Postsynthetic ionization of an imidazole-containing metal-organic framework for the cycloaddition of carbon dioxide and epoxides

Enhancing the Lewis acidity of single atom Tb via introduction of boron to achieve efficient photothermal synergistic CO2 cycloaddition

Nowadays, it is becoming increasingly urgent to lower the escalating carbon dioxide (CO2) to reduce greenhouse effect. Fortunately, it is an ideal strategy by using the inexhaustible solar energy as the driving force to manipulate the cycloaddition reaction, the atomic efficiency of which is 100 %. This work represents the first attempt on utilization of rare-earth metal Tb with atomic dispersion, and the structure of Tb coordinated with 4 N-atoms and 2B-atoms was constructed on interconnected carbon hollow spheres. The introduction of electron-deficient B reduces the electron density of Tb, thereby boosting Lewis acidity and promoting the occurrence of ring-opening reaction. The mechanism exploration enunciates that TbN4B2/C is a photothermal synergistic catalyst, the combined action of photogenerated electrons and strong Lewis acidic site of Tb reduces the free energy of the rate-determining step, and then improving the yield of cyclic carbonate up to 739 mmol g-1h-1.

Rapid visual dual-mode detection of Zr(IV) based on L-histidine functionalized gold nanoparticles

Heavy metal pollution has always been a great threat to human health and safety. Compared with other heavy metals, although zirconium ion (Zr(IV)) is equally harmful, due to the lack of research on Zr(IV) in the biological systems and environment, its detection does not seem to have received the attention it deserves. Herein, a rapid visual dual-mode detection (colorimetric and chrominance method) of Zr(IV) based on L-histidine functionalized gold nanoparticles (HIS-AuNPs) has been reported. AuNPs and HIS-AuNPs before and after adding Zr(IV) were characterized by UV-Vis, TEM, DLS, Zeta potential, EDS and FT-IR, etc. These results showed that L-histidine was successfully modified on the surface of AuNPs by forming a stable Au-N bond, and its modification had little effect on the dispersion degree of AuNPs. After the addition of Zr(IV), interaction of this metal ion with the imidazolyl group on L-histidine can obviously cause the aggregation of HIS-AuNPs within 12 min, and the dispersion state and particle size of HIS-AuNPs can be significantly changed. These two detection modes were established by means of absorbance and color change of solution, and being used in addition and recovery experiments of Zr(IV) in natural water. Under the optimal conditions, these two modes exhibited good linearity within 15-70 and 20-100 μmol L-1, and limit of detection of 2.62 and 6.25 μmol L-1. The proposed method was highly sensitive and selective, which provided a new convenient way to realize the detection of Zr(IV).

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.

Construction of a highly stable natural silicate-supported molybdenum catalyst for efficient epoxidation of olefins

Epoxides are important bulk chemicals, playing irreplaceable role in the chemical industry, but facing serious pollution and low productivity in the production process. Therefore, the development of green and efficient epoxidation of olefins by stable catalysts with low prices is of great significance. In this study, a Mo-MATP catalyst was prepared by modifying Mo(CO)₆ on attapulgite through Si-O bonding. Mo-MATP exhibits excellent performance (99% yield of cyclooctane oxide, CYCO) and stability (80% selectivity of CYCO after 17 cycles), highly tert-butyl hydroperoxide (TBHP) utilization, and extensive substrate scalability. Furthermore, the in-situ Fourier Transform Infrared Spectroscopy (FT-IR), Electron Spin-resonance Spectroscopy (ESR) and High Resolution Mass Spectrometry (HRMS) spectra suggest that TBHP would be activated by Mo-MATP to generate peroxyl radicals, which then oxidize alkenes to their corresponding epoxides. In this study, the stable loading of Mo would largely solve the problem of Mo loss during the catalytic process, thus providing a stable and dispersed Mo active center, enabling the catalyst to possess high catalytic performance and recycling stability.

DFT mechanistic study of the chemical fixation of CO2 by aziridine derivatives

Using density functional theory (DFT), we treat the reaction of coupling of CO2 with aziridine in gas phase, in the presence of water and of a green catalyst (NaBr). Computations show that, in gas phase, this ring-opening conversions to oxazolidinones initiates by coordinating a CO2 molecule to the nitrogen atom of the aziridine. Then, a nucleophilic interaction between one oxygen atom of the coordinated CO2 and the carbon atom of the aziridine occurs. For methyl substituted aziridine, two pathways are proposed leading either to 4-oxazolidinone or to 5-oxazolidinone. Besides, we show that the activation energy of this reaction reduces in aqueous solution, in the presence of a water molecule explicitly or NaBr catalyst. In addition, the corresponding reaction mechanisms and regioselectivity associated with this ring-opening conversions to oxazolidinones, in the presence of carbon dioxide are found to be influenced by solvent and catalyst. The present findings should allow better designing regioisomer oxazolidinones relevant for organic chemistry, medicinal and pharmacological applications.

Ionic liquid post-modified carboxylate-rich MOFs for efficient catalytic CO2 cycloaddition under solvent-free conditions

The synthesis of cyclic carbonates through cycloaddition reactions between epoxides and carbon dioxide (CO2) is an important industrial process. Metal-Organic Frameworks (MOFs) have functional and ordered pore structures, making them attractive catalysts for converting gas molecules into valuable products. One approach to enhance the catalytic activity of MOFs in CO2 cycloaddition reactions is to create open metal sites within MOFs. In this study, the amino-functionalized rare earth Gd-MOF (Gd-TPTC-NH2) and its ionic liquid composite catalysts (Gd-TPTC-NH-[BMIM]Br) were synthesized using 2'-amino-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4TPTC-NH2) as the ligand. The catalytic performance of these two catalysts was observed in the cycloaddition reaction of CO2 and epoxides. Under the optimized reaction conditions, Gd-TPTC-NH-[BMIM]Br can effectively catalyze the cycloaddition reaction of a variety of epoxide substrates with good to excellent yields of cyclic carbonate products. Comparatively, epichlorohydrin and epibromohydrin, which possess halogen substituents, promote higher yields of cyclic carbonates due to the electron-withdrawing nature of Cl and Br substituents. Additionally, the Gd-TPTC-NH-[BMIM]Br catalyst demonstrated good recyclability and reproducibility, maintaining its catalytic activity without any changes in its structure or properties after five reuse cycles.

Bridging the Homogeneous and Heterogeneous Catalysis by Supramolecular Metal-Organic Cages with Varied Packing Modes

Integrating the advantages of homogeneous and heterogeneous catalysis has proved to be an optimal strategy for developing catalytic systems with high efficiency, selectivity, and recoverability. Supramolecular metal-organic cages (MOCs), assembled by the coordination of metal ions with organic linkers into discrete molecules, have performed solvent processability due to their tunable packing modes, endowing them with the potential to act as homogeneous or heterogeneous catalysts in different solvent systems. Here, the design and synthesis of a series of stable {Cu3} cluster-based tetrahedral MOCs with varied packing structures are reported. These MOCs, as homogeneous catalysts, not only show high catalytic activity and selectivity regardless of substrate size during the CO2 cycloaddition reaction, but also can be easily recovered from the reaction media through separating products and co-catalysts by one-step work-up. This is because that these MOCs have varied solubilities in different solvents due to the tunable packing of MOCs in the solid state. Moreover, the entire catalytic reaction system is very clean, and the purity of cyclic carbonates is as high as 97% without further purification. This work provides a unique strategy for developing novel supramolecular catalysts that can be used for homogeneous catalysis and recycled in a heterogeneous manner.

Functional UiO-66 for highly selective adsorption of N-nitrosodipropylamine: adsorption performance and mechanisms

N-Nitrosodipropylamine (NDPA) is a class of nitrogenous disinfection by-products (N-DBPs) with high toxicity. Although NDPA present in water bodies is at relatively low concentrations, the potential risk is high due to its high toxicity and bioaccumulation. Metal-organic frameworks (MOFs), a new type of porous material with remarkable functionality, have shown great performance in a wide variety of applications in adsorption. This is the first study investigating the adsorption of MOFs on NDPA. Herein, UiO-66 with -NH2 and imidazolium functional groups were synthesized by modifying UiO-66 after amination. Adsorption kinetics and isotherm models were used to compare the adsorption properties of the two materials for low-concentration NDPA in water. The results showed that the behavior of all the adsorbents was consistent with the Langmuir model and the pseudo-second-order model and that the adsorption was homogeneous chemisorption. The structures of the nanoparticles were characterized by FTIR, zeta potential, XRD, SEM and BET measurements. Based on the characteristics, four adsorption mechanisms, namely electron conjugation, coordination reaction, anion-π interaction, and van der Waals forces, were simultaneously involved in the adsorption. The influencing factor experiment revealed that the adsorption of UiO-66-NH2 and (I-)Meim-UiO-66 involved hydrogen bonding and electrostatic interactions, respectively.

Coordination Synergy between Iridium Photosensitizers and Metal Nanoclusters Leading to Enhanced CO2 Cycloaddition under Mild Conditions

The achievement of photocatalytic CO2 and epoxide cycloaddition under mild conditions such as room temperature and atmospheric pressure is important for green chemistry, which can be achieved by developing coordination synergies between catalysts and photosensitizers. In this context, we exploit the use of coordinate bonds to connect pyridine-appended iridium photosensitizers and catalysts for CO2 cycloaddition, which is systematically demonstrated by 1H nuclear magnetic resonance titration and X-ray photoelectron spectroscopic measurements. It is shown that the hybrid Ir(Cltpy)2/Mn2Cd4 photocatalytic system with coordination synergy exhibits excellent catalytic performance (yield ≈ 98.2%), which is 3.75 times higher than that of the comparative Ir(Cltpy-Ph)2/Mn2Cd4 system without coordination synergy (yield ≈ 26.2%), under mild conditions. The coordination between the Mn2Cd4 catalyst and the Ir(Cltpy)2 photosensitizer enhances the light absorption and photoresponse properties of the Mn2Cd4 catalyst. This has been confirmed through transient photocurrent, electrochemical impedance, and electron paramagnetic tests. Consequently, the efficiency of cycloaddition was enhanced by utilizing mild conditions.

Recyclable N-Heterocyclic Carbene Porous Coordination Polymers with Two Distinct Metal Sites for Transformation of CO2 to Cyclic Carbonates

Single-component catalysts with integrated multiple reactive centers could work in concert to achieve enhanced activity tailored for specific catalytic reactions, but they remain underdeveloped. Herein, we report the construction of heterogeneous bimetallic porous coordination polymers (PCPs) containing both porphyrin and N-heterocyclic carbene (NHC) metal sites via the coordinative assembly of the NHC functionalities. Three heterobimetallic PCPs (TIPP-Zn-Pd, TIPP-Cu-Pd and TIPP-Ni-Pd) have been prepared to verify this facile synthetic strategy for the first time. In order to establish a cooperative action toward the catalytic CO2 cycloaddition with epoxides, an additional tetraalkylammonium bromide functionality has also been incorporated into these polymeric structures through the N-substituent of the NHC moieties. The resulting heterogeneous bimetallic catalyst TIPP-Zn-Pd exhibits the best catalytic performance in CO2 cycloaddition with styrene oxide (SO) under solvent-free conditions at atmospheric pressure and is applicable to a wide range of epoxides. More importantly, TIPP-Zn-Pd works smoothly and is recyclable in the absence of a cocatalyst under 1.0 MPa of CO2 at 60 °C. This indicates that TIPP-Zn-Pd is quite competitive with the reported heterogeneous catalysts, which typically require a high reaction temperature above 100 °C under cocatalyst-free conditions. Thus, this work provides a new approach to design heterogeneous bimetallic PCP catalysts for high-performance CO2 fixation under mild reaction conditions.

Tuning the Catalytic Activity of Covalent Metal-Organic Frameworks for CO2 Cycloaddition Reactions

The development of efficient, recyclable and low-cost heterogeneous catalysts for conversion of carbon dioxide (CO2 ) into epoxides is highly desired, yet remain a challenge. Herein, we have prepared three two-dimensional (2D) copper(I) cyclic trinuclear units (Cu(I)-CTUs) based covalent metal-organic frameworks (CMOFs), namely JNM-13, JNM-14, and JNM-15, via a one-pot reaction by combination of coordination and dynamic covalent chemistry. Among them, JNM-15 contained the highest density of copper catalytic sites, and exhibited the highest capacity for adsorption of CO2 . More interestingly, JNM-15 delivered the highest catalytic activity for cycloaddition of CO2 to epoxides with good yields (up to 99 %), good substrate compatibility (11 examples) and reusability (four catalytic cycles) under mild condition.

Stable Pyrene-Based Metal-Organic Framework for Cyclization of Propargylic Amines with CO2 and Detection of Antibiotics in Water

A pyrene-based metal-organic framework, Cd2(PTTB)(H2O)2 (WYU-11), was synthesized from the tetracarboxylic pyrene ligand H4PTTB (H4PTTB = 1,3,6,8-tetrakis(3-carboxyphenyl)pyrene) and Cd(NO3)2·4H2O. Powder X-ray diffraction analysis discloses that the framework is stable in acid, base, and various organic solvent environments. WYU-11 shows excellent catalytic performance on the cyclization reaction of propargylic amines with CO2 into 2-oxazolidinones under mild conditions (60 °C, atmospheric CO2). 1H NMR studies unveiled that WYU-11 and 1,1,3,3-tetramethylguanidine (TMG) can synergistically activate the propargylic amine substrate and promote the reaction. Importantly, WYU-11 represents a rare example of noble metal-free heterogeneous catalyst that can catalyze the cyclization of CO2 with propargylic amines. In addition, by virtue of the excellent water stability and luminescence properties, WYU-11 shows excellent detection performance for sulfathiazole (STZ) and ornidazole (ODZ) in water. Investigation reveals that the coexistence of photoinduced electron transfer and internal filtering effect could reasonably explain the luminescence quenching of WYU-11 by the antibiotics.

Metal-Organic Frameworks as a New Platform to Construct Ordered Mesoporous Ce-Based Oxides for Efficient CO2 Fixation under Ambient Conditions

Metal-organic frameworks (MOFs) are proved to be good precursors to derive various nanomaterials with desirable functions, but so far the controllable synthesis of ordered mesoporous derivatives from MOFs has not been achieved. Herein, this work reports, for the first time, the construction of MOF-derived ordered mesoporous (OM) derivatives by developing a facile mesopore-inherited pyrolysis-oxidation strategy. This work demonstrates a particularly elegant example of this strategy, which involves the mesopore-inherited pyrolysis of OM-CeMOF into a OM-CeO2 @C composite, followed by the oxidation removal of its residual carbon, affording the corresponding OM-CeO2 . Furthermore, the good tunability of MOFs helps to allodially introduce zirconium into OM-CeO2 to regulate its acid-base property, thus boosting its catalytic activity for CO2 fixation. Impressively, the optimized Zr-doped OM-CeO2 can achieve above 16 times higher catalytic activity than its solid CeO2 counterpart, representing the first metal oxide-based catalyst to realize the complete cycloaddition of epichlorohydrin with CO2 under ambient temperature and pressure. This study not only develops a new MOF-based platform for enriching the family of ordered mesoporous nanomaterials, but also demonstrates an ambient catalytic system for CO2 fixation.

Ce/Zr-MOF with Dual Cycle Synergistic Catalysis Pathway Enabling Enhanced Peroxidase-like Performance for Wearable Hydrogel Patch Visualization Sensing Platform

Engineering the activity of enzyme-like catalysts should be a top priority to make them superior substitutes for natural enzymes. Herein, a Ce/Zr bimetal-organic framework (Ce/Zr-MOF) was designed and synthesized by a one-pot hydrothermal method, which has enhanced performance in mimicking peroxidase (POD) than its single-metal counterparts. To further comprehend the mechanism of activity enhancement, the role of the bimetallic synergistic catalysis process in H2O2 decomposition and reactive oxygen species formation was elucidated, and the possible dual cycle synergistic catalysis pathway of bimetallic catalysis is proposed for the first time. The enhanced POD-like activity mainly depends on the introduction of Ce, which improved the conductivity and electron-transfer capability of Ce/Zr-MOF and promoted the generation of •OH. Integrated with a hydrogel substrate, a wearable all-solid-state H2O2 sensor for early diagnosis of plant health was produced. The detection limit can be as low as 3.3 μM, which is lower than that of some instrument-based colorimetric methods and has great potential in the development of visualized sensing applications. The concept of dual cycle synergistic catalysis pathway we proposed not only deepens the comprehension regarding sensing and catalytic mechanisms but also provides novel perspectives into the design of enzyme-like catalysts for extensive usage.

Zirconium metal-organic cage decorated with squaramides imparts dual activation for chemical fixation of CO2 under mild conditions

A "two-in-one" dual activation strategy has been developed to give high-density hydrogen-bonding (HB) active units and Lewis acid (LA) active centres by immobilizing squaramides into metal-organic cages (MOCs). The obtained MOC served as an efficient catalyst for the chemical fixation of CO2 under mild conditions up to 99% yields with good recyclability, and the mechanism of high catalytic activity has been further explored.

Confinement of Organic Dyes in UiO-66-Type Metal-Organic Frameworks for the Enhanced Synthesis of [1,2,5]Thiadiazole[3,4-g]benzoimidazoles

Organic dyes as non-noble metal photosensitizers have attracted increasing attention due to their environmental friendliness and sustainability but suffer from fast deactivation and low stability. Here, we reported a fruitful strategy by the confinement and stabilization of visible light-active signal unit organic dyes within the metal-organic frameworks (MOFs) and developed a series of heterogeneous photocatalysts dye@UiO-66s [dye = fluorescein (FL)/rhodamine B (RhB)/eosin Y (EY), UiO-66s = UiO-66, and Bim-UiO-66]. It has been demonstrated that the encapsulated dyes can effectively sensitize MOF hosts and dominate the band structures and photocatalytic activities of dye@UiO-66s regardless of the ligand functionalization of MOFs. Photocatalytic experiments showed that these dye@UiO-66s exhibit enhanced activities relative to free dyes and among them, FL@Bim-UiO-66 displays excellent efficiencies toward the green synthesis of new carbon-bridged annulations, [1,2,5]thiadiazole[3,4-g]benzoimidazoles in the yield of up to 98% at room temperature with outstanding stability and reusability. Furthermore, the intramolecular cyclization intermediate was captured and characterized by the single-crystal X-ray diffraction analysis.

Chemical complexity for targeted function in heterometallic titanium-organic frameworks

Research on metal-organic frameworks is shifting from the principles that control the assembly, structure, and porosity of these reticular solids, already established, into more sophisticated concepts that embrace chemical complexity as a tool for encoding their function or accessing new properties by exploiting the combination of different components (organic and inorganic) into these networks. The possibility of combining multiple linkers into a given network for multivariate solids with tunable properties dictated by the nature and distribution of the organic connectors across the solid has been well demonstrated. However, the combination of different metals remains still comparatively underexplored due to the difficulties in controlling the nucleation of heterometallic metal-oxo clusters during the assembly of the framework or the post-synthetic incorporation of metals with distinct chemistry. This possibility is even more challenging for titanium-organic frameworks due to the additional difficulties intrinsic to controlling the chemistry of titanium in solution. In this perspective article we provide an overview of the synthesis and advanced characterization of mixed-metal frameworks and emphasize the particularities of those based in titanium with particular focus on the use of additional metals to modify their function by controlling their reactivity in the solid state, tailoring their electronic structure and photocatalytic activity, enabling synergistic catalysis, directing the grafting of small molecules or even unlocking the formation of mixed oxides with stoichiometries not accessible to conventional routes.

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.

Aluminosilicate-Supported Catalysts for the Synthesis of Cyclic Carbonates by Reaction of CO2 with the Corresponding Epoxides

Functionalized aluminosilicate materials were studied as catalysts for the conversion of different cyclic carbonates to the corresponding epoxides by the addition of CO₂. Aluminum was incorporated in the mesostructured SBA-15 silica network. Thereafter, functionalization with imidazolium chloride or magnesium oxide was performed on the Al_SBA-15 supports. The isomorphic substitution of Si with Al and the resulting acidity of the supports were investigated via ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and NH₃ adsorption microcalorimetry. The Al content and the amount of MgO were quantified via inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The anchoring of the imidazolium salt was assessed by ²⁹Si and ¹³C MAS NMR spectroscopy and quantified by combustion chemical analysis. Textural and structural properties of supports and catalysts were studied by N₂ physisorption and X-ray diffraction (XRD). The functionalized systems were then tested as catalysts for the conversion of CO₂ and epoxides to cyclic carbonates in a batch reactor at 100 or 125 °C, with an initial CO₂ pressure (at room temperature) of 25 bar. Whereas the activity of the MgO/xAl_SBA-15 systems was moderate for the conversion of glycidol to the corresponding cyclic carbonate, the Al_SBA-15-supported imidazolium chloride catalysts gave excellent results over different epoxides (conversion of glycidol, epichlorohydrin, and styrene oxide up to 89%, 78%, and 18%, respectively). Reusability tests were also performed. Even when some deactivation from one run to the other was observed, a comparison with the literature showed the Al-containing imidazolium systems to be promising catalysts. The fully heterogeneous nature of the present catalysts, where the inorganic support on which the imidazolium species are immobilized also contains the Lewis acid sites, gives them a further advantage with respect to most of the catalytic systems reported in the literature so far.

Self-assembled mononuclear complexes: open metal sites and inverse dimension-dependent catalytic activity for the Knoevenagel condensation and CO2 cycloaddition

To lessen the greenhouse effect, measures such as improving the recovery of crude oil and converting carbon dioxide (CO₂) into valuable chemicals are necessary to create a sustainable low-carbon future. To this end, the development of efficient new oil-displacing agents and CO₂ conversion has aroused great interest in both academia and industry. The Knoevenagel condensation and CO₂ cycloaddition are the key reactions to solve the above problems. Four Cu- or Zn-based molecular complexes built from different ligands possessing hydrophilic-hydrophobic layers and different dimensionalities were chosen as solid catalysts for this study. Structural analysis revealed the presence of hydrophilic-hydrophobic layers and open metal sites in the low-dimensional complexes. To obtain deep insight into the reaction mechanism, first-principles density functional theory (DFT) calculations were carried out. These calculations confirmed that in the Knoevenagel condensation reaction, the final formation of benzylidenemalononitrile is the rate-determining step (an energy barrier (ΔE) value of 73.2 kJ mol^-1). The zero-dimensional (0D) Cu molecular complex with unsaturated metal centers, hydrophilic and hydrophobic layers, exhibited higher catalytic activity (yield: 100%, temperature: room temperature, and time: 2 h) compared with one- and two-dimensional Cu complexes. In the presence of a 0D Zn complex co-catalyzed with Br^- in the CO₂ cycloaddition reaction, the ΔE value reduces to 35.5 kJ mol^-1 for the ring opening of styrene oxide (SO), which is significantly lower than Br^- catalyzed (80.9 kJ mol^-1) reactions. The roles of unsaturated metal centers, hydrophilic-hydrophobic layers and dimensionality in the Knoevenagel condensation and CO₂ cycloaddition were explained in the results of structure-activity relationships.

Thermo-, Electro-, and Photocatalytic CO2 Conversion to Value-Added Products over Porous Metal/Covalent Organic Frameworks

ConspectusThe continuing increase of the concentration of atmospheric CO₂ has caused many environmental issues including climate change. Catalytic conversion of CO₂ using thermochemical, electrochemical, and photochemical methods is a potential technique to decrease the CO₂ concentration and simultaneously obtain value-added chemicals. Due to the high energy barrier of CO₂ however, this method is still far from large-scale applications which requires high activity, selectivity, and stability. Therefore, development of efficient catalysts to convert CO₂ to different products is urgent. With their well-engineered pores and chemical compositions, high surface area, elevated CO₂ adsorption capability, and adjustable active sites, porous crystalline frameworks including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are potential materials for catalytic CO₂ conversion. Here, we summarize our recent work on MOFs and COFs for thermocatalytic, electrocatalytic, and photocatalytic CO₂ conversion and describe the structure-activity relationships that could guide the design of effective catalysts.The first section of this paper describes imidazolium-functionalized porous MOFs, including porous liquid and cationic MOFs with nucleophilic halogen ions, which can promote thermocatalytically CO₂ cycloaddition reaction with epoxides toward cyclic carbonates at one bar pressure. A porous liquid MOF takes on the role of a CO₂ reservoir to tackle the low local CO₂ concentrations in gas-liquid-solid heterogeneous reactions. Imidazolium-functionalized MOFs with halogen ions for CO₂ cycloaddition could avoid the use of cocatalysts, and this leads to milder and more facile experimental conditions and separation processes.In a section dealing with the electrocatalytic CO₂ reduction reaction (CO₂RR), we developed a series of conductive porous framework materials with fast electron transmission capabilities, which afford high current densities and outperform the traditional MOF and COF catalysts that have been reported. The intrinsically conductive two-dimensional 2D MOFs and COFs nanosheets based on the fully π-conjugated phthalocyanine motif with excellent electron transport capability were prepared, and strong electron transporters were also integrated into metalloporphyrin-based COFs for CO₂RR. Cu₂O quantum dots and Cu nanoparticles (NPs) can be uniformly dispersed on porous conductive MOFs/COFs to afford synergistic and/or tandem electrocatalysts, which can achieve highly selective production of CH₄ or C₂H₄ in CO₂RR.A third section describes our efforts to facilitate electron-hole separation in CO₂ photocatalysis. Our focus is on regulation of coordination spheres in MOFs, fabrication of the architecture of MOF heterojunctions, and engineering MOF films to facilitate photocatalytic CO₂ reduction.Finally, we discuss several problems associated with the studies of MOFs and COFs for CO₂ conversion and consider some prospects of the fabrication of effective porous frameworks for CO₂ adsorption and conversion.

Ionic encapsulation of a methanol carbonylation catalyst in a microporous metal-organic framework

The anionic rhodium complex cis-[Rh(CO)₂I₂]^-, active in the Monsanto process for acetic acid production, has been heterogenised via Coulombic interactions in the pores of a UiO-66-type metal-organic framework (MOF). The MOF-supported catalyst is active for the carbonylation of methanol and is recyclable, retaining its framework crystallinity following catalysis. Intermediates in the catalytic cycle observed by IR spectroscopy confirm the same mechanism as the established homogeneous process.

Tetra-Coordinated Boron-Functionalized Phenanthroimidazole-Based Zinc Salen as a Photocatalyst for the Cycloaddition of CO2 and Epoxides

A unique B-N coordinated phenanthroimidazole-based zinc salen was synthesized. The zinc salen thus synthesized acts as a photocatalyst for the cycloaddition of carbon dioxide with terminal epoxides under ambient conditions. DFT study of the cycloaddition of carbon dioxide with terminal epoxide indicates the preference of the reaction pathway when photocatalyzed by zinc salen. We anticipate that this strategy will help to design new photocatalysts for CO₂ fixation.

Two-Dimensional Cationic Aluminoborate as a New Paradigm for Highly Selective and Efficient Cr(VI) Capture from Aqueous Solution

Water pollutants existing in their oxyanion forms have high solubility and environmental mobility. To capture these anionic pollutants, cost-effective inorganic materials with cationic frameworks and outstanding removal performance are ideal adsorbents. Herein, we report that two-dimensional (2D) cationic aluminoborate BAC(10) sets a new paradigm for highly selective and efficient capture of Cr(VI) and other oxyanions from aqueous solution. The structure of Cr(VI)-exchanged BAC(10) sample (Cr(VI)@BAC(10), H0.22·Al2BO4.3·(HCrO4)0.22·2.64H2O) has been successfully solved by continuous rotation electron diffraction. The crystallographic data show that the 2D cationic layer of BAC(10) is built by AlO6 octahedra, BO4 tetrahedra, and BO3 triangles. Partial chromate ions exchanged with Cl- ions are located within the interlayer region, which are chemically bonded to the aluminoborate layer. BAC(10) shows faster adsorption kinetics compared to the commercial anion exchange resin (AER) and layered double hydroxides (LDHs), a higher maximum adsorption capacity of 139.1 mg/g than that of AER (62.77 mg/g), LDHs (81.43 mg/g), and a vast majority of cationic MOFs, and a much broader working pH range (2-10.5) than LDHs. Moreover, BAC(10) also shows excellent Cr(VI) oxyanion removal performance for a solution with a low concentration (1-10 mg/L), and the residual concentration can be reduced to below 0.05 mg/L of the WHO drinking water criterion. These superior properties indicate that BAC(10) is a promising material for remediation of Cr(VI) and other harmful oxyanions from wastewater.

Development of Cationic Benzimidazole-Containing UiO-66 through Step-by-Step Linker Modification to Enhance the Initial Sorption Rate and Sorption Capacities for Heavy Metal Oxo-Anions

Effective and rapid capture of heavy metal oxo-anions from wastewater is a fascinating research topic, but it remains a great challenge. Herein, benzimidazole and -CH₃ groups were integrated into UiO-66 in succession via a step-by-step linker modification strategy that was performed by presynthesis modification (to give Bim-UiO-66) and subsequently by postsynthetic ionization (to give Bim-UiO-66-Me). The UiO-66s (UiO-66, Bim-UiO-66, and Bim-UiO-66-Me) were applied in the removal of heavy metal oxo-anions from water. The two benzimidazole derivatives (Bim-UiO-66 and Bim-UiO-66-Me) showed much better performance than UiO-66, as both the initial sorption rate and sorption capacities decreased in the order Bim-UiO-66-Me > Bim-UiO-66 > UiO-66. The maximum performances of Bim-UiO-66 are 5.1 and 1.7 times those of UiO-66. Remarkably, Bim-UiO-66-Me shows 7.5 and 3.0 times better performance than UiO-66. The higher absorptivity of cationic Bim-UiO-66-Me compared with UiO-66 can be attributed to a strong Coulombic interaction as well as an anion-π interaction and hydrogen bonding between the benzimidazolium functional group and heavy metal oxo-anions. The as-synthesized Bim-UiO-66-Me not only provides a promising candidate for application in removal of heavy metal oxo-anions in wastewater treatment but also opens up a new strategy for the design of high-performance adsorbents.

Site Isolation in Metal-Organic Layers Enhances Photoredox Gold Catalysis

Herein, we report the synthesis of a metal-organic layer, Hf-Ru-Au, containing Ru(bipyridine)₃²⁺-type photosensitizers and (phosphine)-AuCl catalysts for photoredox Au-catalyzed cross-coupling of allenoates, alkenes, or alkynes with aryldiazonium salts to afford furanone, tetrahydrofuran, or aryl alkyne derivatives, respectively. Site isolation of (phosphine)-AuCl complexes in Hf-Ru-Au prevents Au catalyst deactivation via ligand redistribution, Au(I) disproportionation, and aryl-phosphine reductive elimination, while the proximity between the Ru photosensitizers and Au catalysts enhances catalytic efficiency, with 14-200 times higher activity over those of the homogeneous controls in the cross-coupling reactions.

Dynamic Metal-Iodide Bonds in a Tetracoordinated Cadmium-Based Metal-Organic Framework Boosting Efficient CO2 Cycloaddition under Solvent- and Cocatalyst-Free Conditions

Due to the inherent thermodynamic stability and kinetic inertness of CO₂, heterogeneous catalytic conversion of CO₂ to cyclic carbonates often requires harsh operating conditions, high temperature and high pressure, and the addition of cocatalysts. Therefore, the development of efficient heterogeneous catalysts under cocatalyst-free and mild conditions for CO₂ conversion has always been a challenge. Herein, an infrequent tetracoordinated Cd-MOF was synthesized and used to catalyze CO₂ cycloaddition reactions efficiently without the addition of any cocatalyst, and its catalytic mechanism was systematically investigated through a series of experiments, including fluorescence analysis, X-ray photoelectron spectroscopy, microcalorimetry, and density functional theory (DFT) calculation. Cd-MOF features a 3D supermolecule structure with 1D 11.6 × 7.7 Ų channels, and the abundant Lewis acid/base and I^- sites located in the confined channel boost efficient CO₂ conversion with a maximum yield of 98.2% and a turnover number value of 1080.11 at 60 °C and 0.5 MPa, far surpassing most pristine MOF-based catalytic systems. A combined experimental and DFT calculation demonstrates that the exposed Cd(II) Lewis acid sites rapidly participate in coordination to activate the epoxides, and the resulting large steric hindrance facilitates leaving of the coordinated iodide ions in a reversibly dynamic fashion convenient for the rate-determining step ring-opening as a strong nucleophile. Such a pristine MOF catalyst with self-independent catalytic ring-opening overcomes the complicated operation limitation of the traditional cocatalyst-free MOF systems based on encapsulating/postmodifying cocatalysts, providing a whole new strategy for the development of simple, green, and efficient heterogeneous catalysts for CO₂ cycloaddition.

Highly Efficient I2 Sorption, CO2 Capture, and Catalytic Conversion by Introducing Nitrogen Donor Sites in a Microporous Co(II)-Based Metal-Organic Framework

Recently, the development of porous absorbents for efficient CO₂ and I₂ capture has attracted considerable attention because of severe global climate change and environmental issues with the nuclear energy. Hence, a unique porous metal-organic framework (MOF), {[Co(L)]·DMF·2H₂O}_n (1, DMF = N,N-dimethylformamide) with uncoordinated N atoms was rationally constructed via using a heterofunctional 4,6-bis(4'-carboxyphenyl)pyrimidine (H₂L) linker. Interestingly, 1 exhibits exceptional properties for I₂ sorption, CO₂ capture, and catalytic conversion. Particularly, I₂ can be efficiently removed in both vapor and solution forms, and the adsorption amount can reach 676.25 and 345.28 mg g^-1, respectively. Furthermore, complex 1 displays high adsorption capacity for CO₂ (53.78 cm³ g^-1, 273 K). Consequently, 1 is expected to be a promising and practical material for environmental purification due to its excellent adsorption properties.

Effective Dual-Functional Metal-Organic Framework (DF-MOF) as a Catalyst for the Solvent-Free Cycloaddition Reaction

A new porous metal-organic framework, [Co (oba) (bpdh)]·(DMF) (TMU-63), containing accessible nitrogen-rich diazahexadiene groups was successfully prepared with the solvothermal assembly of 5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh), 4,4'-oxybis(benzoic) acid (oba), and Co(II) ions. The combination of Lewis basic functional groups and porosity leads to high performance in CO₂ adsorption and conversion in the cycloaddition reaction of epoxides under solvent-free conditions. To further enhance the catalytic efficiency of TMU-63, we introduced a highly acidic malonamide ligand into the structure via solvent-assisted ligand exchange (SALE) as a postsynthesis method. Incorporating different percentages of N¹,N³-di(pyridine-4-yl) malonamide linker (4-dpm) into TMU-63 created a new porous structure. Powder X-ray diffraction (PXRD) and NMR spectroscopy confirmed that 4-bpdh was successfully replaced with 4-dpm in the daughter MOF, TMU-63S. The catalytic activity of both MOFs was confirmed by significant amounts of CO₂ cycloaddition of epoxides under solvent-free conditions. The catalytic cycloaddition activities were found to be well-correlated with the Lewis base/Brønsted acid distributions of the materials examined in the TMU-63S series, showing that the concurrent presence of both acid and base sites was desirable for high catalytic activity. Furthermore, the heterogeneous catalysts could easily be separated out from the reaction mixtures and reused four times without loss of catalytic activity and with no structural deterioration.

Lanthanide-Organic Frameworks with Uncoordinated Lewis Base Sites: Tunable Luminescence, Antibiotic Detection, and Anticounterfeiting

Several new isostructural lanthanide metal-organic frameworks (Ln-MOFs), {[Ln₂(L)₃DMA₄]·2DMA}_n (1-Ln, where Ln = Eu, Tb, or Eu_xTb_1-x), were first constructed via the solvothermal reactions of 4,6-di(4-carboxyphenyl)pyrimidine and Ln³⁺ ions. 1-Ln exhibits a 4-connected two-dimensional framework endowed with uncoordinated Lewis base sites. An exploration of luminescence sensing demonstrated 1-Eu can be used for the selectivity detection of dimetridazole and metronidazole antibiotics in other antibiotics, blood plasma, and urine, acting as an exceptional recyclable luminescent probe. More importantly, the luminescent inks of 1-Ln are invisible, color adjustable, and stabilized, which may greatly improve their anticounterfeiting applications.

Anion-Afforded Functions of Ionic Metal-Organic Frameworks: Ionochromism, Anion Conduction, and Catalysis

The exchangeable counterions in ionic metal-organic frameworks (IMOFs) provide facile and versatile handles to manipulate functions associated with the ionic guests themselves and host-guest interactions. However, anion-exchangeable stable IMOFs combining multiple anion-related functions are still undeveloped. In this work, a novel porous IMOF featuring unique self-penetration was constructed from an electron-deficient tris(pyridinium)-tricarboxylate zwitterionic ligand. The water-stable IMOF undergoes reversible and single-crystal-to-single-crystal anion exchange and shows selective and discriminative ionochromic behaviors toward electron-rich anions owing to donor-acceptor interactions. The IMOFs with different anions are good ionic conductors with low activation energy, the highest conductivity being observed with chloride. Furthermore, integrating Lewis acidic sites and nucleophilic guest anions in solid state, the IMOFs act as heterogeneous and recyclable catalysts to efficiently catalyze the cycloaddition of CO₂ to epoxides without needing the use of halide cocatalysts. The catalytic activity is strongly dependent upon the guest anions, and the iodide shows the highest activity. The results demonstrate the great potential of developing IMOFs with various functions related to the guest ions included in the porous matrices.

Imidazolium-based poly(ionic liquid)s@MIL-101 for CO2 adsorption and subsequent catalytic cycloaddition without additional cocatalyst and solvent

Poly(ionic liquid)@MIL-101 incorporates an ionic liquid in a MOF and can be used in CO2 capture and conversion.

An yttrium-organic framework based on a hexagonal prism second building unit for luminescent sensing of antibiotics and highly effective CO2 fixation

An yttrium-organic framework based on a hexagonal prism second building unit was constructed from nonanuclear yttrium(iii) and 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine for the luminescent sensing of antibiotics and highly effective CO2 fixation.

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.