Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction

Steering electron flow by constructing an integrated structure in a metal-organic framework (MOF) via iminopyridine units for efficient CO2 photoreduction to C2H4 and C2H6

Selective photoreduction of CO2 to C2+ products has attracted increasing interest, but it remains challenging. Herein, we reported a novel strategy to steer electron flow by constructing integrated photocatalysts to achieve selective photoreduction of CO2 to C2H4 and C2H6. Using iminopyridine units in NH2-MIL-125(Ti) (Ti-MOF) as a tag, both the Ru photosensitizer and CuCo active sites were assembled within the Ti-MOF to form the integrated structure. Due to the strong interaction of the Ru photosensitizer and CuCo with Ti-MOF, the integrated CuCo/Ti-MOF-Py-Ru exhibited accelerated charge separation and migration efficiency, resulting in significant electron accumulation around CuCo. Compared with the discrete counterparts, the CO2 photoreduction products were upgraded from CO and CH4 to C2H4 and C2H6, with a high C2 selectivity of 86.1% over the integrated CuCo/Ti-MOF-Py-Ru, highlighting the critical role of the integrated structure in enhancing the selective CO2 photoreduction to C2+ products.

Stabilizing Polyoxometalate for Enhanced OER Performance Using a Porous Manganese Oxide Support

The oxygen evolution reaction (OER) is a critical challenge in electrocatalytic water splitting, hindered by high energy demands and slow kinetics. Polyoxometalates (POMs), recognized for their unique redox capabilities, structural archetypes, and molecular precision, are promising candidates for the oxygen evolution reaction (OER). Yet, their application is hindered by high water solubility, causing rapid degradation and efficiency loss under harsh OER conditions. This study enhances the performance and stability of polyoxometalates (POMs) for OER by anchoring keggin-type POM [TiCoW11O40]7- nanosheets onto a conductive, carbon-protected manganese oxide (C-Mn2O3) nanospheres support. The acquired porous framework enhances POM/C-Mn₂O₃ (PCM) contact, improving stability, reaction kinetics, and redox activity by offering nucleation sites, electronic pathways, and abundant active sites, significantly boosting OER activity. The resulting PCM nanohybrid demonstrates remarkable OER activity in 1 M KOH, requiring only a 300 mV overpotential to achieve a current density of 10 mA cm-2 with a Tafel slope of 88 mV/dec. The PCM electrocatalyst also shows high mass activity (784 A/g at 1.6 V) and maintains stability over 100 hours at 100 mA cm-2 without performance fatigue. Consequently, this study offers a viable strategy for developing efficient, durable electrocatalysts using low-cost materials.

High-Nuclearity Polyoxometalate-Based Metal-Organic Frameworks for Photocatalytic Oxidative Cleavage of C-C Bond

High-nuclearity polyoxometalate (POM) clusters are attractive building blocks (BBs) for the synthesis of metal-organic frameworks (MOFs) due to their high connectivity and inherently multiple metal centers as functional sites. This work demonstrates a strategy of step-wise growth on ring-shaped [P8W48O184]40- precursor, which produced two new high-nuclearity polyoxotungstates, a half-closed [H16P8W58O218]32- {W58} and a fully-closed [H16P8W64O236]32- {W64}. By in situ synthesis, unique MOFs of copper triazole-benzoic acid (HL) complexes incorporating the negatively-charged {W58} and {W64} as nodes, {Cu11(HL)9W58} HNPOMOF-1 and {Cu9(HL)9W64} HNPOMOF-2, were constructed by delicately tuning the reaction conditions, mainly solution pH, which controls the formation of {W58} and {W64}, and at the same time the protonation of triazole-benzoic acid ligand thus its coordination mode to copper ion that creates the highest nuclearity POM-derived MOFs reported to date. HNPOMOF-1 features 3D framework possessing cage-like cavities filled with exposed carboxyl groups, while the inherent 2D layer-like HNPOMOF-2 allows for facile exfoliation into ultrathin nanosheets, and the resulted HNPOMOF-2NS exhibits superior activity towards photocatalytic oxidative cleavage of C-C bond for a series of lignin models. This work not only provides a strategy to build high-nuclearity POM cluster-based frameworks, but also demonstrates their great potential as functional materials for green catalysis.

Direct Synthesis of Topology-Controlled BODIPY and CO2-Based Zirconium Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction

Boron dipyrromethene (BODIPY)-based zirconium metal-organic frameworks (Zr-MOFs) possess strong light-harvesting capabilities and great potential for artificial photosynthesis without the use of sacrificial reagents. However, their direct preparation has not yet been achieved due to challenges in synthesizing suitable ligands. Herein, we reported the first successful direct synthesis of BODIPY-based Zr-MOFs, utilizing CO2 as a feedstock. By controlling synthetic conditions, we successfully obtained two distinct Zr-MOFs. The first, CO2-Zr6-DEPB, exhibits a face-centered cubic (fcu) topology based on a Zr6(μ3-O)4(μ3-OH)4 node, while the second, CO2-Zr12-DEPB, features a hexagonal closed packed (hcp) topology, structured around a Zr12(μ3-O)8(μ3-OH)8(μ2-OH)6 node. Both MOFs demonstrated excellent crystallinity, as verified through powder X-ray diffraction and high-resolution transmission electron microscopy analyses. These MOF catalysts displayed suitable photocatalytic redox potentials for the reduction of CO2 to CO using H2O as the electron donor in the absence of co-catalyst or toxic sacrificial reagent. Under light irradiation, CO2-Zr12-DEPB and CO2-Zr6-DEPB offered high CO yields of 16.72 and 13.91 μmol g-1 h-1, respectively, with nearly 100 % selectivity. CO2 uptake and photoelectrochemical experiments revealed key insights into the mechanisms driving the different catalytic activities of the two MOFs. These BODIPY and CO2-based, light-responsive Zr-MOFs represent a promising platform for the development of efficient photocatalysts.

Dual Regulation of Sensitizers and Cluster Catalysts in Metal-Organic Frameworks to Boost H2 Evolution

Photocatalytic efficiency is closely correlated to visible-light absorption ability, electron transfer efficiency and catalytic center activity of photocatalysts, nevertheless, the concurrent management of these factors to improve photocatalytic efficiency remains underexplored. Herein, we proposed a sensitizer/catalyst dual regulation strategy on the polyoxometalate@Metal-Organic Framework (POM@MOF) molecular platform to construct highly efficient photocatalysts. Impressively, Ni-Sb9@UiO-Ir-C6, obtained by coupling strong sensitizing [Ir(coumarin 6)2(bpy)]+ with Ni-Sb9 POM with extremely exposed nickel site [NiO3(H2O)3], can drive H2 evolution with a turnover number of 326923, representing a record value among all the POM@MOF composite photocatalysts. This performance is over 34 times higher than that of the typical Ni4P2@UiO-Ir constructed from [Ir(ppy)2(bpy)]+ and Ni4P2 POM. Systematical investigations revealed that dual regulation of sensitizing and catalytic centers endowed Ni-Sb9@UiO-Ir-C6 with strong visible-light absorption, efficient inter-component electron transfer and high catalytic activity to concurrently promote H2 evolution. This work opens up a new avenue to develop highly active POM@MOF photocatalysts by dual regulation of sensitizing/catalytic centers at the molecular level.

Constructing a Polyoxometalate-Based Metal-Organic Framework for Photocatalytic Oxidation of Thioethers to Sulfoxides Utilizing In Situ-Generated Superoxide Radicals

Developing new photocatalysts for the selective oxidation of thioethers to high-value-added sulfoxides under low-oxygen mild conditions is a promising but challenging strategy. Here, a new polyoxometalate-based metal-organic framework (POMOF), CoBW12-TPT, was successfully synthesized, wherein continuous π···π stacking interactions and direct coordination bonds not only strengthen the framework's stability but also accelerate electron transfer. A series of experiments and theoretical studies, including control experiments, kinetic studies, electrochemical spectroscopic analyses, and electron paramagnetic resonance, revealed the synergistic catalytic effect among Co(II) metal centers, BW12O405-, and the photosensitizer TPT. CoBW12-TPT was applied in the photocatalytic oxidation of thioethers to sulfoxides. Under irradiation, the photoinduced electron transfer of POMOF leads to the generation of superoxide radicals from O2, which controls the selective generation of sulfoxide compounds in the photocatalytic desulfurization reaction and shows good activity. In particular, it can be applied to the construction of some drug molecules such as Modafinil and Albendazole Oxide.

Construction of Compartmentalized Meso/Micro Spaces in Hierarchically Porous MOFs with Long-Chain Functional Ligands Inspired by Biological Signal Amplification

The creation of spatially coupled meso-/microenvironments with biomimetic compartmentalized functionalities is of great significance to achieve efficient signal transduction and amplification. Herein, using a soft-template strategy, UiO-67-type hierarchically mesoporous metal-organic frameworks (HMMOFs) were constructed to satisfy the requirements of such an artificial system. The key to the successful synthesis of HMUiO-67 is rooted in the utilization of the preformed cerium-oxo clusters as metal precursors, aligning the growth of MOF crystals with the mild conditions required for the self-assembly of the soft template. The adoption of long-chain functional 2,2'-bipyridine-5,5'-dicarboxylic acid ligands not only resulted in larger microporous sizes, facilitating the transport of various cascade reaction intermediates, but also provided anchorages for the introduction of enzyme-mimicking active sites. A cascade amplification system was designed based on the developed HMUiO-67, in which enzyme cascade reactions were initiated and relayed by a target analyte in the separate but coupled meso/micro spaces. As a proof of concept, natural acetylcholinesterase (AChE) and Cu-based laccase mimetics were integrated into HMMOFs, establishing a spatially coupled nanoreactor. The activity of AChE was triggered by the target analyte of carbaryl, while the amplified products of AChE catalysis mediated the activity of biomimetic enzyme in the closely proximate microporous spaces, producing further amplification of detectable signal. This enabled the entire cascade system to respond to minimal carbaryl with a limit of detection as low as approximately 2 nM. Such a model of cascade amplification is expected to set a conceptual guideline for the rational design of various bioreactors, serving as a sensitive response system for quantifying numerous target analytes.

Modulating the Supramolecular Assembly of α-Cyclodextrin and Anderson-type Polyoxometalate through Covalent Modifications

A series of unprecedented supramolecular complexes of covalently modified Anderson-type polyoxometalates (POMs) and α-cyclodextrins (α-CDs) have been obtained and characterized in solid state by single-crystal X-ray diffraction, and in aqueous solution using various techniques including 1H DOSY NMR, 2D NOESY 1H NMR, isothermal titration calorimetry (ITC), and electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS). It has been demonstrated that the supramolecular assembly process could be modulated by different covalent modification modes of the Anderson POMs, giving rise to a new type of POM/α-CD complexes featuring organic-inorganic pseudo-rotaxane structures, which are in good contrast to those of POM/γ-CD complexes of poly-rotaxane structures. Moreover, it is delighted to find that these pseudo-rotaxanes of POM/α-CD complexes exhibit stable chirality in aqueous solution, which has not been accomplished in previously reported POM/CD assemblies.

Development of polyoxometalate-loaded MOFs for heterogeneous catalysis and enhanced dye adsorption

This study focuses on the enhancement of MIL-117 functionality by incorporating a well-known polyoxometalate (POM), tetrabutylammonium octamolybdate [(n-C4H9)4N]4[Mo8O26]. Using an encapsulation method with conventional heating, Mo8O264- anions were for the first time successfully integrated into MIL-117 tubular channels (Mo8O26@MIL-117). Comprehensive characterization of the material through FTIR, XRD, BET, FE-SEM, EDX, and XPS confirmed the uniform distribution of Mo8O264- within MIL-117 without compromising its structural integrity. The Mo8O26@MIL-117 composite demonstrates exceptional catalytic performance in oxidative C-N bond formation and Paal-Knorr pyrrole synthesis, achieving high yields under optimized conditions with diverse amine substrates. Characterization and stability assessments confirm Mo8O26@MIL-117 as a robust and recyclable catalyst, maintaining structural integrity and catalytic activity over multiple cycles, highlighting its potential for sustainable applications in synthetic chemistry. The composite material was also evaluated for its efficacy in dye removal, specifically targeting methylene blue (MB) and Rhodamine B (RHB) from aqueous solutions. Mo8O26@MIL-117 exhibited superior adsorption capacity for MB compared to MIL-117 alone, demonstrating high efficiency even at elevated concentrations. The composite showed improved selectivity towards MB over RHB, highlighting its potential for selective dye removal in wastewater treatment applications.

Exploring porous structures without crystals: advancements with pair distribution function in metal- and covalent organic frameworks

The pair distribution function (PDF) is a versatile characterisation tool in materials science, capable of retrieving atom-atom distances on a continuous scale (from a few angstroms to nanometres), without being restricted to crystalline samples. Typically, total scattering experiments are performed using high-energy synchrotron X-rays, neutrons or electrons to achieve a high atomic resolution in a short time. Recently, PDF analysis provides a powerful approach to target current characterisation challenges in the field of metal- and covalent organic frameworks. By identifying molecular interactions on the pore surfaces, tracking complex structural transformations involving disorder states, and elucidating nucleation and growth mechanisms, structural analysis using PDF has provided invaluable insights into these materials. This review article highlights the significance of PDF analysis in advancing our understanding of MOFs and COFs, paving the way for innovative applications and discoveries in porous materials research.

Revitalisation of group IV metal-oxo clusters: synthetic approaches, structural motifs and applications

Group (IV) metal oxo clusters represent a unique family of molecular species that are increasingly being utilized in applications ranging from catalysis and materials chemistry to electronics, and sensors. These clusters exhibit distinctive structural features, chemical reactivity, and electronic structure. Nevertheless, their full potential has yet to be fully realized due to the lack of deeper understanding regarding their structure and formation mechanisms, inherent traits, and intricacies in their design, which could ultimately enable significant customization of their properties and overall behaviour. Considering the recently observed reignited interest in the chemistry of group IV molecular species, the scope of this article is to bring to the readers the main chemical characteristics of the family of titanium, zirconium, and hafnium-based clusters, their structural features and their potential in future applications.

Imine bond-directed assembly of polyoxometalate-based metal-organic frameworks

Polyoxometalate-based metal-organic frameworks (POMOFs) are highly effective heterogeneous catalysts that combine the catalytic activity of polyoxometalates (POMs) with the high surface area, tunable porosity, and structural diversity of MOFs. Nevertheless, there is still a lack of a general method to integrate POMs with various transition metal-based building units into POMOFs under mild conditions. In this work, we employed imine bonds to link amino-functionalized Anderson-type POMs with aldehyde-terminated divalent metal clusters, resulting in a series of isostructural POMOFs, M(II)-POMOFs (M = Zn, Co, Mg, or Mn). Furthermore, we used post-synthetic metal exchange and oxidation to transform Zn-POMOF into Fe(III)-POMOF with strong Lewis acidic Fe3+ sites. Notably, both the synthesis and post-synthetic modifications were performed under mild conditions (room temperature, acid-free), preventing the decomposition of the POMs. Compared to M(II)-POMOFs or MOFs without POMs, the combination of Lewis acidic Fe3+ and POMs enhanced its catalytic activity for CO2 cycloaddition with epoxides, enabling efficient synthesis of cyclic carbonates. This versatile synthetic method could broaden the scope of POMOFs, extending their applications in catalysis and beyond.

CH4 Carbonylation to Acetic Acid Using H2O as an Oxidant on a Rh-Functionalized UiO-67 Combined with Oriented External Electric Fields: Selectivity and Mechanistic Insights from DFT Calculations

Acetic acid (CH3COOH), as an industrially important petrochemical product, is predominantly produced via multistep energy-intensive processes. The development of a rhodium single-site heterogeneous catalyst has received considerable attention due to its potential to transform CH4 into CH3COOH in a single step. Herein, the reaction mechanism for the generation of CH3COOH from CH4, CO, and H2O catalyzed by Rh-functionalized metal-organic framework (MOF) UiO-67 and the selectivity of products CH3COOH, formic acid (HCOOH), methanol (CH3OH), and acetaldehyde (CH3CHO) under the oriented external electric fields (OEEFs) were systematically explored by density functional theory (DFT) calculations. The results reveal that the insertion of CO into Rh-CH3 is the rate-determining step with a free energy barrier of 21.0 kcal/mol in CH4 carbonylation to CH3COOH. Upon applying an OEEF of Fx = +0.0050 au along the C-C bond, the rate-determining step shifts toward H2O decomposition with the barrier of 19.6 kcal/mol, significantly improving the selectivity for CH3COOH production, compared to the major competitive HCOOH route. The Brønsted-Evans-Polanyi (BEP) relationships between key transition states, field strength, and NPA charge transfer were established. This study may guide the rational design of atomically dispersed MOF catalysts for the selective coconversion of CH4 and CO to CH3COOH using H2O as the oxidant under the OEEF.

Unveiling Advancements: Trends and Hotspots of Metal-Organic Frameworks in Photocatalytic CO2 Reduction

Metal-organic frameworks (MOFs) are robust, crystalline, and porous materials featured by their superior CO2 adsorption capacity, tunable energy band structure, and enhanced photovoltaic conversion efficiency, making them highly promising for photocatalytic CO2 reduction reaction (PCO2RR). This study presents a comprehensive examination of the advancements in MOFs-based PCO2RR field spanning the period from 2011 to 2023. Employing bibliometric analysis, the paper scrutinizes the widely adopted terminology and citation patterns, elucidating trends in publication, leading research entities, and the thematic evolution within the field. The findings highlight a period of rapid expansion and increasing interdisciplinary integration, with extensive international and institutional collaboration. A notable emphasis on significant research clusters and key terminologies identified through co-occurrence network analysis, highlighting predominant research on MOFs such as UiO, MIL, ZIF, porphyrin-based MOFs, their composites, and the hybridization with photosensitizers and molecular catalysts. Furthermore, prospective design approaches for catalysts are explored, encompassing single-atom catalysts (SACs), interfacial interaction enhancement, novel MOF constructions, biocatalysis, etc. It also delves into potential avenues for scaling these materials from the laboratory to industrial applications, underlining the primary technical challenges that need to be overcome to facilitate the broader application and development of MOFs-based PCO2RR technologies.

Small Conjugated Organic Ligand-Modified Polyoxometalate-Based Hybrid Materials for Boosting Photocatalytic CO2 Reduction

Photocatalytic carbon dioxide (CO2) reduction to value-added chemicals is a multielectron transfer process, and the crucial step is the synthesis of photocatalysts. The introduction of small conjugated organic ligands can make the catalytic active site of the compound easier to be exposed in the reaction system and fully contact with the substrate, accelerating the photocatalytic reaction process. In this paper, we synthesized two isomorphic compounds, namely, {[Co(mtrz)3·(H2O)2]2·[SiW12O40]}·6H2O (1) and {[Ni(mtrz)3·(H2O)2]2·[SiW12O40]}·6H2O (2) (mtrz = 1-methyl-1,2,4-triazole). We found that compound 1 has a great photocatalytic performance through a series of experiments, with a CO reduction yield of 7364.92 μmol g-1 h-1 and a CO selectivity of 82.5%. Furthermore, the high catalytic activity can be maintained over four cycle experiments. The catalytic mechanism of its photocatalytic system is also elucidated, which provides an idea for realizing efficient catalytic reduction of CO2 to CO.

GFP Chromophore Integrated Conjugated Microporous Polymers toward Bioinspired Photocatalytic CO2 Reduction to CO

The development of highly active, durable, and low-cost metal-free catalysts for the photocatalytic CO2 reduction reaction (CO2RR) is an efficient and environmentally friendly solution to address significant problems like global warming and high energy demand. In the present study, we have demonstrated the design and synthesis of a donor-acceptor based conjugated microporous polymer (CMP), TPA-GFP, by integrating an electron donor, tris(4-ethynylphenyl)amine (TPA), with a green fluorescent protein chromophore analogue (Z)-4-(2-hydroxy-3,5-diiodobenzylidene)-1-(4-iodophenyl)-2-methyl-1H-imidazol-5(4H)-one (o-HBDI-I3) (GFP). In comparison to nondonor 1,3,5-triethynylbenzene (TEB) based TEB-GFP CMP, photocatalytic CO2 reduction using donor-acceptor based TPA-GFP CMP displays a 3-fold increment of CO production yield with a maximum CO yield of 1666 μmol g-1 at 12 h. Further, the CO selectivity increases significantly from a mere 54% in TEB-GFP to an impressive 95% in TPA-GFP. The impressive CO2 reduction efficiency and selectivity for TPA-GFP can be attributed to the efficient light-harvesting capability and facile charge separation and migration through donor-acceptor building units of the CMP. The mechanistic aspect of the photocatalytic CO2 reduction process is explored using in situ DRIFTS and DFT calculation, and a plausible photocatalytic mechanism is proposed.

Recent Advances and Challenges in Efficient Selective Photocatalytic CO2 Methanation

Solar-driven carbon dioxide (CO2) methanation holds significant research value in the context of carbon emission reduction and energy crisis. However, this eight-electron catalytic reaction presents substantial challenges in catalytic activity and selectivity. In this regard, researchers have conducted extensive exploration and achieved significant developments. This review provides an overview of the recent advances and challenges in efficient selective photocatalytic CO2 methanation. It begins by discussing the fundamental principles and challenges in detail, analyzing strategies for improving the efficiency of photocatalytic CO2 conversion to CH4 comprehensively. Subsequently, it outlines the recent applications and advanced characterization methods for photocatalytic CO2 methanation. Finally, this review highlights the prospects and opportunities in this area, aiming to inspire CO2 conversion into high-value CH4 and shed light on the research of catalytic mechanisms.

Efficient reduction of CO2 and inhibition of hydrogen precipitation by polyoxometalate photocatalyst modified with the metal Mn

Photocatalytic reduction of CO2 to chemical fuels is attractive for solving both the greenhouse effect and the energy crisis, but the key challenge is to design and synthesize photocatalysts with remarkable performance under visible light irradiation. Efficient catalytic carbon dioxide reduction (CO2RR) with light is considered a promising sustainable and clean approach to solve environmental problems. Herein, we found a new photocatalyst ([Mn(en)2]6[V12B18O54(OH)6]) (abbreviated as Mn6V12) based on the modifiability of polyoxometalates, in which Mn acts as a modifying unit to efficiently reduce CO2 to CO and effectively inhibit the hydrogen precipitation reaction. This Mn modified polyoxometalate catalyst has a maximum CO generation rate of 4625.0 μmol g-1 h-1 and a maximum H2 generation rate of 499.6 μmol g-1 h-1, with a selectivity of 90.3% for CO generation and 9.7% for H2 generation. This polyoxometalate photocatalyst can effectively reduce CO and inhibit the hydrogen precipitation reaction. It provides a new idea for the efficient photocatalytic carbon dioxide reduction (CO2RR) with polyoxometalate catalysts.

Intensifying Photocatalytic Baeyer-Villiger Oxidation of Ketones with the Introduction of Ru Metalloligands and Bimetallic Units in POM@MOF

The synthesis of visible light-responsive and efficient photocatalysts toward green Baeyer-Villiger oxidation organic synthesis is of extraordinary significance. In this work, we have synthesized two examples of visible light responsive crystalline polyoxometalate@metal-organic framework materials Ru-NiMo and Ru-CoMo by introducing Ru metalloligands and {CdM3O12} bimetallic units (M = Ni or Co). This is the first report of metalloligand-modified polyoxometalate@metal-organic framework materials with bimetallic nodes, and the materials form a three-dimensional framework directly through coordination bonds between {CdM3O12} bimetallic units and metalloligands. In particular, Ru-NiMo can achieve efficient photocatalytic conversion of cyclohexanone to ε-caprolactone in yields as high as 95.5% under visible light excitation in the range of λ > 400 nm, achieving a turnover number and turnover frequency of 955 and 440 h-1, respectively, which are the best known photocatalysts for Baeyer-Villiger oxidation, while apparent quantum yield measured at 485 nm is 4.4%. Moreover, Ru-NiMo exhibited excellent structural stability and recyclability, producing a 90.8% yield after five cycles of recycling.

Improved supercapacitors and water splitting performances of Anderson-type manganese(III)-polyoxomolybdate through assembly with Zn-MOF in a host-guest structure

Enhancing performance through the combination of polyoxometalates (POMs) clusters with metal-organic frameworks (MOFs) that contain various transition metals is a challenging task. In this study, we synthesized a polyoxometalate-based metal-organic framework (POMOF) named HRBNU-5 using a solvothermal method. HRBNU-5 is composed of Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]@Zn3(C9H3O6)2·6C3H7NO, which includes two components: Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]·3C3H7NO ({Zn[MnMo6]}) and Zn3(C9H3O6)2·3C3H7NO (Zn-BTC). Structural characterization confirmed the host-guest structure, with Zn-BTC encapsulating {Zn[MnMo6]}. In a three-electrode system, HRBNU-5 exhibited a specific capacitance of 851.3 F g-1 at a current density of 1 A/g and retained high stability (97.2 %) after 5000 cycles. Additionally, HRBNU-5 performed well in aqueous-symmetric/asymmetric supercapacitors (SSC/ASC) in terms of energy density and power density in a double-electrode system. Moreover, it demonstrated excellent catalytic performance in a 1.0 M KOH solution, with low overpotentials and Tafel slopes for hydrogen and oxygen evolution reactions: 177.1 mV (η10 HER), 126.9 mV dec-1 and 370.3 mV (η50 OER), 36.3 mV dec-1, respectively, surpassing its precursors and most reported studies. HRBNU-5's positive performance is attributed to its host-guest structure, high electron-transfer conductivity, and porous structure that enhances efficient mass transport. This work inspires the design of Anderson-type POMOF electrode materials with multiple active sites and a well-defined structure.

Defects-Induced Single-Atom Anchoring on Metal-Organic Frameworks for High-Efficiency Photocatalytic Nitrogen Reduction

Aiming to improve the photocatalytic activity in N2 fixation to produce ammonia, herein, we proposed a photochemical strategy to fabricate defects, and further deposition of Ru single atoms onto UiO-66 (Zr) framework. Electron-metal-support interactions (EMSI) were built between Ru single atoms and the support via a covalently bonding. EMSI were capable of accelerating charge transfer between Ru SAs and UiO-66, which was favorable for highly-efficiently photocatalytic activity. The photocatalytic production rate of ammonia improved from 4.57 μmol g-1  h-1 to 16.28 μmol g-1  h-1 with the fabrication of defects onto UiO-66, and further to 53.28 μmol g-1  h-1 with Ru-single atoms loading. From the DFT results, it was found that d-orbital electrons of Ru were donated to N2 π✶-antibonding orbital, facilitating the activation of the N≡N triple bond. A distal reaction pathway was probably occurred for the photocatalytic N2 reduction to ammonia on Ru1 /d-UiO-66 (single Ru sites decorated onto the nodes of defective UiO-66), and the first step of hydrogenation of N2 was the reaction determination step. This work shed a light on improving the photocatalytic activity via feasibly anchoring single atoms on MOF, and provided more evidences to understand the reaction mechanism in photocatalytic reduction of N2 .

Aryl selenonium vs. aryl sulfonium counterions in polyoxometalate chemistry: the impact of Se+ cationic centers on the photocatalytic reduction of dichromate

A selenonium organic counter ion has been used in polyoxometalate chemistry to develop a new aryl selenonium polyoxometalate (POM) hybrid, and its photocatalytic properties have been explored in comparison with an aryl sulfonium POM-hybrid counterpart for the first time. The chalcogenonium counterions, namely, methyldiphenylsulfonium trifluoromethane sulfonate (MDPST) and methyldiphenylselenonium trifluoromethane sulfonate (MDPSeT), and their octamolybdate ([Mo8O26]4-) hybrids, 1 and 2, with the general formula (C13H13X)4[Mo8O26] (where X = S for 1 and Se for 2) were synthesized and characterized. Hybrids 1 and 2 vary in their chalcogenonium cationic center (S+vs. Se+), which enabled a direct comparison of their photocatalytic properties as a function of the cationic center. The photocatalytic activities of hybrids 1 and 2 were tested using the reduction of dichromate (Cr2O72-) as a model reaction under UV irradiation. A 99% photocatalytic reduction of Cr2O72- with a rate constant of 0.0305 min-1 was achieved with hybrid 2, while only a 67% reduction with a rate constant of 0.0062 min-1 was observed with hybrid 1 in 180 minutes. The better catalytic performance of hybrid 2 may be correlated to the larger atomic radii of Se than S, which helps in better stabilizing the photogenerated electron-hole (e--h+) pair on the POM cluster by polarizing its lone pair more efficiently compared to S. The catalytic recyclability was tested for up to 4 cycles using hybrid 2, and up to 98% reduction was obtained even after the 4th cycle. Recyclability tests and control experiments also indicated the generation of some elemental Se through possible cleavage of some C-Se bonds of MDPSe under prolonged UV exposure during catalysis, and the Se thus generated was found to contribute to the catalytic reduction of dichromate. This study, therefore, opens new avenues for aryl selenonium moieties and their POM hybrids for potential catalytic applications.

Unravelling the Molecular Structure and Confining Environment of an Organometallic Catalyst Heterogenized within Amorphous Porous Polymers

The catalytic activity of multifunctional, microporous materials is directly linked to the spatial arrangement of their structural building blocks. Despite great achievements in the design and incorporation of isolated catalytically active metal complexes within such materials, a detailed understanding of their atomic-level structure and the local environment of the active species remains a fundamental challenge, especially when these latter are hosted in non-crystalline organic polymers. Here, we show that by combining computational chemistry with pair distribution function analysis, 129 Xe NMR, and Dynamic Nuclear Polarization enhanced NMR spectroscopy, a very accurate description of the molecular structure and confining surroundings of a catalytically active Rh-based organometallic complex incorporated inside the cavity of amorphous bipyridine-based porous polymers is obtained. Small, but significant, differences in the structural properties of the polymers are highlighted depending on their backbone motifs.

Coordination Defect-Induced Frustrated Lewis Pairs in Polyoxo-metalate-Based Metal-Organic Frameworks for Efficient Catalytic Hydrogenation

Precise control of the structure and spatial distance of Lewis acid (LA) and Lewis base (LB) sites in a porous system to construct efficient solid frustrated Lewis pair (FLP) catalyst is vital for industrial application but remains challenging. Herein, we constructed FLP sites in a polyoxometalate (POM)-based metal-organic framework (MOF) by introducing coordination-defect metal nodes (LA) and surface-basic POM with abundant oxygen (LB). The well-defined and unique spatial conformation of the defective POM-based MOF ensure that the distance between LA and LB is at ~4.3 Å, a suitable distance to activate H2 . This FLP catalyst can heterolytically dissociate H2 into active Hδ- , thus exhibiting high activity in hydrogenation, which is 55 and 2.7 times as high as that of defect-free POM-based MOF and defective MOF without POM, respectively. This work provides a new avenue toward precise design multi-site catalyst to achieve specific activation of target substrate for synergistic catalysis.

Heterostructured Cobalt Silicide Nanocrystals: Synthesis in Molten Salts, Ferromagnetism, and Electrocatalysis

Nanoscale heterostructures of covalent intermetallics should give birth to a wide range of interface-driven physical and chemical properties. Such a level of design however remains unattainable for most of these compounds, due to the difficulty to reach a crystalline order of covalent bonds at the moderate temperatures required for colloidal chemistry. Herein, we design heterostructured cobalt silicide nanoparticles to trigger magnetic and catalytic properties in silicon-based materials. Our strategy consists in controlling the diffusion of cobalt atoms into silicon nanoparticles, by reacting these particles in molten salts. By adjusting the temperature, we tune the conversion of the initial silicon particles toward homogeneous CoSi nanoparticles and core-shell nanoparticles made of a CoSi shell and a silicon-rich core. The increased interface-to-volume ratio of the CoSi component in the core-shell particles yields distinct properties compared to the bulk and homogeneous nanoparticles. First, the core-shell particles exhibit increased ferromagnetism, despite the bulk diamagnetic properties of cobalt monosilicide. Second, the core-shell nanoparticles act as efficient precatalysts for alkaline water oxidation, where the nanostructure is converted in situ into a layered cobalt silicon oxide/(oxy)hydroxide with high and stable oxygen evolution reaction (OER) electrocatalytic activity. This work demonstrates a route to design heterostructured nanocrystals of covalent intermetallic compounds and shows that these new structures exhibit very rich, yet poorly explored, interface-based physical properties and reactivity.

In Situ Neutron Diffraction of Zn-MOF-74 Reveals Nanoconfinement-Induced Effects on Adsorbed Propene

Even though confinement was identified as a common element of selective catalysis and simulations predicted enhanced properties of adsorbates within microporous materials, experimental results on the characterization of the adsorbed phase are still rare. In this study, we provide experimental evidence of the increase of propene density in the channels of Zn-MOF-74 by 16(2)% compared to the liquid phase. The ordered propene molecules adsorbed within the pores of the MOF have been localized by in situ neutron powder diffraction, and the results are supported by adsorption studies. The formation of a second adsorbate layer, paired with nanoconfinement-induced short intermolecular distances, causes the efficient packing of the propene molecules and results in an increase of olefin density.

Fabrication of Polyoxometalate-Based Metal-Organic Frameworks Integrating Paddlewheel Rh2(OAc)4 for Visible-Light-Driven Oxidative Coupling of Amines

The development of visible-light-responsive, environmentally friendly, and reusable photocatalysts for organic oxidation reactions is of vital significance. Herein, four polyoxometalate-based metal-organic frameworks (POMOFs) were synthesized and systematically characterized by assembling the paddlewheel complex Rh2(OAc)4 and various polyoxometalates (POMs). Single-crystal X-ray diffraction analysis revealed that the four POMOFs were isomorphic and possessed rare structural features among the POMOFs, with POMs as nodes and Rh2(OAc)4 as linkers. As expected, the activities of the four POMOFs for the photocatalytic oxidative coupling of benzylamine were better than that of Rh2(OAc)4 or POMs individually, which was ascribed to the synergistic effect between them, and the intrinsic reasons for the difference in the activity were explained via electrochemical measurements. In particular, the product imine yield reached 96.1% with NaRh-SiW12 as the catalyst and a turnover number and a turnover frequency of 480.5 and 120.5 h-1, respectively, while the product yield remained as high as 92% after three repetitions, evidencing its high stability. Moreover, the higher activities of the four POMOFs for the selective epoxidation of various alkenes reaffirm the synergistic effect between Rh2(OAc)4 and POMs.

Confining charge-transfer complex in a metal-organic framework for photocatalytic CO2 reduction in water

In the quest for renewable fuel production, the selective conversion of CO₂ to CH₄ under visible light in water is a leading-edge challenge considering the involvement of kinetically sluggish multiple elementary steps. Herein, 1-pyrenebutyric acid is post-synthetically grafted in a defect-engineered Zr-based metal organic framework by replacing exchangeable formate. Then, methyl viologen is incorporated in the confined space of post-modified MOF to achieve donor-acceptor complex, which acts as an antenna to harvest visible light, and regulates electron transfer to the catalytic center (Zr-oxo cluster) to enable visible-light-driven CO₂ reduction reaction. The proximal presence of the charge transfer complex enhances charge transfer kinetics as realized from transient absorption spectroscopy, and the facile electron transfer helps to produce CH₄ from CO₂. The reported material produces 7.3 mmol g^-1 of CH₄ under light irradiation in aqueous medium using sacrificial agents. Mechanistic information gleans from electron paramagnetic resonance, in situ diffuse reflectance FT-IR and density functional theory calculation.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Structural Regulation of Two Polyoxometalate-Based Metal-Organic Frameworks for the Heterogeneous Catalysis of Quinazolinones

Two dimeric {ε-Zn₄PMo₁₂}-based metal-organic frameworks (MOFs), [ε-PMo₈^VMo₄^VIO₃₄(OH)₆Zn₄][LO] (SDUT-21, LO = [5-((4'-carboxybenzyl)oxy)isophthalic acid]) and [TBA]₃[ε-PMo₈^VMo₄^VIO₃₇(OH)₃Zn₄][LN] (SDUT-22, TBA⁺ = tetrabutylammonium ion, LN = [5-((4-carboxybenzyl)imino)isophthalic acid]), combining the advantages of polyoxometalates (POMs) and MOFs, were synthesized by the one-pot assembly strategy. The dimeric {ε-Zn₄PMo₁₂} units act as nodes that are linked by the flexible ligands and extended into two- or three-dimensional frameworks. The cyclic voltammetry and proton conductivity measurements of SDUT-21 and SDUT-22 were performed and indicated the high electron and proton transfer abilities. These materials also e xhibited the catalytic performance for the synthesis of quinazolinones in the heterogeneous state, and the different binding capacities toward the substrates caused the catalytic activity of SDUT-21 to be higher than that of SDUT-22 under the same conditions. In addition, the used catalysts could be readily recovered for five successive cycles and maintained high catalytic efficiency.

Charge transfer in metal-organic frameworks

Metal-organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are a novel class of crystalline porous material. The tailorable porous structure, in terms of size, geometry and function, has attracted the attention of researchers across all disciplines of materials science. One of the many exciting aspects of MOFs is that through directional and reversible coordination bonding, organic linkers (chromophores with metal-coordinating functional groups) and metal ions (and clusters) can be spatially organized in a preconceived geometry. The well-defined spatial geometry of the metals and linkers is very advantageous for optoelectronic functions (solar cells, light-emitting diodes, photocatalysts) of the materials. This feature article evaluates the scope of charge transfer (CT) interactions in MOFs, involving the organic linkers and metal ion or cluster components. Irrespective of the type (size, shape, electronic property) of organic chromophores involved, MOFs provide an insightful path to design and make the CT process efficient. The selected examples of MOFs with CT characteristics do not only illustrate the design principles but render a pathway towards understanding the complex photophysical processes and implementing those for future optoelectronic and catalytic applications.

Application of fully conjugated covalent organic frameworks in photocatalytic carbon dioxide reduction performance

Photocatalytic reduction of carbon dioxide into useful feedstocks has attracted increasing attention. In this study, a fully conjugated COF material COF-TMT-A with the main structure containing an alkyne group and triazine part was synthesized using sp²-carbon-carbon double bond (CC) linked COF as a research target. The prepared COF materials were characterized in detail by FT-IR, PXRD, and ¹³C solid-state NMR. The introduction of an alkyne group not only enhanced the conjugated π-electron leaving domain but also optimized the electronic band structure and significantly improved the photocatalytic activity. The selectivity for the product HCOO was as high as 99%. A 10 h photocatalytic CO₂ reduction experiment was carried out, and COF-TMT-A showed a significantly higher HCOO^- yield of about 43 μmol compared with COF-701 and the ligand.

Photocatalytic CO2 -to-Syngas Evolution with Molecular Catalyst Metal-Organic Framework Nanozymes

Syngas, a mixture of CO and H₂ , is a high-priority intermediate for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel sunlight-driven catalytic conversion of CO₂ and protons to syngas is a key step toward a sustainable energy cycle. State-of-the-art catalytic systems and materials often fall short as application-oriented concurrent CO and H₂ evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency. Here a light-harvesting metal-organic framework hosting two molecular catalysts is engineered to yield colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. In-depth fluorescence, X-ray, and microscopic studies paired with kinetic analysis show that the host delivers energy efficiently to active sites, conceptually yielding nanozymes. This unlocked sustained CO₂ reduction and H₂ evolution with benchmark turnover numbers and record incident photon conversions up to 36%, showcasing a highly active and durable all-in-one material toward application in solar energy-driven syngas generation.

Rhodium-Based MOF-on-MOF Difunctional Core-Shell Nanoreactor for NAD(P)H Regeneration and Enzyme Directed Immobilization

An organometallic complex-catalyzed artificial coenzyme regeneration system has attracted widespread attention. However, the combined use of organometallic complex catalysts and natural enzymes easily results in mutual inactivation. Herein, we establish a rhodium-based metal-organic framework (MOF)-on-MOF difunctional core-shell nanoreactor as an artificial enzymatic NAD(P)H regeneration system. UiO67 as the core is used to capture rhodium molecules for catalyzing NAD(P)H regeneration. UiO66 as the shell is used to specifically immobilize His-tagged lactate dehydrogenase (LDH) and serve as a protection shield for LDH and [Cp*Rh(bpy)Cl]⁺ to prevent mutual inactivation. A variety of results indicate that UiO67@Rh@UiO66 has good activity in realizing NAD(P)H regeneration. Noteworthily, UiO67@Rh@UiO66@LDH maintains a high activity level even after 10 cycles. This work reports a novel NAD(P)H regeneration platform to open up a new avenue for constructing chemoenzyme coupling systems.

Postsynthetic Modification of NU-1000 for Designing a Polyoxometalate-Containing Nanocomposite with Enhanced Third-Order Nonlinear Optical Performance

For the advancement of laser technologies and optical engineering, various types of new inorganic and organic materials are emerging. Metal-organic frameworks (MOFs) reveal a promising use in nonlinear optics, given the presence of organic linkers, metal cluster nodes, and possible delocalization of π-electron systems. These properties can be further enhanced by the inclusion of solely inorganic materials such as polyoxometalates as prospective low-cost electron-acceptor species. In this study, a novel hybrid nanocomposite, namely, SiW₁₂@NU-1000 composed of SiW₁₂ (H₄SiW₁₂O₄₀) and Zr-based MOF (NU-1000), was assembled, completely characterized, and thoroughly investigated in terms of its nonlinear optical (NLO) performance. The third-order NLO behavior of the developed system was assessed by Z-scan measurements using a 532 nm laser. The effect of two-photon absorption and self-focusing was significant in both NU-1000 and SiW₁₂@NU-1000. Experimental studies suggested a much superior NLO performance of SiW₁₂@NU-1000 if compared to that of NU-1000, which can be assigned to the charge-energy transfer between SiW₁₂ and NU-1000. Negligible light scattering, good stability, and facile postsynthetic fabrication method can promote the applicability of the SiW₁₂@NU-1000 nanocomposite for various optoelectronic purposes. This research may thus open new horizons to improve and enhance the NLO performance of MOF-based materials through π-electron delocalization and compositing metal-organic networks with inorganic molecules as electron acceptors, paving the way for the generation of novel types of hybrid materials for prospective NLO applications.

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal-organic framework assemblies

Optimising catalyst materials for visible light-driven fuel production requires understanding complex and intertwined processes including light absorption and catalyst stability, as well as mass, charge, and energy transport. These phenomena can be uniquely combined (and ideally controlled) in porous host-guest systems. Towards this goal we designed model systems consisting of molecular complexes as catalysts and porphyrin metal-organic frameworks (MOFs) as light-harvesting and hosting porous matrices. Two MOF-rhenium molecule hybrids with identical building units but differing topologies (PCN-222 and PCN-224) were prepared including photosensitiser-catalyst dyad-like systems integrated via self-assembled molecular recognition. This allowed us to investigate the impact of MOF topology on solar fuel production, with PCN-222 assemblies yielding a 9-fold turnover number enhancement for solar CO2-to-CO reduction over PCN-224 hybrids as well as a 10-fold increase compared to the homogeneous catalyst-porphyrin dyad. Catalytic, spectroscopic and computational investigations identified larger pores and efficient exciton hopping as performance boosters, and further unveiled a MOF-specific, wavelength-dependent catalytic behaviour. Accordingly, CO2 reduction product selectivity is governed by selective activation of two independent, circumscribed or delocalised, energy/electron transfer channels from the porphyrin excited state to either formate-producing MOF nodes or the CO-producing molecular catalysts.

An inorganic-organic hybrid nanomaterial with a core-shell structure constructed by using Mn-BTC and Ag5[BW12O40] for supercapacitors and photocatalytic dye degradation

Creating inorganic-organic hybrids with polyoxometalates (POMs) and metal-organic frameworks (MOFs) as energy storage and dye-degradation materials remains challenging. Here, a new hybrid nanomaterial Mn-BTC@Ag₅[BW₁₂O₄₀] is synthesized by using Ag₅[BW₁₂O₄₀] and Mn₃(BTC)₂(H₂O)₆ (Mn-BTC, BTC = 1,3,5-benzenetricarboxylic acid) through a plain grinding method. The structure and morphology characterization by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) shows that the synthetic products have core-shell construction. Due to its unique structure wherein the core is Mn-BTC and the shell is Ag₅[BW₁₂O₄₀], it exhibits excellent capacitance performance. In a three-electrode system where nickel foam is a collector, at a current density of 1 A g^-1, its specific capacitance is 198.09 F g^-1; after 5000 cycles, the capacitance retention rate is 94.4%. When the power density is 503.1 W kg^-1, the symmetrical supercapacitor reveals a high energy density which is 10.9 W h kg^-1. At the same time, the capacitance retention is 92.9% after 5000 cycles which showed good cycle stability. The photocatalytic degradation efficiencies of rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) dyes exceed 90% after 140 min, and the degradation results remained unchanged after five photocatalytic cycles. The photocatalytic degradation mechanism shows that ˙OH has a major effect. The results show that this research provides a fresh idea for the development of energy storage and dye photocatalytic degradation materials.

Heterogenised Molecular Catalysts for Sustainable Electrochemical CO2 Reduction

There has been a rapid rise in interest regarding the advantages of support materials to protect and immobilise molecular catalysts for the carbon dioxide reduction reaction (CO₂ RR) in order to overcome the weaknesses of many well-known catalysts in terms of their stability and selectivity. In this Review, the state of the art of different catalyst-support systems for the CO₂ RR is discussed with the intention of leading towards standard benchmarking for comparison of such systems across the most relevant supports and immobilisation strategies, taking into account these multiple pertinent metrics, and also enabling clearer consideration of the necessary steps for further progress. The most promising support systems are described, along with a final note on the need for developing more advanced experimental and computational techniques to aid the rational design principles that are prerequisite to prospective industrial upscaling.

Two-Dimensional Porphyrin-Based Covalent Organic Framework with Enlarged Inter-layer Spacing for Tunable Photocatalytic CO2 Reduction

Two-dimensional (2D) porphyrin-based covalent organic frameworks (COFs) are one of the most promising candidates for photocatalytic carbon dioxide reduction reaction (CO₂RR), which however still suffer from the hindered mass transfer during the catalysis procedure associated with the close packing of 2D COF layers due to the strong axial π-π stacking. Herein, condensation between the porphyrinic aldehydes p-MPor-CHO (M = H₂, Co, and Ni) and 3,8-diamino-6-phenyl-phenanthridine (DPP) affords new porphyrin-based 2D COF architecture MPor-DPP-COFs (M = H₂, Co, and Ni). The bulky phenyl substituent at the phenanthridine periphery of the linking unit reduces the axial π-π stacking, providing an enlarged inter-layer spacing of 6.0 Å according to high-resolution transmission electron microscopy. This, in combination with the large surface area (1021 m² g^-1) revealed by nitrogen sorption measurements at 77 K for CoPor-DPP-COF possessing electroactive Co ions, endows it with excellent photocatalytic activity for CO₂RR with a CO generation rate of 10 200 μmol g^-1 h^-1 and a CO selectivity up to 82%. This work affords new ideas for achieving efficient photocatalytic CO₂RR upon fine-tuning the inter-layer spacing of 2D COFs.

An intriguing tetranuclear Rh-based polyoxometalate for the reduction of nitroarene and oxidation of aniline

An organic-inorganic hybrid polyoxometalate Na₅H_5.68 [Na_0.17Rh_0.83^III(C₆H₈N₂)₂Cl₂]₂(C₈H₈N₂)₂[As₄W₄₀O₁₄₀Rh₄^IV(C₆H₄N₂S)₂]·nH₂O (1) containing 5.66 Rh atoms is prepared; to our knowledge, it owns the largest number of Rh atoms in the Rh-POM family. Compound 1 demonstrates good catalytic performance in the reduction of nitrobenzene to aniline and the oxidation of aniline to azobenzene under mild conditions. Moreover, catalyst 1 exhibits high activity, excellent stability and recyclability.

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Switching Excited State Distribution of Metal-Organic Framework for Dramatically Boosting Photocatalysis

Photosensitization associated with electron/energy transfer represents the central science of natural photosynthesis. Herein, we proposed a protocol to dramatically improve the sensitizing ability of metal-organic frameworks (MOFs) by switching their excited state distribution from ³ MLCT (metal-to-ligand charge transfer) to ³ IL (intraligand). The hierarchical organization of ³ IL MOFs and Co/Cu catalysts facilitates electron transfer for efficient photocatalytic H₂ evolution with a yield of 26 844.6 μmol g^-1 and CO₂ photoreduction with a record HCOOH yield of 4807.6 μmol g^-1 among all the MOF photocatalysts. Systematic investigations demonstrate that strong visible-light-absorption, long-lived excited state and ingenious multi-component synergy in the ³ IL MOFs can facilitate both interface and intra-framework electron transfer to boost photocatalysis. This work opens up an avenue to boost solar-energy conversion by engineering sensitizing centers at a molecular level.

Two Copper-Containing Polyoxometalate-Based Metal-Organic Complexes as Heterogeneous Catalysts for the C-H Bond Oxidation of Benzylic Compounds and Olefin Epoxidation

Using a one-pot assembly method, two novel copper-containing Keggin-type polyoxometalates (POMs)-based metal-organic complexes, that is, [Cu^II₂(pbba)₂NO₃^-(H₂O)₂(PW₁₂O₄₀)]·3H₂O [PW₁₂-Cu-pbba, H₂pbba = 1,1'-(1,4-phenylene-bis(methylene))-bis(pyridine-3-carboxylic acid)] and [Cu^II₂(pbba)₂(H₂O)₂(GeW₁₂O₄₀)]·3H₂O (GeW₁₂-Cu-pbba), were successfully synthesized. These two complexes are isostructural, differing only in their POM components. They are applicable as heterogeneous catalysts for the C-H bond oxidation of benzylic compounds and olefin epoxidation under mild conditions, with oxygen as the oxidant and isobutyraldehyde as the coreductant. The catalytic activity of PW₁₂-Cu-pbba was superior to that of GeW₁₂-Cu-pbba. Under the optimal conditions, PW₁₂-Cu-pbba catalyzed the oxidation of indane into 1-indanone with an 81% yield and >99% selectivity within 48 h. As heterogeneous catalysts, both complexes demonstrated excellent recoverability and high stability and could be stably reused five times without significant activity loss.