Creating Well-Defined Hexabenzocoronene in Zirconium Metal–Organic Framework by Postsynthetic Annulation

Metal Center-Tuned Photocatalytic Carbon Dioxide Reduction for Frameworks with the Tetraphenylethene-Imidazole Ligand

As heterogeneous photocatalysts that can effectively transform CO2 to CO, two MOFs with different metal centers, namely, [M(tipe)(H2O)2](ClO4)2·solvent (M = Ni named as Ni-MOF and M = Co referred to as Co-MOF), were synthesized by reactions of 1,1,2,2-tetrakis(4-(imidazole-1-yl)phenyl)ethene (tipe) with the corresponding metal perchlorate. Both Ni-MOF and Co-MOF have 3D structures, in which the metal centers have the same coordination environment with the N4O2 donor set. Driven by visible light, the CO production catalyzed by Co-MOF is 6734.1 μmol g-1 with 45.3% selectivity, and in contrast, Ni-MOF has 4601.3 μmol g-1 CO production with 97.6% selectivity in 5 h. Through photoelectrochemical characterization, DFT calculations, and in situ FT-IR measurements, the photocatalytic CO2 reduction process catalyzed by Ni-MOF and Co-MOF was investigated. The results show that the metal center of the MOF is crucial for photocatalytic CO2 reduction. This work offers an innovative approach for controlling the performance of photocatalytic CO2 reduction through tuning the metal centers of architectures.

Artificial light harvesting system of CM6@Zn-MOF nanosheets with highly enhanced photoelectric performance

As donors for effective energy transfer, metal-organic frameworks (MOFs) have attracted the attention of many experts in the field of artificial light-harvesting materials. This study introduces a novel two-dimensional Zn-MOF, synthesized using flexible 1,3-phenyldiacetic acid (H2mpda) and rigid 1,3,5-tris(1-imidazolyl)benzene (tib) as organic ligands. Through atomic force microscopy (AFM), we have determined the monolayer thickness of this novel material to be 5 nm. Achieving two-dimensional Zn-MOF nanosheets with large BET surface area was made possible by employing ultrasonic stripping techniques. The fluorescence emission spectrum of Zn-MOF nanosheets overlaps with the UV-vis absorption spectrum of coumarin 6 (CM6), so they can be used as a donor and acceptor for fluorescence resonance energy transfer (FRET) to construct an artificial light-harvesting system (ALHS). Compared with single crystal Zn-MOF, CM6@Zn-MOF(2) has a larger BET surface area (41 m2/g), higher quantum yield (Φfl, 30.56 %), narrower energy gap (Eg, 2.87 eV), and the light-harvesting range extends to the visible green light area. Notably, CM6@Zn-MOF(2) demonstrates a robust photocurrent response, characterized by a photocurrent on/off ratio (Ilight/Idark) of 21, and a maximum photocurrent density that surpasses that of pure Zn-MOF (2.25:1). This study successfully designed a high-performance photoelectric conversion material CM6@Zn-MOF(2), which laid a certain theoretical foundation for new artificial optical acquisition systems and electrochemical material selection.

Reticular Chemistry Directed "One-Pot" Strategy to in situ Construct Organic Linkers and Zirconium-Organic Frameworks

Zirconium-based metal-organic frameworks (Zr-MOFs) have emerged as one of the most studied MOFs due to the unlimited numbers of organic linkers and the varying Zr-oxo clusters. However, the synthesis of carboxylic acids, especially multitopic carboxylic acids, is always a great challenge for the discovery of new Zr-MOFs. As an alternative approach, the in situ "one-pot" strategy can address this limitation, where the generation of organic linkers from the corresponding precursors and the sequential construction of MOFs are integrated into one solvothermal condition. Herein, inspired by benzimidazole-contained compounds synthesized via reaction of aldehyde and o-phenylenediamine, tri-, tetra-, penta- and hexa-topic carboxylic acids and a series of corresponding Zr-MOFs can be prepared via the in situ "one-pot" method under the same solvothermal conditions. This strategy can be utilized not only to prepare reported Zr-MOFs constructed using benzimidazole-contained linkers, but also to rationally design, construct and realize functionalities of zirconium-pentacarboxylate frameworks guided by reticular chemistry. More importantly, in situ "one-pot" method can facilitate the discovery of new Zr-MOFs, such as zirconium-hexacarboxylate frameworks. The present study demonstrates the promising potential of benzimidazole-inspired in situ "one-pot" approach in the crystal engineering of structure- and property-specific Zr-MOFs, especially with the guidance of reticular chemistry.

Reticular Chemistry and In Situ "One-Pot" Strategy: A Dream Combination to Construct Metal-Organic Frameworks

The establishment of reticular chemistry has significantly facilitated the development of porous materials, especially for metal-organic frameworks (MOFs). On the other hand, as an alternative approach, in situ "one-pot" strategy has been explored as a promising approach to constructing MOFs, in which the synthesis of organic linkers and the sequential construction of MOFs are integrated into one solvothermal condition. This strategy can efficiently avoid the limitations faced in the traditional construction method, such as time-consuming organic synthesis and multiple separation and purification. Herein, inspired by the reaction of aldehydes and o-phenylenediamine and deep structural analysis of UiO-68, a series of tetra-, hexa-, and octa-topic carboxylic acids are synthesized using 2',3'-diamino-[1,1':4',1'"-terphenyl]-4,4'"-dicarboxylic acid and di-, tri-, and tetra-topic aldehydes as precursor. Then nine multicarboxylate-based zirconium MOFs (Zr-MOFs) are successfully constructed via the combination of reticular chemistry and in situ "one-pot" strategy. The resultant Zr-MOFs can be regarded as the partial face decoration of UiO-68. More importantly, the emission properties of resultant Zr-MOFs can be well controlled using aldehydes with tunable electronic structures. This work provides a new path to rational design and construction of porous materials with specific structures guided by reticular chemistry and conducted using in situ "one-pot" strategy.

Sub-Nanometer Mono-Layered Metal-Organic Frameworks Nanosheets for Simulated Flue Gas Photoreduction

The dilemma between the thickness and accessible active site triggers the design of porous crystalline materials with mono-layered structure for advanced photo-catalysis applications. Here, a kind of sub-nanometer mono-layered nanosheets (Co-MOF MNSs) through the exfoliation of specifically designed Co3 cluster-based metal-organic frameworks (MOFs) is reported. The sub-nanometer thickness and inherent light-sensitivity endow Co-MOF MNSs with fully exposed Janus Co3 sites that can selectively photo-reduce CO2 into formic acid under simulated flue gas. Notably, the production efficiency of formic acid by Co-MOF MNSs (0.85 mmol g-1 h-1) is ≈13 times higher than that of the bulk counterpart (0.065 mmol g-1 h-1) under a simulated flue gas atmosphere, which is the highest in reported works up to date. Theoretical calculations prove that the exposed Janus Co3 sites with simultaneously available sites possess higher activity when compared with single Co site, validating the importance of mono-layered nanosheet morphology. These results may facilitate the development of functional nanosheet materials for CO2 photo-reduction in potential flue gas treatment.

A linker selective retention strategy to construct hierarchical porous metal-organic frameworks with high catalytic activity for oxidative desulfurization

In order to improve the oxidative desulfurization (ODS) performance of MOF materials, an effective way is to convert a microporous MOF into a hierarchical porous MOF (HP-MOF) by utilizing the linker selective retention strategy. Herein, UiO-66 with the introduction of an unstable linker ligand (dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate, dhtz) can selectively remove dhtz ligands to form HP-MOF (HP-UiO-66-dhtz) through heat treatment at high temperature. While maintaining the original structure of UiO-66, HP-UiO-66-dhtz features mesopores and abundant Lewis acid sites, showing excellent ODS performance for diphenylthiophene (DBT).

Dispersive hierarchically porous composites based on defective MOFs as mixed-mode stationary phases for chromatographic separation

Defective metal-organic frameworks-based composites with excellent separation properties were obtained. The mesoporous metal-organic frameworks were selected and deliberately designed to be deficient, and they were then combined with polyacrylamide to be modified on the surface of silica microspheres. The prepared composites were employed as mixed-mode stationary phase in chromatographic separation, and they were compared to both conventional microporous metal-organic framework-based columns and commercial columns. It showed improved selectivity and retention toward both hydrophilic and hydrophobic analytes, allowing for the effective separation of nine nucleosides and nucleobases, eight alkaloids, six antibiotics, and five alkylbenzenes. Additionally, the column was used to effectively separate the active ingredients in the daring substance of honeysuckle, revealing a wide range of possible applications. For the same batch of analytes, three batches of distinct materials demonstrated consistent separation effects. It also demonstrated excellent chromatographic repeatability and stability, with relative standard deviations of the retention time and/or column efficiency being found to be less than 0.8% and 0.9%, respectively. The dispersive hierarchically porous composites were demonstrated to be effective in chromatographic separation, and the results expanded the potential uses of defective MOFs with dispersed multi-level pores.

Reticular chemistry guided precise construction of zirconium-pentacarboxylate frameworks with 5-connected Zr6 clusters

Zirconium-based metal-organic frameworks (Zr-MOFs) have been extensively studied due to their very rich structural chemistry. The combination of nearly unlimited carboxylic acid-based linkers and Zr6 clusters with multiple connectivities has led to diverse structures and specific properties of resultant Zr-MOFs. Herein, we demonstrate the successful use of reticular chemistry to construct two novel Zr-MOFs, HIAM-4040 and HIAM-4040-OH, with zfu topology. Based on a thorough structural analysis of (4,4)-connected lvt-type Zr-tetracarboxylate frameworks and a judicious linker design, we have obtained the first example of a Zr-pentacarboxylate framework featuring unprecedented 5-connected organic linkers and 5-connected Zr6 clusters. Compared with HIAM-4040, a larger Stokes shift is achieved in HIAM-4040-OH via hydroxyl group induced excited-state intramolecular proton transfer (ESIPT). HIAM-4040-OH exhibits high chemical and thermal stability and is used for HClO detection in aqueous solution with excellent sensitivity and selectivity.

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Pore-Confined π-Chromophoric Tetracene as a Visible Light Harvester toward MOF-Based Photocatalytic CO2 Reduction in Water

Integrating photoactive π-chromophoric guest molecules inside the MOF nanopore can result in the emergence of light-responsive features, which in turn can be utilized for developing photoactive materials with inherent properties of MOF. Herein, we report the confining of π-chromophoric tetracene (TET) molecules inside the nanospace of postmodified Zr-MOF-808 (Zr-MOF) with MBA molecules (MBA = 2-(5'-methyl-[2,2'-bipyridine]-5-yl)acetic acid) for effectively utilizing its light-harvesting properties toward photocatalytic CO2 reduction. The confinement of the TET molecules as a photosensitizer and the covalent grafting of a catalytically active [Re(MBA)(CO)3Cl] complex, postsynthetically, result in a single integrated catalytic system named Zr-MBA-TET-Re-MOF. Photoreduction of CO2 over Zr-MBA-TET-Re-MOF showed the evolution of 805 μmol g-1 CO with 99.9% selectivity after 10 h of continuous visible light irradiation in water without any additional sacrificial electron donor and having the apparent quantum efficiency of 1.3%. In addition, the catalyst demonstrated an appreciable activity even under direct sunlight irradiation in aqueous medium with a maximum production of 362.7 μmol g-1 CO, thereby mimicking artificial photosynthesis. Moreover, electron transfer from TET to the catalytic center was supported by the formation of photoinduced TET radical cation, as inferred from in situ UV-vis spectra, electron paramagnetic resonance (EPR) analysis, and transient absorption (TA) studies. Additionally, the in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements support that the photoreduction of CO2 to CO proceeds via *COOH intermediate formation. The close proximity of the light-harvesting molecule and catalytic center facilitated facile electron transfer from the photosensitizer to the catalyst during the CO2 reduction.

Mesoporous MOF Based on a Hexagonal Bipyramid Co8-Cluster: High Catalytic Efficiency on the Cycloaddition Reaction of CO2 with Bulky Epoxides

A mesoporous cobalt-based metal-organic framework (LCU-606) was synthesized based on a hexagonal bipyramid Co8(μ4-O)3 cluster and an N,N,N',N'-tetrakis-(4-benzoic acid)-1,4-phenylenediamine ligand (H4TBAP). LCU-606 featuring large pore diameters of 21.7 Å and exposed Lewis-acid metal sites could serve as an excellent heterogeneous catalyst for CO2 cycloaddition reaction with various epoxide substrates under mild conditions (1 atm CO2, 60 °C, and solvent free). In particular, when extending the substrates to bulkier ones, LCU-606 still shows high catalytic efficiency on account of the large pore aperture. Also, LCU-606 demonstrates high recyclability and stability in consecutive catalytic runs. Therefore, the high efficiency, recyclability, and generality on CO2 catalytic cycloaddition make LCU-606 a very promising heterogeneous catalyst for CO2 chemical fixation.

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.

Postsynthetic Annulation of Three-Dimensional Covalent Organic Frameworks for Boosting CO2 Photoreduction

Three-dimensional covalent organic frameworks (3D COFs), with interconnected pores and exposed functional groups, provide new opportunities for the design of advanced functional materials through postsynthetic modification. Herein, we demonstrate the successful postsynthetic annulation of 3D COFs to construct efficient CO₂ reduction photocatalysts. Two 3D COFs, NJU-318 and NJU-319Fe, were initially constructed by connecting hexaphenyl-triphenylene units with pyrene- or Fe-porphyrin-based linkers. Subsequently, the hexaphenyl-triphenylene moieties within the COFs were postsynthetically transformed into π-conjugated hexabenzo-trinaphthylene (pNJU-318 and pNJU-319Fe) to enhance visible light absorption and CO₂ photoreduction activity. The optimized photocatalyst, pNJU-319Fe, shows a CO yield of 688 μmol g^-1, representing a 2.5-fold increase compared to that of unmodified NJU-319Fe. Notably, the direct synthesis of hexabenzo-trinaphthylene-based COF catalysts was unsuccessful due to the low solubility of conjugated linkers. This study not only provides an effective method to construct photocatalysts but also highlights the unlimited tunability of 3D COFs through structural design and postsynthetic modification.

Minimalist Design for Solar Energy Conversion: Revamping the π-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: the former involves the electron-deficient alkene tetracyanoethylene (TCNE) as the guest reactant; the latter consists of stacked 2D honeycomb covalent networks based on the electron-rich β-ketoenamine hinges that also activate the conjugated, connecting alkyne units. The TCNE/alkyne reaction is a [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) that forms strong push-pull units directly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold, without the need for additional side groups of alkynes or other functions. The ability of the stacked alkyne units (i.e., as part of the honeycomb mass) to undergo such extensive rearrangement highlights the structural flexibility of these covalent organic framework (COF) hosts. The COF solids remain porous, crystalline, and air-/water-stable after the CA-RE modification, while the resulting push-pull units feature distinct open-shell/free-radical character, are strongly light-absorbing, and shift the absorption ends from 590 nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95 eV), so as to better capture sunlight (especially the infrared region which takes up 52% of the solar energy). As a result, the modified COF materials achieve the highest photothermal conversion performances, holding promise in thermoelectric power generation and solar steam generation (e.g., with solar-vapor conversion efficiencies >96%).

Mesoporous Mixed-Metal-Organic Framework Incorporating a [Ru(Phen)3]2+ Photosensitizer for Highly Efficient Aerobic Photocatalytic Oxidative Coupling of Amines

[Ru(Phen)₃]²⁺ (phen = phenanthroline) as a very classical photosensitizer possesses strong absorption in the visible range and facilitates photoinduced electron transfer, which plays a vital role in regulating photochemical reactions. However, it remains a significant challenge to utilize more adequately and exploit more efficiently the ruthenium-based materials due to the uniqueness, scarcity, and nonrenewal of the noble metal. Here, we integrate the intrinsic advantages of the ruthenium-based photosensitizer and mesoporous metal-organic frameworks (meso-MOFs) into a [Ru(Phen)₃]²⁺ photosensitizer-embedded heterometallic Ni(II)/Ru(II) meso-MOF (LTG-NiRu) via the metalloligand approach. LTG-NiRu, with an extremely robust framework and a large one-dimensional (1D) channel, not only makes ruthenium photosensitizer units anchored in the inner wall of meso-MOF tubes to circumvent the problem of product/catalyst separation and recycling of catalysts in heterogeneous systems but also exhibits exceptional activities for the aerobic photocatalytic oxidative coupling of amine derivatives as a general photocatalyst. The conversion of the light-induced oxidative coupling reaction for various benzylamines is ∼100% in 1 h, and more than 20 chemical products generated by photocatalytic oxidative cycloaddition of N-substituted maleimides and N,N-dimethylaniline can be synthesized easily in the presence of LTG-NiRu upon visible light irradiation. Moreover, recycling experiments demonstrate that LTG-NiRu is an excellent heterogeneous photocatalyst with high stability and excellent reusability. LTG-NiRu represents a great potential photosensitizer-based meso-MOF platform with an efficient aerobic photocatalytic oxidation function that is convenient for gram-scale synthesis.

Semirigid Highly Conjugated Zirconium-Organic Framework for the Capture of Micropollutants and Solar-Light Photodegradation

Micro-organic pollutants, particularly organic dyes and personal care products (PPCPs), are widely present in wastewater, and thus pose a serious risk to human health. The capture and solar-light photodegradation of micro-organic pollutants are highly challenging tasks, which require the design and synthesis of microporous materials with specific structures. As we know, organic dyes and PPCPs can be absorbed via π-π* stacking. In this paper, an iron-based metal-organic framework (Fe-UiO-68-terNap) containing semirigid conjugated aromatic ligands is prepared for the capture and solar-light photodegradation of multiple water contaminants. UiO-68-terNap was synthesized based on ternaphthalene with π-π* stacking, which would increase the adsorption capacities of organic micropollutants in wastewater. Additionally, the formation of Fe-O-Zr enhances the charge-separation ability resulting in the successful degradation of micropollutants in 240 min. The novel material has been elucidated by single-crystal X-ray diffraction and Fe K-edge XANES, which provide key insights at a molecular level for the design of novel materials for the capture and photodegradation of organic micropollutants.

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.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Photoreactive Carbon Dioxide Capture by a Zirconium-Nanographene Metal-Organic Framework

The mechanism of photochemical CO₂ reduction to formate by PCN-136, a Zr-based metal-organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a "photoreactive capture" mechanism, where Zr-based nodes serve to capture CO₂ in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a "two-for-one" route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO₂-bound MOF. The mechanistic findings obtained here illustrate several advantages of MOF-based architectures in molecular photocatalyst engineering and provide insights on ways to achieve high formate selectivity.

Zr- and Ti-based metal-organic frameworks: synthesis, structures and catalytic applications

Recently, Zr- and Ti-based metal-organic frameworks (MOFs) have gathered increasing interest in the field of chemistry and materials science, not only for their ordered porous structure, large surface area, and high thermal and chemical stability, but also for their various potential applications. Particularly, the unique features of Zr- and Ti-based MOFs enable them to be a highly versatile platform for catalysis. Although much effort has been devoted to developing Zr- and Ti-based MOF materials, they still suffer from difficulties in targeted synthesis, especially for Ti-based MOFs. In this Feature Article, we discuss the evolution of Zr- and Ti-based MOFs, giving a brief overview of their synthesis and structures. Furthermore, the catalytic uses of Zr- and Ti-based MOF materials in the previous 3-5 years have been highlighted. Finally, perspectives on the Zr- and Ti-based MOF materials are also proposed. This work provides in-depth insight into the advances in Zr- and Ti-based MOFs and boosts their catalytic applications.

Visible-Light-Induced Phenoxyl Radical-based Metal-Organic Framework for Selective Photooxidation of Sulfides

Phenoxyl radicals originating from phenols through oxidation or photoinduction are relatively stable and exhibit mild oxidative activity, which endows them with the potential for photocatalysis. Herein, a stable and recyclable metal-organic framework Zr-MOF-OH constructed of a binaphthol derivative ligand has been synthesized and functions as an efficient heterogeneous photocatalyst. Zr-MOF-OH shows fairly good catalytic activity and substrate compatibility toward the selective oxidation of sulfides to sulfoxides under visible light irradiation. Such irradiation of Zr-MOF-OH converts the phenolic hydroxyl groups of the binaphthol derivative ligand to phenoxyl radicals through excited state intramolecular proton transfer, and the excited state photocatalyst triggers the single-electron oxidation of the sulfide. No reactive oxygen species are produced in the photocatalytic process, and triplet O₂ directly participates in the reaction, endowing Zr-MOF-OH with wide substrate compatibility and high selectivity, which also proposes a promising pathway for the direct activation of substrates via phenoxyl radicals.

Direct Construction of 2D Conductive Metal-Organic Frameworks from a Nonplanar Ligand: In Situ Scholl Reaction and Topological Modulation

Two-dimensional conductive metal-organic frameworks (2D c-MOFs) are an emerging class of promising porous materials with high crystallinity, tunable structures, and diverse functions. However, the limited topologies and difficulties in synthesizing suitable organic linkers remain a great challenge for 2D c-MOFs synthesis and applications. Herein, two layered 2D c-MOF polymorphs with either a rhombus structure (sql-TBA-MOF) or kagome structure (kgm-TBA-MOF) were directly constructed via in situ Scholl reaction and coordination chemistry from a flexible and nonplanar tetraphenylbenzene-based ligand (8OH-TPB) in a one-pot manner. Interestingly, the kgm-TBA-MOF comprising hexagonal and triangular dual pores exhibit higher conductivities of 1.65 × 10^-3 S/cm at 298 K and 3.33 × 10^-2 S/cm at 353 K than that of sql-TBA-MOF (4.48 × 10^-4 and 2.90 × 10^-3 S/cm, respectively). Moreover, the morphology and topology can be modulated via the addition of ammonium hydroxide as modulator. The present work provides a new pathway for design, synthesis, and topological regulation of 2D c-MOFs.

Recent Advances in Tetra- (Ti, Sn, Zr, Hf) and Pentavalent (Nb, V, Ta) Metal-Substituted Molecular Sieve Catalysis

Metal substitution of molecular sieve systems is a major driving force in developing novel catalytic processes to meet current demands of green chemistry concepts and to achieve sustainability in the chemical industry and in other aspects of our everyday life. The advantages of metal-substituted molecular sieves include high surface areas, molecular sieving effects, confinement effects, and active site and morphology variability and stability. The present review aims to comprehensively and critically assess recent advances in the area of tetra- (Ti, Sn, Zr, Hf) and pentavalent (V, Nb, Ta) metal-substituted molecular sieves, which are mainly characterized for their Lewis acidic active sites. Metal oxide molecular sieve materials with properties similar to those of zeolites and siliceous molecular sieve systems are also discussed, in addition to relevant studies on metal-organic frameworks (MOFs) and some composite MOF systems. In particular, this review focuses on (i) synthesis aspects determining active site accessibility and local environment; (ii) advances in active site characterization and, importantly, quantification; (iii) selective redox and isomerization reaction applications; and (iv) photoelectrocatalytic applications.

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.

Engineering the Charge Density on an In2.77S4/Porous Organic Polymer Hybrid Photocatalyst for CO2-to-Ethylene Conversion Reaction

The development of an efficient photocatalyst for C2 product formation from CO₂ is of urgent importance toward the deployment of solar-fuel production. Here, we report a template-free, cost-effective synthetic strategy to develop a carbazole-derived porous organic polymer (POP)-based composite catalyst. The composite catalyst is comprised of In_2.77S₄ and porous organic polymer (POP) and is held together by induced-polarity-driven electrostatic interaction. Utilizing the synergy of the catalytically active In centers and light-harvesting POPs, the catalyst showed 98.9% selectivity toward the generation of C₂H₄, with a formation rate of 67.65 μmol g^-1 h^-1. Two different oxidation states of the In_2.77S₄ spinel were exploited for the C-C coupling process, and this was investigated by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations. The role of POP was elucidated via several photophysical and photoelectrochemical studies. The electron transfer was mapped by several correlated approaches, which assisted in establishing the Z-scheme mechanism. Furthermore, the mechanism of C₂H₄ formation was extensively investigated using density functional theory (DFT) calculations from multiple possible pathways.

Three new Zn(ii) coordination polymers for highly selective and sensitive detection of Fe3+

In this work, three novel Zn(ii)-CPs with diverse structures and fascinating topologies can be highly selective and sensitive luminescent sensors for detection of Fe3+.

Zr-Based Metal-Organic Frameworks with Phosphoric Acids for the Photo-Oxidation of Sulfides

Heterogeneous Brønsted acidic catalysts such as phosphoric acids are the conventional activators for organic transformations. However, the photocatalytic performance of these catalysts is still rarely explored. Herein, a novel Zr-based metal-organic framework Zr-MOF-P with phosphoric acids as a heterogeneous photocatalyst has been fabricated, which shows high selectivity and reactivity towards the photo-oxidation of sulfides under white light illumination. A mechanism study indicates that the selective oxygenation of sulfides occurs with triplet oxygen rather than common reactive oxygen species (ROS). When Zr-MOF-P is irradiated, the hydroxyl group of phosphoric acid is converted into oxygen radical, which takes an electron from the sulfides, and then the activated substrates react with the triplet oxygen to form sulfoxides, avoiding the destruction of the catalysts and endowing the reaction with high substrate compatibility and fine recyclability.

Hexaphenylbenzene-Based Deep Blue-Emissive Metallacages as Donors for Light-Harvesting Systems

We herein report the preparation of a series of hexaphenylbenzene (HPB)-based deep blue-emissive metallacages via multicomponent coordination-driven self-assembly. These metallacages feature prismatic structures with HPB derivatives as the faces and tetracarboxylic ligands as the pillars, as evidenced by NMR, mass spectrometry and X-ray diffraction analysis. Light-harvesting systems were further constructed by employing the metallacages as the donor and a naphthalimide derivative (NAP) as the acceptor, owing to their good spectral overlap. The judiciously chosen metallacage serves as the antenna, providing the suitable energy to excite the non-emissive NAP, and thus resulting in bright emission for NAP in the solid state. This study provides a type of HPB-based multicomponent emissive metallacage and explores their applications as energy donors to light up non-emissive fluorophores in the solid state, which will advance the development of emissive metallacages as useful luminescent materials.

Filling Polyoxoanions into MIL-101(Fe) for Adsorption of Organic Pollutants with Facile and Complete Visible Light Photocatalytic Decomposition

Transition metal-substituted polyoxometalates (POMs) were filled into a metal–organic framework (MOF) to construct a series of POM@MOF composites (PMo₁₂O₄₀@MIL-101, PMo₁₁VO₄₀@MIL-101, PMo₁₀V₂O₄₀@MIL-101). The composite materials possess ultra-high adsorption ability, especially for PMo₁₀V₂O₄₀@MIL-101, with an adsorption capacity of 912.5 mg·g⁻¹ for cationic antibiotic tetracycline in wastewater, much higher than that of isolated MIL-101(Fe) and the commonly used adsorption materials, such as activated carbon and graphene oxide. In particular, they can be used as efficient photocatalysts for the photodegradation of antibiotics under visible light irradiation. The complete photodegradation of the adsorbed species can induce the facile reusability of these composites for multiple cycles. This work opens an avenue to introduce POMs into an MOF matrix for the simultaneous adsorption and photodegradation of antibiotics.

A Sulfur-Tolerant MOF-Based Single-Atom Fe Catalyst for Efficient Oxidation of NO and Hg0

Catalytic oxidation of NO and Hg⁰ is a crucial step to eliminate multiple pollutants from emissions from coal-fired power plants. However, traditional catalysts exhibit low catalytic activity and poor sulfur resistance due to low activation ability and poor adsorption selectivity. Herein, a single-atom Fe decorated N-doped carbon catalyst (Fe₁ -N₄ -C), with abundant Fe₁ -N₄ sites, based on a Fe-doped metal-organic framework is developed to oxidize NO and Hg⁰ . The results demonstrate that the Fe₁ -N₄ -C has ultrahigh catalytic activity for oxidizing NO and Hg⁰ at low and room temperature. More importantly, Fe₁ -N₄ -C exhibits robust sulfur resistance as it preferably adsorbs reactants over sulfur oxides, which has never been achieved before with traditional catalysts. Furthermore, SO₂ boosts the catalytic oxidation of NO over Fe₁ -N₄ -C through accelerating the circulation of active sites. Density functional theory calculations reveal that the Fe₁ -N₄ active sites result in a low energy barrier and high adsorption selectivity, providing detailed molecular-level understanding for its excellent catalytic performance. This is the first report on NO and Hg⁰ oxidation over single-atom catalysts with strong sulfur tolerance. The outcomes demonstrate that single-atom catalysts are promising candidates for catalytic oxidation of NO and Hg⁰ enabling cleaner coal-fired power plant operations.

Metal-organic frameworks and their composites for fuel and chemical production via CO2 conversion and water splitting

Increase in the global energy demand has been leading to major energy crises in recent years. The use of excess fossil fuels for energy production is causing severe global warming, as well as energy shortage. To overcome the global energy crisis, the design of various chemical structures as efficient models for the generation of renewable energy fuels is very much crucial, and will limit the use of fossil fuels. Current challenges involve the design of Metal-Organic Framework (MOF) materials for this purpose to diminish the energy shortage. The large surface area, tunable pore environment, unique structural property and semiconducting nature of the highly porous MOF materials enhance their potential applications towards the production of enhanced energy fuels. This review is focused on the architecture of MOFs and their composites for fuels and essential chemicals production like hydrogen, methane, ethanol, methanol, acetic acid, and carbon monoxide, which can be used as renewable fuel energy sources to limit the use of fossil fuels, thereby reducing global warming.

The Chemistry of Zirconium/Carboxylate Clustering Process: Acidic Conditions to Promote Carboxylate-Unsaturated Octahedral Hexamers and Pentanuclear Species

Clustering chemistry is a key point in the design and synthesis of the secondary building units that comprise metal-organic frameworks (MOFs) based on group IV metals. In this work, the first stages of the zirconium-carboxylate clustering process in alcohol/water mixtures are studied in detail using the monocarboxylic benzoic and hydroxybenzoic acids to avoid the polymerization. Mass spectroscopy measurements performed on the reactions revealed the presence of hexa- and pentanuclear species even at low pH values and also evidenced the acid-base nature and pH dependence of the transformation between both species. The control on the chemistry governing the equilibria between these species has allowed us to isolate six new compounds in the solid state. The single-crystal X-ray diffraction analysis revealed that they are closely related to the well-known [Zr₆(O)₄(OH)₄(OOC)₁₂] secondary building unit found in many MOFs by removing carboxylic ligands in the case of the hexameric species ([Zr₆(O)₄(OH)₄(OOC)₈(H₂O)₈]⁴⁺) or by additionally removing one of the metal centers in the case of the pentameric entities ([Zr₅(O)₂(OH)₆(OOC)₄(H₂O)₁₁(alcohol)]⁶⁺). Going in detail, the unsaturated hexameric clusters exhibit different dispositions of their eight carboxylate ligands in such a way that the remaining four carboxylate-free positions are arranged according to a square planar or tetrahedral symmetry. It should be highlighted that the pentameric complexes imply an unprecedented core nuclearity in zirconium clusters and thus their isolation provides a novel building block for the design of metal-organic materials.

Encapsulation of Polymetallic Oxygen Clusters in a Mesoporous/Microporous Thorium-Based Porphyrin Metal-Organic Framework for Enhanced Photocatalytic CO2 Reduction

Solar-initiated CO₂ reduction is significant for green energy development. Herein, we have prepared a new mesoporous/microporous porphyrin metal-organic framework (MOF), IHEP-20, loaded with polymetallic oxygen clusters (POMs) to form a composite material POMs@IHEP-20 for visible-light-driven photocatalytic CO₂ reduction. The as-made composite material exhibits good stability in water from pH 0 to 11. After POMs were introduced to IHEP-20, they showed superior activity toward photocatalytic CO₂ reduction with a CO production rate of 970 μmol·g^-1·h^-1, which is 3.27 times higher than that of pristine IHEP-20. This study opens a new door for the design and synthesis of high-performance catalysts for the photocatalytic reduction of CO₂.

Syntheses of New Zeolitic Imidazolate Frameworks in Dimethyl Sulfoxide

Presented here are the syntheses of ZIFs in dimethyl sulfoxide (DMSO). A series of new ZIFs with various topologies such as ACO, coi, zni, ANA, GIS, even new topology can...

Building a Pyrazole-Benzothiadiazole-Pyrazole Photosensitizer into Metal-Organic Frameworks for Photocatalytic Aerobic Oxidation

Charge separation plays a crucial role in regulating photochemical properties and therefore warrants consideration in designing photocatalysts. Metal-organic frameworks (MOFs) are emerging as promising candidates for heterogeneous photocatalysis due to their structural designability and tunability of photon absorption. Herein, we report the design of a pyrazole-benzothiadiazole-pyrazole organic molecule bearing a donor-acceptor-donor conjugated π-system for fast charge separation. Further attempts to integrate such a photosensitizer into MOFs afford a more effective heterogeneous photocatalyst (JNU-204). Under visible-light irradiation, three aerobic oxidation reactions involving different oxygenation pathways were achieved on JNU-204. Recycling experiments were conducted to demonstrate the stability and reusability of JNU-204 as a robust heterogeneous photocatalyst. Furthermore, we illustrate its applications in the facile synthesis of pyrrolo[2,1-a]isoquinoline-containing heterocycles, core skeletons of a family of marine natural products. JNU-204 is an exemplary MOF platform with good photon absorption, suitable band gap, fast charge separation, and extraordinary chemical stability for proceeding with aerobic oxidation reactions under visible-light irradiation.

Full-Color Emission in Multicomponent Metal-Organic Frameworks via Linker Installation

Herein, we demonstrate that linker installation (LI) through postsynthesis is an effective strategy to insert emissive second linkers into single-linker-based metal-organic frameworks (MOFs) to tune the emission properties of multicomponent MOFs. Full-color emission, including white-light emission, can be achieved via such a LI process.

Crystal Structure Transformation in Hydrogen-bonded Organic Frameworks via Ion Exchange

Hydrogen-bonded organic frameworks (HOFs) have emerged as rapidly growing porous materials while established permanent porosities are very fragile and difficult to stabilize due to weak hydrogen-bonding interactions among building units. Herein, we report a stable hydrogen-bonded metallotecton framework (termed as HOF-ZJU-102) that was constructed through hydrogen-bonding networks between cationic metal-organic complexes [Cu₂ (Hade)₄ (H₂ O)₂ ]⁴⁺ (Hade=adenine) and GeF₆ ^2- anions. The framework not only shows permanent porosity, but also exhibits efficient separation performance of C₂ H₂ /C₂ H₄ at room temperature. More interestingly, its crystal structure could be irreversibly transformed into isostructural counterpart HOF-ZJU-101 by ion exchange in the SiF₆ ^2- containing solution, evidenced by multiple characterization techniques including gas sorption measurements, ¹⁹ F NMR spectra, FTIR and EDS. Utilizing such an ion exchange mechanism, the collapsed HOF-ZJU-102 could be restored into HOF-ZJU-101 by simply soaking in the salt solution.

High Water Adsorption MOFs with Optimized Pore-Nanospaces for Autonomous Indoor Humidity Control and Pollutants Removal

The indoor air quality is of prime importance for human daily life and health, for which the adsorbents like zeolites and silica-gels are widely used for air dehumidification and harmful gases capture. Herein, we develop a pore-nanospace post-engineering strategy to optimize the hydrophilicity, water-uptake capacity and air-purifying ability of metal-organic frameworks (MOFs) with long-term stability, offering an ideal candidate with autonomous multi-functionality of moisture control and pollutants sequestration. Through variant tuning of organic-linkers carrying hydrophobic and hydrophilic groups in the pore-nanospaces of prototypical UiO-67, a moderately hydrophilic MOF (UiO-67-4Me-NH₂ -38 %) with high thermal, hydrolytic and acid-base stability is screened out, featuring S-shaped water sorption isotherms exactly located in the recommended comfortable and healthy ranges of relative humidity for indoor ventilation (45 %-65 % RH) and adverse health effects minimization (40-60 % RH). Its exceptional attributes of water-uptake working capacity/efficiency, contaminants removal, recyclability and regeneration promise a great potential in confined indoor environment application.

Stepwise Assembly of a Multicomponent Heterometallic Metal-Organic Framework via Th6-Based Metalloligands

Herein we present a new metalloligand, Th₆L₁₂ [IHEP-10; L = 4-pyrazolecarboxylic acid (H₂PyC)], which can be used to generate a novel multicomponent heterometallic metal-organic framework (MOF), [[Cu₃(μ₃-OH)(NO₃)(H₂O)₂]₂Th₆(μ₃-O)₄(μ₃-OH)₄(PyC)₆(HPyC)₆(H₂O)₆](NO₃)₂ (IHEP-11), through further assembly with second [Cu₃(μ₃-OH)(PyC)₃] clusters. In IHEP-11, six Cu₃ clusters are connected by six NO₃^- anions to form an unprecedented annular Cu₁₈ cluster, which can be viewed as a 12-connected node to link with 12 Th₆ clusters, resulting a 4,12-connected shp net. Benefiting from the cationic framework and 3D porous structure, IHEP-11 can efficiently remove ReO₄^- (an analogue of radioactive ⁹⁹TcO₄^-) from aqueous solution in a wide pH range. This work highlights the feasibility of constructing multicomponent MOFs through a step-by-step synthesis strategy based on metalloligands.