Crystal Engineering of an nbo Topology Metal-Organic Framework for Chemical Fixation of CO2under Ambient Conditions

Self-Assembly Bifunctional Tetranuclear Ln2Ni2 Clusters: Magnetic Behaviors and Highly Efficient Conversion of CO2 under Mild Conditions

A series of heterometallic tetranuclear clusters, Ln₂Ni₂(NO₃)₄L₄(μ₃-OCH₃)₂·2(CH₃CN) (Ln = Gd(1), Tb(2), Dy(3), Ho(4), Er(5); HL = methyl 3-methoxysalicylate), were synthesized solvothermally. The intramolecular synergistic effect of two metal centers of Ln(III) and Ni(II) and the exposed multimetallic sites serving as Lewis acid activators greatly increase the efficiency of the CO₂ conversion, and the yield for cluster 3 can be achieved at 96% at atmospheric pressure and low temperature. In particular, the self-assembly multimetal center with polydentate ligand shows good generality and enhanced recyclability. The design of such 3d-4f heterometallic clusters provides an effective strategy for the conversion of CO₂ under greener conditions. Meanwhile, magnetic investigations indicate that cluster 1 is a good candidate for magnetic refrigerant materials with a relatively large magnetocaloric effect (MCE) (-ΔS_m = 28.5 J kg^-1 K^-1 at 3.0 K and 7.0 T), and cluster 3 shows single-molecular magnet behavior under zero dc field. Heterometallic clusters with special magnetic properties and good catalytic behavior for the conversion of CO₂ are rare. Thus, they are potential bifunctional materials applied in practice.

Rational Design of a ZnII MOF with Multiple Functional Sites for Highly Efficient Fixation of CO2 under Mild Conditions: Combined Experimental and Theoretical Investigation

The development of efficient heterogeneous catalysts suitable for carbon capture and utilization (CCU) under mild conditions is a promising step towards mitigating the growing concentration of CO₂ in the atmosphere. Herein, we report the construction of a hydrogen-bonded 3D framework, {[Zn(hfipbba)(MA)]⋅3 DMF}_n (hfipbba=4,4'-(hexaflouroisopropylene)bis(benzoic acid)) (HbMOF1) utilizing Zn^II center, a partially fluorinated, long-chain dicarboxylate ligand (hfipbba), and an amine-rich melamine (MA) co-ligand. Interestingly, the framework possesses two types of 1D channels decorated with CO₂ -philic (-NH₂ and -CF₃ ) groups that promote the highly selective CO₂ adsorption by the framework, which was supported by computational simulations. Further, the synergistic involvement of both Lewis acidic and basic sites exposed in the confined 1D channels along with high thermal and chemical stability rendered HbMOF1 a good heterogeneous catalyst for the highly efficient fixation of CO₂ in a reaction with terminal/internal epoxides at mild conditions (RT and 1 bar CO₂ ). Moreover, in-depth theoretical studies were carried out using periodic DFT to obtain the relative energies for each stage involved in the catalytic reaction and an insight mechanistic details of the reaction is presented. Overall, this work represents a rare demonstration of rational design of a porous Zn^II MOF incorporating multiple functional sites suitable for highly efficient fixation of CO₂ with terminal/internal epoxides at mild conditions supported by comprehensive theoretical studies.

The Significance of Metal Coordination in Imidazole-Functionalized Metal-Organic Frameworks for Carbon Dioxide Utilization

The grafting of imidazole species onto coordinatively unsaturated sites within metal-organic framework MIL-101(Cr) enables enhanced CO₂ capture in close proximity to catalytic sites. The subsequent combination of CO₂ and epoxide binding sites, as shown through theoretical findings, significantly improves the rate of cyclic carbonate formation, producing a highly active CO₂ utilization catalyst. An array of spectroscopic investigations, in combination with theoretical calculations reveal the nature of the active sites and associated catalytic mechanism which validates the careful design of the hybrid MIL-101(Cr).

Mild and selective hydrogenation of CO2 into formic acid over electron-rich MoC nanocatalysts

The direct hydrogenation of CO₂ using H₂ gas is a one-stone-two-birds route to produce highly value-added hydrocarbon compounds and to lower the CO₂ level in the atmosphere. However, the transformation of CO₂ and H₂ into hydrocarbons has always been a great challenge while ensuring both the activity and selectivity over abundant-element-based nanocatalysts. In this work, we designed a Schottky heterojunction composed of electron-rich MoC nanoparticles embedded inside an optimized nitrogen-doped carbon support (MoC@NC) as the first example of noble-metal-free heterogeneous catalysts to boost the activity of and specific selectivity for CO₂ hydrogenation to formic acid (FA) in liquid phase under mild conditions (2 MPa pressure and 70 °C). The MoC@NC catalyst with a high turnover frequency (TOF) of 8.20 mol_FA mol_MoC^-1 h^-1 at 140 °C and an excellent reusability are more favorable for real applications.

Rational Stepwise Construction of Different Heterometallic-Organic Frameworks (HMOFs) for Highly Efficient CO2 Conversion

The coordination preference of different metal ions and ligands have an immense influence on the constructions of functional MOF materials. In this work, two new monometallic complexes, namely [Ag(HL)(bipy)_0.5 ] (1) and {[Tb(L)_1.5 (H₂ O)]⋅4 H₂ O}_n (2) (bipy=4,4-bipyridine), have been synthesized successfully by employing a bifunctional 2-(imidazol-1-yl)terephthalic acid (H₂ L) ligand. After that, two new different heterometallic-organic frameworks (HMOFs), namely {[TbAg(L)₂ (H₂ O)₃ ]⋅H₂ O}_n (3) and [TbAg(L)₂ (H₂ O)]_n (4), were obtained from complexes 1 and 2 as the precursors based on a rational stepwise construction strategy and the theory of hard and soft acids and bases (HSAB principle), respectively. The HMOFs bearing dual metallic catalytic sites (Tb and Ag) can be used as heterogeneous catalysts without losing performance for the chemical fixation of CO₂ with epoxides including the sterically hindered epoxides, demonstrating some of the highest reported catalytic activity values. This work may provide a new synthetic route toward tailoring new HMOFs with excellent catalytic activity.

Formation of CX Bonds in CO2 Chemical Fixation Catalyzed by Metal-Organic Frameworks

Transformation of CO₂ based on metal-organic framework (MOF) catalysts is becoming a hot research topic, not only because it will help to reduce greenhouse gas emission, but also because it will allow for the production of valuable chemicals. In addition, a large number of impressive products have been synthesized by utilizing CO₂ . In fact, it is the formation of new covalent bonds between CO₂ and substrate molecules that successfully result in CO₂ solidly inserting into the products, and only four types of new CX bonds, including CH, CC, CN, and CO bonds, are observed in this exploration. An overview of recent progress in constructing CX bonds for CO₂ conversion catalyzed by various MOF catalysts is provided. The catalytic mechanism of generating different CX bonds is further discussed according to both structural features of MOFs and the interactions among CO₂ , substrates, as well as MOFs. The future opportunities and challenges in this field are also tentatively covered.

A Facile and Versatile "Click" Approach Toward Multifunctional Ionic Metal-organic Frameworks for Efficient Conversion of CO2

Ionic metal-organic frameworks (IMOFs) that integrate synergistic Lewis-acid sites (intrinsic metal centers of the frameworks) and nucleophilic anions (halides encapsulated within pores) are intriguing platforms for the design of fully heterogeneous catalytic systems for cycloaddition of CO₂ to epoxides. A new, facile and versatile synthetic approach has been used to fabricate triazolium-based IMOFs for the first time. The approach makes use of azide-alkyne click chemistry and subsequent N-alkylation to post-synthetically create a cationic triazolium ring and introduce exchangeable counteranions at the same time. The IMOFs are efficient and recyclable heterogeneous catalysts for CO₂ conversion under mild and cocatalyst-free conditions. In particular, the click-accessible triazolium ring provides a handle to incorporate further functionality. The MIL-101-tzmOH-Br catalyst, which integrates hydrogen-bonding hydroxy groups besides metal centers and bromide anions, shows superior catalytic performance under mild conditions.

Robust multivariate metal–porphyrin frameworks for efficient ambient fixation of CO2 to cyclic carbonates

A series of robust polyfluorometalate-metalloporphyrin nets have three-types of orderly distributed metals, high CO₂-trapping capacity and good catalytic properties for coupling CO₂ with epoxides.

Sustainable Heterogeneous Catalysts for CO2 Utilization by Using Dual Ligand ZnII /CdII Metal-Organic Frameworks

Two-dimensional Zn^II /Cd^II -based dual ligand metal-organic frameworks (MOFs) {[M(CHDC)(L)]⋅H₂ O}_n involving 4-pyridyl carboxaldehyde isonicotinoylhydrazone (L) in combination with flexible 1,4-cyclohexanedicarboxylic acid (H₂ CHDC) as linkers have been synthesized by adaptable synthetic protocols including a green mechanochemical (grinding) method. Characterization, chemical/thermal stability, phase purity, and solid-state luminescent properties of both MOFs have been established by various analytical methods. Structural analysis revealed dimeric metal clusters composed of [M₂ (CHDC)₂ ]_n loops doubly pillared with L, generating a 2D framework. Both MOFs can be used as highly active solvent-free binary catalysts for CO₂ cycloaddition with epoxides in the presence of the co-catalyst tetrabutylammonium bromide (TBAB) with good catalytic conversion in up to six catalytic cycles without significant loss of activity. The present investigation demonstrates the application of MOFs as efficient heterogeneous catalysts for CO₂ utilization under moderate reaction conditions. Based on the single-crystal X-ray data, a probable mechanism for the cycloaddition reaction has also been proposed.

Catalytic Space Engineering of Porphyrin Metal-Organic Frameworks for Combined CO2 Capture and Conversion at a Low Concentration

Porous porphyrin metal-organic frameworks (PMOFs) provide promising platforms for studying CO₂ capture and conversion (C3) owing to their versatility in photoelectric, catalytic, and redox activities and porphyrin coordination chemistry. Herein, we report the C3 application of two PMOFs by engineering the coordination space through the introduction of two catalytic metalloporphyrins doped with rhodium or iridium, Rh-PMOF-1 and Ir-PMOF-1, both of which can serve as heterogeneous catalysts for the chemical fixation of CO₂ into cyclic carbonates with yields of up to 99 %. Remarkably, the catalytic reactions can effectively proceed under low CO₂ concentrations and high yields of 83 % and 73 % can be obtained under 5 % CO₂ in the presence of Rh-PMOF-1 and Ir-PMOF-1, respectively. The synergistic effect of the metalloporphyrin ligand and the Zr₆ O₈ cluster, in combination with the CO₂ concentration effect from the pore space, might account for the excellent catalytic performance of Rh-PMOF-1 under low CO₂ concentration. Recycling tests of Rh-PMOF-1 show negligible loss of catalytic activity after 10 runs.

Porous Metal-Organic Polyhedral Framework containing Cuboctahedron Cages as SBUs with High Affinity for H2 and CO2 Sorption: A Heterogeneous Catalyst for Chemical Fixation of CO2

Development of active porous materials that can efficiently adsorb H₂ and CO₂ is needed, due to their practical utilities. Here we present the design and synthesis of an interpenetrated Cu^II metal-organic framework (MOF) that is thermally stable, highly porous and can act as a heterogeneous catalyst. The Cu^II -MOF contains a highly symmetric polyhedral metal cluster (Cu₂₄ ) with cuboctahedron geometry as secondary building unit (SBU). The double interpenetration of such huge cluster-containing nets provides a high density of open metal sites, due to which it exhibits remarkable H₂ storage capacity (313 cm³ g^-1 at 1 bar and 77 K) as well as high CO₂ capture ability (159 cm³ g^-1 at 1 bar and 273 K). Further, its propensity towards CO₂ sorption can be utilized for the heterogeneous catalysis of the chemical conversion of CO₂ into the corresponding cyclic carbonates upon reaction with epoxides, with high turnover number and turnover frequency values.

A Stable Metal-Organic Framework Featuring a Local Buffer Environment for Carbon Dioxide Fixation

A majority of metal-organic frameworks (MOFs) fail to preserve their physical and chemical properties after exposure to acidic, neutral, or alkaline aqueous solutions, therefore limiting their practical applications in many areas. The strategy demonstrated herein is the design and synthesis of an organic ligand that behaves as a buffer to drastically boost the aqueous stability of a porous MOF (JUC-1000), which maintains its structural integrity at low and high pH values. The local buffer environment resulting from the weak acid-base pairs of the custom-designed organic ligand also greatly facilitates the performance of JUC-1000 in the chemical fixation of carbon dioxide under ambient conditions, outperforming a series of benchmark catalysts.

Bifunctional Pyridinium-Based Ionic-Liquid-Immobilized Diindium Tris(diphenic acid) Bis(1,10-phenanthroline) for CO2 Fixation

A pyridinium-based ionic-liquid-decorated 1 D metal-organic framework (MOF; IL-[In₂ (dpa)₃ (1,10-phen)₂ ]; IL=ionic liquid; dpa=diphenic acid; 1,10-phen=1,10-phenanthroline) was developed as a bifunctional heterogeneous catalyst system for CO₂ -oxirane coupling reactions. An aqueous-microwave route was employed to perform the hydrothermal reaction for the synthesis of the [In₂ (dpa)₃ (1,10-phen)₂ ] MOF, and the IL-[In₂ (dpa)₃ (1,10-phen)₂ ] catalyst was synthesized by covalent postfunctionalization. As a result of the synergetic effect of the dual-functional sites, which include Lewis acid sites (coordinatively unsaturated In sites) and the I^- ion in the IL functional sites, IL-[In₂ (dpa)₃ (1,10-phen)₂ ] displayed a high catalytic activity for CO₂ -epoxide cycloaddition reactions under mild and solvent-free conditions. Microwave pulses were employed for the first time in MOF-catalyzed CO₂ -epoxide cycloaddition reactions to result in a high turnover frequency of 2000-3100 h^-1 . The catalyst had an excellent reusability and maintained a continuous high selectivity. Furthermore, only a small amount of leaching was observed from the spent catalyst. A plausible reaction mechanism based on the synergistic effect of the dual-functional sites that catalyze the CO₂ -epoxide cycloaddition reaction effectively is proposed.

Imidazolium- and Triazine-Based Porous Organic Polymers for Heterogeneous Catalytic Conversion of CO2 into Cyclic Carbonates

CO₂ adsorption and concomitant catalytic conversion into useful chemicals are promising approaches to alleviate the energy crisis and effects of global warming. This is highly desirable for developing new types of heterogeneous catalytic materials containing CO₂ -philic groups and catalytic active sites for CO₂ chemical transformation. Here, we present an imidazolium- and triazine-based porous organic polymer with counter chloride anion (IT-POP-1). The porosity and CO₂ affinity of IT-POP-1 may be modulated at the molecular level through a facile anion-exchange strategy. Compared with the post-modified polymers with iodide and hexafluorophosphate anions, IT-POP-1 possesses the highest surface area and the best CO₂ uptake capacity with excellent adsorption selectivity over N₂ . The roles of the task-specific components such as triazine, imidazolium, hydroxyl, and counter anions in CO₂ absorption and catalytic performance were illustrated. IT-POP-1 exhibits the highest catalytic activity and excellent recyclability in solvent- and additive-free cycloaddition reaction of CO₂ with epoxides.

An Uncommon Carboxyl-Decorated Metal-Organic Framework with Selective Gas Adsorption and Catalytic Conversion of CO2

A new three-dimensional (3D) framework, [Ni(btzip)(H₂ btzip)]⋅2 DMF⋅2 H₂ O (1) (H₂ btzip=4,6-bis(triazol-1-yl)isophthalic acid) as an acidic heterogeneous catalyst was constructed by the reaction of nickel wire and a triazolyl-carboxyl linker. Framework 1 possesses intersected 2D channels decorated by free COOH groups and uncoordinated triazolyl N atoms, leading to not only high CO₂ and C₂ H₆ adsorption capacity but also significant selective capture for CO₂ and C₂ H₆ over CH₄ and CO in 273-333 K. Moreover, 1 reveals chemical stability toward water. Grand Canonical Monte Carlo simulations confirmed the multiple CO₂ - and C₂ H₆ -philic sites. As a result of the presence of accessible Brønsted acidic COOH groups in the channels, the activated framework demonstrates highly efficient catalytic activity in the cycloaddition reaction of CO₂ with propylene oxide/4-chloromethyl-1,3-dioxolan-2-one/3-butoxy-1,2-epoxypropane into cyclic carbonates.

Greening the Processes of Metal-Organic Framework Synthesis and their Use in Sustainable Catalysis

Given the shortage of sustainable resources and the increasingly serious environmental issues in recent decades, the demand for clean technologies and sustainable feedstocks is of great interest to researchers worldwide. With regard to the fields of energy saving and environmental remediation, the key point is the development of efficient catalysts, not only in terms of facile synthesis methods, but also the benign utilization of such catalysts. This work reviews the use of metal-organic frameworks (MOFs) and MOF-based materials in these fields. The definition of MOFs and MOF-based materials will be primarily introduced followed by a brief description of the characterization and stability of MOF-related materials under the applied conditions. The greening of MOF synthesis processes will then be discussed and catalogued by benign solvents and conditions and green precursors of MOFs. Furthermore, their suitable application in sustainable catalysis will be summarized, focusing on several typical atom-economic reactions, such as the direct introduction of H₂ or O₂ and C-C bond formation. Approaches towards reducing CO₂ emission by MOF-based catalysts will be described with special emphasis on CO₂ fixation and CO₂ reduction. In addition, driven by the explosive growth of energy consumption in the last century, much research has gone into biomass, which represents a renewable alternative to fossil fuels and a sustainable carbon feedstock for chemical production. The advanced progress of biomass-related transformations is also illustrated herein. Fundamental insights into the nature of MOF-based materials as constitutionally easily recoverable heterogeneous catalysts and as supports for various active sites is thoroughly discussed. Finally, challenges facing the development of this field and the outlook for future research are presented.

Charged Metalloporphyrin Polymers for Cooperative Synthesis of Cyclic Carbonates from CO2 under Ambient Conditions

A facile and one-pot synthesis of metalloporphyrin-based ionic porous organic polymers (M-iPOPs) was performed through a typical Yamamoto-Ullmann coupling reaction for the first time. We used various characterization techniques to demonstrate that these strongly polar Al-based materials (Al-iPOP) possessed a relatively uniform microporosity, good swellable features, and a good CO₂ capture capacity. If we consider the particular physicochemical properties, heterogeneous Al-iPOP, which bears both a metal active center and halogen anion, acted as a bifunctional catalyst for the solvent- and additive-free synthesis of cyclic carbonates from various epoxides and CO₂ with an excellent activity and good recyclability under mild conditions. Interestingly, these CO₂ -philic materials could catalyze the cycloaddition reaction smoothly by using simulated flue gas (15 % CO₂ in N₂ , v/v) as a raw material, which indicates that a stable local microenvironment and polymer swellability might promote the transformation. Thus, the introduction of polar ionic liquid units into metalloporphyrin-based porous materials is regarded as a promising new strategy for the chemical conversion of CO₂ .