K, the adsorption amounts of C2H2 are 118 and 107 cm3 g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively, which are larger than those of the prominent MOFs FeNi-M′MOF (96 cm3 g−1),58 JNU-1 (63 cm3 g−1),59 NKMOF-1-Ni (61 cm3 g−1),60 UTSA-300a (69 cm3 g−1),47 and CPL-NH2 (41 cm3 g−1).39 In contrast, JXNU-11(Fe2M) showed low CO2 uptakes (Figures 2b and 2c). The high uptake of C2H2 is mainly caused by the polarized and fluorophilic microporous environments made of the cage-in-cage structures that are decorated with plenty of highly electronegative fluorine and carboxylate oxygen atoms that have high affinity for the acidic H atoms of C2H2 molecules but low affinity for CO2 molecules with two terminal electronegative O atoms. Such distinct gas sorption behaviors were confirmed by grand canonical Monte Carlo (GCMC) simulations. As depicted in Figures 3a and 3b, the distribution of C2H2 molecules in JXNU-11(Fe2M) is mainly within the octahedral cage and the space between the two cages, whereas much fewer CO2 molecules were distributed within the double-shell nest. Such a phenomenon suggests the double-shell nest is the desirable space for accommodating C2H2 molecules, in agreement with the experimental finding. The linear adsorption isotherms for CO2 further suggest the low affinity to CO2 for the host frameworks. The different adsorption behaviors between C2H2 and CO2 were also evidenced by the isosteric heats of adsorption (Qst) ( Supporting Information Figure S6). The obtained Qst of C2H2 were in the range of 31.6–29.7 kJ mol−1 for JXNU-11(Fe2M), which are notably higher than those of CO2 (16.0–19.6 kJ mol−1), further reflecting the strong affinity toward C2H2. The present Qst(C2H2) values are comparable to those of FJU-6-TATB (29 kJ mol−1),55 UTSA-74 (31 kJ mol−1),37 and JXNU-5a (32.9 kJ mol−1),35 but lower than those of CuI@UIO-66-(COOH)2 (74.5 kJ mol−1),56 JNU-1 (47.6 kJ mol−1),59 and NKMOF-1-Ni (53.9 kJ mol−1).60 The evident difference in adsorption enthalpy endows JXNU-11(Fe2M) with the thermodynamic separation possibility of a C2H2/CO2 mixture. Furthermore, JXNU-11(Fe2M) retained C2H2 storage ability after adsorption/desorption cycling experiments ( Supporting Information Figure S7), confirming their excellent recyclability for C2H2 adsorption. Figure 3 | Grand canonical Monte Carlo (GCMC) adsorption simulation. Computational C2H2 (a) and CO2 (b) distribution in the double-shell nest in JXNU-11(Fe2M) at 298 K and 1 atm. Download figure Download PowerPoint To evaluate the separation performance of JXNU-11(Fe2M), ideal adsorbed solution theory was used to calculate the separation selectivity of the C2H2/CO2 (v∶v = 50∶50) mixture. At 1 bar and 298 K, the adsorption selectivities for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) are 2.7 and 2.5, respectively ( Supporting Information Figure S8), which are comparable to the leading MOFs of UPC-200(Fe)-F-H2O (2.25)32 and FJU-6-TATB (3.1).55 The C2H2/CO2 uptake ratios for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) at 298 K and 1 bar are 2.1 and 1.9, respectively, which are larger than those of noted MOFs FJU-90a (1.75),36 SIFSIX-Cu-TPA (1.7),49 and ATU-Cu (1.2).33The high C2H2 adsorption capacity and moderate selectivity of JXNU-11(Fe2M) at ambient temperature reveal the greatly promising potential for C2H2/CO2 separation. To further investigate the C2H2/CO2 separation performance of JXNU-11(Fe2M), the breakthrough experiments were performed in practical separation processes for a C2H2/CO2 mixture (v∶v = 50∶50) at 298 K and 1 atm. The breakthrough experimental results show that CO2 was eluted first from the packed column, whereas C2H2 was retained in the bed for more than 120 and 108 min g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) (Figure 4a), respectively. The C2H2 breakthrough times for JXNU-11(Fe2M) rank among the leading MOFs ( Supporting Information Table S4). Moreover, the significant roll-up phenomenon of CO2 was observed in the breakthrough experiments, indicating a large proportion of previously adsorbed CO2 molecules can be replaced by the later fed C2H2 molecules. Such a phenomenon means that the C2H2 molecule competes better than CO2 for the binding sites of JXNU-11(Fe2M), highlighting the excellent separation efficiency of JXNU-11(Fe2M) for a C2H2/CO2 mixture. The breakthrough experiments were terminated when the concentrations of effluent gases stabilized. Figure 4 | C2H2 and CO2 separation performances. (a) Breakthrough curves of JXNU-11(Fe2M) at the flow of 2 mL min−1, 298 K, and 1 atm. Breakthrough curves of JXNU-11(Fe2Ni) at different temperatures (b) and different total flow rates at 298 K (c). Download figure Download PowerPoint The captured amounts for C2H2 during the 0∼tbreak time under the dynamic conditions are 4.8 and 4.3 mmol g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively. The C2H2 captured amounts are remarkable and outperform all other MOFs, including the top-performing ZJU-74a (3.64 mmol g−1),42 SNNU-45 (3.5 mmol g−1),41 BSF-3 (2.9 mmol g−1),38 JCM-1 (2.2 mmol g−1),61 and FJU-90a (1.87 mmol g−1)36 (Figure 5 and Supporting Information Table S4). In addition, a long interval time of 55 min g−1 between C2H2 and CO2 breakthrough for JXNU-10(Fe2Ni) was obtained. Such an interval time stands out amongst MOFs and is only shorter than those of SNNU-45 (79 min g−1)41 and SIFSIX-Cu-TPA (69 min g−1).49 Furthermore, their practical separation potentials under different temperatures were evaluated. The C2H2 breakthrough times increased significantly with a decrease of temperature (Figure 4b and Supporting information Figure S9). The breakthrough times of C2H2 reached 146 (283 K) and 165 min g−1 (273 K) with a gas flow of 2 mL min−1. Figure 5 | Experimental breakthrough performance. Comparison of MOFs with top-high C2H2/CO2 breakthrough performance at 298 K and 1 atm. Download figure Download PowerPoint The separation performance of JXNU-11(Fe2M) with different gas flow rates at 298 K were also studied. With the gas flow of 2 mL min−1, the longest C2H2 breakthrough time and the largest amount of the adsorbed C2H2 were achieved for JXNU-11(Fe2M) (Figure 4c and Supporting Information Figure S10). JXNU-11(Fe2M) exhibited no appreciable changes in breakthrough times after three cycles of dynamic breakthrough experiments ( Supporting Information Figure S11), indicative of an excellent recycling separation capability. With the advantage of the modest adsorption heat of C2H2, JXNU-11(Fe2M) materials can be easily regenerated through purging the column with He gas at ambient temperature, as evidenced by the overlapped breakthrough curves ( Supporting Information Figure S11). Such results further indicate JXNU-11(Fe2M) are highly desirable microporous materials for C2H2/CO2 separation. Finally, the desorption experiments were carried out on JXNU-11(Fe2Ni). After reaching the breakthrough equilibrium, the adsorption column was purged with He gas (flow rate: 4 mL min−1) under ambient pressure and 303 K. As depicted in Figure 6a, the adsorbed CO2 molecules were released from the adsorption bed quickly. In contrast, the desorption of C2H2 gas was much slower. The productivity of C2H2 with over 95% purity was estimated from the desorption curve to be 1.98 mmol for 1 g of JXNU-11(Fe2Ni) (Figure 6b). Figure 6 | Breakthrough experiments. (a) and (b) Breakthrough curves and desorption curves of JXNU-11(Fe2Ni) based on the equimolar C2H2/CO2 mixture. Download figure Download PowerPoint To fully understand the underlying mechanism of the preferential adsorption of C2H2 over CO2 in JXNU-11(Fe2M), we performed comprehensive GCMC simulations to investigate the gas adsorption sites for JXNU-11(Fe2M). The calculated preferential binding sites of C2H2 are shown in Figures 7a and 7b. Four primary binding sites (sites I–IV) for C2H2 were found. As expected, the primary binding sites for C2H2 molecules are F and carboxylate O sites. For site I, site II, and site III, strong C−H···F hydrogen bonding notably occur between C2H2 and F atoms (H···F = 2.49–3.01 Å), which confirms the strong binding affinity of JXNU-11(Fe2M) toward C2H2. Compared with these sites, site IV exhibits strong C−H⋯O hydrogen bonds with a pair of carboxylate O atoms with the distances of 2.61 and 2.60 Å. These H⋯F and H⋯O distances are shorter than the sums of the corresponding van der Waals radii of hydrogen and fluorine (2.67 Å) or hydrogen and oxygen (2.72 Å) atoms, suggesting the substantial interactions. Moreover, the C≡C group of C2H2 interacts with the adjacent open metal sites through π⋯M interactions, and weakly interacts with the neighboring aromatic ring units of the framework through π⋯π interactions. Thus these multiple interactions between C2H2 molecules and the framework synergistically result in the remarkable affinity for C2H2. Therefore, the nanosized double-shell nests with many highly electronegative F and O atoms provide the polarized and fluorophilic pore spaces to accommodate C2H2. In contrast, the negatively charged F and O sites are not desirable sites for CO2. The presence of the repulsive interactions between the F/O atoms and the O atoms of CO2 leads to a weaker interaction between CO2 and the framework, which is consistent with the experimental results. The computational results gave an average binding energy of 31.8 kJ mol−1 for C2H2, which is much higher than that of 20.9 kJ mol−1 for CO2, further corroborating the stronger affinity toward C2H2 in the present framework. These calculated results clearly reveal that the rich F and O atoms as well as the open metal sites in the cage-in-cage structures are synergistically responsible for the preferential adsorption of C2H2 over CO2, thus affording the top-level C2H2/CO2 separation performance of JXNU-11(Fe2M). Figure 7 | Mechanism study. adsorption sites of C2H2 in JXNU-11(Fe2M). (a) I and (b) and Ni F and O in and H in C2H2. The labeled is measured in Å. Download figure Download PowerPoint two MOFs M = Ni and featuring nanosized structures and highly polarized and fluorophilic pore Such a cage-in-cage structure formed is composed of an and an large The double-shell nested cages in the MOFs provide the fascinating and the optimized pore spaces for trapping C2H2. The abundant F and O atoms of the organic ligands in JXNU-11(Fe2M) JXNU-11(Fe2M) with multiple binding sites for C2H2. The F and O atoms possessing high in JXNU-11(Fe2M) the pore surfaces of the cages result in the highly dense on the pore which are the desirable sites for trapping C2H2 with charged H atoms. As a JXNU-11(Fe2M) shows a strong binding affinity to C2H2 over CO2. The nanosized double-shell cages in JXNU-11(Fe2M) afford the optimized pore spaces for trapping C2H2, leading to an efficient C2H2/CO2 separation. The practical breakthrough performance of JXNU-11(Fe2Ni) the breakthrough time for C2H2 gas and the C2H2-capture the highest observed at ambient confirming the C2H2/CO2 separation performance of JXNU-11(Fe2M) Thus we a strategy for the design of cage-in-cage structures in MOFs and develop an efficient for the of the highly polarized and fluorophilic pore cages in the fluorous Supporting Information Supporting Information is and detailed experimental crystal data for JXNU-11(Fe2M), adsorption column breakthrough experiments, and GCMC simulation. of is no of to Information This work is by the of and and the for and of of Jiangxi of and of Acetylene and by the of K. Chemistry and of Metal–Organic and of Frameworks to MOF and on Adsorption in a as the in the of the Chemistry of and Metal–Organic Frameworks and Zhang of Frameworks Featuring Separation Metal–Organic Frameworks and for of Surfaces by in Synthesis of in a