Effect of Functionalized Groups on Gas‐Adsorption Properties: Syntheses of Functionalized Microporous Metal–Organic Frameworks and Their High Gas‐Storage Capacity

The microporous metal–organic framework (MMOF) Zn₄O(L1)₂ ? 9 DMF ? 9 H₂O ( 1‐H ) and its functionalized derivatives Zn₄O(L1‐CH₃)₂ ? 9 DMF ? 9 H₂O ( 2‐CH₃ ) and Zn₄O(L1‐Cl)₂ ? 9 DMF ? 9 H₂O ( 3‐Cl ) have been synthesized and characterized (H₃L1=4‐[N,N‐bis(4‐methylbenzoic acid)amino]benzoic acid, H₃L1‐CH₃=4‐[N,N‐bis(4‐methylbenzoic acid)amino]‐2‐methylbenzoic acid, H₃L1‐Cl=4‐[N,N‐bis(4‐methylbenzoic acid)amino]‐2‐chlorobenzoic acid). Single‐crystal X‐ray diffraction analyses confirmed that the two functionalized MMOFs are isostructural to their parent MMOF, and are twofold interpenetrated three‐dimensional (3D) microporous frameworks. All of the samples possess enduring porosity with Langmuir surface areas over 1950 cm² g^?1. Their pore volumes and surface areas decrease in the order 1‐H > 2‐CH₃ > 3‐Cl . Gas‐adsorption studies show that the H₂ uptakes of these samples are among the highest of the MMOFs (2.37 wt % for 3‐Cl at 77 K and 1 bar), although their structures are interpenetrating. Furthermore, this work reveals that the adsorbate–adsorbent interaction plays a more important role in the gas‐adsorption properties of these samples at low pressure, whereas the effects of the pore volumes and surface areas dominate the gas‐adsorption properties at high pressure.     

Ligand Functionalization and Its Effect on CO2 Adsorption in Microporous Metal–Organic Frameworks

We report two new 3D structures, [Zn₃(bpdc)₃(2,2′‐dmbpy)] (DMF)_x(H₂O)_y ( 1 ) and [Zn₃(bpdc)₃(3,3′‐dmbpy)]?(DMF)₄(H₂O)_0.5 ( 2 ), by methyl functionalization of the pillar ligand in [Zn₃(bpdc)₃(bpy)] (DMF)₄?(H₂O) ( 3 ) (bpdc=biphenyl‐4,4′‐dicarboxylic acid; z,z′‐dmbpy=z,z′‐dimethyl‐4,4′‐bipyridine; bpy=4,4′‐bipyridine). Single‐crystal X‐ray diffraction analysis indicates that 2 is isostructural to 3 , and the power X‐ray diffraction (PXRD) study shows a very similar framework of 1 to 2 and 3 . Both 1 and 2 are 3D porous structures made of Zn₃(COO)₆ secondary building units (SBUs) and 2,2′‐ or 3,3′‐dmbpy as pillar ligand. Thermogravimetric analysis (TGA) and PXRD studies reveal high thermal and water stability for both compounds. Gas‐adsorption studies show that the reduction of surface area and pore volume by introducing a methyl group to the bpy ligand leads to a decrease in H₂ uptake for both compounds. However, CO₂ adsorption experiments with 1′ (guest‐free 1 ) indicate significant enhancement in CO₂ uptake, whereas for 2′ (guest‐free 2 ) the adsorbed amount is decreased. These results suggest that there are two opposing and competitive effects brought on by methyl functionalization: the enhancement due to increased isosteric heats of CO₂ adsorption (Q_st), and the detraction due to the reduction of surface area and pore volume. For 1′ , the enhancement effect dominates, which leads to a significantly higher uptake of CO₂ than its parent compound 3′ (guest‐free 3 ). For 2′ , the detraction effect predominates, thereby resulting in reduced CO₂ uptake relative to its parent structure 3′ . IR and Raman spectroscopic studies also present evidence for strong interaction between CO₂ and methyl‐functionalized π moieties. Furthermore, all compounds exhibit high separation capability for CO₂ over other small gases including CH₄, CO, N₂, and O₂.     

Tailor‐Made Metal–Organic Frameworks from Functionalized Molecular Building Blocks and Length‐Adjustable Organic Linkers by Stepwise Synthesis

In this work, we have demonstrated a family of diamondoid metal–organic frameworks (MOFs) based on functionalized molecular building blocks and length‐adjustable organic linkers by using a stepwise synthesis strategy. We have successfully achieved not only “design” and “control” to synthesize MOFs, but also the functionalization of both secondary building units (SBUs) and organic linkers in the same MOF for the first time. Furthermore, the results of N₂ and H₂ adsorption show that their surface areas and hydrogen uptake capacities are determined by the most optimal combination of functional groups from SBUs and organic linkers, interpenetration, and free volume in this system. A member of this series, DMOF‐6 exhibits the highest surface area and H₂ adsorption capacity among this family of MOFs.     

Unprecedented Sulfone‐Functionalized Metal–Organic Frameworks and Gas‐Sorption Properties

Gas storage : A new, sulfone‐functionalized dicarboxylate‐based ligand (see figure) is capable of directing the formation of novel metal–organic frameworks with unprecedented organic and inorganic secondary building units. A high CO₂ uptake with remarkable selectivity over CH₄, N₂, and H₂ was observed at near‐ambient temperature.

     

Extraordinary Separation of Acetylene‐Containing Mixtures with Microporous Metal–Organic Frameworks with Open O Donor Sites and Tunable Robustness through Control of the Helical Chain Secondary Building Units

Acetylene separation is a very important but challenging industrial separation task. Here, through the solvothermal reaction of CuI and 5‐triazole isophthalic acid in different solvents, two metal–organic frameworks (MOFs, FJU‐21 and FJU‐22 ) with open O donor sites and controllable robustness have been obtained for acetylene separation. They contain the same paddle‐wheel {Cu₂(COO₂)₄} nodes and metal–ligand connection modes, but with different helical chains as secondary building units (SBUs), leading to different structural robustness for the MOFs. FJU‐21 and FJU‐22 are the first examples in which the MOFs’ robustness is controlled by adjusting the helical chain SBUs. Good robustness gives the activated FJU‐22 a , which has higher surface area and gas uptakes than the flexible FJU‐21 a . Importantly, FJU‐22 a shows extraordinary separation of acetylene mixtures under ambient conditions. The separation capacity of FJU‐22 a for 50:50 C₂H₂/CO₂ mixtures is about twice that of the high‐capacity HOF‐3, and its actual separation selectivity for C₂H₂/C₂H₄ mixtures containing 1 % acetylene is the highest among reported porous materials. Based on first‐principles calculations, the extraordinary separation performance of C₂H₂ for FJU‐22 a was attributed to hydrogen‐bonding interactions between the C₂H₂ molecules with the open O donors on the wall, which provide better recognition ability for C₂H₂ than other functional sites, including open metal sites and amino groups.     

An Isoreticular Family of Microporous Metal–Organic Frameworks Based on Zinc and 2‐Substituted Imidazolate‐4‐amide‐5‐imidate: Syntheses,Structures and Properties

We report on a new series of isoreticular frameworks based on zinc and 2‐substituted imidazolate‐4‐amide‐5‐imidate (IFP‐1–4, IFP=imidazolate framework Potsdam) that form one‐dimensional, microporous hexagonal channels. Varying R in the 2‐substitued linker (R=Me (IFP‐1), Cl (IFP‐2), Br (IFP‐3), Et (IFP‐4)) allowed the channel diameter (4.0–1.7 Å), the polarisability and functionality of the channel walls to be tuned. Frameworks IFP‐2, IFP‐3 and IFP‐4 are isostructural to previously reported IFP‐1. The structures of IFP‐2 and IFP‐3 were solved by X‐ray crystallographic analyses. The structure of IFP‐4 was determined by a combination of PXRD and structure modelling and was confirmed by IR spectroscopy and ¹H MAS and ¹³C CP‐MAS NMR spectroscopy. All IFPs showed high thermal stability (345–400 °C); IFP‐1 and IFP‐4 were stable in boiling water for 7 d. A detailed porosity analysis was performed on the basis of adsorption measurements by using various gases. The potential of the materials to undergo specific interactions with CO₂ was investigated by measuring the isosteric heats of adsorption. The capacity to adsorb CH₄ (at 298 K), CO₂ (at 298 K) and H₂ (at 77 K) at high pressure were also investigated. In situ IR spectroscopy showed that CO₂ is physisorbed on IFP‐1–4 under dry conditions and that both CO₂ and H₂O are physisorbed on IFP‐1 under moist conditions.     

3D Microporous Base‐Functionalized Covalent Organic Frameworks for Size‐Selective Catalysis

The design and synthesis of 3D covalent organic frameworks (COFs) have been considered a challenge, and the demonstrated applications of 3D COFs have so far been limited to gas adsorption. Herein we describe the design and synthesis of two new 3D microporous base‐functionalized COFs, termed BF‐COF‐1 and BF‐COF‐2, by the use of a tetrahedral alkyl amine, 1,3,5,7‐tetraaminoadamantane (TAA), combined with 1,3,5‐triformylbenzene (TFB) or triformylphloroglucinol (TFP). As catalysts, both BF‐COFs showed remarkable conversion (96 % for BF‐COF‐1 and 98 % for BF‐COF‐2), high size selectivity, and good recyclability in base‐catalyzed Knoevenagel condensation reactions. This study suggests that porous functionalized 3D COFs could be a promising new class of shape‐selective catalysts.     

Metal–Organic Frameworks: Opportunities for Catalysis

The role of metal–organic frameworks (MOFs) in the field of catalysis is discussed, and special focus is placed on their assets and limits in light of current challenges in catalysis and green chemistry. Their structural and dynamic features are presented in terms of catalytic functions along with how MOFs can be designed to bridge the gap between zeolites and enzymes. The contributions of MOFs to the field of catalysis are comprehensively reviewed and a list of catalytic candidates is given. The subject is presented from a multidisciplinary point of view covering solid‐state chemistry, materials science, and catalysis.     

Efficient Separation of n‐Butene and iso‐Butene by Flexible Ultramicroporous Metal‐Organic Frameworks with Pocket‐like Cavities

Efficient separation of n‐butene (n‐C₄H₈) and iso‐butene (iso‐C₄H₈) is of significance for the upgrading of C4 olefins to high‐value end products but remains one of the major challenges in hydrocarbon purifications owing to their similar structures. Herein, we report a flexible metal‐organic framework, MnINA (INA=isonicotinate), featuring one‐dimensional pore channels with periodically large pocket‐like cavities connected by narrow bottlenecks, for the first time for efficient n‐/iso‐C₄H₈ separation. MnINA with smaller pore size (4.62 Å) compared with CuINA (4.84 Å), exhibits steep adsorption isotherms and high capacity of 1.79 mmol g^?1 for n‐C₄H₈ (4.46 Å) through strong host‐guest interactions via C?H???π bonding. The narrow bottlenecks exert barriers for the large molecules of iso‐C₄H₈ (4.84 Å) within the gate‐opening pressure range of 0–0.1 bar. This gives rise to MnINA with excellent separation selectivity of 327.7 for n‐/iso‐C₄H₈ mixture. The adsorption mechanism for n‐C₄H₈ and the gate‐opening effect were investigated by dispersion‐corrected density functional (DFT‐D) theory, verifying the strong interactions between n‐C₄H₈ and the frameworks as well as the gate‐opening effect derived from the rotation of organic linkers. The breakthrough tests confirmed MnINA and CuINA can be promising candidates for n‐/iso‐C₄H₈ separation.     

Unexpected Carbon Dioxide Inclusion in Water‐Saturated Pores of Metal–Organic Frameworks with Potential for Highly Selective Capture of CO2

Unusual CO₂ storage in water‐saturated MOFs was investigated by combining experiment and simulation. It was found that the micropores of HKUST‐1 saturated with water provide an environment that is thermodynamically and kinetically favorable for CO₂ capture, but not for N₂ and H₂ capture. We expect that this phenomenon have potential to be used for successful separation of CO₂ from versatile flue streams and pre‐combustion gas.     

Selective Capture of Toxic Selenite Anions by Bismuth‐based Metal–Organic Frameworks

Chemically durable and effective absorbent materials for selenite (SeO₃^2?) remain highly desirable for contamination remediation. Now a bismuth‐based metal–organic framework (Bi‐MOF, CAU‐17) was used as adsorbent to capture SeO₃^2? anions from aqueous solution with ultrahigh adsorption capacity of 255.3 mg g^?1 and fast kinetics. Furthermore, the adsorbent showed excellent selectivity for SeO₃^2? and was able to work steadily in a broad pH range of 4–11. Density functional theory (DFT) calculation, XANES modeling, and EXAFS fitting suggested that SeO₃^2? anions were immobilized by forming Bi?O?Se bonds (T‐3 structural model) though splitting the O?Bi?O bond in the crystal structure, leading to a structural transformation of CAU‐17 in the solid state.     

Metal‐Ion Metathesis and Properties of Triarylboron‐Functionalized Metal–Organic Frameworks

An anionic metal–organic framework, H₃[(Mn₄Cl)₃ L ₈]?30 H₂O?2.5 DMF?5 Diox ( UPC‐15 ), was successfully prepared by the reaction of MnCl₂ with tris(p‐carboxylic acid)tridurylborane (H₃ L ) under solvothermal conditions. UPC‐15 with wide‐open pores (～18.8 Å) is constructed by packing of octahedral and cuboctahedral cages, and exhibits high gas‐sorption capabilities. Notably, UPC‐15 shows selective adsorption of cationic dyes due to the anion framework. Moreover, the catalytic and magnetic properties were investigated, and UPC‐15 can highly catalyze the cyanosilylation of aromatic aldehydes. UPC‐15 exhibits the exchange of metal ions from Mn to Cu in a single‐crystal‐to‐single‐crystal manner to generate UPC‐16 , which could not be obtained by the direct solvothermal reaction of CuCl₂ and H₃ L. UPC‐16 exhibits similar properties for gas sorption, dye separation, and catalytic activity. However, the magnetic behaviors for UPC‐15 and UPC‐16 are distinct due to the metal‐specific properties. Below 47 K, UPC‐15 exhibits a ferromagnetic coupling but UPC‐16 shows a dominant antiferromagnetic behavior.     

Hierarchical Mesoporous Metal–Organic Frameworks for Enhanced CO2 Capture

Hierarchical porous materials are promising for catalyst, separation and sorption applications. A ligand‐assisted etching process is developed for template‐free synthesis of hierarchical mesoporous MOFs as single crystals and well‐intergrown membranes at 40 °C. At 223 K, the hierarchical porous structures significantly improve the CO₂ capture capacity of HKUST‐1 by more than 44 % at pressures up to 20 kPa and 13 % at 100 kPa. Even at 323 K, the enhancement of CO₂ uptake is above 25 % at pressures up to 20 kPa and 7 % at 100 kPa. The mesoporous structures not only enhance the CO₂ uptake capacity but also improve the diffusion and mass transportation of CO₂. Similarly, well‐intergrown mesoporous HKUST‐1 membranes are synthesized, which hold the potential for film‐like porous devices. Mesoporous MOF‐5 crystals are also obtained by a similar ligand‐assisted etching process. This may provide a facile way to prepare hierarchical porous MOF single crystals and membranes for wide‐ranging applications.     

Controlled Construction of Metal–Organic Frameworks: Hydrothermal Synthesis,X‐ray Structure,and Heterogeneous Catalytic Study

The role of pH in the formation of metal–organic frameworks (MOFs) has been studied for a series of magnesium‐based carboxylate framework systems. Our investigations have revealed the formation of five different zero‐dimensional (0D) to three‐dimensional (3D) ordered frameworks from the same reaction mixture, merely by varying the pH of the medium. The compounds were synthesized by the hydrothermal method and characterized by single‐crystal X‐ray diffraction. Increase of the pH of the medium led to abstraction of the imine hydrogen from the ligand and a concomitant increase in the OH^? ion concentration in the solution, facilitating the construction of higher dimensional framework compounds. A stepwise increase in pH resulted in a stepwise increase in the dimensionality of the network, ultimately leading to the formation of a 3D porous solid. A gas adsorption study of the 3D framework compound confirmed its microporosity with a BET surface area of approximately 450 m² g^?1. Notably, the 3D framework compound catalyzes aldol condensation reactions of various aromatic aldehydes with acetone under heterogeneous conditions.     

Selective Capture of Carbon Dioxide under Humid Conditions by Hydrophobic Chabazite‐Type Zeolitic Imidazolate Frameworks

Hydrophobic zeolitic imidazolate frameworks (ZIFs) with the chabazite ( CHA ) topology are synthesized by incorporating two distinct imidazolate links. Zn(2‐mIm)_0.86(bbIm)_1.14 (ZIF‐300), Zn(2‐mIm)_0.94(cbIm)_1.06 (ZIF‐301), and Zn(2‐mIm)_0.67(mbIm)_1.33 (ZIF‐302), where 2‐mIm=2‐methylimidazolate, bbIm=5(6)‐bromobenzimidazolate, cbIm=5(6)‐chlorobenzimidazolate, and mbIm=5(6)‐methylbenzimidazolate, were prepared by reacting zinc nitrate tetrahydrate and 2‐mIm with the respective bIm link in a mixture of N,N‐dimethylformamide (DMF) and water. Their structures were determined by single‐crystal X‐ray diffraction and their permanent porosity shown. All of these structures are hydrophobic as confirmed by water adsorption isotherms. All three ZIFs are equally effective at the dynamic separation of CO₂ from N₂ under both dry and humid conditions without any loss of performance over three cycles and can be regenerated simply by using a N₂ flow at ambient temperature.     

Selective and Sensitive Aqueous‐Phase Detection of 2,4,6‐Trinitrophenol (TNP) by an Amine‐Functionalized Metal–Organic Framework

Highly selective and sensitive aqueous‐phase detection of nitro explosive 2,4,6‐trinitrophenol (TNP) by a hydrolytically stable 3D luminescent metal–organic framework is reported. The compound senses TNP exclusively even in the presence of other nitro‐compounds, with an unprecedented sensitivity in the MOF regime by means of strategic deployment of its free amine groups. Such an accurate sensing of TNP, widely recognized as a harmful environmental contaminant in water media, establishes this new strategic approach as one of the frontiers to tackle present‐day security and health concerns in a real‐time scenario.     

Adsorption of CO2, CH4, and N2 on Zeolitic Imidazolate Frameworks: Experiments and Simulations

Experimental measurements and molecular simulations were conducted for two zeolitic imidazolate frameworks, ZIF‐8 and ZIF‐76. The transferability of the force field was tested by comparing molecular simulation results of gas adsorption with experimental data available in the literature for other ZIF materials (ZIF‐69). Owing to the good agreement observed between simulation and experimental data, the simulation results can be used to identify preferential adsorption sites, which are located close to the organic linkers. Topological mapping of the potential‐energy surfaces makes it possible to relate the preferential adsorption sites, Henry constant, and isosteric heats of adsorption at zero coverage to the nature of the host–guest interactions and the chemical nature of the organic linker. The role played by the topology of the solid and the organic linkers, instead of the metal sites, upon gas adsorption on zeolite‐like metal–organic frameworks is discussed.     

Thermoplastic Membranes Incorporating Semiconductive Metal–Organic Frameworks: An Advance on Flexible X‐ray Detectors

Semiconductive metal–organic frameworks (MOFs) have emerged in applications such as chemical sensors, electrocatalysts, energy storage materials, and electronic devices. However, examples of semiconductive MOFs within flexible electronics have not been reported. We present flexible X‐ray detectors prepared by thermoplastic dispersal of a semiconductive MOF ( SCU‐13 ) through a commercially available polymer, poly(vinylidene fluoride). The flexible detectors exhibit efficient X‐ray‐to‐electric current conversion with enhanced charge‐carrier mobility and low trap density compared to pelleted devices. A high X‐ray detection sensitivity of 65.86 μCGy_air^?1 cm^?2 was achieved, which outperforms other pelleted devices and commercial flexible X‐ray detectors. We demonstrate that the MOF‐based flexible detectors can be operated at multiple bending angles without a deterioration in detection performance. As a proof‐of‐concept, an X‐ray phase contrast under bending conditions was constructed using a 5×5 pixelated MOF‐based imager.