A Multivariate 2D Metal-Organic Framework with Open Metal Sites for Catalytic CO2 Cycloaddition and Cyanosilylation Reactions

This work reports the synthesis of a dual functional 2D framework, {[Zn2(4-tpom)2(oxdz)2]·4H2O}n (1), at room temperature, where a bent dicarboxylate, oxdz2- (4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoate), and a neutral flexible N-donor linker, 4-tpom (tetrakis(4-pyridyloxymethylene)methane), are utilized. Its single-crystal X-ray analysis indicated a 2-fold interpenetrated 2D framework having tetracoordinated Zn(II) centers and dangling pyridyl groups. Its further characterization was carried out with elemental microanalysis, FTIR spectroscopy, TG analysis, and powder X-ray diffraction. The tetracoordinated Zn(II) centers as active Lewis acidic sites and the N atoms of 4-tpom as Lewis basic sites in 1 are explored for its functioning as a heterogeneous catalyst in two important reactions, (i) cycloaddition of CO2 with various epoxides and (ii) cyanosilylation reaction under solvent-free conditions. We could successfully show the cycloaddition of styrene oxide with CO2 (99% conversion) under balloon pressure with low catalyst (0.2-0.3 mol %) and cocatalyst (0.5-0.75 mol %) loadings, which is otherwise difficult to achieve. It was observed that all the substrates (aromatic and aliphatic), irrespective of their sizes, showed conversion percentage >99%. In the cyanosilylation reaction, a conversion of 96% was obtained with 1.5 mol % of 1 at room temperature for 12 h. This framework emerged as an excellent recyclable catalyst for both the reactions.

Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

Optimized Pore Nanospace through the Construction of a Cagelike Metal-Organic Framework for CO2/N2 Separation

The development of metal-organic framework (MOF) adsorbents with a potential molecule sieving effect for CO₂ capture and separation from flue gas is of critical importance for reducing the CO₂ emissions to the atmosphere yet challenging. Herein, a cagelike MOF with a suitable cage window size falling between CO₂ and N₂ and the cavity has been constructed to evaluate its CO₂/N₂ separation performance. It is noteworthy that the introduction of coordinated dimethylamine (DMA) and N,N'-dimethylformamide (DMF) molecules not only significantly reduces the cage window size but also enhances the framework-CO₂ interaction via C-H···O hydrogen bonds, as proven by molecular modeling, thus leading to an improved CO₂ separation performance. Moreover, transient breakthrough experiments corroborate the efficient CO₂/N₂ separation, revealing that the introduction of DMA and DMF molecules plays a vital role in the separation of a CO₂/N₂ gas mixture.

Covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) as electrochemical sensors for the efficient detection of pharmaceutical residues

Pharmaceutical residues are the undecomposed remains from drugs used in the medical and food industries. Due to their potential adverse effects on human health and natural ecosystems, they are of increasing worldwide concern. The acute detection of pharmaceutical residues can give a rapid examination of their quantity and then prevent them from further contamination. Herein, this study summarizes and discusses the most recent porous covalent-organic frameworks (COFs) and metal-organic frameworks (MOFs) for the electrochemical detection of various pharmaceutical residues. The review first introduces a brief overview of drug toxicity and its effects on living organisms. Subsequently, different porous materials and drug detection techniques are discussed with materials' properties and applications. Then the development of COFs and MOFs has been addressed with their structural properties and sensing applications. Further, the stability, reusability, and sustainability of MOFs/COFs are reviewed and discussed. Besides, COFs and MOFs' detection limits, linear ranges, the role of functionalities, and immobilized nanoparticles are analyzed and discussed. Lastly, this review summarized and discussed the MOF@COF composite as sensors, the fabrication strategies to enhance detection potential, and the current challenges in this area.

Tactile Sensors: Hydroxyl Decorated Silver Metal-Organic Frameworks for Detecting Cr2O72-, MnO4-, Humic Acid, and Fe3+ Ions

Anionic, acidic, and metal ions are common contaminants in water and cause serious concerns for human and aquatic life. With the goal of rapid detection of analytes, we herein design a new array of ligand 5-(4H-1,2,4-triazol-4-yl)pyridin-3-ol-linked silver coordinated metal-organic frameworks Ag-MOFs as a promising sensor for Cr₂O₇^2-, MnO₄^-, humic acid (HA), and Fe³⁺ ions down to the micro level. Furthermore, as evidenced by luminescence, excitation-emission matrix (EEM) spectroscopic, and PXRD measurements, designed metal-organic frameworks (MOFs) can be fast, stable, and reusable for analyte detection in water.

Template-Mediated Synthesis of Hierarchically Porous Metal–Organic Frameworks for Efficient CO2/N2 Separation

Carbon dioxide (CO₂) is generally unavoidable during the production of fuel gases such as hydrogen (H₂) from steam reformation and syngas composed of carbon monoxide (CO) and hydrogen (H₂). Efficient separation of CO₂ from these gases is highly important to improve the energetic utilization efficiency and prevent poisoning during specific applications. Metal–organic frameworks (MOFs), featuring ordered porous frameworks, high surface areas and tunable pore structures, are emerging porous materials utilized as solid adsorbents for efficient CO₂ capture and separation. Furthermore, the construction of hierarchical MOFs with micropores and mesopores could further promote the dynamic separation processes, accelerating the diffusion of gas flow and exposing more adsorptive pore surface. Herein, we report a simple, efficient, one-pot template-mediated strategy to fabricate a hierarchically porous CuBTC (CuBTC-Water, BTC = 1,3,5-benzenetricarboxylate) for CO₂ separation, which demonstrates abundant mesopores and the superb dynamic separation ability of CO₂/N₂. Therefore, CuBTC-Water demonstrated a CO₂ uptake of 180.529 cm³ g⁻¹ at 273 K and 1 bar, and 94.147 cm³ g⁻¹ at 298 K and 1 bar, with selectivity for CO₂/N₂ mixtures as high as 56.547 at 273 K, much higher than microporous CuBTC. This work opens up a novel avenue to facilely fabricate hierarchically porous MOFs through one-pot synthesis for efficient dynamic CO₂ separation.

Effect of Unsaturated Metal Site Modulation in Highly Stable Microporous Materials on CO2 Capture and Fixation

We have designed and synthesized two unprecedented microporous three-dimensional metal-organic frameworks, {[Cd₆(TPOM)₃(L)₆]·12DMF·3H₂O}_n (1) and {[Zn₂(TPOM)(L)₂]·2DMF·H₂O}_n (2), based on a flexible quadritopic ligand, tetrakis(4-pyridyloxymethylene)methane (TPOM), and a bent dicarboxylic acid, 4,4'-(dimethylsilanediyl)bis-benzoic acid (H₂L). The networks of 1 and 2 share a 4-c uninodal net NbO topology but exhibit different metal environments due to coordination preferences of Cd(II) and Zn(II). The Cd(II) center in 1 is six-coordinated, whereas the Zn(II) center in 2 is only four-coordinated, making the latter an unsaturated metal center. Such modulation of coordination atmosphere of metal centers in MOFs with the same topology is possible due to diverse binding of the carboxylate groups of L^2-. Both 1 and 2 have relatively high thermal stability and exhibit permanent porosity after the removal of guest solvent molecules based on variable temperature powder X-ray diffraction and gas adsorption analysis. These materials exhibit similar gas adsorption properties, especially highly selective CO₂ uptake/capture over other gases (N₂ and CH₄). However, because of the presence of an unsaturated Lewis acidic metal site, 2 acts as a very efficient heterogeneous catalyst toward the chemical conversion of CO₂ to cyclic carbonates under mild conditions, whereas 1 shows very less activity. This work provides experimental evidence for the postulate that an unsaturated metal site in MOFs enhances adsoprtion of CO₂ and promotes its conversion via the Lewis-acid catalysis.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Pillared cobalt metal-organic frameworks act as chromatic polarizers

The ease with which molecular building blocks can be ordered in metal-organic frameworks is an invaluable asset for many potential applications. In this work, we exploit this inherent order to produce chromatic polarizers based on visible-light linear dichroism via cobalt paddlewheel chromophores.

Fine-Tuning the Porosities of the Entangled Isostructural Zn(II)-Based Metal-Organic Frameworks with Active Sites by Introducing Different N-Auxiliary Ligands: Selective Gas Sorption and Efficient CO2 Conversion

Three new pairs of 2-fold interpenetrated and self-entangled three-dimensional isostructural porous metal-organic frameworks (MOFs), [Zn(L1)(x)_0.5]·0.5H₂O (x = bipy for 1, bpa for 2, and bpe for 3) and [Zn(L2)(x)_0.5]·0.5H₂O (x = bipy for 4, bpa for 5, and bpe for 6) [bipy = 4,4'-bipyridine, bpa = 1,2-bis(4-pyridyl)ethylene, and bpe = 1,2-bis(4-pyridyl)ethylene], have been created and fine-tuned via similar skeleton ligands 2-(imidazol-1-yl)terephthalic acid (H₂L1) and 2-(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂L2) and N-auxiliary coligands with different linking groups. Interestingly, the porosities of the MOFs can be effectively increased via the insertion of -CH₂CH₂- or -CH═CH- spacers into the N-auxiliary bipy ligand. As a result, complexes 5 and 6 displayed highly enhanced CO₂ uptake capacities. Furthermore, complex 5 also had a higher C₂/C₁ selectivity as well as great CO₂ cycloaddition efficiency.

Effecting structural diversity in a series of Co(II)-organic frameworks by the interplay between rigidity of a dicarboxylate and flexibility of bis(tridentate) spanning ligands

In a one-pot self-assembly reaction of Co(OAc)2·4H2O, thiophene-2,5-dicarboxylic acid (H2tdc) and four different bis(tridentate) polypyridyl spanning ligands under ambient conditions, a series of structurally diverse metal-organic frameworks has been synthesised and characterized by single crystal X-ray diffraction: {[Co2(tdc)2(tpbn)(H2O)2]·solvent}n (solvent = 2H2O, 1; solvent = 2CH3OH, 2H2O, 1a), {[Co2(tdc)2(tphn)]·solvent}n (solvent = H2O, 2; solvent = CH3OH, 2.5H2O, 2a), {[Co2(tdc)2(tpchn)(H2O)2]·solvent}n (solvent = 5H2O, 3; solvent = C2H5OH, 2H2O, 3a), and {[Co2(tdc)2(tpxn)]·solvent}n (solvent = 6H2O, 4; when no solvent, 4a), where tpbn (N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,4-diaminobutane), tphn = N,N',N'',N'''-tetrakis(2-pyridylmethyl)-1,6-diaminohexane, tpchn = N,N'-(cyclohexane-1,4-diylbis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) and tpxn = N,N'-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine). There is a profound effect of the nature of spacer between the alkyl nitrogens in the spanning ligands (flexible vs. semiflexible) on the molecular structures of 1a-4a. The notable differences are (a) the binding mode of the tridentate part of polypyridyl ligands to the Co(ii) center is facial in 1a, 3a and 4a but meridional in 2a, (b) the Co(ii) centers in 1a-3a are hexacoordinated (with a coordinated water in 1a and 3a) but are pentacoordinated in 4a, and (c) the binding mode of tdc linker is bis(monodentate) in 1a, 3a and 4a but chelated in one end and monodentate in the other end in 2a. Thus, the overall framework structure of 1a, 2a, 3a and 4a is cis-decalin type 2D polymer, ladder-shaped 1D polymer, hexagonal 2D net and cis-decalin type 2D polymer, respectively. Their thermal stabilities have been established by thermogravimetric analysis (TGA). The presence of an unsaturated metal center in 4 has provided us an opportunity for its use as an efficient Lewis acid catalyst for the Knoevenagel condensation reaction of malononitrile with various aldehydes (100% conversion in 60 minutes with 2 mol% catalyst in methanol).