Expanded Organic Building Units for the Construction of Highly Porous Metal–Organic Frameworks

Two new organic building units that contain dicarboxylate sites for their self‐assembly with paddlewheel [Cu₂(CO₂)₄] units have been successfully developed to construct two isoreticular porous metal–organic frameworks (MOFs), ZJU‐35 and ZJU‐36, which have the same tbo topologies (Reticular Chemistry Structure Resource (RCSR) symbol) as HKUST‐1. Because the organic linkers in ZJU‐35 and ZJU‐36 are systematically enlarged, the pores in these two new porous MOFs vary from 10.8 Å in HKUST‐1 to 14.4 Å in ZJU‐35 and 16.5 Å in ZJU‐36, thus leading to their higher porosities with Brunauer–Emmett–Teller (BET) surface areas of 2899 and 4014 m² g^?1 for ZJU‐35 and ZJU‐36, respectively. High‐pressure gas‐sorption isotherms indicate that both ZJU‐35 and ZJU‐36 can take up large amounts of CH₄ and CO₂, and are among the few porous MOFs with the highest volumetric storage of CH₄ under 60 bar and CO₂ under 30 bar at room temperature. Their potential for high‐pressure swing adsorption (PSA) hydrogen purification was also preliminarily examined and compared with several reported MOFs, thus indicating the potential of ZJU‐35 and ZJU‐36 for this important application. Studies show that most of the highly porous MOFs that can volumetrically take up the greatest amount of CH₄ under 60 bar and CO₂ under 30 bar at room temperature are those self‐assembled from organic tetra‐ and hexacarboxylates that contain m‐benzenedicarboxylate units with the [Cu₂(CO₂)₄] units, because this series of MOFs can have balanced porosities, suitable pores, and framework densities to optimize their volumetric gas storage. The realization of the two new organic building units for their construction of highly porous MOFs through their self‐assembly with [Cu₂(CO₂)₄] units has provided great promise for the exploration of a large number of new tetra‐ and hexacarboxylate organic linkers based on these new organic building units in which different aromatic backbones can be readily incorporated into the frameworks to tune their porosities, pore structures, and framework densities, thus targeting some even better performing MOFs for very high gas storage and efficient gas separation under high pressure and at room temperature in the near future.     

Preparation of porous monoliths via CO2‐in‐water HIPEs template and the in situ growth of metal organic frameworks on it for multiple applications

Monolithic porous copolymers with 3D structure were prepared via CO₂‐in‐water high internal phase emulsions template by graft copolymerization of sodium methacrylate (MAANa) on to methyl cellulose (MC) backbone. The yielded copolymer monoliths are characterized by Fourier transform infrared spectra, scanning electron microscopy (SEM), and mechanical instrument, the swelling degree of MC‐g‐PMAANa monoliths with different crosslinker in diverse pH were investigated. The adsorption performance of monolith to Cu(II) were conducted to explore its adsorption capacity to heavy metal ions from the wastewater. Then, a strategy of in situ growth of metal‐organic frameworks (MOFs) on MC‐g‐PMAANa that adsorbed with metal ions was proposed first. The X‐ray powder diffraction, SEM, and Brunauer‐Emmett‐Teller (BET) surface area result of MC‐g‐PMAANa/MOFs composites indicated that the MOFs nanoparticles were grown uniformly on the monolith wall without destroying its original 3D porous structure. Compared with MOFs nanoparticle, MC‐g‐PMAANa/MOFs composites have advantages of easy operation and handle, which more conform to practical application. Furthermore, the antibacterial activity of MC‐g‐PMAANa/MOFs was evaluated by disk agar diffusion and optical density methods. In addition, MC‐g‐PMAANa/Cu‐BTC composite was applied to dye adsorption, which has proved the underlying application of such composites in dye removal.     

Gated Channels and Selectivity Tuning of CO2 over N2 Sorption by Post‐Synthetic Modification of a UiO‐66‐Type Metal–Organic Framework

The highly porous and stable metal–organic framework (MOF) UiO‐66 was altered using post‐synthetic modifications (PSMs). Prefunctionalization allowed the introduction of carbon double bonds into the framework through a four‐step synthesis from 2‐bromo‐1,4‐benzenedicarboxylic acid; the organic linker 2‐allyl‐1,4‐benzenedicarboxylic acid was obtained. The corresponding functionalized MOF (UiO‐66‐allyl) served as a platform for further PSMs. From UiO‐66‐allyl, epoxy, dibromide, thioether, diamine, and amino alcohol functionalities were synthesized. The abilities of these compounds to adsorb CO₂ and N₂ were compared, which revealed the structure–selectivity correlations. All synthesized MOFs showed profound thermal stability together with an increased ability for selective CO₂ uptake and molecular gate functionalities at low temperatures.     

Water‐stable Adenine‐based MOFs with Polar Pores for Selective CO2 Capture

Here, we report two novel water‐stable amine‐functionalized MOFs, namely IISERP‐MOF26 ([NH₂(CH₃)₂][Cu₂O(Ad)(BDC)]?(H₂O)₂(DMA), 1 ) and IISERP‐MOF27 ([NH₂(CH₃)₂]_1/2[Zn₄O(Ad)₃(BDC)₂]?(H₂O)₂(DMF)_1/2, 2 ), which show selective CO₂ capture capabilities. They are made by combining inexpensive and readily available terephthalic acid and N‐rich adenine with Cu and Zn, respectively. They possess 1D channels decorated by the free amine group from the adenine and the polarizing oxygen atoms from the terephthalate units. Even more, there are dimethyl ammonium (DMA⁺) cations in the pore rendering an electrostatic environment within the channels. The activated Cu‐ and Zn‐MOFs physisorb about 2.7 and 2.2 mmol g^?1 of CO₂, respectively, with high CO₂/N₂ and moderate CO₂/CH₄ selectivity. The calculated heat of adsorption (HOA=21–23 kJ mol^?1) for the CO₂ in both MOFs suggest optimal physical interactions which corroborate well with their facile on‐off cycling of CO₂. Notably, both MOFs retain their crystallinity and porosity even after soaking in water for 24 hours as well as upon exposure to steam over 24 hours. The exceptional thermal and chemical stability, favorable CO₂ uptakes and selectivity and low HOA make these MOFs promising sorbents for selective CO₂ capture applications. However, the MOF′s low heat of adsorption despite having a highly CO₂‐loving groups lined walls is quite intriguing.     

Studies on Photocatalytic CO2 Reduction over NH2‐Uio‐66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks

Metal–organic framework (MOF) NH₂‐Uio‐66(Zr) exhibits photocatalytic activity for CO₂ reduction in the presence of triethanolamine as sacrificial agent under visible‐light irradiation. Photoinduced electron transfer from the excited 2‐aminoterephthalate (ATA) to Zr oxo clusters in NH₂‐Uio‐66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr^III and its involvement in photocatalytic CO₂ reduction was confirmed by ESR analysis. Moreover, NH₂‐Uio‐66(Zr) with mixed ATA and 2,5‐diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO₂ reduction due to its enhanced light adsorption and increased adsorption of CO₂. This study provides a better understanding of photocatalytic CO₂ reduction over MOF‐based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO₂ reduction.     

Dynamic Entangled Porous Framework for Hydrocarbon (C2–C3) Storage,CO2 Capture,and Separation

Storage and separation of small (C1–C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO₂ from CH₄ is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two‐fold entangled dynamic framework {[Zn₂(bdc)₂(bpNDI)]?4DMF}_n with pore surface carved with polar functional groups and aromatic π clouds is exploited for selective capture of CO₂, C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO₂ and C₂H₂ uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO₂ over CH₄ is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C₂H₂, C₂H₄, C₂H₆, and C₃H₈ over CH₄ at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO₂/CH₄ C₂H₆/CH₄ and CO₂/N₂ separation capability at ambient condition.     

Two highly porous single-crystalline zirconium-based metal-organic frameworks

Herein we report two highly porous Zr-based metal-organic frameworks(MOFs, 1 and 2) constructed by the truncated octahedral secondary building unit(SBU) of Zr_6O_4(OH)_4(CO_2)_(12) and the organic linear ligand of 4,4.-stilbenedicarboxylic acid(H_2sbdc) or4,4′-azobenezenedicarboxylic acid(H_2abdc). Both Zr-based MOFs are obtained as single crystals of suitable size for single-crystal X-ray diffraction analysis. Furthermore, these two Zr-based MOFs have been fully characterized by powder X-ray diffraction(PXRD) studies, thermogravimetric analysis(TGA), infrared spectroscopy(IR) and gas adsorption analysis. In particular, their CO_2 gas adsorption behaviors have been investigated and discussed.     

A PCP Pincer Ligand for Coordination Polymers with Versatile Chemical Reactivity: Selective Activation of CO2 Gas over CO Gas in the Solid State

A tetra(carboxylated) PCP pincer ligand has been synthesized as a building block for porous coordination polymers (PCPs). The air‐ and moisture‐stable PCP metalloligands are rigid tetratopic linkers that are geometrically akin to ligands used in the synthesis of robust metal–organic frameworks (MOFs). Here, the design principle is demonstrated by cyclometalation with Pd^IICl and subsequent use of the metalloligand to prepare a crystalline 3D MOF by direct reaction with Co^II ions and structural resolution by single crystal X‐ray diffraction. The Pd?Cl groups inside the pores are accessible to post‐synthetic modifications that facilitate chemical reactions previously unobserved in MOFs: a Pd?CH₃ activated material undergoes rapid insertion of CO₂ gas to give Pd?OC(O)CH₃ at 1 atm and 298 K. However, since the material is highly selective for the adsorption of CO₂ over CO, a Pd?N₃ modified version resists CO insertion under the same conditions.     

Highly efficient CO2 capture with a metal–organic framework‐derived porous carbon impregnated with polyethyleneimine

Global warming is considered as one of the great challenges of the twenty‐first century. Application of CO₂ capture and storage technologies to flue gas is considered to be a useful method of lessening global warning. Highly porous carbon has played an important role in tackling energy and environmental problems. We attempted to synthesize a highly porous carbon adsorbent by carbonizing a highly crystalline metal–organic framework (MOF) without any carbon precursors and focused on the adsorption of CO₂ and CH₄ gases and CO₂/CH₄ selectivity at 298, 323 and 348 K using a volumetric apparatus. The MOF‐derived porous carbon (MDC) was prepared by direct carbonization of MOF‐199 as a template at 900 °C under nitrogen atmosphere. Amino‐impregnated MDC samples exhibited enhanced adsorption capacities by a combination of physical and chemical adsorption. Polyethyleneimine (PEI) was selected as the amine source, which was found to greatly enhance CO₂ capture when supported on the porous carbon. Novel PEI‐impregnated MDC nanocomposites were synthesized by wetness impregnation and then characterized using various methods.     

Evidence of Amine–CO2 Interactions in Two Pillared‐Layer MOFs Probed by X‐ray Crystallography

Two pillared‐layer metal–organic frameworks (MOFs; PMOF‐55 and NH₂‐PMOF‐55) based on 1,2,4‐triazole and terephthalic acid (bdc)/NH₂‐bdc ligands were assembled and display framework stabilities, to a certain degree, in both acid/alkaline solutions and toward water. They exhibit high CO₂ uptakes and selective CO₂/N₂ adsorption capacities, with CO₂/N₂ selectivity in the range of 24–27, as calculated by the ideal adsorbed solution theory method. More remarkably, the site and interactions between the host network and the CO₂ molecules were investigated by single‐crystal X‐ray diffraction, which showed that the main interaction between the CO₂ molecules and PMOF‐55 is due to multipoint supramolecular interactions of C?H???O, C???O, and O???O. Amino functional groups were shown to enhance the CO₂ adsorption and identified as strong adsorption sites for CO₂ by X‐ray crystallography.     

Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks

Reducing anthropogenic CO₂ emission and lowering the concentration of greenhouse gases in the atmosphere has quickly become one of the most urgent environmental issues of our age. Carbon capture and storage (CCS) is one option for reducing these harmful CO₂ emissions. While a variety of technologies and methods have been developed, the separation of CO₂ from gas streams is still a critical issue. Apart from establishing new techniques, the exploration of capture materials with high separation performance and low capital cost are of paramount importance. Metal-organic frameworks (MOFs), a new class of crystalline porous materials constructed by metal-containing nodes bonded to organic bridging ligands hold great potential as adsorbents or membrane materials in gas separation. In this paper, we review the research progress (from experimental results to molecular simulations) in MOFs for CO₂ adsorption, storage, and separations (adsorptive separation and membrane-based separation) that are directly related to CO₂ capture.     

Adsorption of Carbon Dioxide on Unsaturated Metal Sites in M2(dobpdc) Frameworks with Exceptional Structural Stability and Relation between Lewis Acidity and Adsorption Enthalpy

A series of metal–organic frameworks (MOFs) M₂(dobpdc) (M=Mn, Co, Ni, Zn; H₄dobpdc=4,4′‐dihydroxy‐1,1′‐biphenyl‐3,3′‐dicarboxylic acid), with a highly dense arrangement of open metal sites along hexagonal channels were prepared by microwave‐assisted or simple solvothermal reactions. The activated materials were structurally expanded when guest molecules including CO₂ were introduced into the pores. The Lewis acidity of the open metal sites varied in the order Mn<Co<Ni>Zn, as confirmed by C=O stretching bands in the IR spectra, which are related to the CO₂ adsorption enthalpy. DFT calculations revealed that the high CO₂ binding affinity of transition‐metal‐based M₂(dobpdc) is primarily attributable to the favorable charge transfer from CO₂ (oxygen lone pair acting as a Lewis base) to the open metal sites (Lewis acid), while electrostatic effects, the underlying factor responsible for the particular order of binding strength observed across different transition metals, also play a role. The framework stability against water coincides with the order of Lewis acidity. In this series of MOFs, the structural stability of Ni₂(dobpdc) is exceptional; it endured in water vapor, liquid water, and in refluxing water for one month, and the solid remained intact on exposure to solutions of pH 2–13. The DFT calculations also support the experimental finding that Ni₂(dobpdc) has higher chemical stability than the other frameworks.     

Modeling Adsorption and Optical Properties for the Design of CO2 Photocatalytic Metal-Organic Frameworks

Four Metal-Organic Frameworks (MOFs) were modeled (IRMOF-C-BF₂, IRMOF-C-(2)-BF₂, IRMOF-C’-BF₂, and IRMOF-C-CH₂BF₂) based on IRMOF-1. A series of linkers, based on Frustrated Lewis Pairs and coumarin moieties, were attached to IRMOF-1 to obtain MOFs with photocatalytic properties. Four different linkers were used: (a) a BF₂ attached to a coumarin moiety at position 3, (b) two BF₂ attached to a coumarin moiety in positions 3 and 7, (c) a BF₂ attached in the coumarin moiety at position 7, and (d) a CH₂BF₂ attached at position 3. An analysis of the adsorption properties of H₂, CO₂, H₂O and possible CO₂ photocatalytic capabilities was performed by means of computational modeling using Density Functional Theory (DFT), Time-Dependent Density Functional (TD-DFT) methods, and periodic quantum chemical wave function approach. The results show that the proposed linkers are good enough to improve the CO₂ adsorption, to hold better bulk properties, and obtain satisfactory optical properties in comparison with IRMOF-1 by itself.     

Metal–Organic Frameworks with Target‐Specific Active Sites Switched by Photoresponsive Motifs: Efficient Adsorbents for Tailorable CO2 Capture

Photoresponsive metal–organic frameworks (PMOFs) are of interest for tailorable CO₂ adsorption. However, modulation of CO₂ adsorption on PMOFs is based on steric hindrance or structural change owing to weak interactions between CO₂ and active sites. It is challenging to fabricate PMOFs with strong but tailorable sites for CO₂ adsorption. Now, the construction of PMOFs with target‐specific (strong) active sites is achieved by introducing tetraethylenepentamine into azobenzene‐functionalized MOFs for tailorable CO₂ adsorption. Amines are specific active sites for CO₂, contributing to capture CO₂ selectively. Cis/trans isomerization of azobenzene motifs trigged by UV/Vis light adjusts the electrostatic potential of amines significantly, leading to exposure/shelter of amines and modulation of CO₂ adsorption on strong active sites. This system enables us to design adsorption processes for CO₂ capture from mixtures, which is impossible to realize by traditional PMOFs.     

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.     

Nanocrystal/Metal–Organic Framework Hybrids as Electrocatalytic Platforms for CO2 Conversion

The tunable chemistry linked to the organic/inorganic components in colloidal nanocrystals (NCs) and metal–organic frameworks (MOFs) offers a rich playground to advance the fundamental understanding of materials design for various applications. Herein, we combine these two classes of materials by synthesizing NC/MOF hybrids comprising Ag NCs that are in intimate contact with Al‐PMOF ([Al₂(OH)₂(TCPP)]) (tetrakis(4‐carboxyphenyl)porphyrin (TCPP)), to form Ag@Al‐PMOF. In our hybrids, the NCs are embedded in the MOF while still preserving electrical contact with a conductive substrate. This key feature allows the investigation of the Ag@Al‐PMOFs as electrocatalysts for the CO₂ reduction reaction (CO₂RR). We show that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO₂RR. We also demonstrate a minor contribution of mass‐transfer effects imposed by the porous MOF layer under the chosen testing conditions. Furthermore, we find an increased morphological stability of the Ag NCs when combined with the Al‐PMOF. The synthesis method is general and applicable to other metal NCs, thus revealing a new way to think about rationally tailored electrocatalytic materials to steer selectivity and improve stability.     

Encapsulation of a Porous Organic Cage into the Pores of a Metal–Organic Framework for Enhanced CO2 Separation

We present a facile approach to encapsulate functional porous organic cages (POCs) into a robust MOF by an incipient‐wetness impregnation method. Porous cucurbit[6]uril (CB6) cages with high CO₂ affinity were successfully encapsulated into the nanospace of Cr‐based MIL‐101 while retaining the crystal framework, morphology, and high stability of MIL‐101. The encapsulated CB6 amount is controllable. Importantly, as the CB6 molecule with intrinsic micropores is smaller than the inner mesopores of MIL‐101, more affinity sites for CO₂ are created in the resulting CB6@MIL‐101 composites, leading to enhanced CO₂ uptake capacity and CO₂/N₂, CO₂/CH₄ separation performance at low pressures. This POC@MOF encapsulation strategy provides a facile route to introduce functional POCs into stable MOFs for various potential applications.     

Combination of Optimization and Metalated‐Ligand Exchange: An Effective Approach to Functionalize UiO‐66(Zr) MOFs for CO2 Separation

The strategy to functionalize water‐stable metal–organic frameworks (MOFs) in order to improve their CO₂ uptake capacities for efficient CO₂ separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water‐stable MOF, UiO‐66(Zr), by a combination of optimization and metalated‐ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO‐66(Zr) of the highest BET surface area reported so far (1730 m² g^?1). Moreover, it shows a hybrid Type I/IV N₂ isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO‐66 MOF underwent a metalated‐ligand‐exchange (MLE) process to yield a series of new UiO‐66‐type MOFs, among which UiO‐66‐(COONa)₂‐EX and UiO‐66‐(COOLi)₄‐EX MOFs have both enhanced CO₂ working capacity and IAST CO₂/N₂ selectivity. Our approach has thus suggested an alternative design to achieve water‐stable MOFs with high crystallinity and gas uptake for efficient CO₂ separation.     

Tuning Internal Strain in Metal–Organic Frameworks via Vapor Phase Infiltration for CO2 Reduction

A gas‐phase approach to form Zn coordination sites on metal–organic frameworks (MOFs) by vapor‐phase infiltration (VPI) was developed. Compared to Zn sites synthesized by the solution‐phase method, VPI samples revealed approximately 2.8 % internal strain. Faradaic efficiency towards conversion of CO₂ to CO was enhanced by up to a factor of four, and the initial potential was positively shifted by 200–300 mV. Using element‐specific X‐ray absorption spectroscopy, the local coordination environment of the Zn center was determined to have square‐pyramidal geometry with four Zn?N bonds in the equatorial plane and one Zn‐OH₂ bond in the axial plane. The fine‐tuned internal strain was further supported by monitoring changes in XRD and UV/Visible absorption spectra across a range of infiltration cycles. The ability to use internal strain to increase catalytic activity of MOFs suggests that applying this strategy will enhance intrinsic catalytic capabilities of a variety of porous materials.     

Ligand and Metal Effects on the Stability and Adsorption Properties of an Isoreticular Series of MOFs Based on T‐Shaped Ligands and Paddle‐Wheel Secondary Building Units

The synthesis of stable porous materials with appropriate pore size and shape for desired applications remains challenging. In this work a combined experimental/computational approach has been undertaken to tune the stability under various conditions and the adsorption behavior of a series of MOFs by subtle control of both the nature of the metal center (Co²⁺, Cu²⁺, and Zn²⁺) and the pore surface by the functionalization of the organic linkers with amido and N‐oxide groups. In this context, six isoreticular MOFs based on T‐shaped ligands and paddle‐wheel units with ScD_0.33 topology have been synthesized. Their stabilities have been systematically investigated along with their ability to adsorb a wide range of gases (N₂, CO₂, CH₄, CO, H_2, light hydrocarbons (C₁–C₄)) and vapors (alcohols and water). This study has revealed that the MOF frameworks based on Cu²⁺ are more stable than their Co²⁺ and Zn²⁺ analogues, and that the N‐oxide ligand endows the MOFs with a higher affinity for CO₂ leading to excellent selectivity for this gas over other species.