Pyridinium-Functionalized Ionic Metal-Organic Frameworks Designed as Bifunctional Catalysts for CO2 Fixation into Cyclic Carbonates

Multiple-site activation induced by guanidine ionic liquid decorated chromium (III) terephthalate for coupling of carbon dioxide with epoxides

Cycloaddition, as a sound strategy for high-value utilization of carbon dioxide (CO2), has been long pursued, wherein the challenging substrate activation process is a top priority for devising novel heterogeneous catalysts. In this study, a guanidine-based ionic liquid tethering -NH2 groups was designed and integrated with chromium (III) terephthalate (MIL-101(Cr)) through the coordination with unsaturated Cr3+ centers. The developed [NH2TMG]Br@MIL-101(Cr) (TMG represents tetramethylguanidine) decorated with plentiful basic functional groups created a fast channel for the capturing and binding of CO2, while the highly-accessible Lewis acidic sites (Cr3+) and hydrogen bond donors (N+-H) embedded within the nanocomposite synergized to activate the epoxide, synchronously. Under the reaction conditions optimized by response surface methodology (RSM) (103.2 °C, 1.03 MPa, 1.85 h, and 2.53 wt% of catalyst), a satisfactory chloropropene carbonate (CPC) yield of 98.2 % with a selectivity of 99.2 % were achieved. We further demonstrated the heterogeneity and recyclability of [NH2TMG]Br@MIL-101(Cr), and ascertained the substrate expansibility. Moreover, the in-situ diffuse reflectance infrared Fourier-transform spectra (DRIFTS) and density functional theory (DFT) computations afforded deep insights into the proposed multiple-site activation mechanism for CO2 coupling. This study highlighted an innovative pathway for constructing durable IL@MOFs nanocomposites and presented a tangible route to effectively converting CO2.

NH2-MIL-101(Fe) Nanocrystals Synthesized by the Ionic Liquid-Ethanol Interface for Efficient CO2 Fixation at Mild Conditions

In this study, an ionic liquid-ethanol interface strategy is proposed to synthesize NH2-MIL-101(Fe) nanocrystals at room temperature. The as-synthesized nanocrystals exhibit small crystal sizes, abundant ligand defects, and unsaturated metal sites. The NH2-MIL-101(Fe) nanocrystals present superior catalytic activity for the cycloaddition reaction of CO2 at mild conditions (room temperature and 1 bar CO2 pressure), much higher than the NH2-MIL-101 (Fe) microcrystals synthesized by the conventional solvothermal method. The conversion of propylene oxide catalyzed by NH2-MIL-101(Fe) nanocrystals achieves a 99% yield within 2.5 h, accompanied by a generation rate of carbonate production per gram of catalyst (Rcarbonate) of 52.8 mmol g-1 h-1. In contrast, the system employing NH2-MIL-101(Fe) microcrystals shows a much lower yield of 19.28% and a generation rate (Rcarbonate) of 10.28 mmol g-1 h-1. The NH2-MIL-101(Fe) nanocrystals were further used for the treatment of simulated industrial flue gas conditions with a volume ratio of N2/CO2 = 85:15. The results show the efficient conversion of CO2 at ambient temperature and pressure, even at relatively low CO2 concentrations. This work not only offers a facile, low energy-consumed, and environmentally benign interface method for the fabrication of MOF nanostructures but also provides high-performance systems for CO2 fixation and gas separation.

Enhancement of Two Types of CO2 Conversion by Regulating Functional Thiophene Groups within Zn-MOF

Sustainable conversion of epoxides and propargylic amines using CO2 could produce valuable chemical products. Efficient conversion generally requires harsh conditions such as noble-metal catalysts, cocatalysts, and toxic solvents, thereby underscoring the crucial need for environmentally friendly non-noble-metal metal-organic framework (MOF) catalysts. In this study, we designed a novel zinc-based metal-organic framework (MOF) with a 5-fold interpenetrating diamond framework, specifically {[Zn(DMTDC)(bpy)]·H2O}n (Zn-MOF-2), where H2DMTDC represents 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid and bpy denotes 4,4'-bipyridine. Zn-MOF-2 serves as a bifunctional heterogeneous catalyst to promote the cyclization reaction of epoxides and propargylamine with CO2 under mild conditions. The isolated yields of the resulting cyclic carbonates and oxazolidinones were 92% and 93%, respectively. Notably, the catalyst maintained good catalytic activity after five catalytic cycles in both types of CO2 conversion. Control experiments confirmed the effective catalytic activity of the Zn2+ Lewis acid sites in Zn-MOF-2. The isostructural CPO-5 assembled from 4,4'-biphenyldicarboxylic acid has also been demonstrated to catalyze the cycloaddition of epoxides and CO2 but with an inferior performance in catalyzing the carboxyl cyclization of propargylic amines. This result shows that the thiophene ring in the ligand plays a pronounced role in the catalytic performance. The research will enhance the development of effective MOF catalysts for the conversion of CO2.

Strong Electrochemiluminescence Response Derived from Ionic Chiral Covalent Organic Frameworks for Enantioselective Discrimination of Amino Acid Enantiomers via an Electrostatic Attraction Effect

Porous chiral materials accompanied by electrochemiluminescence (ECL) activity are rarely reported for enantioselective discrimination because of the big challenges to integrate the stereogenic center and ECL-active unit in the backbone. In the present study, ionic chiral covalent organic frameworks (iCCOFs) consisting of the pyridinium unit as the ECL-active species were prepared by a facile strategy. We were amazed that such iCCOFs could display strong cathodic ECL responses. Meanwhile, the as-prepared ECL-active iCCOFs performed enantioselective ECL quenching toward amino acid enantiomers, attributed to the enhanced photoinduced electron transfer process derived from the formed complex between the iCCOFs and amino acids via an electrostatic attraction effect. The iCCOF with an (S)-configuration was prone to interact with l-amino acids, producing a lower ECL intensity. The maximum intensity ratio between the d- and l-enantiomers was 33.0. Finally, the enantiomeric compositions of the measured amino acids presented a good linear relation with the obtained ECL intensity, which was fit for the determination of samples with unknown enantiomeric purity. In brief, the obtained results convince us that this study advances a new generation of ECL-active iCCOFs and displays great potential in enantioselective sensing.

Mechanochemical Synthesis of High-Entropy MOF-74 with Multiple Active Sites for CO2 Adsorption and Synergistic Conversion

Compared with monometallic metal-organic frameworks (MOFs), the synergistic effect of multiple metals significantly enhances the catalytic performance of the CO2 cycloesterification reaction, leading to improved CO2 adsorption and catalytic conversion capabilities. To investigate this concept, a high-entropy MOF-74 (HE-MOF-74) with a uniform distribution of five distinct metal ions (Zn2+, Mg2+, Ni2+, Co2+, and Cu2+) was successfully synthesized using a straightforward mechanical ball milling technique and comprehensively characterized (including structural, morphological, and physicochemical properties). The results reveal that HE-MOF-74 exhibits significantly increased specific surface area and CO2 adsorption capacity compared with those of monometallic MOF-74. The presence of multiple unsaturated metal centers as Lewis acid sites, oxygen atoms linking the metals, and ligand-based hydroxyl groups serving as base sites enable efficient immobilization of CO2 into cyclic carbonate. This study introduces a novel synthetic approach for the green and efficient production of HE-MOF-74 and proposes a new application for CO2 utilization.

Robust {Pb10}-Cluster-Based Metal-Organic Framework for Capturing and Converting CO2 into Cyclic Carbonates under Mild Conditions

Developing a highly active catalyst that can efficiently capture and convert carbon dioxide (CO2) into high-value-added energy materials remains a severe challenge, which inspires us to explore effective metal-organic frameworks (MOFs) with high chemical stability and high-density active sites. Herein, we report a robust 3D lead(II)-organic framework of {(Me2NH2)2[Pb5(PTTPA)2(H2O)3]·2DMF·3H2O}n (NUC-111) with unreported [Pb10(COO)22(H2O)6] clusters (abbreviated as {Pb10}) as nodes (H6PTTPA = 4,4',4″-(pyridine-2,4,6-triyl)triisophthalic acid). After thermal activation, NUC-111a is functionalized by the multifarious symbiotic acid-base active sites of open Pb2+ sites and uncoordinated pyridine groups on the inner surface of the void volume. Gas adsorption tests confirm that NUC-111a displays a higher separation performance for mixed gases of f CO2 and CH4 with the selectivity of CO2/CH4 at 273 K and 101 kPa being 31 (1:99, v/v), 23 (15:85, v/v), and 8 (50:50, v/v), respectively. When the temperature rises to 298 K, the selectivity of CO2/CH4 at 101 kPa is 26 (1:99, v/v), 22 (15:85, v/v), and 11 (50:50, v/v). Moreover, activated NUC-111a exhibited excellent catalytic performance, stability, and recyclability for the cycloaddition of CO2 with epoxides under mild conditions. Hence, this work provides valuable insight into designing MOFs with multifunctionality for CO2 capture, separation, and conversion.

Copper-Based Bio-MOF/GO with Lewis Basic Sites for CO2 Fixation into Cyclic Carbonates and C-C Bond-Forming Reactions

Several measures, including crude oil recovery improvement and carbon dioxide (CO2) conversion into valuable chemicals, have been considered to decrease the greenhouse effect and ensure a sustainable low-carbon future. The Knoevenagel condensation and CO2 fixation have been introduced as two principal solutions to these challenges. In the present study for the first time, bio-metal-organic frameworks (MOF)(Cu)/graphene oxide (GO) nanocomposites have been used as catalytic agents for these two reactions. In view of the attendance of amine groups, biological MOFs with NH2 functional groups as Lewis base sites protruding on the channels' internal surface were used. The bio-MOF(Cu)/20%GO performs efficaciously in CO2 fixation, leading to more than 99.9% conversion with TON = 525 via a solvent-free reaction under a 1 bar CO2 atmosphere. It has been shown that these frameworks are highly catalytic due to the Lewis basic sites, i.e., NH2, pyrimidine, and C═O groups. Besides, the Lewis base active sites exert synergistic effects and render bio-MOF(Cu)/10%GO nanostructures as highly efficient catalysts, significantly accelerating Knoevenagel condensation reactions of aldehydes and malononitrile as substrates, thanks to the high TOF (1327 h-1) and acceptable reusability. Bio-MOFs can be stabilized in reactions using GO with oxygen-containing functional groups that contribute as efficient substitutes, leading to an expeditious reaction speed and facilitating substrate absorption.

The post-synthesis modification (PSM) of MOFs for catalysis

While there are myriad ways to construct metal-organic framework (MOF) based catalysts, the introduction of catalytic functionality via covalent post-synthesis functionalization (PSM) offers multiple advantages: (i) a wide range of different catalyst types are generated from a handful of well-known parent MOFs, (ii) MOF catalyst properties can be systematically tuned while changing few variables, and (iii) catalytically active functional groups that would otherwise interfere with MOF assembly can be introduced facilely. This last advantage is particularly crucial for our quest to generate MOF active sites that are decorated with multiple functional groups capable of cooperative activity, analogous to enzyme active sites.

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.

Advances and Applications of Metal-Organic Frameworks (MOFs) in Emerging Technologies: A Comprehensive Review

Metal-organic frameworks (MOFs) that are the wonder material of the 21st century consist of metal ions/clusters coordinated to organic ligands to form one- or more-dimensional porous structures with unprecedented chemical and structural tunability, exceptional thermal stability, ultrahigh porosity, and a large surface area, making them an ideal candidate for numerous potential applications. In this work, the recent progress in the design and synthetic approaches of MOFs and explore their potential applications in the fields of gas storage and separation, catalysis, magnetism, drug delivery, chemical/biosensing, supercapacitors, rechargeable batteries and self-powered wearable sensors based on piezoelectric and triboelectric nanogenerators are summarized. Lastly, this work identifies present challenges and outlines future opportunities in this field, which can provide valuable references.

Electron-deficient Fe3O4@AC-NH2@Cu-MOF nanoparticles for enhanced degradation of electron-rich benzene derivatives via synergistic adsorption and catalytic oxidation

Benzene derivatives in wastewater have negative impacts on ecosystems and human health, making their removal prior to discharge imperative. In this study, Fe3O4@AC-NH2@Cu-opa (AC-NH2 = aminoclay, Cu-opa = [Cu(opa)(bipy)0.5(H2O)]n (H2opa = 3-(4-oxypyridinium-1-yl) phthalic acid)) nanoparticles (NPs) were synthesized as adsorbent and catalyst for phenolic compound removal from wastewater. Fe3O4@AC-NH2@Cu-opa NPs demonstrated outstanding performance in the adsorption of phenol, exhibiting a remarkable adsorption capacity of up to 166.39 mg g-1 according to the Langmuir model. The composite also exhibited higher Fenton activity toward the degradation of electron-rich organic phenolic pollutants, with a rate approximately 3.4 times higher than that of Fe3O4 alone. The high catalytic activity of the composite was attributed to the large surface area and abundant active sites of the 2D charge-separated Cu-MOF. Meanwhile, the superparamagnetism of the Fe3O4 core enabled magnetic recollection and reuse without any significant loss of activity. Therefore, use of Fe3O4@AC-NH2@Cu-opa/H2O2 shows potential in an efficient method for the removal of phenolic compounds from wastewater.

Embedding Hydrogen Atom Transfer Moieties in Covalent Organic Frameworks for Efficient Photocatalytic C-H Functionalization

Covalent organic frameworks (COFs) have emerged as efficient heterogeneous photocatalysts for a wide range of relatively simple organic reactions, whereas their application in complex organic transformations, like site-selective functionalization of unactivated C-H bonds, is underexplored, which can be mainly attributed to the lack of highly active organophotocatalytic cores. Herein through bonding oxygen atoms at the N-terminus of quinolines in nonsubstituted quinoline-linked COFs (NQ-COFs), we successfully realized the embedding of active hydrogen atom transfer (HAT) moieties into the skeleton of COFs. This novel designed COF (NQ-COFE5 -O), serving as both an excellent photosensitizer and HAT catalyst, exhibited much higher efficiency in C-H functionalization than the corresponding NQ-COFE5 . Specially, we evaluated the photocatalytic performance of NQ-COFE5 -O on ten different substrates, including quinolines, benzothiazole, and benzoxazole, all of which were transferred to desired products in moderate to high yields (up to 93 %). Furthermore, the as-synthesized NQ-COFE5 -O displayed excellent photostability and could be reused with negligible loss of activity for five catalytic cycles.

CO2 Cycloaddition under Ambient Conditions over Cu-Fe Bimetallic Metal-Organic Frameworks

Carbon dioxide cycloaddition into fine chemicals is prospective technology to solve energy crisis and environmental issues. However, high temperature and pressure are usually required in the conventional cycloaddition reactions of CO2 with epoxides. Moreover, metal active sites play a vital role in the CO2 cycloaddition, but it is still unclear. Herein, we select the isostructural MOF-919-Cu-Fe and MOF-919-Cu-Al as models to promote the performance and clarify the effects of metal type on the CO2 cycloaddition. The MOF-919-Cu-Fe with exposed Fe and Cu Lewis acid sites reaches the CO2 cycloaddition with over 99.9% conversion and over 99.9% selectivity at room temperature and a 1 bar CO2 atmosphere, 3.0- and 52.6-fold higher than those of the MOF-919-Cu-Al with Al and Cu sites (33.8%) and the 1H-pyrazole-4-carboxylic acid, Fe, and Cu mixed system (1.9%), respectively. The proposed mechanism demonstrated that the exposed Fe3+ sites facilitate the ring opening of epoxide and CO2 activation to boost the CO2 cycloaddition reaction. This work provides a new insight to tune the catalytic sites of MOFs to achieve high performance for CO2 fixation.

A Dual-Purpose Ce(III)-Organic Framework with Amine Groups and Open Metal Sites: Third-Order Nonlinear Optical Activity and Catalytic CO2 Fixation

The present work focuses on the synthesis and properties of a novel multifunctional cerium(III) MOF, [Ce2(data)3(DMF)4]·DMF (data2-: 2,5-diaminoterephthalate), abbreviated as NH2-Ce-MUM-2. Its crystal structure reveals an intricate 3D 4,5-connected framework with a xah topology. This MOF features unique properties, such as open metal sites, presence of free amino groups, and high stability. Two main applications of NH2-Ce-MUM-2 were investigated: (i) as a heterogeneous catalyst in the CO2 fixation into cyclic carbonates and (ii) as a material with third-order nonlinear optical activity. As a model reaction, the cycloaddition of CO2 to propylene oxide to give the corresponding cyclic carbonate was explored under mild conditions, at the atmospheric pressure of carbon dioxide and in the absence of cocatalyst and added solvent. Various reaction parameters were investigated toward optimization and exploration of substrate scope, revealing up to 99% product yields of cyclic carbonate products. Besides, the structure of NH2-Ce-MUM-2 is highly stable, permitting its recyclability and reusability in further catalytic experiments. The significant contributions of free amino groups and open metal sites within this catalyst were particularly considered when proposing a potential mechanism for the reaction. Z-Scan measurements were used to evaluate the nonlinear optical (NLO) properties of NH2-Ce-MUM-2 at various laser intensities. A high two-photon absorption (TPA) under greater incident intensities shows that NH2-Ce-MUM-2 might be applicable in optical switching devices. Besides, the self-focusing effects of NH2-Ce-MUM-2 under various incident intensities were highlighted by the nonlinear index of refraction (n2). By reporting the synthesis and characterization of a novel MOF, along with its highly promising catalytic and NLO behavior, the current study introduces an additional example of multifunctional material into a growing family of metal-organic frameworks.

Coordination Unsaturation and Basic Site-Immobilized Nanochannel in a Chemorobust MOF for 3-Fold-Increased High-Temperature Selectivity and Fixation of CO2 under Mild Conditions with Nanomolar Recognition of Roxarsone

A multifaceted metal-organic framework (MOF) with task-specific site-engineered pores can promise high-temperature and moisture-tolerant capture and non-redox fixation of CO2 under mild conditions as well as ultrasensitive detection of carcinogenic contaminants in water. Herein, we report a pillar-bilayered MOF that holds a nanochannel with contrasting functionalities for both these sustainable applications with improved performance characteristics. The twofold entangled robust framework exhibits CO2 adsorption at elevated temperatures with considerable MOF-gas interaction. Interestingly, CO2 selectivity unveils nearly a 3-fold improvement upon the rise of temperature, affording a CO2/N2 value of 820 at 313 K, which outperforms many porous adsorbents. Additionally, breakthrough simulation establishes complete separation and attests the potential of this MOF in the separation of flue gas mixture. Importantly, minor CO2 loss during multiple capture-release cycles and under a relative humidity of 75% promise practical usability of the material. Density functional theory (DFT) not only portrays the atomistic level snapshots of temperature-triggered CO2 inclusion inside this microporous vessel alongside the role of diverse CO2-philic sites but also validates the basis of N2-phobicity of an azo-functionalized linker on such increased selectivity. The guest-free MOF further demonstrates non-redox and recyclable CO2 fixation with wide epoxide tolerance under solvent-free mild conditions and even works at atmospheric pressure and room temperature. The crucial roles of high-density acid-base sites in both adsorption and catalysis are supported by control experiments and by comparing the activity of an unfunctionalized MOF. The hydrolytic stability and strong luminescence signature benefit the framework in aqueous-phase selective and fast responsive detection of detrimental roxarsone (ROX) with high quenching (7.56 × 104 M-1) and very low sensitivity (68 nM). Apart from varying degrees of an energy-transfer mechanism, the fluorosensing of ROX is comprehensively supported by in-depth DFT studies that manifest alteration of MOF energy levels in the presence of organoarsenic compounds and depict MOF-analyte supramolecular interactions.

A highly robust cluster-based indium(III)-organic framework with efficient catalytic activity in cycloaddition of CO2 and Knoevenagel condensation

The efficient catalytic performance displayed by MOFs is decided by an appropriate charge/radius ratio of defect metal sites, large enough solvent-accessible channels and Lewis base sites capable of polarizing substrate molecules. Herein, the solvothermal self-assembly led to a highly robust nanochannel-based framework of {[In₄(CPDD)₂(μ₃-OH)₂(DMF)(H₂O)₂]·2DMF·5H₂O}_n (NUC-66) with a 56.8% void volume, which is a combination of a tetranuclear cluster [In₄(μ₃-OH)₂(COO)₁₀(DMF)(H₂O)₂] (abbreviated as {In₄}) and a conjugated tetracyclic pentacarboxylic acid ligand of 4,4'-(4-(4-carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid (H₅CPDD). To the best of our knowledge, NUC-66 is a rarely reported {In₄}-based 3D framework with embedded hierarchical triangular-microporous (2.9 Å) and hexagonal-nanoporous (12.0 Å) channels, which are shaped by six rows of {In₄} clusters. After solvent exchange and vacuum drying, the surface of nanochannels in desolvated NUC-66a is modified by unsaturated In³⁺ ions, N_pyridine atoms and μ₃-OH groups, all of which display polarization ability towards polar molecules due to their Lewis acidity or basicity. The catalytic experiments performed showed that NUC-66a had high catalytic activity in the cycloaddition reactions of epoxides with CO₂ under mild conditions, which should be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas and chemical stability. Moreover, NUC-66a, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work confirms that the development of rigid nanoporous cluster-based MOFs built on metal ions with a high charge and large radius ratio will be more likely to realize practical applications, such as catalysis, adsorption and separation of gas, etc.

A novel zirconium-based metal-organic framework covalently modified by methyl pyridinium bromide for mild and co-catalyst free conversion of CO2 to cyclic carbonates

Building metal-organic frameworks (MOFs) covalently modified by onium halides is a promising approach to develop efficient MOF-based heterogeneous catalysts for the cycloaddition of CO₂ to epoxides (CCE) into cyclic carbonates. Herein, we report a novel zirconium-based MOF covalently modified by methyl pyridinium bromide, Zr₆O₄(OH)₄(MPTDC)_2.2(N-CH₃-MPTDC)_3.8Br_3.8 ((Br-)CH3-Pyridinium-MOF-1), where MPTDC denotes 3-methyl-4-pyridin-4-yl-thieno[2,3-b] thiophene-2,5-dicarboxylate. The structure and composition of this complex were fully characterized with PXRD, NMR, XPS, TEM and so on. CO₂ adsorption experiments show that (Br-)CH3-Pyridinium-MOF-1 has a higher affinity for CO₂ than its electrically neutral precursor, which should be attributed to the fact that charging frameworks containing pyridinium salt have stronger polarization to CO₂. (Br-)CH3-Pyridinium-MOF-1 integrated reactive Lewis acid sites and Br^- nucleophilic anions and exhibited efficient catalytic activity for CCE under ambient pressure in the absence of co-catalysts and solvents. Furthermore, (Br-)CH3-Pyridinium-MOF-1 was recycled after five successive cycles without substantial loss in catalytic activity. The corresponding reaction mechanism also was speculated.

Ionic metal-organic frameworks (iMOFs): progress and prospects as ionic functional materials

Metal-organic frameworks (MOFs) have been a research hotspot for the last two decades, witnessing an extraordinary upsurge across various domains in materials chemistry. Ionic MOFs (both anionic and cationic MOFs) have emerged as next-generation ionic functional materials and are an important subclass of MOFs owing to their ability to generate strong electrostatic interactions between their charged framework and guest molecules. Furthermore, the presence of extra-framework counter-ions in their confined nanospaces can serve as additional functionality in these materials, which endows them a significant advantage in specific host-guest interactions and ion-exchange-based applications. In the present review, we summarize the progress and future prospects of iMOFs both in terms of fundamental developments and potential applications. Furthermore, the design principles of ionic MOFs and their state-of-the-art ion exchange performances are discussed in detail and the future perspectives of these promising ionic materials are proposed.

Alumina-Based Bifunctional Catalyst for Efficient CO2 Fixation into Epoxides at Atmospheric Pressure

The quest toward sustainability and decarbonization demands the development of methods for efficient carbon dioxide capture and utilization. The nonreductive CO₂ fixation into epoxides to prepare cyclic carbonates has gained attention in recent years. In this work, we report the development of guanidine hydrochloride-functionalized γ alumina (γ-Al₂O₃), prepared using green solvents, as an efficient bifunctional catalyst for CO₂ fixation. The resulting guanidine-grafted γ-Al₂O₃ (Al-Gh) proved to be an excellent catalyst to prepare cyclic carbonates from epoxides and CO₂ with high selectivity. The nitrogen-rich Al-Gh shows increased CO₂ adsorption capacity compared to that of γ-Al₂O₃. The as-prepared catalyst was able to carry out CO₂ fixation at 85 °C under atmospheric pressure in the absence of solvents and external additives (e.g., TBAI or KI). The material showed negligible loss of catalytic activity even after five cycles of catalysis. The catalyst successfully converted many epoxides into their respective cyclic carbonates under the optimized conditions. The gram-scale synthesis of commercially important styrene carbonates from styrene oxide and CO₂ using Al-Gh was also achieved. Density functional theory (DFT) calculations revealed the role of alumina in activating the epoxide. This activation facilitated the chloride ion to open the ring to react with CO₂. The DFT studies also validated the role of alumina in stabilizing the electron-rich intermediates during the course of the reaction.

Synthesis of 3D Cadmium(II)-Carboxylate Framework Having Potential for Co-Catalyst Free CO2 Fixation to Cyclic Carbonates

Metal-organic frameworks (MOFs) are porous coordination polymers with interesting structural frameworks, properties, and a wide range of applications. A novel 3D cadmium(II)-carboxylate framework, CdMOF ([Cd2(L)(DMF)(H2O)2]n), was synthesized by the solvothermal method using a tetracarboxylic bridging linker having amide functional moieties. The CdMOF crystal structure exists in the form of a 3D layer structure. Based on the single-crystal X-ray diffraction studies, the supramolecular assembly of CdMOF is explored by Hirshfeld surface analysis. The voids and cavities analysis is performed to check the strength of the crystal packing in CdMOF. The CdMOF followed a multistage thermal degradation pattern in which the solvent molecules escaped around 200 °C and the structural framework remained stable till 230 °C. The main structural framework collapsed (>60 wt.%) into organic volatiles between 400–550 °C. The SEM morphology analyses revealed uniform wedge-shaped rectangular blocks with dimensions of 25–100 μm. The catalytic activity of CdMOF for the solvent and cocatalyst-free cycloaddition of CO2 into epichlorohydrin was successful with 100% selectivity. The current results revealed that this 3D CdMOF is more active than the previously reported CdMOFs and, more interestingly, without using a co-catalyst. The catalyst was easily recovered and reused, having the same performance.

Soybean Oil Epoxidation Catalyzed by a Functionalized Metal–Organic Framework with Active Dioxo-Molybdenum (VI) Centers

In this work, a functionalized gallium metal–organic framework with active dioxo-molybdenum (VI) centers was evaluated as a catalyst in the epoxidation of soybean oil using tert-butyl-hydroperoxide as an oxidizing agent. The influence of the reaction time, temperature, and concentration of the oxidizing agent was studied, and it was demonstrated that the highest epoxide selectivity was obtained at 110 °C after 4 h of reaction (29% conversion and 91% selectivity) using a soybean oil/oxidizing agent ratio of 1/2. The stability of the metal–organic framework was confirmed by infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy EDS. The stability tests demonstrated that the catalyst could be reused in the catalytic process for the recovery of vegetable oils.

Graphical Abstract

Fluorine-Functionalized NbO-Type {Cu2}-Organic Framework: Enhanced Catalytic Performance on the Cycloaddition Reaction of CO2 with Epoxides and Deacetalization-Knoevenagel Condensation

The high catalytic activity of metal-organic frameworks (MOFs) can be realized by increasing their effective active sites, which prompts us to perform the functionalization on selected linkers by introducing a strong Lewis basic group of fluorine. Herein, the exquisite combination of paddle-wheel [Cu₂(CO₂)₄(H₂O)] clusters and meticulously designed fluorine-funtionalized tetratopic 2',3'-difluoro-[p-terphenyl]-3,3″,5,5″-tetracarboxylic acid (F-H₄ptta) engenders one peculiar nanocaged {Cu₂}-organic framework of {[Cu₂(F-ptta)(H₂O)₂]·5DMF·2H₂O}_n (NUC-54), which features two types of nanocaged voids (9.8 Å × 17.2 Å and 10.1 Å × 12.4 Å) shaped by 12 paddle-wheel [Cu₂(COO)₄H₂O)₂] secondary building units, leaving a calculated solvent-accessible void volume of 60.6%. Because of the introduction of plentifully Lewis base sites of fluorine groups, activated NUC-54a exhibits excellent catalytic performance on the cycloaddition reaction of CO₂ with various epoxides under mild conditions. Moreover, to expand the catalytic scope, the deacetalization-Knoevenagel condensation reactions of benzaldehyde dimethyl acetal and malononitrile were performed using the heterogenous catalyst of NUC-54a. Also, NUC-54a features high recyclability and catalytic stability with excellent catalytic performance in subsequent catalytic tests. Therefore, this work not only puts forward a new solution for developing high-efficiency heterogeneous catalysts, but also enriches the functionalization strategies for nanoporous MOFs.

The Al(iii)-based polydentate chelate complex catalyzed cycloaddition of carbon dioxide and epoxides: synthetic optimization and mechanistic study

Aimed at greenhouse gas CO2 high value-added utilization, a N,N′-(propane-1,3-diyl)dipicolinamide (PPPA) supported Al(iii) metal–organic polydentate chelate complex (Al-PPPA) was designed and used to efficiently catalyze CO2 to cyclic carbonates.

An excellent multifunctional photocatalyst cooperated by polyoxometalate-viologen framework for CEES oxidation, Cr(VI) reduction and dyes decolorization under different light regimes

Photocatalytic detoxification of highly toxic agents is a promising approach to protect ecological environment and human health, and the key problem lies in the development of novel efficient photocatalysts. Herein,...

Novel hydroxyl-functionalized ionic liquids as efficient catalysts for the conversion of CO2 into cyclic carbonates under metal/halogen/cocatalyst/solvent-free conditions

Environmentally friendly synthesis route to carbonates from CO2 and epoxides catalysed by novel hydroxyl-functionalized ionic liquids under metal/halogen/cocatalyst/solvent-free conditions.

Pyridine ionic liquid functionalized bimetallic MOF solid-phase extraction coupled with high performance liquid chromatography for separation/analysis sunset yellow

An effective method based on the pyridine ionic liquid functionalized bimetallic MOF solid-phase extractant (Cu/Co-MOF@[PrPy][Br]) coupled with high performance liquid chromatography (HPLC) for the separation/analysis sunset yellow was established. Cu/Co-MOF@[PrPy][Br] was characterized by FTIR, XRD, SEM and TEM. Several important factors, such as pH, amount of extractant, extract time, and types of eluents were investigated in detail. Under the optimal conditions, linear range of the method was 0.05–40.00 μg mL⁻¹, the detection limit was 0.02 μg mL⁻¹, and the linear correlation was good (R² = 0.9992). The analysis of sunset yellow in soda, effervescent tablet and jelly proved that the method was simple and effective.

An effective method based on the pyridine ionic liquid functionalized bimetallic MOF solid-phase extractant (Cu/Co-MOF@[PrPy][Br]) coupled with high performance liquid chromatography (HPLC) for the separation/analysis sunset yellow was established.

Integration of polypyridyl-based ionic liquids into MIL-101 for promoting CO2 conversion into cyclic carbonates under cocatalyst-free and solventless conditions

The polypyridyl-based ionic liquid-functionalized MIL-101(Cr) greatly enhanced the epoxide–CO2 cycloaddition reaction under cocatalyst-free and solventless conditions.

Optimizing the mobility of active species in ionic liquid/MIL-101 composites for boosting carbon dioxide conversion

The mobility of active species has been optimized based on ionic liquid/metal–organic framework (MOF) composites for efficient CO2 chemical fixation.

Facile One-Pot Synthesis of Zn/Mg-MOF-74 with Unsaturated Coordination Metal Centers for Efficient CO2 Adsorption and Conversion to Cyclic Carbonates

Bimetallic metal-organic frameworks (MOFs) containing two different inorganic metal nodes exhibited enhanced properties in CO₂ adsorption and catalytic conversion compared with the corresponding monometallic MOFs. In this work, the novel bimetallic Zn/Mg-MOF-74 with different ratios of Zn/Mg was synthesized successfully by a facile one-pot method. Powder X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, scanning electron microscopy/transmission electron microscopy, N₂/CO₂ adsorption/desorption, and CO₂/NH₃-temperature-programmed desorption techniques thoroughly characterized the structure, morphology, and physicochemical properties of Zn/Mg-MOF-74. Besides the excellent CO₂ adsorption capacity (128.3 cm³/g at 273 K and 1 bar), Zn_0.75Mg_0.25-MOF-74 also showed efficient catalytic activity for the cycloaddition reaction of CO₂ and epoxides to cyclic carbonates with outstanding yield and selectivity all over 99% under solvent-free and mild conditions (60 °C, 0.8 MPa), outperforming the mechanical combination of Zn-MOF-74 and Mg-MOF-74 with the same metal contents, indicating the synergistic effect of two adjacent metals in bimetallic MOF-74. In addition, the Zn_0.75Mg_0.25-MOF-74 catalyst could be recycled for at least five runs and possess good versatility to various substrates. Finally, a feasible mechanism of the catalytic reaction was proposed. Thanks to the high surface area, affinity toward CO₂, and accessibility of multiple active sites of the unsaturated metal centers as active Lewis acid sites and O atoms from Zn-O and Mg-O as Lewis basic sites, efficient chemical fixation of CO₂ to cyclic carbonates was obtained over the Zn_0.75Mg_0.25-MOF-74 catalyst. The present facile synthesis and application of a robust bimetallic MOF catalyst offered a competitive avenue for the integration of CO₂ adsorption and CO₂ catalytic conversion.

Intrinsic-Unsaturation-Enriched Biporous and Chemorobust Cu(II) Framework for Efficient Catalytic CO2 Fixation and Pore-Fitting Actuated Size-Exclusive Hantzsch Condensation with Mechanistic Validation

Carbon dioxide (CO₂) utilization and one-pot Hantzsch condensation denote two important protocols pertinent to sustainable agenda because of the obvious advantages like reduction in chemical usage, short reaction time, and minimum waste generation. To this end, the astute combination of optimum-sized pore structure with built-in Lewis acid center in metal-organic frameworks (MOFs) can bring about such reactions under energetically favorable conditions and offer a step forward to size-exclusive catalysis. The chemoresistant and twofold interpenetrated Cu(II) framework CSMCRI-13 (CSMCRI = Central Salt & Marine Chemicals Research Institute) is built from a C₃-symmetric tricarboxylate ligand and an N,N'-donor linker that undergo incisive amalgamation of the paddle-wheel [Cu₂(COO)₄] secondary building unit (SBU) and the intrinsically unsaturated Cu(II) node with four coordination. The microporous structure features a dual-pore containing cage-like network with free oxygen-atom-enriched cavities and exhibits appreciable CO₂ adsorption with moderate MOF-CO₂ interaction in activated form (13a). Benefitting from both, the coordinatively frustrated metal center containing MOF acts as a highly synergistic and solvent-free catalyst in CO₂ cycloaddition reaction under an 8 bar CO₂ pressure at 70 °C in 6 h. The catalyst furnished admirable reactivity and fair recyclability with a wide range of substrates, wherein sterically encumbered and long-chain epoxides produced poor conversion. This MOF further executes highly regenerable Hantzsch condensation reaction under mild condition with superior activity to contemporary materials, where most of the 1,4-dihydropyridine derivatives are additionally characterized through the single-crystal X-ray diffraction analysis. Importantly, mechanistic proof of the tricomponent condensation involving built-in Lewis acid sites is validated from several control experiments and in-depth analytical studies. To the best of the single-step multicomponent reaction, substrate molecules having incompatible molecular dimension to that of pore size of the framework resulted insignificant conversion and demonstrated the first-ever pore-fitting-induced size selectivity in Hantzsch condensation.

Nanochannel {InZn}-Organic Framework with a High Catalytic Performance on CO2 Chemical Fixation and Deacetalization-Knoevenagel Condensation

The rare combination of In^III 5p and Zn^II 3d in the presence of a structure-oriented TDP^6- ligand led to a robust hybrid material of {(Me₂NH₂)[InZn(TDP)(OH₂)]·4DMF·4H₂O}_n (NUC-42) with the interlaced hierarchical nanochannels (hexagonal and cylindrical) shaped by six rows of undocumented [InZn(CO₂)₆(OH₂)] clusters, which represented the first 5p-3d nanochannel-based heterometallic metal-organic framework. With respect to the multifarious symbiotic Lewis acid-base and Brønsted acid sites in the high porous framework, the catalytic performance of activated NUC-42a upon CO₂ cycloaddition with styrene oxide was evaluated under solvent-free conditions with 1 atm of CO₂ pressure, which exhibited that the reaction could be well completed at ambient temperature within 48 h or at 60 °C within 4 h with high yield and selectivity. Moreover, because of the acidic function of metal sites and a central free pyridine in the TDP^6- ligand, deacetalization-Knoevenagel condensation of acetals and malononitrile could be efficiently facilitated by an activated sample of NUC-42a under lukewarm conditions.

Nickel Nanoparticles Immobilized over Mesoporous SBA-15 for Efficient Carbonylative Coupling Reactions Utilizing CO2: A Spotlight

Ecofriendly routes for the synthesis of carbamates and carbonylative coupling products such as benzyl formate derivatives are very demanding for both academia and industries. Foreseeing a sustainable green future, we systematically analyzed the synthesis history of both these chemicals, mentioning their pros and cons. As a step towards green chemistry, here we have optimized the reaction conditions for the synthesis of various benzyl formates from corresponding benzyl halides and carbamates from substituted anilines and alkyl halides catalyzed by Ni(0) nanoparticles (NPs) immobilized over amine-functionalized ordered mesoporous SBA-15 material in the presence of CO₂ as C1 source. This spotlight on applications is aimed to provide a clear outlook to date regarding the gradual progress in the synthesis of both these aforementioned chemicals and finally addresses further efforts for overcoming the current challenges.

Synthesis of Bifunctional Porphyrin Polymers for Catalytic Conversion of Dilute CO2 to Cyclic Carbonates

Development of efficient solid catalysts for catalytic conversion of dilute CO2 is of extreme importance for carbon capture and utilization. We report the synthesis of bifunctional polymers co-incorporated with porphyrin-Zn as Lewis acid sites and Br- as nucleophiles for the cycloaddition of dilute CO2 with epoxides in this work. It was found that the Br-/Zn ratio has a volcano relation with the activity of bifunctional polymers in a cycloaddition reaction, indicating the synergy effect between Lewis acid sites and nucleophiles. The turnover frequency (TOF) of the bifunctional polymer is more than four-fold that of the physical mixture of tetrabutylammonium bromide and porphyrin-Zn-incorporated polymer, implying the enhanced cooperation between Br- and porphyrin-Zn in the polymer network. The bifunctional polymer with optimized Br-/Zn afforded 99% conversion, 99% selectivity, and a TOF as high as 12,000 h-1 for the cycloaddition of CO2 and propylene oxide, which is among the most active solid catalysts ever reported. Furthermore, the bifunctional polymer could efficiently catalyze the cycloaddition of epichlorohydrin with dilute CO2 (7.5% CO2 balanced by N2) even under ambient conditions, demonstrating its potential application in industrial-scale production.

6s-3d {Ba3Zn4}-Organic Framework as an Effective Heterogeneous Catalyst for Chemical Fixation of CO2 and Knoevenagel Condensation Reaction

The exquisite combination of Ba²⁺ and Zn²⁺ with the aid of 2,4,6-tri(2,4-dicarboxyphenyl)pyridine (H₆TDP) under the condition of solvothermal self-assembly generates one highly robust [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂]-organic framework of {[Ba₃Zn₄(TDP)₂(HCO₂)₂(OH₂)₂]·7DMF·4H₂O}_n (NUC-27), in which adjacent 2D layers are interlaced via hydrogen-bonding interactions to form a 3D skeleton with peapod-like channels and nano-caged voids. It is worth emphasizing that both Ba²⁺ and Zn²⁺ ions in NUC-27 display the extremely low coordination modes: hexa-coordinated [Ba(1)] and tetra-coordinated [Ba(2), Zn(1), and Zn(2)]. Furthermore, to the best our knowledge, NUC-27 is one scarcely reported 2D-based nanomaterial with an unprecedented Z-shaped hepta-nuclear heterometallic cluster of [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂] as SBUs, which not only has plentiful low-coordinated open metal sites but also has the excellent physicochemical properties including omni-directional opening pores, ultrahigh porosity, larger specific surface area, and the coexistence of Lewis acid-base sites. Just as expected, thanks to its rich active metal sites and pyridine groups as strong Lewis acid-base roles, completely activated NUC-27 displays high catalytic efficiency on the chemical transformation of epoxides with CO₂ into cyclic carbonates under mild conditions and effectively accelerates the reaction process of Knoevenagel condensation.

Quest for an Efficient 2-in-1 MOF-Based Catalytic System for Cycloaddition of CO2 to Epoxides under Mild Conditions

We have devised a straightforward tandem postsynthetic modification strategy for Zr-based metal-organic framework (MOF) materials, which resulted in a series of well-defined 2-in-1 heterogeneous catalysts, cat1-cat8, exhibiting high catalytic activity in the synthesis of cyclic carbonates under solvent-free and co-catalyst-free conditions. The materials feature precisely located co-catalyst moieties decorating the metal nodes throughout the bulk of the MOF and yield cyclic carbonates with up to 99% efficiency at room temperature. We use diffuse reflectance infrared Fourier transform (DRIFT) and solid-state nuclear magnetic resonance (NMR) measurements to elucidate the role of each component in this model catalytic reaction. Establishing a method to precisely control the co-catalyst loading allowed us to observe the cooperativity between Lewis acid sites and the co-catalyst in the 2-in-1 heterogeneous system.

One Robust Microporous TmIII-Organic Framework for Highly Catalytic Activity on Chemical CO2 Fixation and Knoevenagel Condensation

In terms of documented references, multifunctional MOFs with high catalytic performance could be constructed from the combination of metal cations and polycarboxyl-pyridine ligands, which could efficiently endow crystallized porous frameworks with the coexisting Lewis acid-base properties. Thus, by employing a ligand-directed synthetic strategy, the exquisite combination of wave-like inorganic chains of [Tm(CO₂)₃(OH₂)]_n and mononuclear units of [Tm(CO₂)₄(OH₂)₂] with the aid of the specially designed ligand of 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H₅BDCP) generates one highly robust microporous framework of {(Me₂NH₂)[Tm₃(BDCP)₂)(H₂O)₃]·4DMF·H₂O}_n (simplified as NUC-25), which contains near-rectangular nanochannels and large solvent-residing voids. Furthermore, the activated state of NUC-25 with the removal of associated water molecules is a rarely reported bifunctional heterogeneous catalyst due to the coexistence of Lewis acid-base sites including 6-coordinated Tm³⁺ ions, uncoordinated carboxyl oxygen atoms, and N_pyridine atoms. Just as expected, NUC-25 exhibits greatly high catalytic activity for the cycloaddition reaction of epoxides with CO₂ into alkyl cyclic carbonates under bland solvent-free conditions, which should be ascribed to the polarity of nitrogen-containing pyridine heterocycles as Lewis base sites on the inner surface of nano-caged voids except for recognized Lewis acid sites of rare earth cations. Moreover, the excellent pore-size-dependent catalytic property for Knoevenagel condensation reactions confirms that NUC-25 can be viewed as a recyclable bifunctional heterogeneous catalyst. Therefore, these results strongly demonstrate that microporous MOFs assembled from pre-designed polycarboxyl-heterocyclic ligands display better catalytic performance not only for chemical CO₂ fixation but also for Knoevenagel condensation reactions.